Persistent effects of acute stress on fear and drug-seeking in a novel model of the comorbidity between post-traumatic stress disorder and addiction

Sex-dependent effects of acute stress in adolescence or adulthood on appetitive motivation

RATIONALE

Intensely stressful experiences can lead to long-lasting changes in appetitive and aversive behaviors. In humans, post-traumatic stress disorder increases the risk of comorbid appetitive disorders including addiction and obesity. We have previously shown that an acute stressful experience in adult male rats suppresses motivation for natural reward.

OBJECTIVES

We examine the impact of sex and age on the effects of intense stress on action-based (instrumental) and stimulus-based (Pavlovian) motivation for natural reward (food).

METHODS

Rats received 15 unsignaled footshocks (stress) in a single session followed by appetitive training and testing in a distinct context. In Experiment 1, stress occurred in either adolescence (PN28) or adulthood (PN70) with appetitive training and testing beginning on PN71 for all rats. In Experiment 2, stress and appetitive training/testing occurred in adolescence.

RESULTS

Acute stress in adolescent females suppressed instrumental motivation assessed with progressive ratio testing when testing occurred in late adolescence or in adulthood, whereas in males stress in adolescence did not suppress instrumental motivation. Acute stress in adulthood did not alter instrumental motivation. In contrast, Pavlovian motivation assessed with single-outcome Pavlovian-to-instrumental transfer (SO-PIT) was consistently enhanced in females following adolescent or adult stress. In males, however, stress in adolescence had no effect, whereas stress in adulthood attenuated SO-PIT.

CONCLUSIONS

Acute stress in adolescence or adulthood altered instrumental motivation and stimulus-triggered Pavlovian motivation in a sex and developmentally specific manner. These findings suggest that the persistent effects of acute stress on Pavlovian and instrumental motivational processes differ in females and males, and that males may be less vulnerable to the deleterious effects of intense stress during adolescence on appetitive motivation.

Perceived stress and renewal: The effects of long-term stress on the renewal effect

Two online experiments evaluated the relationship between long-term stress, as measured with the Perceived Stress Scale-10, and the Renewal Effect. In the first experiment renewal was assessed with a behavioral suppression task in a science-fiction based video game. Participants learned to suppress mouse clicking during a signal for an upcoming attack to avoid losing points. The signal was first paired with an attack in Context A and extinguished in Context B and tested back in Context A. The contexts were different space galaxies where the gameplay took place. Experiment 2 used a food/illness predictive-learning paradigm. Two food items were paired with stomachache in one restaurant (A) and extinguished in Context B prior to testing in both contexts without feedback. Positive correlations were obtained between renewal and stress in each experiment. Unlike acute stress (Drexler et al., 2017), long term stress was associated with greater renewal. The effects of stress, both chronic and punctual, on renewal are discussed.

Prolonged exposure treatment for post-traumatic stress disorder: Single case studies in a sample of adults with mild intellectual disabilities

BACKGROUND

Post-traumatic stress disorder (PTSD) is common in adults with intellectual disabilities. Often there are additional disorders such as substance use, mood and anxiety disorders. The current study focuses on the feasibility and initial efficacy of prolonged exposure (PE) for PTSD in adults with mild intellectual disabilities. The secondary effect of PE on additional mood, anxiety and substance use disorders is also examined.

METHODS

A single case experimental design (N = 12) with an A (baseline)-B (intervention) phase including a follow-up measurement after 3 months was conducted. Time series and single time points measurements were performed.

RESULTS

Six participants dropped-out. The results showed a significant decrease in PTSD symptoms and a significant decrease in additional symptoms (social avoidance, anxiety and stress), among participants who completed treatment.

CONCLUSION

PE appears to be a feasible and effective treatment for PTSD in some adults with mild intellectual disabilities. Suggestions emerge from this study to make standard PE treatment more appropriate for adults with mild intellectual disabilities. Further research is needed to reduce drop-out in trauma treatment. Some suggestions for this are made in this study. Treatment of PTSD with PE did not appear to affect comorbid mood disorders. Further research is needed.

