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Caffeine drinking potentiates cannabinoid transmission in the striatum: Interaction with stress effects
Abstract
Caffeine, the psychoactive ingredient of coffee and of many soft drinks, is frequently abused by humans especially during stressful live events. The endocannabinoid system is involved in the central effects of many psychoactive compounds and of stress. Whether caffeine alters the cannabinoid system and interferes with stress-induced synaptic alterations is however unknown. We have studied electrophysiologically the sensitivity of cannabinoid receptors modulating synaptic transmission in the striatum of mice exposed to caffeine in their drinking solution. Chronic caffeine assumption sensitized GABAergic synapses to the presynaptic effect of cannabinoid CB1 receptor stimulation by exo- and endocannabinoids. Caffeine was conversely unable to affect the sensitivity of cannabinoid receptors modulating glutamate transmission. The synaptic effects of caffeine were slowly reversible after its removal from the drinking solution. Furthermore, although exposure to caffeine for only 24h did not produce measurable changes of the sensitivity of cannabinoid CB1 receptors, it was able to contrast the down-regulation of CB1 receptor-mediated responses after social defeat stress. Our data suggest that the cannabinoid system is implicated in the psychoactive properties of caffeine and in the ability of caffeine to reduce the pathological consequences of stress.
... Recent findings suggested a caffeine-endocannabinoid system interaction in the striatum [15], similar to what is observed with classical substances of abuse [16][17][18]. ...
... Besides the interaction with adenosine neurotransmission, caffeine has been recently showed to induce striatal synaptic adaptations. We have found, by means of neurophysiological recordings from single neurons, a rearrangement of cannabinoid CB1 receptor-mediated control of synaptic transmission in the striatum of mice chronically exposed to caffeine [15]. In this brain area, CB1 receptors are preferentially located on GABAergic nerve terminals [34]. ...
... the striatum [15] (Fig. 3). The caffeine-induced adaptation of the endocannabinoid system was however reversible, since it declined 15 days after drug withdrawal, and was totally reversed after 1 month [15]. ...
Caffeine is the most commonly self-administered psychoactive substance worldwide. At usual doses, the effects of caffeine on vigilance, attention, mood and arousal largely depend on the modulation of central adenosine receptors. The present review article describes the action of caffeine within the striatum, to provide a possible molecular mechanism at the basis of the psychomotor and reinforcing properties of this pharmacological agent. The striatum is in fact a subcortical area involved in sensorimotor, cognitive, and emotional processes, and recent experimental findings showed that chronic caffeine consumption enhances the sensitivity of striatal GABAergic synapses to the stimulation of cannabinoid CB1 receptors. The endocannabinoid system is involved in the psychoactive effects of many compounds, and adenosine A2A receptors (the main receptor target of caffeine) elicit a permissive effect towards CB1 receptors, thus suggesting that A2A-CB1 receptor interaction plays a major role in the generation and maintenance of caffeine reinforcing behavior. Aim of this review is to describe the effects of caffeine on striatal neurotransmission with special reference to the modulation of the endocannabinoid system.
... CB1Rs also control glutamate transmission in the striatum [CB1Rs (Glu) ] by a presynaptic mechanism. In agreement with previous findings (De Chiara et al., 2010a,b;Rossi et al., 2008Rossi et al., , 2009), HU210 inhibited glutamate-mediated sEPSC frequency and eEPSC amplitude in controls (pre-HU210 sEPSC frequency in controls: 2.40 ± 0.40 Hz) (n = 7, P < 0.01 for both experimental conditions). In DAT-CI slices, the effects of HU210 on sEPSCs and eEPSCs were indistinguishable from those in controls (pre-HU210 sEPSC frequency in DAT-CI: 2.50 ± 0.20 Hz) (n = 6, P > 0.05 as compared with HU210 in controls for both sEPSCs and eEPSCs), indicating that experimental ADHD selectively alters the CB1Rs regulating GABA synapses (Figs. ...
... Activation of type 5 metabotropic glutamate receptors (mGluRs) by DHPG mobilizes the endocannabinoid 2-arachidonoylglycerol (2-AG) in the striatum (Jung et al., 2005;Maccarrone et al., 2008Maccarrone et al., , 2009, and this effect results in the inhibition of GABA-mediated mIPSCs or eIPSCs through the stimulation of CB1Rs (Centonze et al., 2007a; Maccarrone et al., 2008Maccarrone et al., , 2009Rossi et al., 2009;De Chiara et al., 2010b). Thus, to determine whether triple point-mutation of DAT altered the sensitivity of GABA synapses not only to the synthetic cannabinoid HU210 but also to endogenous cannabinoids, we measured the effects of DHPG on striatal mIPSCs recorded from WT to DAT-CI mice. ...
Abnormal dopamine (DA) transmission in the striatum plays a pivotal role in attention-deficit/hyperactivity disorder (ADHD). As striatal DA signalling modulates the endocannabinoid system (ECS), the present study was aimed at investigating cannabinoid CB1 receptor (CB1R) function in a model of ADHD obtained by triple point-mutation in the dopamine transporter (DAT) gene in mice, making them insensitive to cocaine [DAT cocaine-insensitive (DAT-CI) mice]. DAT-CI mice had a marked hyperactive phenotype, and neurophysiological recordings revealed that the sensitivity of CB1Rs controlling GABA-mediated synaptic currents [CB1Rs((GABA)) ] in the striatum was completely lost. In contrast, CB1Rs modulating glutamate transmission [CB1Rs((Glu)) ], and GABA(B) receptors were not affected in this model of ADHD. In DAT-CI mice, the blockade of CB1R((GABA)) function was complete even after cocaine or environmental manipulations activating the endogenous DA-dependent reward system, which are known to sensitize these receptors in control animals. Conversely, the hedonic property of sucrose was intact in DAT-CI mice, indicating normal sweet perception in these animals. Our results point to CB1Rs as novel molecular players in ADHD, and suggest that therapeutic strategies aimed at interfering with the ECS might prove effective in this disorder.
... This concurs with the parallel control of CB 1 receptor function by A 1 receptors (Hoffman et al., 2010;Sousa et al., 2011) and by A 2A receptors (Carriba et al., 2007;Martire et al., 2011;Ferreira et al., 2015;Chiodi et al., 2016;Mouro et al., 2017). It also explains the ability of caffeine to enhance the effects of CB 1 receptors on working memory (Panlilio et al., 2012) and short-term memory (Sousa et al., 2011) and on striatal (Rossi et al., 2009) and hippocampal transmission (Hoffman et al., 2010). This interaction between the adenosine and cannabinoid modulation systems to control impulsive behavior paves the way to conceive a simultaneous intervention of these two systems to manage this ADHD trait. ...
Attention deficit and hyperactivity disorder (ADHD) is characterized by impaired levels of hyperactivity, impulsivity and inattention. Adenosine and endocannabinoid systems tightly interact in the modulation of dopamine signaling, involved in the neurobiology of ADHD. In this study, we evaluated the modulating effects of the cannabinoid and adenosine systems in a tolerance to delay of reward task using the most widely used animal model of ADHD. Spontaneous Hypertensive Rats (SHR) and Wistar‐Kyoto (WKY) rats were treated chronically or acutely with caffeine, a non‐selective adenosine receptor antagonist, or acutely with a cannabinoid agonist (WIN55212‐2, WIN) or antagonist (AM251). Subsequently, animals were tested in the tolerance to delay of reward task, in which they had to choose between a small, but immediate, or a large, but delayed, reward. Treatment with WIN decreased while treatment with AM251 increased the choices of the large reward, selectively in SHR rats, indicating a CB1 receptor‐mediated increase of impulsive behavior. An acute pretreatment with caffeine blocked WIN effects. Conversely, a chronic treatment with caffeine increased the impulsive phenotype and potentiated the WIN effects. The results indicate that both cannabinoid and adenosine receptors modulate impulsive behavior in SHR: the antagonism of cannabinoid receptors might be effective in reducing impulsive symptoms present in ADHD; in addition, caffeine showed the opposite effects on impulsive behavior depending on the length of treatment. These observations are of particular importance to consider when therapeutic manipulation of CB1 receptors is applied to ADHD patients who consume coffee.
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... 143 Interestingly, chronic caffeine consumption sensitizes striatal GABAergic synapses to the presynaptic effect of cannabinoid CB 1 receptor stimulation by exo-and endocannabinoids. 144 Blockade of A 2A receptors was also shown to inhibit CB 1 receptor signaling in corticostriatal slices. 145 Furthermore, administration of adenosine A 2A receptor antagonists decrease the motor depressant effects produced by CB 1 receptor agonists. ...
A1 and CB1 receptors are main targets for the cognitive effects of caffeine and Δ9-tetrahydrocannabinol (THC), two of the most heavily consumed psychoactive substances worldwide. Both receptors can coincide in the same neuronal structures and both couple to similar G proteins and transducing pathways. Ex vivo evidence revealed that A1 and CB1 receptors can interact, and recent in vivo studies showed that those interactions can influence the behavioral actions of cannabinoids. In particular, studies on interactions between the adenosine receptor nonselective antagonist caffeine and CB1 receptor agonists, such as THC, showed that these receptor interactions may have relevant consequences for the function of the hippocampus, which impact upon cognition. In addition, interactions between adenosine A2A receptors, also targeted by caffeine, and CB1 receptors may impact upon the motor and addictive actions of cannabinoids. Being so widely consumed, caffeine habits should therefore be taken into account whenever evaluating the influences of cannabinoids upon neuronal function or dysfunction in humans. Manipulation of the degree of activation of adenosine receptors with caffeine or adenosine receptor selective ligands should also be considered to reduce side effects of CB1 receptor ligands with therapeutic potential.
... For all experiments, caffeine (C0750; Sigma-Aldrich, St. Louis, MO) was dissolved in filtered water and administered through drinking bottles at 0, 1.0, or 3.0 mg/mL. The selection of chronic caffeine concentrations was based on previous work (Dall 'Igna et al., 2003;Jaszyna, Gasior, Shoaib, Yasar, & Goldberg, 1998;Rossi et al., 2009) dose analysis indicated that higher concentrations of caffeine were not well tolerated. Bottles were changed every 2-3 days as described previously (Boeck et al., 2009;. ...
Chronic caffeine exerts negligible effects on learning and memory in normal adults, but it is unknown whether this is also true for children and adolescents. The hippocampus, a brain region important for learning and memory, undergoes extensive structural and functional modifications during pre-adolescence and adolescence. As a result, chronic caffeine may have differential effects on hippocampus-dependent learning in pre-adolescents and adolescents compared with adults. Here, we characterized the effects of chronic caffeine and withdrawal from chronic caffeine on hippocampus-dependent (contextual) and hippocampus-independent (cued) fear conditioning in pre-adolescent, adolescent, and adult mice. The results indicate that chronic exposure to caffeine during pre-adolescence and adolescence enhances or impairs contextual conditioning depending on concentration, yet has no effect on cued conditioning. In contrast, withdrawal from chronic caffeine impairs contextual conditioning in pre-adolescent mice only. No changes in learning were seen for adult mice for either the chronic caffeine or withdrawal conditions. These findings support the hypothesis that chronic exposure to caffeine during pre-adolescence and adolescence can alter learning and memory and as changes were only seen in hippocampus-dependent learning, this suggests that the developing hippocampus may be sensitive to the effects of caffeine.
Copyright © 2015. Published by Elsevier B.V.
