Animal Models of Drug Relapse and Craving after Voluntary Abstinence: A Review

Individual differences in prelimbic neural representation of food and cocaine seeking

The prelimbic cortex is involved in operant reward seeking. However, the precise nature of its activity patterns and whether/how they differ between different types of rewards are largely unknown. We use miniscope calcium imaging to observe prelimbic activity during both food and cocaine seeking in freely behaving mice. We find that neurobehavioral representations remain stable across days within individual mice. Unexpectedly, our data reveal significant individual differences: some mice display high similarity in their prelimbic cortex activity patterns for both food and cocaine seeking, while others show no such overlap. These findings suggest that individual differences in the neural mechanisms underlying food and cocaine seeking could be a critical factor to consider when developing future addiction treatment strategies.

Effect of chronic delivery of the NOP/MOR partial agonist AT-201 and NOP antagonist J-113397 on heroin relapse in a rat model of opioid maintenance

RATIONALE

The opioid crisis persists despite availability of effective opioid agonist maintenance treatments (methadone and buprenorphine). Thus, there is a need to advance novel medications for the treatment of opioid use and relapse.

OBJECTIVES

We recently modeled maintenance treatment in rats and found that chronic delivery of buprenorphine and the mu opioid receptor (MOR) partial agonist TRV130 decreases relapse to oxycodone seeking and taking. In contrast, chronic delivery of the buprenorphine analog BU08028 had mixed effects on different heroin relapse-related measures. Here, we tested the effect of the mixed nociceptin (NOP) receptor/MOR partial agonist AT-201 and the NOP receptor antagonist J-113397 on different heroin relapse-related measures.

METHODS

We trained male and female rats to self-administer heroin (6-h/d, 14-d) in context A and then implanted osmotic minipumps containing AT-201 (0, 3.8, or 12 mg/kg/d) or J-113397 (0, 12.6, or 40 mg/kg/d). Next, we tested the effect of chronic delivery of the compounds on (1) incubation of heroin seeking in a non-drug context B, (2) extinction responding reinforced by heroin-associated discrete cues in context B, (3) context A-induced reinstatement of heroin seeking, and (4) reacquisition of heroin self-administration in context A.

RESULTS

In females, AT-201 modestly increased reacquisition of heroin self-administration and J-113397 modestly decreased incubation of heroin seeking. The compounds had no effect on the other relapse-related measures in females, and no effect on any of the measures in males.

CONCLUSION

The NOP/MOR partial agonist AT-201 and the NOP antagonist J-113397 did not mimic buprenorphine's inhibitory effects on relapse in a rat model of opioid maintenance treatment.

Delineating anticipatory arousal from reward consumption: evaluating fixed-intervals in cocaine seeking-taking response chains

RATIONALE

Anticipation is a critical antecedent to drug use, in which the prospect of imminent drug availability can potently motivate instrumental actions directed to procure it. Models that capture the behavioral dynamics that precede drug access may allow for the dissociation of key neural mechanisms underlying appetitive or consummatory processes in drug self-administration.

OBJECTIVES

We aimed to isolate measurements attributed to the procurement and consumption of a reward by defining distinct actions for each using a chain-schedule of reinforcement.

METHODS

Male Long-Evans rats were trained to self-administer cocaine or saccharin under a chained schedule of reinforcement (FI-FR) in order to dissociate appetitive ('seeking') from consummatory ('taking') behaviors. Completion of a fixed-interval (5 min) was followed by 5 min of continuously reinforced responding (FR1) on another lever.

RESULTS

The FI-FR chain procedure appears to provide sensitive and dissociable dimensions of cocaine self-administration within a single experimental session. Importantly, we demonstrate that responding during the FI (i.e., seeking) link tracks with the incentive value of anticipated reward access - whereby response rates corresponded to expected reward magnitude, degree of reward-specific satiety, and general motivational state.

CONCLUSIONS

The FI component is a sensitive and reliable index of motivational changes induced by either the extrinsic incentive value of reinforcement (i.e., anticipated dose) or intrinsic motive states (i.e., satiety or deprivation). This procedure provides a valuable tool for interrogating the neural dynamics of drug-seeking and -taking behavior, in isolation.

Chronic Morphine Leaves a Durable Fingerprint on Whole-Brain Functional Connectivity

BACKGROUND

Opioid use disorder is a chronic relapsing disorder. The brain adapts to opioids that are taken for pain treatment or recreational use so that abstinence becomes a true challenge for individuals with opioid use disorder. Studying brain dysfunction at this stage is difficult, and human neuroimaging has provided highly heterogeneous information.

METHODS

Here, we took advantage of an established mouse model of morphine abstinence together with functional magnetic resonance imaging to investigate whole-brain functional connectivity (FC) first at rest and then in response to an acute morphine challenge during image acquisition.

RESULTS

Hierarchical clustering of seed pair correlation coefficients showed modified FC in abstinent animals, brainwide and regardless of the condition. Seed-to-voxel analysis and random forest classification, performed on data at rest, indicated that the retrosplenial cortex (a core component of the default mode network) and the amygdala (a major aversion center) are the best markers of abstinence, thus validating the translatability of the study. Seed pair network clustering confirmed disruption of a retrosplenial cortex-centered network, reflecting major reorganization of brain FC. The latter analysis also identified a persistent but unreported morphine signature in abstinent mice at rest, which involves cortical and midbrain components and characterizes the enduring morphine footprint. Finally, dynamic FC analysis revealed that the intrascanner acute morphine challenge modified FC faster and more broadly in abstinent animals, demonstrating brainwide adaptations of FC reactivity to an acute opioid challenge.

CONCLUSIONS

This study used a unique experimental design to demonstrate that a prior history of chronic opioid exposure leaves a durable pharmacological signature on brain communication, with implications for pain management and recovery from opioid use disorder.

Differential gene expression and chromatin accessibility in the medial prefrontal cortex associated with individual differences in rat behavioral models of opioid use disorder

Opioid use disorder (OUD) is a neuropsychological disease that has a devastating impact on public health. Substantial individual differences in vulnerability exist, the neurobiological substrates of which remain unclear. To address this question, we investigated genome-wide gene transcription (RNA-seq) and chromatin accessibility (ATAC-seq) in the medial prefrontal cortex (mPFC) of male and female rats exhibiting differential vulnerability in behavioral paradigms modeling different phases of OUD: Withdrawal-Induced Anhedonia (WIA), Demand, and Reinstatement. Ingenuity Pathway Analysis (IPA) of RNA-seq revealed greater changes in canonical pathways in Resilient (vs. Saline) rats in comparison to Vulnerable (vs. Saline) rats across 3 paradigms, suggesting brain adaptations that might contribute to resilience to OUD across its trajectory. Analyses of gene networks and upstream regulators implicated processes involved in oligodendrocyte maturation and myelination in WIA, neuroinflammation in Demand, and metabolism in Reinstatement. Motif analysis of ATAC-seq showed changes in chromatin accessibility to a small set of transcription factor (TF) binding sites as a function either of opioid exposure (i.e., morphine versus saline) generally or of individual vulnerability specifically. Some of these were shared across the 3 paradigms and others were unique to each. In conclusion, we have identified changes in biological pathways, TFs, and their binding motifs that vary with paradigm and OUD vulnerability. These findings point to the involvement of distinct transcriptional and epigenetic mechanisms in response to opioid exposure, vulnerability to OUD, and different stages of the disorder.

The selective D3Receptor antagonist VK4-116 reverses loss of insight caused by self-administration of cocaine in rats

Chronic psychostimulant use causes long-lasting changes to neural and cognitive function that persist after long periods of abstinence. As cocaine users transition from drug use to abstinence, a parallel transition from hyperactivity to hypoactivity has been found in orbitofrontal-striatal glucose metabolism and striatal D2/D3-receptor activity. Targeting these changes pharmacologically, using highly selective dopamine D3-receptor (D3R) antagonists and partial agonists, has shown promise in reducing drug-taking, and attenuating relapse in animal models of cocaine and opioid use disorder. However, much less attention has been paid to treating the loss of insight, operationalized as the inability to infer likely outcomes, associated with chronic psychostimulant use. Here we tested the selective D3R antagonist VK4-116 as a treatment for this loss in rats with a prior history of cocaine use. Male and female rats were first trained to self-administer cocaine or a sucrose liquid for 2 weeks. After 4 weeks of abstinence, performance was assessed using a sensory preconditioning (SPC) learning paradigm. Rats were given VK4-116 (15 mg/kg, i.p.) or vehicle 30 min prior to each SPC training session, thus creating four drug-treatment groups: sucrose-vehicle, sucrose-VK4-116, cocaine-vehicle, cocaine-VK4-116. The control groups (sucrose-vehicle, sucrose-VK4-116) showed normal sensory preconditioning, whereas cocaine use (cocaine-vehicle) selectively disrupted responding to the preconditioned cue, an effect that was reversed in the cocaine-VK4-116 group, which demonstrating responding to the preconditioned cue at levels comparable to controls. These preclinical findings demonstrate that highly selective dopamine D3R antagonists, particularly VK4-116, can reverse the long-term negative behavioral consequences of cocaine use.

