No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Naloxone fails to produce conditioned place aversion in ??-opioid receptor knock-out mice
Abstract
There is growing evidence that tonic activity of the opioid system may be important in the modulation of affective state. Naloxone produces a conditioned place aversion in rodents, an effect that is centrally mediated. Previous pharmacological data using antagonists with preferential actions at mu-, delta-, and kappa-opioid receptors indicate the importance of the mu-opioid receptor in mediating this effect. We sought to test the mu-opioid receptor selectivity of naloxone aversion using mu-opioid receptor knock-out mice. mu-Opioid receptor knock-out and wild-type mice were tested for naloxone (10 mg/kg, s.c.) aversion using a place conditioning paradigm. As a positive control for associative learning, knock-out mice were tested for conditioned place aversion to a kappa agonist, U50,488H (2 mg/kg, s.c.). Naloxone produced a significant place aversion in wild-type mice, but failed to have any effect in mu-opioid receptor knock-out mice. On the other hand, both knock-out and wild-type mice treated with U50,488H spent significantly less time in the drug-paired chamber compared to their respective vehicle controls. We conclude that the mu-opioid receptor is crucial for the acquisition of naloxone-induced conditioned place aversion. Furthermore, in a separate experiment using C57BL/6 mice, the delta-selective antagonist naltrindole (10 or 30 mg/kg, s.c.) failed to produce conditioned place aversion.Taken together, these data further support the notion that naloxone produces aversion by antagonizing tonic opioid activity at the mu-opioid receptor.
... However, the effect of opiate antagonists has also been analyzed in apparently concurrent learning tasks, showing that their administration in neonatal animals blocks the acquisition of olfactory preferences induced by the simultaneous intraoral administration of nutritive substances (Shide & Blass, 1991) or electric shocks (Roth & Sullivan, 2003). In this line, opiate antagonists induce aversion for the place with which they are associated in place conditioning tasks (Mucha, Millan, & Herz, 1985;Shoblock & Maidment, 2006;Stolerman, 1985) and block place preferences induced by morphine (Manzanedo, Serrano, Aguilar, Rodríguez-Arias, & Miñarro, 2001;Mucha, van der Kooy, O'Shaughnessy, & Bucenieks, 1982); cocaine (Mitchem et al., 1999); ethanol (Nizhnikov, Pautassi, Truxell, & Spear, 2009a;Nizhnikov, Varlinskaya, & Spear, 2009b); locomotor activity (Lett, Grant, & Koh, 2001); food (sucrose; Ågmo et al., 1995); and concurrent rewarding intracranial electrical stimulation of the external lateral parabrachial nucleus (Simón, García, & Puerto, 2011;Simón, García, Zafra, Molina, & Puerto, 2007) or insular cortex (García, Simón, & Puerto, 2013;García, Zafra, & Puerto, 2015), brain regions with abundant opiate receptors (Atweh & Kuhar, 1977;Chamberlin, Mansour, Watson, & Saper, 1999;Ding, Kaneko, Nomura, & Mizuno, 1996;Izenwasser, Staley, Cohn, & Mash, 1999;Mansour, Khachaturian, Lewis, Akil, & Watson, 1988;Sales, Riche, Roques, & Denavit-Saubie, 1985;Skoubis, Matthes, Walwyn, Kieffer, & Maidment, 2001;Svingos, Cheng, Clarke, & Pickel, 1995). Moreover, in transgenic animals lacking receptors, opiate antagonists do not induce aversion (Skoubis et al., 2001) and morphine does not induce preference (Matthes et al., 1996) for associated places. ...
... In this line, opiate antagonists induce aversion for the place with which they are associated in place conditioning tasks (Mucha, Millan, & Herz, 1985;Shoblock & Maidment, 2006;Stolerman, 1985) and block place preferences induced by morphine (Manzanedo, Serrano, Aguilar, Rodríguez-Arias, & Miñarro, 2001;Mucha, van der Kooy, O'Shaughnessy, & Bucenieks, 1982); cocaine (Mitchem et al., 1999); ethanol (Nizhnikov, Pautassi, Truxell, & Spear, 2009a;Nizhnikov, Varlinskaya, & Spear, 2009b); locomotor activity (Lett, Grant, & Koh, 2001); food (sucrose; Ågmo et al., 1995); and concurrent rewarding intracranial electrical stimulation of the external lateral parabrachial nucleus (Simón, García, & Puerto, 2011;Simón, García, Zafra, Molina, & Puerto, 2007) or insular cortex (García, Simón, & Puerto, 2013;García, Zafra, & Puerto, 2015), brain regions with abundant opiate receptors (Atweh & Kuhar, 1977;Chamberlin, Mansour, Watson, & Saper, 1999;Ding, Kaneko, Nomura, & Mizuno, 1996;Izenwasser, Staley, Cohn, & Mash, 1999;Mansour, Khachaturian, Lewis, Akil, & Watson, 1988;Sales, Riche, Roques, & Denavit-Saubie, 1985;Skoubis, Matthes, Walwyn, Kieffer, & Maidment, 2001;Svingos, Cheng, Clarke, & Pickel, 1995). Moreover, in transgenic animals lacking receptors, opiate antagonists do not induce aversion (Skoubis et al., 2001) and morphine does not induce preference (Matthes et al., 1996) for associated places. ...
... In this context, studies using place conditioning tasks have shown that opiate antagonists block place preferences associated with morphine (Manzanedo et al., 2001;Mucha et al., 1982), cocaine (Mitchem et al., 1999), ethanol (Nizhnikov et al., 2009a(Nizhnikov et al., , 2009b, locomotor activity (Lett et al., 2001), and food (Ågmo et al., 1995), and also those associated with concurrent rewarding electrical stimulation of the parabrachial complex (Simón et al., 2007(Simón et al., , 2011 and insular cortex (García et al., 2013(García et al., , 2015. Moreover, Nx does not induce place aversion (Skoubis et al., 2001), and place preferences are not induced by morphine (Matthes et al., 1996) in transgenic animals lacking opiate receptors. ...
The role of opiate systems has been extensively studied in relation to learning and memory. Naloxone (Nx), an opiate antagonist, was administrated in concurrent (Experiment 1) and sequential (Experiment 2) flavor aversion learning (FAL) tasks. The outcomes demonstrate that Nx impairs the acquisition of concurrent but not sequential FAL. In the concurrent learning (7 trials), both control (vehicle) and Nx2 (treated with Nx only on the first 2 days) groups learned the task. Furthermore, these 2 groups retained the learning in a discrimination test without drug administration (Day 8) but failed a reversal test (Day 9). In contrast, the Nx group (7 trials with Nx) showed no concurrent learning but correctly performed the discrimination test (Day 8) and, critically, the reversal test. These results suggest that Nx blocks concurrent (implicit) learning in these experiments but induces animals to resort to new strategies that are flexible, a characteristic of explicit learning.
... In contrast to mu opioid receptor agonists such as OXY that produce CPP (Bryant et al., 2012a), opioid receptor antagonists such as naloxone (NAL) produce a conditioned place aversion for the drug-paired environment (Mucha and Iversen, 1984;Mucha et al., 1985;Burgdorf et al., 2001;Murphy, 2004, 2005;Shoblock and Maidment, 2006). NAL-CPA requires mu opioid receptors (Skoubis et al., 2001) and endogenous enkephalins (Skoubis et al., 2005) but does not require D1 or D2 dopamine receptors (Narayanan et al., 2004). Sensitivity to NAL-CPA is greatly enhanced by prior chronic administration of mu opioid receptor agonists (Shoblock and Maidment, 2006) and represents a model for the negative affective-motivational component of opioid withdrawal (Schulteis et al., 1994). ...
... Furthermore, pilot data from both strains indicated a more pronounced CPP and locomotor stimulatory effect of 5 mg/kg OXY relative to lower doses (data not shown). The dose of NAL (4 mg/kg, i.p.) was chosen based on previous studies employing a dose range of 4-10 mg/kg (Skoubis et al., 2001(Skoubis et al., , 2005Narayanan et al., 2004;Sakoori and Murphy, 2004Solecki et al., 2009) and based on pilot studies indicating that this dose was more effective at inducing CPA than 2 mg/kg (data not shown). ...
... The selectivity of the strain difference for the aversive properties of an opioid receptor antagonist and not the rewarding properties of a mu opioid receptor agonist suggests a difference in tonic release of endogenous opioids (Skoubis et al., 2005) or perhaps stress-induced release of endogenous opioids during CPP training that, in turn, leads to an adaptation at the receptor level [e.g., constitutive activity of the mu opioid receptor (Shoblock and Maidment, 2006)] or at the downstream level of the aversion neurocircuitry. In support, endogenous enkephalins are necessary for NAL-CPA (Skoubis et al., 2005) but not opioidinduced CPP (Marquez et al., 2006) whereas mu opioid receptors are necessary for both phenotypes (Matthes et al., 1996;Skoubis et al., 2001). Interestingly, NAL-CPA-prone J mice also drink more ethanol than C57BL/6N mice from the Charles River vendor (Bryant et al., 2008) and this behavior is in part mediated by endogenous opioids (Racz et al., 2008). ...
... We injected the mu-opioid competitive antagonist, the NLX pretesting of morphine conditioning to induce the withdrawal behavioral signs in the morphine-conditioned rats. According to our data, the NLX (0.1-0.4 μg/rat, intra-CeA), when administered before testing of morphine 5 mg/kg (s.c.) response, induced a significant place aversion, reflecting the opioid system involvement, and accords with a previous finding (Skoubis et al. 2001). The previous studies have also shown an aversive response to the morphine due to the blockade of mu-receptors by the NLX (Bie et al. 2009). ...
... Still they protect the animal against the adverse effects of the NLX, the finding which is important in eliminating undesirable morphine withdrawal symptoms. This finding may correlate the presence of the particles in the CeA with the increase in the level of DA, likely by inhibition of GABA neurons (Skoubis et al. 2001), or with the NO production via the oxygen-free radical components. A previous work has shown that the intermediate concentrations of the Ag-NPs can enhance learning and memory formation in the aquatic organisms (Young et al. 2017). ...
Repeated injection of morphine during conditioned place preference (CPP) leads to spatial craving due to high-level nitric oxide (NO) in the central nucleus of amygdala (CeA). Silver nanoparticles (Ag-NPs) can produce oxygen-free radicals that lead to NO formation. We aimed to show the Ag-NPs protective effect on naloxone (NLX)-induced morphine withdrawal in the conditioned rats. Wistar rats (300–350 g) were implanted with cannulae in the CeA. After recovery, they were randomly divided into experimental and saline groups. CPP was conducted by three-phase unbiased program. Morphine (0.5–7.5 mg/kg) was injected subcutaneously (s.c.) once/per day during the conditioning phase. Naloxone (NLX) (0.05–0.4 μg/rat) was given, intra-CeA, 10 min before the CPP test. Ag-NPs (0.0001–0.01 μg/rat) were administered alone or prior to the NLX effective dose (0.4 μg/rat), intra-CeA. Conditioning score and withdrawal signs (wet dog shaking and scratching) were obtained and compared with saline group data. All rats’ brains were collected in formalin 10% and after 48–72 h stained with NADPH-diaphorase, the NO marker. All data were analyzed by one-way or two-way ANOVA. Morphine (2.5–7.5 mg/kg, s.c.) induced a significant CPP vs. saline (1 mL/kg, s.c.). The single Ag-NPs had no significant effect, whereas the NLX caused meaningful WDS and scratching. However, the NLX pre-treatment in combination with Ag-NPs eliminated these signs. Furthermore, the NO level increased in the CeA. The Ag-NPs may protect the morphine-conditioned rats against the NLX-induced withdrawal symptoms due to high-level NO in the CeA.
... More specially, non-peptidyl, selective, and potent MOR antagonists are highly desired for functional characterization of MOR. [26][27][28] Apart from being a pharmacological tool, MOR antagonists may also play a critical role in treatment of substance abuse and addiction. 16,18 Some partially selective MOR antagonists including naloxone, naltrexone and nalmefene have been reported to have beneficial effects including decrease in drug craving and relapse for opiate addicts as well as in treatment of alcoholism. ...
... [29][30][31][32] Although these MOR antagonists are promising, some serious side effects have also been observed. Besides causing depression and dysphoria, [26][27][28][33][34][35] these drugs have also been reported to cause higher incidence of overdoses and suicides among the opiate addict patients 29,31,32,36 due to their affinity to the DOR and KOR and low selectivity to the MOR. Hence, it is plausible that such side effects could be mitigated by development of more selective MOR antagonists. ...
Mu opioid receptor selective antagonists are highly desirable because of their utility as pharmacological probes for receptor characterization and functional studies. Furthermore, the mu opioid receptors act as an important target in drug abuse and addiction treatment. Previously, we reported NAP as a novel lead compound with high selectivity and affinity towards the mu opioid receptor. Based on NAP, we have synthesized its derivatives and further characterized their binding affinities and selectivity towards the receptor. NMP and NGP were identified as the two most selective MOR ligands among NAP derivatives. In the present study, molecular modeling methods were applied to assess the dual binding modes of NAP derivatives, particularly on NMP and NGP, in three opioid receptors, in order to analyze the effects of structural modifications on the pyridyl ring of NAP on the binding affinity and selectivity. The results indicated that the steric hindrance, electrostatic, and hydrophobic effects caused by the substituents on the pyridyl ring of NAP contributed complimentarily on the binding affinity and selectivity of NAP derivatives to three opioid receptors. Analyses of these contributions provided insights on future design of more potent and selective mu opioid receptor ligands.
... Other drugs were injected just before the refeeding test. The doses of drugs have been previously found to be effective on some behavior (Grevert and Goldstein, 1977;Endoh et al., 1992;Suzuki et al., 1993Suzuki et al., , 1996Suzuki et al., , 1997Skoubis et al., 2001;Saitoh et al., 2011) and were optimized to avoid the influence of locomotor activity. ...
... The results showed that the systemic administration of opioid receptor agonists, morphine, KNT-127 and U-50,488H, did not change food intake in food-deprived mice. Since the doses used have been reported to be pharmacologically effective (Suzuki et al., 1996;Skoubis et al., 2001;Saitoh et al., 2011), these results indicate that the stimulation of opioid receptors under conditions of food deprivation does not affect food intake. On the other hand, administration of the non-selective opioid receptor antagonist naloxone significantly reduced food intake in food-deprived mice. ...
Opioid receptors, especially μ-opioid receptors, in the ventral tegmental area (VTA) and nucleus accumbens (NAcc) are reported to regulate food motivation. However, the roles of μ-, δ- and κ-opioid receptors are not fully understood. Moreover, since μ-, δ- and κ-opioid receptors are reported to distribute in the hypothalamus, these receptors in the hypothalamus might regulate feeding behavior. Thus, the present study investigated the role of μ-, δ- and κ-opioid receptors in the VTA, the NAcc and the hypothalamus in the regulation of feeding behavior. Male ICR mice were subjected to a feeding test after food deprivation for 16 h. The mRNA levels of proopiomelanocortin (POMC), preproenkephalin (PENK) and prodynorphin (PDYN), the precursors of endogenous opioid peptides, were measured by RT-PCR. The systemic injection of non-selective (naloxone) and selective μ (β-funaltrexamine; β-FNA), δ (naltrindole) and κ (norbinaltorphimine; norBNI) opioid receptor antagonists markedly reduced food intake. In contrast, the systemic injection of preferential μ (morphine), selective δ (KNT-127) and κ (U-50,488) opioid receptor agonists did not change food intake. The mRNA levels of POMC, PENK and PDYN were decreased in the hypothalamus and the midbrain after food deprivation, whereas the mRNA levels of PENK and PDYN, but not POMC, were decreased in the ventral striatum. The injection of naloxone into the NAcc, VTA and lateral hypothalamus (LH), but not the ventromedial nucleus of the hypothalamus, significantly decreased food intake. The injection of β-FNA and naltrindole into the LH, but not the VTA or NAcc, decreased food intake. The injection of norBNI into the LH and VTA, but not the NAcc, decreased food intake. These results indicate that μ-, δ- and κ-opioid receptors in the LH play more important role in the regulation of feeding behavior than those receptors in the VTA and the NAcc.
... 2006). NAL-CPA requires mu opioid receptors (Skoubis et al. 2001) and endogenous enkephalins (Skoubis et al. 2005) but does not require D1 or D2 dopamine receptors (Narayanan et al. 2004). Sensitivity to NAL-CPA is greatly enhanced by prior chronic administration of mu opioid receptor agonists (Shoblock and Maidment. ...
