Extinction of a cocaine-taking context that protects against drug-primed reinstatement is dependent on the metabotropic glutamate 5 receptor

Examining Sex Differences in Conditioned Place Preference or Aversion to Methamphetamine in Adolescent and Adult Mice

Adolescence marks a particularly vulnerable period to developing substance use disorders. Human and rodent studies suggest that hypersensitivity to reward may contribute towards such vulnerability when adolescents are exposed to casual drug use. Methamphetamine is a popular illicit substance used by male and female youths. However, age- and sex-specific research in methamphetamine is scarce. The present study therefore aimed to examine potential sex differences in methamphetamine-conditioned place preference in adolescent and adult mice. Mice (n = 16–24/group) were conditioned to methamphetamine (0.1 mg/kg). We observed that regardless of age, females were more hyperactive compared to males. Individually normalized score against baseline preference indicated that on average, adolescents formed stronger preference compared to adults in both sexes. This suggests that adolescents are more sensitive to the rewarding effects of methamphetamine compared to adults. Surprisingly, individual data showed that some mice formed a conditioned place aversion instead of preference, with females less likely to form an aversion compared to males. These results suggest that adolescents may be hypersensitive to methamphetamine’s rewarding effects. In addition, female resistance to the aversive effects of methamphetamine may relate to the sex-specific findings in humans, including quicker transition to regular methamphetamine use observed in females compared to males.

Minding the Gap: Leveraging Mindfulness to Inform Cue Exposure Treatment for Substance Use Disorders

Despite extinction-based processes demonstrating efficacy in the animal extinction and human anxiety literatures, extinction for substance use disorders (SUD) has shown poor efficacy (i. e., cue exposure treatment [CET]). Reasons for this lack of success include common threats to extinction, such as renewal and reinstatement. In recent decades, research on mindfulness for SUD has flourished, and a key aspect of these mindfulness-based interventions includes teaching individuals to stay present with whatever experience they have, even if unpleasant, without trying to change/escape/avoid it. Similarly, CET teaches individuals to not escape/avoid conditioned responses (e.g., craving) by engaging in drug use behavior. This paper discusses how mindfulness-based research and practices could positively influence CET through future research (e.g., Could mindfulness practice attenuate renewal? Might mindfulness training + CET enhance the ability to extinguish the most salient or motivational cues?), with the long-term goal of improving SUD treatment.

Pharmacological Treatment of Mood Disorders and Comorbid Addictions: A Systematic Review and Meta-Analysis: Traitement Pharmacologique des Troubles de L'humeur et des Dépendances Comorbides: Une Revue Systématique et une Méta-Analyse

OBJECTIVE

Addiction comorbidity is an important clinical challenge in mood disorders, but the best way of pharmacologically treating people with mood disorders and addictions remains unclear. The aim of this study was to assess the efficacy of pharmacological treatments for mood and addiction symptoms in people with mood disorders and addiction comorbidity.

METHODS

A systematic search of placebo-controlled randomized controlled trials investigating the effects of pharmacological treatments in people with bipolar disorder (BD) or major depressive disorder (MDD), and comorbid addictions was performed. Treatment-related effects on mood and addiction measures were assessed in a meta-analysis, which also estimated risks of participant dropout and adverse effects.

RESULTS

A total of 32 studies met systematic review inclusion criteria. Pharmacological therapy was more effective than placebo for improving manic symptoms (standardized mean difference [SMD] = -0.15; 95% confidence interval [95% CI], -0.29 to -0.02; P = 0.03) but not BD depressive symptoms (SMD = -0.09; 95% CI, -0.22 to 0.03; P = 0.15). Quetiapine significantly improved manic symptoms (SMD = -0.23; 95% CI, -0.39 to -0.06; P = 0.008) but not BD depressive symptoms (SMD = -0.07; 95% CI, -0.23 to 0.10; P = 0.42). Pharmacological therapy was more effective than placebo for improving depressive symptoms in MDD (SMD = -0.16; 95% CI, -0.30 to -0.03; P = 0.02). Imipramine improved MDD depressive symptoms (SMD = -0.58; 95% CI, -1.03 to -0.13; P = 0.01) but Selective serotonin reuptake Inhibitors (SSRI)-based treatments had no effect (SMD = -0.06; 95% CI, -0.30 to 0.17; P = 0.60). Pharmacological treatment improved the odds of alcohol abstinence in MDD but had no effects on opiate abstinence.

