Contributions of nucleus accumbens core and shell GluR1 containing AMPA receptors in AMPA- and cocaine-primed reinstatement of cocaine-seeking behavior

The distribution of neurotransmitters in the brain circuitry: Mesolimbic pathway and addiction

Understanding the central nervous system (CNS) circuitry and its different neurotransmitters that underlie reward is essential to improve treatment for many common health issues, such as addiction. Here, we concentrate on understanding how the mesolimbic circuitry and neurotransmitters are organized and function, and how drug exposure affects synaptic and structural changes in this circuitry. While the role of some reward circuits, like the cerebral dopamine (DA)/glutamate (Glu)/gamma aminobutyric acid (GABA)ergic pathways, in drug reward, is well known, new research using molecular-based methods has shown functional alterations throughout the reward circuitry that contribute to various aspects of addiction, including craving and relapse. A new understanding of the fundamental connections between brain regions as well as the molecular alterations within these particular microcircuits, such as neurotrophic factor and molecular signaling or distinct receptor function, that underlie synaptic and structural plasticity evoked by drugs of abuse has been made possible by the ability to observe and manipulate neuronal activity within specific cell types and circuits. It is exciting that these discoveries from preclinical animal research are now being applied in the clinic, where therapies for human drug dependence, such as deep brain stimulation and transcranial magnetic stimulation, are being tested. Therefore, this chapter seeks to summarize the current understanding of the important brain regions (especially, mesolimbic circuitry) and neurotransmitters implicated in drug-related behaviors and the molecular mechanisms that contribute to altered connectivity between these areas, with the postulation that increased knowledge of the plasticity within the drug reward circuit will lead to new and improved treatments for addiction.

Nicotine but not saline self-administering or yoked control conditions produces sustained neuroadaptations in the accumbens shell

Introduction

Using yoked animals as the control when monitoring operant drug-self-administration is considered the golden standard. However, instrumental learning per se recruits several neurocircuits that may produce distinct or overlapping neuroadaptations with drugs of abuse. The aim of this project was to assess if contingent responding for nicotine or saline in the presence of a light stimulus as a conditioned reinforcer is associated with sustained neurophysiological adaptations in the nucleus accumbens shell (nAcS), a brain region repeatedly associated with reward related behaviors.

Methods

To this end, nicotine-or saline-administrating rats and yoked-saline stimulus-unpaired training conditions were assessed in operant boxes over four consecutive weeks. After four additional weeks of home cage forced abstinence and subsequent cue reinforced responding under extinction conditions, ex vivo electrophysiology was performed in the nAcS medium spiny neurons (MSNs).

Results

Whole cell recordings conducted in voltage and current-clamp mode showed that excitatory synapses in the nAcS were altered after prolonged forced abstinence from nicotine self-administration. We observed an increase in sEPSC amplitude in animals with a history of contingent nicotine SA potentially indicating higher excitability of accumbal MSNs, which was further supported by current clamp recordings. Interestingly no sustained neuroadaptations were elicited in saline exposed rats from nicotine associated visual cues compared to the yoked controls.

Conclusion

The data presented here indicate that nicotine self-administration produces sustained neuroadaptations in the nAcS while operant responding driven by nicotine visual stimuli has no long-term effects on MSNs in nAcS.

Deep brain stimulation of the nucleus accumbens shell attenuates cocaine withdrawal but increases cocaine self-administration, cocaine-induced locomotor activity, and GluR1/GluA1 in the central nucleus of the amygdala in male cocaine-dependent rats

BACKGROUND

Cocaine addiction is a major public health problem. Despite decades of intense research, no effective treatments are available. Both preclinical and clinical studies strongly suggest that deep brain stimulation of the nucleus accumbens (NAcc) is a viable target for the treatment of cocaine use disorder (CUD).

OBJECTIVE

Although previous studies have shown that DBS of the NAcc decreases cocaine seeking and reinstatement, the effects of DBS on cocaine intake in cocaine-dependent animals have not yet been investigated.

METHODS

Rats were made cocaine dependent by allowing them to self-administer cocaine in extended access conditions (6 h/day, 0.5 mg/kg/infusion). The effects of monophasic bilateral high-frequency DBS (60 μs pulse width and 130 Hz frequency) stimulation with a constant current of 150 μA of the NAcc shell on cocaine intake was then evaluated. Furthermore, cocaine-induced locomotor activity, irritability-like behavior during cocaine abstinence, and the levels of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits 1 and 2 (GluR1/GluA1 and GluR2/GluA2) after DBS were investigated.

RESULTS

Contrary to our expectations, DBS of the NAcc shell induced a slight increase in cocaine self-administration, and increased cocaine-induced locomotion after extended access of cocaine self-administration. In addition, DBS decreased irritability-like behavior 18 h into cocaine withdrawal. Finally, DBS increased both cytosolic and synaptosomal levels of GluR1, but not GluR2, in the central nucleus of the amygdala but not in other brain regions.

CONCLUSIONS

These preclinical results with cocaine-dependent animals support the use of high-frequency DBS of the NAcc shell as a therapeutic approach for the treatment of the negative emotional state that emerges during cocaine abstinence, but also demonstrate that DBS does not decrease cocaine intake in active, long-term cocaine users. These data, together with the existing evidence that DBS of the NAcc shell reduces the reinstatement of cocaine seeking in abstinent animals, suggest that NAcc shell DBS may be beneficial for the treatment of the negative emotional states and craving during abstinence, although it may worsen cocaine use if individuals continue drug use.

Maternal separation increases cocaine intake through a mechanism involving plasticity in glutamate signalling

Early-life stress (ELS) is associated with negative consequences, including maladaptive long-lasting brain effects. These alterations seem to increase the likelihood of developing substance use disorders. However, the molecular consequences of ELS are poorly understood. In the present study, we tested the impact of ELS induced by maternal separation with early weaning (MSEW) in CD1 male mice at different phases of cocaine self-administration (SA). We also investigated the subsequent alterations on GluR2, GluR1, cAMP response element-binding (CREB), and CREB-phosphorylation (pCREB) in ventral tegmental area (VTA) and nucleus accumbens (NAc) induced by both MSEW and cocaine SA. Our results show that MSEW animals expressed a higher cocaine intake, an increased vulnerability to the acquisition of cocaine SA, and incapacity to extinguish cocaine SA behaviour. MSEW mice showed decreased GluR2 and increased GluR1 and pCREB in NAc. Also, results displayed reduction of basal levels of GluR1 and CREB and an elevation of GluR1/GluR2 ratio in the VTA. Such results hint at an enhanced glutamatergic function in NAc and increased excitability of VTA DA neurons in maternally separated mice. Altogether, our results suggest that MSEW induces molecular alterations in the brain areas related to reward processing, increasing the vulnerability to depression and cocaine-seeking behaviour.

Prolonged Withdrawal From Escalated Oxycodone Is Associated With Increased Expression of Glutamate Receptors in the Rat Hippocampus

People suffering from opioid use disorder (OUD) exhibit cognitive dysfunctions. Here, we investigated potential changes in the expression of glutamate receptors in rat hippocampi at 2 h and 31 days after the last session of oxycodone self-administration (SA). RNA extracted from the hippocampus was used in quantitative polymerase chain reaction analyses. Rats, given long-access (9 h per day) to oxycodone (LgA), took significantly more drug than rats exposed to short-access (3 h per day) (ShA). In addition, LgA rats could be further divided into higher oxycodone taking (LgA-H) or lower oxycodone taking (LgA-L) groups, based on a cut-off of 50 infusions per day. LgA rats, but not ShA, rats exhibited incubation of oxycodone craving. In addition, LgA rats showed increased mRNA expression of GluA1-3 and GluN2a-c subunits as well as Grm3, Grm5, Grm6, and Grm8 subtypes of glutamate receptors after 31 days but not after 2 h of stopping the SA experiment. Changes in GluA1-3, Grm6, and Grm8 mRNA levels also correlated with increased lever pressing (incubation) after long periods of withdrawal from oxycodone. More studies are needed to elucidate the molecular mechanisms involved in altering the expression of these receptors during withdrawal from oxycodone and/or incubation of drug seeking.

