Susceptibility to traumatic stress sensitizes the dopaminergic response to cocaine and increases motivation for cocaine

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.

Inactivation of ERK1/2 Signaling in Dopaminergic Neurons by Map Kinase Phosphatase MKP3 Regulates Dopamine Signaling and Motivation for Cocaine

The mesolimbic dopamine system is a crucial component of reward and reinforcement processing, including the psychotropic effects of drugs of abuse such as cocaine. Drugs of abuse can activate intracellular signaling cascades that engender long-term molecular changes to brain reward circuitry, which can promote further drug use. However, gaps remain about how the activity of these signaling pathways, such as ERK1/2 signaling, can affect cocaine-induced neurochemical plasticity and cocaine-associated behaviors specifically within dopaminergic cells. To enable specific modulation of ERK1/2 signaling in dopaminergic neurons of the ventral tegmental area, we utilize a viral construct that Cre dependently expresses Map kinase phosphatase 3 (MKP3) to reduce the activity of ERK1/2, in combination with transgenic rats that express Cre in tyrosine hydroxylase (TH)-positive cells. Following viral transfection, we found an increase in the surface expression of the dopamine transporter (DAT), a protein associated with the regulation of dopamine signaling, dopamine transmission, and cocaine-associated behavior. We found that inactivation of ERK1/2 reduced post-translational phosphorylation of the DAT, attenuated the ability of cocaine to inhibit the DAT, and decreased motivation for cocaine without affecting associative learning as tested by conditioned place preference. Together, these results indicate that ERK1/2 signaling plays a critical role in shaping the dopamine response to cocaine and may provide additional insights into the function of dopaminergic neurons. Further, these findings lay important groundwork toward the assessment of how signaling pathways and their downstream effectors influence dopamine transmission and could ultimately provide therapeutic targets for treating cocaine use disorders.

Predator Odor Stressor, 2,3,5-Trimethyl-3-Thiazoline (TMT): Assessment of Stress Reactive Behaviors During an Animal Model of Traumatic Stress in Rats

Animal models utilizing predator odor stress are important in understanding implications for post-traumatic stress disorder. 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) has been used to measure stress reactive behaviors during TMT exposure, indicative of stress coping behaviors. In addition, long-term consequences of stress including contextual-induced stress memory, anxiety-like and hyperarousal behaviors, and subsequent increases in alcohol self-administration can also be examined after TMT exposure. In this article, we describe the TMT exposure protocol used in our lab and how we measure different stress-reactive behaviors that rats engage in during the TMT exposure. Rats are placed in Plexiglass chambers that contain white bedding on the bottom of the chamber and a metal basket in the top right corner containing a filter paper that 10 µl of TMT is pipetted onto. During the 10 min exposure, rats can move around the chamber freely. Exposures are recorded by a video camera for later analysis. During TMT exposure, rats engage in a variety of stress-reactive behaviors, including digging and immobility behavior. These are two distinctly different types of stress-induced behavioral coping strategies to measure individual differences in stress responsivity. To examine individual differences, we group rats into TMT-subgroups based on time spent engaging in digging or immobility behavior. We calculate a digging/immobility ratio score in which we divide the total time spent digging by the total time spent immobile. A cut-off strategy is used such that rats with a criterion ratio score <1.0 are classified as TMT-1 (i.e., low digging/high immobility; greater passive coping) and rats with a ratio score >1.0 are classified as TMT-2 (i.e., high digging/low immobility; greater active coping). Here, we provide a detailed description of the TMT exposure protocol and step-by-step process in evaluation of stress-reactive behaviors. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Predator odor stressor exposure using TMT Basic Protocol 2: Description of stress-reactive behaviors during TMT exposure and formation of TMT-subgroups.