... 2006)] or at the downstream level of the aversion neurocircuitry. In support, endogenous enkephalins are necessary for NAL-CPA (Skoubis et al. 2005) but not opioid-induced CPP (Marquez et al. 2006) whereas mu opioid receptors are necessary for both phenotypes (Matthes et al. 1996;Skoubis et al. 2001). Interestingly, NAL-CPA-prone J mice also drink more ethanol than C57BL/6N mice from the Charles River vendor (Bryant et al. 2008) and this behavior is in part mediated by endogenous opioids (Racz et al. 2008). ...
Drug liking versus drug disliking is a subjective motivational measure in humans that assesses the addiction liability of drugs. Variation in this trait is hypothesized to influence vulnerability versus resilience toward substance abuse disorders and likely contains a genetic component. In rodents and humans, conditioned place preference (CPP) / aversion (CPA) is a Pavlovian conditioning paradigm whereby a learned preference for the drug-paired environment is used to infer drug liking whereas a learned avoidance or aversion is used to infer drug disliking. C57BL/6 inbred mouse substrains are nearly genetically identical, yet demonstrate robust differences in addiction-relevant behaviors, including locomotor sensitization to cocaine and consumption of ethanol. Here, we tested the hypothesis that B6 substrains would demonstrate differences in the rewarding properties of the mu opioid receptor agonist oxycodone (5 mg/kg, i.p.) and the aversive properties of the opioid receptor antagonist naloxone (4 mg/kg, i.p.). Both substrains showed similar degrees of oxycodone-induced CPP; however, there was a three-fold enhancement of naloxone-induced CPA in agonist-naïve C57BL/6J relative to C57Bl/6NJ mice. Exploratory factor analysis of CPP and CPA identified unique factors that explain variance in behavioral expression of reward versus aversion. “Conditioned Opioid-Like Behavior” was a reward-based factor whereby drug-free locomotor variables resembling opioid treatment co-varied with the degree of CPP. “Avoidance and Freezing” was an aversion-based factor, whereby the increase in the number of freezing bouts co-varied with the degree of aversion. These results provide new insight into the behavioral architecture of the motivational properties of opioids. Future studies will use quantitative trait locus mapping in B6 substrains to identify novel genetic factors that contribute to the marked strain difference in NAL-CPA.
... 2002), we wished to determine whether these withdrawal signs would be sufficient to elicit alterations in proactive/operant behavioral choices. We utilized a conditioned place preference/aversion assay (CPP; Skoubis et al., 2001;Weitemier and Murphy, 2009) to test whether a specific environment, paired with naloxone administration during conditioning, would be avoided in favor of a different environment paired with a neutral stimulus (saline) during conditioning in chronically UV-irradiated animals. Due to the kinetics of the UV response, we chose to use naloxone as it allowed an acute effect of limited duration. ...
... The apparatus used consisted of a box with black interior and dim lighting and a box with white interior and bright lighting connected by a smaller gray ''neutral'' box, and procedures were followed as described previously (Skoubis et al., 2001;Weitemier and Murphy, 2009). Mice that had undergone 6 weeks of daily UV exposure or mock exposure were tested for baseline place preferences prior to conditioning (10 min testing time per mouse). ...
UV light is an established carcinogen, yet evidence suggests that UV-seeking behavior has addictive features. Following UV exposure, epidermal keratinocytes synthesize proopiomelanocortin (POMC) that is processed to melanocyte-stimulating hormone, inducing tanning. We show that, in rodents, another POMC-derived peptide, β-endorphin, is coordinately synthesized in skin, elevating plasma levels after low-dose UV. Increases in pain-related thresholds are observed and reversed by pharmacologic opioid antagonism. Opioid blockade also elicits withdrawal signs after chronic UV exposure. This effect was sufficient to guide operant behavioral choices to avoidance of opioid withdrawal (conditioned place aversion). These UV-induced nociceptive and behavioral effects were absent in β-endorphin knockout mice and in mice lacking p53-mediated POMC induction in epidermal keratinocytes. Although primordial UV addiction, mediated by the hedonic action of β-endorphin and anhedonic effects of withdrawal, may theoretically have enhanced evolutionary vitamin D biosynthesis, it now may contribute to the relentless rise in skin cancer incidence in humans. PAPERCLIP:
... Neither noribogaine or its parent drug ibogaine exhibit conditioned place preference seen with potent mu agonists or the conditioned place aversion observed following administration of kappa agonists [67] or mu antagonists [33,68]. In keeping with results from in vitro binding studies, noribogaine does not substitute for the discriminative stimulus of morphine or the kappa agonist U50,488 [34,69]. ...
Ibogaine is a powerful psychoactive substance that not only alters perception, mood and affect, but also stops addictive behaviors. Ibogaine has a very long history of ethnobotanical use in low doses to combat fatigue, hunger and thirst and in high doses as a sacrament in African ritual contexts. In the 1960s, American and European self-help groups provided public testimonials that a single dose of ibogaine alleviated drug craving, opioid withdrawal symptoms and prevented relapse for weeks, months and sometimes years. Ibogaine is rapidly demethylated by first-pass metabolism to a long-acting metabolite noribogaine. Ibogaine and its metabolite interact with two or more CNS targets simultaneously and both drugs have demonstrated predictive validity in animal models of addiction. Online forums endorse the benefits of ibogaine as an "addiction interrupter" and present-day estimates suggest that more than ten thousand people have sought treatment in countries where the drug is unregulated. Open label pilot studies of ibogaine-assisted drug detoxification have shown positive benefit in treating addiction. Ibogaine, granted regulatory approval for human testing in a Phase 1/2a clinical trial, joins the current landscape of psychedelic medicines in clinical development.
... Administration of naloxone during place conditioning induces a place aversion in mice (Cunningham et al., 1995;Solecki et al., 2009). Evidence from Penk and Oprm1 constitutive knockout mice suggests naloxone induces place aversion by blocking the action of enkephalin on MORs (Skoubis et al., 2001(Skoubis et al., , 2005. We reasoned that if MSN-Drd2KO mice have higher endogenous enkephalin tone, they might be more sensitive to developing and expressing place aversion to a low dose of naloxone ( Figure 4A). ...
Low dopamine D2 receptor (D2R) availability in the striatum can predispose for cocaine abuse; though how low striatal D2Rs facilitate cocaine reward is unclear. Overexpression of D2Rs in striatal neurons or activation of D2Rs by acute cocaine suppresses striatal Penk mRNA. Conversely, low D2Rs in D2-striatal neurons increases striatal Penk mRNA and enkephalin peptide tone, an endogenous mu-opioid agonist. In brain slices, met-enkephalin and inhibition of enkephalin catabolism suppresses intra-striatal GABA transmission. Pairing cocaine with intra-accumbens met-enkephalin during place conditioning facilitates acquisition of preference, while mu-opioid receptor antagonist blocks preference in wild-type mice. We propose that heightened striatal enkephalin potentiates cocaine reward by suppressing intra-striatal GABA to enhance striatal output. Surprisingly, a mu-opioid receptor antagonist does not block cocaine preference in mice with low striatal D2Rs, implicating other opioid receptors. The bidirectional regulation of enkephalin by D2R activity and cocaine offers insights into mechanisms underlying the vulnerability for cocaine abuse.
... 125,126 Oprm-1 KO mice demonstrated deficits in social behavior and communication skills, which are deficits usually owned by drug abusers. [127][128][129][130] Moreover, oprm-1 KO mice showed a decreased motivation for food and sucrose selfadministration 131 and failed to demonstrate an aversion to NX. 132 Considering the wide distribution of MORs in the brain, as expected, opioids and MORs intensively regulate the reward circuitry and are thought to play a role in drug addiction. In the following section, we review the reward circuitry and brain regions that are actively involved, followed by discussing the role of MORs in reward and addiction within each brain area. ...
Opioid abuse and addiction have become a global pandemic, posing tremendous health and social burdens. The rewarding effects and the occurrence of withdrawal symptoms are the two mainstays of opioid addiction. Mu-opioid receptors (MORs), a member of opioid receptors, play important roles in opioid addiction, mediating both the rewarding effects of opioids and opioid withdrawal syndrome (OWS). The underlying mechanism of MOR-mediated opioid rewarding effects and withdrawal syndrome is of vital importance to understand the nature of opioid addiction and also provides theoretical basis for targeting MORs to treat drug addiction. In this review, we first briefly introduce the basic concepts of MORs, including their structure, distribution in the nervous system, endogenous ligands, and functional characteristics. We focused on the brain circuitry and molecular mechanism of MORs-mediated opioid reward and withdrawal. The neuroanatomical and functional elements of the neural circuitry of the reward system underlying opioid addiction were thoroughly discussed, and the roles of MOR within the reward circuitry were also elaborated. Furthermore, we interrogated the roles of MORs in OWS, along with the structural basis and molecular adaptions of MORs-mediated withdrawal syndrome. Finally, current treatment strategies for opioid addiction targeting MORs were also presented.
... Such differences are further influenced by chronic pain [9][10][11][12][13][14][15][16], introducing varying capacity to respond to exogenous opioid treatment or perhaps even a predisposition to develop persistent pain. The MOR neurotransmitter system also mediates the reinforcing and hedonic effects of both natural and artificial rewards [17][18][19][20][21]. MORs robustly modulate activity within the mesolimbic pathway-a critical reward circuit consisting of dopamine neuron projections from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) [22]. ...
µ-Opioid receptors (MOR) are a major target of endogenous and exogenous opioids, including opioid pain medications. The µ-opioid neurotransmitter system is heavily implicated in the pathophysiology of chronic pain and opioid use disorder and, as such, central measures of µ-opioid system functioning are increasingly being considered as putative biomarkers for risk to misuse opioids. To explore the relationship between MOR system function and risk for opioid misuse, 28 subjects with chronic nonspecific back pain completed a clinically validated measure of opioid misuse risk, the Pain Medication Questionnaire (PMQ), and were subsequently separated into high (PMQ > 21) and low (PMQ ≤ 21) opioid misuse risk groups. Chronic pain patients along with 15 control participants underwent two separate [¹¹C]-carfentanil positron emission tomography scans to explore MOR functional measures: one at baseline and one during a sustained pain-stress challenge, with the difference between the two providing an indirect measure of stress-induced endogenous opioid release. We found that chronic pain participants at high risk for opioid misuse displayed higher baseline MOR availability within the right amygdala relative to those at low risk. By contrast, patients at low risk for opioid misuse showed less pain-induced activation of MOR-mediated, endogenous opioid neurotransmission in the nucleus accumbens. This study links human in vivo MOR system functional measures to the development of addictive disorders and provides novel evidence that MORs and µ-opioid system responsivity may underlie risk to misuse opioids among chronic pain patients.
... Noribogaine, and ibogaine to a lesser extent, were shown to induce naloxone-sensitive GTPgS binding in rat brain preparations . These drugs however did not induce conditioned place preference like mu agonists or conditioned place aversion like kappa agonists or mu antagonists (Skoubis et al., 2001) in animals ( (Parker et al., 1995) and unpublished results). In addition, neither morphine nor the kappa agonist U50,488 substituted for the discriminative stimulus of noribogaine treatment in rats (Helsley et al., 1998) (Zubaran et al., 1999). ...
Noribogaine is the primary metabolite of the anti‐addictive substance ibogaine, which modulates opiate analgesic activity and the components of drug addiction in animal models at brain concentrations of 0.5‐15 µM. In this study, molecular activities of noribogaine at mu (OPRM) and kappa (OPRK) opioid receptors were characterized. Noribogaine was a moderately potent antagonist of the OPRM G‐protein and β‐arrestin signaling pathways (20 µM; 48 µM). Noribogaine was a partial agonist at the OPRK G‐protein pathway, activating at 75% the maximal efficacy of Dynorphin A (Dyn‐A) at a potency of 9 µM, and had weak inhibitory properties (40 µM, 25% against Dyn‐A). Noribogaine was a biased agonist and poorly activated the OPRK β‐arrestin pathway at 12% of Dyn‐A maximal efficacy. In turn, noribogaine was able to functionally inhibit Dyn‐A‐induced β‐arrestin recruitment (Dyn‐A EC 50 : 82 nM) at physiologically relevant concentrations (IC 50 of 1.45 µM at 370 nM Dyn‐A). Computational simulations indicated that noribogaine may bind to the orthosteric morphinan binding site of the receptor. This study clarifies the action of noribogaine at modulating opioid receptor function, uncovering explanatory mechanisms as well as new avenues of therapeutic development.
... The CeA is an important component of the neural circuitry mediating the aversive experience, and we propose that our observed increases in neuronal activity in the CeA in sham-operated mice are mediated by naltrexone-evoked aversion. Indeed, our naltrexoneinduced CPA results agree with and extend previous studies showing that systemic administration of high doses of naltrexone is aversive in uninjured mice (Boulos et al., 2019;Kirkpatrick & Bryant, 2015;Shoblock & Maidment, 2006;Skoubis et al., 2001Skoubis et al., , 2005. PKCδexpressing neurons of the CeL encode aversive signals during reinforcement learning (Cui et al., 2017). ...
Tissue injury induces a long-lasting latent sensitization (LS) of spinal nociceptive signaling that is kept in remission by an opposing µ-opioid receptor (MOR) constitutive activity. To test the hypothesis that supraspinal sites become engaged, we induced hindpaw inflammation, waited 3 weeks for mechanical hypersensitivity to resolve, and then injected the opioid receptor inhibitors naltrexone, CTOP or β-funaltrexamine subcutaneously, and/or into the cerebral ventricles. Intracerebroventricular injection of each inhibitor reinstated hypersensitivity and produced somatic signs of withdrawal, indicative of LS and endogenous opioid dependence, respectively. In naïve or sham controls, systemic naloxone (3 mg/kg) produced conditioned place aversion, and systemic naltrexone (3 mg/kg) increased Fos expression in the central nucleus of the amygdala (CeA). In LS animals tested 3 weeks after plantar incision, systemic naltrexone reinstated mechanical hypersensitivity and produced an even greater increase in Fos than in sham controls, particularly in the capsular subdivision of the right CeA. One third of Fos+ profiles co-expressed protein kinase C delta (PKCδ), and 35% of PKCδ neurons co-expressed tdTomato+ in Oprm1Cre ::tdTomato transgenic mice. CeA microinjection of naltrexone (1 µg) reinstated mechanical hypersensitivity only in male mice and did not produce signs of somatic withdrawal. Intra-CeA injection of the MOR-selective inhibitor CTAP (300 ng) reinstated hypersensitivity in both male and female mice. We conclude that MORs in the capsular subdivision of the right CeA prevent the transition from acute to chronic postoperative pain.
... In contrast, some studies described aversive property of naloxone administration in drug-naïve animals (Mucha and Walker, 1987;Skoubis et al., 2005;Shoblock and Maidment, 2006;Kirkpatrick and Bryant, 2015). Since naloxone blocks the action of enkephalins at the mu opioid receptor (Skoubis et al., 2001(Skoubis et al., , 2005, this discrepancy suggests differences in the release of endogenous opioids depending on the stress levels during CPA. A previous study (García-Carmona et al., 2015a, 2015b described aversive effects induced by naloxone in morphine-dependent mice together with withdrawal signs such as body weight loss, jumping, rearing, rubbing, grooming, diarrhea, freezing and time to first immobility. ...
Intense associative memories develop between drug-paired contextual cues and the drug withdrawal associated aversive feeling. They have been suggested to contribute to the high rate of relapse. Our study was aimed to elucidate the involvement of hypothalamic-pituitary-adrenocortical (HPA) axis activity in the expression and extinction of aversive memory in Swiss and C57BL/6 J (B6) mice. The animals were rendered dependent on morphine by i.p. injection of increasing doses of morphine (10–60 mg/kg). The negative state associated with naloxone (1 mg/kg s.c.) precipitated morphine withdrawal was examined by using conditioned place aversion (CPA) paradigm. B6 mice obtained a higher aversion score and took longer to extinguish the aversive memory than Swiss mice. In addition, corticosterone levels were increased after CPA expression. Moreover, corticosterone levels were decreased during CPA extinction in Swiss mice without changes in B6 mice. Pre-treatment with the selective CRF1 receptor antagonist CP-154,526 before naloxone, impaired morphine-withdrawal aversive memory acquisition and decreased the extinction period. CP-154,526 also antagonized the increased levels of corticosterone observed after CPA expression in Swiss mice, without any changes in B6 mice. These results indicate that HPA axis could be a critical factor governing opioid withdrawal memory storage and retrieval, but in a strain or stock-specific manner. The differences observed between Swiss and B6 mice suggest that the treatment of addictive disorders should consider different individual predisposition to associate the aversive learning with the context.