Stress, associative learning, and decision-making

Exposure to acute and chronic stress has significant effects on the basic mechanisms of associative learning and memory. Stress can both impair and enhance associative learning depending on type, intensity, and persistence of the stressor, the subject's sex, the context that the stress and behavior is experienced in, and the type of associative learning taking place. In some cases, stress can cause or exacerbate the maladaptive behavior that underlies numerous psychiatric conditions including anxiety disorders, obsessive-compulsive disorder, post-traumatic stress disorder, substance use disorder, and others. Therefore, it is critical to understand how the varied effects of stress, which may normally facilitate adaptive behavior, can also become maladaptive and even harmful. In this review, we highlight several findings of associative learning and decision-making processes that are affected by stress in both human and non-human subjects and how they are related to one another. An emerging theme from this work is that stress biases behavior towards less flexible strategies that may reflect a cautious insensitivity to changing contingencies. We consider how this inflexibility has been observed in different associative learning procedures and suggest that a goal for the field should be to clarify how factors such as sex and previous experience influence this inflexibility.

Acute stress persistently alters instrumental motivation without affecting appetitive Pavlovian conditioning, extinction, or contextual renewal

In two experiments, we adapted the stress-enhanced fear learning approach to evaluate the persistent effects of acute stress on appetitive learning and motivation in adult male Long Evans rats. In Experiment 1, we found that exposure to a battery of footshocks in one context had no effect on the acquisition, extinction, or contextual renewal of an appetitive Pavlovian discrimination in different contexts. However, when rats were subsequently trained to respond on a progressive ratio instrumental schedule, rats with a history of shock showed lower response rates and progressive ratio break points. Extinction of the shock-associated context had little effect on progressive ratio responding. In Experiment 2, we replicated this instrumental responding deficit with a continuous reinforcement schedule when the Pavlovian phases did not intervene in the time between shock and instrumental testing. Our findings here demonstrate that highly stressful acute experiences produce long-lasting deficits in instrumental motivation for food in male rats.

Refinement of the stress-enhanced fear learning model of post-traumatic stress disorder: a behavioral and molecular analysis

Post-traumatic stress disorder (PTSD) is a debilitating mental health condition for which current treatments have long-term efficacy in 50% of patients. There is a clear need for better understanding of the mechanisms underlying PTSD and the development of new treatment approaches. Analog trauma procedures in animals, such as the stress-enhanced fear learning (SEFL) procedure, can be used to produce behavioral and neurobiological changes that have validity in modeling PTSD. However, by necessity, the modeling of PTSD in animals requires them to potentially experience pain and suffering. Consistent with the '3Rs' (reduction, refinement and replacement) of animal research, this study aimed to determine whether the SEFL procedure can be refined to reduce potential animal pain and suffering while retaining the same behavioral and neurobiological changes. Here we showed that PTSD-relevant changes could be produced in both behavior and the brain of rats that were group- rather than single-housed and that received lower-magnitude electric shocks in the 'trauma analog' session. We also varied the number of shock exposures in the trauma analog session, finding SEFL-susceptible and SEFL-resilient populations at all levels of shock exposure, but with greater levels of shock increasing the proportion of rats showing the SEFL-susceptible phenotype. These data demonstrate that the SEFL procedure can be used as an animal analog of PTSD with reduced potential pain and suffering to the animals and that variations in the procedure could be used to generate specific proportions of SEFL-susceptible and SEFL-resilient animals in future studies.

Persistent effects of acute trauma on Pavlovian-to-instrumental transfer

In humans, an acutely traumatic experience can lead to post-traumatic stress disorder (PTSD), which is often characterized by changes in anxiety and motivation months after trauma. There are few demonstrations of the persistent motivational effects of an acute stressor in rodent approaches to PTSD. In two experiments, we evaluated the persistent effects of a battery of footshocks in one context on appetitive Pavlovian conditioning, instrumental learning, and Pavlovian-to-instrumental transfer (PIT) in a different context. In Experiment 1, a battery of footshocks before appetitive training caused deficits in single-outcome PIT (SO-PIT) in male Long Evans rats. The same battery of footshocks after appetitive training, but before testing had little effect on SO-PIT overall, but there were some deficits in within-stimulus expression of SO-PIT. In Experiment 2, the battery of footshocks had no effect on sensory-specific PIT in male or female rats, but two sex differences emerged: males showed more generalized fear from the aversive to the appetitive context compared to females, and females showed less evidence for sensory-specific PIT compared to males. Males showed robust sensory-specific PIT, with clear extinction and spontaneous recovery of the sensory-specific PIT effect across test sessions. These findings show that (a) an acute trauma can have persistent effects on general motivational processes and (b) in sensory-specific PIT, females may show transfer through generalized motivational processes, whereas males may rely on specific features of the cues and outcomes to augment instrumental responding selectively. We discuss implications for current approaches to stress and motivation in preclinical approaches to PTSD.

