[Show abstract][Hide abstract] ABSTRACT: The loss of reproductive capacity during aging involves changes in the neural regulation of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons controlling reproduction. This neuronal circuitry includes glutamate receptors on GnRH neurons. Previously, we reported an increase in the expression of the NR2b subunit protein of the NMDA receptor on GnRH neurons in middle-aged compared to young female rats. Here, we examined the functional implications of the NR2b subunit on the onset of reproductive aging, using an NR2b-specific antagonist ifenprodil.
Young (3-5 months) and middle-aged (10-13 months) female rats were ovariectomized (OVX), 17beta-estradiol (E2) or vehicle (cholesterol) treated, and implanted with a jugular catheter. Serial blood sampling was undertaken every 10 min for 4 h, with ifenprodil (10 mg/kg) or vehicle injected (i.p.) after 1 h of baseline sampling. The pulsatile release of pituitary LH and levels of GnRH mRNA in hypothalamus were quantified as indices of the reproductive axis.
Our results showed effects of ifenprodil on both endpoints. In OVX rats given cholesterol, neither age nor ifenprodil had any effects on LH release. In E2-treated rats, aging was associated with significant decreases in pulsatile LH release. Additionally, ifenprodil stimulated parameters of pulsatile LH release in both young and middle-aged animals. Ifenprodil had few effects on GnRH mRNA; the only significant effect of ifenprodil was found in the middle-aged, cholesterol group.
Together, these findings support a role for the NR2b subunit of the NMDAR in GnRH/LH regulation. Because most of these effects were exhibited on pituitary LH release in the absence of a concomitant change in GnRH gene expression, it is likely that NMDA receptors containing the NR2b subunit play a role in GnRH-induced LH release, independent of de novo GnRH gene expression.
[Show abstract][Hide abstract] ABSTRACT: Experiments were performed to examine the effects of cocaine self-administration and conditioning experience on operant behavior, locomotor activity, and nucleus accumbens (NAcc) and prefrontal cortex (PFC) dopamine (DA) responses. Sensory cues were paired with alternating cocaine and nonreinforcement during 12 (limited training) or 40 (long-term training) daily operant sessions. After limited training, NAcc DA responses to cocaine were significantly enhanced in the presence of cocaine-associated cues compared with nonreward cues and significantly depressed after cocaine-paired cues accompanied a nonreinforced lever response. PFC DA levels were generally nonresponsive to cues after the same training duration. However, after long-term training, cocaine-associated cues increased the magnitude of cocaine-stimulated PFC DA levels significantly over levels observed with nonreinforcement cues. Conversely, conditioned cues no longer influenced NAcc DA levels after long-term training. In addition, cocaine-stimulated locomotor activity was enhanced by cocaine-paired cues after long-term, but not after limited, training. Findings demonstrate that cue-induced cocaine expectation exerts a significant impact on dopaminergic and behavioral systems, progressing from mesolimbic to mesocortical regions and from latent to patent behaviors as cocaine and associative experiences escalate.
[Show abstract][Hide abstract] ABSTRACT: The selective serotonergic neurotoxicity of 3,4-methylenedioxyamphetamine (MDA) and 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) depends on their systemic metabolism. We have recently shown that inhibition of brain endothelial cell gamma-glutamyl transpeptidase (gamma-GT) potentiates the neurotoxicity of both MDMA and MDA, indicating that metabolites that are substrates for this enzyme contribute to the neurotoxicity. Consistent with this view, glutathione (GSH) and N-acetylcysteine conjugates of alpha-methyl dopamine (alpha-MeDA) are selective neurotoxicants. However, neurotoxic metabolites of MDMA or MDA have yet to be identified in brain. Using in vivo microdialysis coupled to liquid chromatography-tandem mass spectroscopy and a high-performance liquid chromatography-coulometric electrode array system, we now show that GSH and N-acetylcysteine conjugates of N-methyl-alpha-MeDA are present in the striatum of rats administered MDMA by subcutaneous injection. Moreover, inhibition of gamma-GT with acivicin increases the concentration of GSH and N-acetylcysteine conjugates of N-methyl-alpha-MeDA in brain dialysate, and there is a direct correlation between the concentrations of metabolites in dialysate and the extent of neurotoxicity, measured by decreases in serotonin (5-HT) and 5-hydroxyindole acetic (5-HIAA) levels. Importantly, the effects of acivicin are independent of MDMA-induced hyperthermia, since acivicin-mediated potentiation of MDMA neurotoxicity occurs in the context of acivicin-mediated decreases in body temperature. Finally, we have synthesized 5-(N-acetylcystein-S-yl)-N-methyl-alpha-MeDA and established that it is a relatively potent serotonergic neurotoxicant. Together, the data support the contention that MDMA-mediated serotonergic neurotoxicity is mediated by the systemic formation of GSH and N-acetylcysteine conjugates of N-methyl-alpha-MeDA (and alpha-MeDA). The mechanisms by which such metabolites access the brain and produce selective serotonergic neurotoxicity remain to be determined.
