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

Predator odor stress reactivity, alcohol drinking and the endocannabinoid system

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are highly comorbid and individual differences in response to stress suggest resilient and susceptible populations. Using animal models to target neurobiological mechanisms associated with individual variability in stress coping responses and the relationship with subsequent increases in alcohol consumption has important implications for the field of traumatic stress and alcohol disorders. The current review discusses the unique advantages of utilizing predator odor stressor exposure models, specifically using 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) on better understanding PTSD pathophysiology and neurobiological mechanisms associated with stress reactivity and subsequent increases in alcohol drinking. Furthermore, there has been increasing interest regarding the role of the endocannabinoid system in modulating behavioral responses to stress with an emphasis on stress coping and individual differences in stress-susceptibility. Therefore, the current review focuses on the topic of endocannabinoid modulation of stress reactive behaviors during and after exposure to a predator odor stressor, with implications on modulating distinctly different behavioral coping strategies.

Effects of Beta Lactams on Behavioral Outcomes of Substance Use Disorders: A Meta-Analysis of Preclinical Studies

INTRODUCTION

Preclinical studies demonstrated that beta-lactams have neuroprotective effects in conditions involving glutamate neuroexcitotoxicity, including substance use disorders (SUDs). This meta-analysis aims to analyze the existing evidences on the effects of beta-lactams as glutamate transporter 1 (GLT-1) upregulators in animal models of SUDs, identification of gaps in the literature, and setting the stage for potential translation into clinical phases.

METHODS

Meta-analysis was conducted on preclinical studies retrieved systematically from MEDLINE and ScienceDirect databases. Abused substances were identified by refereeing to the National Institute on Drug Abuse (NIDA). The results were quantitatively described with a focus on the behavioral outcomes. Treatment effect sizes were described using standardized mean difference, and they were pooled using random effect model. I2-statistic was used to assess heterogeneity, and Funnel plot and Egger's test were used for assessment of publication bias.

RESULTS

Literature search yielded a total of 71 studies that were eligible to be included in the analysis. Through these studies, the effects of beta-lactams were evaluated in animal models of nicotine, cannabis, amphetamines, synthetic cathinone, opioids, ethanol, and cocaine use disorders as well as steroids-related aggressive behaviors. Meta-analysis showed that treatments with beta-lactams consistently reduced the pooled undesired effects of the abused substances in several paradigms, including drug-self administration, conditioned place preference, drug seeking behaviors, hyperlocomotion, withdrawal syndromes, tolerance to analgesic effects, hyperalgesia, and hyperthermia.

CONCLUSION

This meta-analysis revealed that enhancing GLT-1 expression in the brain through beta-lactams seemed to be a promising treatment approach in the context of substance use disorders, as indicated by results in animal models.

Voluntary alcohol intake alters the motivation to seek intravenous oxycodone and neuronal activation during the reinstatement of oxycodone and sucrose seeking

Opioid-alcohol polysubstance use is prevalent and worsens treatment outcomes. Here we assessed whether co-consumption of oxycodone and alcohol influence the intake of one another, demand for oxycodone, and the neurocircuitry underlying cue-primed reinstatement of oxycodone-seeking. Male and female rats underwent oxycodone intravenous self-administration (IVSA) with homecage access to alcohol (20% v/v) and/or water immediately after the IVSA session. Next, economic demand for intravenous oxycodone was assessed while access to alcohol and/or water continued. Control rats self-administered sucrose followed by access to alcohol and/or water. Rats underwent a cue-primed reinstatement test and brains were processed for c-fos mRNA expression. While both sexes decreased oxycodone intake if they had access to alcohol, and decreased alcohol intake if they had access to oxycodone, only female oxycodone + alcohol rats exhibited decreased demand elasticity and increased cue-primed reinstatement. Alcohol consumption increased the number of basolateral and central amygdala neurons activated during sucrose and oxycodone reinstatement and the number of ventral and dorsal striatum neurons engaged by sucrose reinstatement. Nucleus accumbens shell dopamine 1 receptor expressing neurons displayed activation patterns consistent with oxycodone reinstatement. Thus, alcohol alters the motivation to seek oxycodone in a sex-dependent manner and the neural circuitry engaged by cue-primed reinstatement of sucrose and oxycodone-seeking.

Sex-dependent fear memory impairment in cocaine-sired rat offspring

Cocaine self-administration by male rats results in neuronal and behavioral alterations in offspring, including responses to cocaine. Given the high degree of overlap between the brain systems underlying the pathological responses to cocaine and stress, we examined whether sire cocaine taking would influence fear-associated behavioral effects in drug-naïve adult male and female progeny. Sire cocaine exposure had no effect on contextual fear conditioning or its extinction in either male or female offspring. During cued fear conditioning, freezing behavior was enhanced in female, but not male, cocaine-sired progeny. In contrast, male cocaine-sired progeny exhibited enhanced expression of cue-conditioned fear during extinction. Long-term potentiation (LTP) was robust in the basolateral amygdala (BLA), which encodes fear conditioning, of female offspring but was completely absent in male offspring of cocaine-exposed sires. Collectively, these results indicate that cued fear memory is enhanced in the male progeny of cocaine exposed sires, which also have BLA synaptic plasticity deficits.

Oxytocin Reduces Sensitized Stress-Induced Alcohol Relapse in a Model of Posttraumatic Stress Disorder and Alcohol Use Disorder Comorbidity

BACKGROUND

There is high comorbidity of posttraumatic stress disorder (PTSD) and alcohol use disorder with few effective treatment options. Animal models of PTSD have shown increases in alcohol drinking, but effects of stress history on subsequent vulnerability to alcohol relapse have not been examined. Here we present a mouse model of PTSD involving chronic multimodal stress exposure that resulted in long-lasting sensitization to stress-induced alcohol relapse, and this sensitized stress response was blocked by oxytocin (OT) administration.

METHODS

Male and female mice trained to self-administer alcohol were exposed to predator odor (TMT) + yohimbine over 5 consecutive days or left undisturbed. After reestablishing stable alcohol responding/intake, mice were tested under extinction conditions, and then all mice were exposed to TMT or context cues previously associated with TMT before a reinstatement test session. Separate studies examined messenger RNA expression of Oxt and Oxtr in hypothalamus following chronic stress exposure. A final study examined the effects of systemic administration of OT on stress-induced alcohol relapse in mice with and without a history of chronic stress experience.

RESULTS

Chronic stress exposure produced long-lasting sensitization to subsequent stress-induced alcohol relapse that also generalized to stress-related context cues and transcriptional changes in hypothalamic OT system. OT injected before the reinstatement test session completely blocked the sensitized stress-induced alcohol relapse effect.

CONCLUSIONS

Collectively, these results provide support for the therapeutic potential of OT, along with highlighting the value of utilizing this model in evaluating other pharmacological interventions for treatment of PTSD/alcohol use disorder comorbidity.