Hypocretin / Orexin Receptor 1 Knockdown in GABA or Dopamine Neurons in the Ventral Tegmental Area Differentially Impact Mesolimbic Dopamine and Motivation for Cocaine

The hypocretins/orexins (HCRT) have been demonstrated to influence motivation for cocaine through actions on dopamine (DA) transmission. Pharmacological or genetic disruption of the hypocretin receptor 1 (Hcrtr1) reduces cocaine self-administration, blocks reinstatement of cocaine seeking, and decreases conditioned place preference for cocaine. These effects are likely mediated through actions in the ventral tegmental area (VTA) and resulting alterations in DA transmission. For example, HCRT drives VTA DA neuron activity and enhances the effects of cocaine on DA transmission, while disrupting Hcrtr1 attenuates DA responses to cocaine. These findings have led to the perspective that HCRT exerts its effects through Hcrtr1 actions in VTA DA neurons. However, this assumption is complicated by the observation that Hcrtr1 are present on both DA and GABA neurons in the VTA and HCRT drives the activity of both neuronal populations. To address this issue, we selectively knocked down Hcrtr1 on either DA or GABA neurons in the VTA and examined alterations in DA transmission and cocaine self-administration in female and male rats. We found that Hcrtr1 knockdown in DA neurons decreased DA responses to cocaine, increased days to acquire cocaine self-administration, and reduced motivation for cocaine. Although, Hcrtr1 knockdown in GABA neurons enhanced DA responses to cocaine, this manipulation did not affect cocaine self-administration. These observations indicate that while Hcrtr1 on DA versus GABA neurons exert opposing effects on DA transmission, only Hcrtr1 on DA neurons affected acquisition or motivation for cocaine - suggesting a complex interplay between DA transmission and behavior.

On making (and turning adaptive to) maladaptive aversive memories in laboratory rodents

Fear conditioning and avoidance tasks usually elicit adaptive aversive memories. Traumatic memories are more intense, generalized, inflexible, and resistant to attenuation via extinction- and reconsolidation-based strategies. Inducing and assessing these dysfunctional, maladaptive features in the laboratory are crucial to interrogating posttraumatic stress disorder's neurobiology and exploring innovative treatments. Here we analyze over 350 studies addressing this question in adult rats and mice. There is a growing interest in modeling several qualitative and quantitative memory changes by exposing already stressed animals to freezing- and avoidance-related tests or using a relatively high aversive training magnitude. Other options combine aversive/fearful tasks with post-acquisition or post-retrieval administration of one or more drugs provoking neurochemical or epigenetic alterations reported in the trauma aftermath. It is potentially instructive to integrate these procedures and incorporate the measurement of autonomic and endocrine parameters. Factors to consider when defining the organismic and procedural variables, partially neglected aspects (sex-dependent differences and recent vs. remote data comparison) and suggestions for future research (identifying reliable individual risk and treatment-response predictors) are discussed.

Rate of onset of dopamine transporter inhibitors assessed with intracranial self-stimulation and in vivo dopamine photometry in rats

Drug self-administration and intracranial self-stimulation (ICSS) are two preclinical behavioral procedures used to predict abuse potential of drugs, and abuse-related drug effects in both procedures are thought to depend on increased mesolimbic dopamine (DA) signaling. Drug self-administration and ICSS yield concordant metrics of abuse potential across a diverse range of drug mechanisms of action. The "rate of onset," defined as the velocity with which a drug produces its effect once administered, has also been implicated as a determinant of abuse-related drug effects in self-administration procedures, but this variable has not been systematically examined in ICSS. Accordingly, this study compared ICSS effects produced in rats by three DA transporter inhibitors that have different rates of onset (fastest to slowest: cocaine, WIN-35428, RTI-31) and that produced progressively weaker metrics of abuse potential in a drug self-administration procedure in rhesus monkeys. Additionally, in vivo photometry using the fluorescent DA sensor dLight1.1 targeted to the nucleus accumbens (NAc) was used to assess the time course of extracellular DA levels as a neurochemical correlate of behavioral effects. All three compounds produced ICSS facilitation and increased DA levels assessed by dLight. In both procedures, the rank order of onset rate was cocaine > WIN-35428 > RTI-31; however, in contrast to monkey drug self-administration results, maximum effects did not differ across compounds. These results provide additional evidence that drug-induced increases in DA drive ICSS facilitation in rats and illustrate the utility of both ICSS and photometry to evaluate the time course and magnitude of abuse-related drug effects in rats.

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.