Neuroplasticity-related genes correlate with individual differences in distinct phases of oxycodone self-administration in male rats

Opioid use disorder (OUD) is a chronic condition associated with long-lasting molecular and behavioral changes. Animals with prolonged access to opioids develop behaviors similar to human OUD. Identifying associated molecular changes can provide insight to underpinnings that lead to or maintain OUD. In pilot studies, we identified several miRNA targets that are altered by the administration of oxycodone. We selected mir182 for follow up as it was recently shown to be dysregulated in plasma of men administered oxycodone. In addition, mir182 is increased in reward-related brain regions of male rats following exposure to various addictive substances. The present study utilizes a long-access oxycodone self-administration paradigm to examine changes in mir182 and its mRNA targets associated with neuroplasticity, which may be involved in the maintenance of OUD-like phenotype in rats. Male rats were trained to self-administer oxycodone (0.1 mg/kg/infusion, i. v.) for 6 h daily sessions for 12 days. Each animal had a yoked saline control that received matched saline infusions. Animals were then tested on a progressive ratio schedule to measure motivation to obtain a single infusion of oxycodone. Drug seeking was measured following 28 days of forced abstinence using a 90-min cued/test. RTqPCR was utilized to measure mir182 and mRNA targets related to neuroplasticity (wnt3, plppr4, pou3f3, tle4, cacna2d, and bdnf) from the nucleus accumbens. Data revealed that animals responded on a continuum for oxycodone. When divided into two groups termed high- and low responders, animals diverged during self-administration acquisition and maintained differences in behavior and gene expression throughout the study. mir182 was upregulated in the nucleus accumbens of both high and low responders and negatively correlated with tle4, which showed a strong negative correlation with reinstatement behavior. mRNA target levels were correlated with behaviors associated with increased severity of OUD behavior in male rats.

After a period of forced abstinence, rats treated with the norepinephrine neurotoxin DSP-4 still exhibit preserved food-seeking behavior and prefrontal cortex fos-expressing neurons

Aims

Relapse is a common characteristic of compulsive behaviors like addiction, where individuals tend to return to drug use or overeating after a period of abstinence. PFC (prefrontal cortex) neuronal ensembles are required for drug and food-seeking behaviors and are partially regulated by Norepinephrine (NE). However, the contributions of neuromodulators, such as the adrenergic system, in food-seeking behavior are not fully understood.

Main methods

To investigate this, we trained male and female rats to press a lever in an operant chamber to obtain banana-flavored food pellets for ten days. We then administered DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride), a neurotoxin that diminishes norepinephrine levels in the brain. The rats were kept in their home cages for ten more days before being returned to the operant chambers to measure food-seeking behavior.

Key findings

Despite receiving DSP-4, the PFC neuronal ensembles measured by Fos and food-seeking behavior did not differ between groups, but rather sex.

Significance

Although both NE and Fos expressing neurons are implicated in food-seeking, they do not seem to be involved in a cue-contextual induced re-exposure response.

Incubation of methamphetamine craving in punishment-resistant individuals is associated with activation of specific gene networks in the rat dorsal striatum

Methamphetamine use disorder (MUD) is characterized by loss of control over compulsive drug use. Here, we used a self-administration (SA) model to investigate transcriptional changes associated with the development of early and late compulsivity during contingent footshocks. Punishment initially separated methamphetamine taking rats into always shock-resistant (ASR) rats that continued active lever pressing and shock-sensitive (SS) rats that reduced their lever pressing. At the end of the punishment phase, rats underwent 15 days of forced abstinence at the end of which they were re-introduced to the SA paradigm followed by SA plus contingent shocks. Interestingly, 36 percent of the initial SS rats developed delayed shock-resistance (DSR). Of translational relevance, ASR rats showed more incubation of methamphetamine craving than DSR and always sensitive (AS) rats. RNA sequencing revealed increased striatal Rab37 and Dipk2b mRNA levels that correlated with incubation of methamphetamine craving. Interestingly, Bdnf mRNA levels showed HDAC2-dependent decreased expression in the AS rats. The present SA paradigm should help to elucidate the molecular substrates of early and late addiction-like behaviors.

Involvement of Pre-limbic Cortex-Nucleus accumbens projections in Context-Induced alcohol seeking

Alcohol use disorder (AUD) remains a critical public health issue worldwide, characterized by high relapse rates often triggered by contextual cues. This research investigates the neural mechanisms behind context-induced reinstatement of alcohol-seeking behavior, focusing on the nucleus accumbens and its interactions with the prelimbic cortex, employing Male Long-Evans rats in an ABA renewal model. In our experimental setup, rats were trained to self-administer 10 % ethanol in Context A, followed by extinction of lever pressing in the presence of discrete cues in Context B. The context-induced reinstatement of ethanol-seeking was then assessed by re-exposing rats to Context A or B under extinction conditions, aiming to simulate the environmental cues' influence on relapse behaviors. Three experiments were conducted: Experiment 1 utilized Fos-immunohistochemistry to examine neuronal activation in the nucleus accumbens; Experiment 2 applied the baclofen + muscimol inactivation technique to probe the functional importance of the nucleus accumbens core; Experiment 3 used Fos-immunofluorescence along with Retrobeads injection to investigate activation of neurons projecting from the prelimbic cortex to the nucleus accumbens core. Our findings revealed significant increases in Fos-immunoreactive nuclei within the nucleus accumbens core and shell during the reinstatement phase in Context A, underscoring the environment's potent effect on ethanol-seeking behavior. Additionally, inactivation of the nucleus accumbens core markedly reduced reinstatement, and there was a notable activation of neurons from the prelimbic cortex to the nucleus accumbens core in the ethanol-associated context. These results highlight the critical role of the nucleus accumbens core and its corticostriatal projections in the neural circuitry underlying context-driven ethanol seeking.

Efficacy and Neural Mechanisms of Mindfulness Meditation Among Adults With Internet Gaming Disorder: A Randomized Clinical Trial

Importance

The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, Text Revision (DSM-5-TR), recently identified internet gaming disorder (IGD) as a condition warranting more research, and few empirically validated treatments exist. Mindfulness meditation (MM) has multiple health benefits; however, its efficacy in treating IGD and potential neural mechanisms underlying MM treatment of the disorder remain largely unknown.

Objective

To explore the efficacy of MM used to treat adults with IGD and to identify neural mechanisms underlying MM.

Design, Setting, and Participants

This randomized clinical trial was performed from October 1 to November 30, 2023, at Hangzhou Normal University in Hangzhou, China. Adults (aged ≥18 years) who met at least 6 of the 9 DSM-5-TR proposed criteria for IGD were recruited to receive either MM or progressive muscle relaxation (PMR). Data analysis was performed on December 1, 2023.

Intervention

Participants underwent MM training (an 8-session meditation program that focuses on attention and acceptance) and PMR training (an 8-time program for body relaxation) delivered in groups that met 2 times each week for 4 weeks.

Main Outcomes and Measures

This per-protocol analysis included only participants who finished the pretest assessment, 8 training sessions, and posttest assessment. The main outcomes were addiction severity (measured with the DSM-5-TR proposed criteria for IGD and with Internet Addiction Test scores), gaming craving (measured with Questionnaire for Gaming Urges scores), and blood oxygen level-dependent signals assessed with cue-craving tasks on fMRI. Behavioral and brain measurements were compared using analysis of variance. Functional connectivity (FC) among identified brain regions was measured to test connectivity changes associated with MM.

Results

This study included 64 adults with IGD. A total of 32 participants received MM (mean [SD] age, 20.3 [1.9] years; 17 women [53%]) and 32 received PMR (mean [SD] age, 20.2 [1.5] years; 16 women [50%]). The severity of IGD decreased in the MM group (pretest vs posttest: mean [SD], 7.0 [1.1] vs 3.6 [0.8]; P < .001) and in the PMR group (mean [SD], 7.1 [0.9] vs 6.0 [0.9]; P = .04). The MM group had a greater decrease in IGD severity than the PMR group (mean [SD] score change for the MM group vs the PMR group, -3.6 [0.3] vs -1.1 [0.2]; P < .001). Mindfulness meditation was associated with decreased brain activation in the bilateral lentiform nuclei (r = 0.40; 95% CI, 0.19 to 0.60; P = .02), insula (r = 0.35; 95% CI, 0.09 to 0.60; P = .047), and medial frontal gyrus (MFG; r = 0.43; 95% CI, 0.16 to 0.70; P = .01). Increased MFG-lentiform FC and decreased craving (pretest vs posttest: mean [SD], 58.8 [15.7] vs 33.6 [12.0]; t = -8.66; ƞ2 = 0.30; P < .001) was observed after MM, and changes in MFG-lentiform FC mediated the relationship between increased mindfulness and decreased craving (mediate effect, -0.17; 95% CI, -0.32 to -0.08; P = .03).