Role of mesolimbic cannabinoid receptor 1 in stress-driven increases in cocaine self-administration in male rats

Stress is prevalent in the lives of those with substance use disorders (SUDs) and influences SUD outcomes. Understanding the neurobiological mechanisms through which stress promotes drug use is important for the development of effective SUD interventions. We have developed a model wherein exposure to a stressor, uncontrollable electric footshock, daily at the time of cocaine self-administration escalates intake in male rats. Here we test the hypothesis that stress-induced escalation of cocaine self-administration requires the CB1 cannabinoid receptor. Male Sprague-Dawley rats self-administered cocaine (0.5 mg/kg/inf, i.v.) during 2-h sessions comprised of four 30-min self-administration components separated by 5-min shock sequences or 5-min shock-free periods for 14 days. Footshock produced an escalation of cocaine self-administration that persisted following shock removal. Systemic administration of the cannabinoid receptor type 1 (CB1R) antagonist/inverse agonist, AM251, attenuated cocaine intake only in rats with a history of stress. This effect was localized to the mesolimbic system, as intra-nucleus accumbens (NAc) shell and intra-ventral tegmental area (VTA) micro-infusions of AM251 attenuated cocaine intake only in stress-escalated rats. Cocaine self-administration, regardless of stress history, increased CB1R binding site density in the VTA, but not NAc shell. Following extinction, cocaine-primed reinstatement (10 mg/kg, ip) was increased in rats with prior footshock during self-administration. AM251 attenuated reinstatement only in rats with a stress history. Altogether, these data demonstrate that mesolimbic CB1Rs are required to escalate intake and heighten relapse susceptibility and suggest that repeated stress at the time of cocaine use regulates mesolimbic CB1R activity through a currently unknown mechanism.

Brain Monoamine Dysfunction in Response to Predator Scent Stress Accompanies Stress-Susceptibility in Female Rats

Post-traumatic stress disorder (PTSD) is prevalent in women; however, preclinical research on PTSD has predominantly been conducted in male animals. Using a predator scent stress (PSS) rodent model of PTSD, we sought to determine if stress-susceptible female rats show altered monoamine concentrations in brain regions associated with PTSD: the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and dorsal (dHIPP) and ventral (vHIPP) hippocampus. Female Sprague-Dawley rats were exposed to a single, 10-min PSS exposure and tested for persistent anhedonia, fear, and anxiety-like behavior over four weeks. Rats were phenotyped as stress-Susceptible based on sucrose consumption in the sucrose preference task and time spent in the open arms of the elevated plus maze. Brain tissue was collected, and norepinephrine, dopamine, serotonin, and their metabolites were quantified using high-performance liquid chromatography. Stress-susceptibility in female rats was associated with increased dopamine and serotonin turnover in the mPFC. Susceptibility was also associated with elevated dopamine turnover in the NAc and increased norepinephrine in the vHIPP. Our findings suggest that stress-susceptibility after a single stress exposure is associated with long-term effects on monoamine function in female rats. These data suggest interventions that decrease monoamine turnover, such as MAOIs, may be effective in the treatment of PTSD in women.

Fatty acid amide hydrolase inhibition alleviates anxiety-like symptoms in a rat model used to study post-traumatic stress disorder

BACKGROUND AND AIM

Post-traumatic stress disorder (PTSD), a chronic debilitating condition that affects nearly 5-10% of American adults, is treated with a handful of FDA-approved drugs that provide at best symptomatic relief and exert multiple side effects. Preclinical and clinical evidence shows that inhibitors of the enzyme fatty acid amide hydrolase (FAAH), which deactivates the endocannabinoid anandamide, exhibit anxiolytic-like properties in animal models. In the present study, we investigated the effects of two novel brain-permeable FAAH inhibitors - the compounds ARN14633 and ARN14280 - in a rat model of predator stress-induced long-term anxiety used to study PTSD.

METHODS

We exposed male Sprague-Dawley rats to 2,5-dihydro-2,4,5-trimethylthiazoline (TMT), a volatile constituent of fox feces, and assessed anxiety-like behaviors in the elevated plus maze (EPM) test seven days later. We measured FAAH activity using a radiometric assay and brain levels of FAAH substrates by liquid chromatography/tandem mass spectrometry.

RESULTS

Rats challenged with TMT developed persistent (≥ 7 days) anxiety-like symptoms in the EPM test. Intraperitoneal administration of ARN14633 or ARN14280 1 h before testing suppressed TMT-induced anxiety-like behaviors with median effective doses (ED₅₀) of 0.23 and 0.33 mg/kg, respectively. The effects were negatively correlated (ARN14663: R² = 0.455; ARN14280: R² = 0.655) with the inhibition of brain FAAH activity and were accompanied by increases in brain FAAH substrate levels.

CONCLUSIONS

The results support the hypothesis that FAAH-regulated lipid signaling serves important regulatory functions in the response to stress and confirm that FAAH inhibitors may be useful for the management of PTSD.

Sex differences in cocaine self-administration by Wistar rats after predator odor exposure

Traumatic stress disorders are defined in part by persistent avoidance of trauma-related contexts. Our lab uses a preclinical model of traumatic stress using predator odor (i.e., bobcat urine) in which some but not all rats exhibit persistent avoidance of odor-paired stimuli, similar to what is seen in humans. Bobcat urine exposure increases alcohol consumption in male Avoider rats, but it has not been tested for its effects on intake of other drugs. Here, we tested the effect of bobcat urine exposure on cocaine self-administration in adult male and female Wistar rats. We did not observe any effect of bobcat urine exposure on cocaine self-administration in male or female rats. We observed that (1) female rats with long access (6 hours) to cocaine self-administer more cocaine than long-access males, (2) long-access males and females exhibit escalation of cocaine intake over time, (3) stressed rats gain less weight than unstressed rats following acute predator odor exposure, (4) baseline cocaine self-administration is predictive of subsequent cocaine self-administration. The results of this study may inform future work on predator odor effects on cocaine self-administration.

Sex differences in cocaine self-administration by Wistar rats after predator odor exposure

Traumatic stress disorders are defined in part by persistent avoidance of trauma-related contexts. Our lab uses a preclinical model of traumatic stress using predator odor (i.e., bobcat urine) in which some but not all rats exhibit persistent avoidance of odor-paired stimuli, similar to what is seen in humans. Bobcat urine exposure increases alcohol consumption in male Avoider rats, but it has not been tested for its effects on intake of other drugs. Here, we tested the effect of bobcat urine exposure on cocaine self-administration in adult male and female Wistar rats. We did not observe any effect of bobcat urine exposure on cocaine self-administration in male or female rats. We observed that (1) female rats with long access (6 hours) to cocaine self-administer more cocaine than long-access males, (2) long-access males and females exhibit escalation of cocaine intake over time, (3) stressed rats gain less weight than unstressed rats following acute predator odor exposure, (4) baseline cocaine self-administration is predictive of subsequent cocaine self-administration. The results of this study may inform future work on predator odor effects on cocaine self-administration.

Examining mGlu5 Receptor Availability as a Predictor of Vulnerability to PTSD: An [18F]FPEB and PET Study in Male and Female Rats

Background

Females are twice as likely to experience post-traumatic stress disorder (PTSD) than males, yet specific factors contributing to this greater risk are not fully understood. Our clinical and recent preclinical findings suggest a role for the metabotropic glutamate receptor 5 (mGlu5) in PTSD and differential involvement between males and females.

Methods

Here, we further investigate whether mGlu5 receptor availability may contribute to individual and sex differences in PTSD susceptibility by quantifying receptor availability using the mGlu5 receptor-specific radiotracer, [18F]FPEB, and positron emission tomography in male (n = 16) and female (n = 16) rats before and after traumatic footshock exposure (FE) and assessment of stress-enhanced fear learning (SEFL) susceptibility, as compared with no-shock controls (CON; n = 7 male; n = 8 female).

