Upregulation of Arc mRNA expression in the prefrontal cortex following cue-induced reinstatement of extinguished cocaine-seeking behavior

The activity-regulated cytoskeleton-associated protein, Arc, functions in the nucleus accumbens shell to limit multiple triggers of cocaine-seeking behaviour

Use of addictive substances like cocaine produces enduring associations between the drug experience and cues in the drug-taking environment. In individuals with a substance use disorder (SUD) and attempting to remain abstinent, these powerful drug-cue associations can trigger a return to active drug use, but the molecular mechanisms regulating drug-cue associations remain poorly understood. The activity-regulated cytoskeleton-associated protein (Arc) is induced by cocaine in the nucleus accumbens (NAc), an important brain reward region, but Arc's NAc function in SUD-related behaviour remains unclear. We show here that cocaine self-administration (SA) in rats produced a significant upregulation of Arc protein in both the core and shell subregions of the NAc. Subregion-specific Arc reduction (shRNA) in the medial NAc Shell enhanced both context-associated and cue-reinstated cocaine seeking, but without altering the motivation to work for cocaine, the sensitivity to the reinforcing effects of cocaine or the ability of cocaine priming to reinstate drug seeking. In contrast, we observed no effects of Arc knockdown in the NAc core on any aspect of cocaine SA, extinction or reinstated cocaine seeking, suggesting that Arc functions within the medial NAc shell, but not NAc core, to limit the strength of drug-context and drug-cue associations that promote cocaine-seeking behaviour.

Prelimbic Cortex Activity during a Distress Tolerance Task Predicts Cocaine-Seeking Behavior in Male, But Not Female Rats

Distress tolerance (DT) is defined as the ability to persist in challenging goal-directed behavior in the face of stress, and individuals with low DT exhibit heightened drug-seeking behavior. However, no preclinical studies have examined the neurobiology underlying this phenomenon. To assess this, in vivo electrophysiology was used in Long Evans male and female rats during a DT task to record neural activity in the prelimbic cortex (PrL), a brain region implicated in drug-seeking. Rats were first assessed for DT, defined as the amount of time elapsed before rats quit seeking reward in an increasingly difficult operant task. Subsequently, rats underwent 2 weeks of self-administration for either water/saline or cocaine for 6 h/day. Animals then began a 1 month period of experimenter-imposed abstinence to induce heightened drug-seeking behavior. On day 28 of abstinence, DT and neural activity were reassessed; and on day 30, cocaine-seeking behavior was examined under extinction. Males had significantly higher DT than females and exhibited significantly more phasic PrL activity during the DT task. Furthermore, in male rats with a history of cocaine, PrL activity shifted to track DT; and this change in activity significantly correlated with the change in DT. Additionally, male (but not female) rats with low DT after 28 d of abstinence had significantly heightened drug-seeking behavior. Finally, PrL activity during the DT task predicted cocaine-seeking behavior. Collectively, these data demonstrate an important role for the PrL in DT in males, and link this neural activity and behavior to drug-seeking, particularly in males.SIGNIFICANCE STATEMENT Distress tolerance (DT) is defined as the ability to persist in challenging goal-directed behavior in the face of stress, and individuals with low DT exhibit heightened drug-seeking. Here, we investigated the role of the prelimbic cortex (PrL) in DT and its relationship to cocaine-seeking in male and female rats. We found that males had significantly higher DT than females and exhibited significantly more PrL activity during the DT task. Furthermore, male (but not female) rats with low DT after 28 d of abstinence had significantly heightened drug-seeking behavior. Finally, PrL activity during the DT task predicted cocaine-seeking. These data demonstrate an important role for the PrL in DT and link this neural activity and behavior to drug-seeking in males.

Reactivation of cocaine contextual memory engages mechanistic target of rapamycin/S6 kinase 1 signaling

Mechanistic target of rapamycin (mTOR) C1 and its downstream effectors have been implicated in synaptic plasticity and memory. Our prior work demonstrated that reactivation of cocaine memory engages a signaling pathway consisting of Akt, glycogen synthase kinase-3β (GSK3β), and mTORC1. The present study sought to identify other components of mTORC1 signaling involved in the reconsolidation of cocaine contextual memory, including eukaryotic translation initiation factor 4E (eIF4E)-eIF4G interactions, p70 S6 kinase polypeptide 1 (p70S6K, S6K1) activity, and activity-regulated cytoskeleton (Arc) expression. Cocaine contextual memory was established in adult CD-1 mice using conditioned place preference. After cocaine place preference was established, mice were briefly re-exposed to the cocaine-paired context to reactivate the cocaine memory and brains examined. Western blot analysis showed that phosphorylation of the mTORC1 target, p70S6K, in nucleus accumbens and hippocampus was enhanced 60 min following reactivation of cocaine memories. Inhibition of mTORC1 with systemic administration of rapamycin or inhibition of p70S6K with systemic PF-4708671 after reactivation of cocaine contextual memory abolished the established cocaine place preference. Immunoprecipitation assays showed that reactivation of cocaine memory did not affect eIF4E-eIF4G interactions in nucleus accumbens or hippocampus. Levels of Arc mRNA were significantly elevated 60 and 120 min after cocaine memory reactivation and returned to baseline 24 h later. These findings demonstrate that mTORC1 and p70S6K are required for reconsolidation of cocaine contextual memory.

Post-training intra-basolateral complex of the amygdala infusions of clenbuterol enhance memory for conditioned place preference and increase ARC protein expression in dorsal hippocampal synaptic fractions

The basolateral complex of the amygdala (BLA) is critically involved in modulation of memory by stress hormones. Noradrenergic activation of the BLA enhances memory consolidation and plays a necessary role in the enhancing or impairing effects of stress hormones on memory. The BLA is not only involved in the consolidation of aversive memories but can regulate appetitive memory formation as well. Extensive evidence suggests that the BLA is a modulatory structure that influences consolidation of arousing memories through modulation of plasticity and expression of plasticity-related genes, such as the activity regulated cytoskeletal-associated (Arc/Arg 3.1) protein, in efferent brain regions. ARC is an immediate early gene whose mRNA is localized to the dendrites and is necessary for hippocampus-dependent long-term potentiation and long-term memory formation. Post-training intra-BLA infusions of the β-adrenoceptor agonist, clenbuterol, enhances memory for an aversive task and increases dorsal hippocampus ARC protein expression following training on that task. To examine whether this function of BLA noradrenergic signaling extends to the consolidation of appetitive memories, the present studies test the effect of post-training intra-BLA infusions of clenbuterol on memory for the appetitive conditioned place preference (CPP) task and for effects on ARC protein expression in hippocampal synapses. Additionally, the necessity of increased hippocampal ARC protein expression was also examined for long-term memory formation of the CPP task. Immediate post-training intra-BLA infusions of clenbuterol (4 ng/0.2 µL) significantly enhanced memory for the CPP task. This same memory enhancing treatment significantly increased ARC protein expression in dorsal, but not ventral, hippocampal synaptic fractions. Furthermore, immediate post-training intra-dorsal hippocampal infusions of Arc antisense oligodeoxynucleotides (ODNs), which reduce ARC protein expression, prevented long-term memory formation for the CPP task. These results suggest that noradrenergic activity in the BLA influences long-term memory for aversive and appetitive events in a similar manner and the role of the BLA is conserved across classes of memory. It also suggests that the influence of the BLA on hippocampal ARC protein expression and the role of hippocampal ARC protein expression are conserved across classes of emotionally arousing memories.

A sex-dependent role for the prelimbic cortex in impulsive action both before and following early cocaine abstinence

Although impulsive action is strongly associated with addiction, the neural underpinnings of this relationship and how they are influenced by sex have not been well characterized. Here, we used a titrating reaction time task to assess differences in impulsive action in male and female Long Evans rats both before and after short (4-6 days) or long (25-27 days) abstinence from 2 weeks of cocaine or water/saline self-administration (6 h daily access). Neural activity in the prelimbic cortex (PrL) and nucleus accumbens (NAc) core was assessed at each time point. We found that a history of cocaine self-administration increased impulsivity in all rats following short, but not long, abstinence. Furthermore, male rats with an increased ratio of excited to inhibited neurons in the PrL at the start of each trial in the task exhibited higher impulsivity in the naïve state (before self-administration). Following short abstinence from cocaine, PrL activity in males became more inhibited, and this change in activity predicted the shift in impulsivity. However, PrL activity did not track impulsivity in female rats. Additionally, although the NAc core tracked several aspects of behavior in the task, it did not track impulsivity in either sex. Together, these findings demonstrate a sex-dependent role for the PrL in impulsivity both before and after a history of cocaine.

