Prefrontal glutamate release into the core of the nucleus accumbens mediates cocaine-induced reinstatement of drug-seeking behavior

Extinction circuits for fear and addiction overlap in prefrontal cortex

Extinction is a form of inhibitory learning that suppresses a previously conditioned response. Both fear and drug seeking are conditioned responses that can lead to maladaptive behavior when expressed inappropriately, manifesting as anxiety disorders and addiction, respectively. Recent evidence indicates that the medial prefrontal cortex (mPFC) is critical for the extinction of both fear and drug-seeking behaviors. Moreover, a dorsal-ventral distinction is apparent within the mPFC, such that the prelimbic (PL-mPFC) cortex drives the expression of fear and drug seeking, whereas the infralimbic (IL-mPFC) cortex suppresses these behaviors after extinction. For conditioned fear, the dorsal-ventral dichotomy is accomplished via divergent projections to different subregions of the amygdala, whereas for drug seeking, it is accomplished via divergent projections to the subregions of the nucleus accumbens. Given that the mPFC represents a common node in the extinction circuit for these behaviors, treatments that target this region may help alleviate symptoms of both anxiety and addictive disorders by enhancing extinction memory.

Prefrontal cortex AMPA receptor plasticity is crucial for cue-induced relapse to heroin-seeking

Associative learning processes have an important role in the initiation and persistence of heroin-seeking. Here we show in a rat self-administration model that reexposure to cues previously associated with heroin results in downregulation of AMPA receptor subunit GluR2 and concomitant upregulation of clathrin-coat assembly protein AP2ml in synaptic membranes of the medial prefrontal cortex (mPFC). Reduced AMPA receptor expression in synaptic membranes was associated with a decreased AMPA/NMDA current ratio and increased rectification index in mPFC pyramidal neurons. Systemic or ventral (but not dorsal) mPFC injections of a peptide inhibiting GluR2 endocytosis attenuated both the rectification index and cue-induced relapse to heroin-seeking, without affecting sucrose-seeking. We conclude that GluR2 receptor endocytosis and the resulting synaptic depression in ventral mPFC are crucial for cue-induced relapse to heroin-seeking. As reexposure to conditioned stimuli is a major cause for heroin relapse, inhibition of GluR2 endocytosis may provide a new target for the treatment of heroin addiction.

The Roman high- and low-avoidance rat lines differ in the acquisition, maintenance, extinction, and reinstatement of intravenous cocaine self-administration

The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats for, respectively, rapid vs extremely poor acquisition of avoidant behavior in a shuttlebox has produced two phenotypes that differ in temperament traits, in mesocortical/mesolimbic dopamine system function, and in the behavioral and neurochemical responses to the acute and repeated administration of psychostimulants and opiates. The phenotypic traits of the RHA line predict higher susceptibility, compared with RLA rats, to the reinforcing properties of addictive substances like cocaine. The present study was designed to compare the acquisition, maintenance, reinstatement of drug-seeking after long-term extinction, and reacquisition of intravenous cocaine self-administration (SA) behavior in the Roman lines. Compared with RLA rats, the rates of responding during cocaine SA acquisition were higher, extinction from cocaine SA was prolonged, and drug-induced reinstatement of cocaine-seeking behavior was more robust in RHA rats. Moreover, only RHA rats reacquired extinguished lever-pressing activity when a low reinforcing dose of cocaine was available. These findings are consistent with the view that subjects with genetically determined high responsiveness to the acute and chronic (ie, sensitizing) effects of psychostimulants, such as RHA rats, also display a higher propensity to self -administer cocaine. Further comparative studies in the Roman lines, using SA paradigms that distinguish mere drug-taking from the compulsive and uncontrolled drug use that characterizes addiction in humans, may eventually help to characterize the relationships among genotype, temperament traits, and neurobiological mechanisms involved in the individual vulnerability to cocaine addiction.

mGluR5 antagonism attenuates methamphetamine reinforcement and prevents reinstatement of methamphetamine-seeking behavior in rats

Addiction to methamphetamine is a significant public health problem, and there are currently no pharmacological agents that are approved for the treatment of addiction to this powerful psychostimulant. Chronic methamphetamine use leads to cognitive dysfunction as well as numerous psychiatric, neurological, and cardiovascular complications. There is a growing body of literature implicating an important role for glutamate neurotransmission in psychostimulant addiction. In the present study, we examined the effects of the selective type 5 metabotropic glutamate receptor (mGluR5) antagonist 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP) on intravenous self-administration of methamphetamine and reinstatement of methamphetamine-seeking behavior. Adult male Sprague-Dawley rats were trained to respond for intravenous methamphetamine (0.1 or 0.2 mg/kg per infusion) or food pellets and were subsequently administered vehicle or MTEP (0.3-3 mg/kg) before drug or food self-administration on a fixed-ratio 1 (FR1) schedule of reinforcement or a progressive ratio (PR) schedule of reinforcement. We also examined the effects of vehicle or MTEP (0.3-3 mg/kg) on cue- and drug-induced reinstatement of methamphetamine-seeking behavior as well as cue-induced reinstatement of food-seeking behavior. Our results show that MTEP dose dependently reduced the reinforcing effects of methamphetamine under FR1 and PR schedules of reinforcement without altering overall responding for food. MTEP also dose dependently prevented cue- and drug-induced reinstatement of methamphetamine-seeking behavior, but did not alter cue-induced reinstatement of food-seeking behavior. Together, these results indicate that mGluR5 receptors mediate methamphetamine reinforcement and methamphetamine-seeking behavior, and that pharmacological inhibitors of mGluR5 receptor function may represent a novel class of potential therapeutic agents for the treatment of methamphetamine addiction.

Region-specific alterations in glutamate receptor expression and subcellular distribution following extinction of cocaine self-administration

A role for glutamatergic signaling in the nucleus accumbens (NA) in both the expression and extinction of cocaine seeking has been suggested. The effects of extinction following cocaine self-administration on the expression and synaptosomal distribution of GluR1 and NMDAR1 glutamate receptor subunits in the NA shell and core and the dorsolateral striatum were examined. Rats self-administered cocaine or had access to saline for 14 days followed by a period of extinction training, home-cage exposure, or placement in the self-administration chambers with levers retracted in the absence of discrete cues. Self-administration followed by home-cage exposure reduced GluR1 expression in the NA shell and NMDAR1 expression in the dorsolateral striatum without affecting expression in the NA core. These effects were not observed following extinction. Extinction training increased synaptosomal GluR1 in the NA shell and core and NMDAR1 in the dorsolateral striatum while decreasing synaptosomal NMDAR1 in the shell. Extinction but not home-cage exposure was associated with altered expression and synaptosomal content of the scaffolding proteins PICK1 and PSD95.Following extinction, synaptosomal PICK1 decreased in the dorsolateral striatum while total PICK1 expression was increased in the shell. The synaptosomal PSD95 was decreased in the NA shell, while total PSD95 expression was increased in the core. These data suggest that extinguished cocaine seeking is associated with changes in GluR1 and NMDAR1 expression and subcellular distribution that are region-specific and consist of both a reversal of cocaine-induced adaptations and emergent extinction-related alterations that include receptor subunit redistribution and may involve alterations in scaffolding proteins.