Conclusions and Relevance

In this study, MM was more effective in decreasing addiction severity and gaming cravings compared with PMR. These findings indicate that MM may be an effective treatment for IGD and may exert its effects by altering frontopallidal pathways.

Trial Registration

Chinese Clinical Trial Registry Identifier: ChiCTR2300075869.

Relapse after electric barrier-induced voluntary abstinence: A review

Relapse to drug use during abstinence is a defining feature of addiction. To date, however, results from studies using rat relapse/reinstatement models have yet to result in FDA-approved medications for relapse prevention. To address this translational gap, we and others have developed rat models of relapse after voluntary abstinence from drug self-administration. One of these models is the electric barrier conflict model. Here, we introduce the model, and then review studies on behavioral and neuropharmacological mechanisms of cue-induced relapse and incubation of drug seeking (time-dependent increase in drug seeking during abstinence) after electric barrier-induced abstinence. We also briefly discuss future directions and potential clinical implications. One major conclusion of our review is that the brain mechanisms controlling drug relapse after electrical barrier-induced voluntary abstinence are likely distinct from those controlling relapse after homecage forced abstinence.

Opioid craving does not incubate over time in inpatient or outpatient treatment studies: Is the preclinical incubation of craving model lost in translation?

Within addiction science, incubation of craving is an operational label used to describe time-dependent increases in drug seeking during periods of drug deprivation. The purpose of this systematic review was to describe the preclinical literature on incubation of craving and the clinical literature on craving measured over extended periods of abstinence to document this translational homology and factors impacting correspondence. Across the 44 preclinical studies that met inclusion criteria, 31 reported evidence of greater lever pressing, nose pokes, spout licks, or time spent in drug-paired compartments (i.e., drug seeking) relative to neutral compartments after longer periods of abstinence relative to shorter periods of abstinence, labelled as "incubation of craving." In contrast, no clinical studies (n = 20) identified an increase in opioid craving during longer abstinence periods. The lack of clinical evidence for increases in craving in clinical populations weakens the translational utility of operationalizing the time-dependent increase in drug-seeking behavior observed in preclinical models as models of incubation of "craving".

Delay of punishment highlights differential vulnerability to developing addiction-like behavior toward sweet food

Resistance to punishment is commonly used to measure the difficulty in refraining from rewarding activities when negative consequences ensue, which is a hallmark of addictive behavior. We recently developed a progressive shock strength (PSS) procedure in which individual rats can titrate the amount of punishment that they are willing to tolerate to obtain food rewards. Here, we investigated the effects of a range of delays (0-12 s) on resistance to punishment measured by PSS break points. As expected from delay discounting principles, we found that delayed shock was less effective as a punisher, as revealed by higher PSS breakpoints. However, this discounting effect was not equally distributed in the population of rats, and the introduction of a delay highlighted the existence of two populations: rats that were sensitive to immediate punishment were also sensitive to delayed shock, whereas rats that were resistant to immediate punishment showed strong temporal discounting of delayed punishment. Importantly, shock-sensitive rats suppressed responding even in subsequent non-punishment sessions, and they differed from shock-resistant rats in anxiety-like behavior, but not in sensitivity to pain. These results show that manipulation of temporal contingencies of punishment in the PSS procedure provides a valuable tool to identify individuals with a double vulnerability to addiction: low sensitivity to aversion and excessive discounting of negative future consequences. Conversely, the shock-sensitive population may provide a model of humans who are vulnerable to opportunity loss due to excessive anxiety.

Different Effects of Peer Sex on Operant Responding for Social Interaction and Striatal Dopamine Activity

When rats are given discrete choices between social interactions with a peer and opioid or psychostimulant drugs, they choose social interaction, even after extensive drug self-administration experience. Studies show that like drug and nondrug food reinforcers, social interaction is an operant reinforcer and induces dopamine release. However, these studies were conducted with same-sex peers. We examined if peer sex influences operant social interaction and the role of estrous cycle and striatal dopamine in same- versus opposite-sex social interaction. We trained male and female rats (n = 13 responders/12 peers) to lever-press (fixed-ratio 1 [FR1] schedule) for 15 s access to a same- or opposite-sex peer for 16 d (8 d/sex) while tracking females' estrous cycle. Next, we transfected GRAB-DA2m and implanted optic fibers into nucleus accumbens (NAc) core and dorsomedial striatum (DMS). We then retrained the rats for 15 s social interaction (FR1 schedule) for 16 d (8 d/sex) and recorded striatal dopamine during operant responding for a peer for 8 d (4 d/sex). Finally, we assessed economic demand by manipulating FR requirements for a peer (10 d/sex). In male, but not female rats, operant responding was higher for the opposite-sex peer. Female's estrous cycle fluctuations had no effect on operant social interaction. Striatal dopamine signals for operant social interaction were dependent on the peer's sex and striatal region (NAc core vs DMS). Results indicate that estrous cycle fluctuations did not influence operant social interaction and that NAc core and DMS dopamine activity reflect sex-dependent features of volitional social interaction.

Social odor choice buffers drug craving

Social interactions are rewarding and protective against substance use disorders, but it is unclear which specific aspect of the complex sensory social experience drives these effects. Here, we investigated the role of olfactory sensory experience on social interaction, social preference over cocaine, and cocaine craving in rats. First, we conducted bulbectomy on both male and female rats to evaluate the necessity of olfactory system experience on the acquisition and maintenance of volitional social interaction. Next, we assessed the effect of bulbectomy on rats given a choice between social interaction and cocaine. Finally, we evaluated the influence of olfactory sensory experience by training rats on volitional partner-associated odors, assessing their preference for partner odors over cocaine to achieve voluntary abstinence and assessing its effect on the incubation of cocaine craving. Bulbectomy impaired operant social interaction without affecting food and cocaine self-administration. Rats with intact olfactory systems preferred social interaction over cocaine, while rats with impaired olfactory sense showed a preference for cocaine. Providing access to a partner odor in a choice procedure led to cocaine abstinence, preventing incubation of cocaine craving, in contrast to forced abstinence or non-contingent exposure to cocaine and partner odors. Our data suggests the olfactory sensory experience is necessary and sufficient for volitional social reward. Furthermore, the active preference for partner odors over cocaine buffers drug craving. Based on these findings, translational research should explore the use of social sensory-based treatments utilizing odor-focused foundations for individuals with substance use disorders.

IUPHAR Review: New strategies for medications to treat substance use disorders

Substance use disorders (SUDs) and drug overdose are a public health emergency and safe and effective treatments are urgently needed. Developing new medications to treat them is expensive, time-consuming, and the probability of a compound progressing to clinical trials and obtaining FDA-approval is low. The small number of FDA-approved medications for SUDs reflects the low interest of pharmaceutical companies to invest in this area due to market forces, characteristics of the population (e.g., stigma, and socio-economic and legal disadvantages), and the high bar regulatory agencies set for new medication approval. In consequence, most research on medications is funded by government agencies, such as the National Institute on Drug Abuse (NIDA). Multiple scientific opportunities are emerging that can accelerate the discovery and development of new medications for SUDs. These include fast and efficient tools to screen new molecules, discover new medication targets, use of big data to explore large clinical data sets and artificial intelligence (AI) applications to make predictions, and precision medicine tools to individualize and optimize treatments. This review provides a general description of these new research strategies for the development of medications to treat SUDs with emphasis on the gaps and scientific opportunities. It includes a brief overview of the rising public health toll of SUDs; the justification, challenges, and opportunities to develop new medications; and a discussion of medications and treatment endpoints that are being evaluated with support from NIDA.

Shifts in stimulus control over opioid use with increasing periods of recovery

BACKGROUND

Periods of engaging in an alternative behavior diminishes behavioral control by stimuli occasioning alcohol use. This increase in relapse resistance with increasing recovery suggests that changing stimulus control over substance use may be a mechanism responsible for decreased relapse rates with longer recovery. However, the generality of this phenomenon to other drugs of abuse, including opioid self-administration, remains unclear. This study tests the generality of these findings with etonitazene to determine whether the shift in attention represents a behavioral process that generalizes from conditions we previously reported.