The fragile X mental retardation protein promotes adjustments in cocaine self-administration that preserve reinforcement level

The fragile X mental retardation protein (FMRP), an RNA-binding protein, regulates cocaine-induced neuronal plasticity and is critical for the normal development of drug-induced locomotor sensitization, as well as reward-related learning in the conditioned place preference assay. However, it is unknown whether FMRP impacts behaviors that are used to more closely model substance use disorders. Utilizing a cocaine intravenous self-administration (IVSA) assay in Fmr1 knockout (KO) and wild-type (WT) littermate mice, we find that, despite normal acquisition and extinction learning, Fmr1 KO mice fail to make a normal upward shift in responding during dose-response testing. Later, when given access to the original acquisition dose under increasing fixed ratio (FR) schedules of reinforcement (FR1, FR3, and FR5), Fmr1 KO mice earn significantly fewer cocaine infusions than WT mice. Importantly, similar deficits are not present in operant conditioning using a palatable food reinforcer, indicating that our results do not represent broad learning or reward-related deficits in Fmr1 KO mice. Additionally, we find an FMRP target, the activity-regulated cytoskeleton-associated protein (Arc), to be significantly reduced in synaptic cellular fractions prepared from the nucleus accumbens of Fmr1 KO, compared with WT, mice following operant tasks reinforced with cocaine but not food. Overall, our findings suggest that FMRP facilitates adjustments in drug self-administration behavior that generally serve to preserve reinforcement level, and combined with our similar IVSA findings in Arc KO mice may implicate Arc, along with FMRP, in behavioral shifts that occur in drug taking when drug availability is altered.

Neural Substrates and Circuits of Drug Addiction

Drug addiction is a chronic relapsing disorder, and a significant amount of research has been devoted to understand the factors that contribute to the development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide an overview of various theories of addiction to drugs of abuse and the neurobiology involved in elements of the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the role of the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the contribution of these pathways and associated circuits to conditioned responses, drug craving, and loss of behavioral control that may underlie drug relapse. By enhancing the understanding of the neurobiological factors that mediate drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Drug-activated cells: From immediate early genes to neuronal ensembles in addiction

Beyond their rapid rewarding effects, drugs of abuse can durably alter an individual's response to their environment as illustrated by the compulsive drug seeking and risk of relapse triggered by drug-associated stimuli. The persistence of these associations even long after cessation of drug use demonstrates the enduring mark left by drugs on brain reward circuits. However, within these circuits, neuronal populations are differently affected by drug exposure and growing evidence indicates that relatively small subsets of neurons might be involved in the encoding and expression of drug-mediated associations. The identification of sparse neuronal populations recruited in response to drug exposure has benefited greatly from the study of immediate early genes (IEGs) whose induction is critical in initiating plasticity programs in recently activated neurons. In particular, the development of technologies to manipulate IEG-expressing cells has been fundamental to implicate broadly distributed neuronal ensembles coincidently activated by either drugs or drug-associated stimuli and to then causally establish their involvement in drug responses. In this review, we summarize the literature regarding IEG regulation in different learning paradigms and addiction models to highlight their role as a marker of activity and plasticity. As the exploration of neuronal ensembles in addiction improves our understanding of drug-associated memory encoding, it also raises several questions regarding the cellular and molecular characteristics of these discrete neuronal populations as they become incorporated in drug-associated neuronal ensembles. We review recent efforts towards this goal and discuss how they will offer a more comprehensive understanding of addiction pathophysiology.

Effect of pharmacological manipulations on Arc function

Activity-regulated cytoskeleton-associated protein (Arc) is a brain-enriched immediate early gene that regulates important mechanisms implicated in learning and memory. Arc levels are controlled through a balance of induction and degradation in an activity-dependent manner. Arc further undergoes multiple post-translational modifications that regulate its stability, localization and function. Recent studies demonstrate that these features of Arc can be pharmacologically manipulated. In this review, we discuss some of these compounds, with an emphasis on drugs of abuse and psychotropic drugs. We also discuss inflammatory states that regulate Arc.

Genetic Architecture and Molecular Neuropathology of Human Cocaine Addiction

We integrated genomic and bioinformatic analyses, using data from the largest genome-wide association study of cocaine dependence (CD; n = 6546; 82.37% with CD; 57.39% male) and the largest postmortem gene-expression sample of individuals with cocaine use disorder (CUD; n = 36; 51.35% with CUD; 100% male). Our genome-wide analyses identified one novel gene (NDUFB9) associated with the genetic predisposition to CD in African-Americans. The genetic architecture of CD was similar across ancestries. Individual genes associated with CD demonstrated modest overlap across European-Americans and African-Americans, but the genetic liability for CD converged on many similar tissue types (brain, heart, blood, liver) across ancestries. In a separate sample, we investigated the neuronal gene expression associated with CUD by using RNA sequencing of dorsal-lateral prefrontal cortex neurons. We identified 133 genes differentially expressed between CUD case patients and cocaine-free control subjects, including previously implicated candidates for cocaine use/addiction (FOSB, ARC, KCNJ9/GIRK3, NR4A2, JUNB, and MECP2). Differential expression analyses significantly correlated across European-Americans and African-Americans. While genes significantly associated with CD via genome-wide methods were not differentially expressed, two of these genes (NDUFB9 and C1qL2) were part of a robust gene coexpression network associated with CUD involved in neurotransmission (GABA, acetylcholine, serotonin, and dopamine) and drug addiction. We then used a "guilt-by-association" approach to unravel the biological relevance of NDUFB9 and C1qL2 in the context of CD. In sum, our study furthers the understanding of the genetic architecture and molecular neuropathology of human cocaine addiction and provides a framework for translating biological meaning into otherwise obscure genome-wide associations.SIGNIFICANCE STATEMENT Our study further clarifies the genetic and neurobiological contributions to cocaine addiction, provides a rapid approach for generating testable hypotheses for specific candidates identified by genome-wide research, and investigates the cross-ancestral biological contributions to cocaine use disorder/dependence for individuals of European-American and African-American ancestries.

The activity-regulated cytoskeleton-associated protein, Arc/Arg3.1, influences mouse cocaine self-administration

The activity-regulated cytoskeleton-associated protein (Arc, also known as Arg3.1), an immediate early gene and synaptic regulator, is upregulated following a single cocaine exposure. However, there is not much known regarding Arc/Arg3.1's potential contribution to addiction-relevant behaviors. Despite known learning and memory deficits in contextual fear and water-maze reversal learning tasks, we find that mice lacking Arc/Arg3.1 perform conditioned place preference and operant conditioning involving positive reinforcers (food and cocaine) with little-to-no impairment. However, following normal saline-extinction, wild type (WT) mice show a classic inverted-U dose-response function, while Arc/Arg3.1 knockout (KO) mice fail to adjust their intake across multiple doses. Importantly, Arc/Arg3.1 KO and WT mice behave comparably on an increasing cost task (FR1-FR3; acquisition dose), providing evidence that both groups find cocaine reinforcing. Differences in individuals that drive variations in use patterns and particularly, drug intake levels, are critical as they influence the likelihood of developing dependence. Our data suggest that Arc/Arg3.1 may contribute to addiction as a regulator of drug-taking vulnerability under different drug availability conditions.

Neuronal activation in orbitofrontal cortex subregions: Cfos expression following cue-induced reinstatement of cocaine-seeking behavior

Cocaine-use disorders are characterized by repeated relapse to drug-seeking and drug-taking behavior following periods of abstinence. Former drug users display increased activation of the orbitofrontal cortex (OFC) in response to drug-related cues, and similar phenomena are also observed in rodent models of drug relapse. The lateral, but not medial, OFC functionally contributes to the maintenance of cue-drug associations; however, less is known about the role of the ventral OFC in this process. To examine the pattern of neuronal activation in OFC subregions in response to drug-associated cues, rats were trained to respond on a lever for a cocaine infusion paired with a complex cue (2-hr sessions, minimum 10 days). Cocaine self-administration was followed by extinction training, in which lever responses resulted in no consequences (2-hr sessions, minimum 7 days). During a 1-hr reinstatement test, drug-seeking behavior (i.e., responses on the drug-paired lever) was examined in the presence or absence of contingent drug-paired cues (Cue TEST vs. Ext TEST, respectively). Rats were overdosed with a ketamine + xylazine cocktail 30-min post session, and transcardially perfused with 4% paraformaldehyde. Cfos protein expression was utilized to measure potential changes in neural activation between the reinstatement test groups. An increase in the number of Cfos-Immunoreactive cells was observed in the ventral and lateral subregions of the OFC in the Cue TEST group. The present findings provide evidence that the ventral and lateral regions of the rat OFC display similar patterns of neuronal activation in response to cocaine-paired cues. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Neurobiological substrates of persistent working memory deficits and cocaine-seeking in the prelimbic cortex of rats with a history of extended access to cocaine self-administration

Cocaine use disorder (CUD) is associated with prefrontal cortex dysfunction and cognitive deficits that may contribute to persistent relapse susceptibility. As the relationship between cognitive deficits, cortical abnormalities and drug seeking is poorly understood, development of relevant animal models is of high clinical importance. Here, we used an animal model to characterize working memory and reversal learning in rats with a history of extended access cocaine self-administration and prolonged abstinence. We also investigated immediate and long-term functional changes within the prelimbic cortex (PrL) in relation to cognitive performance and drug-seeking. Adult male rats underwent 6 days of short-access (1 h/day) followed by 12 days of long-access (6 h/day) cocaine self-administration, or received passive saline infusions. Next, rats were tested in delayed match-to-sample (DMS) and (non)match-to-sample (NMS) tasks, and finally in a single context + cue relapse test on day 90 of abstinence. We found that a history of chronic cocaine self-administration impaired working memory, though sparing reversal learning, and that the components of these cognitive measures correlated with later drug-seeking. Further, we found that dysregulated metabolic activity and mGlu5 receptor signaling in the PrL of cocaine rats correlated with past working memory performance and/or drug-seeking, as indicated by the analysis of cytochrome oxidase reactivity, mGlu5 and Homer 1b/c protein expression, as well as Arc mRNA expression in mGlu5-positive cells. These findings advocate for a persistent post-cocaine PrL dysfunction, rooted in ineffective compensatory changes and manifested as impaired working memory performance and hyperreactivity to cocaine cues. Considering the possible interplay between the neural correlates underlying post-cocaine cognitive deficits and drug-seeking, cognitive function should be evaluated and considered when developing neurobiologically-based treatments of cocaine relapse.