... While KOR distribution mirrors that of MOR in many regions, oppositional localization in specific regions produces a profound difference in the ultimate effects of KOR stimulation versus MOR stimulation (Table 1). 51 MOR agonist-induced reward and analgesia are maintained in KOR knockout mice 44,70 and prodynorphin knockout mice 79,80 ; conversely, KOR agonist-induced G protein coupling, analgesia, and aversion are unaffected by MOR ablation, 81,82 supporting that the physiological effects of MOR and KOR are distinct and separable. While MOR stimulation produces euphoria, KOR activation produces dysphoria, a negative affective state that includes components of anxiety, anhedonia, and depression. ...
A serious adverse effect of prescription opioid analgesics is addiction, both to these analgesics and to illicit drugs like heroin that also activate the µ-opioid receptor (MOR). Opioid use disorder (OUD) and opioid overdose deaths represent a current American health crisis, and the prescription of opioid analgesics has contributed significantly to this crisis. While prescription opioids are highly effective analgesics, there currently exists no facile way to use them for extended periods without the risk of addiction. If addiction caused by MOR-targeting analgesics could be blocked by blending in a new "antiaddiction" ingredient that does not diminish analgesia and does not introduce its own therapeutically limiting side effects, then continued clinical use of prescription opioids for treating pain could be maintained (or even enhanced) instead of curtailed. In this narrative review, we contextualize this hypothesis, first with a brief overview of the current American opioid addiction crisis. The neurobiology of 2 key receptors in OUD development, MOR and the κ-opioid receptor (KOR), is then discussed to highlight the neuroanatomical features and circuitry in which signal transduction from these receptors lie in opposition-creating opportunities for pharmacological intervention in curtailing the addictive potential of MOR agonism. Prior findings with mixed MOR/KOR agonists are considered before exploring new potential avenues such as biased KOR agonists. New preclinical data are highlighted, demonstrating that the G protein-biased KOR agonist nalfurafine reduces the rewarding properties of MOR-targeting analgesics and enhances MOR-targeting analgesic-induced antinociception. Finally, we discuss the recent discovery that a regulator of G protein signaling (namely, RGS12) is a key component of signaling bias at KOR, presenting another drug discovery target toward identifying a single agent or adjuvant to be added to traditional opioid analgesics that could reduce or eliminate the addictive potential of the latter drug.
... Unfortunately, these KOPr agonists also have side effects. U50,488 and U69,593 produce prodepressive effects (Mague et al., 2003;Zhang et al., 2015), aversion (Mucha and Herz, 1985;Suzuki et al., 1992;Bals-Kubik et al., 1993;Skoubis et al., 2001;Ehrich et al., 2015), anxiety (Privette and Terrian, 1995;Kudryavtseva et al., 2004;Vunck et al., 2011;Wang et al., 2016), muscle weakness, and sedation (Dykstra et al., 1987;Zhang et al., 2015). In clinical studies, spiradoline produced diuresis, sedation, and dysphoria (Ur et al., 1997;Wadenberg, 2003). ...
There is a significant need to find effective, non-addictive pain medications. Kappa opioid receptor (KOPr) agonists have been studied for decades but have recently received increased attention due to their analgesic effects and lack of abuse potential. However, a range of side- effects have limited the clinical development of these drugs. There are several strategies currently used to develop safer and more effective KOPr agonists. These strategies include identifying G-protein biased agonists; developing peripherally-restricted KOPr agonists without centrally-mediated side-effects; and developing mixed opioid agonists, which target multiple receptors at specific ratios to balance side-effect profiles and reduce tolerance. Here, we review the latest developments in research related to KOPr agonists for the treatment of pain. Significance Statement This review discusses strategies for developing safer and therapeutically potential kappa opioid receptor (KOPr) agonists for the treatment of pain. Whilst one strategy is to modify selective KOPr agonists to create peripherally-restricted or G-protein biased structures, another approach is combining KOPr agonists with mu, delta or nociceptin opioid receptor activation to obtain mixed opioid receptor agonists, therefore, negating the adverse effects and retaining the therapeutic effect.
... Despite these advantages, traditional KOPr agonists, such as U50,488, that have balanced signaling properties (Schattauer et al., 2012), are associated with many side effects including sedation, anxiety, aversion, and dysphoria limiting their clinical development (Mucha and Herz, 1985;Pfeiffer et al., 1986;Suzuki et al., 1992;Bals-Kubik et al., 1993;Privette and Terrian, 1995;Pande et al., 1996;Skoubis et al., 2001;Walsh et al., 2001;Mague et al., 2003;Kudryavtseva et al., 2004;Vunck et al., 2011;Ehrich et al., 2015;Zhang et al., 2015;Wang et al., 2016). The concept of biased agonism, whereby activation of a G-protein coupled receptor can result in differential activation of signal transduction pathways, suggests that it may be possible to separate desired physiological effects from adverse side effects. ...
In the search for safer, non-addictive analgesics, kappa opioid receptor (KOPr) agonists are a potential target, as unlike mu-opioid analgesics, they do not have abuse potential. Salvinorin A (SalA) is a potent and selective KOPr agonist, however, clinical utility is limited by the short duration of action and aversive side effects. Biasing KOPr signaling toward G-protein activation has been highlighted as a key cellular mechanism to reduce the side effects of KOPr agonists. The present study investigated KOPr signaling bias and the acute antinociceptive effects and side effects of two novel analogs of SalA, 16-Bromo SalA and 16-Ethynyl SalA. 16-Bromo SalA showed G-protein signaling bias, whereas 16-Ethynyl SalA displayed balanced signaling properties. In the dose-response tail-withdrawal assay, SalA, 16-Ethynyl SalA and 16-Bromo SalA were more potent than the traditional KOPr agonist U50,488, and 16-Ethynyl SalA was more efficacious. 16-Ethynyl SalA and 16-Bromo SalA both had a longer duration of action in the warm water tail-withdrawal assay, and 16-Ethynyl had greater antinociceptive effect in the hot-plate assay, compared to SalA. In the intraplantar 2% formaldehyde test, 16-Ethynyl SalA and 16-Bromo SalA significantly reduced both nociceptive and inflammatory pain-related behaviors. Moreover, 16-Ethynyl SalA and 16-Bromo SalA had no anxiogenic effects in the marble burying task, and 16-Bromo SalA did not alter behavior in the elevated zero maze. Overall, 16-Ethynyl SalA significantly attenuated acute pain-related behaviors in multiple preclinical models, while the biased KOPr agonist, 16-Bromo SalA, displayed modest antinociceptive effects, and lacked anxiogenic effects.
... Moreover, an aversion to the stimulationpaired chamber was induced in rats in the naloxone group, which is consistent with the fact that naloxone is a nonselective antagonist of opioid receptors that can produce aversion. 34 ...
Objective:
Laser stimulation (LS) at both the injury site and specific acupoints may induce analgesic effects. The purpose of this study is to investigate the effects of LS at injury site or acupoint on analgesic-associated approach behavior and determine whether opioid receptors in the anterior cingulate cortex (ACC) were involved.
Methods:
The left hindpaw incision was established in rats. LS (10.6 μm) was performed at the ipsilateral (left) acupoint ST36 (Zusanli) or locally to the incision site. Characteristic guarding pain behavior was measured to assess incision-induced pain. A two-chamber conditioned place preference (CPP) paradigm was used to measure approach behavior induced by pain relief. To inhibit opioid receptors, naloxone was microinjected into the ACC before LS.
Results:
A delayed analgesic effect (24 h after treatment) was induced in both the LS groups (ST36 and incision site) as compared with the sham control or model groups (p < 0.05). An immediate (30 min after the end of the LS) decrease in guarding pain (p < 0.001) and CPP for the LS chamber (p < 0.001) were observed only in the ST36 LS group. The administration of naloxone in ACC inhibited the LS-induced analgesic effect and CPP (p < 0.05).
Conclusions:
Our results highlight the novel approach behavior of pain relief induced by 10.6-μm LS at ST36 in a rat model of incisional pain, and implicate ACC opioid receptor signaling in these actions.
... A recent study analyzed behavioral consequences of a genetic MOR deletion targeted to Chnrb4positive neurons of the MHb. Although rewarding effects of morphine and motivation for palatable food were intact in mutant mice, naloxone-induced place aversion (94) was reduced in both naive and morphine-dependent mice, and naloxone-precipitated physical withdrawal was attenuated in morphine-dependent animals (95). These data reveal a brain site and neuronal population in which MOR blockade Table S1). ...
Opioid use disorder (OUD) is characterized by the development of a negative emotional state that develops after a history of long-term exposure to opioids. OUD represents a true challenge for treatment and relapse prevention. Human research has amply documented emotional disruption in individuals with an opioid substance use disorder, at both behavioral and brain activity levels; however, brain mechanisms underlying this particular facet of OUD are only partially understood. Animal research has been instrumental in elucidating genes and circuits that adapt to long-term opioid use or are modified by acute withdrawal, but research on long-term consequences of opioid exposure and their relevance to the negative affect of OUD remains scarce. In this article, we review the literature with a focus on two questions: 1) Do we have behavioral models in rodents, and what do they tell us? and 2) What do we know about the neuronal populations involved? Behavioral rodent models have successfully recapitulated behavioral signs of the OUD-related negative affect, and several neurotransmitter systems were identified (i.e., serotonin, dynorphin, corticotropin-releasing factor, oxytocin). Circuit mechanisms driving the negative mood of prolonged abstinence likely involve the 5 main reward-aversion brain centers (i.e., nucleus accumbens, bed nucleus of the stria terminalis, amygdala, habenula, and raphe nucleus), all of which express mu opioid receptors and directly respond to opioids. Future work will identify the nature of these mu opioid receptor-expressing neurons throughout reward-aversion networks, characterize their adapted phenotype in opioid abstinent animals, and hopefully position these primary events in the broader picture of mu opioid receptor-associated brain aversion networks.
... That MOR signaling indeed reduces the activity of B4-MHb neurons remains to be demonstrated by electrophysiological measures. Also, future experiments will determine the exact nature of this endogenous MOR activity in the MHb, which may involve either endogenous opioid peptides whose identity and origin remains to be determined, or constitutive MOR activity as was previously suggested [27,28,46]. ...
The medial habenula (MHb) is considered a brain center regulating aversive states. The mu opioid receptor (MOR) has been traditionally studied at the level of nociceptive and mesolimbic circuits, for key roles in pain relief and reward processing. MOR is also densely expressed in MHb, however, MOR function at this brain site is virtually unknown. Here we tested the hypothesis that MOR in the MHb (MHb-MOR) also regulates aversion processing. We used chnrb4-Cre driver mice to delete the Oprm1 gene in chnrb4-neurons, predominantly expressed in the MHb. Conditional mutant (B4MOR) mice showed habenula-specific reduction of MOR expression, restricted to chnrb4-neurons (50% MHb-MORs). We tested B4MOR mice in behavioral assays to evaluate effects of MOR activation by morphine, and MOR blockade by naloxone. Locomotor, analgesic, rewarding, and motivational effects of morphine were preserved in conditional mutants. In contrast, conditioned place aversion (CPA) elicited by naloxone was reduced in both naïve (high dose) and morphine-dependent (low dose) B4MOR mice. Further, physical signs of withdrawal precipitated by either MOR (naloxone) or nicotinic receptor (mecamylamine) blockade were attenuated. These data suggest that MORs expressed in MHb B4-neurons contribute to aversive effects of naloxone, including negative effect and aversive effects of opioid withdrawal. MORs are inhibitory receptors, therefore we propose that endogenous MOR signaling normally inhibits chnrb4-neurons of the MHb and moderates their known aversive activity, which is unmasked upon receptor blockade. Thus, in addition to facilitating reward at several brain sites, tonic MOR activity may also limit aversion within the MHb circuitry.
... The prototypical KOPr agonist U50,488 showed significant reduction in time spent in the U50,488-paired chamber indicating aversive properties (Fig. 5A). These findings are consistent with previous literature reports in rats (Göktalay et al., 2006) and mice (Skoubis et al., 2001). However, rats treated with MP1104 showed no change in time spent in the MP1104-paired chamber, indicating it has no aversive properties at 0.6 mg/kg. ...
Kappa opioid receptor (KOPr) agonists have preclinical anti-cocaine and antinociceptive effects. However, adverse effects including dysphoria, aversion, sedation, anxiety and depression limit their clinical development. MP1104, an analogue of 3-iodobenzoyl naltrexamine, is a potent dual agonist at KOPr and delta opioid receptor (DOPr), with full agonist efficacy at both these receptors. In this study, we evaluate the ability of MP1104 to modulate cocaine-induced behaviors and side-effects preclinically. In male Sprague-Dawley rats trained to self-administer cocaine, MP1104 (0.3 and 1 mg/kg) reduced cocaine-primed reinstatement of drug-seeking behavior and caused significant downward shift of the dose-response curve in cocaine self-administration tests (0.3 and 0.6 mg/kg). The anti-cocaine effects exerted by MP1104 are in part due to increased dopamine (DA) uptake by the dopamine transporter (DAT) in the dorsal striatum (dStr) and nucleus accumbens (NAc). MP1104 (0.3 and 0.6 mg/kg) showed no significant anxiogenic effects in the elevated plus maze, pro-depressive effects in the forced swim test, or conditioned place aversion. Furthermore, pre-treatment with a DOPr antagonist, led to MP1104 producing aversive effects. This data suggests that the DOPr agonist actions of MP1104 attenuate the KOPr-mediated aversive effects of MP1104. The overall results from this study show that MP1104, modulates DA uptake in the dStr and NAc, and exerts potent anti-cocaine properties in self-administration tests with reduced side-effects compared to pure KOPr agonists. This data supports the therapeutic development of dual KOPr/DOPr agonists to reduce the side-effects of selective KOPr agonists.
... conditioned place avoidance, that is known to provide measuring of the aversive properties of conditioning drug i.e. naloxone, is abolished in constitutive MORs knockouts(Skoubis, Matthes et al. 2001) but MORs have not been associated with aversion in general.MHb-On the other hand, growing evidence is accumulating to define the MHb as a center for aversion with emphasis on nicotinic aversion and the B4 subunit. While elevated expression of the B4 subunit increases nicotine aversion in mice by enhancing activity of the MHb to the IPN, nicotine aversion is reversed by increasing α5 expression in the MHb(Frahm, Slimak et al. 2011). ...
Mu opioid receptors (MORs) have been extensively studied for their addictive properties that are thought to operate through the control of reward processes. While the importance of MORs in reward is generally attributed to their presence in the mesocorticolimbic circuitry, their role in the medial habenula (MHb), the structure in which MORs are most densely expressed, remains unexplored to date. This is quite surprising given the increasing literature on the habenula’s role in addiction as well as reward/aversion processes. Here we generated a conditional knockout mouse model that lacks MORs solely in the MH band we investigated the contribution of habenular MORs in brain functions and behavioural out comes with emphasis on reward, aversion and cognition. While the performance of our mutant model did not differ in locomotor, analgesic and reward responses to morphine norincognitive tasks compared to control mice, we uncovered a novel role for MORs in aversive states.This is the first report demonstrating that MORs control both somatic and affective aversion specifically at the level of the MHb. Habenular MORs could thus be crucial to the aversive with drawal stage of addiction cycles that is thought to increase craving and prevent success in quitting.
... Oprm1-null mutants also showed reduced motivation for both food and sucrose in an SA paradigm 51 . Finally, Oprm1-null mutant mice showed no naloxone aversion in a conditioned place aversion (CPA) paradigm, indicating that MORs are crucial in mediating the positive hedonic tone elicited by endogenous opioids 52 . Together, the findings above show that MOR activity both mediates natural rewards and promotes recreational drug use, with the latter in turn favouring the onset of drug abuse. ...
Drug addiction is a worldwide societal problem and public health burden, and results from recreational drug use that develops into a complex brain disorder. The opioid system, one of the first discovered neuropeptide systems in the history of neuroscience, is central to addiction. Recently, opioid receptors have been propelled back on stage by the rising opioid epidemics, revolutions in G protein-coupled receptor research and fascinating developments in basic neuroscience. This Review discusses rapidly advancing research into the role of opioid receptors in addiction, and addresses the key questions of whether we can kill pain without addiction using mu-opioid-receptor-targeting opiates, how mu- and kappa-opioid receptors operate within the neurocircuitry of addiction and whether we can bridge human and animal opioid research in the field of drug abuse.
... To further assess the role of Csnk1e in mu opioid receptor-dependent behaviors, we assessed NAL-CPA , Skoubis et al., 2001) and fentanyl analgesia. There was no significant effect of Genotype on NAL-CPA ( Figure S2) or fentanyl analgesia ( Figure S3). ...