Enhanced psychostimulant response, but not social avoidance, depends on GluA1 AMPA receptors in VTA dopamine neurons following intermittent social defeat stress in rats

Evidence from both human and animal studies demonstrates the importance of social stress in the development of addiction‐related behaviour. In rats, intermittent social defeat stress causes long‐lasting psychostimulant cross‐sensitization. Our recent data reveal heightened expression of AMPA receptor (AMPAR) GluA1 subunit in rat ventral tegmental area (VTA), which occurs concurrently with social stress‐induced amphetamine (AMPH) cross‐sensitization. In addition, social stress in rats induced social avoidance behaviour. The present study evaluated the effects of intermittent social defeat stress on GluA1 expression in VTA dopamine (DA) neurons, then utilized Cre‐dependent virus‐mediated gene transfer to determine the functional role of homomeric GluA1‐AMPARs in these neurons. Social defeat stress exposure induced GluA1 expression in VTA DA neurons, as demonstrated by a greater density of GluA1/tyrosine hydroxylase (TH) double‐labelling in VTA neurons in stressed rats. Additionally, functional inactivation of VTA GluA1 AMPARs in DA neurons prevented stress‐induced cross‐sensitization, or augmented locomotor response to low dose AMPH challenge (1.0 mg/kg, i.p.), but had no effect on social stress‐induced social avoidance behaviour. Furthermore, wild‐type overexpression of GluA1 in VTA DA neurons had the opposite effect; locomotor‐activating effects of AMPH were significantly augmented, even in the absence of stress. Taken together, these results suggest that stress‐induced GluA1 expression in VTA DA neurons is necessary for psychostimulant cross‐sensitization, but not for social avoidance. This differential effect suggests that different neural pathways are implicated in these behaviours. These findings could lead to novel pharmacotherapies to help prevent stress‐induced susceptibility to substance abuse.

Corticostriatal plasticity in the nucleus accumbens core

Small fluctuations in striatal glutamate and dopamine are required to establish goal-directed behaviors and motor learning, while large changes appear to underlie many neuropsychological disorders, including drug dependence and Parkinson's disease. A better understanding of how variations in neurotransmitter availability can modify striatal circuitry will lead to new therapeutic targets for these disorders. Here, we examined dopamine-induced plasticity in prefrontal cortical projections to the nucleus accumbens (NAc) core. We combined behavioral measures of male mice, presynaptic optical studies of glutamate release kinetics from prefrontal cortical projections, and postsynaptic electrophysiological recordings of spiny projection neurons within the NAc core. Our data show that repeated amphetamine promotes long-lasting but reversible changes along the corticoaccumbal pathway. In saline-treated mice, coincident cortical stimulation and dopamine release promoted presynaptic filtering by depressing exocytosis from glutamatergic boutons with a low-probability of release. The repeated use of amphetamine caused a frequency-dependent, progressive, and long-lasting depression in corticoaccumbal activity during withdrawal. This chronic presynaptic depression was relieved by a drug challenge which potentiated glutamate release from synapses with a low-probability of release. D1 receptors generated this synaptic potentiation, which corresponded with the degree of locomotor sensitization in individual mice. By reversing the synaptic depression, drug reinstatement may promote allostasis by returning corticoaccumbal activity to a more stable and normalized state. Therefore, dopamine-induced synaptic filtering of excitatory signals entering the NAc core in novice mice and paradoxical excitation of the corticoaccumbal pathway during drug reinstatement may encode motor learning, habit formation, and dependence.

Normal extinction and reinstatement of morphine-induced conditioned place preference in the GluA1-KO mouse line

Extinction and reinstatement of morphine-induced conditioned place preference were studied in glutamate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor GluA1 subunit-deficient mice (global GluA1-KO mice). In line with previous findings, both acquisition and expression of conditioned place preference to morphine (20 mg/kg, subcutaneously) were fully functional in GluA1 KO mice compared with wild-type littermate controls (GluA1-WT), thus enabling the study of extinction. With a 10-session extinction paradigm, the GluA1 KO mice showed complete extinction similar to that of the GluA1-WT mice. Morphine-induced reinstatement (10 mg/kg, subcutaneously) was detected in both mouse lines. GluA1 KO mice moved more during all the phases of the experiment, including the place conditioning trials, extinction sessions, and place preference tests. The results suggest that the GluA1 subunit may be dispensable or prone to compensation at the neural circuitries delineating extinction and reinstatement. The GluA1 KO mice show altered long-term between-session habituation, which extends longer than previously anticipated.

Pain Therapy Guided by Purpose and Perspective in Light of the Opioid Epidemic

Prescription opioid misuse is an ongoing and escalating epidemic. Although these pharmacological agents are highly effective analgesics prescribed for different types of pain, opioids also induce euphoria, leading to increasing diversion and misuse. Opioid use and related mortalities have developed in spite of initial claims that OxyContin, one of the first opioids prescribed in the USA, was not addictive in the presence of pain. These claims allayed the fears of clinicians and contributed to an increase in the number of prescriptions, quantity of drugs manufactured, and the unforeseen diversion of these drugs for non-medical uses. Understanding the history of opioid drug development, the widespread marketing campaign for opioids, the immense financial incentive behind the treatment of pain, and vulnerable socioeconomic and physical demographics for opioid misuse give perspective on the current epidemic as an American-born problem that has expanded to global significance. In light of the current worldwide opioid epidemic, it is imperative that novel opioids are developed to treat pain without inducing the euphoria that fosters physical dependence and addiction. We describe insights from preclinical findings on the properties of opioid drugs that offer insights into improving abuse-deterrent formulations. One finding is that the ability of some agonists to activate one pathway over another, or agonist bias, can predict whether several novel opioid compounds bear promise in treating pain without causing reward among other off-target effects. In addition, we outline how the pharmacokinetic profile of each opioid contributes to their potential for misuse and discuss the emergence of mixed agonists as a promising pipeline of opioid-based analgesics. These insights from preclinical findings can be used to more effectively identify opioids that treat pain without causing physical dependence and subsequent opioid abuse.