Dopamine modulates an mGluR5-mediated depolarization underlying prefrontal persistent activity

The intrinsic properties of neurons that enable them to maintain depolarized, persistently activated states in the absence of sustained input are poorly understood. In short-term memory tasks, individual prefrontal cortical (PFC) neurons can maintain persistent action potential output during delay periods between informative cues and behavioral responses. Dopamine and drugs of abuse alter PFC function and working memory, possibly by modulating intrinsic neuronal properties. Here we used patch-clamp recording of layer 5 PFC pyramidal neurons to identify a postsynaptic depolarization that was evoked by action potential bursts and mediated by metabotropic glutamate receptor 5 (mGluR5). This depolarization occurred in the absence of recurrent synaptic activity and was reduced by a dopamine D1 receptor (D1R) protein kinase A pathway. After behavioral sensitization to cocaine, the depolarization was substantially diminished and D1R modulation was lost. We propose that burst-evoked intrinsic depolarization is a form of short-term cellular memory that is modulated by dopamine and cocaine experience.

N-Acetylcysteine reverses cocaine-induced metaplasticity

Cocaine addiction is characterized by an impaired ability to develop adaptive behaviors that can compete with cocaine seeking, implying a deficit in the ability to induce plasticity in cortico-accumbens circuitry critical for regulating motivated behavior. RWe found that rats withdrawn from cocaine self-administration had a marked in vivo deficit in the ability to develop long-term potentation (LTP) and depression (LTD) in the nucleus accumbens core subregion following stimulation of prefrontal cortex. N-acetylcysteine treatment prevents relapse in animal models and craving in humans by activating cystine-glutamate exchange and thereby stimulating extrasynaptic metabotropic glutamate receptors (mGluR). N-acetylcysteine treatment restored the ability to induce LTP and LTD by indirectly stimulating mGluR2/3 and mGluR5, respectively. Cocaine self-administration induces metaplasticity that inhibits the further induction of synaptic plasticity, and this impairment can be reversed by N-acetylcysteine, a drug that also prevents relapse.

Targeting Homer genes using adeno-associated viral vector: lessons learned from behavioural and neurochemical studies

Over a decade of in-vitro data support a critical role for members of the Homer family of postsynaptic scaffolding proteins in regulating the functional architecture of glutamate synapses. Earlier studies of Homer knockout mice indicated a necessary role for Homer gene products in normal mesocorticolimbic glutamate transmission and behaviours associated therewith. The advent of adeno-associated viral vectors carrying cDNA for, or short hairpin RNA against, specific Homer isoforms enabled the site-directed targeting of Homers to neurons in the brain. This approach has allowed our groups to address developmental issues associated with conventional knockout mice, to confirm active roles for distinct Homer isoforms in regulating glutamate transmission in vivo, as well as in mediating a variety of behavioural processes. This review summarizes the existing data derived from our studies using adeno-associated viral vector-mediated neuronal targeting of Homer in rodents, implicating this family of proteins in drug and alcohol addiction, learning/memory and emotional processing.

Role of the glutamatergic system in nicotine dependence : implications for the discovery and development of new pharmacological smoking cessation therapies

Preclinical research findings in laboratory animals indicate that the glutamatergic system is critically involved in nicotine dependence. In animals, compounds that decrease glutamatergic neurotransmission, such as antagonists at postsynaptic NMDA receptors, antagonists at excitatory postsynaptic metabotropic glutamate (mGlu) 5 receptors, or agonists at inhibitory presynaptic mGlu(2) and mGlu(3) receptors, decreased nicotine self-administration or reinstatement of nicotine-seeking behaviour. These findings suggest that medications that decrease glutamatergic transmission overall may reduce the reinforcing effects of tobacco smoking and prevent relapse to tobacco smoking in humans. Furthermore, compounds that increase glutamate release, such as antagonists at mGlu(2) and mGlu(3) receptors, ameliorated reward deficits associated with nicotine withdrawal in animals, and thus may alleviate the depression-like symptoms associated with nicotine withdrawal in humans. Animal studies also showed that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors did not appear to be involved in mediating the primary reinforcing effects of nicotine but that they may be involved in the development of nicotine dependence and withdrawal.Taken together, the preclinical data indicate that different glutamatergic receptors are involved in the mediation of different aspects of nicotine dependence. These findings have implications for the discovery and development of new pharmacotherapies that target the glutamatergic system to aid in smoking cessation. At present, very few clinical studies have addressed the effects of glutamatergic compounds on cigarette smoking. Clinical studies involving compounds that have actions at ionotropic glutamate receptors are briefly discussed in this review and suggest the potential of glutamatergic compounds as pharmacotherapies to aid in smoking cessation. Medications that target mGlu receptors have recently been tested in human phase II trials for various indications; however, the potential of these mGlu compounds as medications for nicotine dependence remains to be evaluated in humans. The preclinical data evaluated in this review indicate that such clinical trials for smoking cessation with mGlu compounds are clearly warranted and may reveal novel treatments for nicotine dependence.

Distinct contributions of dopamine receptors in the nucleus accumbens core or shell to established cocaine reinforcement under a second-order schedule

Nucleus accumbens dopamine is implicated in the primary and conditioned reinforcing properties of abused drugs. In the present study, specific impairments in responding for intravenous cocaine (0.3 mg/inf/0.1 ml/5 s) under a fixed-ratio 1 (FR-1) or second-order schedule (FI 15 min (FR10:S)) were investigated following infusion of the dopamine antagonist, alpha-flupenthixol, into either the nucleus accumbens core or shell. Infusion of alpha-flupenthixol into the core decreased cocaine intake under the FR-1 and second-order schedules. By comparison, blockade of nucleus accumbens shell dopamine receptors increased cocaine intake under the FR-1 schedule. Under the second-order schedule, cocaine intake and the number of responses was decreased. Effects on responding were more apparent after self-administered cocaine, when impairments in the latency to receive cocaine infusions were no longer evident. These results are discussed with reference to a role for nucleus accumbens shell dopamine in instrumental responding, and a role of nucleus accumbens core dopamine in incentive motivation, perhaps under the control of contextual stimuli.

Computational model of extracellular glutamate in the nucleus accumbens incorporates neuroadaptations by chronic cocaine

Chronic cocaine administration causes instability in extracellular glutamate in the nucleus accumbens that is thought to contribute to the vulnerability to relapse. A computational framework was developed to model glutamate in the extracellular space, including synaptic and nonsynaptic glutamate release, glutamate elimination by glutamate transporters and diffusion, and negative feedback on synaptic release via metabotropic glutamate receptors (mGluR2/3). This framework was used to optimize the geometry of the glial sheath surrounding excitatory synapses, and by inserting physiological values, accounted for known stable extracellular, extrasynaptic concentrations of glutamate measured by microdialysis and glutamatergic tone on mGluR2/3. By using experimental values for cocaine-induced reductions in cystine-glutamate exchange and mGluR2/3 signaling, and by predicting the down-regulation of glutamate transporters, the computational model successfully represented the experimentally observed increase in glutamate that is seen in rats during cocaine-seeking. This model provides a mathematical framework for describing how pharmacological or pathological conditions influence glutamate transmission measured by microdialysis.

Review. Psychological and neural mechanisms of relapse

Relapse, the resumption of drug taking after periods of abstinence, remains the major problem for the treatment of addiction. Even when drugs are unavailable for long periods or when users are successful in curbing their drug use for extended periods, individuals remain vulnerable to events that precipitate relapse. Behavioural studies in humans and laboratory animals show that drug-related stimuli, drugs themselves and stressors are powerful events for the precipitation of relapse. Molecular, neurochemical and anatomical studies have identified lasting neural changes that arise from mere exposure to drugs and other enduring changes that arise from learning about the relationship between drug-related stimuli and drug effects. Chronic drug exposure increases sensitivity of some systems of the brain to the effects of drugs and stressful events. These changes, combined with those underlying conditioning and learning, perpetuate vulnerability to drug-related stimuli. Circuits of the brain involved are those of the mesocorticolimbic dopaminergic system and its glutamatergic connections, and the corticotropin-releasing factor and noradrenergic systems of the limbic brain. This paper reviews advances in our understanding of how these systems mediate the effects of events that precipitate relapse and of how lasting changes in these systems can perpetuate vulnerability to relapse.