Genetic and pharmacological studies indicate that casein kinase-1 epsilon (Csnk1e) contributes to psychostimulant, opioid, and ethanol motivated behaviors. We previously used pharmacological inhibition to demonstrate that Csnk1e negatively regulates the locomotor stimulant properties of opioids and psychostimulants. Here, we tested the hypothesis that Csnk1e negatively regulates opioid and psychostimulant reward using genetic inhibition and the conditioned place preference assay in Csnk1e knockout mice. Similar to pharmacological inhibition, Csnk1e knockout mice showed enhanced opioid-induced locomotor activity with the mu opioid receptor agonist fentanyl (0.2 mg/kg i.p.) as well as enhanced sensitivity to low-dose fentanyl reward (0.05 mg/kg). Interestingly, female knockout mice also showed a markedly greater escalation in consumption of sweetened palatable food - a behavioral pattern consistent with binge eating that also depends on mu opioid receptor activation. No difference was observed in fentanyl analgesia in the 52.5 °C hot plate assay (0-0.4 mg/kg), naloxone conditioned place aversion (4 mg/kg), or methamphetamine conditioned place preference (0-4 mg/kg). To identify molecular adaptations associated with increased drug and food behaviors in knockout mice, we completed transcriptome analysis via mRNA sequencing of the striatum. Enrichment analysis identified terms associated with myelination and axon guidance and pathway analysis identified a differentially expressed gene set predicted to be regulated by the Wnt signaling transcription factor, Tcf7l2. To summarize, Csnk1e deletion increased mu opioid receptor-dependent behaviors, supporting previous studies indicating an endogenous negative regulatory role of Csnk1e in opioid behavior.
... No. 14124; 1:1000). Subsequently, the sections were rinsed 5 times in PBS and 157 incubated with the following secondary antibodies: donkey anti-mouse Alexa were selected based on previous reports (Skoubis et al., 2001; Rodriguez Parkitna et al., 2012; 216 Rodriguez Parkitna et al., 2013). The dose of cocaine 25 mg/kg was reported to produce robust 217 preference and no seizures (Piechota et al., 2010). ...
Plasticity of the brain’s dopamine system plays a crucial role in adaptive behavior by regulating appetitive motivation and the control of reinforcement learning. In this study, we investigated drug- and natural-reward conditioned behaviors in a mouse model in which the NMDA receptor-dependent plasticity of dopaminoceptive neurons was disrupted. We generated a transgenic mouse line with inducible selective inactivation of the NR1 subunit in neurons expressing dopamine D1 receptors (the NR1D1CreERT2 mice). Whole-cell recordings of spontaneous EPSCs on neurons in the nucleus accumbens confirmed that a population of neurons lacked the NMDA receptor-dependent component of the current. This effect was accompanied by impaired long-term potentiation in the nucleus accumbens and in the CA1 area of the ventral, but not the dorsal, hippocampus. Mutant mice did not differ from control animals when tested for pavlovian or instrumental conditioning. However, NR1D1CreERT2 mice acquired no preference for a context associated with administration of drugs of abuse. In the conditioned place preference paradigm, mutant mice did not spend more time in the context paired with cocaine, morphine, or ethanol, although these mice acquired a preference for sucrose jelly and an aversion to naloxone injections, as normal. Thus, we observed that the selective inducible ablation of the NMDA receptors specifically blocks drug-associated context memory with no effect on positive reinforcement in general.
... Peripheral blockade of kappa-(but not mu) opioid receptors prevented aversive social motivation in pair bonded male prairie voles as indicated by a decrease in selective aggression. It is not surprising that peripheral blockade of mu-opioid receptors failed to inhibit selective aggression since previous studies have demonstrated that blockade of these receptors is aversive (van Ree et al., 1999;Skoubis et al., 2001;. In contrast to negative affective states induced by mu-opioid-receptor blockade, activation of these receptors is associated with positive hedonics and mediates the rewarding properties of positive social incentives such as play and contact comfort . ...
The socially monogamous prairie vole is an excellent animal model for studies of the neurobiology of selective social attachment. Here, we have demonstrated that monogamous bonds are formed and maintained by a balance between mu- and kappa-opioid receptor activation within motivational circuitry. We first show that the formation of a pair bond requires the activation of mu-opioid receptors that mediate both motivational and positive hedonic aspects of reward processing. We hypothesize that these two populations of mu-opioid receptors work in parallel to motivate social interactions with a potential mating partner and subsequently encode these interactions as rewarding through the induction of a positive hedonic state. In contrast to pair bond formation, which is associated with affiliative social encounters generally categorized as positive, the maintenance of a pair bond is associated with aversive social encounters, such as the aggressive rejection of novel conspecifics, that acts to both guard the mate and prevent the formation of a subsequent alternate bond. In the present body of work, we demonstrate that selective aggression, an established behavioral indicator of pair bond maintenance, is mediated by interactions between D1-like dopamine receptors and kappa-opioid receptors within the nucleus accumbens shell and that direct activation of kappa-opioid receptors within this region encodes social aversion. These data suggest that the encoding of social stimuli besides the mating partner as aversive and subsequently aggressively rejecting this stimulus is important for the maintenance of the original pair bond. Additionally, we show that the establishment of a pair bond is associated with a dramatic reorganization of the dopamine and kappa-opioid receptor systems, which likely acts to mediate the transition from generally affiliative to selectively aggressive. Together, these data suggest that interactions between valence coding opioid receptors and motivational circuitry are critical for guiding the direction of socially motivated behaviors, such as the motivation to form and maintain bonds. Importantly, an increased understanding of neural mechanisms that mediate social attachment behavior of prairie voles may provide insight into neural mechanisms that regulate attachment behavior in our own species.
... Neither noribogaine or its parent drug ibogaine exhibit conditioned place preference seen with potent mu agonists or the conditioned place aversion observed following administration of kappa agonists [67] or mu antagonists [33,68]. In keeping with results from in vitro binding studies, noribogaine does not substitute for the discriminative stimulus of morphine or the kappa agonist U50,488 [34,69]. ...
Introduction Ibogaine is a naturally occurring psychoactive indole alkaloid derived from the roots of the rainforest shrub Tabernanthe iboga.Ibogaine is used by indigenous peoples of Western Africa in low doses to combat fatigue, hunger, and thirst, and in higher doses as a sacrament in religious rituals (Goutarel et al., 1991). The use of ibogaine for the treatment of drug dependence was based on anecdotal reports by groups of self-treating addicts that the drug blocked opiate withdrawal and reduced craving for opiates, cocaine, and other illicit drugs for extended time periods (Shepard, 1994; Alper et al., 1999). Preclinical studies supported these early claims and provided proof-of-concept in animal models (Dzoljic et al., 1988; Glick et al., 1992). Addiction is a behavioral pattern of drug abuse characterized by compulsive use, loss of behavioral control, and a high tendency to relapse. An integrated medical, psychosocial, and spiritual treatment is often needed to achieve recovery in addicted patients. Ibogaine is a unique pharmacotherapy for the treatment of substance abuse disorders because it fosters a life change or may work as a transition-based therapy similar to the goals set in the 12-step fellowship programs. While ibogaine's effects on behavior are complex, the beneficial actions of the drug on withdrawal symptoms and cravings are because of an interaction of the active metabolite noribogaine with neurotransmitters in the brain reward and addiction circuit.
... Noribogaine, and ibogaine to a lesser extent, were shown to induce naloxone-sensitive GTPγS binding in rat brain preparations . These drugs however did not induce conditioned place preference like mu agonists or conditioned place aversion like kappa agonists or mu antagonists (Skoubis et al., 2001) in animals ( (Parker et al., 1995) and unpublished results). In addition, neither morphine nor the kappa agonist U50,488 substituted for the discriminative stimulus of noribogaine treatment in rats (Helsley et al., 1998) (Zubaran et al., 1999. ...
Noribogaine is the long-lived human metabolite of the anti-addictive substance ibogaine. Noribogaine efficaciously reaches the brain with concentrations up to 20 μM after acute therapeutic dose of 40 mg/kg ibogaine in animals. Noribogaine displays atypical opioid-like components in vivo, anti-addictive effects and potent modulatory properties of the tolerance to opiates for which the mode of action remained uncharacterized thus far. Our binding experiments and computational simulations indicates that noribogaine may bind to the orthosteric morphinan binding site of the opioid receptors. Functional activities of noribogaine at G-protein and non G-protein pathways of the mu and kappa opioid receptors were characterized. Noribogaine was a weak mu antagonist with a functional inhibition constants (Ke) of 20 μM at the G-protein and β-arrestin signaling pathways. Conversely, noribogaine was a G-protein biased kappa agonist 75% as efficacious as dynorphin A at stimulating GDP-GTP exchange (EC50 = 9 μM) but only 12% as efficacious at recruiting β-arrestin, which could contribute to the lack of dysphoric effects of noribogaine. In turn, noribogaine functionally inhibited dynorphin-induced kappa β-arrestin recruitment and was more potent than its G-protein agonistic activity with an IC50 of 1 μM. This biased agonist/antagonist pharmacology is unique to noribogaine in comparison to various other ligands including ibogaine, 18-MC, nalmefene, and 6'-GNTI. We predict noribogaine to promote certain analgesic effects as well as anti-addictive effects at effective concentrations >1 μM in the brain. Because elevated levels of dynorphins are commonly observed and correlated with anxiety, dysphoric effects, and decreased dopaminergic tone, a therapeutically relevant functional inhibition bias to endogenously released dynorphins by noribogaine might be worthy of consideration for treating anxiety and substance related disorders.
Copyright © 2015. Published by Elsevier Ltd.
... As expected, salvinorin A produced robust CPA. This is not surprising as salvinorin A is a KOR agonist and compounds with this mechanism of action, such as U50,488H, produce place aversion (Skoubis et al., 2001;Tzschentke, 2007). In addition, this work is consistent with research showing salvinorin A produces CPA in mice (Zhang et al., 2005). ...
Ethnopharmacological relevance: Consumer use of botanicals has increased despite, in many instances, the paucity of research demonstrating efficacy or identifying liabilities. This research employed the place preference/aversion paradigm to characterize the psychoactive properties of Salvia divinorum extract (10, 30, 100 mg/kg), salvinorin A (0.1, 0.3, 1.0 mg/kg), Mitragyna speciosa MeOH extract (50, 100, 300 mg/kg), Mitragyna speciosa alkaloid-enriched fraction (12.5, 25, 75 mg/kg) and mitragynine (5, 10, 30 mg/kg) in rats. Material and methods: Following apparatus habituation and baseline preference scores, male Sprague-Dawley rats were given eight counterbalanced drug versus vehicle conditioning trials followed by a preference test conducted under drug-free states. S(þ)-amphetamine (1 mg/kg) served as the positive control (in Exp. 2) and haloperidol (0.8, 1.0 mg/kg) served as the negative control in both studies. Results: Rats displayed place aversion to both Salvia divinorum and salvinorin A that exceeded that of haloperidol. Rats showed place preference to mitragynine that was similar to that of S(þ)-amphetamine. This CPP effect was much less pronounced with the Mitragyna speciosa extract and its fraction. Conclusions: These findings suggest that both botanicals possess liabilities, albeit somewhat different, that warrant caution in their use.
... This involvement was first attributed to the rewarding effects of morphine and to the ability of naloxone and naltrexone to produce conditioned aversive responses (Bals-Kubik et al., 1989;Parker and Rennie, 1992;Gerrits et al., 2003). Recent research has extended such findings by showing that both enkephalin and µ-opioid receptor knockout mice fail to display conditioned aversion to naloxone (Skoubis et al., 2001;Skoubis et al., 2005). Additionally, studies of µ-opioid receptor knockout mice show that the µ-receptor is crucial for the analgesic and rewarding effects of morphine (Matthes et al., 1996). ...
... Enkephalin forms a part of the endogenous anti-nociceptive system (Akil et al., 1978;Inturrisi et al., 1980) and enkephalin can bind to μ-opioid receptors (Waldhoer et al., 2004), which have been suggested to be important in enkephalin induced analgesia (Gacel et al., 1981). Enkephalin and μ-opioid receptors have also been implicated in stimulating food intake and contributing to a basal positive hedonic state of the organism (Skoubis et al., 2001;Wilson et al., 2003;Skoubis et al., 2005) and the μ-opioid receptor is essential for the hedonic and analgesic effects of morphine (Matthes et al., 1996). Interestingly, many μ-opioid receptor-expressing neurons can be found in the PBelo. ...
... As expected, salvinorin A produced robust CPA. This is not surprising as salvinorin A is a KOR agonist and compounds with this mechanism of action, such as U50,488H, produce place aversion (Skoubis et al., 2001;Tzschentke, 2007). In addition, this work is consistent with research showing salvinorin A produces CPA in mice (Zhang et al., 2005). ...
... This pathway could be implicated in the maintenance of a basal hedonic state as well. For instance, systemic injections of the non-specific opioid antagonist naloxone induce a robust conditioned place aversion, which is absent in ENK deficient mice and MOR deficient mice (Skoubis et al., 2001(Skoubis et al., , 2005. Moreover, local injections of naloxone in the VP can induce a conditioned place avoidance (Skoubis & Maidment, 2003). ...
Abstract Restraint and immobilization have been extensively used to study habituation of the neuroendocrine response to a repeated stressor, but behavioral consequences of this stress regimen remain largely uncharacterized. In this study, we used sucrose preference and the elevated-plus maze to probe behavioral alterations resulting from 14 days of restraint in rats. We observed a decrease in sucrose preference in stressed animals, particularly in a subgroup of individuals, but no alteration in anxiety behaviors (as measured in the elevated-plus maze) four days following the last restraint. In these low sucrose preference animals, we observed a downregulation of the expression of preproenkephalin mRNA in the nucleus accumbens. Furthermore, we observed a strong correlation between enkephalin expression and sucrose preference in the shell part of the nucleus accumbens, with a lower level of enkephalin expression being associated with lower sucrose preference. Interestingly, quantification of the corticosterone response revealed a delayed habituation to restraint in the low sucrose preference population, which suggests that vulnerability to stress-induced deficits might be associated with prolonged exposure to glucocorticoids. The induction of ΔFosB is also reduced in the nucleus accumbens shell of the low sucrose preference population and this transcription factor is expressed in enkephalin neurons. Taken together, these results suggest that a ΔFosB-mediated downregulation of enkephalin in the nucleus accumbens might underlie the susceptibility to chronic stress. Further experiments will be needed to determine causality between these two phenomena.
... [1][2][3] More specifically, characterization of the mu opioid receptor (MOR) structure-function relationship is essential because the analgesic effect, addictive properties, and notorious side effects (such as addiction/abuse liability, respiratory depression, and constipation) of the key drug morphine are abolished in MOR knock-out mice, indicating that these side effects are primarily due to its interaction with the MOR. [4][5][6] Yet, the lack of a non-peptidyl, highly selective, reversible, and potent MOR antagonist limits our understanding of the structure-function relationship of the MOR, the interaction of nonpeptidyl MOR agonists with it, and more significantly, the activation mechanism of the receptor with respect to its role in drug abuse and addiction. ...
Highly selective opioid receptor antagonists are essential pharmacological probes in opioid receptor structural characterization and opioid agonist functional studies. Currently, there is no highly selective, nonpeptidyl and reversible mu opioid receptor antagonist available. Among a series of naltrexamine derivatives that have been designed and synthesized, two compounds, NAP and NAQ, were previously identified as novel leads for this purpose based on their in vitro and in vivo pharmacological profiles. Both compounds displayed high binding affinity and selectivity to the mu opioid receptor. To further study the interaction of these two ligands with the three opioid receptors, the recently released opioid receptor crystal structures were employed in docking studies to further test our original hypothesis that the ligands recognize a unique 'address' domain in the mu opioid receptor involving Trp318 that facilitates their selectivity. These modeling results were supported by site-directed mutagenesis studies on the mu opioid receptor, where the mutants Y210A and W318A confirmed the role of the latter in binding. Such work not only enriched the 'message-address' concept, also facilitated our next generation ligand design and development.
... 35,36 In agreement with our previous studies, 37 the administration of naloxone by itself induced CPA in mice, an effect mediated by the central nervous system. 38 BDL also reduced the CPA response that was induced in sham-operated animals. A small right-shift was observed in the aversive effects of naloxone (intermediate dose in sham versus higher dose in BDL animals), which could be related to down-regulated opioid receptors in the brain. ...
Several investigations have indicated that cholestasis decreases opioid receptor expression in the brain following increased opioidergic neurotransmission. The opioidergic system plays an important role in regulation of reward circuits that may be produced via dopamine-dependent mechanisms. It has been suggested that the dopaminergic system of the nucleus accumbens is necessary in conditioned place preference (CPP). The aim of this study is, therefore, to test if cholestasis can alter the reward system and the involvement of opioidergic and dopaminergic systems in this phenomenon.