Journal of Pharmacology and Experimental Therapeutics 05/2005; 313(1):422-31. DOI:10.1124/jpet.104.077628 · 3.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cocaine is rapidly absorbed through many routes of administration and enters the brain, where DA neural pathways are the primary sites for its effects. Increased DA levels resulting from cocaine binding to the DAT is convincingly associated with the rewarding effects of cocaine (Ritz et al., 1987; Volkow et al., 1997). However, since genetically altered mice that lack DAT will self-administer cocaine (Rocha et al., 1998), some have questioned the necessity of DAT in mediating the cocaine reward. Nevertheless, even in the absence of DAT, cocaine increases extracellular DA levels and retains reinforcing properties (Carboni et al., 2001). The NAcc and the PFC are particular targets of current cocaine investigation. Increased NAcc DA after cocaine use is most commonly linked to cocaine reward (e.g., Hoebel et al., 1983; Pettit and Justice, 1991; White and Kalivas, 1998). However, within the NAcc brain region, the shell and core subregions appear to respond differently to cocaine. For instance, greater dopaminergic responses to cocaine in the shell than the core region have been observed, although higher doses (Pontieri et al., 1995) and the mode of administration (e.g., experimenter- or self-administered) may influence this effect (Ikegami and Duvauchelle, 2004). In the PFC, normal cognitive function is proposed as dependent on optimal levels of D1 DA receptor activation (Granon et al., 2000). Although the PFC DA is less responsive than the NAcc to initial cocaine use (Ikegami and Duvauchelle, 2004), DA manipulations in this region affect cocaine-associated behaviors (Goeders and Smith, 1986; Schenk et al., 1991; Tzschentke and Schmidt, 1998). After chronic cocaine use, morphological, metabolic, and DA-receptor alterations are observed in this region, revealing a particular vulnerability of the PFC to cocaine use (Robinson et al., 2001; Volkow et al., 1992, 1993). An emerging approach in drug abuse and addiction research is to study cocaine use in the context of learning (e.g., Di Chiara, 1998, 1999; Nestler, 2001b; Spanagel and Weiss, 1999). Several lines of evidence support the view that enhanced DA levels attained by cocaine use recruits common molecular events that occur with learning. For example, repeated cocaine administration results in synaptic changes similar to those seen with LTP. Also, activity of the DA- and cAMP-regulated phosphoprotein, DARPP-32 and the transcription factor CREB are regulated during DA-mediated signaling cascades, and are largely implicated in synaptic plasticity, learning, and memory (Frey et al., 1993; Greengard et al., 1999; Guzowski and McGaugh, 1997; Nestler, 2001a; Silva et al., 1998; Yin et al., 1994). Indeed, formation of synaptic plasticity is impaired in DARPP-32 KO mice (Calabresi et al., 2000), and these animals have deficits in learning certain operant tasks (Heyser et al., 2000). Midbrain DA neurons are tonically active but also respond to certain external stimuli with burst discharges. Novel and salient stimuli, as well as conventional rewards and reward-paired stimuli, cause such changes in DA neuronal activity (Horvitz et al., 1997; Ljungberg et al., 1992; Mirenowicz and Schultz, 1994; Schultz, 1998; Schultz and Dickinson, 2000). Responses to conventional rewards and associated stimuli are altered by repetition and experience (Ljungberg et al., 1992; Redgrave et al., 1999; Schultz, 1998; Schultz and Dickinson, 2000), indicating an influence of learning on DA neuronal activity. Whether DA neuronal responses to cocaine or cocaine-conditioned rewards follow similar patterns after repetitive conditioning trials is currently unknown. However, future testing of this hypothesis can be performed within the context of learning. Learned associations formed between the pharmacological actions of cocaine and environmental stimuli present during cocaine-taking can trigger relapse to cocaine use (Wallace, 1989). Although many researchers have assessed DA and behavioral responses to cocaine-associated stimuli, findings are not in complete consensus (Bradberry et al., 2000; Brown and Fibiger, 1992; Ciccocioppo et al., 2001; Duvauchelle et al., 2000b; Fontana et al., 1993; Kiyatkin and Stein, 1996). Discrepancies may be the result of procedural differences, including response artifacts caused by route and mode of drug administration and varied levels of cocaine and conditioning experience. For instance, intraperitoneal-administered cocaine can result in stress-induced behavioral and hormonal responses (DeVries and Pert, 1998; Yang et al., 1992). In addition, the dopaminergic response to cocaine differs between rats that receive cocaine noncontingently and those that self-administer cocaine (Hemby et al., 1997; Kiyatkin and Stein, 1995). The self-administration methodology appears an excellent means to model neurochemical and behavioral changes specifically associated with voluntary drug use. Using cocaine self-administration in conjunction with conditioning procedures, effects of varied levels of cocaine experience and conditioning on DA responses were examined. A compilation of data showed that