Results

Overall, FE rats displayed greater fear generalization as compared with CON (p < .001). Further, greater mGlu5 receptor availability at baseline (p = .003) and post-test (p = .005) was significantly associated with expression of the SEFL phenotype. Notably, FE female rats displayed a shift to more passive coping (ie, freezing), and displayed greater SEFL susceptibility (p = .01), and had lower baseline mGlu5 availability (p = .03) relative to their FE male rat counterparts.

Conclusion

Results are consistent with clinical findings of higher mGlu5 receptor availability in PTSD, and add to growing evidence implicating these receptors in the pathophysiology of PTSD and sex-differences in susceptibility for this disorder.

The metabotropic glutamate receptor 5 as a biomarker for psychiatric disorders

The role of glutamate system in the etiology and pathophysiology of psychiatric disorders has gained considerable attention in the past two decades, including dysregulation of the metabotropic glutamatergic receptor subtype 5 (mGlu5). Thus, mGlu5 may represent a promising therapeutic target for psychiatric conditions, particularly stress-related disorders. Here, we describe mGlu5 findings in mood disorders, anxiety, and trauma disorders, as well as substance use (specifically nicotine, cannabis, and alcohol use). We highlight insights gained from positron emission tomography (PET) studies, where possible, and discuss findings from treatment trials, when available, to explore the role of mGlu5 in these psychiatric disorders. Through the research evidence reviewed in this chapter, we make the argument that, not only is dysregulation of mGlu5 evident in numerous psychiatric disorders, potentially functioning as a disease "biomarker," the normalization of glutamate neurotransmission via changes in mGlu5 expression and/or modulation of mGlu5 signaling may be a needed component in treating some psychiatric disorders or symptoms. Finally, we hope to demonstrate the utility of PET as an important tool for investigating mGlu5 in disease mechanisms and treatment response.

Steady electrocorticogram characteristics predict specific stress-induced behavioral phenotypes

Introduction

Depending on the individual, exposure to an intense stressor may, or may not, lead to a stress-induced pathology. Predicting the physiopathological evolution in an individual is therefore an important challenge, at least for prevention. In this context, we developed an ethological model of simulated predator exposure in rats: we call this the multisensorial stress model (MSS). We hypothesized that: (i) MSS exposure can induce stress-induced phenotypes, and (ii) an electrocorticogram (ECoG) recorded before stress exposure can predict phenotypes observed after stress.

Methods

Forty-five Sprague Dawley rats were equipped with ECoG telemetry and divided into two groups. The Stress group (n = 23) was exposed to an MSS that combined synthetic fox feces odor deposited on filter paper, synthetic blood odor, and 22 kHz rodent distress calls; the Sham group (n = 22) was not exposed to any sensorial stimulus. Fifteen days after initial exposure, the two groups were re-exposed to a context that included a filter paper soaked with water as a traumatic object (TO) reminder. During this re-exposure, freezing behavior and avoidance of the filter paper were measured.

Results

Three behaviors were observed in the Stress group: 39% developed a fear memory phenotype (freezing, avoidance, and hyperreactivity); 26% developed avoidance and anhedonia; and 35% made a full recovery. We also identified pre-stress ECoG biomarkers that accurately predicted cluster membership. Decreased chronic 24 h frontal Low θ relative power was associated with resilience; increased frontal Low θ relative power was associated with fear memory; and decreased parietal β2 frequency was associated with the avoidant-anhedonic phenotype.

Discussion

These predictive biomarkers open the way to preventive medicine for stress-induced diseases.

The role of mGlu receptors in susceptibility to stress-induced anhedonia, fear, and anxiety-like behavior

Stress and trauma exposure contribute to the development of psychiatric disorders such as post-traumatic stress disorder (PTSD) and major depressive disorder (MDD) in a subset of people. A large body of preclinical work has found that the metabotropic glutamate (mGlu) family of G protein-coupled receptors regulate several behaviors that are part of the symptom clusters for both PTSD and MDD, including anhedonia, anxiety, and fear. Here, we review this literature, beginning with a summary of the wide variety of preclinical models used to assess these behaviors. We then summarize the involvement of Group I and II mGlu receptors in these behaviors. Bringing together this extensive literature reveals that mGlu5 signaling plays distinct roles in anhedonia, fear, and anxiety-like behavior. mGlu5 promotes susceptibility to stress-induced anhedonia and resilience to stress-induced anxiety-like behavior, while serving a fundamental role in the learning underlying fear conditioning. The medial prefrontal cortex, basolateral amygdala, nucleus accumbens, and ventral hippocampus are key regions where mGlu5, mGlu2, and mGlu3 regulate these behaviors. There is strong support that stress-induced anhedonia arises from decreased glutamate release and post-synaptic mGlu5 signaling. Conversely, decreasing mGlu5 signaling increases resilience to stress-induced anxiety-like behavior. Consistent with opposing roles for mGlu5 and mGlu2/3 in anhedonia, evidence suggests that increased glutamate transmission may be therapeutic for the extinction of fear learning. Thus, a large body of literature supports the targeting of pre- and post-synaptic glutamate signaling to ameliorate post-stress anhedonia, fear, and anxiety-like behavior.

Predator odor (TMT) exposure potentiates interoceptive sensitivity to alcohol and increases GABAergic gene expression in the anterior insular cortex and nucleus accumbens in male rats

Post-traumatic stress disorder (PTSD) confers enhanced vulnerability to developing comorbid alcohol use disorder (AUD). Exposure to the scent of a predator, such as the fox odor TMT, has been used to model a traumatic stressor with relevance to PTSD symptomatology. Alcohol produces distinct interoceptive (subjective) effects that may influence vulnerability to problem drinking and AUD. As such, understanding the lasting impact of stressors on sensitivity to the interoceptive effects of alcohol is clinically relevant. The present study used a 2-lever, operant drug discrimination procedure to train male Long-Evans rats to discriminate the interoceptive effects of alcohol (2 g/kg, i.g. [intragastrically]) from water. Upon stable performance, rats underwent a 15-min exposure to TMT. Two weeks later, an alcohol dose-response curve was conducted to evaluate the lasting effects of the TMT stressor on the interoceptive effects of alcohol. The TMT group showed a leftward shift in the effective dose (ED₅₀) of the dose-response curve compared to controls, reflecting potentiated interoceptive sensitivity to alcohol. TMT exposure did not affect response rate. GABAergic signaling in both the anterior insular cortex (aIC) and the nucleus accumbens (Acb) is involved in the interoceptive effects of alcohol and stressor-induced adaptations. As such, follow-up experiments in alcohol-naïve rats examined neuronal activation (as measured by c-Fos immunoreactivity) following TMT and showed that TMT exposure increased c-Fos expression in the aIC and the nucleus accumbens core (AcbC). Two weeks after TMT exposure, Gad-1 gene expression was elevated in the aIC and Gat-1 was increased in the Acb, compared to controls. Lastly, the alcohol discrimination and alcohol-naïve groups displayed dramatic differences in stress reactive behaviors during the TMT exposure, suggesting that alcohol exposure may alter the behavioral response to predator odor. Together, these data suggest that predator odor stressor results in potentiated sensitivity to alcohol, possibly through GABAergic adaptations in the aIC and Acb, which may be relevant to understanding PTSD-AUD comorbidity.