CONCLUSIONS

Pharmacological treatments were significantly better than placebo in improving manic symptoms, MDD depressive symptoms, and alcohol abstinence but were not better for bipolar depression symptoms. Importantly, quetiapine was not more effective than placebo in improving bipolar depression symptoms nor were SSRI's for the treatment of MDD depression. Our findings highlight the need for further high-quality clinical trials of treatments for mood disorders and comorbid addictions.

Molecular and circuit mechanisms regulating cocaine memory

Risk of relapse is a major challenge in the treatment of substance use disorders. Several types of learning and memory mechanisms are involved in substance use and have implications for relapse. Associative memories form between the effects of drugs and the surrounding environmental stimuli, and exposure to these stimuli during abstinence causes stress and triggers drug craving, which can lead to relapse. Understanding the neural underpinnings of how these associations are formed and maintained will inform future advances in treatment practices. A large body of research has expanded our knowledge of how associative memories are acquired and consolidated, how they are updated through reactivation and reconsolidation, and how competing extinction memories are formed. This review will focus on the vast literature examining the mechanisms of cocaine Pavlovian associative memories with an emphasis on the molecular memory mechanisms and circuits involved in the consolidation, reconsolidation, and extinction of these memories. Additional research elucidating the specific signaling pathways, mechanisms of synaptic plasticity, and epigenetic regulation of gene expression in the circuits involved in associative learning will reveal more distinctions between consolidation, reconsolidation, and extinction learning that can be applied to the treatment of substance use disorders.

Extinction and drug-induced reinstatement of cocaine seeking following self-administration or conditioned place preference in adolescent and adult rats

Adolescence marks a particularly vulnerable period to developing substance use disorders, and people who start using drugs in adolescence are more likely to relapse. A limited number of studies have investigated age difference in relapse following re-exposure to the drug after a period of abstinence. Using a cocaine self-administration paradigm, we showed no age difference in acquisition or extinction of self-administration. Interestingly, adolescent rats displayed impaired cocaine-primed reinstatement of cocaine seeking. Using the same dose as that self-administered in the first experiment, we then investigated age differences in acquisition and extinction of conditioned place preference, as well as locomotor sensitization. While there were no differences in locomotor activity or acquisition of preference, adolescents failed to extinguish their preference, even when the number of extinction sessions was doubled from what adults received. Taken together, these results suggest that while cocaine has similar rewarding and reinforcing effects regardless of age, adolescents may attribute stronger salience to the drug-associated context. In addition, re-exposure to cocaine itself may not be a strong relapse trigger in adolescence. Overall, these findings suggest that we should focus more on alleviating drug-context salience compared to re-exposure to substance in order to reduce relapse of drug seeking in adolescents.

The cognitive cost of reducing relapse to cocaine-seeking with mGlu5 allosteric modulators

RATIONALE

Cocaine use disorder (CUD) remains difficult to treat with no FDA-approved medications to reduce relapse. Antagonism of metabotropic glutamate receptor 5 (mGlu5) has been demonstrated to decrease cocaine-seeking but may also further compromise cognitive function in long-term cocaine users.

OBJECTIVES

Here we assessed the effect of repeated administration of negative or positive allosteric modulators (NAM or PAM) of mGlu5 on both cognitive performance and (context+cue)-primed cocaine-seeking after prolonged abstinence (≥ 45 days).