Contrasting the Role of xCT and GLT-1 Upregulation in the Ability of Ceftriaxone to Attenuate the Cue-Induced Reinstatement of Cocaine Seeking and Normalize AMPA Receptor Subunit Expression

Long-term treatment with ceftriaxone attenuates the reinstatement of cocaine seeking while increasing the function of the glutamate transporter 1 (GLT-1) and system xC- (Sxc) in the nucleus accumbens core (NAc). Sxc contributes the majority of nonsynaptic extracellular glutamate in the NAc, while GLT-1 is responsible for the majority of glutamate uptake. Here we used antisense to decrease the expression of GLT-1 and xCT (a catalytic subunit of Sxc) to determine the relative importance of both proteins in mediating the ability of ceftriaxone to prevent cue-induced reinstatement of cocaine seeking and normalize glutamatergic proteins in the NAc of rats. Intra-NAc xCT knockdown prevented ceftriaxone from attenuating reinstatement and from upregulating GLT-1 and resulted in increased surface expression of AMPA receptor subunits GluA1 and GluA2. Intra-NAc GLT-1 knockdown also prevented ceftriaxone from attenuating reinstatement and from upregulating xCT expression, without affecting GluA1 and GluA2 expression. In the absence of cocaine or ceftriaxone treatment, xCT knockdown in the NAc increased the expression of both GluA1 and GluA2 without affecting GLT-1 expression while GLT-1 knockdown had no effect. PCR and immunoprecipitation of GLT-1 revealed that ceftriaxone does not upregulate GLT-1 and xCT through a transcriptional mechanism, and their coregulation by ceftriaxone is not mediated by physical interaction. These data support important and distinct roles for xCT and GLT-1 in the actions of ceftriaxone and add to a body of literature finding evidence for coregulation of these transporters. Our results also point to xCT expression and subsequent basal glutamate levels as being a key mediator of AMPA receptor expression in the NAc.SIGNIFICANCE STATEMENT Ceftriaxone attenuates the reinstatement of cocaine, alcohol, and heroin seeking. The mechanism of action of this behavioral effect has been attributed to glutamate transporter 1 (GLT-1) and xCT (a catalytic subunit of Sxc)/Sxc upregulation in the nucleus accumbens core. Here we used an antisense strategy to knock down GLT-1 or xCT in the nucleus accumbens core and examined the behavioral and molecular consequences. While upregulation of both xCT and GLT-1 are essential to the ability of ceftriaxone to attenuate cue-induced reinstatement of cocaine seeking, each protein uniquely affects the expression of other glutamate receptor and transporter proteins. We also report that reducing basal glutamate levels through the manipulation of xCT expression increases the surface expression of AMPA receptor subunits, providing insight to the mechanism by which cocaine alters AMPA surface expression.

The Nucleus Accumbens: Mechanisms of Addiction across Drug Classes Reflect the Importance of Glutamate Homeostasis

The nucleus accumbens is a major input structure of the basal ganglia and integrates information from cortical and limbic structures to mediate goal-directed behaviors. Chronic exposure to several classes of drugs of abuse disrupts plasticity in this region, allowing drug-associated cues to engender a pathologic motivation for drug seeking. A number of alterations in glutamatergic transmission occur within the nucleus accumbens after withdrawal from chronic drug exposure. These drug-induced neuroadaptations serve as the molecular basis for relapse vulnerability. In this review, we focus on the role that glutamate signal transduction in the nucleus accumbens plays in addiction-related behaviors. First, we explore the nucleus accumbens, including the cell types and neuronal populations present as well as afferent and efferent connections. Next we discuss rodent models of addiction and assess the viability of these models for testing candidate pharmacotherapies for the prevention of relapse. Then we provide a review of the literature describing how synaptic plasticity in the accumbens is altered after exposure to drugs of abuse and withdrawal and also how pharmacological manipulation of glutamate systems in the accumbens can inhibit drug seeking in the laboratory setting. Finally, we examine results from clinical trials in which pharmacotherapies designed to manipulate glutamate systems have been effective in treating relapse in human patients. Further elucidation of how drugs of abuse alter glutamatergic plasticity within the accumbens will be necessary for the development of new therapeutics for the treatment of addiction across all classes of addictive substances.

Disrupting GluA2 phosphorylation potentiates reinstatement of cocaine seeking

Addiction is associated with changes in synaptic plasticity mediated, in part, by alterations in the trafficking and stabilization of AMPA receptors at synapses within the nucleus accumbens. Exposure to cocaine can lead to protein kinase C-mediated phosphorylation of GluA2 AMPA subunits and this phosphorylation event leads to the internalization of GluA2-containing AMPARs, which are calcium-impermeable. However, it is not clear whether this internalization is necessary for the expression of addictive phenotypes. Utilizing a mouse with a point mutation within the GluA2 subunit c-terminus, the current study demonstrates that disrupting PKC-mediated GluA2 phosphorylation potentiates reinstatement of both cue-induced cocaine seeking and cocaine conditioned reward without affecting operant learning, food self-administration or cocaine sensitization. Electrophysiological recordings revealed increased GluA2-mediated AMPA transmission as evidenced by increased sEPSC amplitude without any changes in sEPSC frequency or rectification. In support of this increase in GluA2 activity mediating the augmented cocaine reinstatement, we found that accumbal overexpression of GluA2 recapitulated this behavioral effect in wildtype mice while not altering reinstatement behavior in the GluA2 K882A knock-in mice. In addition, disrupting GluA2 phosphorylation was associated with blunted long-term depression in the nucleus accumbens, mimicking the anaplasticity seen following cocaine self-administration. Taken together these results indicate that disrupting GluA2 phosphorylation and increasing GluA2-mediated transmission in the nucleus accumbens leads to increased vulnerability to cocaine relapse. Further, these results indicate that modulating GluA2-containing AMPAR trafficking can contribute to addictive phenotypes in the absence of alterations in GluA2-lacking receptors. These results highlight the GluA2 phosphorylation site as a novel target for the development of cocaine addiction therapeutics.

NMDA antagonist MK 801 in nucleus accumbens core but not shell disrupts the restraint stress-induced reinstatement of extinguished cocaine-conditioned place preference in rats

Relapse is a common feature of cocaine addiction. In rodents, it can be elicited by cues, stress or the drug. Restraint stress-induced reinstatement of cocaine-conditioned place preference (CPP) is a useful model to study the mechanisms involved in stress-induced relapse of drug-seeking behavior. There is evidence that the glutamate NMDA receptors are critically involved in drug- and cue-induced reinstatement of seeking behavior and drug-CPP responses. The aim of this study was to investigate the contribution of NMDA receptors within core vs. shell nucleus accumbens (NAc) subregions to restraint stress-induced reinstatement of extinguished cocaine-CPP. After extinction of cocaine-conditioned preference, animals were administered MK 801 systemically or directly into intra-core or intra-shell, and restrained for 30min or left undisturbed in their home-cages. First, we demonstrated that restraint stress-induced reinstatement of extinguished cocaine-CPP depends on the duration of restraint as well as on the context in which it is applied. Second, this effect was blocked by systemic MK 801 administration either before or after restraint. Third, intra-core but not intra-shell administration abrogated the restraint stress-induced reinstatement. These findings show that NMDA receptors within NAc core, but not shell, play a critical role in restraint stress-induced reinstatement of cocaine-CPP.

A Critical Role for the GluA1 Accessory Protein, SAP97, in Cocaine Seeking

A growing body of evidence indicates that the transport of GluA1 subunit-containing calcium-permeable AMPA receptors (CP-AMPARs) to synapses in subregions of the nucleus accumbens promotes cocaine seeking. Consistent with these findings, the present results show that administration of the CP-AMPAR antagonist, Naspm, into the caudal lateral core or caudal medial shell of the nucleus accumbens attenuated cocaine priming-induced reinstatement of drug seeking. Moreover, viral-mediated overexpression of 'pore dead' GluA1 subunits (via herpes simplex virus (HSV) GluA1-Q582E) in the lateral core or medial shell attenuated the reinstatement of cocaine seeking. The overexpression of wild-type GluA1 subunits (via HSV GluA1-WT) in the medial shell, but not the lateral core, enhanced the reinstatement of cocaine seeking. These results indicate that activation of GluA1-containing AMPARs in subregions of the nucleus accumbens reinstates cocaine seeking. SAP97 and 4.1N are proteins involved in GluA1 trafficking to and stabilization in synapses; SAP97-GluA1 interactions also influence dendritic growth. We next examined potential roles of SAP97 and 4.1N in cocaine seeking. Viral-mediated expression of a microRNA that reduces SAP97 protein expression (HSV miSAP97) in the medial accumbens shell attenuated cocaine seeking. In contrast, a virus that overexpressed a dominant-negative form of a 4.1N C-terminal domain (HSV 4.1N-CTD), which prevents endogenous 4.1N binding to GluA1 subunits, had no effect on cocaine seeking. These results indicate that the GluA1 subunit accessory protein SAP97 may represent a novel target for pharmacotherapeutic intervention in the treatment of cocaine craving.