The effects of predator odor (TMT) exposure and mGlu3 NAM pretreatment on behavioral and NMDA receptor adaptations in the brain

A stressor can trigger lasting adaptations that contribute to neuropsychiatric disorders. Predator odor (TMT) exposure is an innate stressor that may activate the metabotropic glutamate receptor 3 (mGlu3) to produce stress adaptations. To evaluate functional involvement, the mGlu3 negative allosteric modulator (NAM, VU6010572; 3 mg/kg, i.p.) was administered before TMT exposure in male, Long Evans rats. Two weeks after, rats underwent context re-exposure, elevated zero maze (ZM), and acoustic startle (ASR) behavioral tests, followed by RT-PCR gene expression in the insular cortex and bed nucleus of the stria terminalis (BNST) to evaluate lasting behavioral and molecular adaptations from the stressor. Rats displayed stress-reactive behaviors in response to TMT exposure that were not affected by VU6010572. Freezing and hyperactivity were observed during the context re-exposure, and mGlu3-NAM pretreatment during stressor prevented the context freezing response. TMT exposure did not affect ZM or ASR measures, but VU6010572 increased time spent in the open arms of the ZM and ASR habituation regardless of stressor treatment. In the insular cortex, TMT exposure increased expression of mGlu (Grm3, Grm5) and NMDA (GriN2A, GriN2B, GriN2C, GriN3A, GriN3B) receptor transcripts, and mGlu3-NAM pretreatment blocked GriN3B upregulation. In the BNST, TMT exposure increased expression of GriN2B and GriN3B in vehicle-treated rats, but decreased expression in the mGlu3-NAM group. Similar to the insular cortex, mGlu3-NAM reversed the stressor-induced upregulation of GriN3B in the BNST. mGlu3-NAM also upregulated GriN2A, GriN2B, GriN3B and Grm2 in the control group, but not the TMT group. Together, these data implicate mGlu3 receptor signaling in some lasting adaptations of predator odor stressor and anxiolytic-like effects.

A Rat Model of Post-Traumatic Stress Syndrome Causes Phenotype-Associated Morphological Changes and Hypofunction of the Adrenal Gland

Background: Rats exposed to chronic predator scent stress mimic the phenotype of complex post-traumatic stress disorder (PTSD) in humans, including altered adrenal morphology and function. High- and low-anxiety phenotypes have been described in rats exposed to predator scent stress (PSS). This study aimed to determine whether these high- and low-anxiety phenotypes correlate with changes in adrenal histomorphology and corticosteroid production. Methods: Rats were exposed to PSS for ten days. Thirty days later, the rats’ anxiety index (AI) was assessed with an elevated plus-maze test. Based on differences in AI, the rats were segregated into low- (AI ≤ 0.8, n = 9) and high- (AI > 0.8, n = 10) anxiety phenotypes. Plasma corticosterone (CORT) concentrations were measured by ELISA. Adrenal CORT, desoxyCORT, and 11-dehydroCORT were measured by high-performance liquid chromatography. After staining with hematoxylin and eosin, adrenal histomorphometric changes were evaluated by measuring the thickness of the functional zones of the adrenal cortex. Results: Decreased plasma CORT concentrations, as well as decreased adrenal CORT, desoxyCORT and 11-dehydroCORT concentrations, were observed in high- but not in low-anxiety phenotypes. These decreases were associated with increases in AI. PSS led to a significant decrease in the thickness of the zona fasciculata and an increase in the thickness of the zona intermedia. The increase in the thickness of the zona intermedia was more pronounced in low-anxiety than in high-anxiety rats. A decrease in the adrenal capsule thickness was observed only in low-anxiety rats. The nucleus diameter of cells in the zona fasciculata of high-anxiety rats was significantly smaller than that of control or low-anxiety rats. Conclusion: Phenotype-associated changes in adrenal function and histomorphology were observed in a rat model of complex post-traumatic stress disorder.

The role of the nucleus reuniens in regulating contextual conditioning with the predator odor TMT in female rats

RATIONALE

Experiencing intrusive distressing memories of a traumatic event(s) is a prominent symptom profile for post-traumatic stress disorder (PTSD). Understanding the neurobiological mechanisms associated with this symptom profile can be invaluable for effective treatment for PTSD.

OBJECTIVES

Here, we investigated the functional role of the nucleus reuniens (RE), a midline thalamic in modulating stressor-related memory.

METHODS

Female Long Evans rats were implanted with a cannula aimed at the RE. The RE was pharmacologically inactivated via muscimol (0.5 mM) prior to exposure to the predator odor stressor trimethylthiazoline (TMT; synthetically derived fox feces component) or water (controls) in a distinct context with bedding material (experiment 1) or no bedding (experiment 2). To measure context reactivity, the index of the contextual memory, 2 weeks following exposure to TMT, rats were re-exposed to the TMT-paired context (in the absence of TMT).

RESULTS

In experiment 1, during context re-exposure (with bedding), inactivation of the RE had no effect on context reactivity. In experiment 2, during context re-exposure (no bedding), rats previously exposed to TMT showed decreased immobility compared to controls, indicating reactivity to the context and likely related to theincreased exploration of the environment. Rats in the TMT group that received RE inactivation showed increased immobility relative to rats that received aCSF, suggesting that muscimol pre-treatment blunted context reactivity.

CONCLUSION

In conclusion, recruitment of the RE in stressor-related contextual memory appears to be dependent on the contextual environment and whether the animal is able to engage in different stress coping strategies.

Increased mGlu5 mRNA expression in BLA glutamate neurons facilitates resilience to the long-term effects of a single predator scent stress exposure

Post-traumatic stress disorder (PTSD) develops in a subset of individuals exposed to a trauma with core features being increased anxiety and impaired fear extinction. To model the heterogeneity of PTSD behavioral responses, we exposed male Sprague-Dawley rats to predator scent stress once for 10 min and then assessed anxiety-like behavior 7 days later using the elevated plus maze and acoustic startle response. Rats displaying anxiety-like behavior in both tasks were classified as stress Susceptible, and rats exhibiting behavior no different from un-exposed Controls were classified as stress Resilient. In Resilient rats, we previously found increased mRNA expression of mGlu5 in the amygdala and prefrontal cortex (PFC) and CB1 in the amygdala. Here, we performed fluorescent in situ hybridization (FISH) to determine the subregion and cell-type-specific expression of these genes in Resilient rats 3 weeks after TMT exposure. Resilient rats displayed increased mGlu5 mRNA expression in the basolateral amygdala (BLA) and the infralimbic and prelimbic regions of the PFC and increased BLA CB1 mRNA. These increases were limited to glutamatergic cells. To test the necessity of mGlu5 for attenuating TMT-conditioned contextual fear 3 weeks after TMT conditioning, intra-BLA infusions of the mGlu5 negative allosteric modulator MTEP were administered prior to context re-exposure. In TMT-exposed Resilient rats, but not Controls, MTEP increased freezing on the day of administration, which extinguished over two additional un-drugged sessions. These results suggest that increased mGlu5 expression in BLA glutamate neurons contributes to the behavioral flexibility observed in stress-Resilient animals by facilitating a capacity for extinguishing contextual fear associations.