Extracellular amino acid levels in the striatum of the dt(sz) mutant, a model of paroxysmal dystonia

The pathophysiology of idiopathic dystonia is still unknown, but it is regarded as a basal ganglia disorder. Previous studies indicated an involvement of a striatal GABAergic disinhibition and a cortico-striatal glutamatergic overactivity in the manifestation of stress-inducible dystonic episodes in the dt(sz) hamster, a model of idiopathic paroxysmal dystonia. These investigations were carried out postmortem or in anesthetized animals. In the present study, in vivo microdialysis in conscious, freely-moving dt(sz) and non-dystonic control hamsters was used to examine the levels of GABA, aspartate, glutamate, glutamine, glycine and taurine in each animal during following conditions: (1) at baseline in the absence of dystonia, (2) during an episode of paroxysmal dystonia precipitated by stressful stimuli, (3) during a recovery period and (4) at baseline after complete recovery. In comparison to non-dystonic controls, which were treated in the same manner as the dystonic animals, no differences could be detected under basal conditions. The induction of a dystonic episode in mutant hamsters led to higher contents of glycine in these animals in comparison to stressed but non-dystonic controls. Significant changes of glycine levels within the animal groups were not detected. The levels of the excitatory amino acids glutamate, glutamine and aspartate as well as the levels of the inhibitory amino acids GABA and taurine did not differ between the animal groups or between the periods of measurement. The higher levels of glycine might contribute to the manifestation of paroxysmal dystonia in dt(sz) hamsters, although unaltered glutamate, glutamine and aspartate levels do not support the hypothesis of a critical involvement of a cortico-striatal overactivity. It seems that a deficiency of GABAergic interneurons, found by previous immunohistochemical examinations, does not lead to reduced extracellular GABA levels in the striatum.

Proton magnetic resonance spectroscopy in alcohol use disorders: a potential new endophenotype?

BACKGROUND

Current effort is directed at defining new classification schemes for alcohol use disorders (AUD) based on genetic/biological, physiological, and behavioral endophenotypes.

METHODS

We describe briefly findings of in vivo brain proton magnetic resonance spectroscopy ((1)H MRS) studies in AUD and propose that they be further explored and expanded regarding their value as a potential endophenotype for AUD.

RESULTS

In vivo (1)H MRS, as part of the emerging field of "imaging genomics," may provide readily accessible, objective, functionally significant and region-specific neurobiological measures that successfully link genotypes to neurocognition and to psychiatric symptomatology in relatively small patient cohorts. We discuss several functional gene variants that may affect specific (1)H MRS-detectable metabolites and provide recent data from our own work that supports the view of genetic effects on metabolite measures.

CONCLUSIONS

MRS-genetics research will not only offer clues to the functional significance and downstream effects of genetic differences in AUD, but, via monitoring and/or predicting the efficacy of pharmacological and behavioral interventions as a function of genotype, has the potential to influence future clinical management of AUD.

Effects of chronic buprenorphine treatment on levels of nucleus accumbens glutamate and on the expression of cocaine-induced behavioral sensitization in rats

RATIONALE

Chronic treatment with the mu-opioid receptor agonist, buprenorphine, reduces cocaine-induced behaviors in rats with a history of cocaine self-administration. The mechanisms underlying these actions of buprenorphine remain unclear.

OBJECTIVES

The objective of this study is to investigate the effects of chronic buprenorphine treatment on cocaine-induced activity and levels of glutamate and dopamine (DA) in the nucleus accumbens (NAc) in rats that were preexposed to cocaine or drug-naïve.

MATERIALS AND METHODS

In experiment 1, basal levels of NAc glutamate were assessed using in vivo microdialysis in cocaine-naïve rats that were treated chronically with buprenorphine (3.0 mg/kg per day) via osmotic minipumps or that underwent sham surgery. In experiment 2, rats were preexposed to seven daily injections of cocaine or saline. After a 12-16-day drug-free period, extracellular levels of NAc glutamate and DA and locomotor activity were assessed simultaneously, before and after an acute injection of cocaine (15 mg/kg, intraperitoneal), in rats under sham and chronic buprenorphine (3.0 mg/kg per day) treatment.

RESULTS

Chronic buprenorphine treatment increased basal levels of glutamate in drug-naïve and cocaine-preexposed rats, blocked the expression of locomotor sensitization to cocaine, and potentiated the NAc DA response to acute cocaine in cocaine-preexposed rats.

CONCLUSIONS

These findings suggest that buprenorphine may block the expression of cocaine sensitization and other cocaine-related behaviors by increasing basal levels of glutamate in the NAc, which would serve to decrease the effectiveness of cocaine or cocaine-associated cues.

Gamma-vinyl GABA inhibits cocaine-triggered reinstatement of drug-seeking behavior in rats by a non-dopaminergic mechanism

Relapse to drug use is a core feature of addiction. Previous studies demonstrate that gamma-vinyl GABA (GVG), an irreversible GABA transaminase inhibitor, attenuates the acute rewarding effects of cocaine and other addictive drugs. We here report that systemic administration of GVG (25-300 mg/kg) dose-dependently inhibits cocaine- or sucrose-induced reinstatement of reward-seeking behavior in rats. In vivo microdialysis data indicated that the same doses of GVG dose-dependently elevate extracellular GABA levels in the nucleus accumbens (NAc). However, GVG, when administered systemically or locally into the NAc, failed to inhibit either basal or cocaine-priming enhanced NAc dopamine in either naïve rats or cocaine extinction rats. These data suggest that: (1) GVG significantly inhibits cocaine- or sucrose-triggered reinstatement of reward-seeking behavior; and (2) a GABAergic-, but not dopaminergic-, dependent mechanism may underlie the antagonism by GVG of cocaine-triggered reinstatement of drug-seeking behavior, at least with respect to GVG's action on the NAc.

Attenuation of cue-induced heroin-seeking behavior by cannabinoid CB1 antagonist infusions into the nucleus accumbens core and prefrontal cortex, but not basolateral amygdala

As with other drugs of abuse, heroin use is characterized by a high incidence of relapse following detoxification that can be triggered by exposure to conditioned stimuli previously associated with drug availability. Recent findings suggest that cannabinoid CB(1) receptors modulate the motivational properties of heroin-conditioned stimuli that induce relapse behavior. However, the neural substrates through which CB(1) receptors modulate cue-induced heroin seeking have not been elucidated. In this study, we evaluated alterations in cue-induced reinstatement of heroin-seeking behavior produced by infusions of the CB(1) receptor antagonist SR 141716A (0, 0.3 and 3 microg per side) delivered into the prefrontal cortex (PFC), nucleus accumbens (NAC), and basolateral amygdala (BLA) of rats. Results show that following extinction of operant behavior the presentation of a discriminative stimulus conditioned to heroin availability reinstated nonreinforced lever pressing to levels comparable to preextinction levels. Intra-PFC SR 141716A dose-dependently reduced cue-induced reinstatement of heroin seeking, with a significant reduction following the 3 microg per side dose. In the NAC, both SR 141716A doses induced a significant reduction in cue-induced reinstatement, with the highest dose completely blocking the effect of the cue. In contrast, intra-BLA SR 141716A did not alter cue-induced reinstatement of responding while systemic administration of this antagonist (3 mg/kg, i.p.) significantly blocked cue-induced reinstatement in all three-placement groups (BLA, PFC, and NAC). These findings provide new insights into the neural mechanisms through which CB(1) receptors modulate the motivational properties of heroin-associated cues inducing relapse.