Common neurocircuitry mediating drug and fear relapse in preclinical models

BACKGROUND

Comorbidity of anxiety disorders, stressor- and trauma-related disorders, and substance use disorders is extremely common. Moreover, therapies that reduce pathological fear and anxiety on the one hand, and drug-seeking on the other, often prove short-lived and are susceptible to relapse. Considerable advances have been made in the study of the neurobiology of both aversive and appetitive extinction, and this work reveals shared neural circuits that contribute to both the suppression and relapse of conditioned responses associated with trauma or drug use.

OBJECTIVES

The goal of this review is to identify common neural circuits and mechanisms underlying relapse across domains of addiction biology and aversive learning in preclinical animal models. We focus primarily on neural circuits engaged during the expression of relapse.

KEY FINDINGS

After extinction, brain circuits involving the medial prefrontal cortex and hippocampus come to regulate the expression of conditioned responses by the amygdala, bed nucleus of the stria terminalis, and nucleus accumbens. During relapse, hippocampal projections to the prefrontal cortex inhibit the retrieval of extinction memories resulting in a loss of inhibitory control over fear- and drug-associated conditional responding.

CONCLUSIONS

The overlapping brain systems for both fear and drug memories may explain the co-occurrence of fear and drug-seeking behaviors.

The role of the orbitofrontal cortex in alcohol use, abuse, and dependence

One of the major functions of the orbitofrontal cortex (OFC) is to promote flexible motivated behavior. It is no surprise, therefore, that recent work has demonstrated a prominent impact of chronic drug use on the OFC and a potential role for OFC disruption in drug abuse and addiction. Among drugs of abuse, the use of alcohol is particularly salient with respect to OFC function. Although a number of studies in humans have implicated OFC dysregulation in alcohol use disorders, animal models investigating the association between OFC and alcohol use are only beginning to be developed, and there is still a great deal to be revealed. The goal of this review is to consider what is currently known regarding the role of the OFC in alcohol use and dependence. I will first provide a brief, general overview of current views of OFC function and its contributions to drug seeking and addiction. I will then discuss research to date related to the OFC and alcohol use, both in human clinical populations and in non-human models. Finally I will consider issues and strategies to guide future study that may identify this brain region as a key player in the transition from moderated to problematic alcohol use and dependence.

ARC and BDNF expression after cocaine self-administration or cue-induced reinstatement of cocaine seeking in adolescent and adult male rats

Recreational drug use peaks during adolescence. Our research with adolescent vs adult male rats, however, shows that rats taking cocaine as adolescents have lower levels of cue-induced reinstatement of drug-seeking than adults, despite similar levels of intravenous (i.v.) cocaine self-administration. Lower rates of reinstatement in younger rats could be explained by higher levels of brain plasticity. Two neuroplasticity-related genes, activity-regulated cytoskeletal-associated gene (Arc) and brain-derived neurotrophic factor (Bdnf), influence cocaine self-administration and cue-induced reinstatement. We tested whether reinstatement of cocaine seeking correlates with expression of these genes in reinforcement-related brain regions. Adolescent and adult male rats (postnatal day 35 or 83-95 at start) were allowed to acquire lever-pressing maintained by i.v. infusions of cocaine in daily 2-h sessions over 13 days. At one of three experimental time points, rats were sacrificed and tissue collected to analyze Arc and Bdnf mRNA by in situ hybridization in the entire medial prefrontal cortex and entire nucleus accumbens, as well as relevant subregions: prelimbic cortex, infralimbic cortex, and nucleus accumbens core and shell. Despite taking similar amounts of cocaine, adolescents reinstated less than adults. Gene expression was most notable in the prelimbic cortex, was generally higher in adolescent-onset groups, and was higher with longer abstinence. These data partially support the hypothesis that higher levels of Arc and/or Bdnf gene expression in reinforcement-related brain regions of younger animals contribute to lower rates of extinction responding and/or reinstatement. Future studies should include mechanistic analysis of Arc, Bdnf, and their signaling pathways in age-dependent effects of cocaine.

The hot 'n' cold of cue-induced drug relapse

Environmental cues associated with rewards can acquire motivational properties. However, there is considerable variation in the extent to which a reward cue gains motivational control over behavior, depending on the individual and the form of the cue. When a discrete cue is paired with food reward, it acquires greater control over motivated behavior in some rats (sign-trackers, STs) than others (goal-trackers, GTs) as indicated by the propensity to approach the cue, the willingness to work to obtain it, and its ability to reinstate reward-seeking behavior. Here, we review studies that employ this ST/GT animal model to investigate characteristics of individuals that are especially susceptible to reward cue-elicited behavior and the involvement of dopamine and acetylcholine neuromodulator systems in the susceptibility to cue-induced drug relapse. First, we discuss individual differences in the attribution of incentive salience to different forms of reward cues and the involvement of the mesolimbic dopamine system. We then discuss individual differences in cognitive/attentional control and the contributions of the cholinergic system in processing reward cues. It is suggested that in STs a propensity to attribute motivational properties to a drug cue is combined with poor attentional control in the face of these cues, making them particularly vulnerable to transition from casual/experimental patterns of drug use to addiction and to cue-induced relapse.

Cued Reinstatement of Cocaine but Not Sucrose Seeking Is Dependent on Dopamine Signaling in Prelimbic Cortex and Is Associated with Recruitment of Prelimbic Neurons That Project to Contralateral Nucleus Accumbens Core

BACKGROUND

Drug cues recruit prelimbic cortex neurons that project to ipsilateral nucleus accumbens core. However, it is not known if the same is true for prelimbic cortex projections that decussate to innervate contralateral nucleus accumbens core. Further, a role for prelimbic cortex dopamine signaling in cued reinstatement of cocaine seeking has not been shown.

METHODS

We assessed Fos expression in prelimbic cortex neurons that project to contralateral nucleus accumbens core following cued reinstatement of cocaine or sucrose seeking. We also tested the effect of intra-prelimbic cortex infusions of the D1/D2 antagonist fluphenazine on cued cocaine and sucrose seeking.

RESULTS

Prelimbic cortex-contralateral nucleus accumbens core projections were activated by cocaine cues but not sucrose cues, and this activation correlated with reinstatement behavior. Blockade of prelimbic cortex dopamine signaling prevented cued reinstatement of cocaine- but not sucrose-seeking behavior.

CONCLUSIONS

Cued cocaine seeking is associated with activation of the prelimbic cortex-contralateral nucleus accumbens core pathway. Prelimbic cortex dopamine signaling is necessary for cues to reinstate drug-seeking behavior.

Endogenous glutamate within the prelimbic and infralimbic cortices regulates the incubation of cocaine-seeking in rats

The incubation of cue-reinforced cocaine-seeking coincides with increased extracellular glutamate within the ventromedial prefrontal cortex (vmPFC). The vmPFC is comprised of two subregions that oppositely regulate drug-seeking, with infralimbic (IL) activity inhibiting, and prelimibic (PL) activity facilitating, drug-seeking. Thus, we hypothesized that increasing and decreasing endogenous glutamate within the IL would attenuate and potentiate, respectively, cue-reinforced drug-seeking behavior, with the converse effects observed upon manipulations of endogenous glutamate within the PL. Male Sprague-Dawley rats were trained to self-administer cocaine (0.25 mg/infusion; 6 h/day X 10 days), the delivery of which was signaled by a tone-light cue. Rats were then subdivided into 3 or 30 day withdrawal groups. For testing, rats were microinjected with vehicle, 20 mM of the mGlu2/3 agonist LY379268 (to lower endogenous glutamate), or 300 μM of the excitatory amino acid transporter inhibitor threo-β-benzyloxyaspartate (TBOA; to raise endogenous glutamate) into either the IL or PL (0.5 μl/side) and then given a 30-min test for cue-reinforced drug-seeking. Vehicle-infused rats exhibited incubated responding on the cocaine-associated lever. Neither LY379268 nor TBOA altered behavior at 3 days withdrawal, indicating that glutamate within neither subregion regulates cue-reinforced drug-seeking during early withdrawal. At 30 days withdrawal, intra-PL LY379268 microinjection significantly decreased drug-seeking behavior, while the effect was more modest when infused intra-IL. Interestingly, intra-IL TBOA attenuated incubated drug-seeking during protracted withdrawal, but did not affect behavior when infused intra-PL. These results argue that glutamate release within the PL in response to drug-seeking likely drives the manifestation of incubated cocaine-seeking during protracted withdrawal.