Glutamatergic synaptic plasticity in the mesocorticolimbic system in addiction

Addictive drugs remodel the brain’s reward circuitry, the mesocorticolimbic dopamine (DA) system, by inducing widespread adaptations of glutamatergic synapses. This drug-induced synaptic plasticity is thought to contribute to both the development and the persistence of addiction. This review highlights the synaptic modifications that are induced by in vivo exposure to addictive drugs and describes how these drug-induced synaptic changes may contribute to the different components of addictive behavior, such as compulsive drug use despite negative consequences and relapse. Initially, exposure to an addictive drug induces synaptic changes in the ventral tegmental area (VTA). This drug-induced synaptic potentiation in the VTA subsequently triggers synaptic changes in downstream areas of the mesocorticolimbic system, such as the nucleus accumbens (NAc) and the prefrontal cortex (PFC), with further drug exposure. These glutamatergic synaptic alterations are then thought to mediate many of the behavioral symptoms that characterize addiction. The later stages of glutamatergic synaptic plasticity in the NAc and in particular in the PFC play a role in maintaining addiction and drive relapse to drug-taking induced by drug-associated cues. Remodeling of PFC glutamatergic circuits can persist into adulthood, causing a lasting vulnerability to relapse. We will discuss how these neurobiological changes produced by drugs of abuse may provide novel targets for potential treatment strategies for addiction.

Cocaine-elicited imbalances in ventromedial prefrontal cortex Homer1 versus Homer2 expression: implications for relapse

Withdrawal from a history of extended access to self-administered cocaine produces a time-dependent intensification of drug seeking, which might relate to a cocaine-induced imbalance in the relative expression of constitutively expressed Homer1 versus Homer2 isoforms within the ventromedial aspect of the prefrontal cortex (vmPFC). Thus, we employed immunoblotting to examine the relation between cue-reinforced lever pressing at 3- versus 30-day withdrawal from a 10-day history of extended access (6 hours/day) to intravenous cocaine (0.25 mg/infusion) or saline (Sal6h), and the expression of Homer1b/c and Homer2a/b within the vmPFC versus the more dorsomedial aspect of this structure (dmPFC). Behavioral studies employed adeno-associated virus (AAV) vectors to reverse cocaine-elicited changes in the relative expression of Homer1 versus Homer2 isoforms and tested animals for cocaine prime-, and cue-induced responding following extinction training. Cocaine self-administration elevated both Homer1b/c and Homer2a/b levels within the vmPFC at 3-day withdrawal, and the rise in Homer2a/b persisted for at least 30 days. dmPFC Homer levels did not change as a function of self-administration history. Reversing the relative increase in Homer2 versus Homer1 expression via Homer1c overexpression or Homer2b knockdown failed to influence cue-reinforced lever pressing when animals were tested in a drug-free state, but both AAV treatments prevented cocaine-primed reinstatement of lever-pressing behavior. These data suggest that a cocaine-elicited imbalance in the relative expression of constitutively expressed Homer2 versus Homer1 within the vmPFC is necessary for the capacity of cocaine to reinstate drug-seeking behavior, posing drug-induced changes in vmPFC Homer expression as a molecular trigger contributing to drug-elicited relapse.

Oxytocin reduces cocaine seeking and reverses chronic cocaine-induced changes in glutamate receptor function

BACKGROUND

Oxytocin, a neurohypophyseal neuropeptide, is a potential mediator and regulator of drug addiction. However, the cellular mechanisms of oxytocin in drug seeking remain unknown.

METHODS

In the present study, we used a self-administration/reinstatement model to study the effects of oxytocin on cocaine seeking and its potential interaction with glutamate function at the receptor level.

RESULTS

Systemic oxytocin dose-dependently reduced cocaine self-administration during various schedules of reinforcement, including fixed ratio 1, fixed ratio 5, and progressive ratio. Oxytocin also attenuated reinstatement to cocaine seeking induced by cocaine prime or conditioned cues. Western-blot analysis indicated that oxytocin increased phosphorylation of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptor GluA1 subunit at the Ser 845 site with or without accompanying increases in phosphorylation of extracellular signal-regulated kinase, in several brain regions, including the prefrontal cortex, bed nucleus of the stria terminalis, amygdala, and dorsal hippocampus. Immunoprecipitation of oxytocin receptor and GluA1 subunit receptors further demonstrated a physical interaction between these 2 receptors, although the interaction was not influenced by chronic cocaine or oxytocin treatment. Oxytocin also attenuated sucrose seeking in a GluA1- or extracellular-signal-regulated kinase-independent manner.

CONCLUSIONS

These findings suggest that oxytocin mediates cocaine seeking through interacting with glutamate receptor systems via second messenger cascades in mesocorticolimbic regions.

A shift in the role of glutamatergic signaling in the nucleus accumbens core with the development of an addicted phenotype

BACKGROUND

While dopamine signaling in the nucleus accumbens (NAc) plays a well-established role in motivating cocaine use in early nonaddicted stages, recent evidence suggests that other signaling pathways may be critical once addiction has developed. Given the importance of glutamatergic signaling in the NAc for drug seeking and relapse, here we examined its role in motivating cocaine self-administration under conditions known to produce either a nonaddicted or an addicted phenotype.

METHODS

Following acquisition, male and female Sprague Dawley rats were given either short access (three fixed-ratio 1 sessions, 20 infusions/day) or extended 24-hour access (10 days; 4 trials/hour; up to 96 infusions/day) to cocaine. Following a 14-day abstinence period, motivation for cocaine was assessed under a progressive-ratio schedule, and once stable, the effects of intra-NAc infusions of the glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate/kainate receptor antagonist CNQX (0, .01, .03, .1 μg/side) were determined. As an additional measure for the development of an addicted phenotype, separate groups of rats were screened under an extinction/cue-induced reinstatement procedure following abstinence from short-access versus extended-access self-administration.

RESULTS

Motivation for cocaine and levels of extinction and reinstatement responding were markedly higher following extended-access versus short-access self-administration, confirming the development of an addicted phenotype in the extended-access group. CNQX dose-dependently reduced motivation for cocaine in the extended-access group but was without effect in the short-access group.

CONCLUSIONS

These results suggest that the role of glutamatergic signaling in the NAc, though not essential for motivating cocaine use in nonaddicted stages, becomes critical once addiction has developed.

Chronic unilateral stimulation of the nucleus accumbens at high or low frequencies attenuates relapse to cocaine seeking in an animal model

BACKGROUND

Deep brain stimulation (DBS), a form of neurosurgical intervention that is used to modulate the electrophysiological activity of specific brain areas, has emerged as a form of therapy for severe cases of treatment-refractory addiction.

OBJECTIVE/HYPOTHESIS

Recent research suggests that the nucleus accumbens (NAC) is a promising target area for DBS in addiction. The current experiments were designed to determine optimal parameters of stimulation and long-term efficacy of NAC DBS in an animal model of cocaine addiction.

METHODS

Rats were implanted with a stimulating electrode in the right NAC and exposed to chronic cocaine self-administration (0.5 mg/kg/infusion). Rats underwent drug seeking tests by exposing them to the self-administration context paired with cocaine challenge (5 mg/kg i.p.) on days 1, 15 and 30 after withdrawal from cocaine self-administration. Low-frequency (LF, 20 Hz) or high-frequency (HF, 160 Hz) DBS was applied for 30 min daily for 14 consecutive days starting one day after drug withdrawal.