METHODS

Adult male Sprague-Dawley rats underwent 6 days of short-access (1 h/day) and 12 days of long-access (6 h/day) cocaine self-administration. Rats were then trained and tested in a delayed match-to-sample (DMS) task to establish baseline working memory performance over a 5-day block of testing. Next, rats received daily systemic administration of the mGlu5 NAM 3-((2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine hydrochloride (MTEP; 3 mg/kg), the mGlu5 PAM 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB; 30 mg/kg) or vehicle prior to DMS testing during a block of 5 days, followed by a 5-day washout DMS testing block.

RESULTS

MTEP and CDPPB decreased drug-seeking in response to cocaine-associated cues after prolonged abstinence. However, repeated treatment with MTEP impaired working memory, while CDPPB had no effects on performance.

CONCLUSIONS

These results emphasize the relevance of evaluating cognitive function within the context of investigating pharmacotherapies to treat CUD. Further research is needed to determine how two mechanistically different pharmacological compounds can exert the same behavioral effects to reduce cocaine-seeking.

Extinction training following cocaine or MDMA self-administration produces discrete changes in D2-like and mGlu5 receptor density in the rat brain

BACKGROUND

Several studies strongly support the role of the dopamine D2-like and glutamate mGlu5 receptors in psychostimulant reward and relapse.

METHODS

The present study employed cocaine or MDMA self-administration with yoked-triad procedure in rats to explore whether extinction training affects the drug-seeking behavior and the D2-like and mGlu5 receptor Bmax and Kd values in several regions of the animal brain.

RESULTS

Both cocaine and MDMA rats developed maintenance of self-administration, but MDMA evoked lower response rates and speed of self-administration acquisition. During reinstatement tests, cocaine or MDMA seeking behavior was produced by either exposure to the drug-associated cues or drug-priming injections. The extinction training after cocaine self-administration did not alter significantly D2-like receptor expression the in the limbic and subcortical brain areas, while MDMA yoked rats showed a decrease of the D2-like binding density in the nucleus accumbens and increase in the hippocampus and a rise of affinity in the striatum and hippocampus. Interestingly, in the prefrontal cortex a reduction in the mGlu5 receptor density in cocaine- or MDMA-abstinent rats was demonstrated, with significant effects being observed after previous MDMA exposure. Moreover, rats self-administered cocaine showed a rise in the density of mGlu5 receptor for the nucleus accumbens.

CONCLUSION

This study first time shows that abstinence followed extinction training after cocaine or MDMA self- or passive-injections changes the D2-like and mGlu5 density and affinity. The observed changes in the expression of both receptors are brain-region specific and related to either pharmacological and/or motivational features of cocaine or MDMA.

Mechanisms underlying the efficacy of exercise as an intervention for cocaine relapse: a focus on mGlu5 in the dorsal medial prefrontal cortex

RATIONALE

Exercise shows promise as a treatment option for addiction; but in order to prevent relapse, it may need to be introduced early in the course of treatment.

OBJECTIVE

We propose that exercise, by upregulating dorsal medial prefrontal cortex (dmPFC)-nucleus accumbens (NAc) transmission, offsets deficits in pathways targeting glutamate, BDNF, and dopamine during early abstinence, and in doing so, normalizes neuroadaptations that underlie relapse.

METHODS

We compared the effects of exercise (wheel running, 2-h/day) during early (days 1-7), late (days 8-14), and throughout abstinence (days 1-14) to sedentary conditions on cocaine-seeking and gene expression in the dmPFC and NAc core of male rats tested following 24-h/day extended-access cocaine (up to 96 infusions/day) or saline self-administration and protracted abstinence (15 days). Based on these data, we then used site-specific manipulation to determine whether dmPFC metabotropic glutamate receptor5 (mGlu5) underlies the efficacy of exercise.