... This increase should not necessarily be interpreted as beneficial for bone growth because any imbalance in cellular function, whether an increase or decrease, may impair skeletogenesis. Caffeine binds to adenosine receptors, and modulate several others receptors including glucocorticoid receptors, insulin, estrogen, androgen, vitamin D, cannabinoid, glutamate and adrenergic receptors, all of which are expressed in osteoblasts or osteoprogenitor cells and have important functions during osteoblast differentiation [23,[58][59][60][61][62][63][64][65][66][67][68][69][70][71][72]. However, little is known about the action of caffeine on these receptors or the resultant effects on endochondral bone formation and growth. ...
Background
Caffeine is an active alkaloid that can cause damage to bones in formation during prenatal life into adulthood. This compound can pass across the placenta and into the mother¿s milk, causing a reduction in bone formation, growth and mass. The objective of this study was to examine the osteogenic potential of osteoblasts extracted from neonatal rats born to mothers treated with caffeine throughout pregnancy.Methods
Twenty-four adult Wistar rats were randomly divided into four groups, consisting of one control group and three groups that were treated with 25, 50, or 100 mg/kg of caffeine by an oral-gastric probe throughout the duration of the experimental period (pregnancy). At birth, three puppies from each dam in each group were euthanized, and osteoblasts were extracted from the calvaria of these pups for in vitro testing.ResultsThe osteoblasts extracted from the pups of rats that received 50 mg/kg caffeine during pregnancy exhibited increased expression of osteocalcin, osteopontin, sialoprotein, runx-2, alkaline phosphatase and type I collagen transcripts, resulting in increased synthesis of mineralization nodules.Conclusions
Neonates from rats treated with 50 mg/kg caffeine during pregnancy contained osteoblasts with a higher osteogenic potential characterized by increased expression of osteocalcin, osteopontin, sialoprotein, runx-2, alkaline phosphatase and type I collagen and increased synthesis of mineralization nodules.
... In rabbits, an acute dose of caffeine antagonized THC-induced changes in cortico-hippocampal electroencephalogram recordings [276]. In mice, chronic caffeine at high doses potentiated CB 1 -dependent stimulation by eCBs and HU210 at striatal GABAergic, but not glutamatergic, synapses [277]. A single dose or a subacute dose (one day of caffeine in water) rescued the sensitivity of GABAergic synapses to HU210 in mice exposed to chronic stress. ...
The "classic" endocannabinoid (eCB) system includes the cannabinoid receptors CB1 and CB2, the eCB ligands anandamide (AEA) and 2-arachidonoylglycerol (2-AG), and their metabolic enzymes. An emerging literature documents the "eCB deficiency syndrome" as an etiology in migraine, fibromyalgia, irritable bowel syndrome, psychological disorders, and other conditions. We performed a systematic review of clinical interventions that enhance the eCB system-ways to upregulate cannabinoid receptors, increase ligand synthesis, or inhibit ligand degradation.
We searched PubMed for clinical trials, observational studies, and preclinical research. Data synthesis was qualitative. Exclusion criteria limited the results to 184 in vitro studies, 102 in vivo animal studies, and 36 human studies. Evidence indicates that several classes of pharmaceuticals upregulate the eCB system, including analgesics (acetaminophen, non-steroidal anti-inflammatory drugs, opioids, glucocorticoids), antidepressants, antipsychotics, anxiolytics, and anticonvulsants. Clinical interventions characterized as "complementary and alternative medicine" also upregulate the eCB system: massage and manipulation, acupuncture, dietary supplements, and herbal medicines. Lifestyle modification (diet, weight control, exercise, and the use of psychoactive substances-alcohol, tobacco, coffee, cannabis) also modulate the eCB system.
Few clinical trials have assessed interventions that upregulate the eCB system. Many preclinical studies point to other potential approaches; human trials are needed to explore these promising interventions.
... This may be a consequence of the usually higher age, lower weight and particular body composition (low body fat) of ALS patients. However, a higher sensitivity of the CB-1 cannabinoid receptor in this group of patients may also play a role, as has been shown in a rodent model of ALS [26]. Given the substantial interindividual variability of the pharmacology of THC and the individual sensitivity to THC, future studies may include up-titration to the optimal individual dose of THC rather than a fixed dosing regimen. ...
Background:
Cannabinoids exert neuroprotective and symptomatic effects in amyotrophic lateral sclerosis (ALS). We assessed the pharmacokinetics (PK) and tolerability of delta-9-tetrahydrocannabinol (THC) in ALS patients.
Methods:
Nine patients received THC single oral doses of 5mg and 10mg, separated by a wash-out period of two weeks. Blood samples for the determination of THC, 11-nor-9-carboxy-THC (THC-COOH) and hydroxy-THC (THC-OH) were taken up to 8 hours after intake. Adverse events were assessed by visual analogue scales (VAS). Plasma concentrations of the active metabolite THC-OH were submitted to sequential pharmacokinetic-pharmacodynamic population modeling on individual heart rate as a proxy for THC's cardiovasculatory effects.
Results:
Drowsiness, euphoria, orthostasis, sleepiness, vertigo and weakness were significantly more frequent in patients receiving 10mg compared to 5 mg THC. A marked interindividual variability was found for the absorption of oral THC (84%) and elimination of THC-COOH (45%). PK data did not support any clinically relevant deviation from linear PK in the investigated range of concentrations. Plasma concentrations of THC-OH were positively correlated with the individual heart rate. An E(max-model) was successfully fitted to individual heart rate, with a THC-OH plasma concentration of 3.2 x 10(-4) μmol/L for EC(50) and an E(max) of 93 bpm for heart rate.
Conclusions:
The higher 10mg dose of THC was dose-limiting in patients with ALS. High interindividual PK variability requires individuell titration of THC for potential therapeutic use in patients with ALS.
... The maximal response of striatal CB1Rs(GABA) is significantly enhanced by chronic cocaine, when rodents develop overt addictive behaviours, but not by a single administration of the psychostimulant (Centonze et al., 2007a; Rossi et al., 2008; De Chiara et al., 2010a,b). Similar potentiation of CB1R(GABA) signalling is seen in mice drinking chronically another psychoactive compound, caffeine (Rossi et al., 2009), or exposed to running wheel or given access to a drinking solution containing sucrose (De Chiara et al., 2010a). Of note, voluntary running activity has strong rewarding and reinforcing properties in rodents, and shares many neurochemical and behavioural characteristics with drug-induced reward situations, through the activation of dopamine signalling and the modulation of striatal neuron activity (Werme et al., 2000; 2002; Lett et al., 2001; De Visser et al., 2007). ...
Type‐1 cannabinoid receptor (CB 1 ) is the most abundant G‐protein‐coupled receptor (GPCR) in the brain. CB 1 and its endogenous agonists, the so‐called ‘endocannabinoids (eCBs)’, belong to an ancient neurosignalling system that plays important functions in neurodegenerative and neuroinflammatory disorders like Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis. For this reason, research on the therapeutic potential of drugs modulating the endogenous tone of eCBs is very intense. Several GPCRs reside within subdomains of the plasma membranes that contain high concentrations of cholesterol: the lipid rafts. Here, the hypothesis that changes in membrane fluidity alter function of the endocannabinoid system, as well as progression of particular neurodegenerative diseases, is described. To this end, the impact of membrane cholesterol on membrane properties and hence on neurodegenerative diseases, as well as on CB 1 signalling in vitro and on CB 1 ‐dependent neurotransmission within the striatum, is discussed. Overall, present evidence points to the membrane environment as a critical regulator of signal transduction triggered by CB 1 , and calls for further studies aimed at better clarifying the contribution of membrane lipids to eCBs signalling. The results of these investigations might be exploited also for the development of novel therapeutics able to combat disorders associated with abnormal activity of CB 1 .
LINKED ARTICLES This article is part of a themed issue on Cannabinoids in Biology and Medicine. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2011.163.issue‐7
... Interestingly, the over-activity of the endocannabinoid system acting through CB 1 R has been identified as one of the major pathologic mechanisms of several acute and chronic symptoms related to psychoses and schizophrenia; therefore CB 1 R antagonists are being tested as a novel antipsychotic strategy (Laviolette and Grace 2006). Notably, A 2A R ligands are also proposed as antipsychotic agents, targeting conditions such as schizophrenia (Ferré 1997;see Cunha et al. 2008) and it was recently described that caffeine potentiates cannabinoid signalling upon stress (Rossi et al. 2009). Furthermore, both CB 1 R activation (de Lago and Fernández-Ruiz 2007) and A 2A R inhibition (Popoli et al. 2007;Schiffmann et al. 2007) are neuroprotective in motor disorders. ...
J. Neurochem. (2011) 116, 273–280.
An interaction between adenosine A2A receptors (A2ARs) and cannabinoid CB1 receptors (CB1Rs) has been consistently reported to occur in the striatum, although the precise mechanisms are not completely understood. As both receptors control striatal glutamatergic transmission, we now probed the putative interaction between pre-synaptic CB1R and A2AR in the striatum. In extracellular field potentials recordings in corticostriatal slices from Wistar rats, A2AR activation by CGS21680 inhibited CB1R-mediated effects (depression of synaptic response and increase in paired-pulse facilitation). Moreover, in superfused rat striatal nerve terminals, A2AR activation prevented, while A2AR inhibition facilitated, the CB1R-mediated inhibition of 4-aminopyridine-evoked glutamate release. In summary, the present study provides converging neurochemical and electrophysiological support for the occurrence of a tight control of CB1R function by A2ARs in glutamatergic terminals of the striatum. In view of the key role of glutamate to trigger the recruitment of striatal circuits, this pre-synaptic interaction between CB1R and A2AR may be of relevance for the pathogenesis and the treatment of neuropsychiatric disorders affecting the basal ganglia.
... It is therefore not surprising that acute caffeine administration prevents THC-induced effects (Consroe et al, 1976), whereas chronic caffeine exposure exacerbates the memory disruption induced by CB 1 receptor agonists (present work). In a recent study, chronic administration of a high dose of caffeine (210 mg/kg/day) in rats was shown to potentiate CB 1 -dependent effects at striatal GABAergic, but not glutamatergic, synapses (Rossi et al, 2009). However, it is difficult to draw a comparison with this study, given the differences in the experimental approach, namely the dose of caffeine used, which is not adenosine receptor-selective and is more than about 70 times higher than the equivalent daily human intake. ...
The cannabinoid CB(1) receptor-mediated modulation of γ-aminobutyric acid (GABA) release from inhibitory interneurons is important for the integrity of hippocampal-dependent spatial memory. Although adenosine A(1) receptors have a central role in fine-tuning excitatory transmission in the hippocampus, A(1) receptors localized in GABAergic cells do not directly influence GABA release. CB(1) and A(1) receptors are the main targets for the effects of two of the most heavily consumed psychoactive substances worldwide: Δ(9)-tetrahydrocannabinol (THC, a CB(1) receptor agonist) and caffeine (an adenosine receptor antagonist). We first tested the hypothesis that an A(1)-CB(1) interaction influences GABA and glutamate release in the hippocampus. We found that A(1) receptor activation attenuated the CB(1)-mediated inhibition of GABA and glutamate release and this interaction was manifested at the level of G-protein activation. Using in vivo and in vitro approaches, we then investigated the functional implications of the adenosine-cannabinoid interplay that may arise following chronic caffeine consumption. Chronic administration of caffeine in mice (intraperitoneally, 3 mg/kg/day, for 15 days, >12 h before trials) led to an A(1)-mediated enhancement of the CB(1)-dependent acute disruptive effects of THC on a short-term spatial memory task, despite inducing a reduction in cortical and hippocampal CB(1) receptor number and an attenuation of CB(1) coupling with G protein. A(1) receptor levels were increased following chronic caffeine administration. This study shows that A(1) receptors exert a negative modulatory effect on CB(1)-mediated inhibition of GABA and glutamate release, and provides the first evidence of chronic caffeine-induced alterations on the cannabinoid system in the cortex and hippocampus, with functional implications in spatial memory.