Activity-Regulated Cytoskeleton-Associated Protein Accumulates in the Nucleus in Response to Cocaine and Acts as a Brake on Chromatin Remodeling and Long-Term Behavioral Alterations

BACKGROUND

Addiction relies on persistent alterations of neuronal properties, which depends on gene regulation. Activity-regulated cytoskeleton-associated protein (Arc) is an immediate early gene that modulates neuronal plasticity underlying learning and memory. Its role in cocaine-induced neuronal and behavioral adaptations remains elusive.

METHODS

Acute cocaine-treated mice were used for quantitative reverse-transcriptase polymerase chain reaction, immunocytochemistry, and confocal imaging from striatum. Live imaging and transfection assays for Arc overexpression were performed from primary cultures. Molecular and behavioral adaptations to cocaine were studied from Arc-deficient mice and their wild-type littermates.

RESULTS

Arc messenger RNA and proteins are rapidly induced in the striatum after acute cocaine administration, via an extracellular-signal regulated kinase-dependent de novo protein synthesis. Although detected in dendrites, Arc accumulates in the nucleus in active zones of transcription, where it colocalizes with phospho-Ser10-histone H3, an important component of nucleosomal response. In vitro, Arc overexpression downregulates phospho-Ser10-histone H3 without modifying extracellular-signal regulated kinase phosphorylation in the nucleus. In vivo, Arc-deficient mice display decreased heterochromatin domains, a high RNA-polymerase II activity and enhanced c-Fos expression. These mice presented an exacerbated psychomotor sensitization and conditioned place preference induced by low doses of cocaine.

CONCLUSIONS

Cocaine induces the rapid induction of Arc and its nuclear accumulation in striatal neurons. Locally, it alters the nucleosomal response, and acts as a brake on chromatin remodeling and gene regulation. These original observations posit Arc as a major homeostatic modulator of molecular and behavioral responses to cocaine. Thus, modulating Arc levels may provide promising therapeutic approaches in drug addiction.

Developmental Exposure to Cocaine Dynamically Dysregulates Cortical Arc/Arg3.1 Modulation in Response to a Challenge

During adolescence, the medial prefrontal cortex (mPFC) is still developing. We have previously shown that developmental cocaine exposure alters mPFC's ability to cope with challenging events. In this manuscript, we exposed rats developmentally treated with cocaine to a novelty task and analyzed the molecular changes of mPFC. Rats were exposed to cocaine from post-natal day (PND) 28 to PND 42 and sacrificed at PND 43, immediately after the novel object recognition (NOR) test. Cocaine-treated rats spent more time exploring the novel object than saline-treated counterparts, suggesting an increased response to novelty. The messenger RNA (mRNA) and protein levels of the immediate early gene Arc/Arg3.1 were reduced in both infralimbic (IL) and prelimbic (PL) cortices highlighting a baseline reduction of mPFC neuronal activity as a consequence of developmental exposure to cocaine. Intriguingly, significant molecular changes were observed in the IL, but not PL, cortex in response to the combination of cocaine exposure and test such as a marked upregulation of both Arc/Arg3.1 mRNA and protein levels only in cocaine-treated rats. As for proteins, such increase was observed only in the post-synaptic density and not in the whole homogenate, suggesting psychostimulant-induced changes in trafficking of Arc/Arg3.1 or an increased local translation. Notably, the same profile of Arc/Arg3.1 was observed for post-synaptic density (PSD)-95 leading to the possibility that Arc/Arg3.1 and PSD-95 bridge together to promote aberrant synaptic connectivity in IL cortex following repeated exposure to cocaine during brain development.

Cocaine Self-Administration Elevates GluN2B within dmPFC Mediating Heightened Cue-Elicited Operant Responding

Cue-elicited drug-craving correlates with hyperactivity within prefrontal cortex (PFC), which is theorized to result from dysregulated excitatory neurotransmission. The NMDA glutamate receptor is highly implicated in addiction-related neuroplasticity. As NMDA receptor function is regulated critically by its GluN2 subunits, herein, we assayed the relation between incubated cue-elicited cocaine-seeking following extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) and the expression of GluN2A/B receptor subunits within PFC sub regions during early versus late withdrawal (respectively, 3 vs. 30 days). Cocaine-seeking rats exhibited elevated GluN2B expression within the dorsomedial aspect of the PFC (dmPFC); this effect was apparent at both 3 and 30 days withdrawal and occurred in cocaine-experienced rats, regardless of experiencing an extinction test or not. Thus, elevated dmPFC GluN2B expression appears to reflect a pharmacodynamic response to excessive cocaine intake that is independent of the duration of drug withdrawal or re-exposure to drug-taking context. The functional relevance of elevated dmPFC GluN2B expression for drug-seeking was assessed by the local infusion of the prototypical GluN2B-selective antagonist ifenprodil (1.0 µg/side). Ifenprodil did not alter cue-elicited responding in animals with a history of saline self-administration. In contrast, ifenprodil lowered cue-elicited cocaine-seeking, while potentiating cue-elicited sucrose-seeking. Thus, the effects of an intra-dmPFC ifenprodil infusion upon cue reactivity are reinforcer-specific, arguing in favor of targeting GluN2B-containing NMDA receptors as a pharmacological strategy for reducing behavioral reactivity to drug-associated cues with the potential benefit of heightening the reinforcing properties of cues associated with non-drug primary rewards.

Differential roles of medial prefrontal subregions in the regulation of drug seeking

The prefrontal cortex plays an important role in shaping cognition and behavior. Many studies have shown that medial prefrontal cortex (mPFC) plays a key role in seeking, extinction, and reinstatement of cocaine seeking in rodent models of relapse. Subregions of mPFC appear to play distinct roles in these behaviors, such that the prelimbic cortex (PL) is proposed to drive cocaine seeking and the infralimbic cortex (IL) is proposed to suppress cocaine seeking after extinction. This dichotomy of mPFC function may be a general attribute, as similar dorsal-ventral distinctions exist for expression vs. extinction of fear conditioning. However, other results indicate that the role of mPFC neurons in reward processing is more complex than a simple PL-seek vs. IL-extinguish dichotomy. Both PL and IL have been shown to drive and inhibit drug seeking (and other types of behaviors) depending on a range of factors including the behavioral context, the drug-history of the animal, and the type of drug investigated. This heterogeneity of findings may reflect multiple subcircuits within each of these PFC areas supporting unique functions. It may also reflect the fact that the mPFC plays a multifaceted role in shaping cognition and behavior, including those overlapping with cocaine seeking and extinction. Here we discuss research leading to the hypothesis that dorsal and ventral mPFC differentially control drug seeking and extinction. We also present recent results calling the absolute nature of a PL vs. IL dichotomy into question. Finally, we consider alternate functions for mPFC that correspond less to response execution and inhibition and instead incorporate the complex cognitive behavior for which the mPFC is broadly appreciated.

Revealing Behavioral Learning Deficit Phenotypes Subsequent to In Utero Exposure to Benzo(a)pyrene

To characterize behavioral deficits in pre-adolescent offspring exposed in utero to Benzo(a)pyrene [B(a)P], timed-pregnant Long Evans Hooded rats were treated with B(a)P (150, 300, 600, and 1200 µg/kg BW) or peanut oil (vehicle) on E14, 15, 16, and 17. Following birth, during the pre-weaning period, B(a)P metabolites were examined in plasma and whole brain or cerebral cortex from exposed and control offspring. Tissue concentrations of B(a)P metabolites were (1) dose-dependent and (2) followed a time-dependence for elimination with ∼60% reduction by PND5 in the 1200 µg/kg BW experimental group. Spatial discrimination-reversal learning was utilized to evaluate potential behavioral neurotoxicity in P40-P60 offspring. Late-adolescent offspring exposed in utero to 600 and 1200 µg/kg BW were indistinguishable from their control counterparts for ability to acquire an original discrimination (OD) and reach criterion. However, a dose-dependent effect of in utero B(a)P-exposure was evident upon a discrimination reversal as exposed offspring perseverated on the previously correct response. This newly characterized behavioral deficit phenotype for the first reversal was not apparent in either the (1) OD or (2) subsequent reversal sessions relative to the respective control offspring. Furthermore, the expression of activity related-cytoskeletal-associated protein (Arc), an experience-dependent cortical protein marker known to be up-regulated in response to acquisition of a novel behavior, was greater in B(a)P-exposed offspring included in the spatial discrimination cohort versus home cage controls. Collectively, these findings support the hypothesis that in utero exposure to B(a)P during critical windows of development representing peak periods of neurogenesis results in behavioral deficits in later life.