Repeated cocaine exposure prior to fear conditioning induces persistency of PTSD-like symptoms and enhancement of hippocampal and amygdala cell density in male rats

Pre- and post-trauma drug use can interfere with recovery from post-traumatic stress disorder (PTSD). However, the biological underpinnings of this interference are poorly understood. Here we examined the effect of pre-fear conditioning cocaine self-administration on PTSD-like symptoms in male rats, and defined impairment of fear extinction as difficulty to recover from PTSD. We also examined cell density changes in brain regions suspected of being involved in resistance to PTSD recovery. Before footshock stress testing, rats were trained to self-administer cocaine during 20 consecutive days, after which they were exposed to footshocks, while other rats continued to self-administer cocaine until the end of the experiment. Upon assessment of three PTSD-like symptoms (fear during situational reminders, anxiety-like behavior, and impairment of recognition memory) and fear extinction learning and memory, changes in cell density were measured in the medial prefrontal cortex, hippocampus, and amygdala. Results show that pre-footshock cocaine exposure did not affect fear during situational reminders. Fear conditioning did not lead to an increase in cocaine consumption. However, in footshock stressed rats, cocaine induced a reduction of anxiety-like behavior, an aggravation of recognition memory decline, and an impairment of extinction memory. These behavioral alterations were associated with increased cell density in the hippocampal CA1, CA2, and CA3 regions and basolateral amygdala, but not in the medial prefrontal cortex. Our findings suggest that enhancement of cell density in the hippocampus and amygdala may be changes associated with drug use, interfering with PTSD recovery.

N-Acetylcysteine treatment during acute stress prevents stress-induced augmentation of addictive drug use and relapse

Converging epidemiological studies show that a life-threatening event increases the incidence of posttraumatic stress disorder (PTSD), which carries 30% to 50% comorbidity with substance use disorders (SUDs). Such comorbidity results in greater drug use and poorer treatment outcomes. There is overlap between the enduring synaptic neuroadaptations produced in nucleus accumbens core (NAcore) by acute restraint stress and cocaine self-administration. Because of these coincident neuroadaptations, we hypothesized that an odor paired with acute restraint stress would reinstate drug seeking and chose two mechanistically distinct drugs of abuse to test this hypothesis: alcohol and cocaine. Rats were trained to self-administer either drug beginning 3 weeks after odor pairing with acute stress or sham, and acute restraint stress increased alcohol consumption. Following context extinction training, the stress-paired odor reinstated both alcohol and cocaine seeking, while an unpaired odor had no effect. N-Acetylcysteine (NAC) restores drug and stress-induced reductions in glial glutamate transporter-1 and has proven effective at reducing cue-induced reinstatement of drug seeking. We administered NAC for 5 days prior to reinstatement testing and abolished the capacity of the stress-paired odor to increase alcohol and cocaine seeking. Importantly, daily NAC given during or just following experiencing acute restraint stress also prevented the capacity of stress-paired odors to reinstate alcohol and cocaine seeking and prevented stress-induced deficits in behavioral flexibility. These data support using daily NAC treatment during or immediately after experiencing a strong acute stress to prevent subsequent conditioned stress responding, in particular relapse and cognitive deficits induced by stress-conditioned stimuli.

Partial MHC/neuroantigen peptide constructs attenuate methamphetamine-seeking and brain chemokine (C-C motif) ligand 2 levels in rats

There are no medications that target the neurotoxic effects or reduce the use of methamphetamine. Recombinant T-cell receptor ligand (RTL) 1000 [a partial major histocompatibility complex (pMHC) class II construct with a tethered myelin peptide], addresses the neuroimmune effects of methamphetamine addiction by competitively inhibiting the disease-promoting activity of macrophage migration inhibitory factor to CD74, a key pathway involved in several chronic inflammatory conditions, including substance use disorders. We previously reported that RTL constructs improve learning and memory impairments and central nervous system (CNS) inflammation induced by methamphetamine in mouse models. The present study in Lewis rats evaluated the effects of RTL1000 on maintenance of self-administration and cue-induced reinstatement using operant behavioral methods. Post-mortem brain and serum samples were evaluated for the levels of inflammatory factors. Rats treated with RTL1000 displayed significantly fewer presses on the active lever as compared to rats treated with vehicle during the initial extinction session, indicating more rapid extinction in the presence of RTL1000. Immunoblotting of rat brain sections revealed reduced levels of the pro-inflammatory chemokine (C-C motif) ligand 2 (CCL2) in the frontal cortex of rats treated with RTL1000, as compared to vehicle. Post hoc analysis identified a positive association between the levels of CCL2 detected in the frontal cortex and the number of lever presses during the first extinction session. Taken together, results suggest that RTL1000 may block downstream inflammatory effects of methamphetamine exposure and facilitate reduced drug seeking-potentially offering a new strategy for the treatment of methamphetamine-induced CNS injury and neuropsychiatric impairments.