Axonal Tract Reconstruction Using a Tissue-Engineered Nigrostriatal Pathway in a Rat Model of Parkinson's Disease

Parkinson's disease (PD) affects 1-2% of people over 65, causing significant morbidity across a progressive disease course. The classic PD motor deficits are caused by the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc), resulting in the loss of their long-distance axonal projections that modulate striatal output. While contemporary treatments temporarily alleviate symptoms of this disconnection, there is no approach able to replace the nigrostriatal pathway. We applied microtissue engineering techniques to create a living, implantable tissue-engineered nigrostriatal pathway (TE-NSP) that mimics the architecture and function of the native pathway. TE-NSPs comprise a discrete population of dopaminergic neurons extending long, bundled axonal tracts within the lumen of hydrogel micro-columns. Neurons were isolated from the ventral mesencephalon of transgenic rats selectively expressing the green fluorescent protein in dopaminergic neurons with subsequent fluorescent-activated cell sorting to enrich a population to 60% purity. The lumen extracellular matrix and growth factors were varied to optimize cytoarchitecture and neurite length, while immunocytochemistry and fast-scan cyclic voltammetry (FSCV) revealed that TE-NSP axons released dopamine and integrated with striatal neurons in vitro. Finally, TE-NSPs were implanted to span the nigrostriatal pathway in a rat PD model with a unilateral 6-hydroxydopamine SNpc lesion. Immunohistochemistry and FSCV established that transplanted TE-NSPs survived, maintained their axonal tract projections, extended dopaminergic neurites into host tissue, and released dopamine in the striatum. This work showed proof of concept that TE-NSPs can reconstruct the nigrostriatal pathway, providing motivation for future studies evaluating potential functional benefits and long-term durability of this strategy. This pathway reconstruction strategy may ultimately replace lost neuroarchitecture and alleviate the cause of motor symptoms for PD patients.

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.

Incubation of cocaine craving coincides with changes in dopamine terminal neurotransmission

Relapse to drug use is one of the major challenges in treating substance use disorders. Exposure to drug-related cues and contexts triggers drug craving, which drives cocaine seeking, and increases the probability of relapse. Clinical and animal studies have shown a progressive intensification of cocaine seeking and craving that develops over the course of abstinence, a phenomenon commonly referred to as incubation of cocaine craving. Although the neurobiology underlying incubation of cocaine craving has been examined – particularly within the context of glutamate plasticity– the extent to which increased cocaine craving engenders mesolimbic dopamine (DA) changes has received relatively little attention. To assess whether incubation of cocaine craving is associated with alterations in DA terminal neurotransmission in the nucleus accumbens core (NAc), we used ex vivo fast scan cyclic voltammetry in female and male rats to assess DA dynamics following short access, long access, or intermittent access to cocaine self-administration followed by 28 days of abstinence. Results indicated that both long access and intermittent access to cocaine produced robust incubation of cocaine craving, which was associated with increases in cocaine potency. In addition, intermittent access self-administration also produced a robust increase in DA uptake rate at baseline. In contrast, short access to cocaine did not engender incubation of cocaine craving, nor produce changes in DA neurotransmission. Together these observations indicate that incubation of cocaine craving coincides with changes in DA transmission, suggesting that underlying changes in mesolimbic DA signaling may contribute to the progressive intensification of drug craving that occurs across periods of abstinence.

Impairment of glutamate homeostasis in the nucleus accumbens core underpins cross-sensitization to cocaine following chronic restraint stress

Though the facilitating influence of stress on drug abuse is well documented, the mechanisms underlying this interaction have yet to be fully elucidated. The present study explores the neurobiological mechanisms underpinning the sensitized response to the psychomotor-stimulating effects of cocaine following chronic restraint stress (CRS), emphasizing the differential contribution of both subcompartments of the nucleus accumbens (NA), the core (NAcore) and shell (NAshell), to this phenomenon. Adult male Wistar rats were restrained for 2 h/day for 7 days and, 2 weeks after the last stress exposure (day 21), all animals were randomly assigned to behavioral, biochemical or neurochemical tests. Our results demonstrated that the enduring CRS-induced increase in psychostimulant response to cocaine was paralleled by an increase of extracellular dopamine levels in the NAcore, but not the NAshell, greater than that observed in the non-stress group. Furthermore, we found that CRS induced an impairment of glutamate homeostasis in the NAcore, but not the NAshell. Its hallmarks were increased basal extracellular glutamate concentrations driven by a CRS-induced downregulation of GLT-1, blunted glutamate levels in response to cocaine and postsynaptic structural remodeling in pre-stressed animals. In addition, ceftriaxone, a known GLT-1 enhancer, prevented the CRS-induced GLT-1 downregulation, increased basal extracellular glutamate concentrations and changes in structural plasticity in the NAcore as well as behavioral cross-sensitization to cocaine, emphasizing the biological importance of GLT-1 in the comorbidity between chronic stress exposure and drug abuse. A future perspective concerning the paramount relevance of the stress-induced disruption of glutamate homeostasis as a vulnerability factor to the development of stress and substance use disorders during early life or adulthood of descendants is provided.