METHODS

Five adult male Lewis rats were trained to respond on levers under two stimulus conditions; high-cost food (food FR150 and etonitazene FR5) and low-cost food (both food and etonitazene FR 5). Next, only the high-cost food stimulus (occasioning etonitazene responding) was presented for 20 sessions (Use Phase) followed by 9 sessions in which only the low-cost food stimulus (occasioning food responding) was presented (Recovery Phase). During the Recovery Phase, testing occurred during the first component of sessions 0, 1, 2, 4, and 8 when rats were re-exposed to the high-cost food stimulus. The number of food responses prior to completing the etonitazene response requirement during this stimulus exposure was the primary measure.

RESULTS

Food responses during stimulus re-exposure increased significantly as a function of recovery sessions completed with a slope [95 % CI] of 2.49 responses/recovery session [0.16, 4.81]. The average number of etonitazene deliveries per use session was 32 ± 6.6 or an average daily dose of 48.8 ± 10.1 μg/kg. During Recovery Phase, etonitazene deliveries decreased to 2.4 ± 1 or 3.6 ± 1.5 μg/kg.

CONCLUSION

The decrease in stimulus control observed for ethanol self-administration appears to generalize to opioid self-administration, indicating this change in stimulus control may play a general role in recovery.

Neurobiology of Stress-Induced Nicotine Relapse

Tobacco smoking is the leading cause of preventable death and disease. Although there are some FAD-approved medicines for controlling smoking, the relapse rate remains very high. Among the factors that could induce nicotine relapse, stress might be the most important one. In the last decades, preclinical studies have generated many new findings that lead to a better understanding of stress-induced relapse of nicotine-seeking. Several molecules such as α3β4 nicotinic acetylcholine receptor, α2-adrenergic receptors, cannabinoid receptor 1, trace amine-associated receptor 1, and neuropeptide systems (corticotropin-releasing factor and its receptors, dynorphine and kappa opioid receptor) have been linked to stress-induced nicotine relapse. In this review, we discuss recent advances in the neurobiology, treatment targets, and potential therapeutics of stress-induced nicotine relapse. We also discuss some factors that may influence stress-induced nicotine relapse and that should be considered in future studies. In the final section, a perspective on some research directions is provided. Further investigation on the neurobiology of stress-induced nicotine relapse will shed light on the development of new medicines for controlling smoking and will help us understand the interactions between the stress and reward systems in the brain.

An automated group-housed oral fentanyl self-administration method in mice

RATIONALE AND OBJECTIVES

Social factors play a critical role in human drug addiction, and humans often consume drugs together with their peers. In contrast, in traditional animal models of addiction, rodents consume or self-administer the drug in their homecage or operant self-administration chambers while isolated from their peers. Here, we describe HOMECAGE ("Home-cage Observation and Measurement for Experimental Control and Analysis in a Group-housed Environment"), a translationally relevant method for studying oral opioid self-administration in mice. This setting reduces experimental confounds introduced by social isolation or interaction with the experimenter.

METHODS

We have developed HOMECAGE, a method in which mice are group-housed and individually monitored for their consumption of a drug vs. a reference liquid.

RESULTS

Mice in HOMECAGE preserve naturalistic aspects of behavior, including social interactions and circadian activity. The mice showed a preference for fentanyl and escalated their fentanyl intake over time. Mice preferred to consume fentanyl in bouts during the dark cycle. Mice entrained to the reinforcement schedule of the task, optimizing their pokes to obtain fentanyl rewards, and maintained responding for fentanyl under a progressive ratio schedule. HOMECAGE also enabled the detection of cage-specific and individual-specific behavior patterns and allowed the identification of differences in fentanyl consumption between co-housed control and experimental mice.

CONCLUSIONS

HOMECAGE serves as a valuable procedure for translationally relevant studies on oral opioid intake under conditions that more closely mimic the human condition. The method enables naturalistic investigation of factors contributing to opioid addiction-related behaviors and can be used to identify novel treatments.

Resurgence of goal-directed actions and habits

This study investigated how goal-directed and habitual behaviors recover after extinction within the context of the resurgence effect, a form of relapse induced by the removal or worsening of alternative reinforcement. Rats were trained to press a target lever with one reinforcer (O1) for either minimal (4) or extended (16) sessions. An extinction test after the completion of O1 devaluation confirmed that minimal and extended training formed goal-directed and habitual behaviors, respectively. Then, pressing an alternative lever was reinforced with a second reinforcer (O2) while the target response was placed on extinction. When O2 was discontinued, the minimally trained target response resurged with goal-directed status as in the extinction test. However, the extinguished habitual behavior in the extensively trained rats did not recover as a habit but instead with goal-directed status, possibly due to the context specificity of habits or the introduction of a new response-reinforcer contingency. The critical finding that reinforcer devaluation consistently led to less resurgence regardless of the amount of acquisition training provides a clinical implication that coupling differential-reinforcement-of-alternative-behavior (DRA) treatments with the devaluation of the associated reinforcer of problematic behavior could effectively diminish its recurrence.

Advances in the characterization of negative affect caused by acute and protracted opioid withdrawal using animal models

Opioid use disorder (OUD) is a chronic brain disease which originates from long-term neuroadaptations that develop after repeated opioid consumption and withdrawal episodes. These neuroadaptations lead among other things to the development of a negative affect, which includes loss of motivation for natural rewards, higher anxiety, social deficits, heightened stress reactivity, an inability to identify and describe emotions, physical and/or emotional pain, malaise, dysphoria, sleep disorders and chronic irritability. The urge for relief from this negative affect is one of major causes of relapse, and thus represents a critical challenge for treatment and relapse prevention. Animal models of negative affect induced by opioid withdrawal have recapitulated the development of a negative emotional state with signs such as anhedonia, increased anxiety responses, increased despair-like behaviour and deficits in social interaction. This research has been critical to determine neurocircuitry adaptations during chronic opioid administration or upon withdrawal. In this review, we summarize the recent literature of rodent models of (i) acute withdrawal, (ii) protracted abstinence from passive administration of opioids, (iii) withdrawal or protracted abstinence from opioid self-administration. Finally, we describe neurocircuitry involved in acute withdrawal and protracted abstinence. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

THC exposure during adolescence increases impulsivity-like behavior in adulthood in a WIN 55,212-2 self-administration mouse model

Background

Cannabis addiction is a chronically relapsing disorder lacking effective treatment. Regular cannabis consumption typically begins during adolescence, and this early cannabinoid exposure may increase the risk for drug addiction in adulthood.

Objective

This study investigates the development of cannabis addiction-like behavior in adult mice after adolescent exposure to the main psychoactive component of cannabis, Δ⁹-tetrahydrocannabinol (THC).

Methods

Adolescent male mice were exposed to 5 mg/kg of THC from postnatal days 37 to 57. Operant self-administration sessions of WIN 55,212-2 (12.5 μg/kg/infusion) were conducted for 10 days. Mice were tested for three addiction-like criteria (persistence of response, motivation, and compulsivity), two parameters related to craving (resistance to extinction and drug-seeking behavior), and two phenotypic vulnerability traits related to substance use disorders (impulsivity and reward sensitivity). Additionally, qPCR assays were performed to detect differentially expressed genes in medial prefrontal cortex (mPFC), nucleus accumbens (NAc), dorsal striatum, and hippocampus (HPC) of "addicted" and "non-addicted" mice.

Results

Adolescent THC exposure did not modify WIN 55,212-2 reinforcement nor the development of cannabis addiction-like behavior. Inversely, THC pre-exposed mice displayed impulsive-like behavior in adulthood, which was more pronounced in mice that developed the addiction-like criteria. Moreover, downregulated drd2 and adora2a gene expression in NAc and HPC was revealed in THC pre-exposed mice, as well as a downregulation of drd2 expression in mPFC of vehicle pre-treated mice that developed addiction-like behaviors.

Discussion

These findings suggest that adolescent THC exposure may promote impulsivity-like behavior in adulthood, associated with downregulated drd2 and adora2a expression in NAc and HPC.

Morphine exposure during adolescence induces enduring social changes dependent on adolescent stage of exposure, sex, and social test

Drug exposure during adolescence, when the 'reward' circuitry of the brain is developing, can permanently impact reward-related behavior. Epidemiological studies show that opioid treatment during adolescence, such as pain management for a dental procedure or surgery, increases the incidence of psychiatric illness including substance use disorders. Moreover, the opioid epidemic currently in the United States is affecting younger individuals raising the impetus to understand the pathogenesis of the negative effects of opioids. One reward-related behavior that develops during adolescence is social behavior. We previously demonstrated that social development occurs in rats during sex-specific adolescent periods: early to mid-adolescence in males (postnatal day (P)30-40) and pre-early adolescence in females (P20-30). We thus hypothesized that morphine exposure during the female critical period would result in adult sociability deficits in females, but not males, and morphine administered during the male critical period would result in adult sociability deficits in males, but not females. We found that morphine exposure during the female critical period primarily resulted in deficits in sociability in females, while morphine exposure during the male critical period primarily resulted in deficits in sociability primarily in males. However, depending on the test performed and the social parameter measured, social alterations could be found in both sexes that received morphine exposure at either adolescent stage. These data indicate that when drug exposure occurs during adolescence, and how the endpoint data are measured, will play a large role in determining the effects of drug exposures on social development.