We used CPP and hole-board paradigms to measure the reward effect and exploratory behaviors, respectively, in mice. Cholestasis was induced by ligation of the main bile duct, using two ligatures and transecting the duct between them (BDL mice).
The data showed that morphine (1 and 2 mg/kg), sulpiride (80 mg/kg) and SKF38393 (20 mg/kg) produced CPP, while naloxone (1 mg/kg) and SCH23390 (1mg/kg) produced conditioned place aversion (CPA), whereas quinpirole had no effect in sham-operated mice. However, morphine (2 mg/kg, i.p.), sulpiride (40 mg/kg) and? SKF38393 (10 mg/kg) induced CPP in BDL mice compared to sham-operated mice. Naloxone- or SCH23390-induced CPA was reduced in BDL mice compared with the respective sham-operated mice. Quinpirole tended to induce aversion in BDL mice which was, however, not significant. In addition, quinpirole 1 mg/kg) and SCH23390 (1 mg/kg) increased head-dip exploratory behavior, whereas naloxone (2 mg/kg) caused a decrease in head-dip exploratory behavior in sham-operated mice. Morphine (2 mg/kg), SCH23390 (1 mg/kg) and quinpirole (0.25 and 0.5 mg/kg) induced anxiogenic-like behavior in BDL mice.
It can be concluded that cholestasis differentially alters the reward effects of opioidergic and dopaminergic agents.
... This suggests that tonic activation of endogenous opioid receptors contributes to hedonic homeostasis in mice. A series of studies using opioid receptor knockout animals has demonstrated that Mu opioid receptors, but not kappa or delta receptors, mediate this antagonist-induced aversion [34]. Furthermore, this aversion is eliminated in proenkephalin knockout mice yet is completely retained in proopiomelanocortin knockout mice, suggesting that in order for naloxone to produce aversion, it must disrupt opioid peptides derived from proenkephalin activating Mu opioid receptors [35]. ...
Conventional strategies for treating opioid abuse include cognitive/behavioral therapy, maintenance therapy and managed withdrawal. These strategies deal with the addictive state, but much effort has shifted toward developing preventative measures. One approach is to reduce the rewarding properties of abused drugs, which in the case of prescription pharmaceuticals such as opioid analgesics is clearly a worthwhile endeavor. First, we will review recent studies demonstrating that mice lacking various G-protein coupled receptors and their ligands show diminished opioid reward. These studies point to considerable interdependence among different neuromodulatory systems for establishing drug reward. Thus, we will highlight the potential for combining opioids with other receptor targets to maximize analgesia and minimize reward in order to prevent the establishment of euphoric memories and opponent processes driving addiction. A more complex issue is treating opioid addicts where many physiological shifts have developed to oppose the acute drug effects. These opponent processes occur both at the cellular level (e.g. adenylate cyclase supersensitivity) and behavioral level (e.g. dysphoria and hyperalgesia). Evidence will be presented showing that even weak opioid agonists can effectively induce the intracellular process of adenylate cyclase supersensitivity. Furthermore, opioid-induced hyperalgesia, a behavioral opponent process, may manifest differently depending on the pain stimulus, and may require a priming drug dose in some cases. Environmental stress interacts with these opponent processes and drives relapse and thus, is another variable that needs to be considered in the development of innovative pharmacotherapy options for opioid addicts.
... Naloxone (0.05-0.4 mg/kg, i.p.) injection prior to morphine (5 mg/kg) response testing induced a significant place aversion according to our data reflecting an opioid system involvement [24]. Previous studies proposed that an aversive effect of naloxone can be seen by using either a biased paradigm [25] or a balanced biased procedure with no strong preconditioning preferences [26]. ...
Role of nitric oxide (NO) on expression of morphine conditioning using a solely classic task has been proposed previously. In this work, the involvement of NO on the expression of opioid-induced conditioning in the task paired with an injection of naloxone was investigated.
Conditioning was established in adult male Wistar rats (weighing 200-250 g) using an unbiased procedure. Naloxone (0.05-0.4 mg/kg, i.p.), a selective antagonist of mu-opioid receptor, was administered once prior to morphine response testing. NO agents were administered directly into the central amygdala (CeA) prior to naloxone injection pre-testing.
Morphine (2.5-10 mg/kg, s.c.) produced a significant dose-dependent place preference in experimental animals. When naloxone (0.05-0.4 mg/kg, i.p.) was injected before testing of morphine (5 mg/kg, s.c.) response, the antagonist induced a significant aversion. This response was reversed due to injection of L-arginine (0.3-3 microg/rat), intra-CeA prior to naloxone administration. However, pre-injection of L-NAME (intra-CeA), an inhibitor of NO production, blocked this effect.
The finding may reflect that NO in the nucleus participates in morphine plus naloxone interaction.
... In agreement with earlier studies (Blokhina et al. 2000), the present results also demonstrated that place conditioning with the opioid antagonist naloxone in wild-type and CRF1R-deficient mice was significantly potentiated by morphine administration prior to conditioning trial with the opioid antagonist. The aversive effects of naloxone in saline-treated mice are thought to be produced by blocking the action of tonically released, endogenous opioid peptides that act through the mu opioid receptors in various brain regions, including the VTA and the NAc ( Skoubis et al. 2001Skoubis et al. , 2005). ...
Evidence suggests that corticotropin-releasing factor (CRF) system is an important mediator in the negative symptoms of opioid withdrawal.
We used genetically engineered mice lacking functional CRF receptor-1 (CRF1R) levels to study the role for CRF/CRF1R pathways in the negative affective states of opioid withdrawal.
Wild-type and CRF1R(-/-) offspring of CRF1R(+/-) breeders were identified by PCR analysis of tail DNA and were rendered dependent on morphine via intraperitoneal injection of increasing doses of morphine (10-60 mg/kg). Negative state associated with opioid withdrawal was examined by using conditioned place aversion (CPA), TH expression and TH phosphorylation were measured in different brain regions involved in addictive behaviours using immunohistochemistry.
The weight loss in morphine withdrawn CRF1R(-/-) animals was significantly (p < 0.05) lower versus wild-type. The aversion for environmental cues paired with opioid withdrawal was lower (p < 0.001) in the CRF1R-deficient versus wild-type. Using dual immunolabeling for c-Fos, data show that naloxone-induced withdrawal increases the number of TH positive neurons phosphorylated at Ser40 or Ser31 that coexpress c-Fos in the nucleus of tractus solitarius (NTS)-A2 from wild-type and CRF(-/-) deficient mice. By contrast, the number of phospho-Ser40 or phospho-Ser31 positive neurons expressing c-Fos was lower in the ventrolateral medulla (VLM)-A1 in CRF(-/-)-deficient mice.
Our study demonstrates an increased activity of brainstem catecholaminergic neurons after CPA induced by morphine withdrawal suggesting that CRF1R is implicated in the activation of A1 neurons and provides evidence that this receptor is involved in the body weight loss and in the negative aversive effects of morphine withdrawal.
Kappa opioid receptor (KOR) agonists have preclinical antipsychostimulant effects; however, adverse side effects have limited their therapeutic development. In this preclinical study, conducted in Sprague Dawley rats, B6-SJL mice, and non-human primates (NHPs), we evaluated the G-protein-biased analogue of salvinorin A (SalA), 16-bromo salvinorin A (16-BrSalA), for its anticocaine effects, side effects, and activation of cellular signaling pathways. 16-BrSalA dose-dependently decreased the cocaine-primed reinstatement of drug-seeking behavior in a KOR-dependent manner. It also decreased cocaine-induced hyperactivity, but had no effect on responding for cocaine on a progressive ratio schedule. Compared to SalA, 16-BrSalA had an improved side effect profile, with no significant effects in the elevated plus maze, light–dark test, forced swim test, sucrose self-administration, or novel object recognition; however, it did exhibit conditioned aversive effects. 16-BrSalA increased dopamine transporter (DAT) activity in HEK-293 cells coexpressing DAT and KOR, as well as in rat nucleus accumbens and dorsal striatal tissue. 16-BrSalA also increased the early phase activation of extracellular-signal-regulated kinases 1 and 2, as well as p38 in a KOR-dependent manner. In NHPs, 16-BrSalA caused dose-dependent increases in the neuroendocrine biomarker prolactin, similar to other KOR agonists, at doses without robust sedative effects. These findings highlight that G-protein-biased structural analogues of SalA can have improved pharmacokinetic profiles and fewer side effects while maintaining their anticocaine effects.
Endogenous opioids have been implicated in cocaine reward. However, the role of each opioid peptide in this regard is unknown. Notably, the role of each peptide in extinction and reinstatement is not fully characterized. Thus, we assessed whether cocaine-induced conditioned place preference (CPP) and its extinction and reinstatement would be altered in the absence of beta-endorphin. We also examined if sex-related differences would exist in these processes. Male and female mice lacking beta-endorphin and their respective controls were tested for baseline place preference on day 1. On day 2, mice were treated with saline/cocaine (15 mg/kg) and confined to the vehicle- or drug-paired chamber for 30 min, respectively. In the afternoon, mice were treated with the alternate treatment and confined to the opposite chamber. Mice were then tested for CPP on day 3. Mice then received additional conditioning on this day as well as on day 4. Mice were then tested for CPP on day 5. Mice then received extinction training on day 9. On day 10, mice were tested for extinction and then reinstatement of CPP following a priming dose of cocaine (7.5 mg/kg). Male and female mice lacking beta-endorphin did not exhibit CPP following single conditioning with cocaine. On the other hand, only male mice lacking beta-endorphin failed to show CPP after repeated conditioning. Nonetheless, reinstatement of CPP was blunted in both male and female mice lacking beta-endorphin compared to controls. The present results suggest that beta-endorphin plays a functional role in cocaine-induced CPP and its reinstatement, and sex-related differences exist in the regulatory action of beta-endorphin on the acquisition but not reinstatement of cocaine CPP.
Novelty- and sensation-seeking behaviors induce activity of the brain reward system and are associated with increased susceptibility to drug abuse. Endogenous opioids have been implicated in reward-related behavior; however, the involvement of specific opioid receptors in the mechanism of sensation seeking is unknown. Here, we show that selective inhibition of opioid receptors reduce operant sensation seeking in mice. Administration of naltrexone (a nonselective opioid antagonist) reduced instrumental responding for sensory stimuli at one of the tested doses (2 mg/kg). More robust effects were observed in the case of cyprodime, a selective μ opioid receptor antagonist, which reduced instrumental responses by ∼50% at doses of 0.5 mg/kg and larger. Conversely, selective δ and κ receptor antagonists (naltrindole and nor-binaltorphimine, respectively) had no effect on sensation-seeking behavior. Importantly, while naltrexone produces aversion in the conditioned place preference test, cyprodime had no such effect. Therefore, reduced instrumental responding was not correlated with aversive effects of the opioid antagonists. In conclusion, our results revealed a novel mechanism of action of selective opioid receptors antagonists, which may have relevance for their efficacy in the treatment of drug abuse.
Evidence has shown that downstream signaling by mu opioid receptor (MOR) agonists that recruit β-arrestin2 may lead to the development of tolerance. Also, it has been suggested that opioid receptor desensitization and cyclic AMP overshoot contributes to the development of tolerance and occurrence of withdrawal, respectively. Therefore, studies were conducted with 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6α-(isoquinoline-3-carboxamido)morphinan (NAQ), a MOR selective partial agonist discovered in our laboratory, to characterize its effect on β-arrestin2 recruitment and precipitation of a cyclic AMP overshoot. DAMGO, a MOR full agonist dose-dependently increased β-arrestin2 association with the MOR, whereas NAQ did not. Moreover, NAQ displayed significant, concentration-dependent antagonism of DAMGO-induced β-arrestin2 recruitment. After prolonged morphine treatment of mMOR-CHO cells, there was a significant overshoot of cAMP upon exposure to naloxone, but not NAQ. Moreover, prolonged incubation of mMOR-CHO cells with NAQ did not result in desensitization nor downregulation of the MOR. In functional studies comparing NAQ with nalbuphine in the cAMP inhibition, Ca2+flux and [35S]GTPγS binding assays, NAQ did not show agonism in the Ca2+flux assay but showed partial agonism in the cAMP and [35S]GTPγS assays. Also, NAQ significantly antagonized DAMGO-induced intracellular Ca2+increase. In conclusion, NAQ is a low efficacy MOR modulator that lacks β-arrestin2 recruitment function and does not induce cellular hallmarks of MOR adaptation and fails to precipitate a cellular manifestation of withdrawal in cells pretreated with morphine. These characteristics are desirable if NAQ is pursued for opioid abuse treatment development.
This review covers β-phenylethylamines and isoquinoline alkaloids derived from them, including further products of oxidation, condensation with formaldehyde and rearrangement, some of which do not contain an isoquinoline system, together with naphthylisoquinoline alkaloids, which have a different biogenetic origin. The occurrence of the alkaloids, with the structures of new bases, together with their reactions, syntheses and biological activities are reported. The literature from July 2002 to June 2003 is reviewed, with 568 references cited.
Neuropathic pain develops in up to two-thirds of people following spinal cord injury (SCI). Opioids are among the most effective treatments for this pain, and are commonly prescribed. There is concern surrounding the use of these analgesics, however, because use is often associated with the development of addiction. Previous data suggests that this concern may not be relevant in the presence of neuropathic pain. Yet, despite the common prescription of opioids for the treatment of SCI related pain, there has only been one previous study examining the addictive potential of morphine following spinal injury. To address this, the present studies used a self-administration paradigm to examine the addictive potential of morphine in a rodent model of SCI. Animals were placed into self-administration chambers 24-hours, 14-days, or 35-days following a moderate spinal contusion injury. They were placed into the chambers for 7, 12-hour sessions with access to 1.5 mg morphine / lever depression, up to 30 mg/day. In the acute phase of SCI, contused animals self-administered significantly less morphine than their sham counterparts, as previously shown. However, contused animals showing signs of neuropathic pain did not self-administer less morphine than their sham counterparts when administration began 14- or 35-days after injury. Instead, these animals administered nearly the full amount of morphine available each session. This amount of morphine did not affect recovery of locomotor function, but did cause significant weight loss. We suggest caution is warranted when prescribing opioids for the treatment of neuropathic pain resulting from SCI, as the addictive potential is not reduced in this model.
The production and analysis of opioid system knockout (KO) mice deficient in one or more of the three opioid receptors (mu
(MOR-1), delta (DOR-1), and kappa (KOR-1)) and the three precursor proteins encoding opioid system ligands (proenkephalin,
prodynorphin, and proopiomelanocortin) have considerably enhanced our understanding of the roles of this complex system. All
single and combinatorial KO mice with mutations in this system are viable with receptor mutations, in particular, generally
accompanied by only minor compensations in other opioid system genes. These novel strains have been used to examine the contributions
of the endogenous opioid system to both baseline behaviors and to actions of exogenous compounds. The effects of opioid system
mutation on endogenous behavior are generally modest, but there are demonstrated effects of specific opioid system gene deletions
on locomotion as well as emotional behaviors, and there is evidence for opposing actions of specific receptor systems on these
processes. Exogenous administration of opioid system ligands has complemented and extended the findings of traditional pharmacology
while unanticipated observations continue to emerge. For example, extensive analysis in multiple paradigms continues to demonstrate
that essentially all actions of morphine are abolished following deletion of MOR-1, despite its significant binding to other
receptor subtypes in vitro. In addition, requirements of DOR-1 and preproenkephalin for morphine tolerance but not dependence
have been demonstrated, as have alternate systems that can mediate analgesic responses to delta ligands, and recent findings
indicate that at least some traditional opioid receptor ligands retain activity even in mice lacking all classic opioid receptors.
These recent results should help focus analysis of the genomic and proteomic alterations that accompany drug administration
and opioid system mutation as well as characterization of conditional mutations of opioid system genes.
Previous studies have reported that some of the central effects of morphine are counteracted by the administration of the methanolic extract of the root of Indian ginseng, Withania somnifera Dunal (WSE). The present study sought to determine whether WSE affects acquisition and expression of morphine-elicited conditioned place preference (CPP) in CD-1 mice. In CPP acquisition experiments, WSE (0, 25, 50, and 100 mg/kg) was administered, during conditioning, 30 min before morphine (10 mg/kg), whereas in expression experiments, WSE (0, 25, 50, and 100 mg/kg) was administered 30 min before the postconditioning test. The results demonstrate (i) that WSE was devoid of motivational properties; (ii) that WSE (100 mg/kg) was devoid of effects on spontaneous and morphine-stimulated motor activity and on spatial memory; and (iii) that WSE (50 and 100 mg/kg) significantly prevented the acquisition and expression of CPP. Further, to characterize the receptor(s) involved in these effects, we studied, by receptor-binding assay, the affinity of WSE for µ-opioid and γ-aminobutyric acid B receptors. These experiments revealed a higher affinity of WSE for γ-aminobutyric acid B than for µ-opioid receptors. Overall, these results point to WSE as an interesting alternative tool, worthy of further investigation, to study opiate addiction.