the magnitude of DA response to cocaine and conditioned cues in the NAcc and PFC brain regions were altered by experience in opposing manners. Early in training, DA responses to cocaine in the presence of cocaine-associated cues were significantly greater in the NAcc, but not in the PFC, compared to the responses observed in the presence of saline-associated cues. After extensive training, however, conditioned cues had no influence on the NAcc DA response to cocaine, but the presence of cocaine-associated cues resulted in significantly higher levels of cocaine-stimulated PFC DA. In addition, cocaine-induced hyperlocomotor activity was unaffected by conditioned cues early in training, but was enhanced by cocaine-associated cues after extensive cocaine training sessions (Ikegami, 2003). Deeper understanding of cocaine abuse and addiction comes about by continuing to expand cocaine research beyond the pharmacological effects at neural terminal regions. By considering cocaine use within the context of learning paradigms, interactions between the environment and in vivo experiences are revealed as crucial factors in the development of cocaine addiction.
International Review of Neurobiology 02/2004; 62:45-94. DOI:10.1016/S0074-7742(04)62002-2 · 1.92 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cocaine reinforcement is strongly associated with increased nucleus accumbens dopamine (NAcc DA). The involvement of medial prefrontal cortex (mPFC) DA in cocaine reward is less defined, but substantial evidence indicates that increased mPFC DA may suppress NAcc DA levels. Using in vivo microdialysis, NAcc or mPFC DA was determined in cocaine-naive rats after a self-administered cocaine injection (3.0 mg/kg). Extracellular levels of NAcc DA were dramatically enhanced 10 min post-cocaine injection, but dropped significantly at each subsequent assessment. mPFC DA also increased significantly, but to a lesser extent than observed in the NAcc. Findings of prominent DA increases in both the NAcc and mPFC terminals during the test session indicate that NAcc DA responses do not appear to be inhibited by increased mPFC DA during cocaine self-administration.
[Show abstract][Hide abstract] ABSTRACT: Separate lines of evidence suggest that neuroadaptations associated with ethanol (EtOH) reinforcement can be initiated by chronic EtOH preexposure and a signaling pathway activated by dopamine (DA) D1 receptor stimulation. We have previously shown that rewarding and locomotor effects of EtOH alone [Pharmacol. Biochem. Behav. 72 (2002) 787] are enhanced after chronic exposure to self-administered EtOH/cocaine combinations. To determine the importance of chronic EtOH exposure, dopamine D1 receptor activation and mode of drug administration in EtOH reward, animals were given daily intravenous infusions of experimenter-administered saline, EtOH (2.0 g/kg), the DA D1 receptor agonist, SKF81297 (0.2 mg/kg), or EtOH+SKF81297 over a 4-week period. Compared to other groups, animals preexposed to EtOH+SKF81297 self-administered significantly greater amounts of intravenous EtOH and showed greater enhancement and less suppression of locomotor activity in response to a range of intravenous EtOH dosages (0.125, 0.25, 0.5, 1.0 and 1.5 g/kg). Since chronic treatment with EtOH alone did not enhance EtOH-induced reinforcement or locomotor activity, it is unlikely that these effects were due to EtOH tolerance. These findings suggest that chronic D1 receptor activation combined with EtOH administration alters neural responsiveness to EtOH and support the notion that D1 activation is important to EtOH reward.
Pharmacology Biochemistry and Behavior 10/2003; 76(2):335-42. · 2.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The nucleus accumbens, a major component of the ventral striatum, and the dorsal striatum are primary targets of the mesolimbic dopamine pathway, which is a pathway that plays a critical role in reward and addiction. The shell compartment of the nucleus accumbens and the ventromedial striatum, in particular, receive extensive afferent projections from the ventral tegmental area, which is the major afferent source of the mesolimbic pathway [Prog Brain Res 99 (1993) 209; J Neurosci 7 (1987) 3915]. The present study focused on striatal cholinergic interneurons as potential key neurons involved in the neural basis of drug reinforcement. The main finding of this study is that cholinergic interneurons located in the shell compartment of the nucleus accumbens and the ventromedial striatum were activated, as measured by Fos labeling, following a 1 h session of the self-administration of cocaine in rats. A direct correlation existed between the percent of cholinergic interneurons that were activated and the amount of cocaine that was self-administered. The greatest amount of administered cocaine (approximately 10 mg/kg) resulted in the activation of approximately 80% of the cholinergic neurons. No such correlation existed in the group of animals that self-administered saline. In addition, activation was not found in the core compartment of the nucleus accumbens or the dorsolateral striatum, which receive extensive innervation from the substantia nigra and thus are more closely tied to the motor effects of the drug.