Higher anhedonia during withdrawal from initial opioid exposure is protective against subsequent opioid self-administration in rats

RATIONALE

Understanding factors contributing to individual differences in vulnerability to opioid addiction is essential for developing more effective preventions and treatments, yet few reliable behavioral predictors of subsequent opioid self-administration have been identified in rodents. Sensitivity to the acute effects of initial drug exposure predicts later addiction vulnerability in both humans and animals, but the relationship between sensitivity to withdrawal from initial drug exposure and later drug use vulnerability is unclear.

OBJECTIVE

The goal of the current study was to evaluate whether the degree of anhedonia experienced during withdrawal from early opioid exposure predicts subsequent vulnerability to opioid self-administration.

METHODS

Rats were first tested for withdrawal sensitivity following acute injections of morphine (i.e., "acute dependence"), measured as elevations in intracranial self-stimulation (ICSS) thresholds (anhedonia-like behavior) during naloxone-precipitated and spontaneous withdrawal. Rats were then tested for addiction-like behavior using various measures of i.v. morphine self-administration (MSA) including acquisition, demand, extinction, and reinstatement induced by morphine, stress, and/or drug-associated cues.

RESULTS

Greater naloxone-precipitated withdrawal across repeated morphine injections and greater peak spontaneous withdrawal severity following a single morphine injection were associated with lower addiction-like behavior on multiple MSA measures. Withdrawal-induced anhedonia predicted a wider range of MSA measures than did any individual measure of MSA itself.

CONCLUSIONS

Our data establish WIA as one of the first behavioral measures to predict individual differences in opioid SA in rodents. This model promises to be useful for furthering our understanding of behavioral and neurobiological mechanisms underlying vulnerability to opioid addiction.

Drug instrumentalization

Psychoactive drugs with addiction potential are widely used by people of virtually all cultures in a non-addictive way. In order to understand this behaviour, its population penetrance, and its persistence, drug instrumentalization was suggested as a driving force for this consumption. Drug instrumentalization theory holds that psychoactive drugs are consumed in a very systematic way in order to make other, non-drug-related behaviours more efficient. Here, we review the evolutionary origin of this behaviour and its psychological mechanisms and explore the neurobiological and neuropharmacological mechanisms underlying them. Instrumentalization goals are discussed, for which an environmentally selective and mental state-dependent consumption of psychoactive drugs can be learned and maintained in a non-addictive way. A small percentage of people who regularly instrumentalize psychoactive drugs make a transition to addiction, which often starts with qualitative and quantitative changes in the instrumentalization goals. As such, addiction is proposed to develop from previously established long-term drug instrumentalization. Thus, preventing and treating drug addiction in an individualized medicine approach may essentially require understanding and supporting personal instrumentalization goals.

Non-pharmacological factors that determine drug use and addiction

Based on their pharmacological properties, psychoactive drugs are supposed to take control of the natural reward system to finally drive compulsory drug seeking and consumption. However, psychoactive drugs are not used in an arbitrary way as pure pharmacological reinforcement would suggest, but rather in a highly specific manner depending on non-pharmacological factors. While pharmacological effects of psychoactive drugs are well studied, neurobiological mechanisms of non-pharmacological factors are less well understood. Here we review the emerging neurobiological mechanisms beyond pharmacological reinforcement which determine drug effects and use frequency. Important progress was made on the understanding of how the character of an environment and social stress determine drug self-administration. This is expanded by new evidence on how behavioral alternatives and opportunities for drug instrumentalization generate different patterns of drug choice. Emerging evidence suggests that the neurobiology of non-pharmacological factors strongly determines pharmacological and behavioral drug action and may, thus, give rise for an expanded system's approach of psychoactive drug use and addiction.