Objective(s)Single injection of naloxone, a selective antagonist of morphine, prior to the drug conditioning testing was used to investigate on morphine dependence.Materials and Methods
Conditioning to morphine (2.5-10 mg/kg, s.c.) was established in adult male Wistar rats (weighing 200-250 g) using an unbiased procedure. Nitric oxide agents were microinjected into the central amygdala prior to naloxone-paired place conditioning testing.ResultsThe results showed that morphine produced a significant dose-dependent place preference in animals. Naloxone (0.1-0.4 mg/kg, i.p.) injections pre-testing of the response to morphine (7.5 mg/kg, s.c.) caused a significant aversion at the higher doses (0.4 mg/kg, i.p.). This response was reversed by microinjection of L-arginine (0.3-3 µg/rat, intra-central amygdala) prior to naloxone on the day of the testing. The response to L-arginine was blocked by pre-injection of NG-nitro-L-arginine methyl ester (L-NAME) (intra-central amygdala).ConclusionA single injection of naloxone on the test day of morphine place conditioning may simply reveal the occurrence of morphine dependence in rats, and that the nitric oxide in the central amygdala most likely plays a key role in this phenomenon.
The motivational component of drug withdrawal may contribute to drug seeking and relapse through the negative reinforcement-related process; thus, it is important to understand the mechanisms that mediate affective withdrawal behaviors. The present study was undertaken to examine the calcium-dependent mechanism of negative motivational symptoms of nicotine and morphine withdrawal using the conditioned place aversion (CPA) paradigm. Rats were chronically treated with nicotine (1.168 mg/kg, free base, s.c., 11 days, three times daily) or morphine (10 mg/kg,s.c., 11 days, twice daily). Then, during conditioning, rats pre-treated with nicotine or morphine received a nicotinic receptor antagonist mecamylamine (3.5 mg/kg) or an opioid receptor antagonist naloxone (1 mg/kg) to precipitate withdrawal in their initially preferred compartment, or saline in their non-preferred compartment. Our results demonstrated that after three conditioning sessions, mecamylamine induced a clear place aversion in rats that had previously received nicotine injections, and naloxone induced a significant place aversion in rats that had previously received morphine injections. Further, the major findings showed that calcium channel antagonists, i.e., nimodipine, verapamil and flunarizine (5 and 10 mg/kg, i.p.), injected before the administration of mecamylamine or naloxone, attenuated nicotine or morphine place aversion. As an outcome, these findings support the hypothesis that similar calcium-dependent mechanisms are involved in aversive motivational component associated with nicotine a morphine withdrawal. We can suggest that calcium channel blockers have potential for alleviating nicotine and morphine addiction by selectively decreasing the incentive motivational properties of both drugs, and may be beneficial as smoking cessation or opioid dependence pharmacotherapies.
Naltrexone is an opioid receptor antagonist that has been shown to be effective for maintaining abstinence in alcohol-dependent persons. It is particularly effective in a subset of persons who suffer from high craving, as it reduces craving for alcohol. Family history has been shown to be a predictor of treatment response and, indeed, allelic variation in the mu opioid receptor gene predicts treatment response to naltrexone. The therapeutic effects of naltrexone are mediated by blockade of central mu opioid receptors. The site of action is under investigation but evidence supports a role of mu opioid receptors in the central nucleus of the amygdala, nucleus accumbens, and ventral tegmental area in the therapeutic actions of naltrexone for alcohol dependence. This article reviews the role of the endogenous opioid system in addictive diseases, especially alcoholism and discusses the pharmacological basis for the use of naltrexone in the treatment of alcohol dependence.
These experiments aimed to elucidate feeding-associated behavioral roles of globus pallidus (GP) neurons in gustatory functions: The effects of bilateral microiontophoretic kainate (KA) lesions of the ventromedial pallidal (vmGP) region on neophobia and conditioned taste aversion (CTA) were studied. Lesioned rats displayed strong and persistent neophobia to a mild citric acid solution. Neuron-specific damage to the vmGP also prevented rats from proper acquisition of CTA. Rats that previously showed normal neophobia and successfully learned CTA demonstrated difficulties in CTA retention after GP lesions. KA-lesioned rats, in addition, exhibited deficits in orientation reactions but did not have aphagia, adipsia, or motor disturbances seen after larger pallidal lesions. These findings suggest that neurons of the GP are significant in acquisition, memory storage, and retrieval mechanisms of feeding-associated taste information.
Albino rats implanted with intracerebroventricular (ICV) cannulae were used to investigate the effects of ICV administration of d-ala2-methionine enkephalin. Potentially positive affective consequences were assessed by observing rats movements in an alley, one compartment of which had previously been paired with drug administration. Like morphine (10 mg/kg, intraperitoneally - IP), this enkephalin analogue (10 μg, ICV) produced a tendency for rats to move toward the place where they had previously experienced the drug's effects. In another experiment, the same dose of d-ala2-methionine enkephalin was not sufficient to produce a conditioned taste aversion, as did the 10-mg/kg IP dose of morphine. Rats with a prior history of administration of either d-ala2-methionine enkephalin or systemic morphine subsequently consumed significantly more sweetened morphine solution than control animals in a voluntary oral consumption situation with tap water also available. Collectively, these results suggest that enkephalin administration may produce a positive affective state without aversive components and potentiate voluntary consumption of morphine.
Chronic administration of narcotic μ opioid agonists results in tolerance and dependence. We propose that agonist stimulation causes a gradual conversion of μ receptors to a constitutively active state μ∗ as a key step in tolerance and physical dependence. We provide evidence in support of the existence of μ∗ in human neuroblastoma cells, SH-SY5Y, and μ∗ upregulation during morphine treatment. Naloxone blocked μ∗ activity, acting as an antagonist with negative intrinsic activity which accounts for its high potency in eliciting withdrawal. In contrast, the μ selective antagonist CTAP did not affect μ∗ activity but inhibited naloxone's effect. The protein kinase inhibitor H7 was found to suppress μ∗ formation, suggesting that μ∗ is phosphorylated. In a model of acute morphine tolerance/dependence in mice, H7 prevented naloxone induced withdrawal jumping and reversed morphine (antinociceptive) tolerance. CTAP cause only mild withdrawal and attenuated naloxone induced withdrawal, as predicted for an antagonist without negative activity. These results support a role for constitutive μ receptor activation in narcotic tolerance and dependence, affording potential separation of acute and chronic narcotic effects.
The authors explored naltriben's (NTB, a delta opioid antagonist) potential to be a pharmacotherapy for cocaine abuse. NTB (3–6 mg/kg) reduced rats' (
Rattus norvegicus) intravenous, self-administration of cocaine, daily, across 5 days. NTB did not, however, interfere with rats' pressing for water. NTB (3 mg/kg) blocked cocaine's facilitation of pressing for brain stimulation, daily, for 5 days. As doses of NTB were explored, it was found that 10 mg/kg of NTB was lethal for about one third of the rats. Smaller doses (e.g., 3 mg/kg) gave some indications of toxicity as indexed by NTB's tendency to reduce pressing for brain stimulation by itself. NTB (3 mg/kg) induced a mild conditioned taste aversion but had no negative effects on rats' ability to learn and remember a sequence of mazes. Taken together, these results lead to the suggestion that opioidergic processes play an important role in mediating cocaine's reinforcement. Although NTB may not be the ideal opioid antagonist for treating people, it has many positive properties supporting further investigation of opioid antagonists as agents for treating cocaine addiction. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Microinfusions of the endogenous neurohumor (endocoid), the opioid methionine-enkephalin, into the forebrain structure nucleus accumbens initiated a reinforcing stimulus in a dose-related manner. The reinforcing nature of this intracranial self-administration was evaluated with intermittent schedules of reinforcement and a two-lever discrimination procedure. Opiate receptors are likely responsible for the initiation since the opiate antagonist naloxone (Narcan) effectively blocked self-administration. These data suggest the mediation of opiate reinforcement through interactions with opiate receptors in brain regions outside of the ventral tegmentum, a brain region thought to be the site of the opiate reinforcement produced by exogenous morphine. Endocoids play a critical role in reinforcement.
A conditioned place preference paradigm was employed to demonstrate the rewarding property of unilateral injections of 200 ng (D-ala2)-met5-enkephalinamide into the ventral tegmental area of the rat brain. This effect was attenuated in a dose-related manner by systemic injections of the dopamine receptor blocker haloperidol. In addition, selective lesions of the ascending dopamine (DA) pathways ipsilateral to the injection site blocked the rewarding effect when DA levels were reduced by more than 90%. Similar lesions in the contralateral hemisphere had no influence on this behavior. These data suggest that forebrain DA pathways can mediate some of the rewarding properties of opioid drugs.
Opiates are known to be reinforcing when injected into the ventral tegmental area (VTA). The present study produced conditioned reinforcement with local injections of exogenous d-ala2-met5-enkephalinamide (DALA), a potent analogue of met-enkephalin, and with thiorphan , an enkephalinase inhibitor which protects endogenous opiates from enzymic degradation. In a conditioned place preference paradigm, rats received injections of DALA (1.0, 3.0, or 8.0 micrograms), thiorphan (60 micrograms), and/or naloxone (10 micrograms), or saline vehicle. Conditioned reinforcement was obtained with 8.0 micrograms of DALA and also with thiorphan but not with thiorphan plus naloxone. This suggests that reward can be generated by endogenous opiates in the VTA. Tests during the light phase and dark phase suggested that diurnal periodicity may play a role in opiate reward. It is concluded that the VTA can generate conditioned reward through transmitter-receptor interaction involving an endogenous opiate substrate which is probably enkephalinergic.
Intracranial self-administration of mu- and delta-opioid agonists was demonstrated in male Long-Evans rats. Independent groups were allowed to lever-press for ventral tegmental area (VTA) microinfusions of morphine, the selective mu agonist [D-Ala2,N-Me-Phe4-Gly5-ol]-enkephalin (DAMGO), the selective delta-agonist [D-Pen2,D-Pen5]-enkephalin (DPDPE), or ineffective drug vehicle. Morphine, DAMGO, and DPDPE were each effective in establishing and maintaining lever-pressing habits. Lever-pressing responses were extinguished during a session when vehicle was substituted for drug, and reinstated when drug reinforcement was reestablished. Thus, it appears that VTA mu- and delta-opioid receptors are each involved in reinforcement of opiate self-administration. The effective dose of DAMGO--both for establishing and for maintaining the lever-press habit--was 100 times lower than the effective doses for DPDPE and morphine, suggesting that the major contribution of VTA mechanisms to intravenous heroin self-administration involves an action on mu-opioid receptors.
Opiate analgesics are widely used in the treatment of severe pain. Because of their importance in therapy, different strategies have been considered for making opiates more effective while curbing their liability to be abused. Although most opiates exert their analgesic effects primarily via mu opioid receptors, a number of studies have shown that delta receptor-selective drugs can enhance their potency. The molecular basis for these findings has not been elucidated previously. In the present study, we examined whether heterodimerization of mu and delta receptors could account for the cross-modulation previously observed between these two receptors. We find that co-expression of mu and delta receptors in heterologous cells followed by selective immunoprecipitation results in the isolation of mu-delta heterodimers. Treatment of these cells with extremely low doses of certain delta -selective ligands results in a significant increase in the binding of a mu receptor agonist. Similarly, treatment with mu -selective ligands results in a significant increase in the binding of a delta receptor agonist. This robust increase is also seen in SKNSH cells that endogenously express both mu and delta receptors. Furthermore, we find that a delta receptor antagonist enhances both the potency and efficacy of the mu receptor signaling; likewise a mu antagonist enhances the potency and efficacy of the d receptor signaling. A combination of agonists (mu and delta receptor selective) also synergistically binds and potentiates signaling by activating the mu-delta heterodimer. Taken together, these studies show that heterodimers exhibit distinct ligand binding and signaling characteristics. These findings have important clinical ramifications and may provide new foundations for more effective therapies.
Understanding the neurobiological mechanisms of addiction requires an integration of basic neuroscience with social psychology, experimental psychology, and psychiatry. Addiction is presented as a cycle of spiralling dysregulation of brain reward systems that progressively increases, resulting in compulsive drug use and a loss of control over drug-taking. Sensitization and counteradaptation are hypothesized to contribute to this hedonic homeostatic dysregulation, and the neurobiological mechanisms involved, such as the mesolimbic dopamine system, opioid peptidergic systems, and brain and hormonal stress systems, are beginning to be characterized. This framework provides a realistic approach to identifying the neurobiological factors that produce vulnerability to addiction and to relapse in individuals with a history of addiction.
A fundamental concept of modern pharmacology is that the biological activity of a drug results from its interaction with a macromolecular tissue component. Such a receptor must be defined in terms of its pharmacology: it encompasses both a recognition site to which a drug binds, and
a factor which translates binding into the events which lead ultimately to a biological response. A drug which binds specifically to the recognition site has an affinity for that receptor, usually expressed as the equilibrium dissociation constant K
D, which is the ratio of the association and dissociation rate constants. Agonists have, in addition to affinity for the recognition site, the ability to trigger the translational phase to produce a biological effect, while antagonists fail to elicit a biological response. This chapter will deal primarily with the biochemical characterization of the opiate receptors. Since binding studies yield information only about the recognition site of the receptor, they cannot distinguish between the agonist or antagonist nature of the interactions. We shall therefore correlate the biochemical data with results from pharmacological assays.
Naltrindole, a selective δ-opioid receptor antagonist, was evaluated for its potential to block the reinforcing properties of cocaine using a conditioned place pairing paradigm in Lewis rats. Cocaine HCl (15 mg/kg s.c.) produced a strong place preference which was significantly blocked in animals pretreated with naltrindole (3 mg/kg i.p.); naltrindole alone showed no reinforcing or aversive effects. The results suggest a novel approach for the treatment of cocaine abuse in man.
Guidelines for submitting commentsPolicy: Comments that contribute to the discussion of the article will be posted within approximately three business days. We do not accept anonymous comments. Please include your email address; the address will not be displayed in the posted comment. Cell Press Editors will screen the comments to ensure that they are relevant and appropriate but comments will not be edited. The ultimate decision on publication of an online comment is at the Editors' discretion. Formatting: Please include a title for the comment and your affiliation. Note that symbols (e.g. Greek letters) may not transmit properly in this form due to potential software compatibility issues. Please spell out the words in place of the symbols (e.g. replace “α” with “alpha”). Comments should be no more than 8,000 characters (including spaces ) in length. References may be included when necessary but should be kept to a minimum. Be careful if copying and pasting from a Word document. Smart quotes can cause problems in the form. If you experience difficulties, please convert to a plain text file and then copy and paste into the form.
Although compounds with relative selectivity for the mu and kappa opiate receptors subtypes have been reported to condition taste aversions, it is not known whether systemically administered delta compounds have the ability to produce aversions. To that end, female Long-Evans rats were adapted to water deprivation and were given pairings of a novel saccharin solution and various doses of the selective delta agonist SNC 80 (0.32–10.0 mg/kg; Experiment 1) or the selective delta antagonist naltrindole (1.0–18.0 mg/kg; Experiment 2). For comparison, the relatively selective mu agonist morphine (Experiment 1) and mu antagonist naloxone (Experiment 2) were assessed under identical conditions. Both SNC 80 (Experiment 1) and naltrindole (Experiment 2) were effective as unconditioned stimuli within this design, inducing dose-dependent taste aversions with repeated conditioning trials. Although at no dose did animals injected with SNC 80 differ from those injected with morphine, aversions induced by SNC 80 were acquired at a faster rate than those induced by morphine. Subjects injected with naloxone drank significantly less than those injected with naltrindole at the 10 mg/kg dose, and aversions induced by naloxone at 5.6 and 10 mg/kg were acquired at a faster rate than those induced by naltrindole. Although the basis for opioid agonist- and antagonist-induced taste aversions is not known, the differences between aversions induced by SNC 80 and naltrindole and those induced by morphine and naloxone, respectively, may be a function of their relative selectivity for specific opiate receptor subtypes.