RESULTS

Exercise initiated during early, but not late abstinence, reduced cocaine-seeking; this effect was strongly associated with dmPFC Grm5 expression (gene encoding mGlu5), and modestly associated with dmPFC Grin1 and Bdnf-IV expression. Activation of mGlu5 in the dmPFC during early abstinence mimicked the efficacy of early-initiated exercise; however, inhibition of these receptors prior to the exercise sessions did not block its efficacy indicating that there may be redundancy in the mechanisms through which exercise reduces cocaine-seeking.

CONCLUSION

These findings indicate that addiction treatments, including exercise, should be tailored for early versus late phases of abstinence since their effectiveness will vary over abstinence due to the dynamic nature of the underlying neuroadaptations.

A medium throughput rodent model of relapse from addiction with behavioral and pharmacological specificity

One of most formidable problems in the treatment of addiction is the high rate of relapse. The discovery of medicines to help mitigate relapse are aided by animal models that currently involve weeks of training and require surgical preparations and drug delivery devices. The present set of experiments was initiated to investigate a rapid 8-day screening method that utilizes food instead of intravenous drug administration. Male Sprague-Dawley rats were trained in a reinstatement paradigm in which every lever press produced a 45 mg food pellet concurrently paired with a light and tone. Behavior was subsequently extinguished with lever responses producing neither food nor food-associated stimuli. Reinstatement of responding was evaluated under conditions in which the first three responses of every 5 min time bin produced a food pellet along with food-associated stimuli. The mGlu₅ receptor antagonists MPEP and MTEP produced a significant reduction in reinstatement while failing to alter responding where every response produced food. The cannabinoid CB₁ receptor antagonist rimonabant and the mGlu_2/3 receptor agonist LY379268 also selectively reduced reinstatement. Other compounds including clozapine, d-amphetamine, chlordiazepoxide, ABT-431, naltrexone and citalopram were without effect. The results suggest that relapse-like behavioral effects can be extended to non-pharmacological reinforcers. Drug effects demonstrated both behavioral and pharmacological specificity. The present experimental design thus allows for efficient and rapid assessment of the effects of drugs that might be useful in the treatment of addiction-associated relapse.

Drug Refraining and Seeking Potentiate Synapses on Distinct Populations of Accumbens Medium Spiny Neurons

Cocaine-associated cues and contexts can precipitate drug seeking in humans and in experimental animals. Glutamatergic synapses in the core subcompartment of the nucleus accumbens (NAcore) undergo transient potentiation in response to presenting drug-associated cues. The NAcore contains two populations of medium spiny neurons (MSNs) that differentially express D1 or D2 dopamine receptors. By recording the ratio of AMPA and NMDA glutamate receptor currents (AMPA/NMDA ratio) from MSNs in NAcore tissue slices, we endeavored to understand which subpopulation of MSNs was undergoing transient potentiation. Transgenic female and male mice differentially expressing fluorescent reporters in D1 or D2 MSNs were withdrawn for 2-3 weeks after being trained to self-administer cocaine. In some mice, discrete cocaine-conditioned cues were isolated from the drug-associated context via extinction training, which causes rodents to refrain from drug seeking in the extinguished context. By measuring AMPA/NMDA ratios in the drug context with or without contextual or discrete cues, and with or without extinction training, we made the following three discoveries: (1) mice refraining from cocaine seeking in the extinguished context showed selective elevation in AMPA/NMDA ratios in D2 MSNs; (2) without extinction training, the drug-associated context selectively increased AMPA/NMDA ratios in D1 MSNs; (3) mice undergoing cue-induced cocaine seeking after extinction training in the drug-associated context showed AMPA/NMDA ratio increases in both D1 and D2 MSNs. These findings reveal that the NAcore codes drug seeking through transient potentiation of D1 MSNs, and that refraining from cocaine seeking in an extinguished context is coded through transient potentiation of D2 MSNs.SIGNIFICANCE STATEMENT Relapse is a primary symptom of addiction that can involve competition between the desire to use drugs and the desire to refrain from using drugs. Drug-associated cues induce relapse, which is correlated with transiently potentiated glutamatergic synapses in the nucleus accumbens core. We determined which of two cell populations in the accumbens core, D1-expressing or D2-expressing neurons, undergo transient synaptic potentiation. After being trained to self-administer cocaine, mice underwent withdrawal, some with and others without extinguishing responding in the drug-associated context. Extinguished mice showed transient potentiation in D2-expressing neurons in the extinguished environment, and all mice engaged in context-induced or cue-induced drug seeking showed transient potentiation of D1-expressing neurons. A simple binary engram in accumbens for seeking drugs and refraining from drugs offers opportunities for cell-specific therapies.