Chronic opioid pretreatment potentiates the sensitization of fear learning by trauma

Despite the large comorbidity between PTSD and opioid use disorders, as well as the common treatment of physical injuries resulting from trauma with opioids, the ability of opioid treatments to subsequently modify PTSD-related behavior has not been well studied. Using the stress-enhanced fear learning (SEFL) model for PTSD, we characterized the impact of chronic opioid regimens on the sensitization of fear learning seen following traumatic stress in mice. We demonstrate for the first time that chronic opioid pretreatment is able to robustly augment associative fear learning. Highlighting aversive learning as the cognitive process mediating this behavioral outcome, these changes were observed after a considerable period of drug cessation, generalized to learning about multiple aversive stimuli, were not due to changes in stimulus sensitivity or basal anxiety, and correlated with a marker of synaptic plasticity within the basolateral amygdala. Additionally, these changes were not observed when opioids were given after the traumatic event. Moreover, we found that neither reducing the frequency of opioid administration nor bidirectional manipulation of acute withdrawal impacted the subsequent enhancement in fear learning seen. Given the fundamental role of associative fear learning in the generation and progression of PTSD, these findings are of direct translational relevance to the comorbidity between opioid dependence and PTSD, and they are also pertinent to the use of opioids for treating pain resulting from traumas involving physical injuries.

Sex differences and the modulating effects of gonad intactness on behavioral conformity in a mouse model

Although gender differences in conformity are noticed in human studies, cultural norms and psychosocial factors inevitably affect such differences. Biological factors, especially the gonadal hormones and the brain regions involved, contributing to the sex differences in behavioral conformity remained scarcely explored. To prevent psychosocial and cultural norm confounds, intact and gonadectomized male and female mice were used to assess the modulating effects of gonadal hormones on behavioral conformity and such conformity-related brain regions using an approach of choice paradigm. Intact and gonadectomized mice' choices for the nonrewarded runway were assessed when these experimental mice were alone versus with a group, consisting of three same-sex noncagemates choosing the respective experimental mice' nonrewarded runway, in a double-J-shaped maze test. Although male and female mice exhibited comparable rewarded runway choices at the conclusion of the operant training procedures and in the test individually, male mice demonstrated greater conformity index as compared to female mice when group tested. Gonadectomy, done at their 4 or 9 weeks of age, decreased males' conformity index but did not affect females' when both sexes were group tested. Such gonadectomy did not affect the conditioning or conformity index when tested individually in either sex. Female mice had higher serum corticosterone (CORT) levels when group tested as compared to the female mice tested individually and male mice. Finally, the number of FOS-staining cells in high conformity-displaying mice was found less than it in the low conformity-performing mice in the nucleus accumbens. Taken together, we conclude that testis-derived hormones, at least, play a role in enhancing behavioral conformity in male mice. CORT and nucleus accumbal neuronal activity deserve further investigation for their involvement in behavioral conformity.

A mouse model of stress-enhanced fear learning demonstrates extinction-sensitive and extinction-resistant effects of footshock stress

Stressful experiences can cause long-lasting sensitization of fear and anxiety that extends beyond the circumstances of the initial trauma. The neural mechanisms of these stress effects have been studied extensively in rats using the stress-enhanced fear learning (SEFL) paradigm, in which exposure to footshock stress potentiates subsequent fear conditioning. Here we establish a mouse version of the SEFL paradigm. Male and female 129s6 mice received four 1-mA footshocks or equivalent context exposure without shock. Shock exposure induced Pavlovian fear conditioning to the shock context and produced three more general effects: (1) suppression of open field exploration, (2) potentiated unconditioned fear of a novel tone stimulus, and (3) enhanced fear conditioning in a novel context. To determine whether these effects of footshock stress reflect generalized Pavlovian fear conditioning versus nonassociative fear sensitization, some mice received extinction training in the footshock stress context, which reduced contextual fear to the levels of unstressed control mice. Extinction restored normal open field exploration, suggesting that this effect of stress reflects generalized Pavlovian fear. In contrast, extinction failed to attenuate stress-enhanced fear, indicating that stress-enhanced fear is nonassociative and mechanistically distinct from Pavlovian fear conditioning. The effects of footshock stress were similar in male and female mice, although female mice displayed larger acute responses to fear-inducing stimuli than did males. The results demonstrate that footshock stress influences emotional behavior through distinct associative and nonassociative mechanisms, which likely involve unique neural underpinnings.

