Nathan J. Marchant

The reinstatement model of drug relapse: recent neurobiological findings, emerging research topics, and translational research.

Background and rationaleResults from many clinical studies suggest that drug relapse and craving are often provoked by acute exposure to the self-administered drug or related drugs, drug-associated cues or contexts, or certain stressors. During the last two decades, this clinical scenario has been studied in laboratory animals by using the reinstatement model. In this model, reinstatement of drug seeking by drug priming, drug cues or contexts, or certain stressors is assessed following drug self-administration training and subsequent extinction of the drug-reinforced responding.ObjectiveIn this review, we first summarize recent (2009–present) neurobiological findings from studies using the reinstatement model. We then discuss emerging research topics, including the impact of interfering with putative reconsolidation processes on cue- and context-induced reinstatement of drug seeking, and similarities and differences in mechanisms of reinstatement across drug classes. We conclude by discussing results from recent human studies that were inspired by results from rat studies using the reinstatement model.ConclusionsMain conclusions from the studies reviewed highlight: (1) the ventral subiculum and lateral hypothalamus as emerging brain areas important for reinstatement of drug seeking, (2) the existence of differences in brain mechanisms controlling reinstatement of drug seeking across drug classes, (3) the utility of the reinstatement model for assessing the effect of reconsolidation-related manipulations on cue-induced drug seeking, and (4) the encouraging pharmacological concordance between results from rat studies using the reinstatement model and human laboratory studies on cue- and stress-induced drug craving.

Role of Projections from Ventral Medial Prefrontal Cortex to Nucleus Accumbens Shell in Context-Induced Reinstatement of Heroin Seeking

In humans, exposure to contexts previously associated with heroin use can provoke relapse. In rats, exposure to heroin-paired contexts after extinction of drug-reinforced responding in different contexts reinstates heroin seeking. This effect is attenuated by inhibition of glutamate or dopamine transmission in nucleus accumbens shell, or inactivation of ventral medial prefrontal cortex (mPFC). Here, we used an anatomical asymmetrical disconnection procedure to demonstrate that an interaction between glutamatergic projections from ventral mPFC to accumbens shell and local dopamine D1 postsynaptic receptors contributes to context-induced reinstatement of heroin seeking. We also combined the marker of neuronal activity, Fos, with the retrograde tracer Fluoro-Gold to assess activation in this pathway during context-induced reinstatement. Rats were trained to self-administer heroin for 12 d; drug infusions were paired with a discrete tone–light cue. Lever pressing was subsequently extinguished in a nondrug-associated context in the presence of the discrete cue. Rats were then tested in the heroin- or extinction-associated contexts under extinction conditions. Injections of muscimol + baclofen into ventral mPFC in one hemisphere and D1-family receptor antagonist SCH 23390 into the contralateral or ipsilateral accumbens shell decreased context-induced reinstatement. Unilateral injections of muscimol + baclofen into ventral mPFC or SCH 23390 into the accumbens shell had no effect. Context-induced reinstatement was associated with increased Fos expression in ventral mPFC neurons, including those projecting to accumbens shell, with higher double-labeling in the ipsilateral projection than in the contralateral projection. Our results demonstrate that activation of glutamatergic projections from ventral mPFC to accumbens shell, previously implicated in inhibition of cocaine relapse, promotes heroin relapse.

Recent developments in animal models of drug relapse

Drug craving and relapse to drug use during abstinence are defining features of addiction. Evidence indicates that drug craving and relapse in humans are often provoked by acute exposure to the self-administered drug, drug-associated cues, or stress. During the last two decades, this clinical scenario has been primarily studied at the preclinical level using the classical reinstatement model. However, a single preclinical model cannot capture the complicated nature of human drug relapse. Therefore, more recently, we and others have developed several other models to study different facets of human drug relapse. In this review, we introduce and discuss recent findings from these other relapse models, including incubation of drug craving, reacquisition and resurgence models, and punishment-based and conflict-based relapse models.

Extinction of drug seeking

Drug seeking behavior can be reduced or inhibited via extinction. The brain mechanisms for extinction of drug seeking are poorly understood but are of significant interest because of their potential to identify novel approaches that promote abstinence from drug taking. Here we review recent literature on the neural mechanisms for extinction in drug self-administration paradigms. First, we consider the brain regions important for extinction of drug seeking. Functional inactivation studies have identified infralimbic prefrontal cortex, nucleus accumbens shell, as well as medial dorsal hypothalamus in the expression of extinction of drug seeking. These structures have been implicated in extinction expression across several reinforcers including cocaine, heroin, and alcohol. Second, we consider molecular studies which show that extinction training is associated with plasticity in glutamatergic signaling in both nucleus accumbens shell and core, and that this training may reverse or ameliorate the neuroadaptations produced by chronic drug exposure and spontaneous withdrawal. Finally, we consider the neural circuitry for extinction of drug seeking. Functional disconnection and neuroanatomical tracing studies show that extinction expression depends, at least in part, on cortico-striatal-hypothalamic and cortico-hypothalalmic-thalamic pathways. Moreover, they indicate that the expression of extinction and reinstatement of drug seeking may depend on parallel pathways that converge within lateral hypothalamus and paraventricular thalamus.