Role of Piriform Cortex and Its Afferent Projections in Relapse to Fentanyl Seeking after Food Choice-Induced Voluntary Abstinence

We previously demonstrated a role of piriform cortex (Pir) in relapse to fentanyl seeking after food choice-induced voluntary abstinence. Here, we used this model to further study the role of Pir and its afferent projections in fentanyl relapse. We trained male and female rats to self-administer palatable food pellets for 6 d (6 h/day) and fentanyl (2.5 µg/kg/infusion, i.v.) for 12 d (6 h/day). We assessed relapse to fentanyl seeking after 12 voluntary abstinence sessions, achieved through a discrete choice procedure between fentanyl and palatable food (20 trials/session). We determined projection-specific activation of Pir afferents during fentanyl relapse with Fos plus the retrograde tracer cholera toxin B (injected into Pir). Fentanyl relapse was associated with increased Fos expression in anterior insular cortex (AI) and prelimbic cortex (PL) neurons projecting to Pir. We next used an anatomical disconnection procedure to determine the causal role of these two projections (AI→Pir and PL→Pir) in fentanyl relapse. Contralateral but not ipsilateral disconnection of AI→Pir projections decreased fentanyl relapse but not reacquisition of fentanyl self-administration. In contrast, contralateral but not ipsilateral disconnection of PL→Pir projections modestly decreased reacquisition but not relapse. Fluorescence-activated cell sorting and quantitative PCR data showed molecular changes within Pir Fos-expressing neurons associated with fentanyl relapse. Finally, we found minimal or no sex differences in fentanyl self-administration, fentanyl versus food choice, and fentanyl relapse. Our results indicate that AI→Pir and PL→Pir projections play dissociable roles in nonreinforced relapse to fentanyl seeking versus reacquisition of fentanyl self-administration after food choice-induced voluntary abstinence.SIGNIFICANCE STATEMENT We previously showed a role of Pir in fentanyl relapse after food choice-induced voluntary abstinence in rats, a procedure mimicking human abstinence or a significant reduction in drug self-administration because of the availability of alternative nondrug rewards. Here, we aimed to further characterize the role of Pir in fentanyl relapse by investigating the role of Pir afferent projections and analyzing molecular changes in relapse-activated Pir neurons. We identified dissociable roles of two Pir afferent projections (AI→Pir and PL→Pir) in relapse to fentanyl seeking versus reacquisition of fentanyl self-administration after voluntary abstinence. We also characterized molecular changes within Pir Fos-expressing neurons associated with fentanyl relapse.

A male mouse model of WIN 55,212-2 self-administration to study cannabinoid addiction

We have established for the first time a mouse model of cannabinoid addiction using WIN 55,212-2 intravenous self-administration (0.0125 mg/kg/infusion) in C57Bl/6J mice. This model allows to evaluate the addiction criteria by grouping them into 1) persistence of response during a period of non-availability of the drug, 2) motivation for WIN 55,212-2 with a progressive ratio, and 3) compulsivity when the reward is associated with a punishment such as an electric foot-shock, in agreement with the Diagnostic and Statistical Manual of Mental Disorders 5th edition (DSM-5). This model also allows to measure two parameters that have been related with the DSM-5 diagnostic criteria of craving, resistance to extinction and reinstatement, and two phenotypic traits suggested as predisposing factors, impulsivity and sensitivity to reward. We found that 35.6% of mice developed the criteria of cannabinoid addiction, allowing to differentiate between resilient and vulnerable mice. Therefore, we have established a novel and reliable model to study the neurobiological correlates underlying the resilience or vulnerability to develop cannabinoid addiction. This model included the chemogenetic inhibition of neuronal activity in the medial prefrontal cortex to the nucleus accumbens pathway to assess the neurobiological substrate of cannabinoid addiction. This model will shed light on the neurobiological substrate underlying cannabinoid addiction.

Amphetamine-induced prolonged disturbances in tissue levels of dopamine and serotonin in the rat brain

BACKGROUND

A hallmark of psychostimulants is the persistence of neurobiological changes they produce. The difficulty in reversing long-time effects of psychostimulants use is why addiction therapy is so ineffective. This study aimed to look for such drug-induced changes that can be detected even after many weeks of abstinence.

METHODS

Rats were given 12 doses of amphetamine (Amph) at 1.5 mg/kg. The rewarding effect of Amph was assessed using ultrasonic vocalization. After 14 and 28 days of abstinence, tissue levels of dopamine (DA), serotonin (5-HT), and their metabolites were measured in the prefrontal cortex (PFC), nucleus accumbens (Acb), dorsomedial (CPuM), and dorsolateral (CPuL) striatum.

RESULTS

After 28 days of abstinence, DA levels were increased in the dorsal striatum while 5-HT levels were decreased in all brain regions studied. The opposite direction of changes in DA and 5-HT tissue levels observed in the dorsal striatum may be related to the changes in the emotional state during abstinence and may contribute to the incubation of craving and relapses. Tissue levels of 5-HT and DA showed intra- and inter-structural correlations, most pronounced after 14 days of abstinence. Most of them were absent in the control group (ctrl), which may indicate that their appearance was related to the changes induced by earlier Amph administration. We did not find any associations between reward sensitivity and the persistence of Amph-induced neurochemical disturbances.

CONCLUSIONS

Administration of 12 moderate doses of Amph causes prolonged changes in DA and 5-HT tissue levels. The direction and severity of the changes are dependent on the brain region and the neurotransmitter studied.

Neural circuits controlling choice behavior in opioid addiction

As the opioid epidemic presents an ever-expanding public health threat, there is a growing need to identify effective new treatments for opioid use disorder (OUD). OUD is characterized by a behavioral misallocation in choice behavior between opioids and other rewards, as opioid use leads to negative consequences, such as job loss, family neglect, and potential overdose. Preclinical models of addiction that incorporate choice behavior, as opposed to self-administration of a single drug reward, are needed to understand the neural circuits governing opioid choice. These choice models recapitulate scenarios that humans suffering from OUD encounter in their daily lives. Indeed, patients with substance use disorders (SUDs) exhibit a propensity to choose drug under certain conditions. While most preclinical addiction models have focused on relapse as the outcome measure, our data suggest that choice is an independent metric of addiction severity, perhaps relating to loss of cognitive control over choice, as opposed to excessive motivational drive to seek drugs during relapse. In this review, we examine both preclinical and clinical literature on choice behavior for drugs, with a focus on opioids, and the neural circuits that mediate drug choice versus relapse. We argue that preclinical models of opioid choice are needed to identify promising new avenues for OUD therapy that are translationally relevant. Both forward and reverse translation will be necessary to identify novel treatment interventions. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Acute food deprivation-induced relapse to heroin seeking after short and long punishment-imposed abstinence in male rats

RATIONAL

Stress is a major trigger for drug relapse in humans and animal models, even after prolonged abstinence. However, animal models for stress-induced relapse were criticized for the lack of predictive and face validity.

OBJECTIVES

Here we investigated the effect of acute food deprivation stress in a novel stress-induced relapse model using voluntary, punishment-imposed abstinence from heroin. We also performed a detailed characterization of the development of punishment-imposed abstinence.

METHODS

Male rats were trained to self-administered heroin (0.1 mg/kg/infusion) for 2 weeks, using the seeking-taking chained schedule. Pressing the 'seeking' lever led to the insertion of the 'taking' lever and pressing the take lever resulted in heroin infusion. Following self-administration training, rats were exposed to 8 or 21 days of heroin-seeking punishment. During punishment, 30% of the completed seek links resulted in a mild escalating footshock instead of take lever presentation. Next, rats were tested for heroin seeking under extinction conditions after 24 h of food deprivation and sated conditions.

RESULTS

Probabilistic punishment of seeking lever responses resulted in gradual suppression of heroin seeking and taking. Exposure to food-deprivation stress induced a robust relapse to heroin seeking after short and long punishment-imposed abstinence periods, without significant effects of time, i.e., no incubation of heroin seeking. Individual differences were observed in the development of punishment-induced abstinence and stress-induced relapse.

CONCLUSIONS

These results suggest that stress is a reliable trigger to relapse even after a prolonged period of punishment-induced, voluntary abstinence.