Prefrontal Cortex KCa2 Channels Regulate mGlu5-Dependent Plasticity and Extinction of Alcohol-Seeking Behavior

Identifying novel treatments that facilitate extinction learning could enhance cue-exposure therapy and reduce high relapse rates in alcoholics. Activation of mGlu5 receptors in the infralimbic prefrontal cortex (IL-PFC) facilitates learning during extinction of cue-conditioned alcohol-seeking behavior. Small-conductance calcium-activated potassium (KCa2) channels have also been implicated in extinction learning of fear memories, and mGlu5 receptor activation can reduce KCa2 channel function. Using a combination of electrophysiological, pharmacological, and behavioral approaches, this study examined KCa2 channels as a novel target to facilitate extinction of alcohol-seeking behavior in rats. This study also explored related neuronal and synaptic mechanisms within the IL-PFC that underlie mGlu5-dependent enhancement of extinction learning. Using whole-cell patch-clamp electrophysiology, activation of mGlu5 in ex vivo slices significantly reduced KCa2 channel currents in layer V IL-PFC pyramidal neurons, confirming functional downregulation of KCa2 channel activity by mGlu5 receptors. Additionally, positive modulation of KCa2 channels prevented mGlu5 receptor-dependent facilitation of long-term potentiation in the IL-PFC. Systemic and intra-IL-PFC treatment with apamin (KCa2 channel allosteric inhibitor) significantly enhanced extinction of alcohol-seeking behavior across multiple extinction sessions, an effect that persisted for 3 weeks, but was not observed after apamin microinfusions into the prelimbic PFC. Positive modulation of IL-PFC KCa2 channels significantly attenuated mGlu5-dependent facilitation of alcohol cue-conditioned extinction learning. These data suggest that mGlu5-dependent facilitation of extinction learning and synaptic plasticity in the IL-PFC involves functional inhibition of KCa2 channels. Moreover, these findings demonstrate that KCa2 channels are a novel target to facilitate long-lasting extinction of alcohol-seeking behavior.SIGNIFICANCE STATEMENT Alcohol use disorder is a chronic relapsing disorder that is associated with compulsive alcohol-seeking behavior. One of the main causes of alcohol relapse is the craving caused by environmental cues that are associated with alcohol. These cues are formed by normal learning and memory principles, and the understanding of the brain mechanisms that help form these associations can lead to the development of drugs and/or behavior therapies that reduce the impact that these cues have on relapse in alcoholics.

Stages of dysfunctional decision-making in addiction

Drug use is a choice with immediate positive outcomes, but long-term negative consequences. Thus, the repeated use of drugs in the face of negative consequences suggests dysfunction in the cognitive mechanisms underpinning decision-making. This cognitive dysfunction can be mapped into three stages: the formation of preferences involving valuation of decision options; choice implementation including motivation, self-regulation and inhibitory processes; and feedback processing implicating reinforcement learning. This article reviews behavioral studies that have examined alterations in these three stages of decision-making in people with substance use disorders. Relative to healthy individuals, those with alcohol, cannabis, stimulant and opioid use disorders value risky options more highly during the formation of preferences; have a greater appetite for superficially attractive rewards during choice implementation; and are both more efficient in learning from rewards and less efficient in learning from losses during feedback processing. These observed decision-making deficits are most likely due to both premorbid factors and drug-induced effects. Because decision-making deficits have been prospectively associated with a greater risk of drug relapse, we advocate for greater research on modulating the component stages that give rise to dysfunctional decision-making in disorders of addiction.