... In summary, the above mentioned data suggests that adenosine A2A receptors facilitate CB1 receptor signalling as well as the interplay between CB1 receptors and other key receptors and pathways involved in drug addiction (Fig. 3). Surprisingly, however, chronic caffeine consumption, therefore chronic blockade of adenosine receptors, sensitizes GABAergic synapses to the CB1 receptor mediated presynaptic inhibitory action of exo-and endocannabinoids at the striatum [128]. Though the detailed receptor mechanisms responsible for these observations remain unknown, they reinforce previous evidence [43] that chronic and acute blockade of adenosine receptors may lead to opposite changes in the homeostatic balance mediated by adenosine. ...
The 'omnipresence' of adenosine in all nervous system cells (neurons and glia) together with the intensive release of adenosine following insults, makes adenosine as a sort of 'maestro' of synapses leading to the homeostatic coordination of brain function. Besides direct actions of adenosine on the neurosecretory mechanisms, where adenosine operates to tune neurotransmitter release, receptor-receptor interactions as well as interplays between adenosine receptors and transporters occur as part of the adenosine's attempt to fine tuning synaptic transmission. This review will focus on the different ways adenosine can use to trigger or brake the action of several neurotransmitters and neuromodulators. Adenosine receptors cross talk with other G protein coupled receptors (GPCRs), with ionotropic receptors and with receptor kinases. Most of these interactions occur through A2A receptors, which in spite their low density in some brain areas, such as the hippocampus, may function as metamodulators. Tonic adenosine A2A receptor activity is a required step to allow synaptic actions of neurotrophic factors, namely upon synaptic transmission at both pre- and post-synaptic level as well as upon synaptic plasticity and neuronal survival. The implications of these interactions in normal brain functioning and in neurologic and psychiatric dysfunction will be discussed.
... Endocannabinoid signaling is prominent in the striatum and represents a major downstream target of D 2 receptor activation in indirect-pathway MSNs (Giuffrida et al., 1999;Kreitzer and Malenka, 2007;Shen et al., 2008b), raising the possibility that it could be influenced by A 2A receptors (Rossi et al., 2009). Two major endocannabinoids have been identified thus far: AEA and 2-AG. ...
Adenosine A(2A) receptor antagonists are psychomotor stimulants that also hold therapeutic promise for movement disorders. However, the molecular mechanisms underlying their stimulant properties are not well understood. Here, we show that the robust increase in locomotor activity induced by an A(2A) antagonist in vivo is greatly attenuated by antagonizing cannabinoid CB(1) receptor signaling or by administration to CB(1)(-/-) mice. To determine the locus of increased endocannabinoid signaling, we measured the amount of anandamide [AEA (N-arachidonoylethanolamine)] and 2-arachidonoylglycerol (2-AG) in brain tissue from striatum and cortex. We find that 2-AG is selectively increased in striatum after acute blockade of A(2A) receptors, which are highly expressed by striatal indirect-pathway medium spiny neurons (MSNs). Using targeted whole-cell recordings from direct- and indirect-pathway MSNs, we demonstrate that A(2A) receptor antagonists potentiate 2-AG release and induction of long-term depression at indirect-pathway MSNs, but not direct-pathway MSNs. Together, these data outline a molecular mechanism by which A(2A) antagonists reduce excitatory synaptic drive on the indirect pathway through CB(1) receptor signaling, thus leading to increased psychomotor activation.
Flow is an intrinsically rewarding state characterised by positive affect and total task absorption. Because cognitive and physical performance are optimal in flow, chemical means to facilitate this state are appealing. Caffeine, a non-selective adenosine receptor antagonist, has been emphasized as a potential flow-inducer. Thus, we review the psychological and biological effects of caffeine that, conceptually, enhance flow. Caffeine may facilitate flow through various effects, including: i) upregulation of dopamine D1/D2 receptor affinity in reward-associated brain areas, leading to greater energetic arousal and ‘wanting’; ii) protection of dopaminergic neurons; iii) increases in norepinephrine release and alertness, which offset sleep-deprivation and hypoarousal; iv) heightening of parasympathetic high frequency heart rate variability, resulting in improved cortical stress appraisal, v) modification of striatal endocannabinoid-CB1 receptor-signalling, leading to enhanced stress tolerance; and vi) changes in brain network activity in favour of executive function and flow. We also discuss the application of caffeine to treat attention deficit hyperactivity disorder and caveats. We hope to inspire studies assessing the use of caffeine to induce flow.
Drug development was historically started by targeting protein active sites as means to pharmacologically modulate the functional properties of the target. However, with high attrition rates, pharmacologists and medicinal chemists must begin thinking outside the box more earnestly when designing new drugs. Such thinking has created an impetus toward the discovery of “biased” or “allosteric” modulators to fine-tune activity, selecting for a desired therapeutic profile. This has become a sought-after approach for the therapeutic targeting of G protein-coupled receptors (GPCRs). Structure-based studies have greatly increased our mechanistic understanding of GPCR activation in large part due to the use of single domain antibodies (or nanobodies). Even if nanobodies were initially developed to assist in receptor stabilization for structure determination of GPCRs, they have now proven to be more than simple chaperones for crystallization. Nanobodies have helped elucidate key features of GPCR biological responses, as they also behave are allosteric regulators of GPCR pharmacology. For many years, optical biosensors relying on resonance energy transfer have conventionally been used to quantify allosteric regulation. Yet, the transformation of nanobodies into biosensors, sensing distinct receptor conformation or activation states, has generated unprecedented knowledge of GPCR biology and signal transduction. Further, peptidomimetic ligands or pepducins generated from the primary and secondary structures of GPCRs have also been used as key allosteric tools to understand and drive GPCR signaling. In this review, we discuss how intracellular allosteric modulators, such as intracellular ions, and tool compounds, such as pepducins and nanobodies, have informed our understanding of G protein-coupled receptors from structural studies, to the generation of receptor conformational and signaling signatures.
Type 1 and 2 cannabinoid receptors respond to endocannabinoids and contribute broadly to the function of the nervous system and immune system. Cannabinoid receptors participate in higher-order complex with other class A G protein-coupled receptors in both heterologous expression systems and native mammalian tissues. Receptor heteromers engage in protein-protein allosteric interactions that influence the conformation of receptor monomers within the complex. These altered conformations cause receptor heteromers to respond differently to cognate ligands compared to the individual receptors. This creates additional diversity in cell signaling that must be considered pharmacologically. In this chapter, we provide an overview of known cannabinoid receptor heteromers and signaling changes related to heteromer function. We also discuss the allosteric processes responsible for these effects and the therapeutic potential of this rapidly expanding group of receptor heteromers.
Adenosine A 2A receptors (A 2 A R s) and cannabinoid CB 1 receptors ( CB 1 Rs) are highly expressed in the striatum, where they functionally interact and form A 2A / CB 1 heteroreceptor complexes. We investigated the effects of CB 1 R stimulation in a transgenic rat strain over‐expressing A 2 A R s under the control of the neural‐specific enolase promoter ( NSEA 2A rats) and in age‐matched wild‐type ( WT ) animals. The effects of the CB 1 R agonist WIN 55,212‐2 ( WIN ) were significantly lower in NSEA 2A rats than in WT animals, as demonstrated by i) electrophysiological recordings of synaptic transmission in corticostriatal slices; ii) the measurement of glutamate outflow from striatal synaptosomes and iii) in vivo experiments on locomotor activity. Moreover, while the effects of WIN were modulated by both A 2 A R agonist ( CGS 21680) and antagonists ( ZM 241385, KW ‐6002 and SCH ‐442416) in WT animals, the A 2 A R antagonists failed to influence WIN ‐mediated effects in NSEA 2A rats. The present results demonstrate that in rats with genetic neuronal over‐expression of A 2 A R s, the effects mediated by CB 1 R activation in the striatum are significantly reduced, suggesting a change in the stoichiometry of A 2A and CB 1 receptors and providing a strategy to dissect the involvement of A 2 A R forming or not forming heteromers in the modulation of striatal functions. These findings add additional evidence for the existence of an interaction between striatal A 2 A R s and CB 1 Rs, playing a fundamental role in the regulation of striatal functions.
image We studied A 2A ‐CB 1 receptor interaction in transgenic rats over‐expressing adenosine A 2A receptors under the control of the neuron‐specific enolase promoter (NSEA 2A ). In these rats, we demonstrated a reduced effect of the CB 1 receptor agonist WIN 55,212‐2 in the modulation of corticostriatal synaptic transmission and locomotor activity, while CB 1 receptor expression level did not change with respect to WT rats. A reduction in the expression of A 2A ‐CB 1 receptor heteromers is postulated.
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Endocannabinoid system is involved in the neurobiological mechanism underlying drug addiction in all known kinds of drugs including nicotine and alcohol. Recently, relationships between endocannabinoids and biological nature of depression and eating disorders were recognised. Polymorphisms of genes encoding CB1 receptors and genes encoding main degrading enzyme FAAH responsible for pathology in motivation and cognition were identified.
This chapter examines the psychostimulant actions of methylxanthines, with a focus on the consequences of their excessive use. Consumption of methylxanthines is pervasive and their use is often associated with that of substances known to produce dependence and to have abuse potential. Therefore, the consequences of this combined use are taken into consideration in order to evaluate whether, and to what extent, methylxanthines could influence dependence on or abuse of other centrally active substances, leading to either amplification or attenuation of their effects. Since the methylxanthine that mostly influences mental processes and readily induces psychostimulation is caffeine, this review mainly focuses on caffeine as a prototype of methylxanthine-produced dependence, examining, at the same time, the risks related to caffeine use.
A widely prescribed and potent short-acting hypnotic, zolpidem has become the mainstay for the treatment of middle-of-the-night sleeplessness. It is expected to be antagonized by caffeine. Paradoxically, in some cases caffeine appears to slightly enhance zolpidem sedation. The pharmacokinetic and pharmacodynamic nature of this odd effect remains unexplored. The purpose of this study is to reproduce a hypothetical molecular network recruited by caffeine when co-administered with zolpidem using Ingenuity Pathway Analysis. Thus generated, network drew attention to several possible contributors to caffeine sedation, such as tachykinin precursor 1, cannabinoid, and GABA receptors. The present overview is centered on the possibility that caffeine potentiation of zolpidem sedation does not involve a centralized interaction of specific neurotransmitters, but rather is contributed by its antioxidant capacity. It is proposed that by modifying the cellular redox state, caffeine ultimately reduces the pool of reactive oxygen species, thereby increasing the bioavailability of endogenous melatonin for interaction with zolpidem. This side effect of caffeine encourages further studies of multiple antioxidants as an attractive way to potentially increasing somnolence.
Adenosine A(1) and cannabinoid CB(1) receptors are affected by drugs widely consumed by humans, as it is the case for caffeine, an adenosine receptor antagonist, and tetrahydrocannabinol, a cannabinoid receptor agonist. These receptors are present in the hippocampus and inhibit neurotransmitter release by operating similar transduction mechanisms. We, therefore, evaluated if they cross-talk to modulate synaptic transmission in the hippocampus. To do so, field excitatory postsynaptic potentials (fEPSPs) were recorded from the CA1 area of rat hippocampal slices and the consequences of activation or blockade of cannabinoid CB(1) or adenosine A(1) receptors upon neuromodulation exerted by the other receptor were assessed. The cannabinoid CB(1) receptor agonist, WIN55212-2 (300nM), slowly decreased ( congruent with40%) the fEPSP slope, while the adenosine A(1) receptor agonist, cyclopenthyladenosine (CPA, 15nM) rapidly decreased ( congruent with50%) it. Blockade of cannabinoid CB(1) receptors with AM251 (1microM) did not influence the adenosine A(1) receptor-mediated inhibition of synaptic transmission. Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 50nM) did not affect the inhibition perpetrated by cannabinoid CB(1) receptor activation. When both receptors were simultaneously activated (5nM CPA plus 300nM WIN55212-2) the net inhibition of synaptic transmission was about the sum of the effect of each drug applied independently. These results indicate independent synaptic transmission modulation by adenosine A(1) and cannabinoid CB(1) receptors at the hippocampus, suggesting that availability of G-proteins coupled to each receptor and availability of other signalling molecules involved in their transducing pathways, are not limiting factors for their modulatory role.