Epigenetic Regulation of Circadian Clocks and Its Involvement in Drug Addiction

Based on studies describing an increased prevalence of addictive behaviours in several rare sleep disorders and shift workers, a relationship between circadian rhythms and addiction has been hinted for more than a decade. Although circadian rhythm alterations and molecular mechanisms associated with neuropsychiatric conditions are an area of active investigation, success is limited so far, and further investigations are required. Thus, even though compelling evidence connects the circadian clock to addictive behaviour and vice-versa, yet the functional mechanism behind this interaction remains largely unknown. At the molecular level, multiple mechanisms have been proposed to link the circadian timing system to addiction. The molecular mechanism of the circadian clock consists of a transcriptional/translational feedback system, with several regulatory loops, that are also intricately regulated at the epigenetic level. Interestingly, the epigenetic landscape shows profound changes in the addictive brain, with significant alterations in histone modification, DNA methylation, and small regulatory RNAs. The combination of these two observations raises the possibility that epigenetic regulation is a common plot linking the circadian clocks with addiction, though very little evidence has been reported to date. This review provides an elaborate overview of the circadian system and its involvement in addiction, and we hypothesise a possible connection at the epigenetic level that could further link them. Therefore, we think this review may further improve our understanding of the etiology or/and pathology of psychiatric disorders related to drug addiction.

MC-100093, a Novel β-Lactam Glutamate Transporter-1 Enhancer Devoid of Antimicrobial Properties, Attenuates Cocaine Relapse in Rats

Cocaine use disorder currently lacks Food and Drug Administration-approved treatments. In rodents, the glutamate transporter-1 (GLT-1) is downregulated in the nucleus accumbens after cocaine self-administration, and increasing the expression and function of GLT-1 reduces the reinstatement of cocaine seeking. The β-lactam antibiotic ceftriaxone upregulates GLT-1 and attenuates cue- and cocaine-induced cocaine seeking without affecting motivation for natural rewards. Although ceftriaxone shows promise for treating cocaine use disorder, it possesses characteristics that limit successful translation from bench to bedside, including poor brain penetration, a lack of oral bioavailability, and a risk of bacterial resistance when used chronically. Thus, we aimed to develop novel molecules that retained the GLT-1-enhancing effects of ceftriaxone but displayed superior drug-like properties. Here, we describe a new monocyclic β-lactam, MC-100093, as a potent upregulator of GLT-1 that is orally bioavailable and devoid of antimicrobial properties. MC-100093 was synthesized and tested in vitro and in vivo to determine physiochemical, pharmacokinetic, and pharmacodynamic properties. Next, adult male rats underwent cocaine self-administration and extinction training. During extinction training, rats received one of four doses of MC-100093 for 6-8 days prior to a single cue-primed reinstatement test. Separate cohorts of rats were used to assess nucleus accumbens GLT-1 expression and MC-100093 effects on sucrose self-administration. We found that 50 mg/kg MC-100093 attenuated cue-primed reinstatement of cocaine seeking while upregulating GLT-1 expression in the nucleus accumbens core. This dose did not produce sedation, nor did it decrease sucrose consumption or body weight. Thus, MC-100093 represents a potential treatment to reduce cocaine relapse. SIGNIFICANCE STATEMENT: Increasing GLT-1 activity reliably reduces drug-seeking across classes of drugs; however, existing GLT1-enhancers have side effects and lack oral bioavailability. To address this issue, novel GLT-1 enhancers were synthesized, and the compound with the most favorable pharmacokinetic and pharmacodynamic properties, MC-100093, was selected for further testing. MC-100093 attenuated cued cocaine seeking without reducing food seeking or locomotion and upregulated GLT-1 expression in the nucleus accumbens.

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.

Extinction vs. Abstinence: A Review of the Molecular and Circuit Consequences of Different Post-Cocaine Experiences

The intravenous cocaine self-administration model is widely used to characterize the neurobiology of cocaine seeking. When studies are aimed at understanding relapse to cocaine-seeking, a post-cocaine abstinence period is imposed, followed by “relapse” tests to assess the ability of drug-related stimuli (“primes”) to evoke the resumption of the instrumental response previously made to obtain cocaine. Here, we review the literature on the impact of post-cocaine abstinence procedures on neurobiology, finding that the prelimbic and infralimbic regions of the prefrontal cortex are recruited by extinction training, and are not part of the relapse circuitry when extinction training does not occur. Pairing cocaine infusions with discrete cues recruits the involvement of the NA, which together with the dorsal striatum, is a key part of the relapse circuit regardless of abstinence procedures. Differences in molecular adaptations in the NA core include increased expression of GluN1 and glutamate receptor signaling partners after extinction training. AMPA receptors and glutamate transporters are similarly affected by abstinence and extinction. Glutamate receptor antagonists show efficacy at reducing relapse following extinction and abstinence, with a modest increase in efficacy of compounds that restore glutamate homeostasis after extinction training. Imaging studies in humans reveal cocaine-induced adaptations that are similar to those produced after extinction training. Thus, while instrumental extinction training does not have face validity, its use does not produce adaptations distinct from human cocaine users.

Neuroimmune Mechanisms as Novel Treatment Targets for Substance Use Disorders and Associated Comorbidities

Recent studies examining the neurobiology of substance abuse have revealed a significant role of neuroimmune signaling as a mechanism through which drugs of abuse induce aberrant changes in synaptic plasticity and contribute to substance abuse-related behaviors. Immune signaling within the brain and the periphery critically regulates homeostasis of the nervous system. Perturbations in immune signaling can induce neuroinflammation or immunosuppression, which dysregulate nervous system function including neural processes associated with substance use disorders (SUDs). In this review, we discuss the literature that demonstrates a role of neuroimmune signaling in regulating learning, memory, and synaptic plasticity, emphasizing specific cytokine signaling within the central nervous system. We then highlight recent preclinical studies, within the last 5 years when possible, that have identified immune mechanisms within the brain and the periphery associated with addiction-related behaviors. Findings thus far underscore the need for future investigations into the clinical potential of immunopharmacology as a novel approach toward treating SUDs. Considering the high prevalence rate of comorbidities among those with SUDs, we also discuss neuroimmune mechanisms of common comorbidities associated with SUDs and highlight potentially novel treatment targets for these comorbid conditions. We argue that immunopharmacology represents a novel frontier in the development of new pharmacotherapies that promote long-term abstinence from drug use and minimize the detrimental impact of SUD comorbidities on patient health and treatment outcomes.

Evidence for Modulation of Substance Use Disorders by the Gut Microbiome: Hidden in Plain Sight

The gut microbiome modulates neurochemical function and behavior and has been implicated in numerous central nervous system (CNS) diseases, including developmental, neurodegenerative, and psychiatric disorders. Substance use disorders (SUDs) remain a serious threat to the public well-being, yet gut microbiome involvement in drug abuse has received very little attention. Studies of the mechanisms underlying SUDs have naturally focused on CNS reward circuits. However, a significant body of research has accumulated over the past decade that has unwittingly provided strong support for gut microbiome participation in drug reward. β-Lactam antibiotics have been employed to increase glutamate transporter expression to reverse relapse-induced release of glutamate. Sodium butyrate has been used as a histone deacetylase inhibitor to prevent drug-induced epigenetic alterations. High-fat diets have been used to alter drug reward because of the extensive overlap of the circuitry mediating them. This review article casts these approaches in a different light and makes a compelling case for gut microbiome modulation of SUDs. Few factors alter the structure and composition of the gut microbiome more than antibiotics and a high-fat diet, and butyrate is an endogenous product of bacterial fermentation. Drugs such as cocaine, alcohol, opiates, and psychostimulants also modify the gut microbiome. Therefore, their effects must be viewed on a complex background of cotreatment-induced dysbiosis. Consideration of the gut microbiome in SUDs should have the beneficial effects of expanding the understanding of SUDs and aiding in the design of new therapies based on opposing the effects of abused drugs on the host's commensal bacterial community. SIGNIFICANCE STATEMENT: Proposed mechanisms underlying substance use disorders fail to acknowledge the impact of drugs of abuse on the gut microbiome. β-Lactam antibiotics, sodium butyrate, and high-fat diets are used to modify drug seeking and reward, overlooking the notable capacity of these treatments to alter the gut microbiome. This review aims to stimulate research on substance abuse-gut microbiome interactions by illustrating how drugs of abuse share with antibiotics, sodium butyrate, and fat-laden diets the ability to modify the host microbial community.