Association between posttraumatic stress disorder and nonfatal drug overdose

OBJECTIVE

North America is in the midst of a growing drug overdose crisis. While prescription opioid misuse and synthetic opioids such as fentanyl have been implicated in the overdose crisis, less attention has been given to the role that posttraumatic stress disorder (PTSD) may play in this crisis. As such, this study sought to examine the relationship between PTSD and risk of nonfatal overdose among people who use drugs (PWUD).

METHOD

Data were derived from three prospective cohorts of PWUD in Vancouver, Canada. For each participant, PTSD was assessed using the PTSD Checklist for the DSM-5. Multivariate logistic regression analysis was used to estimate the relationship between PTSD and nonfatal overdose, adjusting for potential confounders.

RESULTS

Between 2016 and 2018 among 1,059 PWUD, including 363 (34%) nonmale participants, 171 (16%) experienced a nonfatal drug overdose in the past 6 months, and 414 (39%) met criteria for a provisional PTSD diagnosis. In multivariate analysis, PTSD (adjusted odds ratio = 1.98, 95% confidence interval [1.4, 2.79]) remained independently associated with nonfatal overdose after adjustment for a range of confounders.

CONCLUSIONS

Among participants in these community-recruited cohorts of PWUD, having a provisional PTSD diagnosis nearly doubled the risk of nonfatal overdose. The findings from this study support the need to incorporate a trauma-informed approach within the current overdose prevention framework. Education and training relating to trauma and PTSD should be prioritized for health care professionals who work with and treat PWUD. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Context-Dependent and Context-Independent Effects of D1 Receptor Antagonism in the Basolateral and Central Amygdala during Cocaine Self-Administration

One way that drugs of abuse perturb the dopamine system is by triggering large amounts of extracellular dopamine to efflux into limbic regions. The basolateral (BLA) and central (CeA) nuclei of the amygdala have been shown to play distinct roles in value representation of primary and conditioned reward. However, the precise role of dopaminergic receptors in the BLA and the CeA during reward-related behaviors remains to be determined. Here we investigate the effects of dopamine D1 receptor blockade in the BLA and the CeA during asymptotic performance of cocaine self-administration and in a novel application of contextual renewal under continued access conditions. After more than three weeks of chained seek-take self-administration of cocaine, male Long Evans rats were given a bilateral intra-BLA or intra-CeA infusion of the D1 antagonist SCH-23390 (2 µg/0.3 µl) for multiple days. Intra-BLA D1 receptor blockade before, but not after the self-administration session, gradually suppressed drug seeking and taking responses and persisted with a change in context with continued D1 blockade. In contrast, intra-CeA D1 receptor blockade caused a rapid reduction in self-administration that showed renewal with a change in context with continued D1 blockade. Further, conditioned place aversion developed with intra-BLA but not intra-CeA infusions. Collectively, these results demonstrate that dopamine D1 receptors in the BLA and CeA both contribute to drug seeking and taking, but may do so through distinct mechanisms.

Effects of a histone deacetylase 3 inhibitor on extinction and reinstatement of cocaine self-administration in rats

RATIONALE

A challenge in treating substance use disorder is that successful treatment often does not persist, resulting in relapse and continued drug seeking. One approach to persistently weaken drug-seeking behaviors is to pair exposure to drug-associated cues or behaviors with delivery of a compound that may strengthen the inhibition of the association between drug cues and behavior.

OBJECTIVES

We evaluated whether a selective histone deacetylase 3 (HDAC3) inhibitor could promote extinction and weaken contextual control of operant drug seeking after intravenous cocaine self-administration.

METHODS

Male Long-Evans rats received a systemic injection of the HDAC3 inhibitor RGFP966 either before or immediately after the first extinction session. Persistence of extinction was tested over subsequent extinction sessions, as well as tests of reinstatement that included cue-induced reinstatement, contextual renewal, and cocaine-primed reinstatement. Additional extinction sessions occurred between each reinstatement test. We also evaluated effects of RGFP966 on performance and motivation during stable fixed ratio operant responding for cocaine and during a progressive ratio of reinforcement.

RESULTS

RGFP966 administered before the first extinction session led to significantly less responding during subsequent extinction and reinstatement tests compared to vehicle-injected rats. Follow-up studies found that these effects were not likely due to a performance deficit or a change in motivation to self-administer cocaine, as injections of RGFP966 had no effect on stable responding during a fixed or progressive ratio schedule. In addition, RGFP966 administered just after the first extinction session had no effect during early extinction and reinstatement tests, but weakened long-term responding during later extinction sessions.