The endocannabinoid system (ECS) plays a fundamental role in the regulation of synaptic transmission. Exposure to stressful events triggers synaptic adaptations in many brain areas. The activity of the ECS in stress-responsive neural circuits suggests that it may be involved in the behavioral responses and synaptic effects typical of stress. In this review, we discuss evidence demonstrating that striatal ECS is modulated by stress. Chronic stress exposure alters endocannabinoid levels, cannabinoid CB1 receptor binding and cannabinoid CB1 receptor-mediated control of inhibitory synaptic transmission in the striatum. Recent studies have shown that impairment of endocannabinoid signalling is associated with inability to adapt to chronic stress and to the development of maladaptive behaviors. The ECS represents a novel potential pharmacological target to treat stress-associated neuropsychiatric conditions.
The spiny projection neurons are by far the most numerous type of striatal neuron. In addition to being the principal projection neurons of the striatum, the spiny projection neurons also have an extensive network of local axon collaterals by which they make synaptic connections with other striatal projection neurons. However, up to now there has been no direct physiological evidence for functional inhibitory interactions between spiny projection neurons. Here we present new evidence that striatal projection neurons are interconnected by functional inhibitory synapses. To examine the physiological properties of unitary inhibitory postsynaptic potentials (IPSPs), dual intracellular recordings were made from pairs of spiny projection neurons in brain slices of adult rat striatum. Synaptic interactions were found in 9 of 45 pairs of neurons using averages of 200 traces that were triggered by a single presynaptic action potential. In all cases, synaptic interactions were unidirectional, and no bidirectional interactions were detected. Unitary IPSPs evoked by a single presynaptic action potential had a peak amplitude ranging from 157 to 319 μV in different connections (mean: 277 ± 46 μV, n = 9). The percentage of failures of single action potentials to evoke a unitary IPSP was estimated and ranged from 9 to 63% (mean: 38 ± 14%, n = 9). Unitary IPSPs were reversibly blocked by bicuculline ( n = 4) and had a reversal potential of −62.4 ± 0.7 mV ( n = 5), consistent with GABA-mediated inhibition. The findings of the present study correlate very well with anatomical evidence for local synaptic connectivity between spiny projection neurons and suggest that lateral inhibition plays a significant role in the information processing operations of the striatum.
The function of the central cannabinoid receptor (CB1) was investigated by invalidating its gene. Mutant mice did not respond to cannabinoid drugs, demonstrating the exclusive
role of the CB1 receptor in mediating analgesia, reinforcement, hypothermia, hypolocomotion, and hypotension. The acute effects of opiates
were unaffected, but the reinforcing properties of morphine and the severity of the withdrawal syndrome were strongly reduced.
These observations suggest that the CB1 receptor is involved in the motivational properties of opiates and in the development of physical dependence and extend the
concept of an interconnected role of CB1 and opiate receptors in the brain areas mediating addictive behavior.
Exposure to stressful events has a myriad of consequences in animals and in humans, and triggers synaptic adaptations in many brain areas. Stress might also alter cannabinoid-receptor-mediated transmission in the brain, but no physiological study has addressed this issue so far. In the present study, we found that social defeat stress, induced in mice by exposure to aggression, altered cannabinoid CB(1)-receptor-mediated control of synaptic transmission in the striatum. In fact, the presynaptic inhibition of GABAergic IPSCs induced by the cannabinoid CB(1) receptor agonist HU210 [(6aR)-trans-3-(1,1-dimethylheptyl)-6a,7,10,10a-tetrahydro-1-hydroxy-6,6-dimethyl-6H-dibenzo[b,d]pyran-9-methanol] was reduced after a single stressful episode and fully abolished after 3 and 7 d of stress exposure. Repeated psychoemotional stress also impaired the sensitivity of GABA synapses to endocannabinoids mobilized by group I metabotropic glutamate receptor stimulation, whereas the cannabinoid CB(1)-mediated control of glutamate transmission was unaffected by repeated exposure to an aggressor. Corticosteroids released in response to the activation of the hypothalamic-pituitary-adrenal axis played a major role in the synaptic defects observed in stressed animals, because these alterations were fully prevented by pharmacological blockade of glucocorticoid receptors and were mimicked by corticosterone injections. The recovery of stress-induced synaptic defects was favored when stressed mice were given access to a running wheel or to sucrose consumption, which function as potent natural rewards. A similar rescuing effect was obtained by a single injection of cocaine, a psychostimulant with strong rewarding properties. Targeting cannabinoid CB(1) receptors or endocannabinoid metabolism might be a valuable option to treat stress-associated neuropsychiatric conditions.
Mice experiencing repeated aggression develop a long-lasting aversion to social contact, which can be normalized by chronic,
but not acute, administration of antidepressant. Using viral-mediated, mesolimbic dopamine pathway–specific knockdown of brain-derived
neurotrophic factor (BDNF), we showed that BDNF is required for the development of this experience-dependent social aversion.
Gene profiling in the nucleus accumbens indicates that local knockdown of BDNF obliterates most of the effects of repeated
aggression on gene expression within this circuit, with similar effects being produced by chronic treatment with antidepressant.
These results establish an essential role for BDNF in mediating long-term neural and behavioral plasticity in response to
aversive social experiences.
Stress has been shown to impair subsequent learning. To determine whether stress would impair classical conditioning, rats were exposed to inescapable, low-intensity tail shock and subsequently classically conditioned under freely moving conditions with a brief periorbital shock unconditioned stimulus and a white noise conditioned stimulus. Unexpectedly stressed rats exhibited significantly more conditioned eyeblink responses and the magnitude of their individual responses was also enhanced. These results stand in contrast to the learning deficits typically observed and suggest that stress can enhance the acquisition of discrete conditioned responses.
In vivo intracellular recordings of spontaneous activity of neostriatal spiny cells revealed two-state behavior, i.e., characteristic shifts of membrane potential between two preferred levels. The more polarized level, called the Down state, varied among neurons from -61 to -94 mV. The more depolarized level, called the Up state, varied among neurons form -71 to -40 mV. For any one neuron, the membrane potential in the Up and Down states was constant over the period of observation (from 15 min to 4 hr), and the cells spent little time in transition between states. The level of membrane potential noise was higher in the Up state than in the Down state. Spontaneous membrane potential fluctuations were not abolished by experimental alteration of the membrane potential, but the time spent in each state was altered when intracellular current was used to vary the baseline membrane potential. Neither the sodium nor the calcium action potential that could be evoked by depolarization of spiny neurons was required for the occurrence of spontaneous shifts of membrane potential. Blockade of these action potentials using intracellular injection of QX314 and D890, respectively, altered neither the incidence of the membrane potential shifts nor the preferred membrane potential in either state. In contrast, antagonism of voltage-dependent potassium channels with intracellular cesium altered membrane potential shifts. In the presence of QX314 and D890, intracellular injection of cesium caused little or no change in the Down state and a large depolarizing shift in the Up state (to about -20 mV). Under these circumstances, the neuron responded to current in a nearly linear manner, and membrane conductance was found to be increased in the Up state, attributable to a membrane conductance with the same reversal potential as that of the synaptic potential evoked by cortical stimulation. These results indicate that the event underlying the Up state is a maintained barrage of synaptic excitation, but that the membrane potential achieved during the Up state in neostriatal spiny neurons is determined by dendritic potassium channels that clamp the membrane potential at a level determined by their voltage sensitivity. Neostriatal spiny neurons ordinarily receive enormously powerful excitation, which would drive the cells to saturation, and probably destroy them, if it were not for these potassium currents.
Behavioral stress has detrimental effects on subsequent cognitive performance in many species, including humans. For example, humans exposed to stressful situations typically exhibit marked deficits in various learning and memory tasks. However, the underlying neural mechanisms by which stress exerts its effects on learning and memory are unknown. We now report that in adult male rats, stress (i.e., restraint plus tailshock) impairs long-term potentiation (LTP) but enhances long-term depression (LTD) in the CA1 area of the hippocampus, a structure implicated in learning and memory processes. These effects on LTP and LTD are prevented when the animals were given CGP39551 (the carboxyethylester of CGP 37849; DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, before experiencing stress. In contrast, the anxiolytic drug diazepam did not block the stress effects on hippocampal plasticity. Thus, the effects of stress on subsequent LTP and LTD appear to be mediated through the activation of the NMDA subtype of glutamate receptors. Such modifications in hippocampal plasticity may contribute to learning and memory impairments associated with stress.
The induction of activity-dependent persistent increases in synaptic efficacy, such as long-term potentiation (LTP), is inhibited by behavioural stress. The question arises whether stress also affects the ability to induce persistent decreases in synaptic efficacy, such as long-term depression (LTD). We now report that the induction of stable homosynaptic LTD in the CA1 area of the hippocampus of awake adult rats is facilitated, rather than inhibited, by exposure to mild naturalistic stress. The same stress blocked the induction of LTP. The effects of such stress were short lasting: acclimatization to, or removal from, the conditions that facilitated LTD induction led to a rapid loss of the ability to elicit this form of plasticity. The time window in which LTD could be reliably elicited was prolonged by inducing anaesthesia immediately after the stress. These data reveal that even brief exposure to mild stress can produce a striking shift in the susceptibility to synaptic plasticity in the awake animal.
The basal ganglia are an interconnected set of subcortical regions whose established role in cognition and motor control remains poorly understood. An important nucleus within the basal ganglia, the striatum, receives cortical afferents that convey sensorimotor, limbic and cognitive information. The activity of medium-sized spiny neurons in the striatum seems to depend on convergent input within these information channels. To determine the degree of correlated input, both below and at threshold for the generation of action potentials, we recorded intracellularly from pairs of spiny neurons in vivo. Here we report that the transitions between depolarized and hyperpolarized states were highly correlated among neurons. Within individual depolarized states, some significant synchronous fluctuations in membrane potential occurred, but action potentials were not synchronized. Therefore, although the mean afferent signal across fibres is highly correlated among striatal neurons, the moment-to-moment variations around the mean, which determine the timing of action potentials, are not. We propose that the precisely timed, synchronous component of the membrane potential signals activation of cell assemblies and enables firing to occur. The asynchronous component, with low redundancy, determines the fine temporal pattern of spikes.
The function of the central cannabinoid receptor (CB1) was investigated by invalidating its gene. Mutant mice did not respond to cannabinoid drugs, demonstrating the exclusive role of the CB1 receptor in mediating analgesia, reinforcement, hypothermia, hypolocomotion, and hypotension. The acute effects of opiates were unaffected, but the reinforcing properties of morphine and the severity of the withdrawal syndrome were strongly reduced. These observations suggest that the CB1 receptor is involved in the motivational properties of opiates and in the development of physical dependence and extend the concept of an interconnected role of CB1 and opiate receptors in the brain areas mediating addictive behavior.