The μ opioid receptor gene (MOR) was mutated in mice by a gene targeting procedure. In these MOR-knockout mice, the analgesic effects of morphine, its major metabolites, morphine-6-glucuronide (M-6-G) and morphine-6-ethereal sulfate (M-6-S), and endomorphin-2, as well as morphine-induced lethality, were drastically reduced, whereas the effects of DPDPE and U50488 remained unchanged. It is concluded that analgesic effects of μ-specific opioid ligands and acute morphine lethality are mediated by the μ receptor.
The role of central σ-opioid receptors in the mediation of opioid reinforcement and endogenous reward processes was examined using a non-biased place-preference conditioning procedure. Intracerebroventricular (i.c.v.) administration of the selective σ-receptor agonists, [d-Pen2, d-Pen5]-enkephalin (DPDPE, 10.0–25.0 μg) produced a significant preference for the drug-associated place and a similar effect was observed following i.c.v. injections of morphine (10.0 μg). Administration of the σ-receptor antagonist, ICI 174,864, at doses (1.0–5.0 μg, i.c.v.) which had no aversive effects when tested alone, abolished the reinforcing properties of DPDPE. Such treatment did not, however, modify the effect of morphine. These findings demonstrate the involvement σ- as well as μ-receptors in the motivational properties of opioids and suggest that the activation of either receptor type is sufficient for the elicitation of appetively reinforcing effects.
Reward effects of enkephalin were demonstrated by establishing a conditioned place preference to a distinctive location paired with microinjection of (d-Ala2),Met5-enkephalinamide (d-Ala) into the ventral tegmental area. Bilateral injections of vehicle and 50 ng d-Ala were ineffective, whereas 100–250 ng produced a significant conditioned place preference. Naloxone (2 mg/kg) successfully antagonized the reward effect of 100 ng D-Ala.
An unbiased place preference conditioning procedure was used to identify the central opioid receptor types through which the endogenous opioid peptide, β-endorphin, acts to exert its reinforcing effects in rats in vivo. The intracerebroventricular administration of β-endorphin, and selective μ (DAGO) or δ (DPDPE) opioid receptor agonists produced marked preferences for the drug-associated place. Intracerebroventricular pretreatment with the selective μ antagonist, CTOP, eliminated the place preference produced by DAGO but not that produced by DPDPE. Pretreatment with the selective δ antagonist, ICI 174,864, abolished the place preference induced by DPDPE. It did not modify the effect of DAGO. In contrast, pretreatment with either ICI 174,864 or CTOP abolished the effects of β-endorphin. These data demonstrate that both μ and δ receptors are involved in mediating the reinforcing effect of β-endorphin and indicate that the activation of both receptor types is required for the expression of the motivational effects of β-endorphin. Further they suggest that β-endorphin produces its motivational effects via an interaction with an opioid receptor complex composed of both μ and δ receptors.
The hypothesis that painful stimuli activate the endogenous opioid (endorphin) system in humans was tested by examining the effect of the opiate antagonist naloxone on experimentally induced ischemic pain and on subjective mood ratings. Intravenous injections of saline or naloxone hydrochloride (2 and 10 mg) were administered under double-blind conditions to 12 subjects. Naloxone did not affect the pain ratings. However, a significant dose-related effect of naloxone on tension-anxiety was found, suggesting that the endorphins. like exogenously administered opiates, may have antianxiety properties.
The effect of naloxone on the latency of mice to enter a dark box was determined and the study of Jacob et al. (1974) on the effect of naloxone on hotplate behavior was replicated. Mice took significantly longer to enter a dark box after receiving 10 mg/kg of naloxone than did the saline controls. With the hot-plate technique, at all doses (0.1, 0.3, 1, 3, and 10 mg/kg) except the lowest (0.03 mg/kg), naloxone-treated mice jumped from the hot plate significantly sooner than did the saline controls. No effect of naloxone on paw-lick latency was found. All the data confirm the findings of Jacob et al. (1974). The hotplate results suggest that prolonged exposure to a noxious stimulus may be necessary to activate the endorphin system.
The effects of morphine, pentazocine, naloxone and combinations of these drugs on schedule-controlled behavior were examined in rhesus monkeys and pigeons. The order of potency in decreasing response rates under a multiple 5-minute fixed-interval, 30-response fixed-ratio schedule in both monkeys and pigeons was morphine greater than pentazocine greater than naloxone. Compared to monkeys, pigeons were less sensitive to morphine and pentazocine and slightly more sensitive to naloxone. In monkeys and pigeons, as the morphine dose increased, higher naloxone doses were required to restore responding to or near control levels. In pigeons, however, the response rate decreases caused by naloxone prevented the complete antagonism of the highest morphine dose and limited the range of naloxone doses over which complete antagonism of morphine occurred. Antagonism of pentazocine by naloxone generally was greatest at the lowest pentazocine dose tested in both monkeys and pigeons. At higher pentazocine doses, if any antagonism was obtained, it was slight, and the amount of antagonism typically did not increase as the naloxone dose increased. Pentazocine generally failed to antagonize morphine in monkeys and pigeons.
The aim of this study was to investigate the neuroanatomical regions implicated in the expression of the physical signs of morphine withdrawal by using local intracerebral injections of methylnaloxonium in dependent rats. Guide cannulas were implanted and aimed at the lateral ventricle, nucleus accumbens, central amygdala, anterior hypothalamus, medial thalamus, periaqueductal gray matter, locus coeruleus and nucleus raphe magnus. After surgery, rats were made physically dependent by s.c. implantation of two 75-mg morphine pellets. Methylnaloxonium, a quaternary derivative of naloxone (31-1000 ng), was administered 72 hr after pellet implantation. Methylnaloxonium administered i.c.v. induced a withdrawal syndrome similar to systemic naloxone, although several signs such as diarrhea, salivation, lacrimation and rhinorrhea did not appear, suggesting possible peripheral mediation. The most sensitive site for methylnaloxonium-precipitated withdrawal was the locus coeruleus. Signs such as jumping, rearing and locomotor activity were particularly frequent after methylnaloxonium injections into the locus coeruleus. Rearing and locomotor activity were also strongly increased after methylnaloxonium administration into the periaqueductal gray matter. Wet dog shakes were mainly observed after methylnaloxonium administration into the anterior preoptic hypothalamus and nucleus raphe magnus. Injections of methylnaloxonium into the amygdala produced a weak withdrawal syndrome, and the nucleus accumbens and medial thalamus were the least sensitive structures. These results suggest that the locus coeruleus, and secondarily the periaqueductal gray matter, play an important role in the precipitation of the physical signs of opiate withdrawal, mainly in the expression of its motor component. The expression of other signs of withdrawal appear to be multisite determined.
Specific brain sites for the opiate abstinence syndrome syndrome have been elusive to delineate, and the classic overt signs of withdrawal such as wet dog shakes, ptosis and teeth chattering appear to be widely represented in the brain. Using a more general motivational test involving a disruption of operant behavior in dependent rats, the brain site most sensitive to the response disruptive effects of intracerebral administration of the opiate antagonist, methylnaloxonium, was the region of the nucleus accumbens, a site also implicated in the acute reinforcing properties of opiates. This disruption of operant responding was hypothesized to reflect the aversive properties of opiate withdrawal. The present study directly tested that hypothesis by exploring whether intercerebral administration of methylnaloxonium produced aversive stimulus effects as measured by the formation of place aversions. Rats implanted intracerebroventricularly or with bilateral cannulae aimed at the medial dorsal thalamus, periaqueductal gray, ventral tegmental area, amygdala or nucleus accumbens were made dependent on morphine by subcutaneous implantation of two 75-mg morphine pellets. The animals were then subjected to place aversion training by pairing of a distinct environment (one of three arms of a three-armed box with distinct texture, markings and smell) with a single injection of methylnaloxonium intracerebroventricularly or intracerebrally. Results showed that at high doses of methylnaloxonium (1000-2000 ng) all sites produced a place aversion. However, lower doses (250-500 ng) produced a significant brain site selectivity with the region of the nucleus accumbens the most sensitive. Observational measurements taken during the postinjection period with the high dose of methylnaloxonium showed that agitation was particularly observed following methylnaloxonium administration into the nucleus accumbens and periaqueductal gray.(ABSTRACT TRUNCATED AT 250 WORDS)
The motivational effects of exclusively peripheral or central opiate receptor blockade were studied using place conditioning. Place aversions were observed with intraventricular (i.c.v.) methylnaloxone (MN) in both naive (200-1000 ng) and morphine-dependent rats (50-500 ng). Subcutaneous MN (0.03-10 mg/kg) was ineffective in naive rats; in dependent rats a small aversion was seen at the highest dose. Place aversions were not necessarily associated with behavioral signs of withdrawal. The data suggest that the aversive properties of opioid receptor antagonism are centrally mediated in both naive and dependent rats, and that their enhancement in morphine-dependent subjects results from a sensitized central mechanism rather than from the recruitment of a peripheral component.
The role of central versus peripheral opioid receptors in mediating the aversive effects of opioids was examined by use of an unbiased place preference conditioning procedure in rats. The non-selective opioid antagonist naloxone (NLX) produced conditioned aversions for the drug-associated place after subcutaneous (SC) as well as intracerebroventricular (ICV) administration. Place aversions were also observed in response to the ICV administration of the selective mu-antagonist CTOP. In contrast, the selective delta-antagonist ICI 174,864 and the selective kappa-antagonist norbinaltorphimine (nor-BNI) (ICV) were without effect. Place aversions were also produced by central applications of the selective kappa-agonist U50,488H and the dynorphin derivative E-2078. For those opioid ligands tested, the doses required to produce place aversions were substantially lower following ICV as compared to SC administration. These data confirm that kappa-agonists and opioid antagonists produce aversive states in the drug-naive animal and demonstrate that this effect is centrally mediated. Furthermore, the ability of NLX and CTOP, in contrast to both ICI 174,864 and nor-BNI, to produce place aversions suggests that the aversive effects of opioid antagonists result from the blockade of mu-receptors.
The motivational effect of naloxone administration in the non-dependent laboratory mouse was examined with taste and place conditioning procedures. Thus, male CD1 mice without any history of drug exposure avoided a cue paired with three SC injections of as little as 0.1 mg/kg naloxone HCl. The aversive effect of naloxone was also seen in DBA/2 and C57BL/6 mice. In addition, it only occurred with the minus isomer and not the plus isomer, and it was potentiated by implantation, 3 days prior to training, of a morphine-containing (37.5 mg) but not a placebo pellet. Naloxone injection, therefore, acts as an aversive stimulus in naive mice and this is probably produced by decreases in activity of endogenous opioid peptide systems. Together with other data, the present results support the conclusion that the aversive effect of opioid receptor blockade in the opiate non-dependent organism may be general to a wide range of species including primates. The importance of training and testing variables for observing the naloxone aversive effect is discussed. Advantages of studying preference conditioning with mice are also given.
The role of central delta-opioid receptors in the mediation of opioid reinforcement and endogenous reward processes was examined using a non-biased place-preference conditioning procedure. Intracerebroventricular (i.c.v.) administration of the selective delta-receptor agonist, [D-Pen2, D-Pen5]-enkephalin (DPDPE, 10.0-25.0 micrograms) produced a significant preference for the drug-associated place and a similar effect was observed following i.c.v. injections of morphine (10.0 micrograms). Administration of the delta-receptor antagonist, ICI 174,864, at doses (1.0-5.0 micrograms, i.c.v.) which had no aversive effects when tested alone, abolished the reinforcing properties of DPDPE. Such treatment did not, however, modify the effect of morphine. These findings demonstrate the involvement delta- as well as mu-receptors in the motivational properties of opioids and suggest that the activation of either receptor type is sufficient for the elicitation of appetitively reinforcing effects.
The role of beta-endorphin and mu-opioid receptors in mediating the motivational effect of lithium was examined by use of an unbiased place-preference conditioning procedure. Administration of lithium to drug-naive rats resulted in a dose-related aversion for the drug-associated place. Radiofrequency lesions of the medio-basal arcuate hypothalamus, which markedly reduced the levels of immunoreactive beta-endorphin in the hypothalamus, abolished the lithium-induced aversion. However, suppression of circulating beta-endorphin levels by chronic dexamethasone treatment was without effect. Infusion of the opioid antagonist, naloxone, throughout the conditioning procedure at a dose (0.5 mg/kg per h) that blocks mu- but not kappa-opioid receptors, resulted in the complete abolition of the lithium-induced place aversion. These data demonstrate an involvement of endogenous opioidergic systems in the motivational effect of lithium and indicate that the aversive properties of this drug result from its interactions with beta-endorphin and mu-opioid receptors in the CNS.
The present study examines the influence of destruction of the medio-basal arcuate hypothalamus (MBH), the primary site of synthesis of central pools of beta-endorphin (beta-EP), upon the aversive properties of naloxone in a conditioned place preference paradigm. Bilateral radiofrequency lesions of the MBH resulted in a pronounced fall in levels of immunoreactive beta-EP in the brain. Lesioned rats, in contrast to non-operated animals, showed a clear reduction in the conditioned place aversion produced by naloxone. However, they showed no loss of the conditioned preference produced by the mu-selective opioid receptor agonist, morphine, or the conditioned aversion produced by the kappa-selective agonist, U50-488. In contrast to the effect of the lesions, suppression of circulating beta-EP by dexamethasone treatment failed to influence conditioning produced by naloxone. Thus, the data indicate that the aversive properties of naloxone are attenuated by disruption of central (but not peripheral) beta-EP activity. We suggest that these properties of naloxone reflect an antagonism of beta-EP activity in the brain. In addition, the data indicate that differing mechanisms underlie the aversive actions of naloxone as compared to U50-488.
The reinforcing properties of various opioid agonists acting preferentially on the kappa and mu opioid receptors were assessed using taste and place preference conditioning procedures.
Kappa receptor agonists produced conditioned aversions. Taste aversions were produced by all of the drugs used, including racemic mixtures of ethylketazocine, tifluadom, and U50-488, and active isomers (+)-tifluadom, (-)-bremazocine, and Mr 2034; corresponding inactive isomers either produced no effect of were less potent. Place aversions were produced by U50-488 and (-)-bremazocine, but not (+)-bremazocine or any of the other kappa receptor agonists tested with the taste procedure. The mu agonists produced predominantly conditioned preferences. Place preferences were produced by morphine, fentanyl and sufentanil. Taste preferences were produced by low doses of these substances; at higher doses the taste preferences were absent or replaced by aversions. Finally, with naloxone and lithium chloride it was shown that the taste procedure was more sensitive to punishing effects than the place procedure.
It is concluded that kappa and mu opioid receptor agonists are effective unconditioned stimuli. From the lower portions of the dose response curves it is further concluded that activation of kappa opioid receptors has aversive properties and activation of mu receptors appetitive reinforcing properties. The findings are also discussed with regard to the prevailing notions of taste conditioning with opiates, and the reinforcing properties of activity of the endogenous opioid peptide systems.
Rats display a preference for an environment in which they previously received morphine. The present report provides behavioral and pharmacological data for this simple model of reinforcement produced by opiates and describes an aversion in rats for an environment in which they previously received naloxone. Preferences were produced with intravenous (i.v.) morphine sulfate at doses of 0.08-15 mg/kg and durations of the pairing between environment and morphine of 10 min to 1.5 h. Preferences were also seen with other opiate agonists (etorphine-HCl and levorphanol-tartrate), another route of drug administration (subcutaneous), and after 1-4 administrations of morphine. Cocaine-HCl (i.v.), a non-narcotic drug, known to be self-administered by humans, also produced a place preference. Lithium chloride (i.v.), an agent found to be a punishing stimulus in other situations, produced a place aversion. There was no appreciable preference for an environment paired with dextrorphan-tartrate and naloxone-HCl (2 mg/kg, i.p.) blocked the production of the preference produced by i.v. morphine. In contrast to the effect produced by morphine, aversions were produced with (-)-naloxone-HCl alone at doses of 0.1-45 mg/kg (i.v.). The aversion was not produced at (+)-naloxone. Implantation of rats with a 75 mg morphine pellet 3 days prior to place conditioning potentiated the aversive effect of naloxone. It was concluded that place conditioning produced by morphine and naloxone is mediated by specific opiate receptors and that stimulating and decreasing activity of the endogenous opioid peptide system with systemically administered drugs is positively reinforcing and aversive, respectively. The discussion emphasizes application of the simple and sensitive place conditioning model to drug reinforcement research, including analyses of reinforcement produced by microinjection of opiates into the brain.
Microinfusions of the endogenous opiate neurohumor, methionine enkephalin, into the nucleus accumbens initiated a reinforcing stimulus in a dose-related manner. The reinforcing nature of this intracranial self-administration was evaluated with intermittent schedules of reinforcement and a two-lever discrimination procedure. Opiate receptors are likely responsible for the initiation of this reinforcing stimulus since naloxone effectively blocked self-administration. These data suggest the mediation of opiate reinforcement through interactions with opiate receptors in brain regions outside the ventral tegmental area, questioning the current dopamine hypothesis for the initiation of these reinforcement processes.