CONCLUSIONS

These results suggest that a systemic injection of a selective HDAC3 inhibitor can enhance extinction and suppress reinstatement after cocaine self-administration. The finding that behavioral and pharmacological manipulations can be combined to decrease drug seeking provides further potential for treatment by epigenetic modulation.

CREST in the Nucleus Accumbens Core Regulates Cocaine Conditioned Place Preference, Cocaine-Seeking Behavior, and Synaptic Plasticity

Epigenetic mechanisms result in persistent changes at the cellular level that can lead to long-lasting behavioral adaptations. Nucleosome remodeling is a major epigenetic mechanism that has not been well explored with regards to drug-seeking behaviors. Nucleosome remodeling is performed by multi-subunit complexes that interact with DNA or chromatin structure and possess an ATP-dependent enzyme to disrupt nucleosome-DNA contacts and ultimately regulate gene expression. Calcium responsive transactivator (CREST) is a transcriptional activator that interacts with enzymes involved in both histone acetylation and nucleosome remodeling. Here, we examined the effects of knocking down CREST in the nucleus accumbens (NAc) core on drug-seeking behavior and synaptic plasticity in male mice as well as drug-seeking in male rats. Knocking down CREST in the NAc core results in impaired cocaine-induced conditioned place preference (CPP) as well as theta-induced long-term potentiation in the NAc core. Further, similar to the CPP findings, using a self-administration procedure, we found that CREST knockdown in the NAc core of male rats had no effect on instrumental responding for cocaine itself on a first-order schedule, but did significantly attenuate responding on a second-order chain schedule, in which responding has a weaker association with cocaine. Together, these results suggest that CREST in the NAc core is required for cocaine-induced CPP, synaptic plasticity, as well as cocaine-seeking behavior.SIGNIFICANCE STATEMENT This study demonstrates a key role for the role of Calcium responsive transactivator (CREST), a transcriptional activator, in the nucleus accumbens (NAc) core with regard to cocaine-induced conditioned place preference (CPP), self-administration (SA), and synaptic plasticity. CREST is a unique transcriptional regulator that can recruit enzymes from two different major epigenetic mechanisms: histone acetylation and nucleosome remodeling. In this study we also found that the level of potentiation in the NAc core correlated with whether or not animals formed a CPP. Together the results indicate that CREST is a key downstream regulator of cocaine action in the NAc.

A novel rat model of comorbid PTSD and addiction reveals intersections between stress susceptibility and enhanced cocaine seeking with a role for mGlu5 receptors

PTSD is highly comorbid with cocaine use disorder (CUD), and cocaine users with PTSD + CUD are more resistant to treatment. Here we sought to develop a rat model of PTSD + CUD in order to identify the neurobiological changes underlying such comorbidity and screen potential medications for reducing cocaine seeking in the PTSD population. We utilized a predator scent stress model of PTSD, wherein rats received a single exposure to the fox pheromone 2,5-dihydro-2,4,5-trimethylthiazoline (TMT). One week after TMT exposure, stress-susceptible (susceptible), intermediate, and resilient phenotypes were detected and were consistent with behavioral, corticosterone, and gene expression profiles 3 weeks post TMT. We assessed phenotypic differences in cocaine self-administration, extinction, and cue-primed reinstatement. Susceptible rats exhibited deficits in extinction learning and increased cue-primed reinstatement that was not prevented by Ceftriaxone, an antibiotic that consistently attenuates the reinstatement of cocaine seeking. TMT-exposed resilient rats displayed increased mGlu5 gene expression in the amygdala and medial prefrontal cortex and did not display the enhanced cocaine seeking observed in susceptible rats. Combined treatment with the mGlu5 positive allosteric modulator 3-Cyano-N-(1,3-diphenyl-1 H-pyrazol-5-yl)benzamide (CDPPB), fear extinction, and ceftriaxone prevented the reinstatement of cocaine seeking in susceptible rats with fear extinction an important mediating condition. These results highlight the need for animal models of PTSD to consider stress-responsivity, as only a subset of trauma-exposed individuals develop PTSD and these individuals likely exhibit distinct neurobiological changes compared with trauma-exposed populations who are resilient to stress. This work further identifies glutamate homeostasis and mGlu5 as a target for treating relapse in comorbid PTSD-cocaine addiction.