Caffeine is the most widely used psychoactive substance and has been considered occasionally as a drug of abuse. The present paper reviews available data on caffeine dependence, tolerance, reinforcement and withdrawal. After sudden caffeine cessation, withdrawal symptoms develop in a small portion of the population but are moderate and transient. Tolerance to caffeine-induced stimulation of locomotor activity has been shown in animals. In humans, tolerance to some subjective effects of caffeine seems to occur, but most of the time complete tolerance to many effects of caffeine on the central nervous system does not occur. In animals, caffeine can act as a reinforcer, but only in a more limited range of conditions than with classical drugs of dependence. In humans, the reinforcing stimuli functions of caffeine are limited to low or rather moderate doses while high doses are usually avoided. The classical drugs of abuse lead to quite specific increases in cerebral functional activity and dopamine release in the shell of the nucleus accumbens, the key structure for reward, motivation and addiction. However, caffeine doses that reflect the daily human consumption, do not induce a release of dopamine in the shell of the nucleus accumbens but lead to a release of dopamine in the prefrontal cortex, which is consistent with caffeine reinforcing properties. Moreover, caffeine increases glucose utilization in the shell of the nucleus accumbens only at rather high doses that stimulate most brain structures, non-specifically, and likely reflect the side effects linked to high caffeine ingestion. That dose is also 5-10-fold higher than the one necessary to stimulate the caudate nucleus, which mediates motor activity and the structures regulating the sleep-wake cycle, the two functions the most sensitive to caffeine. In conclusion, it appears that although caffeine fulfils some of the criteria for drug dependence and shares with amphetamines and cocaine a certain specificity of action on the cerebral dopaminergic system, the methylxanthine does not act on the dopaminergic structures related to reward, motivation and addiction.
We sought neurochemical correlates to the stimulatory action of caffeine in rats and to adaptations during development of tolerance. Acute intraperitoneal injections of caffeine (7.5 mg/kg) increased locomotion and NGFI-A mRNA, a marker of neuronal activity, in the hippocampal area CA1, but decreased NGFI-A mRNA in rostral striatum and nucleus accumbens. Rats that received caffeine (0.3 gm/l) in their drinking water for 14 d developed tolerance to the stimulatory effect of a challenge with caffeine (7.5 mg/kg) and responded with a less pronounced decrease of NGFI-A mRNA in rostral striatum and nucleus accumbens. Metabolism of caffeine to its active metabolites was increased in tolerant animals, but the total level of active metabolites in brain was not significantly altered. Thus, there are changes in caffeine metabolism after long-term caffeine treatment, but they cannot explain development of tolerance.
Caffeine-tolerant animals had downregulated levels of adenosine A 2A receptors and the corresponding mRNA in rostral parts of striatum, but an increased expression of adenosine A 1 receptor mRNA in the lateral amygdala. No changes in mesencephalic tyrosine hydroxylase mRNA were found in caffeine-tolerant rats.
Thus, we have identified neuronal pathways that are regulated by adenosine A 1 and/or A 2A receptors and are targets for the stimulatory action of caffeine. Furthermore, adaptive changes in gene expression in these brain areas were associated with the development of locomotor tolerance to caffeine.
CB1 cannabinoid receptors in the neostriatum mediate profound motor deficits induced when cannabinoid drugs are administered to rodents. Because the CB1 receptor has been shown to inhibit neurotransmitter release in various brain areas, we investigated the effects of CB1 activation on glutamatergic synaptic transmission in the dorsolateral striatum of the rat where the CB1 receptor is highly expressed. We performed whole cell voltage-clamp experiments in striatal brain slices and applied the CB1 agonists HU-210 or WIN 55,212-2 during measurement of synaptic transmission. Excitatory postsynaptic currents (EPSCs), evoked by electrical stimulation of afferent fibers, were significantly reduced in a dose-dependent manner by CB1 agonist application. EPSC inhibition was accompanied by an increase in two separate indices of presynaptic release, the paired-pulse response ratio and the coefficient of variation, suggesting a decrease in neurotransmitter release. These effects were prevented by application of the CB1 antagonist SR141716A. When Sr(2+) was substituted for Ca(2+) in the extracellular solution, application of HU-210 (1 microM) significantly reduced the frequency, but not amplitude, of evoked, asynchronous quantal release events. Spontaneous release events were similarly decreased in frequency with no change in amplitude. These findings further support the interpretation that CB1 activation leads to a decrease of glutamate release from afferent terminals in the striatum. These results reveal a novel potential role for cannabinoids in regulating striatal function and thus basal ganglia output and may suggest CB1-targeted drugs as potential therapeutic agents in the treatment of Parkinson's disease and other basal ganglia disorders.
How do drugs of abuse modify neural circuitry and thereby lead to addictive behaviour? As for many forms of experience-dependent plasticity, modifications in glutamatergic synaptic transmission have been suggested to be particularly important. Evidence of such changes in response to in vivo administration of drugs of abuse is lacking, however. Here we show that a single in vivo exposure to cocaine induces long-term potentiation of AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid)-receptor-mediated currents at excitatory synapses onto dopamine cells in the ventral tegmental area. Potentiation is still observed 5 but not 10 days after cocaine exposure and is blocked when an NMDA (N-methyl-d-aspartate) receptor antagonist is administered with cocaine. Furthermore, long-term potentiation at these synapses is occluded and long-term depression is enhanced by in vivo cocaine exposure. These results show that a prominent form of synaptic plasticity can be elicited by a single in vivo exposure to cocaine and therefore may be involved in the early stages of the development of drug addiction.
The present study was designed to explore the relationship between the cannabinoid and opioid receptors in animal models of opioid-induced reinforcement. The acute administration of SR141716A, a selective central cannabinoid CB1 receptor antagonist, blocked heroin self-administration in rats, as well as morphine-induced place preference and morphine self-administration in mice. Morphine-dependent animals injected with SR141716A exhibited a partial opiate-like withdrawal syndrome that had limited consequences on operant responses for food and induced place aversion. These effects were associated with morphine-induced changes in the expression of CB1 receptor mRNA in specific nuclei of the reward circuit, including dorsal caudate putamen, nucleus accumbens, and septum. Additionally, the opioid antagonist naloxone precipitated a mild cannabinoid-like withdrawal syndrome in cannabinoid-dependent rats and blocked cannabinoid self-administration in mice. Neither SR141716A nor naloxone produced any intrinsic effect on these behavioral models. The present results show the existence of a cross-interaction between opioid and cannabinoid systems in behavioral responses related to addiction and open new strategies for the treatment of opiate dependence.
Stress is a biologically significant factor that, by altering brain cell properties, can disturb cognitive processes such as learning and memory, and consequently limit the quality of human life. Extensive rodent and human research has shown that the hippocampus is not only crucially involved in memory formation, but is also highly sensitive to stress. So, the study of stress-induced cognitive and neurobiological sequelae in animal models might provide valuable insight into the mnemonic mechanisms that are vulnerable to stress. Here, we provide an overview of the neurobiology of stress memory interactions, and present a neural endocrine model to explain how stress modifies hippocampal functioning.
The spiny projection neurons are by far the most numerous type of striatal neuron. In addition to being the principal projection neurons of the striatum, the spiny projection neurons also have an extensive network of local axon collaterals by which they make synaptic connections with other striatal projection neurons. However, up to now there has been no direct physiological evidence for functional inhibitory interactions between spiny projection neurons. Here we present new evidence that striatal projection neurons are interconnected by functional inhibitory synapses. To examine the physiological properties of unitary inhibitory postsynaptic potentials (IPSPs), dual intracellular recordings were made from pairs of spiny projection neurons in brain slices of adult rat striatum. Synaptic interactions were found in 9 of 45 pairs of neurons using averages of 200 traces that were triggered by a single presynaptic action potential. In all cases, synaptic interactions were unidirectional, and no bidirectional interactions were detected. Unitary IPSPs evoked by a single presynaptic action potential had a peak amplitude ranging from 157 to 319 microV in different connections (mean: 277 +/- 46 microV, n = 9). The percentage of failures of single action potentials to evoke a unitary IPSP was estimated and ranged from 9 to 63% (mean: 38 +/- 14%, n = 9). Unitary IPSPs were reversibly blocked by bicuculline (n = 4) and had a reversal potential of -62.4 +/- 0.7 mV (n = 5), consistent with GABA-mediated inhibition. The findings of the present study correlate very well with anatomical evidence for local synaptic connectivity between spiny projection neurons and suggest that lateral inhibition plays a significant role in the information processing operations of the striatum.
DeltaFosB is a transcription factor that accumulates in a region-specific manner in the brain after chronic perturbations. For example, repeated administration of drugs of abuse increases levels of DeltaFosB in the striatum. In the present study, we analyzed the effect of spontaneous wheel running, as a model for a natural rewarding behavior, on levels of DeltaFosB in striatal regions. Moreover, mice that inducibly overexpress DeltaFosB in specific subpopulations of striatal neurons were used to study the possible role of DeltaFosB on running behavior. Lewis rats given ad libitum access to running wheels for 30 d covered what would correspond to approximately 10 km/d and showed increased levels of DeltaFosB in the nucleus accumbens compared with rats exposed to locked running wheels. Mice that overexpress DeltaFosB selectively in striatal dynorphin-containing neurons increased their daily running compared with control littermates, whereas mice that overexpress DeltaFosB predominantly in striatal enkephalin-containing neurons ran considerably less than controls. Data from the present study demonstrate that like drugs of abuse, voluntary running increases levels of DeltaFosB in brain reward pathways. Furthermore, overexpression of DeltaFosB in a distinct striatal output neuronal population increases running behavior. Because previous work has shown that DeltaFosB overexpression within this same neuronal population increases the rewarding properties of drugs of abuse, results of the present study suggest that DeltaFosB may play a key role in controlling both natural and drug-induced reward.
Dopamine is a critical modulator of striatal function; its absence produces Parkinson's disease. Most cellular actions of dopamine are still unknown. This work describes the presynaptic actions of dopaminergic receptor agonists on GABAergic transmission between neostriatal projection neurons. Axon collaterals interconnect projection neurons, the main axons of which project to other basal ganglia nuclei. Most if not all of these projecting axons pass through the globus pallidus. Thus, we lesioned the intrinsic neurons of the globus pallidus and stimulated neostriatal efferent axons antidromically with a bipolar electrode located in this nucleus. This maneuver revealed a bicuculline-sensitive synaptic current while recording in spiny cells. D1 receptor agonists facilitated whereas D2 receptor agonists depressed this synaptic current. In contrast, a bicuculline-sensitive synaptic current evoked by field stimulation inside the neostriatum was not consistently modulated, in agreement with previous studies. The data are discussed in light of the most recent experimental and modeling results. The conclusion was that inhibition of spiny cells by axon collaterals of other spiny cells is quantitatively important; however, to be functionally important, this inhibition might be conditioned to the synchronized firing of spiny neurons. Finally, dopamine exerts a potentially important role regulating the extent of lateral inhibition.
A major goal of research on addiction is to identify the molecular mechanisms of long-lasting behavioural alterations induced by drugs of abuse. Cocaine and delta-9-tetrahydrocannabinol (THC) activate extracellular signal-regulated kinase (ERK) in the striatum and blockade of the ERK pathway prevents establishment of conditioned place preference to these drugs. However, it is not known whether activation of ERK in the striatum is specific for these two drugs and/or this brain region. We studied the appearance of phospho-ERK immunoreactive neurons in CD-1 mouse brain following acute administration of drugs commonly abused by humans, cocaine, morphine, nicotine and THC, or of other psychoactive compounds including caffeine, scopolamine, antidepressants and antipsychotics. Each drug generated a distinct regional pattern of ERK activation. All drugs of abuse increased ERK phosphorylation in nucleus accumbens, lateral bed nucleus of the stria terminalis, central amygdala and deep layers of prefrontal cortex, through a dopamine D1 receptor-dependent mechanism. Although some non-addictive drugs moderately activated ERK in a few of these areas, they never induced this combined pattern of strong activation. Antidepressants and caffeine activated ERK in hippocampus and cerebral cortex. Typical antipsychotics mildly activated ERK in dorsal striatum and superficial prefrontal cortex, whereas clozapine had no effect in the striatum, but more widespread effects in cortex and amygdala. Our results outline a subset of structures in which ERK activation might specifically contribute to the long-term effects of drugs of abuse, and suggest mapping ERK activation in brain as a way to identify potential sites of action of psychoactive drugs.