Silicon Wafer-Based Platinum Microelectrode Array Biosensor for Near Real-Time Measurement of Glutamate in Vivo

Using Micro-Electro-Mechanical-Systems (MEMS) technologies, we have developed silicon wafer-based platinum microelectrode arrays (MEAs) modified with glutamate oxidase (GluOx) for electroenzymatic detection of glutamate in vivo. These MEAs were designed to have optimal spatial resolution for in vivo recordings. Selective detection of glutamate in the presence of the electroactive interferents, dopamine and ascorbic acid, was attained by deposition of polypyrrole and Nafion. The sensors responded to glutamate with a limit of detection under 1muM and a sub-1-second response time in solution. In addition to extensive in vitro characterization, the utility of these MEA glutamate biosensors was also established in vivo. In the anesthetized rat, these MEA glutamate biosensors were used for detection of cortically-evoked glutamate release in the ventral striatum. The MEA biosensors also were applied to the detection of stress-induced glutamate release in the dorsal striatum of the freely-moving rat.

Cocaine but not natural reward self-administration nor passive cocaine infusion produces persistent LTP in the VTA

Persistent drug-seeking behavior is hypothesized to co-opt the brain's natural reward-motivational system. Although ventral tegmental area (VTA) dopamine (DA) neurons represent a crucial component of this system, the synaptic adaptations underlying natural rewards and drug-related motivation have not been fully elucidated. Here, we show that self-administration of cocaine, but not passive cocaine infusions, produced a persistent potentiation of VTA excitatory synapses, which was still present after 3 months abstinence. Further, enhanced synaptic function in VTA was evident even after 3 weeks of extinction training. Food or sucrose self-administration induced only a transient potentiation of VTA glutamatergic signaling. Our data show that synaptic function in VTA DA neurons is readily but reversibly enhanced by natural reward-seeking behavior, while voluntary cocaine self-administration induced a persistent synaptic enhancement that is resistant to behavioral extinction. Such persistent synaptic potentiation in VTA DA neurons may represent a fundamental cellular phenomenon driving pathological drug-seeking behavior.

Blunted cystine-glutamate antiporter function in the nucleus accumbens promotes cocaine-induced drug seeking

Repeated cocaine alters glutamate neurotransmission, in part, by reducing cystine-glutamate exchange via system xc-, which maintains glutamate levels and receptor stimulation in the extrasynaptic compartment. In the present study, we undertook two approaches to determine the significance of plasticity involving system xc-. First, we examined whether the cysteine prodrug N-acetylcysteine attenuates cocaine-primed reinstatement by targeting system xc-. Rats were trained to self-administer cocaine (1 mg/kg/200 microl, i.v.) under extended access conditions (6 h/day). After extinction training, cocaine (10 mg/kg, i.p.) primed reinstatement was assessed in rats pretreated with N-acetylcysteine (0-60 mg/kg, i.p.) in the presence or absence of the system xc- inhibitor (S)-4-carboxyphenylglycine (CPG; 0.5 microM; infused into the nucleus accumbens). N-acetylcysteine attenuated cocaine-primed reinstatement, and this effect was reversed by co-administration of CPG. Secondly, we examined whether reduced system xc- activity is necessary for cocaine-primed reinstatement. To do this, we administered N-acetylcysteine (0 or 90 mg/kg, i.p.) prior to 12 daily self-administration sessions (1 mg/kg/200 microl, i.v.; 6 h/day) since this procedure has previously been shown to prevent reduced activity of system xc-. On the reinstatement test day, we then acutely impaired system xc- in some of the rats by infusing CPG (0.5 microM) into the nucleus accumbens. Rats that had received N-acetylcysteine prior to daily self-administration sessions exhibited diminished cocaine-primed reinstatement; this effect was reversed by infusing the cystine-glutamate exchange inhibitor CPG into the nucleus accumbens. Collectively these data establish system xc- in the nucleus accumbens as a key mechanism contributing to cocaine-primed reinstatement.

A unified framework for addiction: vulnerabilities in the decision process

The understanding of decision-making systems has come together in recent years to form a unified theory of decision-making in the mammalian brain as arising from multiple, interacting systems (a planning system, a habit system, and a situation-recognition system). This unified decision-making system has multiple potential access points through which it can be driven to make maladaptive choices, particularly choices that entail seeking of certain drugs or behaviors. We identify 10 key vulnerabilities in the system: (1) moving away from homeostasis, (2) changing allostatic set points, (3) euphorigenic "reward-like" signals, (4) overvaluation in the planning system, (5) incorrect search of situation-action-outcome relationships, (6) misclassification of situations, (7) overvaluation in the habit system, (8) a mismatch in the balance of the two decision systems, (9) over-fast discounting processes, and (10) changed learning rates. These vulnerabilities provide a taxonomy of potential problems with decision-making systems. Although each vulnerability can drive an agent to return to the addictive choice, each vulnerability also implies a characteristic symptomology. Different drugs, different behaviors, and different individuals are likely to access different vulnerabilities. This has implications for an individual's susceptibility to addiction and the transition to addiction, for the potential for relapse, and for the potential for treatment.

Glutamate transmission in addiction

Cortico-striatal glutamate transmission has been implicated in both the initiation and expression of addiction related behaviors, such as locomotor sensitization and drug-seeking. While glutamate transmission onto dopamine cells in the ventral tegmental area undergoes transient plasticity important for establishing addiction-related behaviors, glutamatergic plasticity in the nucleus accumbens is critical for the expression of these behaviors. This information points to the value of exploring pharmacotherapeutic manipulation of glutamate plasticity in treating drug addiction.

Dissecting motivational circuitry to understand substance abuse

An important goal of cocaine addiction research is to understand the neurobiological mechanisms underlying this disease state. Here, we review studies from our laboratory that examined nucleus accumbens (NAc) cell firing and rapid dopamine signaling using electrophysiological and electrochemical recordings in behaving rodents. A major advantage of these techniques is that they allow for the characterization of NAc activity and rapid dopamine release during specific phases of motivated behavior. Moreover, each approach enables an examination of the dynamic nature of NAc signaling as a function of factors such as hedonics and associative learning. We show that NAc neurons differentially respond to rewarding and aversive stimuli and their predictors in a bivalent manner. This differential responding is modifiable and can be altered by the presentation of other natural rewards or cocaine. Likewise, the dynamic nature of NAc cell firing is also reflected in the differential activation of distinct populations of NAc neurons during goal-directed behaviors for natural versus drug rewards, and the heightened activation of some NAc neurons following cocaine abstinence. Our electrochemical data also show that rapid dopamine signaling in the NAc reflects primary rewards and their predictors and appears to modulate specific NAc neuronal responses. In some cases, these influences are observed in a regionally specific manner that matches previous pharmacological manipulations. Collectively, these findings provide critical insight into the functional organization of the NAc that can be used to guide additional studies aimed at dissecting the neural code underlying compulsive drug-seeking behavior.

Methamphetamine induces chronic corticostriatal depression: too much of a bad thing

Leading theories of drug addiction propose that repeated drug exposure produces a long-lasting homeostatic dysregulation in brain reward processing that is normalized by drug readministration. In this issue of Neuron, Bamford and colleagues describe a novel neurobiological substrate that may contribute to this effect.

Repeated exposure to methamphetamine causes long-lasting presynaptic corticostriatal depression that is renormalized with drug readministration

Addiction-associated behaviors such as drug craving and relapse are hypothesized to result from synaptic changes that persist long after withdrawal and are renormalized by drug reinstatement, although such chronic synaptic effects have not been identified. We report that exposure to the dopamine releaser methamphetamine for 10 days elicits a long-lasting (>4 month) depression at corticostriatal terminals that is reversed by methamphetamine readministration. Both methamphetamine-induced chronic presynaptic depression and the drug's selective renormalization in drug-experienced animals are independent of corresponding long-term changes in synaptic dopamine release but are due to alterations in D1 dopamine and cholinergic receptor systems. These mechanisms might provide a synaptic basis that underlies addiction and habit learning and their long-term maintenance.