Recent developments in the behavioural and pharmacological enhancement of extinction of drug seeking

One of the principal barriers to overcoming addiction is the propensity to relapse, even after months or years of abstinence. Relapse can be precipitated by cues and contexts associated with drug use; thus, decreasing the conditioned properties of these cues and contexts may assist in preventing relapse. The predictive power of drug cues and contexts can be reduced by repeatedly presenting them in the absence of the drug reinforcer, a process known as extinction. The potential of extinction to limit relapse has generated considerable interest and research over the past few decades. While pre-clinical animal models suggest extinction learning assists relapse prevention, treatment efficacy is often lacking when extinction learning principles are translated into clinical trials. Conklin and Tiffany (Addiction, 2002) suggest the lack of efficacy in clinical practice may be due to limited translation of procedures demonstrated through animal research and propose several methodological improvements to enhance extinction learning for drug addiction. This review will examine recent advances in the behavioural and pharmacological manipulation of extinction learning, based on research from pre-clinical models. In addition, the translation of pre-clinical findings-both those suggested by Conklin and Tiffany () and novel demonstrations from the past 13 years-into clinical trials and the efficacy of these methods in reducing craving and relapse, where available, will be discussed. Finally, we highlight areas where promising pre-clinical models have not yet been integrated into current clinical practice but, if applied, could improve upon existing behavioural and pharmacological methods.

Cocaine self-administration, extinction training and drug-induced relapse change metabotropic glutamate mGlu5 receptors expression: Evidence from radioligand binding and immunohistochemistry assays

Several behavioral findings highlight the importance of glutamatergic transmission and its metabotropic receptor type 5 (mGlu₅) in the controlling of cocaine reward and seeking behaviors. The molecular or neurochemical nature of such interactions is not well recognized, so in the present paper we determine if cocaine self-administration and extinction/reinstatement models with the yoked triad control procedure alter mGlu₅ receptor density in rats. [³H]MPEP was used to evaluate mGlu₅ receptors density and affinity in selected brain structures, while immunofluorescence analysis was used to detect changes in mGlu₅ receptors' brain location. Cocaine self-administration and yoked cocaine delivery evoked a significant elevation in mGlu₅ receptors' density in the dorsal striatum, while receptor protein expression was importantly elevated in the substantia nigra and reduced in the nucleus accumbens shell. Cocaine administration followed by 10 extinction training sessions resulted in biphasic mGlu₅ receptor density changes in the prefrontal cortex-nucleus accumbens pathway. mGlu₅ receptors' up-regulation was noted for cocaine self-administration and extinction training in the hippocampus and in yoked cocaine controls following drug abstinence in the dorsal striatum. A cocaine priming dose (but not a saline priming) resulted in a significant decrease of mGlu₅ receptors' density in the nucleus accumbens of rats previously treated with the drug and in the hippocampus of rats previously self-administered cocaine. The latter decrease in mGlu₅ receptors' density and protein expression in the hippocampus was parallel to an increase in [³H]MPEP affinity and opposite to a rise observed after single cocaine administration (ip) to drug-naïve yoked saline controls. Additionally, we also observed a significant elevation in the protein expression of the tested receptors in the limbic cortex in both cocaine groups. The present results shown modality dependent and brain-region specific changes in mGlu₅ receptors' localization and membrane specific binding.