RESULTS

Rats exhibited robust drug-seeking 1, 15 and 30 days after withdrawal from cocaine self-administration, with responding being highest on day 15. Both LF and HF attenuated cocaine seeking on day 15 post-withdrawal by 36 and 48%, respectively. Both forms of stimulation were ineffective on the tests conducted on days 1 and 30.

CONCLUSION

The present data showed that unilateral DBS of the NAC effectively attenuated cocaine relapse after 15 days of drug withdrawal, with therapeutic-like effects seemingly diminishing after DBS discontinuation. This evidence provides support for DBS as a promising intervention in intractable cases of stimulant addiction.

Deep brain stimulation for the treatment of addiction: basic and clinical studies and potential mechanisms of action

RATIONALE

Deep brain stimulation (DBS) has achieved substantial success as a treatment for movement disorders such as Parkinson's disease. The therapeutic efficacy and relative lack of serious side effects resulted in the expansion of DBS into the treatment of many other diseases, including obsessive-compulsive disorder, Tourette's, and depression, among others. More recently, a limited number of basic and clinical studies indicated that DBS may also be useful in the treatment of various addictions.

OBJECTIVES

Here, we briefly summarize the history of DBS and review the basic and clinical studies focused on DBS and addiction. We also examine the potential mechanisms that may underlie the effects of DBS.

RESULTS AND CONCLUSIONS

The available data indicate that DBS is a promising therapeutic modality for the treatment of addiction. Thus far, the nucleus accumbens and subthalamic nucleus are the most promising sites for DBS, reversing aspects of addiction. The mechanisms underlying DBS are complex and likely vary from region to region. Emerging evidence indicates that DBS of the nucleus accumbens produces its effects, at least in part, by antidromic activation of cortico-accumbal afferents that stimulate inhibitory medial prefrontal cortex interneurons via recurrent collaterals.

Deep brain stimulation of the nucleus accumbens shell attenuates cocaine reinstatement through local and antidromic activation

Accumbal deep brain stimulation (DBS) is a promising therapeutic modality for the treatment of addiction. Here, we demonstrate that DBS in the nucleus accumbens shell, but not the core, attenuates cocaine priming-induced reinstatement of drug seeking, an animal model of relapse, in male Sprague Dawley rats. Next, we compared DBS of the shell with pharmacological inactivation. Results indicated that inactivation using reagents that influenced (lidocaine) or spared (GABA receptor agonists) fibers of passage blocked cocaine reinstatement when administered into the core but not the shell. It seems unlikely, therefore, that intrashell DBS influences cocaine reinstatement by inactivating this nucleus or the fibers coursing through it. To examine potential circuit-wide changes, c-Fos immunohistochemistry was used to examine neuronal activation following DBS of the nucleus accumbens shell. Intrashell DBS increased c-Fos induction at the site of stimulation as well as in the infralimbic cortex, but had no effect on the dorsal striatum, prelimbic cortex, or ventral pallidum. Recent evidence indicates that accumbens DBS antidromically stimulates axon terminals, which ultimately activates GABAergic interneurons in cortical areas that send afferents to the shell. To test this hypothesis, GABA receptor agonists (baclofen/muscimol) were microinjected into the anterior cingulate, and prelimbic or infralimbic cortices before cocaine reinstatement. Pharmacological inactivation of all three medial prefrontal cortical subregions attenuated the reinstatement of cocaine seeking. These results are consistent with DBS of the accumbens shell attenuating cocaine reinstatement via local activation and/or activation of GABAergic interneurons in the medial prefrontal cortex via antidromic stimulation of cortico-accumbal afferents.

Bidirectional modulation of cocaine expectancy by phasic glutamate fluctuations in the nucleus accumbens

While glutamate in the nucleus accumbens (NAS) contributes to the promotion of drug-seeking by drug-predictive cues, it also appears to play a role in the inhibition of drug-seeking following extinction procedures. Thus we measured extracellular fluctuations of NAS glutamate in response to discriminative stimuli that signaled either cocaine availability or cocaine omission. We trained rats to self-administer intravenous cocaine and then to recognize discriminative odor cues that predicted either sessions where cocaine was available or alternating sessions where it was not (saline substituted for cocaine). Whereas responding in cocaine availability sessions remained stable, responding in cocaine omission sessions progressively declined to chance levels. We then determined the effects of each odor cue on extracellular glutamate in the core and shell subregions of NAS preceding and accompanying lever pressing under an extinction condition. Glutamate levels were elevated in both core and shell by the availability odor and depressed in the core but not the shell by the omission odor. Infusion of kynurenic acid (an antagonist for ionotropic glutamate receptors) into core but not shell suppressed responding associated with the availability odor, but had no effect on the suppression associated with the omission odor. Thus cocaine-predictive cues appear to promote cocaine seeking in part by elevating glutamatergic neurotransmission in the core of NAS, whereas cocaine-omission cues appear to suppress cocaine seeking in part by depressing glutamatergic receptor activation in the same region.

Psychostimulant-induced neuroadaptations in nucleus accumbens AMPA receptor transmission

Medium spiny neurons of the nucleus accumbens serve as the interface between corticolimbic regions that elicit and modulate motivated behaviors, including those related to drugs of abuse, and motor regions responsible for their execution. Medium spiny neurons are excited primarily by AMPA-type glutamate receptors, making AMPA receptor transmission in the accumbens a key regulatory point for addictive behaviors. In animal models of cocaine addiction, changes in the strength of AMPA receptor transmission onto accumbens medium spiny neurons have been shown to underlie cocaine-induced behavioral adaptations related to cocaine seeking. Here we review changes in AMPA receptor levels and subunit composition that occur after discontinuing different types of cocaine exposure, as well as changes elicited by cocaine reexposure following abstinence or extinction. Signaling pathways that regulate these cocaine-induced adaptations will also be considered, as they represent potential targets for addiction pharmacotherapies.

Role of medial prefrontal cortex Narp in the extinction of morphine conditioned place preference

Narp knockout (KO) mice demonstrate an impaired extinction of morphine conditioned place preference (CPP). Because the medial prefrontal cortex (mPFC) has been implicated in extinction learning, we tested whether Narp cells in this region play a role in the extinction of morphine CPP. We found that intracranial injections of adenoassociated virus (AAV) expressing wild-type (WT) Narp into the mPFC of Narp KO mice rescued the extinction and the injection of AAV expressing a dominant negative form of Narp (NarpN) into the mPFC of WT mice impaired the extinction of morphine CPP. These findings suggest that Narp in the mPFC mediates the extinction of morphine CPP.

Neuroplasticity in addiction: cellular and transcriptional perspectives

Drug addiction is a chronic, relapsing brain disorder which consists of compulsive patterns of drug-seeking and taking that occurs at the expense of other activities. The transition from casual to compulsive drug use and the enduring propensity to relapse is thought to be underpinned by long-lasting neuroadaptations in specific brain circuitry, analogous to those that underlie long-term memory formation. Research spanning the last two decades has made great progress in identifying cellular and molecular mechanisms that contribute to drug-induced changes in plasticity and behavior. Alterations in synaptic transmission within the mesocorticolimbic and corticostriatal pathways, and changes in the transcriptional potential of cells by epigenetic mechanisms are two important means by which drugs of abuse can induce lasting changes in behavior. In this review we provide a summary of more recent research that has furthered our understanding of drug-induced neuroplastic changes both at the level of the synapse, and on a transcriptional level, and how these changes may relate to the human disease of addiction.