Neuroadaptive changes in the mesocortical glutamatergic system during chronic nicotine self-administration and after extinction in rats

Nicotine self-administration causes adaptation in the mesocorticolimbic glutamatergic system, including the up-regulation of ionotropic glutamate receptor subunits. We therefore determined the effects of nicotine self-administration and extinction on NMDA-induced glutamate neurotransmission between the medial prefrontal cortex (mPFC) and ventral tegmental area (VTA). On day 19 of nicotine SA, both regions were microdialyzed for glutamate while mPFC was sequentially perfused with Kreb's Ringer buffer (KRB), 200 microM NMDA, KRB, 500 microM NMDA, KRB, and 100 mM KCl. Basal glutamate levels were unaffected, but nicotine self-administration significantly potentiated mPFC glutamate release to 200 microM NMDA, which was ineffective in controls. Furthermore, in VTA, nicotine self-administration significantly amplified glutamate responses to both mPFC infusions of NMDA. This hyper-responsive glutamate neurotransmission and enhanced glutamate subunit expression were reversed by extinction. Behavioral studies also showed that a microinjection of 2-amino-5-phosphonopentanoic acid (NMDA-R antagonist) into mPFC did not affect nicotine or sucrose self-administration. However, in VTA, NBQX (AMPA-R antagonist) attenuated both nicotine and sucrose self-administration. Collectively, these studies indicate that mesocortical glutamate neurotransmission adapts to chronic nicotine self-administration and VTA AMPA-R may be involved in the maintenance of nicotine self-administration.

Neurotensin in the ventral pallidum increases extracellular gamma-aminobutyric acid and differentially affects cue- and cocaine-primed reinstatement

Cocaine-primed reinstatement is an animal model of drug relapse. The neurocircuitry underlying cocaine-primed reinstatement includes a decrease in GABA in the ventral pallidum (VP) that is inhibited by a mu opioid receptor antagonist, suggesting that opioid peptides colocalized with GABA in the projection from the nucleus accumbens to the VP may mediate this effect. Neurotensin is also colocalized with GABA and has been shown to increase GABA release in several brain regions. Therefore, the present study determined whether neurotensin increases GABA release in the VP, antagonizes cocaine-induced decreases in GABA, and prevents reinstatement of cocaine seeking. In vivo microdialysis revealed that the neurotensin agonist neurotensin peptide fragment 8-13 [NT(8-13)] increased GABA in the VP in a neurotensin receptor and tetrodotoxin-dependent manner and blocked the cocaine-induced decrease in GABA. NT(8-13) (3 nmol) microinjected into the VP prevented cue-induced reinstatement without affecting cocaine self-administration. In contrast, 3 nmol NT(8-13) potentiated cocaine-primed reinstatement. The neurotensin antagonist SR142948 (2-[[[5-(2,6-dimethoxyphenyl)-1-[4-[[[3-(dimethylamino)propyl]methylamino]carbonyl]-2-(1-methylethyl)phenyl]-1H -pyrazol-3-yl]carbonyl]amino]-tricyclo-[3.3.1.13,7]decane-2-carboxylic acid) had no effect on any behavioral measure when infused in the VP at the dose tested but attenuated cocaine-primed reinstatement when administered systemically. In contrast to reinstatement, NT(8-13) did not alter the motor response to acute cocaine or the development of motor sensitization by chronic cocaine. Three conclusions can be drawn from these data: 1) neurotensin promotes GABA release in the VP and correspondingly inhibits cue-induced reinstatement, 2) neurotensin and cocaine interact in a manner that countermands the neurotensin-induced increase in GABA and promotes reinstatement, and 3) endogenous release of neurotensin in the VP is not necessary for reinstatement.

Developmental lead exposure alters methamphetamine self-administration in the male rat: acquisition and reinstatement

The rate of acquisition of drug self-administration and the return to drug seeking are important elements of the overall drug profile, and are essential factors in understanding risks associated with drug abuse. Experiment 1 examined the effects of perinatal (gestation/lactation) lead exposure on adult rates of acquisition of intravenous (i.v.) methamphetamine self-administration. Experiment 2 investigated the effects of perinatal lead exposure on drug-maintained responding in a reinstatement (relapse) paradigm. In Experiment 1, female rats were gavaged daily with 0 or 16 mg lead for 30 days prior to breeding with nonexposed males. Lead exposure continued through gestation and lactation and was discontinued at weaning (postnatal day [PND] 21). Male rats born to control or lead-exposed dams were tested daily as adults in an acquisition paradigm that incorporated both Pavlovian and operant components. An initial 3-h autoshaping period preceded a 3-h self-administration period. For 35 daily training sessions i.v. methamphetamine infusions [inf] (0.02 mg/kg) were paired with the extension and retraction of a lever (autoshaping), while inf occurred during self-administration only when a lever press was executed (FR-1). In Experiment 2 animals developmentally exposed to lead were trained on an FR-2 to self-administer methamphetamine (0.04 mg/kg/inf) and then placed on an extinction schedule prior to receiving intraperitoneal (i.p.) priming injections of saline, 0.50, 1.00, or 1.50 mg/kg methamphetamine. The findings from Experiment 1 showed that acquisition was delayed in rats born to lead-exposed dams gavaged daily with 16 mg lead throughout gestation and lactation when a 0.02-mg/kg/inf of methamphetamine served as the reinforcement outcome. Additional data from Experiment 2 indicated priming cues (injections of methamphetamine [i.p.]) administered after extinction were less likely to occasion a return to drug seeking (relapse) in the 16-mg group relative to the 0-mg control group. These results suggest perinatal lead exposure alters patterns of methamphetamine self-administration during the adult cycle.

Neuroplasticity in the mesolimbic dopamine system and cocaine addiction

The main characteristics of cocaine addiction are compulsive drug use despite adverse consequences and high rates of relapse during periods of abstinence. A current popular hypothesis is that compulsive cocaine use and cocaine relapse is due to drug-induced neuroadaptations in reward-related learning and memory processes, which cause hypersensitivity to cocaine-associated cues, impulsive decision making and abnormal habit-like learned behaviours that are insensitive to adverse consequences. Here, we review results from studies on the effect of cocaine exposure on selected signalling cascades, growth factors and physiological processes previously implicated in neuroplasticity underlying normal learning and memory. These include the extracellular signal-regulated kinase (ERK) signalling pathway, brain-derived neurotrophic factor (BDNF), glutamate transmission, and synaptic plasticity (primarily in the form of long-term potentiation and depression, LTP and LTD). We also discuss the degree to which these cocaine-induced neuroplasticity changes in the mesolimbic dopamine system mediate cocaine psychomotor sensitization and cocaine-seeking behaviours, as assessed in animal models of drug addiction. Finally, we speculate on how these factors may interact to initiate and sustain cocaine psychomotor sensitization and cocaine seeking.