To investigate whether an elevated plus-maze consisting of two open and two closed arms could be used as a model of anxiety in the mouse, NIH Swiss mice were tested in the apparatus immediately after a holeboard test. Factor analysis of data from undrugged animals tested in the holeboard and plus-maze yielded three orthogonal factors interpreted as assessing anxiety, directed exploration and locomotion. Anxiolytic drugs (chlordiazepoxide, sodium pentobarbital and ethanol) increased the proportion of time spent on the open arms, and anxiogenic drugs (FG 7142, caffeine and picrotoxin) reduced this measure. Amphetamine and imipramine failed to alter the indices of anxiety. The anxiolytic effect of chlordiazepoxide was reduced in mice that had previously experienced the plus-maze in an undrugged state. Testing animals in the holeboard immediately before the plus-maze test significantly elevated both the percentage of time spent on the open arms and the total number of arm entries, but did not affect the behavioral response to chlordiazepoxide. The plus-maze appears to be a useful test with which to investigate both anxiolytic and anxiogenic agents.
1. Neostriatal neurones were recorded intracellularly from a rat corticostriatal slice preparation. Depolarizing postsynaptic potentials (DPSPs) were evoked by either cortical or intrastriatal stimulation. 2. Kynurenic acid (600 microM), an antagonist of excitatory amino acids, reduced the cortically-evoked DPSPs by 88% while the intrastriatally evoked potentials were reduced by 48%. Bicuculline (100 microM) produced only a slight inhibition of the cortically evoked DPSPs (12%), but clearly depressed intrastriatal potentials (52%). 3. The effects of (-)-baclofen, a gamma-aminobutyric acid (GABA)B receptor agonist, were studied on the cortically evoked DPSPs. In all the tested neurones (-)-baclofen, added to the superfusion medium, caused a concentration-dependent decrease of these potentials (half-maximal effect (EC50) = 800 nM). This effect was not affected by bicuculline. (-)-Baclofen did not change the membrane potential, the input resistance, current-evoked firing frequency, or postsynaptic responses to exogenously applied glutamate. 4. The effects of (-)-baclofen on the DPSPs were compared to those produced by application of GABA and muscimol. GABA and muscimol decreased the DPSPs and caused a membrane depolarization coupled with a decrease of the membrane resistance. Bicuculline (100 microM) blocked the GABA-induced changes of the membrane potential and of the resistance, but not the decrease of the synaptic potentials. All the effects produced by muscimol were blocked by bicuculline. 5. Following intrastriatal stimulation a residual kynurenate-insensitive potential persisted; this potential was blocked by bicuculline (100 microM). (-)-Baclofen produced a dose-dependent decrease of this potential (EC50 = 800 nM). The postsynaptic responses to exogenously applied GABA were unchanged by (-)-baclofen. 6. The amplitude of kynurenate and bicuculline-sensitive DPSPs were stable at a frequency of 0.1 Hz. At frequencies between 0.3 and 3 Hz both these potentials were attenuated with the second stimulus and after about five stimuli a steady state was reached. Membrane potential and input resistance were not affected by these frequencies of stimulation. 7. Application of the GABA uptake inhibitor nipecotic acid (100-300 microM) clearly reduced the amplitude of both kynurenate-and bicuculline-sensitive DPSPs evoked at low frequencies of stimulation (0.01-0.3 Hz), but had lower effects at higher stimulation rates (1-3 Hz). Application of nipecotic acid increased the duration of membrane responses to exogenously applied GABA.(ABSTRACT TRUNCATED AT 400 WORDS)
Cannabinoid receptors have recently been characterized and localized using a high-affinity radiolabeled cannabinoid analog in section binding assays. In rat brain, the highest receptor densities are in the globus pallidus and substantia nigra pars reticulata. Receptors are also dense in the caudate-putamen. In order to determine the neuronal localization of these receptors, selective lesions of key striatal afferent and efferent systems were made. Striatal neurons and efferent projections were selectively destroyed by unilateral infusion of ibotenic acid into the caudate-putamen. The nigrostriatal pathway was selectively destroyed in another set of animals by infusion of 6-hydroxydopamine into the medial forebrain bundle. After 2- or 4-week survivals, slide-mounted brain sections were incubated with ligands selective for cannabinoid ([3H]CP 55,940), dopamine D1 3H]SCH-23390) and D2 ([3H]raclopride) receptors, and dopamine uptake sites ([3H]GBR-12935). Slides were exposed to 3H-sensitive film. The resulting autoradiography showed ibotenate-induced losses of cannabinoid, D1 and D2 receptors in the caudate-putamen and topographic losses of cannabinoid and D1 receptors in the globus pallidus, entopeduncular nucleus, and substantia nigra pars reticulata at both survivals. Four weeks after medial forebrain bundle lesions (which resulted in amphetamine-induced rotations), there was loss of dopamine uptake sites in the striatum and substantia nigra pars compacta but no change in cannabinoid receptor binding. The data show that cannabinoid receptors in the basal ganglia are neuronally located on striatal projection neurons, including their axons and terminals. Cannabinoid receptors may be co-localized with D1 receptors on striatonigral neurons. Cannabinoid receptors are not localized on dopaminergic nigrostriatal cell bodies or terminals.
Four lines of mutant mice were used to investigate (1) the neuronal localization of cannabinoid receptors in the cerebellar molecular layer and (2) the anatomical association of these receptors with elements of the two second messenger systems in the brain. Two of the mutant lines--Purkinje cell degeneration and nervous--are selectively deficient in Purkinje cells; the other two--weaver and reeler--are deficient in granule cells. In the heterozygous mice, [3H]CP 55,940 binding to cannabinoid receptors was discretely and densely localized to the molecular layer, as was [3H]forskolin binding to adenylate cyclase and [3H]phorbol 12,13-dibutyrate binding to protein kinase C, a component of the phosphoinositide cycle. [3H]CP 55,940 and [3H]forskolin binding was selectively reduced in weaver and reeler homozygous mice but unchanged in Purkinje cell deficient and nervous homozygotes. No decreases in [3H]phorbol 12,13-dibutyrate binding were found in any of the homozygous mutants relative to the heterozygous littermates. The results suggest that cannabinoid receptors and adenylate cyclase are localized to granule cell axons in the molecular layer, whereas protein kinase C is equally distributed in parallel fibers and Purkinje cell dendrites.
Behavioral and neurochemical evidence indicates interactions between caffeine and other adenosine receptor ligands and the gamma-aminobutyric acid (GABA)-benzodiazepine system. To assess the effects of caffeine on binding and function at the GABAA receptor, we studied the effects of behaviorally-active doses of caffeine on benzodiazepine and Cl- channel binding and on overall function of the GABAA receptor as measured by Cl- uptake. There was no effect of caffeine on benzodiazepine receptor binding in cortical synaptosomal membranes at concentrations of 1-100 microM. No effects on benzodiazepine binding were found ex vivo in mice treated with caffeine, 20 and 40 mg/kg. At the putative Cl- channel site labeled by t-butylbicyclophosphorothionate (TBPS), binding was unchanged in vitro after caffeine treatment (1 and 10 microM) in washed and unwashed membranes. However, in ex vivo studies caffeine (20 and 40 mg/kg) increased numbers of TBPS sites in unwashed but not washed membranes. Muscimol-stimulated Cl- uptake into cortical synaptoneurosomes was decreased in mice treated with caffeine, 20 and 40 mg/kg. Similar results were observed in in vitro preparations treated with 50 microM but not 100 microM caffeine. These results indicate that caffeine administration significantly alters the Cl- transport function of the GABAA receptor complex.
A group of rats was trained to escape low-intensity shock in a shuttle-box test, while another group of yoked controls could not escape but was exposed to the same amount and regime of shock. After 1 week of training, long-term potentiation (LTP) was measured in vitro in hippocampal slices. Exposure to uncontrollable shock massively impaired LTP relative to exposure to the same amount and regime of controllable shock. These results provide evidence that controllability modulates plasticity at the cellular-neuronal level.
The pharmacologic specificity of tolerance to caffeine-induced stimulation of locomotor activity was studied in adult male rats that were given access to either caffeine solution (0.5 or 1.0 mg/ml) or plain water for 10 min every 6 h on a chronic daily basis; daily caffeine intake averaged 41 and 62 mg/kg, respectively. Dose-effect curves were determined for behavioral stimulant and depressant drugs in control and caffeine-treated groups. Drugs were injected IP and locomotor activity was measured for 30 min beginning 35 min later. Rats tolerant to stimulation of locomotor activity by caffeine were also tolerant to theophylline and 7-(2-chloroethyl)theophylline, but not to any of six nonxanthine stimulants, including cocaine, methylphenidate, and d-amphetamine. The adenosine analogs, R(-)-N6-2-(phenylisopropyl)adenosine(R(-)-PIA) and 5'-(N-ethyl)carboxamidoadenosine (NECA), decreased locomotor activity of control and caffeine-treated (0.5 mg/ml) rats; dose-effect curves in rats consuming caffeine chronically were displaced to the right of the control curves by 10-fold for R(-)-PIA and 100-fold for NECA. Dose-effect curves for the nonadenosine behavioral depressants chlorpromazine and diazepam were unchanged by chronic treatment with caffeine, but the curve for pentobarbital, which is thought to inhibit adenosine receptor binding, was shifted to the right by a factor of 3. Rats withdrawn from chronic caffeine for 24 h were still completely tolerant to caffeine-induced stimulation of locomotor activity. Dose-effect curves for R(-)-PIA and d-amphetamine in rats withdrawn from chronic caffeine for 24 h were not different from curves in control animals.(ABSTRACT TRUNCATED AT 250 WORDS)
The effects of caffeine and several selective phosphodiesterase (PDE) inhibitors on ventilation and on schedule-controlled behavior were studied in rhesus monkeys. In seated, unanesthetized monkeys prepared with a head plethysmograph, ventilation during exposure to air (normocapnia) and to elevated levels of CO2 (3, 4 and 5%) mixed in air (hypercapnia) was measured after cumulative doses of each drug. In other monkeys, behavioral effects were studied by administering cumulative doses preceding sequential periods of fixed-ratio or fixed-interval responding. The nonselective PDE inhibitors, caffeine and 3-isobutyl-1-methylxanthine, and the type IV-selective PDE inhibitors, rolipram and Ro 20-1724, had pronounced respiratory-stimulant effects during conditions of normocapnia and hypercapnia, and their potencies in increasing ventilation corresponded with their potencies as PDE inhibitors. The type III-selective PDE inhibitor, CI-930, had only modest respiratory-stimulant effects at the highest dose studied, and the type V-selective PDE inhibitor, zaprinast, had no respiratory effect. CGS 15943, a selective adenosine antagonist lacking PDE-inhibitory effects, also had only modest respiratory-stimulant effects at the highest dose studied. In contrast to their relative potencies and efficacies in stimulating respiration, caffeine and 3-isobutyl-1-methylxanthine were less efficacious than CGS 15943 in increasing fixed-interval responding, and CI-930, rolipram and Ro 20-1724 only decreased fixed-interval responding. Zaprinast had little or no behavioral effect. The results support the interpretation that inhibition of type IV PDE plays a prominent role in the respiratory-stimulant effects of xanthines, whereas the behavioral-stimulant effects are more closely related to antagonism of adenosine receptors.