Subregion-specific role of glutamate receptors in the nucleus accumbens on drug context-induced reinstatement of cocaine-seeking behavior in rats

The functional integrity of the nucleus accumbens (NAC) core and shell is necessary for contextual cocaine-seeking behavior in the reinstatement animal model of drug relapse; however, the neuropharmacological mechanisms underlying this phenomenon are poorly understood. The present study evaluated the contribution of metabotropic glutamate receptor subtype 1 (mGluR1) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor populations to drug context-induced reinstatement of cocaine-seeking behavior. Rats were trained to lever press for un-signaled cocaine infusions in a distinct context followed by extinction training in a different context. Cocaine-seeking behavior (non-reinforced active lever pressing) was then assessed in the previously cocaine-paired and extinction contexts after JNJ16259685 (mGluR1 antagonist: 0.0, 0.6, or 30 pg/0.3 µl/hemisphere) or CNQX (AMPA/kainate receptor antagonist: 0.0, 0.03, or 0.3 µg/0.3 µl /hemisphere) administration into the NAC core, medial or lateral NAC shell, or the ventral caudate-putamen (vCPu, anatomical control). JNJ16259685 or CNQX in the NAC core dose-dependently impaired contextual cocaine-seeking behavior relative to vehicle. Conversely, CNQX, but not JNJ16259685, in the lateral or medial NAC shell attenuated, whereas CNQX or JNJ16259685 in vCPu failed to inhibit, this behavior. The manipulations failed to alter instrumental behavior in the extinction context, general motor activity or food-reinforced instrumental behavior in control experiments. Thus, glutamate-mediated changes in drug context-induced motivation for cocaine involve distinct neuropharmacological mechanisms within the core and shell subregions of the NAC, with the stimulation of mGlu1 and AMPA/kainate receptors in the NAC core and the stimulation of AMPA/kainate, but not mGlu1, receptors in the NAC shell being necessary for this phenomenon.

Pharmacotherapeutics directed at deficiencies associated with cocaine dependence: focus on dopamine, norepinephrine and glutamate

Much effort has been devoted to research focused on pharmacotherapies for cocaine dependence yet there are no FDA-approved medications for this brain disease. Preclinical models have been essential to defining the central and peripheral effects produced by cocaine. Recent evidence suggests that cocaine exerts its reinforcing effects by acting on multiple neurotransmitter systems within mesocorticolimibic circuitry. Imaging studies in cocaine-dependent individuals have identified deficiencies in dopaminergic signaling primarily localized to corticolimbic areas. In addition to dysregulated striatal dopamine, norepinephrine and glutamate are also altered in cocaine dependence. In this review, we present these brain abnormalities as therapeutic targets for the treatment of cocaine dependence. We then survey promising medications that exert their therapeutic effects by presumably ameliorating these brain deficiencies. Correcting neurochemical deficits in cocaine-dependent individuals improves memory and impulse control, and reduces drug craving that may decrease cocaine use. We hypothesize that using medications aimed at reversing known neurochemical imbalances is likely to be more productive than current approaches. This view is also consistent with treatment paradigms used in neuropsychiatry and general medicine.

The synaptic pathology of drug addiction

A hallmark of drug addiction is the uncontrollable desire to consume drugs at the expense of severe negative consequences. Moreover, addicts that successfully refrain from drug use have a high vulnerability to relapse even after months or years of abstinence. In this chapter, we will discuss the current understanding of drug-induced neuroplasticity within the mesocorticolimbic brain system that contributes to the development of addiction and the persistence of relapse to drug seeking. I particular, we will focus at animal models that can be translated to human addiction. Although dopaminergic transmission is important for the acute effects of drug intake, the long-lived behavioral abnormalities associated with addiction are thought to arise from pathological plasticity in glutamatergic neurotransmission. The nature of changes in excitatory synaptic plasticity depends on several factors, including the type of drug, the brain area, and the time-point studied in the transition of drug exposure to withdrawal and relapse to drug seeking. Identification of drug-induced neuroplasticity is crucial to understand how molecular and cellular adaptations contribute to the end stage of addiction, which from a clinical perspective, is a time-point where pharmacotherapy may be most effectively employed.

Striatal signal transduction and drug addiction

Drug addiction is a severe neuropsychiatric disorder characterized by loss of control over motivated behavior. The need for effective treatments mandates a greater understanding of the causes and identification of new therapeutic targets for drug development. Drugs of abuse subjugate normal reward-related behavior to uncontrollable drug-seeking and -taking. Contributions of brain reward circuitry are being mapped with increasing precision. The role of synaptic plasticity in addiction and underlying molecular mechanisms contributing to the formation of the addicted state are being delineated. Thus we may now consider the role of striatal signal transduction in addiction from a more integrative neurobiological perspective. Drugs of abuse alter dopaminergic and glutamatergic neurotransmission in medium spiny neurons of the striatum. Dopamine receptors important for reward serve as principle targets of drugs abuse, which interact with glutamate receptor signaling critical for reward learning. Complex networks of intracellular signal transduction mechanisms underlying these receptors are strongly stimulated by addictive drugs. Through these mechanisms, repeated drug exposure alters functional and structural neuroplasticity, resulting in transition to the addicted biological state and behavioral outcomes that typify addiction. Ca²⁺ and cAMP represent key second messengers that initiate signaling cascades, which regulate synaptic strength and neuronal excitability. Protein phosphorylation and dephosphorylation are fundamental mechanisms underlying synaptic plasticity that are dysregulated by drugs of abuse. Increased understanding of the regulatory mechanisms by which protein kinases and phosphatases exert their effects during normal reward learning and the addiction process may lead to novel targets and pharmacotherapeutics with increased efficacy in promoting abstinence and decreased side effects, such as interference with natural reward, for drug addiction.

Drug wanting: behavioral sensitization and relapse to drug-seeking behavior

Repeated exposure to drugs of abuse enhances the motor-stimulant response to these drugs, a phenomenon termed behavioral sensitization. Animals that are extinguished from self-administration training readily relapse to drug, conditioned cue, or stress priming. The involvement of sensitization in reinstated drug-seeking behavior remains controversial. This review describes sensitization and reinstated drug seeking as behavioral events, and the neural circuitry, neurochemistry, and neuropharmacology underlying both behavioral models will be described, compared, and contrasted. It seems that although sensitization and reinstatement involve overlapping circuitry and neurotransmitter and receptor systems, the role of sensitization in reinstatement remains ill-defined. Nevertheless, it is argued that sensitization remains a useful model for determining the neural basis of addiction, and an example is provided in which data from sensitization studies led to potential pharmacotherapies that have been tested in animal models of relapse and in human addicts.

Females are more vulnerable to drug abuse than males: evidence from preclinical studies and the role of ovarian hormones

Human and animal research indicates the presence of sex differences in drug abuse. These data suggest that females, compared to males, are more vulnerable to key phases of the addiction process that mark transitions in drug use such as initiation, drug bingeing, and relapse. Recent data indicate that the female gonadal hormone estrogen may facilitate drug abuse in women. For example, phases of the menstrual cycle when estrogen levels are high are associated with enhanced positive subjective measures following cocaine and amphetamine administration in women. Furthermore, in animal research, the administration of estrogen increases drug taking and facilitates the acquisition, escalation, and reinstatement of cocaine-seeking behavior. Neurobiological data suggest that estrogen may facilitate drug taking by interacting with reward- and stress-related systems. This chapter discusses sex differences in and hormonal effects on drug-seeking behaviors in animal models of drug abuse. The neurobiological basis of these differences and effects are also discussed.