Role of GluR1 expression in nucleus accumbens neurons in cocaine sensitization and cocaine-seeking behavior

Chronic cocaine use reduces glutamate levels in the nucleus accumbens (NAc), and is associated with experience-dependent changes in (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) glutamate receptor membrane expression in NAc neurons. These changes accompany behavioral sensitization to cocaine and increased susceptibility to cocaine relapse. The functional relationship between neuroplasticity in AMPA receptors and the behavioral manifestation of cocaine addiction remains unclear. Thus, we examined the behavioral effects of up- and downregulating basal AMPA receptor function in the NAc core and shell using viral-mediated gene transfer of wild-type glutamate receptor 1 (wt-GluR1) or a dominant-negative pore-dead GluR1 (pd-GluR1), respectively. Transient increases in wt-GluR1 during or after cocaine treatments diminished the development of cocaine sensitization, while pd-GluR1 expression exacerbated cocaine sensitization. Parallel changes were found in D2, but not D1, receptor-mediated behavioral responses. As a correlate of the sensitization experiments, we overexpressed wt- or pd-GluR1 in the NAc core during cocaine self-administration, and tested the effects on subsequent drug-seeking behavior 3 weeks after overexpression declined. wt-GluR1 overexpression during self-administration had no effect on cocaine intake, but subsequently reduced cocaine seeking in extinction and cocaine-induced reinstatement, whereas pd-GluR1 facilitated cocaine-induced reinstatement. When overexpressed during reinstatement tests, wt-GluR1 directly attenuated cocaine- and D2 agonist-induced reinstatement, while pd-GluR1 enhanced reinstatement. In both experimental procedures, neither wt- nor pd-GluR1 expression affected cue-induced reinstatement. Together, these results suggest that degrading basal AMPA receptor function in NAc neurons is sufficient to facilitate relapse via sensitization in D2 receptor responses, whereas elevating basal AMPA receptor function attenuates these behaviors.

Cocaine and amphetamine-like psychostimulants: neurocircuitry and glutamate neuroplasticity

Although the pharmacology of amphetamine-like psychostimulants at dopamine transporters is well understood, addiction to this class of drugs has proven difficult to deal with. The reason for this disconnection is that while the molecular mechanism of amphetamine action is critical to reinforce drug use, it is only the first step in a sequence of widespread neuroplastic events in brain circuitry. This review outlines the affect of psychostimulants on mesocorticolimbic dopamine projections that mediate their reinforcing effect, and how this action ultimately leads to enduring pathological neuroplasticity in glutamatergic projections from the prefrontal cortex to the nucleus accumbens. Molecular neuroadaptations induced by psychostimulant abuse are described in glutamate neurotransmission, and from this information potential pharmacotherapeutic targets are identified, based upon reversing or countermanding psychostimulant-induced neuroplasticity.

The glutamatergic projection from the prefrontal cortex to the nucleus accumbens core is required for cocaine-induced decreases in ventral pallidal GABA

Cocaine-primed reinstatement of drug seeking is associated with a decrease in extracellular GABA in the ventral pallidum (VP). The present study investigated the neural mechanism of this cocaine-induced decrease in VP GABA by determining if activity of the glutamatergic projection from the medial prefrontal cortex (PFC) to the nucleus accumbens is required for the effect. Microdialysis was performed to measure extracellular GABA in the VP while simultaneously, either a combination of the GABA agonists baclofen and muscimol was microinjected into the PFC, or the AMPA/kainate glutamate receptor antagonist CNQX was microinjected into the accumbens core. Inhibition of the PFC with GABA agonists and blockade of AMPA glutamate receptors in the accumbens core were both sufficient to prevent the cocaine-induced decrease in VP GABA, further implicating increased activity of the cortico-striato-pallidal circuit in relapse to drug seeking.

Opposing roles for the ventral prefrontal cortex and the basolateral amygdala on the spontaneous recovery of cocaine-seeking in rats

RATIONALE

The neural circuitry subserving cocaine-seeking after extinction vs abstinence alone requires different constituent brain structures. Spontaneous recovery of cocaine-seeking, a model, which incorporates both extinction and abstinence, depends on an unknown neural circuit.

OBJECTIVES

The present study examined the hypothesis that the spontaneous recovery of cocaine-seeking would require overlapping but distinct neural circuits compared to models that incorporate either extinction or abstinence alone.

MATERIAL AND METHODS

Rats were trained to self-administer cocaine (0.2 mg/inf), then responding on the cocaine-paired lever was extinguished, followed by an additional period of abstinence in the home cage. Finally, rats were returned to the self-administration context for a test of spontaneous recovery (SR TEST). Just before the SR TEST, discrete brain regions were inactivated with a GABA agonist cocktail (1 mM baclofen + 0.1mM muscimol) to determine the relative importance of these brain regions in the spontaneous recovery of cocaine-seeking.

RESULTS

The inactivation of the ventromedial prefrontal cortex (vPFC) enhanced cocaine-seeking, whereas the inactivation of the basolateral amygdala (BLA) attenuated spontaneous recovery. Inactivation of the nucleus accumbens core (Core) resembled the effects of BLA inactivation, but these results were confounded by an inhibitory effect of the vehicle treatment. Finally, the spontaneous recovery of cocaine-seeking was unaltered by manipulations of the dorsomedial prefrontal cortex (dPFC) and the nucleus accumbens shell (Shell).

CONCLUSIONS

The neural circuitry subserving cocaine-seeking behavior in a spontaneous recovery model requires the BLA and possibly the Core, like extinction models. In addition, this behavior is subject to regulation by vPFC, in a manner functionally opposite to that of the BLA.

Subcellular and subsynaptic localization of group I metabotropic glutamate receptors in the nucleus accumbens of cocaine-treated rats

There is significant pharmacological and behavioral evidence that group I metabotropic glutamate receptors (mGluR1a and mGluR5) in the nucleus accumbens play an important role in the neurochemical and pathophysiological mechanisms that underlie addiction to psychostimulants. To further address this issue, we undertook a detailed ultrastructural analysis to characterize changes in the subcellular and subsynaptic localization of mGluR1a and mGluR5 in the core and shell of nucleus accumbens following acute or chronic cocaine administration in rats. After a single cocaine injection (30 mg/kg) and 45 min withdrawal, there was a significant decrease in the proportion of plasma membrane-bound mGluR1a in accumbens shell dendrites. Similarly, the proportion of plasma membrane-bound mGluR1a was decreased in large dendrites of accumbens core neurons following chronic cocaine exposure (i.e. 1-week treatment followed by 3-week withdrawal). However, neither acute nor chronic cocaine treatments induced significant change in the localization of mGluR5 in accumbens core and shell, which is in contrast with the significant reduction of plasma membrane-bound mGluR1a and mGluR5 induced by local intra-accumbens administration of the group I mGluR agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG). In conclusion, these findings demonstrate that cocaine-induced glutamate imbalance has modest effects on the trafficking of group I mGluRs in the nucleus accumbens. These results provide valuable information on the neuroadaptive mechanisms of accumbens group I mGluRs in response to cocaine administration.

The neurocircuitry of addiction: an overview

Drug addiction presents as a chronic relapsing disorder characterized by persistent drug-seeking and drug-taking behaviours. Given the significant detrimental effects of this disease both socially and economically, a considerable amount of research has been dedicated to understanding a number of issues in addiction, including behavioural and neuropharmacological factors that contribute to the development, loss of control and persistence of compulsive addictive behaviours. In this review, we will give a broad overview of various theories of addiction, animal models of addiction and relapse, drugs of abuse, and the neurobiology of drug dependence and relapse. Although drugs of abuse possess diverse neuropharmacological profiles, activation of the mesocorticolimbic system, particularly the ventral tegmental area, nucleus accumbens, amygdala and prefrontal cortex via dopaminergic and glutamatergic pathways, constitutes a common pathway by which various drugs of abuse mediate their acute reinforcing effects. However, long-term neuroadaptations in this circuitry likely underlie the transition to drug dependence and cycles of relapse. As further elucidated in more comprehensive reviews of various subtopics on addiction in later sections of this special issue, it is anticipated that continued basic neuroscience research will aid in the development of effective therapeutic interventions for the long-term treatment of drug-dependent individuals.