Deprenyl reduces inflammation during acute SIV infection

In the era of antiretroviral therapy, inflammation is a central factor in numerous HIV-associated comorbidities, such as cardiovascular disease, cognitive impairment, and neuropsychiatric disorders. This highlights the value of developing therapeutics that both reduce HIV-associated inflammation and treat associated comorbidities. Previous research on monoamine oxidase inhibitors (MAOIs) suggests this class of drugs has anti-inflammatory properties in addition to neuropsychiatric effects. Therefore, we examined the impact of deprenyl, an MAOI, on SIV-associated inflammation during acute SIV infection using the rhesus macaque model of HIV infection. Our results show deprenyl decreased both peripheral and CNS inflammation but had no effect on viral load in either the periphery or CNS. These data show that the MAOI deprenyl may have broad anti-inflammatory effects when given during the acute stage of SIV infection, suggesting more research into the anti-inflammatory effects of this drug could result in a beneficial adjuvant for antiretroviral therapy.

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.

Resilience to the effects of social stress on vulnerability to developing drug addiction

We review the still scarce but growing literature on resilience to the effects of social stress on the rewarding properties of drugs of abuse. We define the concept of resilience and how it is applied to the field of drug addiction research. We also describe the internal and external protective factors associated with resilience, such as individual behavioral traits and social support. We then explain the physiological response to stress and how it is modulated by resilience factors. In the subsequent section, we describe the animal models commonly used in the study of resilience to social stress, and we focus on the effects of chronic social defeat (SD), a kind of stress induced by repeated experience of defeat in an agonistic encounter, on different animal behaviors (depression- and anxiety-like behavior, cognitive impairment and addiction-like symptoms). We then summarize the current knowledge on the neurobiological substrates of resilience derived from studies of resilience to the effects of chronic SD stress on depression- and anxiety-related behaviors in rodents. Finally, we focus on the limited studies carried out to explore resilience to the effects of SD stress on the rewarding properties of drugs of abuse, describing the current state of knowledge and suggesting future research directions.

New directions in modelling dysregulated reward seeking for food and drugs

Behavioral models are central to behavioral neuroscience. To study the neural mechanisms of maladaptive behaviors (including binge eating and drug addiction), it is essential to develop and utilize appropriate animal models that specifically focus on dysregulated reward seeking. Both food and cocaine are typically consumed in a regulated manner by rodents, motivated by reward and homeostatic mechanisms. However, both food and cocaine seeking can become dysregulated, resulting in binge-like consumption and compulsive patterns of intake. The speakers in this symposium for the 2021 International Behavioral Neuroscience Meeting utilize behavioral models of dysregulated reward-seeking to investigate the neural mechanisms of binge-like consumption, enhanced cue-driven reward seeking, excessive motivation, and continued use despite negative consequences. In this review, we outline examples of maladaptive patterns of intake and explore recent animal models that drive behavior to become dysregulated, including stress exposure and intermittent access to rewards. Lastly, we explore select behavioral and neural mechanisms underlying dysregulated reward-seeking for both food and drugs.

Repeated ethanol exposure following avoidance conditioning impairs avoidance extinction and modifies conditioning-associated prefrontal dendritic changes in a mouse model of post-traumatic stress disorder

Treatment of post-traumatic stress disorder is complicated by the presence of alcohol use disorder comorbidity. Little is known about the underlying brain mechanisms. We have recently shown, in mice, that the post-traumatic stress disorder-like phenotype is characterised by the increase and decrease in total dendritic number and length in the prelimbic and infralimbic areas of the medial prefrontal cortex, respectively. Here, we examined whether repeated ethanol exposure would exacerbate these changes and whether this would be associated with difficulty to extinguish passive avoidance behaviour, as an indicator of treatment resistance. We also analysed whether other known trauma-associated changes, like increased or decreased corticosterone and decreased brain-derived neurotrophic factor levels, would also be exacerbated. Male mice underwent trauma exposure (1.5-mA footshock), followed, 8 days later, by a conditioned place preference training with ethanol. Tests for fear sensitization, passive avoidance, anxiety-like behaviour, extinction acquisition and relapse susceptibility were used to assess behaviour changes. Plasma corticosterone and brain-derived neurotrophic factor levels and prefrontal dendritic changes were subsequently measured. Trauma-susceptible mice exposed to ethanol acquired a strong place preference and behaved differently from those not exposed to ethanol, with delayed avoidance extinction and higher avoidance relapse vulnerability. Ethanol potentiated trauma-associated dendritic changes in the prelimbic area and suppressed trauma-associated dendritic changes in the infralimbic area. However, ethanol had no effect on trauma-induced increased corticosterone and decreased brain-derived neurotrophic factor levels. These data suggest that the modification of prefrontal trauma-related changes, due to alcohol use, can characterise, and probably support, treatment-resistant post-traumatic stress disorder.