Extinction of conditioned cues attenuates incubation of cocaine craving in adolescent and adult rats

Relapse to drug use is often precipitated by exposure to drug associated cues that evoke craving. Cue-induced drug craving has been observed in both animals and humans to increase over the first few weeks of abstinence and remain high over extended periods, a phenomenon known as 'incubation of craving'. As adolescence represents a period of vulnerability to developing drug addiction, potentially due to persistent reactivity to drug associated cues, we first compared incubation of cocaine craving in adolescent and adult rats. Adolescent (P35) and adult (P70) rats were trained to lever press to obtain intravenous cocaine, with each drug delivery accompanied by a light cue that served as the conditioned stimulus (CS). Following acquisition of stable responding, rats were tested for cue-induced cocaine-seeking after either 1 or 30days of abstinence. Additional groups of rats were also tested after 30days of abstinence, however these rats were subjected to a cue extinction session 1week into the abstinence period. Rats were injected with aripiprazole, a dopamine 2 receptor (D2R)-like partial agonist, or vehicle, 30min prior to cue extinction. We found that adolescent and adult rats acquired and maintained a similar level of cocaine self-administration, and rats of both ages exhibited a higher level of cue-induced cocaine-seeking if they were tested after 30days of abstinence compared to 1day. Incubation of cocaine craving was significantly reduced to 1day levels in both adults and adolescents that received cue extinction training. Administration of aripiprazole prior to cue extinction did not further reduce cue-induced drug-seeking. These results indicate that cue extinction training during abstinence may effectively reduce cue-induced relapse at a time when cue-induced drug craving is usually high.

Role of Dopamine 2 Receptor in Impaired Drug-Cue Extinction in Adolescent Rats

Adolescent drug users display resistance to treatment such as cue exposure therapy (CET), as well as increased liability to relapse. The basis of CET is extinction learning, which involves dopamine signaling in the medial prefrontal cortex (mPFC). This system undergoes dramatic alterations during adolescence. Therefore, we investigated extinction of a cocaine-associated cue in adolescent and adult rats. While cocaine self-administration and lever-alone extinction were not different between the two ages, we observed that cue extinction reduced cue-induced reinstatement in adult but not adolescent rats. Infusion of the selective dopamine 2 receptor (D2R)-like agonist quinpirole into the infralimbic cortex (IL) of the mPFC prior to cue extinction significantly reduced cue-induced reinstatement in adolescents. This effect was replicated by acute systemic treatment with the atypical antipsychotic aripiprazole (Abilify), a partial D2R-like agonist. These data suggest that adolescents may be more susceptible to relapse due to a deficit in cue extinction learning, and highlight the significance of D2R signaling in the IL for cue extinction during adolescence. These findings inspire new tactics for improving adolescent CET, with aripiprazole representing an exciting potential pharmacological adjunct for behavioral therapy.

The metabotropic glutamate 5 receptor is necessary for extinction of cocaine-associated cues

BACKGROUND AND PURPOSE

There is currently no medication approved specifically to treat cocaine addiction. Behavioural interventions such as cue exposure therapy (CET) rely heavily on new learning. Antagonism of the metabotropic glutamate 5 (mGlu5 ) receptor has emerged as a potential treatment, by reducing the reinforcing properties of cocaine. However, mGlu5 receptor activity is necessary for learning; therefore, such agents could interfere with behavioural treatments. We used a novel rodent model of CET to test the effects of mGlu5 negative and positive allosteric modulators (NAM and PAM) on behavioural therapy.

EXPERIMENTAL APPROACH

Rats were trained to press a lever for cocaine in the presence of a discrete cue [conditioned stimulus (CS)] and then extinguished in the absence of the CS. Following lever extinction, half the rats received CS extinction in the same chambers but with the levers withdrawn; the remaining rats received no CS extinction. Before this session, rats received a systemic administration of either vehicle or a mGlu5 NAM (MTEP, experiment 1) or PAM (CDPPB, experiment 2). Cue-induced reinstatement was tested in a drug-free session the following day.

KEY RESULTS

At reinstatement, rats that had received CS extinction showed reduced responding. This effect was attenuated by MTEP treatment before CS extinction. In contrast, administration of CDPPB (PAM) led to decreased reinstatement the following day, regardless of extinction condition.