Among the many reported central nervous system effects of long-term caffeine use is improvement in mood.
To examine prospectively the relationship of coffee and caffeine intake to risk of death from suicide.
We conducted a 10-year follow-up study (1980 to 1990) in an ongoing cohort of 86 626 US female registered nurses aged 34 to 59 years in 1980, who were free of diagnosed coronary heart disease, stroke, or cancer. Information on coffee and caffeine intake was collected by a semiquantitative food frequency questionnaire in 1980. Deaths from suicide were determined by physician review of death certificates.
Fifty-six cases of suicide occurred during 832 704 person-years of observation. Compared with non-drinkers of coffee, the age-adjusted relative risk of suicide in women who consumed two to three cups per day was 0.34 (95% confidence interval [CI, 0.17 to 0.68) and 0.42 (95% CI, 0.21 to 0.86) in women who consumed four or more cups per day (P for linear trend=.002). These findings remained essentially unchanged after adjusting for a broad range of potential confounding factors, including smoking habit, alcohol intake, medication use (diazepam and phenothiazine), history of comorbid disease (hypertension, hypercholesterolemia, or diabetes), marital status, and self-reported stress. A strong inverse relationship was similarly found for caffeine intake from all sources and risk of suicide.
The data suggest a strong inverse association between coffee intake and risk of suicide. Whether regular intake of coffee or caffeine has clinically significant effects on the maintenance of affect or the prevention of depression merits further investigation in clinical trials and population-based prospective studies.
Electrophysiological consequences of activation of cannabinoid receptors have been mostly investigated on neuronal cell lines and on cells transfected with cannabinoid receptors. The aim of the present experiments was to study cannabinoid effects on identified neurons in situ. Electrically-evoked postsynaptic currents and voltage-dependent calcium currents were investigated in the principal neurons of the corpus striatum, the medium spiny neurons, with the patch-clamp method for brain slices. These neurons were chosen because they produce messenger RNA for cannabinoid receptors and because the density of cannabinoid binding sites in the striatum is high. Activation of muscarinic receptors by carbachol (10−5 M) reduced inhibitory postsynaptic current amplitude by 67%. The synthetic cannabinoid receptor agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphtalenyl)methanone (WIN55212-2; 10−8 to 10−5 M) dose-dependently reduced striatal inhibitory postsynaptic currents; the maximum effect, inhibition by 52%, was observed at 10−6 M. Another cannabinoid agonist, (−)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP55940; 10−6 M), also reduced inhibitory postsynaptic currents, by 50%. The CB1 cannnabinoid receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazolecarboxamide (SR141716A; 10−6 M) had no effect when given alone but abolished the effect of WIN55212-2 (10−6 M). WIN55212-2 (10−6 M) did not change the current evoked by the GABAA-receptor agonist muscimol (10−6 M). Activation of muscarinic receptors by carbachol (10−5 M) inhibited voltage-dependent calcium currents by 21%, but the cannabinoid receptor agonist WIN55212-2 (10−6 M) was without effect.
Conditioned place preference (CPP) was employed to clarify the reinforcing and locomotor stimulating effects of several doses of cocaine and caffeine (0.32, 1.0, 3.2, 5.6, and 10.0 mg/kg) and to explore the possibility of additive effects between the two drugs. Additionally, the hypothesis that the reinforcing effects of psychostimulants are mediated by the same systems that control psychostimulant-induced locomotor activity was examined by conducting correlational studies between drug-induced locomotor activity and time spent in the drug-conditioned compartments. Several doses of cocaine (1.0, 3.0, 5.6, 10.0 mg/kg), and caffeine (0.32, 1.0, 3.2, 5.6, 10.0) were found to condition place preference and stimulate locomotor activity. A combination of low doses (0.32 mg/kg) of each drug appeared to be additive. A positive relationship between locomotor activity observed during conditioning and time spent in the conditioned compartment during testing was found for cocaine but not caffeine or the low-dose combination of cocaine and caffeine.
Exposure of rats to sustained stress has been associated with behavioural impairments, the degree of impairment being greater with increasing age of the subject. Although the behavioural deficits have been frequently attributed to stress-induced neuronal loss in the hippocampus, the validity of that view may be disputed since it is based on data collected using conventional morphometric methods which are subject to bias. The question of whether stress per se does indeed induce hippocampal cell losses was therefore re-examined using unbiased stereological tools in the present work. Specifically, we used the optical fractionator and the Cavalieri principle, to respectively estimate the total number of neurons and volumes of the main divisions of the hippocampal formation of young and old rats which had been exposed for 1 month to an unpredictable stress paradigm. The efficacy of the treatment was confirmed by elevated serum corticosterone levels measured at various intervals during the experimental period. In order to evaluate whether any deleterious effects might have occurred merely due to the stress-induced elevations in corticosterone secretion, we conducted a parallel study on animals that were injected with corticosterone over a similar duration. Neither stress nor treatment with corticosterone was found to result in significant cell losses in any division of the hippocampal formation; likewise, neither treatment produced significant volumetric differences. Further, these results were not influenced by age of the experimental subjects. The present findings therefore call for a reappraisal of the hypothesis that hippocampal cell loss accounts for the behavioural impairments observed by others following prolonged stress and/or chronic elevation of serum corticosterone levels.
The purpose of this study was to investigate the cannabinoid and opioid mediated regulation on the effects of central Delta(9)-tetrahydrocannabinol (Delta(9)-THC) administration on hypothalamus-pituitary-adrenal (HPA) axis activity in the male rat. Intracerebroventricular (i.c.v.) administration of delta(9)-THC (25, 50, 100 microg/rat) markedly increased plasma adrenocorticotropin hormone (ACTH) and corticosterone concentrations. Time course effect studies revealed that both hormones secretion peaked at 60 min after Delta(9)-THC i.c.v. administration (50 microg/rat), decreased gradually and returned to baseline levels by 480 min. The i.c.v. administration of the specific cannabinoid receptor antagonist SR-141716A (3 microg/rat) significantly attenuated the increase of both hormones secretion induced by Delta(9)-THC (50 microg/rat). Nevertheless, higher doses (12.5 and 50 microg/rat) of this compound increased both ACTH and corticosterone plasma concentrations. Subcutaneous (s.c.) administration with the opiate receptor antagonist naloxone (0.3 mg/kg) was without effect but significantly diminished the increase of both hormones secretion induced by Delta(9)-THC (50 microg/rat). Taken together, these results indicate that opiate and cannabinoid receptors are involved in the activation of the HPA axis induced by Delta(9)-THC. Furthermore, the increase of ACTH and corticosterone secretion after the administration of higher doses of SR-141716A than those required to block such activation, suggests that endogenous cannabinoids are tonically inhibiting the release of both hormones or that this agonist-like activity may be part of an uncharacterized action of this compound not mediated by cannabinoid receptors.
Double-label in situ hybridization was used to identify the phenotypes of striatal neurons that express mRNA for cannabinoid CB(1) receptors. Simultaneous detection of multiple mRNAs was performed by combining a (35)S-labeled ribonucleotide probe for CB(1) mRNA with digoxigenin-labeled riboprobes for striatal projection neurons (preprotachykinin A, prodynorphin, and preproenkephalin mRNAs) and interneurons (vesicular acetylcholine transporter (VAChT), choline acetyltransferase (ChAT), somatostatin, and glutamic acid decarboxylase (Mr 67,000; GAD67) mRNAs). To ascertain whether CB(1) mRNA was a marker for striatal efferents, digoxigenin-labeled probes for mRNA markers of both striatonigral (prodynorphin or preprotachykinin A mRNAs), and striatopallidal (proenkephalin mRNAs) projection neurons were combined with the (35)S-labeled probe for CB(1). A mediolateral gradient in CB(1) mRNA expression was observed at rostral and mid-striatal levels; in the same coronal sections the number of silver grains per cell ranged from below the threshold of detectability at the medial and ventral poles to saturation at the dorsolateral boundary bordered by the corpus callosum. At the caudal level examined, CB(1) mRNA was denser in the ventral sector relative to the dorsal sector. Virtually all neurons expressing mRNA markers for striatal projection neurons colocalized CB(1) mRNA. Combining a (35)S-labeled riboprobe for CB(1) with digoxigenin-labeled riboprobes for both preproenkephalin and prodynorphin confirmed localization of CB(1) mRNA to striatonigral and striatopallidal neurons expressing prodynorphin and preproenkephalin mRNAs, respectively. However, CB(1) mRNA-positive cells that failed to coexpress the other markers were also apparent. CB(1) mRNA was localized to putative GABAergic interneurons that express high levels of GAD67 mRNA. These interneurons enable functional interactions between the direct and indirect striatal output pathways. By contrast, aspiny interneurons that express preprosomatostatin mRNA and cholinergic interneurons that coexpress ChAT and VAChT mRNAs were CB(1) mRNA-negative. The present data provide direct evidence that cannabinoid receptors are synthesized in striatonigral neurons that contain dynorphin and substance P and striatopallidal neurons that contain enkephalin. By contrast, local circuit neurons in striatum that contain somatostatin or acetylcholine do not synthesize cannabinoid receptors. Published 2000 Wiley-Liss, Inc.
Physical activities such as long-distance running can be habit forming and associated with a sense of well-being to a degree that justifies comparison with drug-induced addictive behaviours. To understand molecular similarities and dissimilarities controlling these behaviours in humans we compared the effects of running in running wheels to the effects of chronic cocaine or morphine administration on mRNA levels in brain reward pathways in the inbred Fischer and Lewis rat strains. These strains are both inbred from the Sprague-Dawley strain; Lewis rats display a higher preference towards addictive drugs and running than do Fischer rats. After chronic cocaine or running a similar increase of dynorphin mRNA in medial caudate putamen was found in the Lewis rat, suggesting common neuronal adaptations in this brain region to both cocaine and running. Fischer and Lewis rats both responded to cocaine with increased dynorphin mRNA levels in medial caudate putamen. However, only Lewis rats increased dynorphin mRNA after running, possibly reflecting the much higher degree of running by the Lewis strain as compared to the Fischer strain. Moreover, the running-induced upregulation of dynorphin mRNA was blocked by the opioid receptor antagonist naloxone. We suggest that running increases dynorphin mRNA by a mechanism that involves endogenous opioids. The voluntary wheel-running model in rats might be used to study natural reward and compulsive behaviours and possibly also to screen candidate drugs for treatment of compulsive disorders.
The striatum is a crucial site of action for the motor effects of cannabinoids (CBs). However, the electrophysiological consequences of activation of CB receptors on the striatal neurons have not been established. Here we report for the first time that the cannabimimetic aminoalkylindole WIN 55,212-2 and the endogenous cannabinoid anandamide substantially depress corticostriatal glutamatergic synaptic transmission onto striatal neurons in the brain slice preparation. The selective CB1 receptor antagonist SR 141716 effectively reversed this inhibition. WIN 55,212-2 significantly increased the paired-pulse facilitation of synaptically evoked EPSCs, while having no effect on the sensitivity of postsynaptic neurons to [alpha]-amino-3-hydroxy-5-methylisoxazole-4-propionic acid. WIN 55,212-2 also reduced the frequency of spontaneous, action potential-dependent EPSCs (sEPSCs) without altering their amplitude distribution. Superfusion of WIN 55,212-2 elicited a membrane hyperpolarization accompanied by a decrease in input resistance. Both effects were blocked by intracellular caesium. In contrast, intracellular caesium failed to affect WIN 55,212-2-mediated synaptic inhibition. The WIN 55,212-2-mediated synaptic inhibition was blocked by the Gi/o protein inhibitor pertussis toxin (PTX), but not by the GABA(A) receptor antagonist bicuculline or GABA(B) receptor antagonist SCH 50911. Pretreatment with the N-type Ca2+ channel antagonist [omega]-conotoxin GVIA selectively abolished the WIN-55,212-2-mediated synaptic inhibition. These results suggest that cannabinoids depress the corticostriatal glutamatergic synaptic transmission through the activation of presynaptic CB1 receptors to inhibit N-type Ca2+ channel activity, which in turn reduces glutamate release. The presynaptic action of cannabinoids is mediated by a PTX-sensitive Gi/o protein-coupled signalling pathway.