[Show abstract][Hide abstract] ABSTRACT: Evidence suggests that ethanol (EtOH) preexposure influences the rewarding valence of subsequent EtOH use. This study was conducted to determine if EtOH preexposure through EtOH/cocaine self-administration facilitates the motivational effects of EtOH alone. Rats self-administered intravenous (iv) EtOH/cocaine combinations (EtOH/Cocaine Fading group; EtOH 125.0 mg/kg/inj+Cocaine 0.1-0.75 mg/kg/inj) during a preexposure period. Consequently, these rats self-administered intravenous EtOH alone (62.5, 125.0, 250.0 and 500.0 mg/kg/inj) significantly more than a control group with prior cocaine self-administration experience (0.1-0.75 mg/kg/inj). In addition, at equal EtOH intake levels, locomotor activity was significantly enhanced in the EtOH/Cocaine Fading group but not the Cocaine Control animals (P=.01). The amount of EtOH self-administered in the EtOH/Cocaine Fading group during 1-h sessions (approximately 0.5-2.0 g/kg) corresponded with blood alcohol levels (BAL) ranging from 44 to 221 mg/dl. The highest BALs reported here have not previously been demonstrated after voluntary EtOH intake through any route of administration. These data suggest that preexposure to EtOH during EtOH/cocaine self-administration sessions modified neural substrates underlying both the reinforcing and locomotor responses to EtOH alone. Further studies utilizing intravenous EtOH self-administration will allow identification of various long-term behavioral and neural consequences of voluntary high EtOH intake.
[Show abstract][Hide abstract] ABSTRACT: In vivo microdialysis, behavioral activity assessments, and a conditioned place preference (CPP) test were used to investigate dopaminergic correlates of cocaine-conditioned behaviors. Over 12 days, rats were given either intravenous cocaine (4.2 mg/kg) or saline (6 cocaine and 6 saline infusions) daily in distinctively different environments. The following day, rats were tested in the cocaine- and saline-paired environments; 48 hr later, CPP was determined. The cocaine-associated environment elicited greater nucleus accumbens dopamine (NAcc DA) levels, hyperactivity, and place preference, though the emergence of DA increases was not in synchrony with peak behavioral activation. Although conditioned behavioral effects after repeated cocaine are well documented, direct evidence of increased NAcc DA in response to a cocaine-paired environment has not been previously reported. Discrepancies with previous work are attributed to a number of methodological differences.
[Show abstract][Hide abstract] ABSTRACT: In two conditioning experiments, identical procedures (previously shown to produce place preferences for a cocaine-paired environment) were used to assess dopaminergic and behavioral activity correlates of cocaine reward conditioning and sensitization. In these experiments, animals received repeated injections of intravenous cocaine (4.2 mg/kgx6) or saline (0.2 mlx6) on alternating days. One group in each of these experiments ('Cocaine Cues') occupied a consistent distinctive environment during cocaine treatments and testing sessions. For the other conditioned group ('Novel'), all procedures were the same, except that the last cocaine injection was administered while animals were occupying a novel environment. During day 1 and day 6 of the cocaine treatment, behavioral activity was assessed in experiment 1 and in vivo microdialysis procedures were conducted in experiment 2. Over the course of the conditioning sessions, cocaine-induced behavioral activity (locomotion and rearing) increased significantly in the Cocaine Cues group, but not in the Novel group. In addition, cocaine-induced increases in NAcc dopamine levels were significantly greater when cocaine-experienced animals were tested in a cocaine-paired environment compared to equally experienced and cocaine-naive animals tested in a novel environment. Context-dependent behavioral sensitization is a well-documented phenomenon. The observation of a corresponding enhancement of dopamine efflux in lieu of a lengthy withdrawal period is uncommon, but can be attributed to methodological differences across studies. The present study uniquely demonstrates concurrent context-dependent potentiation of behavioral and dopaminergic responses to cocaine occurring in conjunction with cocaine reward.
Brain Research 05/2000; 862(1-2):49-58. DOI:10.1016/S0006-8993(00)02091-6 · 2.84 Impact Factor