Medial Dorsal Hypothalamus Mediates the Inhibition of Reward Seeking after Extinction

Extinction promotes abstinence from drug seeking. Extinction expression is an active process, dependent on infralimbic prefrontal cortex (ilPFC). However, the neurocircuitry mediating extinction expression is unknown. Here we studied the neural mechanisms for expression of extinction of alcoholic beer seeking in rats. We first examined the pattern of activation in prefrontal cortex projections to medial dorsal hypothalamus (MDH) (i.e., perifornical and dorsomedial nuclei) during extinction expression. Double labeling for retrograde tracer cholera toxin B subunit (CTb) and the neuronal activity marker c-Fos revealed significant recruitment of MDH projecting ilPFC neurons during extinction expression. We then studied the causal role of MDH in inhibiting alcoholic beer seeking during extinction expression. MDH infusion of the inhibitory neuropeptide cocaine- and amphetamine-regulated transcript prevented extinction expression, showing that MDH is necessary for extinction expression. Next we examined the pattern of activation in MDH projections to paraventricular thalamus (PVT) during extinction expression. Double labeling for CTb and c-Fos revealed significant recruitment of PVT projecting MDH neurons during extinction expression. We also showed, using triple-label immunofluorescence, that the majority of PVT projecting extinction neurons express prodynorphin, suggesting that actions at κ opioid receptors (KORs) in PVT may be critical for inhibiting alcoholic beer seeking. Consistent with this, infusions of a KOR agonist into PVT prevented reinstatement of alcoholic beer seeking showing that PVT KOR activation is sufficient to inhibit alcoholic beer seeking. Together, these findings identify a role for MDH and its ilPFC afferents and PVT efferents in inhibiting alcoholic beer seeking during extinction expression.

Lateral Hypothalamus Is Required for Context-Induced Reinstatement of Extinguished Reward Seeking

We studied the role of lateral hypothalamus (LH) in context-induced reinstatement (renewal) of reward seeking. Rats were trained to respond for 4% (v/v) alcoholic beer or 10% (w/v) sucrose reward in one context (Context A) before extinction training in a second context (Context B). On test, rats were returned to the training context, A (ABA), or the extinction context, B (ABB). Return to the training context (ABA) produced robust reinstatement. Reversible inactivation of LH via baclofen/muscimol infusion prevented context-induced reinstatement of beer and sucrose seeking. This prevention was specific to bilateral infusions into LH. We then used the retrograde neuronal tracer cholera toxin b subunit (CTb) combined with detection of the c-Fos protein to identify activated afferents to LH during context-induced reinstatement of beer seeking. Double labeling for c-Fos and CTb revealed a significant recruitment of LH-projecting neurons in nucleus accumbens shell (AcbSh) during reinstatement. These afferents could be classified into two anatomically and functionally distinct groups. First, afferents in the ventral AcbSh projecting to LH were activated during reinstatement. Second, afferents in the dorsomedial AcbSh projecting to LH were activated during test in the extinction context. These recruitments were specific to an AcbSh–LH pathway because they were not observed following CTb injection into the immediately adjacent perifornical hypothalamus. These results show that LH is critical for context-induced reinstatement of reward seeking and that parallel striatal-hypothalamic pathways are recruited following return to the training versus extinction contexts.

Effect of the Novel Positive Allosteric Modulator of Metabotropic Glutamate Receptor 2 AZD8529 on Incubation of Methamphetamine Craving After Prolonged Voluntary Abstinence in a Rat Model

BACKGROUND Cue-induced methamphetamine craving increases after prolonged forced (experimenter-imposed) abstinence from the drug (incubation of methamphetamine craving). Here, we determined whether this incubation phenomenon would occur under conditions that promote voluntary (self-imposed) abstinence. We also determined the effect of the novel metabotropic glutamate receptor 2 positive allosteric modulator, AZD8529, on incubation of methamphetamine craving after forced or voluntary abstinence. METHODS We trained rats to self-administer palatable food (6 sessions) and then to self-administer methamphetamine under two conditions: 12 sessions (9 hours/day) or 50 sessions (3 hours/day). We then assessed cue-induced methamphetamine seeking in extinction tests after 1 or 21 abstinence days. Between tests, the rats underwent either forced abstinence (no access to the food- or drug-paired levers) or voluntary abstinence (achieved via a discrete choice procedure between methamphetamine and palatable food; 20 trials per day) for 19 days. We also determined the effect of subcutaneous injections of AZD8529 (20 and 40 mg/kg) on cue-induced methamphetamine seeking 1 day or 21 days after forced or voluntary abstinence. RESULTS Under both training and abstinence conditions, cue-induced methamphetamine seeking in the extinction tests was higher after 21 abstinence days than after 1 day (incubation of methamphetamine craving). AZD8529 decreased cue-induced methamphetamine seeking on day 21 but not day 1 of forced or voluntary abstinence. CONCLUSIONS We introduce a novel animal model to study incubation of drug craving and cue-induced drug seeking after prolonged voluntary abstinence, mimicking the human condition of relapse after successful contingency management treatment. Our data suggest that positive allosteric modulators of metabotropic glutamate receptor 2 should be considered for relapse prevention.