Systemic administration of caffeine reinstates extinguished cocaine self-administration behavior in rats, but the mechanism mediating this behavioral effect has not been established. The present study examined the role of adenosinergic A2 and dopaminergic mechanisms in caffeine-produced cocaine seeking. Following extinction of cocaine self-administration, experimenter-administered injections of caffeine (1.25-20 mg/kg) and theophylline (1-10 mg/kg) dose-dependently reinstated extinguished cocaine-seeking behavior. Administration of the adenosinergic A2 antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX; 0.546-2.18 microg/kg), failed to produce cocaine seeking. Pretreatment with doses of the adenosine A1/A2 agonist 5'-N-ethylcarboxamidoadenosine (NECA; 0.003-0.03 mg/kg) that were below those that produced marked sedation failed to block reinstatement. These data suggest that methylxanthine-produced cocaine seeking is not due to adenosine A2 receptor antagonism. In contrast, pretreatment with the dopaminergic D1-like antagonist SCH 23390 (0.005-0.02 mg/kg) or the D2-like antagonist eticlopride (0.03-0.3 mg/kg) produced a dose-dependent attenuation of caffeine-produced reinstatement at doses that did not decrease cocaine self-administration. These findings suggest that dopaminergic mechanisms underlie the ability of caffeine to reinstate extinguished cocaine-taking behavior.
The effects of caffeine on extracellular dopamine and acetylcholine have been studied in freely moving rats implanted with concentric microdialysis probes in the nucleus accumbens shell and core and in the medial prefrontal cortex. Intravenous administration of caffeine (0.25, 0.5, 1.0, 2.5 and 5.0 mg/kg) dose-dependently increased dopamine and acetylcholine dialysate concentrations in the medial prefrontal cortex, while it did not affect dialysate dopamine in the shell and core of the nucleus accumbens. Intraperitoneal administration of caffeine (1.5, 3, 10, 30 mg/kg) also failed to affect DA in the shell and core of the nucleus accumbens. Such effects were duplicated by intravenous administration of DPCPX, a selective antagonist of adenosine A1 receptors, and of SCH 58261, an antagonist of A2a receptors. The effect of caffeine on prefrontal dopamine and acetylcholine transmission was also studied in rats chronically administered with caffeine (25 mg/kg, twice a day for seven days). At the end of this treatment rats became tolerant to the locomotor stimulating effects of a dose of 1 and 2.5 mg/kg i.v. of caffeine; these doses, however, still increased dialysate acetylcholine but did not affect dopamine in the prefrontal cortex. Therefore, in rats made tolerant to the locomotor stimulant effects of caffeine, tolerance developed to the dopamine stimulant but not to the acetylcholine stimulant effect of caffeine in the prefrontal cortex. The lack of acute stimulation of dopamine release in the nucleus accumbens shell by caffeine is relevant to the issue of its addictive properties and of the role of DA in drug- and substance-addiction. On the other hand, the dissociation between tolerance to the locomotor effects of caffeine and stimulation of acetylcholine release in the prefrontal cortex suggests that this effect might be correlated to the arousing effects of caffeine as distinct from its locomotor stimulant properties.
Dopamine transmission acting at either the D(1) or D(2) receptor level is known to influence the stimulant properties of caffeine, an antagonist of adenosine A(1) and A(2) receptors. In contrast, how caffeine influences the motor stimulant properties of selective D(1) and D(2) receptor agonists is still undefined.
In this study the acute motor response to the dopamine D(1) receptor agonist SKF 77434 and the D(2)/D(3) receptor agonist quinpirole was studied in rats treated subchronically with caffeine (15 mg/kg i.p., on alternate days) or vehicle, either in the test cage (paired group) or in the home cage (home group).
Repeated caffeine administration did not induce any significant increase in motor activity or in stereotyped behavior during the course of treatment, indicating that the response to caffeine itself did not develop sensitization. Three days after the last caffeine or vehicle administration, rats were challenged with caffeine, SKF 77434, or quinpirole. Caffeine (5 mg/kg i.p.) elicited the same motor stimulant effects in both caffeine- and vehicle-pretreated rats, confirming the presence of neither tolerance nor sensitization to caffeine itself. SKF 77434 (3 mg/kg s.c.) elicited a higher locomotor activation in caffeine- than in vehicle-pretreated rats, whereas quinpirole (0.15 mg/kg s.c.) induced a similar locomotor activation and a higher stereotyped behavior in caffeine-pretreated rats as compared to rats pretreated with vehicle. The sensitized response to SKF 77434 and quinpirole was not due to environmental conditioning since the responses were similar in either paired or home group.
The results provide support for the presence of long-term functional interactions between drugs acting at the adenosine and dopamine receptor levels. Subchronic caffeine, by sensitizing the motor stimulant effects of dopamine D(1) and D(2) receptor agonists, produces adaptive changes which might result in a potentiation of the dopaminergic component of drugs of abuse.
We studied whether coffee and its components (caffeine and chlorogenic acid) have stress-relaxing effects. In vivo brain microdialysis was used to characterize the effects of coffee, stress, and their interaction on the serotonergic and dopaminergic systems in the rat hippocampus. Restraint stress for 100 min caused a marked increase in dopamine and serotonin (5-HT) levels in the hippocampus, and then, 100 min resting (freely-moving) time reduced them to basal levels. Pretreatment with 33 mg/kg coffee or 1.7 mg/kg caffeine reduced the second restraint-induced increase in the neurotransmitters, especially 5-HT, but neither saline nor 1.7 mg/kg chlorogenic acid did. These results suggest that coffee contributes to the reduction of restraint-induced stress and that these effects could be due to caffeine. Possible mechanisms of the effects are considered.
Even though 85% of adults drink caffeinated beverages daily, very limited studies on plasma caffeine concentration in the US population have been published. Smoking induces cytochrome P450 1A2 (CYP1A2), which is the main enzyme involved in caffeine metabolism. The current naturalistic pilot study explores plasma caffeine concentrations in a US sample, and presents a mathematical model of the relationship between caffeine intake and plasma concentrations for smokers and nonsmokers. Caffeine intake and average plasma caffeine concentrations from morning (7:30-9:30 a.m.) and afternoon (2:00-4:00 p.m.) samples were studied in 69 volunteers (21 smokers and 48 nonsmokers). The mean caffeine intake obtained from caffeinated beverages was 3.02 mg/kg/day, which is similar to the intake in the US population. Almost all subjects in the present sample (99%; 95% confidence interval [CI]: 96-100) had detectable plasma caffeine concentrations. Smokers had significantly higher caffeine intake than nonsmokers. The ratio of concentration/dose of caffeine from caffeinated beverages was approximately four-fold higher in nonsmokers (1.33 kgxday/l) than in smokers (0.29 kgxday/l). According to the model, the median plasma caffeine concentration was two- to three-fold higher in nonsmokers for each level of caffeine intake. Our model improves our understanding of the interactions between caffeine and smoking. Additional studies are needed to replicate the model. This model may help epidemiologists to correct for the effects of smoking on caffeine intake and pharmacologists to screen for the activity of CYP1A2.
Administration of N-methyl- d-aspartate (NMDA) receptor antagonists produce hyperlocomotion and cognitive deficits in rodents. Activation of NMDA receptors promotes adenosine release, and adenosine agonists prevent central effects of NMDA receptor antagonists. We hypothesized that if NMDA receptor antagonists require adenosine to produce behavioral effects, mice tolerant to the adenosine receptor antagonist caffeine would have a diminished response to NMDA receptor antagonists.
To evaluate MK-801-induced hyperlocomotion and cognitive deficits after chronic caffeine treatment in mice.
Locomotor activity was analyzed in a computerized system, spontaneous alternation was assessed in the Y-maze and long-term memory was assessed with the inhibitory avoidance task in mice.
Mice chronically treated with caffeine in drinking solution (1 mg/ml for 7 days) presented normal habituation and substantial tolerance to acute caffeine (30 mg/kg, i.p.) locomotor effects. MK-801 (0.25 mg/kg, i.p.) produced pronounced hyperlocomotion in water-treated mice, but this effect was abolished in caffeine-drinking mice. Chronic caffeine treatment had no influence on either normal or MK-801-induced deficits in spontaneous alternation and inhibitory avoidance tasks.
Hyperlocomotion induced by MK-801 may be mediated by reduced adenosinergic activity. These results also suggest that locomotor and cognitive effects of MK-801 can be dissociated and are distinctly modulated. Finally, these findings agree with the adenosine hypofunction model of schizophrenia, since NMDA receptor antagonists are a pharmacological model for this disorder.
Drug seeking and drug self-administration in both animals and humans can be triggered by drugs of abuse themselves or by stressful events. Here, we demonstrate that in vivo administration of drugs of abuse with different molecular mechanisms of action as well as acute stress both increase strength at excitatory synapses on midbrain dopamine neurons. Psychoactive drugs with minimal abuse potential do not cause this change. The synaptic effects of stress, but not of cocaine, are blocked by the glucocorticoid receptor antagonist RU486. These results suggest that plasticity at excitatory synapses on dopamine neurons may be a key neural adaptation contributing to addiction and its interactions with stress and thus may be an attractive therapeutic target for reducing the risk of addiction.
These experiments examined the mnemonic functions of hippocampus and dorsal striatum in Pavlovian aversive conditioning. Rats were trained in a single session by presenting three paired tones and footshocks. Immediately after training, they were given microinjections of D-amphetamine or vehicle into either dorsal hippocampus or dorsal striatum. Twenty-four hours later conditioned freezing (measured as cessation of movement by infrared motion detectors) to the experimental context and to the tone were measured separately. Compared to vehicle injections, amphetamine injections into dorsal hippocampus significantly increased conditioned freezing to the context but not to the tone. Injections into dorsal striatum increased conditioned freezing to both the context and the tone. These results converge with findings from lesion experiments suggesting that hippocampus is involved in aversive conditioning with contextual CSs only, and that dorsal striatum is involved in aversive conditioning with both contextual and discrete cue CSs. The functions of the these two structures in relation to that of the amygdala in the mediation of conditioned freezing are discussed.
Theories about basal ganglia function have always been driven by our knowledge about the spiny projection neurons of the striatum. At the core of these theories lies the question of how, precisely, spiny projection neurons process cortical inputs. Most recently, studies demonstrating the role of spiny projection neurons in local synaptic GABA transmission have provided several new avenues for exploring striatal dynamics. They have also suggested new experimental directives for examining the specific ways in which spiny projection neurons both compete and cooperate through their local axon collaterals during cortical input processing.
The endocannabinoids are a family of lipid messengers that engage the cell surface receptors that are targeted by Δ9-tetrahydrocannabinol, the active principle in marijuana (Cannabis). They are made on demand through cleavage of membrane precursors and are involved in various short-range signalling processes. In the brain, they combine with CB1 cannabinoid receptors on axon terminals to regulate ion channel activity and neurotransmitter release. Their ability to modulate synaptic efficacy has a wide range of functional consequences and provides unique therapeutic possibilities.