Neurochemical mechanisms and neurocircuitry underlying the contribution of stress to cocaine seeking

In individuals with substance use disorders, stress is a critical determinant of relapse susceptibility. In some cases, stressors directly trigger cocaine use. In others, stressors interact with other stimuli to promote drug seeking, thereby setting the stage for relapse. Here, we review the mechanisms and neurocircuitry that mediate stress-triggered and stress-potentiated cocaine seeking. Stressors trigger cocaine seeking by activating noradrenergic projections originating in the lateral tegmentum that innervate the bed nucleus of the stria terminalis to produce beta adrenergic receptor-dependent regulation of neurons that release corticotropin releasing factor (CRF) into the ventral tegmental area (VTA). CRF promotes the activation of VTA dopamine neurons that innervate the prelimbic prefrontal cortex resulting in D1 receptor-dependent excitation of a pathway to the nucleus accumbens core that mediates cocaine seeking. The stage-setting effects of stress require glucocorticoids, which exert rapid non-canonical effects at several sites within the mesocorticolimbic system. In the nucleus accumbens, corticosterone attenuates dopamine clearance via the organic cation transporter 3 to promote dopamine signaling. In the prelimbic cortex, corticosterone mobilizes the endocannabinoid, 2-arachidonoylglycerol (2-AG), which produces CB1 receptor-dependent reductions in inhibitory transmission, thereby increasing excitability of neurons which comprise output pathways responsible for cocaine seeking. Factors that influence the role of stress in cocaine seeking, including prior history of drug use, biological sex, chronic stress/co-morbid stress-related disorders, adolescence, social variables, and genetics are discussed. Better understanding when and how stress contributes to drug seeking should guide the development of more effective interventions, particularly for those whose drug use is stress related.

Neuroinflammatory and behavioral susceptibility profile of mice exposed to social stress towards cocaine effects

Using the social defeat (SD) model, numerous studies have shown that stressed mice display an enhanced response to the motivational effects of cocaine in the self-administration (SA) and conditioned-place preference (CPP) paradigms. However, not all subjects exposed to stress express its harmful effects. Some are particularly susceptible to the deleterious effects of repeated SD, while resilient mice successfully cope with stressful experiences and display adjusted psychological functioning after stress. Vulnerability to develop stress-related disorders, such as depression, has been linked to coping strategies and more recently to individual differences in the immune system. However, no studies have evaluated if coping strategies and immune system reactivity to social stress experiences can also predict susceptibility to stress-induced enhancement of the cocaine response. We evaluated cocaine response in socially defeated mice in the CPP and SA paradigms. To evaluate neuroimmune reactivity to stress the pro-inflammatory cytokine IL-6 and the chemokine CX3CL1 were measured in the striatum and hippocampus. Behavioral phenotype during and after SD episodes was also evaluated. Our results showed that susceptible mice to the depressive-like behaviors effects of stress showed increased conditioned rewarding effects of cocaine in the CPP. In addition, susceptible mice displayed passive-reactive coping behavior during social stress episodes and more pronounced changes in neuroinflammatory markers after the last SD episode, which lasted for one month. Although the complex mechanisms underlying susceptibility or resilience to social stress are still unclear, our results point to multiple adaptive stress responses expressed at different phenotypic levels.

The synthetically produced predator odor 2,5-dihydro-2,4,5-trimethylthiazoline increases alcohol self-administration and alters basolateral amygdala response to alcohol in rats

Post-traumatic stress disorder (PTSD) is a psychiatric illness that can increase the risk for developing an alcohol use disorder (AUD). While clinical data has been useful in identifying similarities in the neurobiological bases of these disorders, preclinical models are essential for understanding the mechanism(s) by which stressors increase the risk for escalated alcohol consumption. The purpose of these studies was to examine if exposure of male Long-Evans rats to the synthetically derived predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT; a component of fox feces) would increase sweetened alcohol self-administration, potentially by facilitating transfer of salience towards cues, and alter neuronal response to alcohol as measured by the immediate early gene c-Fos. In experiment 1, rats exposed to repeated (4×) TMT showed reductions in port entries in Pavlovian conditioned approach and increases in sweetened alcohol self-administration. In experiment 2, rats exposed to repeated TMT showed blunted basolateral amygdala c-Fos response to alcohol. In experiment 3, rats exposed to single, but not repeated TMT, showed increases in sweetened alcohol self-administration, and no change in anxiety-like behavior or hyperarousal. In experiment 4, rats continued to show a significant corticosterone response to TMT after repeated exposures. In summary, exposure of male rats to TMT can cause escalations in sweetened alcohol self-administration and reduction in BLA response to alcohol. These studies outline and utilize a novel preclinical model that can be used to further neurobiological understanding of the emergence of escalated alcohol consumption following stress exposure.

Increased alcohol self-administration following exposure to the predator odor TMT in active coping female rats

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are highly comorbid. Additionally, individual differences in response to stress suggest resilient and susceptible populations. The current study exposed male and female Long Evans rats to the synthetically produced predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) to examine individual differences in stress-reactive behaviors (digging and immobility) and whether these differences were related to subsequent alcohol drinking. Male and female Long Evans rats were trained on operant alcohol self-administration. After 9 sessions, rats underwent exposure to TMT or water (Control) in a distinct context. 6 days after TMT exposure, rats underwent re-exposure to the TMT-paired context (without TMT), and a series of behavioral assessments (acoustic startle, zero maze, light/dark box), after which rats resumed alcohol self-administration. TMT subgroups were created using a ratio of digging to immobility behavior during TMT exposure and rats with a ratio score < 1.0 or> 1.0 were grouped into TMT-1 (low digging/high immobility) or TMT-2 (high digging/low immobility), respectively. All male rats exposed to TMT met criteria for TMT-1, while female rats were divided into the two subgroups. In females, high digging/low immobility behavior during TMT exposure (TMT-2) was related to increased alcohol self-administration, but this was not observed in males or females that engaged in low digging/high immobility (TMT-1). These data show that individual differences in stress-reactivity can lead to lasting behavioral changes which may lead to a better understanding of increases in alcohol drinking following stress in females.