Acute amphetamine exposure selectively desensitizes kappa-opioid receptors in the nucleus accumbens

In the present study, we investigated the effects of psychostimulant exposure on kappa-opioid peptide (KOP) receptor signaling in the rat mesolimbic system. A single subcutaneous (s.c.) injection of amphetamine (2.5 mg/kg) reduced the KOP receptor-mediated inhibition of glutamate release in the nucleus accumbens shell, as a consequence of KOP receptor desensitization. This effect was blocked by dopamine (DA) receptor antagonists or the nonselective opioid antagonist, naltrexone (1 mg/kg, s.c.), and mimicked by the KOP receptor agonists U69593 (0.32 mg/kg, s.c.) and dynorphin (1 microM), indicating that an amphetamine-induced release of dynorphin is producing a long-lasting desensitization of the KOP receptor. Despite the fact that amphetamine also increases dynorphin release in the ventral tegmental area (VTA), KOP receptor function in this region was not affected by amphetamine; there was no difference in the KOP receptor-mediated change in firing rate or resting membrane potential measured in VTA neurons from saline- or amphetamine-treated animals. This study demonstrates that amphetamine can produce regionally selective adaptations in KOP receptor signaling, which may, in turn, alter the effects of subsequent drug exposure.

Prenatal stress enhances responsiveness to cocaine

Early environmental events have profound influences on a wide range of adult behavior. In the current study, we assessed the influence of maternal stress during gestation on psychostimulant and neurochemical responsiveness to cocaine, cocaine self-administration, and reinstatement of cocaine-seeking in adult offspring. Pregnant, female Sprague-Dawley rats were subjected to either no treatment or to restraint stress three times per day for the last 7 days of gestation and cocaine-related behavior was assessed in offspring at 10 weeks of age. Relative to controls, a noncontingent cocaine injection elevated locomotor activity as well as nucleus accumbens levels of extracellular dopamine and glutamate to a greater extent in both cocaine-naive and cocaine-experienced prenatal stress (PNS) rats and elevated prefrontal cortex dopamine in cocaine-experienced PNS rats. To assess the impact of PNS on cocaine addiction-related behavior, rats were trained to lever press for intravenous (i.v.) infusions of cocaine (0.25, 0.5, or 1 mg/kg/infusion), with each infusion paired with a light+tone-conditioned stimulus. Lever-pressing was extinguished and cocaine-seeking reinstated by re-exposure to the conditioned cues or by intraperitoneal cocaine-priming injections (5 or 10 mg/kg). PNS elevated active lever responding both during extinction and cocaine-primed reinstatement, but not during self-administration or conditioned-cued reinstatement. PNS also did not alter intake during self-administration. These findings demonstrate that PNS produces enduring nervous system alterations that increase the psychomotor stimulant, motivational, and neurochemical responsiveness to noncontingent cocaine. Thus, early environmental factors contribute to an individual's initial responsiveness to cocaine and propensity to relapse to cocaine-seeking.

CaMKII: a biochemical bridge linking accumbens dopamine and glutamate systems in cocaine seeking

Increases in dopamine and glutamate transmission in the nucleus accumbens independently promote the reinstatement of cocaine seeking, an animal model of relapse. Here we have tested whether cocaine reinstatement in rats depends on interactions between accumbal dopamine and glutamate systems that are mediated by Ca(2+)/calmodulin-mediated kinase II (CaMKII). We show that stimulation of D1-like dopamine receptors in the nucleus accumbens shell reinstates cocaine seeking by activating L-type Ca(2+) channels and CaMKII. Cocaine reinstatement is associated with D1-like dopamine receptor-dependent increases in accumbens shell CaMKII phosphorylated on Thr286 and glutamate receptor 1 (GluR1) phosphorylated on Ser831 (a known CaMKII phosphorylation site), in addition to increases in cell-surface expression of GluR1-containing AMPA receptors in the shell. Consistent with these findings, cocaine reinstatement is attenuated by intra-shell administration of AAV10-GluR1-C99, a vector that impairs the transport of GluR1-containing AMPA receptors. Thus, CaMKII may be an essential link between accumbens shell dopamine and glutamate systems involved in the neuronal plasticity underlying cocaine craving and relapse.

Effects of adolescent exposure to cocaine on locomotor activity and extracellular dopamine and glutamate levels in nucleus accumbens of DBA/2J mice

Adolescents differ from adults in their acute sensitivity to several drugs of abuse, but little is known about the long-term neurobehavioral effects of adolescent drug exposure. To explore this further, we evaluated the locomotor responses to repeated cocaine administration in adolescent and adult male DBA/2J mice and alterations in extracellular levels of dopamine (DA) and glutamate (GLU) in the nucleus accumbens (NAc) in response to a subsequent cocaine challenge. Adolescent and adult mice were treated daily with saline or cocaine (10 mg/kg, i.p) for 9 consecutive days. Ten days following the last injection, animals were implanted with microdialysis probes and 24 h later microdialysis samples were collected before and after an acute cocaine challenge. Adolescents but not adults demonstrated development of behavioral sensitization to cocaine. Microdialysis procedures revealed that cocaine-treated mice displayed greater peak increases in extracellular DA in response to a subsequent cocaine challenge as compared to saline-treated mice, in contrast with lower peak increases in extracellular GLU. While adults exhibited greater peaks in extracellular DA in response to cocaine than adolescents did, adolescent mice presented a more rapid onset of peak extracellular DA levels than adults. Our results indicate differences in the behavioral and neurochemical responses to cocaine in adolescent versus adult mice, which may be relevant to the increased risk of developing addiction in humans who are exposed to drugs of abuse during adolescence.

Glutamate and aspartate levels in the nucleus accumbens during cocaine self-administration and extinction: a time course microdialysis study

RATIONALE

Accumbal excitatory amino acid (EAA) transmission has been implicated in cocaine addiction. However, the time course effects of extinction of cocaine self-administration on EAAs are unknown.

OBJECTIVES

The objective of this study was to define the time course of cocaine self-administration and extinction effects on glutamate and aspartate levels in the nucleus accumbens.

MATERIALS AND METHODS

Rats were trained to self-administer cocaine for 20 days, and the levels of extracellular glutamate and aspartate were measured by in vivo microdialysis both during cocaine self-administration and after a priming cocaine injection at different time points after extinction (1, 5, or 10 days). A food-reinforced control group was also included in this study. Furthermore, the effect of acute i.v. cocaine administration (0, 1, 2, or 4 mg/kg) on glutamate and aspartate levels was also evaluated.

RESULTS

At any of the dose tested, acute i.v. cocaine did not affect the levels of glutamate or aspartate in the Nacc. In contrast, glutamate levels were reduced in animals trained to self-administer cocaine, although they augmented substantially during a subsequent session of cocaine self-administration, and similar changes were not observed in food-reinforced controls. After 1 or 5, but not after 10 days of extinction, the glutamate levels were also reduced, and the ability of i.v. cocaine priming injections to increase glutamate levels followed a similar time course. These effects were specific, as aspartate levels were not affected by any administration protocol.

CONCLUSIONS

These data suggest that glutamatergic transmission could be involved in the maintenance of cocaine self-administration and in the early phases of abstinence.