Dyadic social interaction inhibits cocaine-conditioned place preference and the associated activation of the accumbens corridor

Impaired social interaction is a hallmark symptom of many psychiatric disorders. In substance use disorders, impaired social interaction is triply harmful (a) because addicts increasingly prefer the drug of abuse to the natural reward of drug-free social interaction, thus worsening the progression of the disease by increasing their drug consumption, (b) because treatment adherence and, consequently, treatment success itself depends on the ability of the recovering addict to maintain social interaction and adhere to treatment, and (c) because socially interacting with an individual suffering from a substance use disorder may be harmful for others. Helping the addict reorient his/her behavior away from the drug of abuse toward social interaction would therefore be of considerable therapeutic benefit. This article reviews our work on the neural basis of such a reorientation from cocaine, as a prototypical drug of abuse, toward dyadic (i.e. one-to-one) social interaction and compares our findings with the effects of other potentially beneficial interventions, that is, environmental enrichment or paired housing, on the activation of the accumbens and other brain regions involved in behavior motivated by drugs of abuse or nondrug stimuli. Our experimental models are based on the conditioned place preference paradigm. As the therapeutically most promising finding, only four 15 min episodes of dyadic social interaction were able to inhibit both the subsequent reacquisition/re-expression of preference for cocaine and the neural activation associated with this behavior, that is, an increase in the expression of the immediate early gene Early Growth Response protein 1 (EGR1, Zif268) in the nucleus accumbens, basolateral and central amygdala, and the ventral tegmental area. The time spent in the cocaine-associated conditioning compartment was correlated with the density of EGR1-activated neurons not only in the medial core (AcbCm) and medial shell (AcbShm) of the nucleus accumbens, but was observed in all regions medial to the anterior commissure ('accumbens corridor'), including (from medial to lateral), the vertical limb of the diagonal band and the medial septum (VDB+MS), the major island of Calleja and the intermediate nucleus of the lateral septum (ICjM+LSI), the AcbShm, and the AcbCm. All effects were limited to GABAergic projection neurons (called 'medium spiny neurons', in the accumbens), encompassing both dopamine D1 receptor-expressing and D2 receptor-expressing medium spiny neuron subtypes. Our EGR1 expression findings were mirrored in multielectrode array recordings. Finally, we have validated our paradigm in C57BL/6 mice to make use of the plethora of transgenic models available in this genus.

The Dorsal Agranular Insular Cortex Regulates the Cued Reinstatement of Cocaine-Seeking, but not Food-Seeking, Behavior in Rats

Prior studies suggest that the insular cortex (IC), and particularly its posterior region (the PIc), is involved in nicotine craving and relapse in humans and rodents. The present experiments were conducted to determine whether the IC and its different subregions regulate relapse to cocaine-seeking behavior in rats. To address this issue, male Sprague-Dawley rats underwent cocaine self-administration followed by extinction training and reinstatement tests. Before each reinstatement, the PIc or the more anterior dorsal agranular IC (AId) was inactivated to determine their roles in the reinstatement to cocaine seeking. In contrast to the nicotine findings, PIc inactivation had no effect on cue-induced reinstatement for cocaine seeking. However, AId inactivation reduced cued reinstatement while having no effect on cocaine-prime reinstatement. AId inactivation had no effect on reinstatement of food-seeking behavior induced by cues, a food-prime, or cues+food-prime. Based on previous work hypothesizing a role for corticotropin-releasing factor (CRF) in the IC during craving and relapse, a subsequent experiment found that CRF receptor-1 (CRF1) blockade in the AId similarly reduced cued reinstatement. Our results suggest that the AId, along with CRF1 receptors in this region, regulates reinstatement to cocaine seeking, but not food seeking, depending on the type of reinstatement, whereas PIc activity does not influence cue-induced reinstatement.

Synaptic Cytoskeletal Plasticity in the Prefrontal Cortex Following Psychostimulant Exposure

Addiction is characterized by maladaptive decision-making, a loss of control over drug consumption and habit-like drug seeking despite adverse consequences. These cognitive changes may reflect the effects of drugs of abuse on prefrontal cortical neurobiology. Here, we review evidence that amphetamine and cocaine fundamentally remodel the structure of excitatory neurons in the prefrontal cortex. We summarize evidence in particular that these psychostimulants have opposing effects in the medial and orbital prefrontal cortices ('mPFC' and 'oPFC', respectively). For example, amphetamine and cocaine increase dendrite length and spine density in the mPFC, while dendrites are impoverished and dendritic spines are eliminated in the oPFC. We will discuss evidence that certain cytoskeletal regulatory proteins expressed in the oPFC and implicated in postnatal (adolescent) neural development also regulate behavioral sensitivity to cocaine. These findings potentially open a window of opportunity for the identification of novel pharmacotherapeutic targets in the treatment of drug abuse disorders in adults, as well as in drug-vulnerable adolescent populations. Finally, we will discuss the behavioral implications of drug-related dendritic spine elimination in the oPFC, with regard to reversal learning tasks and tasks that assess the development of reward-seeking habits, both used to model aspects of addiction in rodents.

Cortico-striatal circuits: Novel therapeutic targets for substance use disorders

It is widely believed that substance use disorder (SUD) results from both pre-alterations (vulnerability) and/or post-alterations (drug effects) on cortico-striatal circuits. These circuits are essential for cognitive control, motivation, reward dependent learning, and emotional processing. As such, dysfunctions in cortico-striatal circuits are thought to relate to the core features of SUD, which include compulsive drug use, loss of the ability to control drug intake, and the emergence of negative emotional states (Koob and Volkow, 2010. Neuropsychopharmacology 35(1), 217-238). While the brain circuits underlying SUD have been studied in human patients largely through imaging studies, experiments in animals have allowed researchers to examine the specific cell-types within these circuits to reveal their role in behavior relevant to SUD. Here, we will review imaging studies on cortico-striatal systems that are altered in SUD, and describe animal experiments that relate SUD to specific neural projections and cell types within this circuitry. We will end with a discussion of novel clinical approaches such as deep brain stimulation (DBS), repeated transcranial magnetic stimulation (rTMS), and pharmacological targeting of G protein-coupled receptor (GPCR) heteromers that may provide promising avenues for modulating these circuits to combat SUD in humans.

Cocaine self-administration causes signaling deficits in corticostriatal circuitry that are reversed by BDNF in early withdrawal

Cocaine self-administration disturbs intracellular signaling in prefrontal cortical neurons that regulate neurotransmission in the nucleus accumbens. The deficits in dorsomedial prefrontal cortex (dmPFC) signaling change over time, resulting in different neuroadaptations during early withdrawal from cocaine self-administration than after one or more weeks of abstinence. Within the first few hours of withdrawal, there is a marked decrease in tyrosine phosphorylation of critical intracellular and membrane-bound proteins in the dmPFC that include ERK/MAP kinase and the NMDA receptor subunits, GluN1 and GluN2B. These changes are accompanied by a marked increase in STEP tyrosine phosphatase activation. Simultaneously, ERK and PKA-dependent synapsin phosphorylation in presynaptic terminals of the nucleus accumbens is increased that may have a destabilizing impact on glutamatergic transmission. Infusion of brain-derived neurotrophic factor (BDNF) into the dmPFC immediately following a final session of cocaine self-administration blocks the cocaine-induced changes in phosphorylation and attenuates relapse to cocaine seeking for as long as three weeks. The intra-dmPFC BDNF infusion also prevents cocaine-induced deficits in prefronto-accumbens glutamatergic transmission that are implicated in cocaine seeking. Thus, intervention with BDNF in the dmPFC during early withdrawal has local and distal effects in target areas that are critical to mediating cocaine-induced neuroadaptations that lead to cocaine seeking.

Reacquisition of cocaine conditioned place preference and its inhibition by previous social interaction preferentially affect D1-medium spiny neurons in the accumbens corridor

We investigated if counterconditioning with dyadic (i.e., one-to-one) social interaction, a strong inhibitor of the subsequent reacquisition of cocaine conditioned place preference (CPP), differentially modulates the activity of the diverse brain regions oriented along a mediolateral corridor reaching from the interhemispheric sulcus to the anterior commissure, i.e., the nucleus of the vertical limb of the diagonal band, the medial septal nucleus, the major island of Calleja, the intermediate part of the lateral septal nucleus, and the medial accumbens shell and core. We also investigated the involvement of the lateral accumbens core and the dorsal caudate putamen. The anterior cingulate 1 (Cg1) region served as a negative control. Contrary to our expectations, we found that all regions of the accumbens corridor showed increased expression of the early growth response protein 1 (EGR1, Zif268) in rats 2 h after reacquisition of CPP for cocaine after a history of cocaine CPP acquisition and extinction. Previous counterconditioning with dyadic social interaction inhibited both the reacquisition of cocaine CPP and the activation of the whole accumbens corridor. EGR1 activation was predominantly found in dynorphin-labeled cells, i.e., presumably D1 receptor-expressing medium spiny neurons (D1-MSNs), with D2-MSNs (immunolabeled with an anti-DRD2 antibody) being less affected. Cholinergic interneurons or GABAergic interneurons positive for parvalbumin, neuropeptide Y or calretinin were not involved in these CPP-related EGR1 changes. Glial cells did not show any EGR1 expression either. The present findings could be of relevance for the therapy of impaired social interaction in substance use disorders, depression, psychosis, and autism spectrum disorders.