Individual differences in dopamine uptake in the dorsomedial striatum prior to cocaine exposure predict motivation for cocaine in male rats

A major theme of addiction research has focused on the neural substrates of individual differences in the risk for addiction; however, little is known about how vulnerable populations differ from those that are relatively protected. Here, we prospectively measured dopamine (DA) neurotransmission prior to cocaine exposure to predict the onset and course of cocaine use. Using in vivo voltammetry, we first generated baseline profiles of DA release and uptake in the dorsomedial striatum (DMS) and nucleus accumbens of drug-naïve male rats prior to exposing them to cocaine using conditioned place preference (CPP) or operant self-administration. We found that the innate rate of DA uptake in the DMS strongly predicted motivation for cocaine and drug-primed reinstatement, but not CPP, responding when "price" was low, or extinction. We then assessed the impact of baseline variations in DA uptake on cocaine potency in the DMS using ex vivo voltammetry in naïve rats and in rats with DA transporter (DAT) knockdown. DA uptake in the DMS of naïve rats predicted the neurochemical response to cocaine, such that rats with innately faster rates of DA uptake demonstrated higher cocaine potency at the DAT and rats with DAT knockdown displayed reduced potency compared to controls. Together, these data demonstrate that inherent variability in DA uptake in the DMS predicts the behavioral response to cocaine, potentially by altering the apparent potency of cocaine.

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.

Dopamine transporter function fluctuates across sleep/wake state: potential impact for addiction

The dopamine transporter (DAT) has been implicated in a variety of arousal-related processes including the regulation of motor activity, learning, motivated behavior, psychostimulant abuse, and, more recently, sleep/wake state. We previously demonstrated that DAT uptake regulates fluctuations in extracellular dopamine (DA) in the striatum across the light/dark cycle with DA levels at their highest during the dark phase and lowest during the light phase. Despite this evidence, whether fluctuations in DA uptake across the light/dark cycle are associated with changes in sleep/wake has not been tested. To address this, we employed a combination of sleep/wake recordings, fast scan cyclic voltammetry, and western blotting to examine whether sleep/wake state and/or light/dark phase impact DA terminal neurotransmission in male rats. Further, we assessed whether variations in plasma membrane DAT levels and/or phosphorylation of the threonine 53 site on the DAT accounts for fluctuations in DA neurotransmission. Given the extensive evidence indicating that psychostimulants increase DA through interactions with the DAT, we also examined to what degree the effects of cocaine at inhibiting the DAT vary across sleep/wake state. Results demonstrated a significant association between individual sleep/wake states and DA terminal neurotransmission, with higher DA uptake rate, increased phosphorylation of the DAT, and enhanced cocaine potency observed after periods of sleep. These findings suggest that sleep/wake state influences DA neurotransmission in a manner that is likely to impact a host of DA-dependent processes including a variety of neuropsychiatric disorders.

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.

The neural, behavioral, and epidemiological underpinnings of comorbid alcohol use disorder and post-traumatic stress disorder

Alcohol use disorder (AUD) and (PTSD) frequently co-occur and individuals suffering from this dual diagnosis often exhibit increased symptom severity and poorer treatment outcomes than those with only one of these diseases. Although there have been significant advances in our understanding of the neurobiological mechanisms underlying each of these disorders, the neural underpinnings of the comorbid condition remain poorly understood. This chapter summarizes recent epidemiological findings on comorbid AUD and PTSD, with a focus on vulnerable populations, the temporal relationship between these disorders, and the clinical consequences associated with the dual diagnosis. We then review animal models of the comorbid condition and emerging human and non-human animal research that is beginning to identify maladaptive neural changes common to both disorders, primarily involving functional changes in brain reward and stress networks. We end by proposing a neural framework, based on the emerging field of affective valence encoding, that may better explain the epidemiological and neural findings on AUD and PTSD.