Hypothalamic-pituitary-adrenal axis activity in post-traumatic stress disorder and cocaine use disorder

Post-traumatic stress disorder (PTSD) is often comorbid with cocaine use disorder (CUD), but little is known about hypothalamic-pituitary-adrenal (HPA) axis function in PTSD + CUD. Here we review the clinical and pre-clinical literature of PTSD and CUD with the goal of generating hypotheses about HPA axis activity in comorbid PTSD + CUD. Low glucocorticoid (CORT) levels immediately after trauma exposure are associated with PTSD. CORT administered within 12 h of trauma exposure reduces later PTSD symptoms. Weeks-years after trauma, meta-analyses find lower CORT levels in patients with PTSD relative to never-traumatized controls; the same is found in a pre-clinical model of PTSD. In rodents, reduced basal CORT levels are consistently found after chronic cocaine self-administration. Conversely, increased CORT levels are found in CUD patients during the first 2 weeks of cocaine abstinence. There is evidence for CORT hyper-suppression after dexamethasone, high glucocorticoid receptor (GR) number pre-trauma, and increased GR translocation to the nucleus in PTSD. Hyper-suppression of HPA axis activity after dexamethasone suggests that PTSD individuals may have increased anterior pituitary GR. Given evidence for decreased anterior pituitary GR in rats that self-administer cocaine, PTSD + CUD individuals may have normal GR density and low basal CORT levels during late abstinence. Future studies should aim to reconcile the differences in pre-clinical and clinical basal CORT levels during cocaine and assess HPA axis function in both rodent models of CUD that consider stress-susceptibility and in PTSD + CUD individuals. Although additional studies are necessary, individuals with PTSD + CUD may benefit from behavioral and psychopharmacological treatments to normalize HPA axis activity. LAY SUMMARY Post-traumatic stress disorder (PTSD) is often comorbid with cocaine use disorder (CUD), but little is known about the hypothalamic-pituitary-adrenal (HPA) axis function in PTSD + CUD. The current review provides a synthesis of available clinical and pre-clinical data on PTSD and CUD with the goal of generating hypotheses about HPA axis activity in comorbid PTSD + CUD. While this review finds ample evidence supporting aberrant HPA axis activity in both PTSD and CUD, it suggests that more research is needed to understand the unique changes HPA axis activity in PTSD + CUD, as well as the bidirectional relationship between stress-susceptibility and motivation to seek cocaine.

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.

Exposure to the predator odor TMT induces early and late differential gene expression related to stress and excitatory synaptic function throughout the brain in male rats

Persistent changes in brain stress and glutamatergic function are associated with post-traumatic stress disorder (PTSD). Rodent exposure to the predator odor trimethylthiazoline (TMT) is an innate stressor that produces lasting behavioral consequences relevant to PTSD. As such, the goal of the present study was to assess early (6 hours and 2 days-Experiment 1) and late (4 weeks-Experiment 2) changes to gene expression (RT-PCR) related to stress and excitatory function following TMT exposure in male, Long-Evans rats. During TMT exposure, rats engaged in stress reactive behaviors, including digging and immobility. Further, the TMT group displayed enhanced exploration and mobility in the TMT-paired context 1 week after exposure, suggesting a lasting contextual reactivity. Gene expression analyses revealed upregulated FKBP5 6 hours post-TMT in the hypothalamus and dorsal hippocampus. Two days after TMT, GRM3 was downregulated in the prelimbic cortex and dorsal hippocampus, but upregulated in the nucleus accumbens. This may reflect an early stress response (FKBP5) that resulted in later glutamatergic adaptation (GRM3). Finally, another experiment 4 weeks after TMT exposure showed several differentially expressed genes known to mediate excitatory tripartite synaptic function in the prelimbic cortex (GRM5, DLG4 and SLC1A3 upregulated), infralimbic cortex (GRM2 downregulated, Homer1 upregulated), nucleus accumbens (GRM7 and SLC1A3 downregulated), dorsal hippocampus (FKBP5 and NR3C2 upregulated, SHANK3 downregulated) and ventral hippocampus (CNR1, GRM7, GRM5, SHANK3 and Homer1 downregulated). These data show that TMT exposure induces stress and excitatory molecular adaptations, which could help us understand the persistent glutamatergic dysfunction observed in PTSD.

A Rat Model of Cocaine-Alcohol Polysubstance Use Reveals Altered Cocaine Seeking and Glutamate Levels in the Nucleus Accumbens

Preclinical models of cocaine use disorder are widely utilized to identify neuroadaptations underlying cocaine seeking and to screen medications to reduce seeking. However, while the majority of cocaine users engage in poly-substance use (PSU), a minority of preclinical studies employ PSU models. We previously reported that when rats consume alcohol after daily intravenous cocaine self-administration, nucleus accumbens (NA) core basal glutamate levels are reduced below those of rats that consumed only cocaine, and do not increase during cue + cocaine-primed reinstatement of cocaine-seeking. Here we used the same model of sequential cocaine and alcohol self-administration to test the hypothesis that a similar pattern of glutamate changes would be observed in the NA core prior to and during a cocaine-primed reinstatement test. Rats underwent intravenous cocaine self-administration followed by access to unsweetened alcohol in the home cage for 12 days. Rats underwent a minimum of 12 daily extinction sessions prior to a cocaine-primed reinstatement test conducted during microdialysis procedures. Contrary to our previous work using the same model, here we found that access to alcohol increased cocaine intake and increased responding during early extinction training. We found that as in our previous work, cocaine + alcohol-consuming rats displayed basal glutamate levels below those of rats that self-administered only cocaine. During the cocaine-primed reinstatement test, rats that consumed only cocaine displayed increased glutamate efflux in the NA core while those that consumed cocaine + alcohol did not. These results indicate that preclinical models of PSU should be utilized to develop experimental therapeutics for the reduction of cocaine seeking.

Molecular changes evoked by the beta-lactam antibiotic ceftriaxone across rodent models of substance use disorder and neurological disease

Ceftriaxone is a beta-lactam antibiotic that increases the expression of the major glutamate transporter, GLT-1. As such, ceftriaxone ameliorates symptoms across multiple rodent models of neurological diseases and substance use disorders. However, the mechanism behind GLT-1 upregulation is unknown. The present review synthesizes this literature in order to identify commonalities in molecular changes. We find that ceftriaxone (200 mg/kg for at least two days) consistently restores GLT-1 expression in multiple rodent models of neurological disease, especially when GLT-1 is decreased in the disease model. The same dose given to healthy/drug-naive rodents does not reliably upregulate GLT-1 in any brain region except the hippocampus. Increased GLT-1 expression does not consistently arise from transcriptional regulation, and is likely to be due to trafficking changes. In addition to altered transporter expression, ceftriaxone ameliorates neuropathologies (e.g. tau, amyloid beta, cell death) and aberrant protein expression associated with a number of neurological disease models. Taken together, these results indicate that ceftriaxone remains a strong candidate for treatment of multiple disorders in the clinic.

Rodent models of post-traumatic stress disorder: behavioral assessment

Although the etiology and expression of psychiatric disorders are complex, mammals show biologically preserved behavioral and neurobiological responses to valent stimuli which underlie the use of rodent models of post-traumatic stress disorder (PTSD). PTSD is a complex phenotype that is difficult to model in rodents because it is diagnosed by patient interview and influenced by both environmental and genetic factors. However, given that PTSD results from traumatic experiences, rodent models can simulate stress induction and disorder development. By manipulating stress type, intensity, duration, and frequency, preclinical models reflect core PTSD phenotypes, measured through various behavioral assays. Paradigms precipitate the disorder by applying physical, social, and psychological stressors individually or in combination. This review discusses the methods used to trigger and evaluate PTSD-like phenotypes. It highlights studies employing each stress model and evaluates their translational efficacies against DSM-5, validity criteria, and criteria proposed by Yehuda and Antelman's commentary in 1993. This is intended to aid in paradigm selection by informing readers about rodent models, their benefits to the clinical community, challenges associated with the translational models, and opportunities for future work. To inform PTSD model validity and relevance to human psychopathology, we propose that models incorporate behavioral test batteries, individual differences, sex differences, strain and stock differences, early life stress effects, biomarkers, stringent success criteria for drug development, Research Domain Criteria, technological advances, and cross-species comparisons. We conclude that, despite the challenges, animal studies will be pivotal to advances in understanding PTSD and the neurobiology of stress.