Retrieval-induced NMDA receptor-dependent Arc expression in two models of cocaine-cue memory

The association of environmental cues with drugs of abuse results in persistent drug-cue memories. These memories contribute significantly to relapse among addicts. While conditioned place preference (CPP) is a well-established paradigm frequently used to examine the modulation of drug-cue memories, very few studies have used the non-preference-based model conditioned activity (CA) for this purpose. Here, we used both experimental approaches to investigate the neural substrates of cocaine-cue memories. First, we directly compared, in a consistent setting, the involvement of cortical and subcortical brain regions in cocaine-cue memory retrieval by quantifying activity-regulated cytoskeletal-associated (Arc) protein expression in both the CPP and CA models. Second, because NMDA receptor activation is required for Arc expression, we investigated the NMDA receptor dependency of memory persistence using the CA model. In both the CPP and CA models, drug-paired animals showed significant increases in Arc immunoreactivity in regions of the frontal cortex and amygdala compared to unpaired controls. Additionally, administration of a NMDA receptor antagonist (MK-801 or memantine) immediately after cocaine-CA memory reactivation impaired the subsequent conditioned locomotion associated with the cocaine-paired environment. The enhanced Arc expression evident in a subset of corticolimbic regions after retrieval of a cocaine-context memory, observed in both the CPP and CA paradigms, likely signifies that these regions: (i) are activated during retrieval of these memories irrespective of preference-based decisions, and (ii) undergo neuroplasticity in order to update information about cues previously associated with cocaine. This study also establishes the involvement of NMDA receptors in maintaining memories established using the CA model, a characteristic previously demonstrated using CPP. Overall, these results demonstrate the utility of the CA model for studies of cocaine-context memory and suggest the involvement of an NMDA receptor-dependent Arc induction pathway in drug-cue memory interference.

Prelimbic and infralimbic cortical regions differentially encode cocaine-associated stimuli and cocaine-seeking before and following abstinence

Cocaine stimuli often trigger relapse of drug-taking, even following periods of prolonged abstinence. Here, electrophysiological recordings were made in rats (n = 29) to determine how neurons in the prelimbic (PrL) or infralimbic (IL) regions of the medial prefrontal cortex (mPFC) encode cocaine-associated stimuli and cocaine-seeking, and whether this processing is differentially altered after 1 month of cocaine abstinence. After self-administration training, neurons (n = 308) in the mPFC were recorded during a single test session conducted either the next day or 1 month later. Test sessions consisted of three phases during which (i) the tone-houselight stimulus previously paired with cocaine infusion during self-administration was randomly presented by the experimenter, (ii) rats responded on the lever previously associated with cocaine during extinction and (iii) the tone-houselight was presented randomly between cocaine-reinforced responding during resumption of cocaine self-administration. PrL neurons showed enhanced encoding of the cocaine stimulus and drug-seeking behavior (under extinction and self-administration) following 30 days of abstinence. In contrast, although IL neurons encoded cocaine cues and cocaine-seeking, there were no pronounced changes in IL responsiveness following 30 days of abstinence. Importantly, cue-related changes do not represent a generalised stimulus-evoked discharge as PrL and IL neurons in control animals (n = 4) exhibited negligible recruitment by the tone-houselight stimulus. The results support the view that, following abstinence, neural encoding in the PrL but not IL may play a key role in enhanced cocaine-seeking, particularly following re-exposure to cocaine-associated cues.

Contribution of a mesocorticolimbic subcircuit to drug context-induced reinstatement of cocaine-seeking behavior in rats

Cocaine-seeking behavior triggered by drug-paired environmental context exposure is dependent on orbitofrontal cortex (OFC)-basolateral amygdala (BLA) interactions. Here, we present evidence supporting the hypothesis that dopaminergic input from the ventral tegmental area (VTA) to the OFC critically regulates these interactions. In experiment 1, we employed site-specific pharmacological manipulations to show that dopamine D1-like receptor stimulation in the OFC is required for drug context-induced reinstatement of cocaine-seeking behavior following extinction training in an alternate context. Intra-OFC pretreatment with the dopamine D1-like receptor antagonist, SCH23390, dose-dependently attenuated cocaine-seeking behavior in an anatomically selective manner, without altering motor performance. Furthermore, the effects of SCH23390 could be surmounted by co-administration of a sub-threshold dose of the D1-like receptor agonist, SKF81297. In experiment 2, we examined effects of D1-like receptor antagonism in the OFC on OFC-BLA interactions using a functional disconnection manipulation. Unilateral SCH23390 administration into the OFC plus GABA agonist-induced neural inactivation of the contralateral or ipsilateral BLA disrupted drug context-induced cocaine-seeking behavior relative to vehicle, while independent unilateral manipulations of these brain regions were without effect. Finally, in experiment 3, we used fluorescent retrograde tracers to demonstrate that the VTA, but not the substantia nigra, sends dense intra- and interhemispheric projections to the OFC, which in turn has reciprocal bi-hemispheric connections with the BLA. These findings support that dopaminergic input from the VTA, via dopamine D1-like receptor stimulation in the OFC, is required for OFC-BLA functional interactions. Thus, a VTA-OFC-BLA neural circuit promotes drug context-induced motivated behavior.

Relapse to cocaine-seeking after abstinence is regulated by cAMP-dependent protein kinase A in the prefrontal cortex

Abstinence from cocaine self-administration (SA) is associated with neuroadaptations in the prefrontal cortex (PFC) and nucleus accumbens (NAc) that are implicated in cocaine-induced neuronal plasticity and relapse to drug-seeking. Alterations in cAMP-dependent protein kinase A (PKA) signaling are prominent in medium spiny neurons in the NAc after repeated cocaine exposure but it is unknown whether similar changes occur in the PFC. Because cocaine SA induces disturbances in glutamatergic transmission in the PFC-NAc pathway, we examined whether dysregulation of PKA-mediated molecular targets in PFC-NAc neurons occurs during abstinence and, if so, whether it contributes to cocaine-seeking. We measured the phosphorylation of cAMP response element binding protein (Ser133) and GluA1 (Ser845) in the dorsomedial (dm) PFC and the presynaptic marker, synapsin I (Ser9, Ser62/67, Ser603), in the NAc after 7 days of abstinence from cocaine SA with or without cue-induced cocaine-seeking. We also evaluated whether infusion of the PKA inhibitor, 8-bromo-Rp-cyclic adenosine 3', 5'-monophosphorothioate (Rp-cAMPs), into the dmPFC after abstinence would affect cue-induced cocaine-seeking and PKA-regulated phosphoprotein levels. Seven days of forced abstinence increased the phosphorylation of cAMP response element binding protein and GluA1 in the dmPFC and synapsin I (Ser9) in the NAc. Induction of these phosphoproteins was reversed by a cue-induced relapse test of cocaine-seeking. Bilateral intra-dmPFC Rp-cAMPs rescued abstinence-elevated PKA-mediated phosphoprotein levels in the dmPFC and NAc and suppressed cue-induced relapse. Thus, by inhibiting abstinence-induced PKA molecular targets, relapse reverses abstinence-induced neuroadaptations in the dmPFC that are responsible, in part, for the expression of cue-induced cocaine-seeking.

Lack of increased immediate early gene expression in rats reinstating cocaine-seeking behavior to discrete sensory cues

Drug-seeking behavior elicited by drug-associated cues contributes to relapse in addiction; however, whether relapse elicited by drug-associated conditioned reinforcers (CR) versus discriminative stimuli (DS) involves distinct or overlapping neuronal populations is unknown. To address this question, we developed a novel cocaine self-administration and cue-induced reinstatement paradigm that exposed the same rats to distinct cocaine-associated CR and DS. Rats were trained to self-administer cocaine in separate sessions. In one, a DS signaled cocaine availability; in the other, cocaine delivery was paired with a different CR. After extinction training and reinstatement testing, where both cues were presented in separate sessions, rats were sacrificed and processed for cellular analysis of temporal activity by fluorescent in situ hybridization (CatFISH) for activity regulated cytoskeleton-associated protein (Arc) mRNA and for radioactive in situ hybridization for Arc and zif268 mRNAs. CatFISH did not reveal significant changes in Arc mRNA expression. Similar results were obtained with radioactive in situ hybridization. We have shown that while rats reinstate drug seeking in response to temporally discrete presentations of distinct drug-associated cues, such reinstatement is not associated with increased transcriptional activation of Arc or zif268 mRNAs, suggesting that expression of these genes may not be necessary for cue-induced reinstatement of drug-seeking behavior.

Systems level neuroplasticity in drug addiction

Drug addiction is a chronic relapsing disorder for which research has been dedicated to understand the various factors that contribute to development, loss of control, and persistence of compulsive addictive behaviors. In this review, we provide a broad overview of various theories of addiction, drugs of abuse, and the neurobiology involved across the addiction cycle. Specific focus is devoted to the role of the mesolimbic pathway in acute drug reinforcement and occasional drug use, the mesocortical pathway and associated areas (e.g., the dorsal striatum) in escalation/dependence, and the involvement of these pathways and associated circuits in mediating conditioned responses, drug craving, and loss of behavioral control thought to underlie withdrawal and relapse. With a better understanding of the neurobiological factors that underlie drug addiction, continued preclinical and clinical research will aid in the development of novel therapeutic interventions that can serve as effective long-term treatment strategies for drug-dependent individuals.