Learning processes in relapse to alcohol use: lessons from animal models

RATIONALE

Alcohol use is reliably preceded by discrete and contextual stimuli which, through diverse learning processes, acquire the capacity to promote alcohol use and relapse to alcohol use.

OBJECTIVE

We review contemporary extinction, renewal, reinstatement, occasion setting, and sex differences research within a conditioning framework of relapse to alcohol use to inform the development of behavioural and pharmacological therapies.

KEY FINDINGS

Diverse learning processes and corresponding neurobiological substrates contribute to relapse to alcohol use. Results from animal models indicate that cortical, thalamic, accumbal, hypothalamic, mesolimbic, glutamatergic, opioidergic, and dopaminergic circuitries contribute to alcohol relapse through separable learning processes. Behavioural therapies could be improved by increasing the endurance and generalizability of extinction learning and should incorporate whether discrete cues and contexts influence behaviour through direct excitatory conditioning or occasion setting mechanisms. The types of learning processes that most effectively influence responding for alcohol differ in female and male rats.

CONCLUSION

Sophisticated conditioning experiments suggest that diverse learning processes are mediated by distinct neural circuits and contribute to relapse to alcohol use. These experiments also suggest that gender-specific behavioural and pharmacological interventions are a way towards efficacious therapies to prevent relapse to alcohol use.

Persistent Neuroadaptations in the Nucleus Accumbens Core Accompany Incubation of Methamphetamine Craving in Male and Female Rats

Relapse is a major problem in treating methamphetamine use disorder. "Incubation of craving" during abstinence is a rat model for persistence of vulnerability to craving and relapse. While methamphetamine incubation has previously been demonstrated in male and female rats, it has not been demonstrated after withdrawal periods greater than 51 d and most mechanistic work used males. Here, we address both gaps. First, although methamphetamine intake was higher in males during self-administration training (6 h/d × 10 d), incubation was similar in males and females, with "incubated" craving persisting through withdrawal day (WD)100. Second, using whole-cell patch-clamp recordings in medium spiny neurons (MSNs) of the nucleus accumbens (NAc) core, we assessed synaptic levels of calcium-permeable AMPA receptors (CP-AMPARs), as their elevation is required for expression of incubation in males. In both sexes, compared with saline-self-administering controls, CP-AMPAR levels were significantly higher in methamphetamine rats across withdrawal, although this was less pronounced in WD100-135 rats than WD15-35 or WD40-75 methamphetamine rats. We also examined membrane properties and NMDA receptor (NMDAR) transmission. In saline controls, MSNs from males exhibited lower excitability than females. This difference was eliminated after incubation because of increased excitability of MSNs from males. NMDAR transmission did not differ between sexes and was not altered after incubation. In conclusion, incubation persists for longer than previously described and equally persistent CP-AMPAR plasticity in NAc core occurs in both sexes. Thus, abstinence-related synaptic plasticity in NAc is similar in males and females although other methamphetamine-related behaviors and neuroadaptations show differences.

Context-drug-associations and reinstatement of drug-seeking behavior in male rats: Adolescent and adult time-dependent effects

RATIONALE

Drug use during adolescence results in a life-long risk to develop substance-use disorders. Adolescent rats are sensitive to different drug-associated cues, compared to adults; however, the contribution of adolescent-formed context-drug-associations to elicit relapse-like behavior is underexplored.

OBJECTIVES

The present study compared the effect of adolescent vs adult-formed context-drug associations to elicit time-dependent increases in cocaine-seeking behavior. This objective was accomplished using an abbreviated (ABRV) operant cocaine self-administration (Coc-SA), Extinction (EXT) paradigm, with cocaine-seeking tests occurring 1 day after training (T1, early relapse) or following 15 days of abstinence (T15, late relapse).

METHODS

Adolescent and adult rats received ABRV Coc-SA in a distinct context (2 hr, 2x/day over 5 days) then EXT in a second context (2 hr, 2x/day over 4 days). Adolescent or adult cocaine-exposed rats were then tested (2 hr, non-rewarded) in either the previous EXT or Coc-paired contexts during early or late relapse.

RESULTS & CONCLUSIONS

As previously reported, both adolescent and adult cocaine-exposed rats displayed similar magnitudes of cocaine intake and lever presses during Coc-SA, EXT, and early relapse. Independent analysis of adolescent and adult groups revealed differences in lever responding, specifically rats with cocaine exposure during adolescence showed time-dependent increases in lever responding during late relapse. These data suggest that cocaine-context associations formed during adolescence can elicit craving during adulthood and that these age-specific differences in contextual sensitivity may not be immediately observed at early relapse periods.

Role of ventral subiculum neuronal ensembles in incubation of oxycodone craving after electric barrier-induced voluntary abstinence

High relapse rate is a key feature of opioid addiction. In humans, abstinence is often voluntary due to negative consequences of opioid seeking. To mimic this human condition, we recently introduced a rat model of incubation of oxycodone craving after electric barrier-induced voluntary abstinence. Incubation of drug craving refers to time-dependent increases in drug seeking after cessation of drug self-administration. Here, we used the activity marker Fos, muscimol-baclofen (GABAa + GABAb receptor agonists) global inactivation, Daun02-selective inactivation of putative relapse-associated neuronal ensembles, and fluorescence-activated cell sorting of Fos-positive cells and quantitative polymerase chain reaction to demonstrate a key role of vSub neuronal ensembles in incubation of oxycodone craving after voluntary abstinence, but not homecage forced abstinence. We also used a longitudinal functional magnetic resonance imaging method and showed that functional connectivity changes in vSub-related circuits predict opioid relapse after abstinence induced by adverse consequences of opioid seeking.

Chronic restraint stress increases sensitivity to punishment during cocaine self-administration via a dopamine D1-like receptor-mediated mechanism in prelimbic medial prefrontal cortex

We recently reported that male rats displayed less sensitivity to punishment during cocaine self-administration compared to females. Moreover, daily restraint stress increased sensitivity to punishment in males, while having no effect in females. The purpose of the present study was to extend these findings by determining whether chronic stress-induced dopamine release in prelimbic medial prefrontal cortex mediates the effect of stress on punished cocaine self-administration. Thus, male rats were trained to press a lever for i.v. cocaine infusions (0.50 mg/kg/infusion) paired with a discrete tone + light cue in daily 3-hr sessions. Subsequently, 50 % of the lever presses were punished by a mild footshock that gradually increased in intensity over 7 days. During the punishment phase, rats were exposed to a chronic restraint stress procedure (3 h/day) or control procedure (unstressed). Rats also received bilateral microinjections of the D1-like receptor antagonist SCH-23390 (0.25 μg/0.5 μl/side) or vehicle (0.5 μl/side) delivered to prelimbic cortex prior to daily treatments. Relapse tests were conducted 1 and 8 days after the last punishment session. Chronically stressed rats displayed reduced cocaine self-administration during punishment relative to unstressed rats, an effect prevented by co-administration of SCH-23390 to prelimbic cortex with daily restraint. Neither stress nor SCH-23390 treatment had significant effects on subsequent relapse-like behavior. These results establish a specific role for prelimbic D1-like receptors in chronic stress-induced suppression of punished cocaine self-administration in male rats. As such, these findings may inform novel methods to facilitate self-imposed abstinence in cocaine-dependent men.

Selecting the appropriate hurdles and endpoints for pentilludin, a novel antiaddiction pharmacotherapeutic targeting the receptor type protein tyrosine phosphatase D

Substance use disorders provide challenges for development of effective medications. Use of abused substances is likely initiated, sustained and "quit" by complex brain and pharmacological mechanisms that have both genetic and environmental determinants. Medical utilities of prescribed stimulants and opioids provide complex challenges for prevention: how can we minimize their contribution to substance use disorders while retaining medical benefits for pain, restless leg syndrome, attention deficit hyperactivity disorder, narcolepsy and other indications. Data required to support assessments of reduced abuse liability and resulting regulatory scheduling differs from information required to support licensing of novel prophylactic or therapeutic anti-addiction medications, adding further complexity and challenges. I describe some of these challenges in the context of our current efforts to develop pentilludin as a novel anti-addiction therapeutic for a target that is strongly supported by human and mouse genetic and pharmacologic studies, the receptor type protein tyrosine phosphatase D (PTPRD).