Accelerated development of cocaine-associated dopamine transients and cocaine use vulnerability following traumatic stress

Post-traumatic stress disorder and cocaine use disorder are highly co-morbid psychiatric conditions. The onset of post-traumatic stress disorder generally occurs prior to the development of cocaine use disorder, and thus it appears that the development of post-traumatic stress disorder drives cocaine use vulnerability. We recently characterized a rat model of post-traumatic stress disorder with segregation of rats as susceptible and resilient based on anxiety-like behavior in the elevated plus maze and context avoidance. We paired this model with in vivo fast scan cyclic voltammetry in freely moving rats to test for differences in dopamine signaling in the nucleus accumbens core at baseline, in response to a single dose of cocaine, and in response to cocaine-paired cues. Further, we examined differences in the acquisition of cocaine self-administration across groups. Results indicate that susceptibility to traumatic stress is associated with alterations in phasic dopamine signaling architecture that increase the rate at which dopamine signals entrain to cocaine-associated cues and increase the magnitude of persistent cue-evoked dopamine signals following training. These changes in phasic dopamine signaling correspond with increases in the rate at which susceptible rats develop excessive cocaine-taking behavior. Together, our studies demonstrate that susceptibility to traumatic stress is associated with a cocaine use-vulnerable phenotype and suggests that differences in phasic dopamine signaling architecture may contribute to the process by which this vulnerability occurs.

Glutamatergic postsynaptic density in early life stress programming: Topographic gene expression of mGlu5 receptors and Homer proteins

Type-5 metabotropic glutamate receptors (mGlu5) have been implicated in the mechanism of resilience to stress. They form part of the postsynaptic density (PSD), a thickening of the glutamatergic synapse that acts as a multimodal hub for multiple cellular signaling. Perinatal stress in rats triggers alterations that make adult offspring less resilient to stress. In the present study, we examined the expression of gene encoding the mGlu5 (Grm5), as well as those encoding the short and long isoforms of Homer proteins in different brain regions of the offspring of dams exposed to repeated episodes of restraint stress during pregnancy ("perinatally stressed" or PRS offspring). To this end, we investigated unconditioned behavioral response using the light/dark box test, as well as the expression of PSD genes (Homer1a, Homer1b, and Grm5), in the medial prefrontal cortex, cortex, caudate-putamen, amygdala, and dorsal hippocampus. PRS rats spent significantly less time in the light area than the control group. In the amygdala, Homer1a mRNA levels were significantly increased in PRS rats, whereas Homer1b and Grm5 mRNA levels were reduced. In contrast, the transcript encoding for Homer1a was significantly reduced in the medial prefrontal cortex, caudate-putamen, and dorsal hippocampus of PRS rats. We also evaluated the relative ratio between Homer1a and Homer1b/Grm5 expression, finding a significant shift toward the expression of Homer1a in the amygdala and toward Homer1b/Grm5 in the other brain regions. These topographic patterns of Homer1a, Homer1b, and mGlu5 gene expression were significantly correlated with risk-taking behavior measured in the light/dark box test. Remarkably, in the amygdala and in other brain regions, Homer1b and Grm5 expression showed positive correlation with time spent in the light box, whereas Homer1a in the amygdala showed a negative correlation with risk-taking behavior, in contrast with all other brain regions analyzed, wherein these correlations were positive. These results suggest that perinatal stress programs the developmental expression of PSD molecules involved in mGlu5 signaling in discrete brain regions, with a predominant role for the amygdala.

The Divergent Effects of CDPPB and Cannabidiol on Fear Extinction and Anxiety in a Predator Scent Stress Model of PTSD in Rats

Post-traumatic stress disorder (PTSD) currently has no FDA-approved treatments that reduce symptoms in the majority of patients. The ability to extinguish fear memory associations is impaired in PTSD individuals. As such, the development of extinction-enhancing pharmacological agents to be used in combination with exposure therapies may benefit the treatment of PTSD. Both mGlu5 and CB1 receptors have been implicated in contextual fear extinction. Thus, here we tested the ability of the mGlu5 positive allosteric modulator 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) and cannabidiol (CBD) to reduce both conditioned and unconditioned fear. We used a predator-threat animal model of PTSD which we and others have previously shown to capture the heterogeneity of anxiety responses observed in humans exposed to trauma. Here, 1 week following a 10-min exposure to predator scent stress, rats were classified into stress-Susceptible and stress-Resilient phenotypes using behavioral criteria for elevated plus maze and acoustic startle response performance. Two weeks after classification, rats underwent 3 days of contextual fear extinction and were treated with vehicle, CDPPB or CBD prior to each session. Finally, the light-dark box test was employed to assess phenotypic differences and the effects of CDPPB and CBD on unconditioned anxiety. CDPBB but not CBD, reduced freezing in Susceptible rats relative to vehicle. In the light-dark box test for unconditioned anxiety, CBD, but not CDPPB, reduced anxiety in Susceptible rats. Resilient rats displayed reduced anxiety in the light-dark box relative to Susceptible rats. Taken together, the present data indicate that enhancement of mGlu5 receptor signaling in populations vulnerable to stress may serve to offset a resistance to fear memory extinction without producing anxiogenic effects. Furthermore, in a susceptible population, CBD attenuates unconditioned but not conditioned fear. Taken together, these findings support the use of predator-threat stress exposure in combination with stress-susceptibility phenotype classification as a model for examining the unique drug response profiles and altered neuronal function that emerge as a consequence of the heterogeneity of psychophysiological response to stress.

The effects of clavulanic acid and amoxicillin on cue-primed reinstatement of cocaine seeking

Research using the cocaine self-administration and reinstatement animal model of relapse finds that the beta-lactam antibiotic, ceftriaxone, attenuates cocaine-primed reinstatement of cocaine seeking and upregulates two proteins that regulate glutamate release and reuptake (xCT and GLT-1, respectively) in the nucleus accumbens core (NAc). We tested three compounds with beta-lactam rings for their ability to attenuate cue-primed reinstatement and increase GLT-1 and xCT expression in the NAc and prefrontal cortex (PFC). Rats self-administered intravenous cocaine for 1 hr/day for 7 days then 6 hrs/day for 10 days. Cue-primed reinstatement tests began after 8-9 days of extinction training. Rats received oral vehicle, clavulanic acid (CA), amoxicillin (AMX), or CA + AMX (Augmentin; AUG) for 5 days prior to testing. Only AMX-treated rats demonstrated a reduction of cocaine-seeking that trended toward significance, warranting future investigation of a wider range of doses. In the NAc, GLT-1a expression was reduced in vehicle-treated rats relative to cocaine-naïve controls and was not restored by AMX or AUG. CA-treated rats reinstated more than vehicle-treated rats and exhibited GLT-1a and xCT expression intermediate between cocaine-naïve controls and vehicle-treated cocaine rats. In agreement with our previous work, cocaine did not decrease PFC GLT-1a expression. Cocaine reduced xCT expression in the PFC that was unchanged by any of the three compounds. These results indicate that AMX may be another beta-lactam that attenuates cocaine relapse. Furthermore, the upregulation of both GLT-1 and xCT in the NAc may be needed to attenuate cocaine seeking. (PsycINFO Database Record (c) 2019 APA, all rights reserved).