Deficits in ventromedial prefrontal cortex group 1 metabotropic glutamate receptor function mediate resistance to extinction during protracted withdrawal from an extensive history of cocaine self-administration

Anomalies in prefrontal cortex (PFC) function are posited to underpin difficulties in learning to suppress drug-seeking behavior during abstinence. Because group 1 metabotropic glutamate receptors (mGluRs) regulate drug-related learning, we assayed the consequences of extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) on the PFC expression of group 1 mGluRs and the relevance of observed changes for cocaine seeking. After protracted withdrawal, cocaine-experienced animals exhibited a time-dependent intensification of cue-induced cocaine-seeking behavior and an impaired extinction of this behavior. These behavioral phenomena were associated with a time-dependent reduction in mGluR1/5 expression within ventromedial PFC (vmPFC) of cocaine-experienced animals exposed to extinction testing but not in untested ones. Interestingly, pharmacological manipulations of vmPFC mGluR1/5 produced no immediate effects on cue-induced cocaine-seeking behavior but produced residual effects on a subsequent test for cocaine seeking. At 3 d withdrawal, cocaine-experienced rats infused intra-vmPFC with mGluR1/5 antagonists, either before or after an initial test for cocaine seeking, persisted in their cocaine seeking akin to cocaine-experienced rats in protracted withdrawal. Conversely, cocaine-experienced rats infused with an mGluR1/5 agonist before the initial test for cocaine-seeking at 30 d withdrawal exhibited a facilitation of extinction learning. These data indicate that cue-elicited deficits in vmPFC group 1 mGluR function mediate resistance to extinction during protracted withdrawal from a history of extensive cocaine self-administration and pose pharmacological stimulation of these receptors as a potential approach to facilitate learned suppression of drug-seeking behavior that may aid drug abstinence.

Exaggerated cue-induced reinstatement of cocaine seeking but not incubation of cocaine craving in a developmental rat model of schizophrenia

RATIONALE

Patients with schizophrenia exhibit high comorbidity for substance abuse, but the biological underpinnings of this dual-diagnosis condition are still unclear. Previous studies have shown that rats with a neonatal ventral hippocampal lesion (NVHL), a widely used developmental animal model of schizophrenia, exhibit increased cocaine and methamphetamine self-administration and cocaine-induced reinstatement.

OBJECTIVE

Here, we assessed whether a NVHL would also potentiate cue-induced reinstatement of cocaine seeking and the time-dependent increases in cue-induced cocaine seeking after withdrawal (incubation of cocaine craving) in adult rats.

METHODS

Rats were trained to self-administer cocaine (3 or 6 h/day with 0.75 mg kg(-1) infusion(-1) paired with a tone-light cue) for 10 days, followed by extinction training (3 h/day) and cue-induced reinstatement of cocaine seeking. Other rats were tested for incubation of cocaine craving, assessed in extinction tests 1 and 30 days after the last self-administration session.

RESULTS

Although there was no significant difference in cocaine intake between NVHL and sham controls, NVHL rats took significantly longer to reach an a priori set extinction criterion and exhibited enhanced cue-induced reinstatement. However, while cue-induced cocaine seeking was higher after 30 days than after 1 day of withdrawal (incubation of cocaine craving), the NVHL had no effect on this incubation.

CONCLUSION

These data confirm previous reports on enhanced resistance to extinction after NVHL and demonstrate that NVHL rats exhibit enhanced cue-induced reinstatement of cocaine seeking after extinction, a measure of drug relapse.

Fos activation of selective afferents to ventral tegmental area during cue-induced reinstatement of cocaine seeking in rats

Ventral tegmental area (VTA) dopamine neurons are crucial for appetitive responses to Pavlovian cues, including cue-induced reinstatement of drug seeking. However, it is unknown which VTA inputs help activate these neurons, transducing stimuli into salient cues that drive drug-seeking behavior. Here we examined 56 VTA afferents from forebrain and midbrain that are Fos activated during cue-induced reinstatement. We injected the retrograde tracer cholera toxin β subunit (CTb) unilaterally into rostral or caudal VTA of male rats. All animals were trained to self-administer cocaine, then extinguished of this behavior. On a final test day, animals were exposed to response-contingent cocaine-associated cues, extinction conditions, a non-cocaine-predictive CS-, or a novel environment, and brains were processed to visualize CTb and Fos immunoreactivity to identify VTA afferents activated in relation to behaviors. VTA-projecting neurons in subregions of medial accumbens shell, ventral pallidum, elements of extended amygdala, and lateral septum (but not prefrontal cortex) were activated specifically during cue-induced cocaine seeking, and some of these were also activated proportionately to the degree of cocaine seeking. Surprisingly, though efferents from the lateral hypothalamic orexin field were also Fos activated during reinstatement, these were largely non-orexinergic. Also, VTA afferents from the rostromedial tegmental nucleus and lateral habenula were specifically activated during extinction and CS- tests, when cocaine was not expected. These findings point to a select set of subcortical nuclei which provide reinstatement-related inputs to VTA, translating conditioned stimuli into cocaine-seeking behavior.

Pavlovian-to-instrumental transfer in cocaine seeking rats

Drug-associated cues are believed to be important mediators of addiction and drug relapse. Although such cues may influence drug-seeking behavior through multiple routes, it is their putative incentive motivational properties-their ability to elicit "craving"-that interests many addiction researchers. The Pavlovian-to-instrumental transfer paradigm is commonly used to assay cue-evoked incentive motivation in situations involving natural rewards, but has not been widely applied to the study of drug self-administration. We used this paradigm to determine whether cues paired with intravenous cocaine could promote performance of an independently trained task in which rats self-administered cocaine by completing a chain of two different lever press actions, a procedure used to parse behavior into cocaine seeking (first action) and cocaine taking (second action). Rats showed significant transfer, increasing task performance during cocaine-paired cues. This effect was observed for both seeking and taking actions, although a trend toward greater cocaine taking was observed, a result that is consistent with studies using natural rewards. Our results demonstrate that cocaine-paired cues can provoke the pursuit of cocaine through a Pavlovian motivational process. This phenomenon may provide a useful new tool for modeling drug relapse, particularly as a method for targeting the response-invigorating effects of stimulus-drug learning.

A food predictive cue must be attributed with incentive salience for it to induce c-fos mRNA expression in cortico-striatal-thalamic brain regions

Cues associated with rewards acquire the ability to engage the same brain systems as rewards themselves. However, reward cues have multiple properties. For example, they not only act as predictors of reward capable of evoking conditional responses (CRs), but they may also acquire incentive motivational properties. As incentive stimuli they can evoke complex emotional and motivational states. Here we sought to determine whether the predictive value of a reward cue is sufficient to engage brain reward systems, or whether the cue must also be attributed with incentive salience. We took advantage of the fact that there are large individual differences in the extent to which reward cues are attributed with incentive salience. When a cue (conditional stimulus, CS) is paired with delivery of food (unconditional stimulus, US), the cue acquires the ability to evoke a CR in all rats; that is, it is equally predictive and supports learning the CS-US association in all. However, only in a subset of rats is the cue attributed with incentive salience, becoming an attractive and desirable incentive stimulus. We used in situ hybridization histochemistry to quantify the ability of a food cue to induce c-fos mRNA expression in rats that varied in the extent to which they attributed incentive salience to the cue. We found that a food cue induced c-fos mRNA in the orbitofrontal cortex, striatum (caudate and nucleus accumbens), thalamus (paraventricular, intermediodorsal and central medial nuclei), and lateral habenula, only in rats that attributed incentive salience to the cue. Furthermore, patterns of "connectivity" between these brain regions differed markedly between rats that did or did not attribute incentive salience to the food cue. These data suggest that the predictive value of a reward cue is not sufficient to engage brain reward systems-the cue must also be attributed with incentive salience.

Stress and cocaine interact to modulate Arc/Arg3.1 expression in rat brain

RATIONALE

The interaction between stress and drugs of abuse is a critical component of drug addiction, but the underlying molecular mechanisms remain elusive. Arc/Arg3.1 is an effector immediate early gene that may represent a bridge connecting short- and long-term neuronal modifications associated with exposure to stress and drugs of abuse.

OBJECTIVES

This research aims to study the modulation of Arc/Arg3.1 expression as a marker of neuronal changes associated with exposure to stress and cocaine.

MATERIALS AND METHODS

Rats exposed to either single or repeated stress sessions were subjected to a single intraperitoneal injection of cocaine hydrochloride (10 mg/kg) and sacrificed 2 h later. RNase protection assay was used to determine changes in Arc/Arg3.1 gene expression in different brain regions.