Cocaine- and morphine-induced synaptic plasticity in the nucleus accumbens

The critical brain areas and molecular mechanisms involved in drug abuse and dependence have been extensively studied. Drug-induced persistent behaviors such as sensitization, tolerance, or relapse, however, far outlast any previously reported mechanisms. A challenge in the field of addiction, therefore, has been to identify drug-induced changes in brain circuitry that may subserve long-lasting changes in behavior. This study examined behavioral changes and electron microscopic evidence of altered synaptic connectivity within the nucleus accumbens (NAc) following repeated administration of cocaine or morphine. The unbiased quantitative stereological physical disector method was used to estimate the number of synapses per neuron. Increases in the synapse-to-neuron ratio were found in the NAc shell of cocaine-treated (49.1%) and morphine-treated (55.1%) rats and in the NAc core of cocaine-treated animals (49.1%). This study provides direct ultrastructural evidence of drug-induced synaptic plasticity and identifies synaptic remodeling as a potential neural substrate underlying drug-induced behavioral sensitization.

The dopamine D1-D2 receptor heteromer localizes in dynorphin/enkephalin neurons: increased high affinity state following amphetamine and in schizophrenia

The distribution and function of neurons coexpressing the dopamine D1 and D2 receptors in the basal ganglia and mesolimbic system are unknown. We found a subset of medium spiny neurons coexpressing D1 and D2 receptors in varying densities throughout the basal ganglia, with the highest incidence in nucleus accumbens and globus pallidus and the lowest incidence in caudate putamen. These receptors formed D1-D2 receptor heteromers that were localized to cell bodies and presynaptic terminals. In rats, selective activation of D1-D2 heteromers increased grooming behavior and attenuated AMPA receptor GluR1 phosphorylation by calcium/calmodulin kinase IIα in nucleus accumbens, implying a role in reward pathways. D1-D2 heteromer sensitivity and functional activity was up-regulated in rat striatum by chronic amphetamine treatment and in globus pallidus from schizophrenia patients, indicating that the dopamine D1-D2 heteromer may contribute to psychopathologies of drug abuse, schizophrenia, or other disorders involving elevated dopamine transmission.

AMPA receptor synaptic plasticity induced by psychostimulants: the past, present, and therapeutic future

Experience-dependent plasticity at excitatory synapses of the mesocorticolimbic system is a fundamental brain mechanism that enables adaptation to an ever-changing environment. These synaptic responses are critical for the planning and execution of adaptive behaviors that maximize survival. The mesocorticolimbic system mediates procurement of positive reinforcers such as food and sex; however, drugs of abuse resculpt this crucial circuitry to promote compulsive drug-seeking behavior. This review will discuss the long-term changes in glutamatergic neurotransmission that occur within the mesolimbic system following cocaine exposure. In addition, we will examine how these long-lasting neuroadaptations may drive the pathology of psychostimulant addiction. Finally, we review clinical trials that highlight antagonists at excitatory AMPA receptors as promising targets against cocaine abuse.

Regulation of AMPA receptor trafficking in the nucleus accumbens by dopamine and cocaine

Nucleus accumbens (NAc) neurons are excited primarily by AMPA-type glutamate receptors (AMPAR). This is required for cocaine seeking in animal models of cocaine addiction, suggesting AMPAR transmission in the NAc as a key control point for cocaine-related behaviors. This review will briefly describe AMPAR properties and trafficking, with a focus on studies in NAc neurons, and then consider mechanisms by which cocaine may alter AMPAR transmission. Two examples will be discussed that may be important in two different stages of addiction: learning about drugs and drug-related cues during the period of drug exposure, and persistent vulnerability to craving and relapse after abstinence is achieved. The first example is drawn from studies of cultured NAc neurons. Elevation of dopamine levels (as would occur following cocaine exposure) facilitates activity-dependent strengthening of excitatory synapses onto medium spiny neurons, the main cell type and projection neuron of the NAc. This occurs because activation of D1-class dopamine receptors primes AMPAR for synaptic insertion. This may create a temporal window in which stimuli related to cocaine-taking are more efficacious at eliciting synaptic plasticity and thus being encoded into memory. The second example involves rat models of cocaine addiction. Cell surface and synaptic expression of AMPAR on NAc neurons is persistently increased after withdrawal from repeated cocaine exposure. We hypothesize that this increases the reactivity of NAc neurons to glutamate inputs from cortex and limbic structures, facilitating the ability of these inputs to trigger cocaine seeking and thus contributing to the persistent vulnerability to relapse that characterizes addiction.

Cocaine-induced neuroadaptations in glutamate transmission: potential therapeutic targets for craving and addiction

A growing body of evidence indicates that repeated exposure to cocaine leads to profound changes in glutamate transmission in limbic nuclei, particularly the nucleus accumbens. This review focuses on preclinical studies of cocaine-induced behavioral plasticity, including behavioral sensitization, self-administration, and the reinstatement of cocaine seeking. Behavioral, pharmacological, neurochemical, electrophysiological, biochemical, and molecular biological changes associated with cocaine-induced plasticity in glutamate systems are reviewed. The ultimate goal of these lines of research is to identify novel targets for the development of therapies for cocaine craving and addiction. Therefore, we also outline the progress and prospects of glutamate modulators for the treatment of cocaine addiction.

Synaptic plasticity in the mesolimbic system: therapeutic implications for substance abuse

In an ever-changing environment, animals must learn new behavioral strategies for the successful procurement of food, sex, and other needs. Synaptic plasticity within the mesolimbic system, a key reward circuit, affords an animal the ability to adapt and perform essential goal-directed behaviors. Ironically, drugs of abuse can also induce synaptic changes within the mesolimbic system, and such changes are hypothesized to promote deleterious drug-seeking behaviors in lieu of healthy, adaptive behaviors. In this review, we will discuss drug-induced neuroadaptations in excitatory transmission in the ventral tegmental area and the nucleus accumbens, two critical regions of the mesolimbic system, and the possible role of dopamine receptors in the development of these neuroadaptations. In particular, we will focus our discussion on recent studies showing changes in AMPA receptor function as a common molecular target of addictive drugs, and the possible behavioral consequences of such neuroadaptations.

The role of glutamate receptor redistribution in locomotor sensitization to cocaine

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR) surface expression in the nucleus accumbens (NAc) is enhanced after withdrawal from repeated cocaine exposure. However, it is unclear whether this contributes to the expression of locomotor sensitization and whether similar changes can be observed in other striatal regions. In this study we examined the relationship between AMPAR surface expression in the NAc and locomotor sensitization. We also examined AMPAR distribution in the dorsolateral striatum (DS) and NMDA receptor (NMDAR) distribution in the NAc and DS. Trends but no significant changes in NMDAR distribution were found in the NAc after withdrawal. No NMDAR changes were observed in the DS. AMPAR surface expression was increased in the NAc 15 days after the last exposure to cocaine, but decreased in the DS. Re-exposure to cocaine on withdrawal day 14 decreased AMPAR surface expression in the NAc 24 h, but not 30 min, after challenge, but increased it in the DS 24 h and 30 min after challenge. Locomotor sensitization was evaluated at times associated with increased or decreased AMPAR surface expression in the NAc. The magnitude of sensitization did not vary with changes in the level of AMPAR surface expression, nor was it significantly reduced by decreasing AMPAR transmission through intra-NAc infusion of CNQX before cocaine challenge. On the basis of our results, and other findings, we suggest that the expression of sensitization has no clear relationship to altered AMPAR surface expression in the NAc, although the latter may have a role in the enhanced pursuit and self-administration of drugs observed in sensitized rats.