In rats, conditioned place preferences are produced by morphine and conditioned place aversions produced by naloxone. In the present studies, several issues concerning the demonstration and interpretation of place conditioning findings were examined in a two-compartment (black and white) tilt box: (1) the responses of naive rats to testing, (2) place conditioning in rats with strong unconditioned biases to one of the sides, and (3) modifications of the testing situation so that naive rats respond to the black and white sides with a minimum of initial bias. Experiments involving manipulation of the conditions of training and testing, use of pentobarbital, and use of a three-compartment test box helped to control for morphine's ability to produce state dependent learning as an explanation of its conditioned place preference. In addition, we examined previous place conditioning studies that failed to show aversive effects of naloxone. These negative findings were suggested to be due to the use or procedures insensitive to aversive stimuli and to the IP administration of naloxone. Finally, in the course of the experiments, novel data on general parameters of the place conditioning were provided. Dose-response curves for subcutaneous (SC) morphine (0.04-5.0 mg/kg) and naloxone (0.02-5.0 mg/kg) were established. Conditioned preferences were also shown to occur after at three pairings of SC drug, and they were retained for at least 1 month.
Naltrexone was given to ten opiate-free volunteer subjects following the same dosage schedule used for initiating treatment of opiate-dependent persons. During the three-week initiation period, three subjects dropped from the study owing to aversive effects of the drug. The remaining seven subjects reported similar unpleasant but tolerable effects. A separate group of ten volunteer subjects was given single doses of 50 or 100 mg of naltrexone or a naltrexone placebo on three separate occasions using blind controls. These subjects also reported aversive effects. The principal symptoms reported were loss of energy, gastrointestinal disturbances and mental depression. It is possible that these aversive reactions of naltrexone have limited acceptance of the drug as a treatment for opiate-dependent persons.
Chronic administration of narcotic mu opioid agonists results in tolerance and dependence. We propose that agonist stimulation causes a gradual conversion of mu receptors to a constitutively active state (mu*) as a key step in tolerance and physical dependence. We provide evidence in support of the existence of mu* in human neuroblastoma cells, SH-SY5Y, and mu* upregulation during morphine treatment. Naloxone blocked mu* activity, acting as an antagonist with negative intrinsic activity which accounts for its high potency in eliciting withdrawal. In contrast, the mu selective antagonist CTAP did not affect mu* activity but inhibited naloxone's effect. The protein kinase inhibitor H7 was found to suppress mu* formation, suggesting that mu* is phosphorylated. In a model of acute morphine tolerance/dependence in mice, H7 prevented naloxone induced withdrawal jumping and reversed morphine (antinociceptive) tolerance. CTAP caused only mild withdrawal and attenuated naloxone induced withdrawal, as predicted for an antagonist without negative activity. These results support a role for constitutive mu receptor activation in narcotic tolerance and dependence, affording potential separation of acute and chronic narcotic effects.
We propose the hypothesis that certain G protein coupled receptors can become constitutively activated during agonist stimulation so that the receptor remains active even after the agonist is removed. This new paradigm of receptor regulation may account for some long term effects of neurotransmitters and hormones. We have tested the hypothesis that constitutive mu receptor activation represents a crucial step driving narcotic tolerance and dependence. Our results indeed support the conversion of mu to a constitutively active state, mu*, observed in neuroblastoma SK-N-SH and SH-SY5Y tissue culture, in U293 cells transfected with the mu receptor gene, and in vivo. Constitutive mu activation may result from receptor phosphorylation to yield mu*, and further, in vivo studies indicate that formation of mu* could account for narcotic tolerance and dependence.
Conditioned taste aversions (CTAs) are well known to be robust and long-lasting instances of learning induced by a single CS (taste)-US (malaise) pairing. CTA can be taken as a general model to search for neural mechanisms of learning and memory. In spite of extensive research on CTAs using a variety of approaches during the last three decades, the neural mechanisms of taste aversion learning still remain unsolved. In this article we propose a model of neural substrates of CTAs on the basis of our recent studies incorporating previous findings by other workers. Our studies mainly included experiments using ibotenic acid injections into various parts of the rat brain as a lesion technique, and c-fos immunohistochemistry in naive and CTA trained rats. CTAs were established by pairing the ingestion of saccharin (CS) with an ip injection of LiCl (US). Behavioral studies have shown that the parabrachial nucleus (PBN), medial thalamus, and basolateral nucleus of the amygdala are essential for both acquisition and retention of CTAs. C-fos studies suggested that association between gustatory CS and visceral US takes place in the PBN. The gustatory cortex (GC) may modify the strength of this association depending on the nature of the CS, viz., novel or familiar. The amygdala is indispensable for the expressions of CTAs. Tastes with hedonic values are stored in the GC in a long-term manner.
This study investigated diurnal variations in the affective and endocrine response to opioid blockade in man and whether there were effects related either to the dose of naloxone or the time of day at which it was given. Normal male subjects were given an intravenous bolus of either 0.2 mg/kg (study 1) or 1 mg/kg naloxone (study 2) or control infusions at two time points (0900 or 1800 hours) in a single-blind crossover design. Before and following each infusion, mood was measured by the Profile of Mood States (POMS) and a visual analogue scale (VAS), and blood samples taken at 15-min intervals. Cortisol, LH ACTH and vasopressin (study 2 only) were measured. Blood pressure and heart rate were also monitored. The lower dose of naloxone had no effect on overall mood (POMS), though tension and confusion were increased in the afternoon. The VAS showed increased depression in the afternoon, and heightened tension, sleepiness and reduced ability to concentrate at both times of day. The higher dose increased overall dysphoria at both time points, though the tension and depression subscales were not altered. VAS depression and tension were increased, and there were changes in sleepiness. Subjective reports showed that 45% of the subjects correctly identified the drug treatment at the lower dose compared with 89% at the higher one. ACTH increased after both doses of naloxone irrespective of time of day. Cortisol was also raised by naloxone; the effect was greater in the afternoon for the lower dose, but not the higher.(ABSTRACT TRUNCATED AT 250 WORDS)
Antagonists selective for mu, delta and kappa-opioid receptors were evaluated for their effects on responding maintained by i.v. injections of heroin (60.0 micrograms/kg/injection) in rats during daily 3-hr sessions. Under base-line conditions, rats self-administered 10 to 20 heroin injections during each session, and injections were separated by relatively constant interinjection intervals of about 10 to 20 min. The mu-selective antagonist beta-funaltrexamine (beta-FNA; 5.0-20.0 mg/kg, s.c.) produced a dose-dependent increase in responding for heroin, with some doses of beta-FNA producing an extinction-like pattern of responding. These results were qualitatively similar to the effect obtained by lowering the unit dose per injection of heroin. The mu 1-selective antagonist naloxonazine (NXZ; 7.5-30.0 mg/kg, i.v.) and the delta-selective antagonist naltrindole (1.0-17.0 mg/kg) also produced dose-dependent increases in heroin self-administration, but neither naloxonazine nor naltrindole produced extinction-like patterns of responding. The kappa-selective antagonist nor-binaltorphimine (nor-BNI; 5.0-10.0 mg/kg, s.c.) had no effect on heroin self-administration. These results indicate that mu receptors play an important role in mediating the reinforcing effects of heroin in the rat. Delta and mu 1 receptors, but not kappa receptors, may also be involved.
The non-selective opioid antagonist naltrexone and the partial agonist buprenorphine have been reported to reduce cocaine self-administration (SA) and relapse in both humans and rhesus monkeys. Data suggesting an involvement of delta-opioid receptors in modulating the conditioned rewarding effects of cocaine were also recently presented. In view of such findings, the present SA and place conditioning studies were conducted to examine the influence of the selective delta-opioid receptor antagonist naltrindole upon the rewarding effects of cocaine. Sprague-Dawley rats were trained to self-administer cocaine (1.0 mg/kg per infusion) on an FR2 schedule of reinforcement. Dose-response and antagonist testing commenced once stable rates of cocaine SA were achieved. For antagonist testing, rats received naltrindole (0.03-10.0 mg/kg, IP) 30 min prior to the start of 2-h SA sessions. SA behavior in response to cocaine delivery (0.25 and 1.0 mg/kg per infusion) was then determined. Naltrindole in doses of 0.03-3.0 mg/kg did not alter the number of cocaine infusions taken by the rats. A higher dose of naltrindole (10.0 mg/kg), which markedly depressed locomotor activity, resulted in a 16% reduction of cocaine (0.25 mg/kg per infusion) SA behavior. When SA sessions were terminated and naltrindole (1.0 mg/kg) was administered repeatedly for 3 days, no alterations in the re-acquisition of cocaine SA were seen. Place conditioning studies also failed to find an effect of naltrindole (0.1-3.0 mg/kg) on cocaine (10 mg/kg)-induced conditioned place preferences. Naltrindole, by itself, did not induce significant place conditioning.(ABSTRACT TRUNCATED AT 250 WORDS)
CRF in the central nervous system appears to hve activating properties on behavior and to coordinate behavioral responses to stressors. These behavioral effects of CRF appear to be independent of the pituitary-adrenal axis and can be reversed by CRF antagonists. CRF antagonist administration reverses not only decreases in behavior associated with stress, but also increases in behavior associated with stress, thus suggesting that the role of CRF is stress dependent and not intrinsic to a given behavioral response. Further, microinjection of alpha-helical CRF 9-41 and immunotargeting of CRF neurons in separate brain compartments reveal a link between the anatomical sites that contain CRF and the nature of the behavioral response to stressors that can be modified by suppression of endogenous CRF activity therein. These actions of CRF in coordinating coping responses to stress at several bodily levels are consistent with a role for CRF similar to the dual role of other hypothalamic releasing factors in integrating hormonal and neural mechanisms by acting both as secretagogues for anterior pituitary hormones and as extrapituitary peptide neurotransmitters. Moreover, dysfunction in such a fundamental homeostatic system may be the key to a variety of pathophysiological conditions including mental disorders.
To test if naloxone behaved as an inverse agonist rather than as an antagonist we evaluated its responses in guinea-pig ilea with and without morphine (480 nM, 24 h). In control ilea, naloxone (100 nM) had no effect. In morphine-treated ilea, naloxone as a bolus, but not as an infusion, elicited an abstinence response. Preadministration of naloxone blocked the response to subsequent administrations. Similarly, naloxone failed to produce an abstinence response in ilea pretreated with kappa compounds (bremazocine, U50488 or xorphanol 100 nM) or with kinase inhibitors (H7 or H8 30 microM). These findings can be interpreted in the light of the two-state receptor model if naloxone behaves as an inverse agonist: Incubation with morphine increased the active state of receptors making them susceptible to the inverse agonist (naloxone); exposure to naloxone favored the inactive conformation making them insensitive to further administration of naloxone; kappa compounds behaved as antagonists preventing the response to naloxone; and kinase inhibitors interfered with the active conformation making the system insensitive to naloxone. According to this model, dependence can be viewed as an overexpression of the active receptors and withdrawal as an abrupt change from the active to the inactive state.
An unbiased place preference conditioning procedure was used to examine the role of delta-opioid receptors in mediating the aversive effects of opioid withdrawal. Rats were implanted s.c. with two pellets each containing placebo or 75 mg morphine. Single-trial conditioning sessions with saline and the opioid receptor antagonists naloxone (0.001-1.0 mg/kg, s.c.), naltrindole (0.01-3.0 mg/kg, s.c.) or naltriben (0.01-3.0 mg/kg, s.c.) commenced 4 days later. During these conditioning sessions, physical signs of withdrawal were also quantified. Tests of conditioning were conducted on day 5. Naloxone in doses of 0.01-1.0 mg/kg produced significant conditioned place aversions in morphine-implanted animals. A dose of 0.01 mg/kg produced few physical withdrawal signs whereas higher doses resulted in marked wet dog shakes, body weight loss ptosis and diarrhea. No such effects were observed in control (placebo-implanted) animals. Administration of the selective delta-opioid receptor antagonists naltrindole and naltriben produced dose-related place aversions in morphine-implanted animals. The magnitude of these effects did not differ from that observed with naloxone. The minimum effective doses of naltrindole and naltriben were 0.1 mg/kg. Doses of 0.1-1.0 mg/kg produced few, if any, somatic signs of withdrawal whereas higher doses of these antagonists only produced diarrhea and wet-dog shakes. Other withdrawal signs were absent. In contrast to the opioid receptor antagonists tested, the dopamine D1 receptor antagonist SCH23390 failed to produced conditioned place aversions or physical signs of withdrawal in morphine-pelleted animals. These data demonstrate that the selective blockade of either delta- or mu-opioid receptors is sufficient to induce conditioned aversive effects in morphine-dependent animals. They also indicate that physical symptoms associated with precipitated morphine withdrawal differ depending upon the opioid receptor antagonist employed.
Despite tremendous efforts in the search for safe, efficacious and non-addictive opioids for pain treatment, morphine remains the most valuable painkiller in contemporary medicine. Opioids exert their pharmacological actions through three opioid-receptor classes, mu, delta and kappa, whose genes have been cloned. Genetic approaches are now available to delineate the contribution of each receptor in opioid function in vivo. Here we disrupt the mu-opioid-receptor gene in mice by homologous recombination and find that there are no overt behavioural abnormalities or major compensatory changes within the opioid system in these animals. Investigation of the behavioural effects of morphine reveals that a lack of mu receptors abolishes the analgesic effect of morphine, as well as place-preference activity and physical dependence. We observed no behavioural responses related to delta- or kappa-receptor activation with morphine, although these receptors are present and bind opioid ligands. We conclude that the mu-opioid-receptor gene product is the molecular target of morphine in vivo and that it is a mandatory component of the opioid system for morphine action.
Peptides have been identified in mammalian brain that are considered to be endogenous agonists for the delta (enkephalins) and kappa (dynorphins) opiate receptors, but none has been found to have any preference for the mu receptor. Because morphine and other compounds that are clinically useful and open to abuse act primarily at the mu receptor, it could be important to identify endogenous peptides specific for this site. Here we report the discovery and isolation from brain of such a peptide, endomorphin-1 (Tyr-Pro-Trp-Phe-NH2), which has a high affinity (Ki = 360 pM) and selectivity (4,000- and 15,000-fold preference over the delta and kappa receptors) for the mu receptor. This peptide is more effective than the mu-selective analogue DAMGO in vitro and it produces potent and prolonged analgesia in mice. A second peptide, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), which differs by one amino acid, was also isolated. The new peptides have the highest specificity and affinity for the mu receptor of any endogenous substance so far described and they may be natural ligands for this receptor.
These experiments aimed to elucidate feeding-associated behavioral roles of globus pallidus (GP) neurons in gustatory functions: The effects of bilateral microiontophoretic kainate (KA) lesions of the ventromedial pallidal (vmGP) region on neophobia and conditioned taste aversion (CTA) were studied. Lesioned rats displayed strong and persistent neophobia to a mild citric acid solution. Neuron-specific damage to the vmGP also prevented rats from proper acquisition of CTA. Rats that previously showed normal neophobia and successfully learned CTA demonstrated difficulties in CTA retention after GP lesions. KA-lesioned rats, in addition, exhibited deficits in orientation reactions but did not have aphagia, adipsia, or motor disturbances seen after larger pallidal lesions. These findings suggest that neurons of the GP are significant in acquisition, memory storage, and retrieval mechanisms of feeding-associated taste information.
The opioid receptor antagonist naloxone is known to influence a wide range of behavioral effects of cocaine, including its addictive property. In the present study the effects of different doses of naloxone and naloxone-methyl-iodide, a methylated analogon of naloxone that does not penetrate the blood-brain barrier, on the action of cocaine in the intravenous self-administration and conditioned place preference (CCP) paradigm were assessed. Systemic naloxone, but not naloxone-methyl-iodide, dose-dependently suppressed cocaine intake during self-administration and decreased the preference for the cocaine-associated compartment in the CCP paradigm. A significant blockade of cocaine's effects was only present at a relatively high dose of NLX (1.0 mg/kg, s.c.). In addition, NLX produced a rightward shift in the inverted U-shaped dose-response curve for cocaine reward during self-administration, indicating a decrease in sensitivity for the reinforcing effects of cocaine. These data demonstrate that blockade of opioid receptors in the brain block both the reinforcing and conditioned motivational effects of cocaine. An interaction between endogenous opioid systems and local dopaminergic systems is suggested in mediating the effects of NLX on cocaine.