Demand and cross-price elasticity of cocaine and social contact in a free-operant procedure of nonexclusive choice

Research examining the social determinants of addiction has advanced significantly with the recent development of preclinical models of drug use and the social environment. These models reveal that drug use and social contact compete with one another for behavioral expression in discrete-trial choice procedures using concurrent schedules of reinforcement. The purpose of this study was to determine how concurrent access to cocaine and a social partner influences the demand for each alternative under free-operant conditions in which responding maintained by each reinforcer is independent and nonexclusive of the other. To this end, male rats were trained under a free-operant, concurrent schedule of reinforcement in which responding maintained by cocaine and access to a social partner operated independently of one another. Measures of economic demand (e.g., intensity, Omax, cross-price elasticity) were determined by manipulating the response requirement (i.e., fixed ratio value) across sessions. Tests were conducted in which the social partner was either treated or not treated with cocaine to determine whether the intoxication state of the partner influenced demand. The principal findings of this study are (1) demand for a cocaine-treated partner is greater than demand for a cocaine-free partner, (2) demand for cocaine is greater in the presence of a cocaine-treated partner than a cocaine-free partner, and (3) concurrent access to cocaine decreases demand for social contact. Notably, measures of cross-price elasticity indicated that social contact is a robust economic substitute for cocaine.

Corticotropin releasing factor and drug seeking in substance use disorders: Preclinical evidence and translational limitations

The neuropeptide, corticotropin releasing factor (CRF), has been an enigmatic target for the development of medications aimed at treating stress-related disorders. Despite a large body of evidence from preclinical studies in rodents demonstrating that CRF receptor antagonists prevent stressor-induced drug seeking, medications targeting the CRF-R1 have failed in clinical trials. Here, we provide an overview of the abundant findings from preclinical rodent studies suggesting that CRF signaling is involved in stressor-induced relapse. The scientific literature that has defined the receptors, mechanisms and neurocircuits through which CRF contributes to stressor-induced reinstatement of drug seeking following self-administration and conditioned place preference in rodents is reviewed. Evidence that CRF signaling is recruited with repeated drug use in a manner that heightens susceptibility to stressor-induced drug seeking in rodents is presented. Factors that may determine the influence of CRF signaling in substance use disorders, including developmental windows, biological sex, and genetics are examined. Finally, we discuss the translational failure of medications targeting CRF signaling as interventions for substance use disorders and other stress-related conditions. We conclude that new perspectives and research directions are needed to unravel the mysterious role of CRF in substance use disorders.

Oxytocin and orexin systems bidirectionally regulate the ability of opioid cues to bias reward seeking

As opioid-related fatalities continue to rise, the need for novel opioid use disorder (OUD) treatments could not be more urgent. Two separate hypothalamic neuropeptide systems have shown promise in preclinical OUD models. The oxytocin system, originating in the paraventricular nucleus (PVN), may protect against OUD severity. By contrast, the orexin system, originating in the lateral hypothalamus (LH), may exacerbate OUD severity. Thus, activating the oxytocin system or inhibiting the orexin system are potential therapeutic strategies. The specific role of these systems with regard to specific OUD outcomes, however, is not fully understood. Here, we probed the therapeutic efficacy of pharmacological interventions targeting the orexin or oxytocin system on two distinct metrics of OUD severity in rats-heroin choice (versus choice for natural reward, i.e., food) and cued reward seeking. Using a preclinical model that generates approximately equal choice between heroin and food reward, we examined the impact of exogenously administered oxytocin, an oxytocin receptor antagonist (L-368,899), and a dual orexin receptor antagonist (DORA-12) on opioid choice. Whereas these agents did not alter heroin choice when rewards (heroin and food) were available, oxytocin and DORA-12 each significantly reduced heroin seeking in the presence of competing reward cues when no rewards were available. In addition, the number of LH orexin neurons and PVN oxytocin neurons correlated with specific behavioral economic variables indicative of heroin versus food motivation. These data identify a novel bidirectional role of the oxytocin and orexin systems in the ability of opioid-related cues to bias reward seeking.

Sex-dependent effects of chronic stress on punished cocaine self-administration and cue-induced relapse to cocaine seeking

We tested the effects of chronic stress on cocaine relapse after drug-reinforced responding was suppressed by punishment, an animal model of human relapse after self-imposed abstinence due to the negative consequences of drug use. Male rats displayed greater resistance to punishment than females, but daily stress decreased this resistance. Only female rats with a history of chronic stress displayed increased responding for cocaine cues from abstinence Day 1 to Day 8. Thus, the effects of chronic stress on punished cocaine self-administration and cue-induced relapse are dependent on biological sex, and may have implications for more targeted treatments for cocaine addiction.

Lateral septum inputs to nucleus accumbens mediates stress induced suppression of natural reward seeking

Stress alters the level of reward evaluation and seeking. However, the neural circuitry mechanisms underlying stress induced effects on natural reward seeking remain unclear. Here we report a septal-accumbens pathway that mediates the effects of acute stress on reward seeking suppression. We first established the sucrose oral self-administration paradigm and measured the effects of acute stress on reward seeking behavior after 21 days of abstinence. Both forced swimming stress and foot shock stress significantly suppressed the natural reward seeking. Among a variety of brain regions, intermediolateral septum (LSi) appear as a strong stress-responsive area containing abundant c-Fos positive cells; chemogenetic inactivation of LSi reinstated the reward seeking behavior. To elucidate the downstream targets receiving LSi projections, we combined pathway-specific retro-labeling and chemogenetic manipulation to confirm the involvement of LSi-nucleus accumbens (NAc) rather than the Ventral tegmental area (VTA) in mediating the observed behavioral responses. In conclusion, the septal-accumbal projection constitute a discrete circuit dictating the stress evoked alterations on reward seeking and may implicate in treatment of stress induced anhedonia.

Ventral pallidum GABA neurons bidirectionally control opioid relapse across rat behavioral models

Opioid addiction is a chronic, relapsing disorder. Whether addicted individuals are forced to abstain or they decide themselves to quit using drugs, relapse rates are high-especially upon encountering contexts and stimuli associated with prior opioid use. Rodents similarly show context- and cue-induced reinstatement of drug seeking following abstinence, and intriguingly, the neural circuits underlying these relapse-like behaviors differ when abstinence is involuntarily imposed, responding is extinguished, or animals decide themselves to cease taking drug. Here, we employ two complementary rat behavioral models of relapse-like behavior for the highly reinforcing opioid drug remifentanil, and asked whether GABAergic neurons in the ventral pallidum (VPGABA) control opioid seeking under these behavioral conditions. Specifically, we asked how chemogenetically stimulating VPGABA neurons with clozapine-N-oxide (CNO) influences the ability of contextual or discrete remifentanil-paired cues to reinstate drug seeking following either voluntary abstinence (punishment-induced; GroupPunish), or extinction training (GroupExt). In GroupPunish rats, we also chemogenetically inhibited VPGABA neurons, and examined spontaneous VP activity (Fos) during cued reinstatement. In both GroupPunish and GroupExt rats, stimulating Gq-signaling in VPGABA neurons augmented remifentanil reinstatement in a cue- and context-dependent manner. Conversely, engaging inhibitory Gi-signaling in VPGABA neurons in GroupPunish suppressed cue-induced reinstatement, and cue-triggered seeking was correlated with Fos expression in rostral, but not caudal VP. Neither stimulating nor inhibiting VPGABA neurons influenced unpunished remifentanil self-administration. We conclude that VPGABA neurons bidirectionally control opioid seeking regardless of the specific relapse model employed, highlighting their fundamental role in opioid relapse-like behavior across behavioral models, and potentially across species.

Age-related differences in the effect of chronic alcohol on cognition and the brain: a systematic review

Adolescence is an important developmental period associated with increased risk for excessive alcohol use, but also high rates of recovery from alcohol use-related problems, suggesting potential resilience to long-term effects compared to adults. The aim of this systematic review is to evaluate the current evidence for a moderating role of age on the impact of chronic alcohol exposure on the brain and cognition. We searched Medline, PsycInfo, and Cochrane Library databases up to February 3, 2021. All human and animal studies that directly tested whether the relationship between chronic alcohol exposure and neurocognitive outcomes differs between adolescents and adults were included. Study characteristics and results of age-related analyses were extracted into reference tables and results were separately narratively synthesized for each cognitive and brain-related outcome. The evidence strength for age-related differences varies across outcomes. Human evidence is largely missing, but animal research provides limited but consistent evidence of heightened adolescent sensitivity to chronic alcohol's effects on several outcomes, including conditioned aversion, dopaminergic transmission in reward-related regions, neurodegeneration, and neurogenesis. At the same time, there is limited evidence for adolescent resilience to chronic alcohol-induced impairments in the domain of cognitive flexibility, warranting future studies investigating the potential mechanisms underlying adolescent risk and resilience to the effects of alcohol. The available evidence from mostly animal studies indicates adolescents are both more vulnerable and potentially more resilient to chronic alcohol effects on specific brain and cognitive outcomes. More human research directly comparing adolescents and adults is needed despite the methodological constraints. Parallel translational animal models can aid in the causal interpretation of observed effects. To improve their translational value, future animal studies should aim to use voluntary self-administration paradigms and incorporate individual differences and environmental context to better model human drinking behavior.