Incubation of methamphetamine and palatable food craving after punishment-induced abstinence.

In a rat model of drug craving and relapse, cue-induced drug seeking progressively increases after withdrawal from methamphetamine and other drugs, a phenomenon termed ‘incubation of drug craving’. However, current experimental procedures used to study incubation of drug craving do not incorporate negative consequences of drug use, which is a common factor promoting abstinence in humans. Here, we studied whether incubation of methamphetamine craving is observed after suppression of drug seeking by adverse consequences (punishment). We trained rats to self-administer methamphetamine or palatable food for 9 h per day for 14 days; reward delivery was paired with a tone-light cue. Subsequently, for one group within each reward type, 50% of the lever-presses were punished by mild footshock for 9–10 days, whereas for the other group lever-presses were not punished. Shock intensity was gradually increased over time. Next, we assessed cue-induced reward seeking in 1-h extinction sessions on withdrawal days 2 and 21. Response-contingent punishment suppressed extended-access methamphetamine or food self-administration; surprisingly, food-trained rats showed greater resistance to punishment than methamphetamine-trained rats. During the relapse tests, both punished and unpunished methamphetamine- and food-trained rats showed significantly higher cue-induced reward seeking on withdrawal day 21 than on day 2. These results demonstrate that incubation of both methamphetamine and food craving occur after punishment-induced suppression of methamphetamine or palatable food self-administration. Our procedure can be used to investigate mechanisms of relapse to drug and palatable food seeking under conditions that more closely approximate the human condition.

Context-Induced Relapse to Alcohol Seeking After Punishment in a Rat Model

BACKGROUND Rat studies have demonstrated that exposure to environments associated with alcohol intake reinstates alcohol seeking after extinction of alcohol-reinforced responding in a different context. However, extinction is limited as an abstinence model, because humans typically abstain because of negative consequences associated with excessive drinking. It is currently unknown whether alcohol-associated contexts can provoke relapse to alcohol seeking after alcohol-taking behavior is suppressed by adverse consequences in a different context. METHODS Alcohol-preferring P rats were first given home-cage access to 20% ethanol. Next, they were trained to self-administer 20% ethanol in one context (context A). Subsequently, all rats continued to self-administer alcohol in a different context (context B). For one group, 50% of alcohol-reinforced responses were punished by mild footshock; two other groups either received noncontingent shocks or no shock. A fourth group was given extinction training in context B. All rats were then tested for relapse to alcohol seeking under extinction conditions in contexts A and B. RESULTS In Context B, alcohol-taking behavior was suppressed by contingent shock (punishment) and extinction training but not by noncontingent shock. In Context A, relapse to alcohol seeking was reliably observed in the punished and extinction groups; a context switch had no effect on alcohol seeking in the no-shock or noncontingent shock groups. CONCLUSIONS Our data indicate that punishment-induced suppression of alcohol-taking behavior is context-dependent. We propose that our procedure can be used to explore mechanisms of context-induced relapse to alcohol seeking after alcohol-taking behavior is suppressed by adverse consequences.

Optogenetic Inhibition of Dorsal Medial Prefrontal Cortex Attenuates Stress-Induced Reinstatement of Palatable Food Seeking in Female Rats

Relapse to maladaptive eating habits during dieting is often provoked by stress. Recently, we identified a role of dorsal medial prefrontal cortex (mPFC) neurons in stress-induced reinstatement of palatable food seeking in male rats. It is unknown whether endogenous neural activity in dorsal mPFC drives stress-induced reinstatement in female rats. Here, we used an optogenetic approach, in which female rats received bilateral dorsal mPFC microinjections of viral constructs coding light-sensitive eNpHR3.0–eYFP or control eYFP protein and intracranial fiber optic implants. Rats were food restricted and trained to lever press for palatable food pellets. Subsequently, pellets were removed, and lever pressing was extinguished; then the effect of bilateral dorsal mPFC light delivery on reinstatement of food seeking was assessed after injections of the pharmacological stressor yohimbine (an α-2 andrenoceptor antagonist) or pellet priming, a manipulation known to provoke food seeking in hungry rats. Dorsal mPFC light delivery attenuated yohimbine-induced reinstatement of food seeking in eNpHR3.0-injected but not eYFP-injected rats. This optical manipulation had no effect on pellet-priming-induced reinstatement or ongoing food-reinforced responding. Dorsal mPFC light delivery attenuated yohimbine-induced Fos immunoreactivity and disrupted neural activity during in vivo electrophysiological recording in awake rats. Optical stimulation caused significant outward currents and blocked electrically evoked action potentials in eNpHR3.0-injected but not eYFP-injected mPFC hemispheres. Light delivery alone caused no significant inflammatory response in mPFC. These findings indicate that intracranial light delivery in eNpHR3.0 rats disrupts endogenous dorsal mPFC neural activity that plays a role in stress-induced relapse to food seeking in female rats.