Homers regulate drug-induced neuroplasticity: implications for addiction

Drug addiction is a chronic, relapsing disorder, characterized by an uncontrollable motivation to seek and use drugs. Converging clinical and preclinical observations implicate pathologies within the corticolimbic glutamate system in the genetic predisposition to, and the development of, an addicted phenotype. Such observations pose cellular factors regulating glutamate transmission as likely molecular candidates in the etiology of addiction. Members of the Homer family of proteins regulate signal transduction through, and the trafficking of, glutamate receptors, as well as maintain and regulate extracellular glutamate levels in corticolimbic brain regions. This review summarizes the existing data implicating the Homer family of protein in acute behavioral and neurochemical sensitivity to drugs of abuse, the development of drug-induced neuroplasticity, as well as other behavioral and cognitive pathologies associated with an addicted state.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Drug addiction as a pathology of staged neuroplasticity

Using addictive drugs can evolve from controlled social use into the compulsive relapsing disorder that characterizes addiction. This transition to addiction results from genetic, developmental, and sociological vulnerabilities, combined with pharmacologically induced plasticity in brain circuitry that strengthens learned drug-associated behaviors at the expense of adaptive responding for natural rewards. Advances over the last decade have identified the brain circuits most vulnerable to drug-induced changes, as well as many associated molecular and morphological underpinnings. This growing knowledge has contributed to an expanded understanding of how drugs usurp normal learning circuitry to create the pathology of addiction, as evidenced by involuntary activation of reward circuits in response to drug-associated cues and simultaneous reports of drug craving. This new understanding provides unprecedented potential opportunities for novel pharmacotherapeutic targets in treating addiction. There appears to be plasticity associated with the addiction phenomenon in general as well as changes produced by addiction to a specific class of addicting drugs. These findings also provide the basis for the current understanding of addiction as a chronic, relapsing disease of the brain with changes that persist long after the last use of the drug. Here, we describe the neuroplasticity in brain circuits and cell function induced by addictive drugs that is thought to underlie the compulsions to resume drug-taking, and discuss how this knowledge is impelling exploration and testing of novel addiction therapies.

Cross-sensitization of the reinforcing effects of cocaine and amphetamine in rats

RATIONALE

Cross-sensitization between cocaine and amphetamine has been demonstrated in different behavioral paradigms. There is a relative paucity of studies examining whether cross-sensitization occurs between amphetamine and cocaine when both are self-administered.

OBJECTIVE

The current study was designed to test whether animals sensitized to the reinforcing effects of cocaine would show cross-sensitization of the reinforcing effects of amphetamine, using a self-administration paradigm.

MATERIALS AND METHODS

Male, Sprague-Dawley rats were trained to self-administer cocaine and given limited or high exposure to cocaine under a fixed ratio (FR) 1 procedure. After the initial exposure to cocaine, animals self-administered cocaine (1.5 mg/kg per injection) under a progressive ratio (PR) procedure. Subsequently, breakpoints on a PR schedule and rates of intake on an FR schedule maintained by different doses of amphetamine were assessed.

RESULTS

Animals with high initial exposure to cocaine (40 injections of 1.5 mg/kg per injection per day for 5 days) showed stable breakpoints throughout testing. Animals given limited initial cocaine exposure (20 injections of 0.75 mg/kg per injection for 1 day) produced a gradual increase in breakpoints maintained by cocaine over time (i.e., sensitization of the reinforcing effects of cocaine). When subsequently tested with amphetamine, the dose-effect curve was shifted upward in the limited-exposure group relative to the high-exposure group, suggesting cross-sensitization of the reinforcing effects of amphetamine.

CONCLUSIONS

Sensitization of the reinforcing effects of cocaine resulted in cross-sensitization of the reinforcing effects of amphetamine. This phenomenon occurs even when both drugs are self-administered.

Acamprosate attenuates cocaine- and cue-induced reinstatement of cocaine-seeking behavior in rats

RATIONALE

Acamprosate (calcium acetylhomotaurinate) is a glutamatergic neuromodulator used for the treatment of alcoholism, but its potential efficacy in the treatment of psychostimulant addiction has not been explored.

OBJECTIVES

The purpose of this study was to assess the effects of acamprosate on cocaine-stimulated locomotor activity, cocaine self-administration, and cue- and cocaine-induced reinstatement of cocaine-seeking behavior.

MATERIALS AND METHODS

All experiments utilized once-daily treatment for 5 consecutive days. First, the effects of saline or acamprosate (100, 300, or 500 mg/kg intraperitoneally) on body weight were examined. On the last day of treatment, locomotor activity was assessed before and after drug treatment, after which all animals received an acute challenge of cocaine (10 mg/kg). Next, a separate group of rats were trained to intravenously (IV) self-administer cocaine (0.6 mg/kg per infusion), subjected to extinction procedures, and then tested for effects of acamprosate on cue- or cocaine-induced reinstatement. A third group of rats was trained to self-administer cocaine as described above and were treated with saline or acamprosate before daily IV self-administration sessions.

RESULTS

Repeated administration of 500 mg/kg acamprosate but not lower doses produced reductions in both body weight and spontaneous locomotor activity, and thus this dose was not tested further. Acamprosate at 300 mg/kg but not 100 mg/kg attenuated both cocaine- and cue-induced reinstatement without altering baseline patterns of cocaine self-administration or cocaine-stimulated hyperlocomotion.

CONCLUSIONS

Acamprosate attenuates both drug- and cue-induced reinstatement of cocaine-seeking behavior, suggesting that this compound may serve as a potential treatment for preventing relapse in cocaine-addicted humans.

Dopamine D(1) and D(3) receptors oppositely regulate NMDA- and cocaine-induced MAPK signaling via NMDA receptor phosphorylation

Development of drug addiction involves complex molecular changes in the CNS. The mitogen-activated protein kinase (MAPK) signaling pathway plays a key role in mediating neuronal activation induced by dopamine, glutamate, and drugs of abuse. We previously showed that dopamine D(1) and D(3) receptors play different roles in regulating cocaine-induced MAPK activation. Although there are functional and physical interactions between dopamine and glutamate receptors, little is known regarding the involvement of D(1) and D(3) receptors in modulating glutamate-induced MAPK activation and underlying mechanisms. In this study, we show that D(1) and D(3) receptors play opposite roles in regulating N-methyl-d-aspartate (NMDA) -induced activation of extracellular signal-regulated kinase (ERK) in the caudate putamen (CPu). D(3) receptors also inhibit NMDA-induced activation of the c-Jun N-terminal kinase and p38 kinase in the CPu. NMDA-induced activation of the NMDA-receptor R1 subunit (NR1), Ca(2+)/calmodulin-dependent protein kinase II and the cAMP-response element binding protein (CREB), and cocaine-induced CREB activation in the CPu are also oppositely regulated by dopamine D(1) and D(3) receptors. Finally, the blockade of NMDA-receptor reduces cocaine-induced ERK activation, and inhibits phosphorylation of NR1, Ca(2+)/calmodulin-dependent protein kinase II, and CREB, while inhibiting ERK activation attenuates cocaine-induced CREB phosphorylation in the CPu. These results suggest that dopamine D(1) and D(3) receptors oppositely regulate NMDA- and cocaine-induced MAPK signaling via phosphorylation of NR1.

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.

How to make a rat addicted to cocaine

Procedures have been developed which provide extremely stable patterns of cocaine self-administration in rats and these have been useful in lesion and drug pretreatment studies aimed at understanding the neurobiology of cocaine reinforcement. The issue now is whether studying the neurobiology of reinforcement is the same as studying the neurobiology of addiction. If the goal is to understand a progressive and deteriorating disorder, then the self-administration procedures should model specific aspects of the progressive stages of the addiction process. Here we review theoretical strategies for modeling the addiction process and present data from a series of experiments from our laboratory showing conditions which produce a progressive change in the motivation to self-administer cocaine in rats. This phenomenon is revealed by an escalation in breakpoints on a progressive ratio schedule. The effect, which is robust and persistent, depends on dose and speed of injection. Interestingly, high drug intake can retard the development of this effect, which we argue indicates that the addiction process has a developmental sequence. Finally, we suggest that specific parameters (dose, price and availability) can be used to examine the transition from recreational use to binge-like intake.