RESULTS

We found significant stress-cocaine interactions in the prefrontal cortex (p < 0.001) and hypothalamus (p < 0.05). In the prefrontal cortex, acute stress potentiated cocaine-induced Arc/Arg3.1 mRNA elevation, whereas prolonged stress attenuated the response to cocaine. In the hypothalamus, although markedly reduced by acute stress, Arc/Arg3.1 gene expression was still increased by cocaine. No interaction was observed following repeated stress. Notably, cocaine-induced Arc/Arg3.1 mRNA levels were not influenced by stress in striatum and hippocampus.

CONCLUSIONS

In our experimental model, stress interacted with cocaine to alter Arc/Arg3.1 expression in a regionally selective fashion and in a way that depended on whether stress was acute or repeated. These results point to Arc/Arg3.1 as a potential molecular target modulated by stress to alter cellular sensitivity to cocaine.

Suppression of activity-regulated cytoskeleton-associated gene expression in the dorsal striatum attenuates extinction of cocaine-seeking

The caudate putamen (CPu) has been implicated in habit learning and neuroadaptive changes that mediate the compulsive nature of drug-seeking following chronic cocaine self-administration. Re-exposure to an operant chamber previously associated with cocaine, but not yoked-saline, increases activity-regulated cytoskeleton-associated (Arc) gene mRNA expression within the dorsolateral (dl) CPu following prolonged abstinence. In this study, we tested the hypothesis that antisense gene knockdown of Arc within the dlCPu would alter cocaine-seeking. Initial studies showed that a single infusion of Arc antisense oligodeoxynucleotide (ODN) into the dlCPu significantly attenuated the induction of Arc mRNA and Arc protein by a single cocaine exposure (20 mg/kg i.p.) compared to scrambled-ODN-infused controls. In cocaine self-administering rats, infusion of Arc antisense ODN into the dlCPu 3 h prior to a test of context-driven drug-seeking significantly attenuated Arc protein induction, but failed to alter responding during testing, suggesting striatal Arc does not facilitate context-induced drug-seeking following prolonged abstinence. However, Arc antisense ODN infusion blunted the decrease in responding during subsequent 1-h extinction tests 24 and 48 h later. Following re-exposure to a cocaine-paired context, surface expression of the AMPA-type glutamate receptor GluR1 was significantly reduced whereas GluR2 was significantly increased in the dlCPu, independent of Arc antisense ODN infusion. Together, these findings indicate an important role for Arc in neuroadaptations within brain regions responsible for drug-seeking after abstinence and direct attention to changes occurring within striatal circuitry that are necessary to break down the habitual behaviour that leads to relapse.

Regulation of the immediate-early genes arc and zif268 in a mouse operant model of cocaine seeking reinstatement

Reinstatement of extinguished operant responding for drug is an appropriate model of relapse to drug abuse. Due to the difficulty of implementing in mice the procedure of instrumental intravenous self-administration, mechanisms of reinstatement have so far been studied almost exclusively in rats. A mouse model of reinstatement of cocaine seeking has recently been characterized (Soria et al. 2008). The aim of the present study was to assess regional brain activation, as measured by induction of the immediate early genes (IEG) arc and zif268, during priming- or cue-elicited reinstatement of cocaine seeking using this new mouse model and the in situ hybridization technique. We have demonstrated that cue-elicited reinstatement of cocaine seeking was associated with induction of the IEG in the medial prefrontal cortex (prelimbic and infralimbic) and basolateral amygdala. Priming-induced reinstatement produced a more widespread up-regulation of those genes in forebrain regions including medial prefrontal, orbitofrontal and motor cortex, dorsal striatum and basolateral amygdala. These patterns of IEG expression are in agreement with previous results obtained in rats and thus indicate that the new mouse model of reinstatement is functionally equivalent to rat models. That comparability adds to the usefulness of the mouse model as a tool for addressing neurobiological mechanisms of addiction.

Blockade of 5-HT2A receptors in the medial prefrontal cortex attenuates reinstatement of cue-elicited cocaine-seeking behavior in rats

RATIONALE

The action of serotonin (5-HT) at the 5-HT(2A) receptor subtype is thought to be involved in cocaine-seeking behavior that is motivated by exposure to drug-associated cues and drug priming. 5-HT(2A) receptors are densely clustered in the ventromedial prefrontal cortex (vmPFC), an area that plays a role in mediating cocaine-seeking behavior.

OBJECTIVES

This study examined the hypothesis that M100907, a 5-HT(2A) receptor antagonist, infused directly in the vmPFC attenuates cue- and cocaine-primed reinstatement of cocaine-seeking behavior.

METHODS

Rats trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues underwent extinction training during which operant responses produced no consequences. Once behavior extinguished, rats were tested for reinstatement of responding elicited by either response-contingent presentations of the cocaine-paired light/tone cues or by cocaine-priming injections (10 mg/kg, i.p.) within 1 min after pretreatment with microinfusions of M100907 (0.1, 0.3, 1.0, or 1.5 μg/0.2 μl/side) into the vmPFC.

RESULTS

Intra-vmPFC M100907 decreased cue-elicited reinstatement at the two highest doses (1.0 and 1.5 μg) but produced only a slight decrease in cocaine-primed reinstatement that was not dose dependent. The decrease in cue reinstatement was not likely due to impaired ability to respond since intra-vmPFC M100907 infusions had minimal effect on cocaine self-administration and no effect on cue-elicited sucrose-seeking behavior, or spontaneous or cocaine-induced locomotion. M100907 infusions into the adjacent anterior cingulate cortex had no effect on cue reinstatement.

CONCLUSIONS

The results suggest that the blockade of 5-HT(2A) receptors in the vmPFC selectively attenuates the incentive motivational effects of cocaine-paired cues.

Interaction of the basolateral amygdala and orbitofrontal cortex is critical for drug context-induced reinstatement of cocaine-seeking behavior in rats

The basolateral amygdala (BLA) and lateral orbitofrontal cortex (OFC) are critical elements of the neural circuitry that regulates drug context-induced reinstatement of cocaine-seeking behavior. Given the existence of dense reciprocal anatomical connections between these brain regions, this study tested the hypothesis that serial information processing by the BLA and OFC is necessary for drug context-induced cocaine-seeking behavior. Male Sprague-Dawley rats were trained to lever press for un-signaled cocaine infusions (0.15 mg/infusion, i.v.) in a distinct environment (cocaine-paired context) then underwent extinction training in a different environment (extinction context). During four subsequent test sessions, rats were re-exposed to the cocaine-paired and extinction contexts in order to assess cocaine-seeking behavior (non-reinforced active lever responding). Immediately before each test session, rats received microinfusions of the GABA(A)/GABA(B) agonist cocktail, baclofen+muscimol (BM: 1.0/.01 mM), or vehicle unilaterally into the BLA plus the contralateral or ipsilateral OFC, or unilaterally into the OFC alone. Exposure to the previously cocaine-paired context, but not the extinction context, reinstated extinguished cocaine-seeking behavior. BM-induced unilateral OFC inactivation failed to alter this behavior, similar to the effect of unilateral BLA inactivation in our previous study (Fuchs et al, 2007). Conversely, neural inactivation of the BLA plus the contralateral or ipsilateral OFC equally attenuated drug context-induced cocaine seeking without altering food-reinforced instrumental responding, relative to vehicle pretreatment. These findings suggest that the BLA and OFC co-regulate drug context-induced motivation for cocaine either through sequential information processing via intra- and interhemispheric connections or by providing converging input to a downstream brain region.

Early methylphenidate exposure enhances cocaine self-administration but not cocaine-induced conditioned place preference in young adult rats

RATIONALE

Previous studies in rodents show that early exposure to methylphenidate alters later responsiveness to drugs of abuse. An interesting feature of these studies is that early methylphenidate treatment decreases the rewarding value of cocaine when measured by conditioned place preference (CPP), but the same treatment increases cocaine self-administration.

OBJECTIVE

The goal of the present study was to examine the effects of early methylphenidate exposure on cocaine-induced responding using both reward paradigms.

METHODS

Rats were treated with methylphenidate (0, 2, or 5 mg/kg) from postnatal days (PDs) 11 to 20, and then cocaine-induced CPP or cocaine self-administration was measured in separate groups of rats in adulthood. The CPP procedure included 8 days of acquisition training, 8 days of extinction training, and a reinstatement test. Rats were conditioned with 0, 10, or 20 mg/kg cocaine. Reinstatement was assessed after a priming dose of cocaine (10 mg/kg). For the self-administration experiment, a jugular catheter was implanted and rats were trained to press a lever reinforced with cocaine (0.25 or 0.75 mg/kg/infusion) on a fixed ratio (FR) one schedule. Rats were gradually moved from an FR1 to an FR10 schedule and, after criterion was reached, rats were placed on a progressive ratio schedule for 5 days.

RESULTS

Cocaine produced robust rewarding effects as determined by both the CPP and self-administration experiments; however, early methylphenidate exposure only enhanced the reinforcing effects of cocaine on the self-administration paradigm. Interestingly, this methylphenidate enhancement was only seen in male rats.

CONCLUSIONS

These data suggest that in males, methylphenidate enhances the reinforcing value of cocaine, but not cocaine-associated cues.