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.

Chemogenetic Manipulation of Dopamine Neurons Dictates Cocaine Potency at Distal Dopamine Transporters

The reinforcing efficacy of cocaine is largely determined by its capacity to inhibit the dopamine transporter (DAT), and emerging evidence suggests that differences in cocaine potency are linked to several symptoms of cocaine use disorder. Despite this evidence, the neural processes that govern cocaine potency in vivo remain unclear. In male rats, we used chemogenetics with intra-VTA microinfusions of the agonist clozapine-n-oxide to bidirectionally modulate dopamine neurons. Using ex vivo fast scan cyclic voltammetry, pharmacological probes of the DAT, biochemical assessments of DAT membrane availability and phosphorylation, and cocaine self-administration, we tested the effects of chemogenetic manipulations on cocaine potency at distal DATs in the nucleus accumbens as well as the behavioral economics of cocaine self-administration. We discovered that chemogenetic manipulation of dopamine neurons produced rapid, bidirectional modulation of cocaine potency at DATs in the nucleus accumbens. We then provided evidence that changes in cocaine potency are associated with alterations in DAT affinity for cocaine and demonstrated that this change in affinity coincides with DAT conformation biases and changes in DAT phosphorylation state. Finally, we showed that chemogenetic manipulation of dopamine neurons alters cocaine consumption in a manner consistent with changes in cocaine potency at distal DATs. Based on the spatial and temporal constraints inherent to our experimental design, we posit that changes in cocaine potency are driven by alterations in dopamine neuron activity. When considered together, these observations provide a novel mechanism through which GPCRs regulate cocaine's pharmacological and behavioral effects.SIGNIFICANCE STATEMENT Differences in the pharmacological effects of cocaine are believed to influence the development and progression of cocaine use disorder. However, the biological and physiological processes that determine sensitivity to cocaine remain unclear. In this work, we use a combination of chemogenetics, fast scan cyclic voltammetry, pharmacology, biochemistry, and cocaine self-administration with economic demand analysis to demonstrate a novel mechanism by which cocaine potency is determined in vivo These studies identify a novel process by which the pharmacodynamics of cocaine are derived in vivo, and thus this work has widespread implications for understanding the mechanisms that regulate cocaine consumption across stages of addiction.

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.

Effects of Alcohol and Cocaine in a Mutant Mouse Model of Predisposition to Post-Traumatic Stress Disorder

Comorbidity between drug abuse and post-traumatic stress disorder (PTSD), a stress-related dysregulation of fear responses, is very high. While some drugs are known to increase fear and anxiety, there are only few data regarding interactions between voluntary drug consumption and fear memory. The spontaneous chronic consumption of either alcohol or cocaine under a 3-week free-choice progressive paradigm of alcohol (3/6/10%) or cocaine (0.2/0.4/0.6 mg/ml), was assessed in VGV transgenic mice, having full 5-HT_2C receptor editing and displaying PTSD-like behaviors. The consequences of these drug consumptions on the potentiated contextual and cued fear conditioning responses of VGV mice were assessed. The effects of drugs on hippocampal brain-derived neurotrophic factor (Bdnf) mRNA were measured as its expression was previously found to be decreased in VGV mice. Chronic alcohol consumption was similar in WT and VGV mice. In the alcohol condition, fear acquisition was not different at the end of the learning session and cue-fear extinction was facilitated. Regarding cocaine, in contrast to WT mice, VGV mice did not increase their drug consumption along with increasing doses, an effect that might be related with enhanced drug stimuli discrimination via increased 5-HT_2C receptors. Cocaine-intake VGV mice did not display the contextual fear generalization usually observed in control VGV mice. In addition, Bdnf expression was upregulated after either chronic alcohol or cocaine intake. Altogether, these results suggest that both chronic alcohol and cocaine voluntary oral consumptions can exert some therapeutic-like effects in a mutant model of PTSD predisposition.

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.