AMPA receptor plasticity in the nucleus accumbens after repeated exposure to cocaine

This review focuses on cocaine-induced postsynaptic plasticity in the nucleus accumbens (NAc) involving changes in AMPA receptor (AMPAR) transmission. First, fundamental properties of AMPAR in the NAc are reviewed. Then, we provide a detailed and critical analysis of literature demonstrating alterations in AMPAR transmission in association with behavioral sensitization to cocaine and cocaine self-administration. We conclude that cocaine exposure leads to changes in AMPAR transmission that depend on many factors including whether exposure is contingent or non-contingent, the duration of withdrawal, and whether extinction training has occurred. The relationship between changes in AMPAR transmission and responding to cocaine or cocaine-paired cues can also be affected by these variables. However, after prolonged withdrawal in the absence of extinction training, our findings and others lead us to propose that AMPAR transmission is enhanced, resulting in stronger responding to drug-paired cues. Finally, many results indicate that the state of synaptic transmission in the NAc after cocaine exposure is associated with impairment of AMPAR-dependent plasticity. This may contribute to a broad range of addiction-related behavioral changes.

Sensitization processes in drug addiction

In 1993, Robinson and Berridge published their first review that laid out the incentive sensitization theory of addiction (Robinson and Berridge 1993 Brain Res Rev 18:247). Its basic point is that repeated exposure to drugs of abuse causes hypersensitivity to drugs and drug-associated stimuli of the neural circuits mediating incentive salience, an important way in which motivational stimuli influence behavior. In laymen's terms, it states that this drug-induced hypersensitivity of motivational circuitry would mediate an increase in drug "wanting," thus being responsible for the dramatically exaggerated motivation for drugs displayed by addicts. This theory has been exceptionally influential, as evidenced by the fact that the original review paper about this theory (Robinson and Berridge 1993 Brain Res Rev 18:247) has been cited 2,277 times so far, and subsequent updates of this view (Robinson and Berridge 2000 Addiction 95(Suppl 2):S91; Robinson and Berridge 2001 Addiction 96:103; Robinson and Berridge 2003 Ann Rev Psychol 54:25) have been cited 274, 297, and 365 times, respectively, adding up to more than 3,200 citations within 15 years. The present chapter aims to delineate the merits and limitations of the incentive sensitization view of addiction, and whether incentive sensitization occurs in humans. We conclude that since incentive sensitization most prominently occurs after the first few drug exposures, it may represent an important initial step in the addiction process. During the expression of full-blown addiction, characterized by loss of control over drug intake and use of large quantities of drugs, the expression of incentive sensitization may be transiently suppressed. However, detoxification and the gradual disappearance of tolerance and withdrawal symptoms may unmask sensitization, which could then play an important role in the high risk of relapse.

Cocaine-evoked synaptic plasticity: persistence in the VTA triggers adaptations in the NAc

Addictive drugs hijack mechanisms of learning and memory that normally underlie reinforcement of natural rewards and induce synaptic plasticity of glutamatergic transmission in the mesolimbic dopamine (DA) system. In the ventral tegmental area (VTA), a single exposure to cocaine efficiently triggers NMDA receptor-dependent synaptic plasticity in DA neurons, whereas plasticity in the nucleus accumbens (NAc) occurs only after repeated injections. Whether these two forms of plasticity are independent or hierarchically organized remains unknown. We combined ex vivo electrophysiology in acute brain slices with behavioral assays modeling drug relapse in mice and found that the duration of the cocaine-evoked synaptic plasticity in the VTA is gated by mGluR1. Overriding mGluR1 in vivo made the potentiation in the VTA persistent. This led to synaptic plasticity in the NAc, which contributes to cocaine-seeking behavior after protracted withdrawal. Impaired mGluR1 function in vulnerable individuals could represent a first step in the recruitment of the neuronal network that underlies drug addiction.

Signaling pathway adaptations and novel protein kinase A substrates related to behavioral sensitization to cocaine

Behavioral sensitization is an animal model for aspects of cocaine addiction. Cocaine-sensitized rats exhibit increased AMPA receptor (AMPAR) surface expression in the nucleus accumbens (NAc) which may in turn enhance drug seeking. To identify signaling pathways contributing to AMPAR up-regulation, we measured AMPAR surface expression and signaling pathway activation in the NAc of cocaine-sensitized rats, cocaine-exposed rats that failed to sensitize and saline controls on withdrawal days (WD) 1, 7, and 21. We focused on calcium/calmodulin-dependent protein kinase II (CaMKII), extracellular signal-regulated protein kinase (ERK), and protein kinase A (PKA). In sensitized rats, AMPAR surface expression was elevated on WD7 and WD21 but not WD1. ERK2 activation followed a parallel time-course, suggesting a role in AMPAR up-regulation. Both sensitized and non-sensitized rats exhibited CaMKII activation on WD7, suggesting that CaMKII activation is not sufficient for AMPAR up-regulation. PKA phosphorylation, measured using an antibody recognizing phosphorylated PKA substrates, increased gradually over withdrawal in sensitized rats, from below control levels on WD1 to significantly greater than controls on WD21. Using proteomics, novel sensitization-related PKA substrates were identified, including two structural proteins (CRMP-2 and alpha-tubulin) that we speculate may link PKA signaling to previously reported dendritic remodeling in NAc neurons of cocaine-sensitized rats.

Synaptic AMPA receptor plasticity and behavior

The ability to change behavior likely depends on the selective strengthening and weakening of brain synapses. The cellular models of synaptic plasticity, long-term potentiation (LTP) and depression (LTD) of synaptic strength, can be expressed by the synaptic insertion or removal of AMPA receptors (AMPARs), respectively. We here present an overview of studies that have used animal models to show that such AMPAR trafficking underlies several experience-driven phenomena-from neuronal circuit formation to the modification of behavior. We argue that monitoring and manipulating synaptic AMPAR trafficking represents an attractive means to study cognitive function and dysfunction in animal models.

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.

Paraventricular thalamus mediates context-induced reinstatement (renewal) of extinguished reward seeking

Paraventricular thalamus (PvTh) is uniquely placed to contribute to reinstatement of drug and reward seeking. It projects extensively to regions implicated in reinstatement including accumbens shell (AcbSh), prefrontal cortex (PFC) and basolateral amygdala (BLA), and receives afferents from other regions important for reinstatement such as lateral hypothalamus. We used complementary neuroanatomical and functional approaches to study the role of PvTh in context-induced reinstatement (renewal) of extinguished reward-seeking. Rats were trained to respond for a reward in context A, extinguished in context B and tested in context A or B. We applied the neuronal tracer cholera toxin B subunit (CTb) to AcbSh and examined retrograde-labelled neurons, c-Fos immunoreactivity (IR) and dual c-Fos/CTb labelled neurons in PvTh and other AcbSh afferents. In PvTh there was c-Fos IR in CTb-positive neurons associated with renewal showing activation of a PvTh-AcbSh pathway during renewal. In PFC there was little c-Fos IR in CTb-positive or negative neurons associated with renewal. In BLA, two distinct patterns of activation and retrograde labelling were observed. In rostral BLA there was significant c-Fos IR in CTb-negative neurons associated with renewal. In caudal BLA there was significant c-Fos IR in CTb-positive neurons associated with being tested in either the extinction (ABB) or training (ABA) context. We then studied the functional role of PvTh in renewal. Excitotoxic lesions of PvTh prevented renewal. These lesions had no effect on the acquisition of reward seeking. These results show that PvTh mediates context-induced reinstatement and that this renewal is associated with recruitment of a PvTh-AcbSh pathway.