CONCLUSION AND IMPLICATIONS

These results suggest that mGlu5 receptor activity is both necessary and sufficient for efficient extinction of a cocaine-associated CS. Therefore, mGlu5 PAMs could enhance the efficacy of CET.

mGlu5 Receptors and Relapse to Cocaine-Seeking: The Role of Receptor Trafficking in Postrelapse Extinction Learning Deficits

We have previously demonstrated that MTEP, an allosteric antagonist of mGlu5, infused into the nucleus accumbens attenuates relapse after abstinence from cocaine self-administration. MTEP infused into the dorsolateral striatum (dlSTR) does not alter relapse but has long-lasting effects on subsequent extinction learning. Here we tested whether systemic MTEP would prevent relapse after abstinence or alter extinction learning. We also investigated the mechanism of action by which intra-dlSTR MTEP on test day alters extinction on subsequent days. Animals self-administered cocaine for 12 days followed by abstinence for 20-21 days. MTEP (0.5–5 mg/kg IP) was administered prior to placement into the operant chamber for a context-primed relapse test. A separate group of animals received intra-dlSTR MTEP prior to the relapse test and were sacrificed day later. Systemic administration of MTEP attenuated abstinent-relapse without significantly affecting extinction learning. Surface biotinylation analysis of protein expression in the dlSTR revealed that, in cocaine animals, intra-dlSTR MTEP administration decreased mGlu5 surface expression and prevented changes in Arc and GluA1/GluA2 observed in their vehicle counterparts. Thus, blockade of mGlu5 receptors may be utilized in future treatment strategies for relapse prevention in humans, although the effects of chronic blockade on extinction learning should be further evaluated.

Distinct miRNA expression in dorsal striatal subregions is associated with risk for addiction in rats

Recently, we published data using an animal model that allowed us to characterize animals into two groups, addiction vulnerable and addiction resilient, where we identified that addiction/relapse vulnerability was associated with deficits in synaptic plasticity-associated gene expression in the dorsal striatum (DS). Notable was the strong reduction in expression for activity-regulated cytoskeleton-associated protein (Arc) considered a master regulator of synaptic plasticity. In the present study, we confirmed that Arc messenger RNA was significantly decreased in the DS, but importantly, we identified that this reduction was restricted to the dorsomedial (DMS) and not dorsolateral striatum (DLS). There is recent evidence of microRNA (miRNA)-associated posttranscriptional suppression of Arc and animal models of addiction have identified a key role for miRNA in the regulation of addiction-relevant genes. In further support of this link, we identified several differentially expressed miRNA with the potential to influence addiction-relevant plasticity genes, including Arc. A key study recently reported that miR-212 expression is protective against compulsive cocaine-seeking. Supporting this hypothesis, we found that miR-212 expression was significantly reduced in the DMS but not DLS of addiction-vulnerable animals. Together, our data provide strong evidence that miRNA promote ongoing plasticity deficits in the DS of addiction-vulnerable animals.

Role of cues and contexts on drug-seeking behaviour

Environmental stimuli are powerful mediators of craving and relapse in substance-abuse disorders. This review examined how animal models have been used to investigate the cognitive mechanisms through which cues are able to affect drug-seeking behaviour. We address how animal models can describe the way drug-associated cues come to facilitate the development and persistence of drug taking, as well as how these cues are critical to the tendency to relapse that characterizes substance-abuse disorders. Drug-associated cues acquire properties of conditioned reinforcement, incentive motivation and discriminative control, which allow them to influence drug-seeking behaviour. Using these models, researchers have been able to investigate the pharmacology subserving the behavioural impact of environmental stimuli, some of which we highlight. Subsequently, we examine whether the impact of drug-associated stimuli can be attenuated via a process of extinction, and how this question is addressed in the laboratory. We discuss how preclinical research has been translated into behavioural therapies targeting substance abuse, as well as highlight potential developments to therapies that might produce more enduring changes in behaviour.