Characterization of operant social interaction in rats: effects of access duration, effort, peer familiarity, housing conditions, and choice between social interaction vs. food or remifentanil

RATIONALE AND OBJECTIVE

Social factors play a critical role in drug addiction. We recently showed that rats will abstain from methamphetamine, cocaine, heroin, and remifentanil self-administration when given a choice between the addictive drug and operant social interaction. Here, we further characterized operant social interaction by determining the effects of access duration, effort, peer familiarity, and housing conditions. We also determined choice between social interaction vs. palatable food or remifentanil.

METHODS

We first trained single-housed male and female rats to lever-press for social interaction with a sex- and age-matched peer. Next, we determined effects of access duration (3.75 to 240 s), effort (increasing fixed-ratio schedule requirements or progressive ratio schedule), peer familiarity (familiar vs. unfamiliar), and housing conditions (single vs. paired housing) on social self-administration. We also determined choice between social interaction vs. palatable food pellets or intravenous remifentanil (0, 1, 10 µg/kg/infusion).

RESULTS

Increasing access duration to a peer decreased social self-administration under fixed ratio but not progressive ratio schedule; the rats showed similar preference for short vs. long access duration. Social self-administration under different fixed ratio requirements was higher in single-housed than in paired-housed rats and higher for a familiar vs. unfamiliar partner in single-housed but not paired-housed rats. Response rates of food-sated rats under increasing fixed-ratio requirements were higher for palatable food than for social interaction. The rats strongly preferred palatable food over social interaction and showed dose-dependent preference for social interaction vs. remifentanil.

CONCLUSIONS

We identified parameters influencing the reinforcing effects of operant social interaction and introduce a choice procedure sensitive to remifentanil self-administration dose.

Is addiction a brain disease? A plea for agnosticism and heterogeneity

RATIONALE

Although increasingly subject to criticism, the brain disease model of addiction (BDMA) remains dominant within addiction science. Yet few advocates or critics of the BDMA have provided an account of what a brain disease is. The aim of this review is to conceptually clarify what it would mean for the BDMA to be true, rather than to argue decisively for or against it.

OBJECTIVES

Conceptual clarification of the BDMA requires consideration of possible models of disease and their relationship to the BDMA. A barrier to such consideration is belief that the BDMA is necessary to combatting addiction stigma. To address this barrier, I begin with discussion of what we know about the effects of the brain disease label on addiction stigma, and why labelling effects should have no bearing on the validity of the BDMA. I then distinguish strong, minimal, network, and mismatch models of disease, and I argue that the BDMA aligns with a strong disease model. This means that underlying brain pathology is hypothesized to be the cause of the personal-level observable signs and experienced symptoms characteristic of addiction. Evaluation of the BDMA therefore requires analysis of the concepts of brain dysfunction and causation, and their application to addiction science.

RESULTS

Brain dysfunction cannot be analyzed merely as brain changes or brain differences; nor can it be inferred merely from the presence of personal-level signs and symptoms. It is necessary to have an account of normal brain function by which to measure it. The theoretical and empirical challenges to developing such an account are not insurmountable, but they are substantial. Although there exist competing analyses of causation, there is a relatively standard method used to establish it within experimental science: intervention. Using this method, the causal significance of brain states, such as, e.g., extensive gray matter loss and/or neuroadapations in the mesocorticolimbic dopamine system, is not yet fully demonstrated. Further studies are necessary to determine their effect compared with other possible variables, such as, e.g., alternative reinforcers.

CONCLUSIONS

Conceptual clarification and preliminary empirical assessment of the BDMA recommends agnosticism about its validity and an openness to heterogeneity; in some cases addiction may be a brain disease, in others not. Either way, addiction stigma can be combatted by fighting moralism about drugs and moralistic drug policies directly, as opposed to resting hopes on the brain disease label.

Different brain oxidative and neuroinflammation status in rats during prolonged abstinence depending on their ethanol relapse-like drinking behavior: Effects of ethanol reintroduction

RATIONALE

Accumulating evidence suggests that chronic alcohol consumption is associated with excessive oxidative damage and neuroinflammatory processes and these events have been associated to early alcohol withdrawal. In the present research we wonder if brain oxidative stress and neuroinflammation remains altered during prolonged withdrawal situations and whether these alterations can be correlated with relapse behavior in alcohol consumption. The effects of alcohol reintroduction were also evaluated METHODS: We have used a model based on the alcohol deprivation effect (ADE) within a cohort of wild-type male Wistar rats. Two subpopulations were identified according to the alcohol relapse-like drinking behavior displayed (ADE and NO-ADE subpopulations). Oxidized and reduced glutathione content was determined within the hippocampus and the amygdala using a mass spectrometry method. The levels of mRNA of seven different inflammatory mediators in the prefrontal cortex of rats were quantified. All the analyses were performed in two different conditions: after 21-day alcohol deprivation (prolonged abstinence) and after 24 h of ethanol reintroduction in both subpopulations.

RESULTS

ADE and NO-ADE rats showed different endophenotypes. ADE rats always displayed a significant lower alcohol intake rate and ethanol preference than NO-ADE rats. The results also demonstrated the existence of altered brain redox and neuroinflammation status after prolonged abstinence exclusively in ADE rats. Moreover, when ethanol was reintroduced in the ADE subpopulation, altered oxidative stress and neuroinflammatory markers were restored.

CONCLUSIONS

Present findings provide new mechanisms underlying the neurobiology of relapse behavior and suggest the development of new pharmacological approaches to treat alcohol-induced relapse.

Dissociation Between Incubation of Cocaine Craving and Anxiety-Related Behaviors After Continuous and Intermittent Access Self-Administration

Studies using either continuous or intermittent access cocaine self-administration procedures showed that cocaine seeking increases during abstinence (incubation of cocaine craving), and that this effect is higher after intermittent cocaine access. Other studies showed that cocaine abstinence is characterized by the emergence of stress- and anxiety-related states which were hypothesized to increase relapse vulnerability. We examined whether incubation of cocaine craving and anxiety-related behaviors are correlated and whether intermittent cocaine self-administration would potentiate these behaviors during abstinence. Male rats self-administered cocaine either continuously (6 h/day) or intermittently (5 min ON, 25 min OFF × 12) for 14 days, followed by relapse tests after 1 or 21 abstinence days. A group of rats that self-administered saline served as a control. Anxiety-related behaviors were measured on the same abstinence days, using the novelty induced-hypophagia test. Finally, motivation for cocaine was measured using a progressive ratio reinforcement schedule. Lever-presses after 21 abstinence days were higher than after 1 day and this incubation effect was higher in the intermittent access group. Progressive ratio responding was also higher after intermittent cocaine access. Intermittent and continuous cocaine access did not induce anxiety-like responses in the novelty-induced hypophagia test after 1 or 21 abstinence days. Independent of the access condition, incubation of cocaine seeking was not correlated with the novelty-induced hypophagia measures. Results suggest that cocaine-induced anxiety-related states during protracted abstinence do not contribute to incubation of cocaine craving. However, this conclusion is tentative because we used a single anxiety-related measure and did not test female rats.

Contextual extinction of drug-associated discriminative stimuli fails to attenuate drug-vs-food choice in rhesus monkeys

Relapse within the context of a substance use disorder can be triggered by cues that function as discriminative stimuli to signal contingencies of drug availability and promote drug-taking behavior. Extinction procedures can weaken this association between drug-associated cues and drug-taking behavior and may reduce the probability of relapse. This study evaluated a regimen of extinction training on cocaine and heroin self-administration in rhesus monkeys under a drug-vs-food choice procedure. Behavior was initially maintained under a concurrent schedule of food (1-g food pellets; fixed-ratio 100 schedule) and cocaine injections (0-0.1 mg/kg/injection; fixed-ratio 10) (n = 4 males) or heroin injections (0-0.01 mg/kg/injection; fixed-ratio 10) (n = 3 females and 1 male) during daily 2-hr choice sessions. Subsequently, choice sessions were supplemented by daily 20-hr saline self-administration sessions for 14 consecutive days. During saline self-administration sessions, only drug-associated discriminative stimuli were presented and responding produced saline injections. Drug continued to be available during choice sessions. Prior to extinction training, both cocaine and heroin maintained dose-dependent increases in drug-vs-food choice. Exposure to 14 saline self-administration sessions failed to significantly decrease drug choice and increase food choice. These preclinical results do not support the effectiveness of extinguishing drug-associated discriminative stimuli as a nonpharmacological treatment strategy for reducing drug choice.