Behavioral Traits Associated With Resilience to the Effects of Repeated Social Defeat on Cocaine-Induced Conditioned Place Preference in Mice

The relationship between stress and drug use is well demonstrated. Stress-induced by repeated social defeat (RSD) enhances the conditioned place preference (CPP) induced by cocaine in mice. The phenomenon of resilience understood as the ability of subjects to overcome the negative effects of stress is the focus of increasing interest. Our aim is to characterize the behavior of resilient animals with respect to the effects of RSD on the CPP induced by cocaine. To this end, 25 male C57BL/6 mice were exposed to stress by RSD during late adolescence, while other 15 male mice did not undergo stress (controls). On the 2 days following the last defeat, all the animals carried out the elevated plus maze (EPM) and Hole Board, Social Interaction, Tail Suspension and Splash tests. Three weeks later, all the animals performed the CPP paradigm with a low dose of cocaine (1 mg/kg). Exposure to RSD decreased all measurements related to the open arms of the EPM. It also reduced social interaction, immobility in the tail suspension test (TST) and grooming in the splash test. RSD exposure also increased the sensitivity of the mice to the rewarding effects of cocaine, since only defeated animals acquired CPP. Several behavioral traits were related to resilience to the potentiating effect of RSD on cocaine CPP. Mice that showed less submission during defeat episodes, a lower percentage of time in the open arms of the EPM, low novelty-seeking, high social interaction, greater immobility in the TST and a higher frequency of grooming were those that were resilient to the long-term effects of social defeat on cocaine reward since they behaved like controls and did not develop CPP. These results suggest that the behavioral profile of resilient defeated mice is characterized by an active coping response during episodes of defeat, a greater concern for potential dangers, less reactivity in a situation of inevitable moderate stress and fewer depressive-like symptoms after stress. Determining the neurobehavioral substrates of resilience is the first step towards developing behavioral or pharmacological interventions that increase resilience in individuals at a high risk of suffering from stress.

Pre- and postnatal alcohol exposure delays, in female but not in male rats, the extinction of an auditory fear conditioned memory and increases alcohol consumption

Repeated exposure to alcohol increases retrieval of fear-conditioned memories, which facilitates, among other factors, the emergence of post-traumatic stress disorder (PTSD). Individuals with PTSD are more likely to develop alcohol and substance abuse related disorders. We assessed if prenatal and early postnatal alcohol exposure (PAE) increased the susceptibility to retain aversive memories and if this was associated with subsequent heightened alcohol consumption. Pregnant Sprague-Dawley rats were exposed for 22 hr/day, throughout pregnancy and until postnatal Day 7 to a single bottle of sucralose - sweetened 10% alcohol solution (PAE Group), or to a single bottle of tap water and sucralose (Control Group). Auditory fear conditioning (AFC) was performed in the adolescent offspring at postnatal Day 40. Freezing was measured during acquisition, retention and extinction phases, followed by 3 weeks of free choice alcohol intake. Female, but not male, PAE rats exhibited impaired extinction of the aversive memory, a finding associated with higher levels of 3-4 Dihidroxyphenylacetic acid (DOPAC) in the nucleus accumbens and heightened alcohol intake, respect to controls. These findings suggest that PAE makes females more vulnerable to long-term retention of aversive memories, which coexist with heightened alcohol intake. These findings are reminiscent of those of PTSD.

Susceptibility and Resilience to PTSD-Like Symptoms in Mice Are Associated with Opposite Dendritic Changes in the Prelimbic and Infralimbic Cortices Following Trauma

Post-traumatic stress disorder (PTSD) is triggered by exposure to traumatic events, but not everyone who experiences trauma develops this disorder. Like humans, PTSD-like symptoms develop in some laboratory rodents (susceptible individuals), while others express less or no symptoms (resilient individuals). Here, considering (i) the putative causal role of fear conditioning in PTSD development and (ii) the involvement of the medial prefrontal cortex (mPFC) in the regulation of conditioned fear response, we tested whether trauma-associated changes in the mPFC may discriminate stress-resilient from stress-susceptible mice. From data on avoidance behavior (as a major symptom), we found that trauma-exposed mice displayed a bimodal distribution in their step-through latency, with low avoider (stress-resilient) individuals and high avoider (stress-susceptible) individuals. Dendrites of Golgi-Cox-stained neurons were analyzed in two parts of the mPFC: the prelimbic (PrL) and infralimbic (IL) areas. In the resilient phenotype, the total number of dendrites decreased in the PrL and increased in the IL; however, it decreased only in the IL in the susceptible phenotype compared to controls. These findings demonstrate that the type of post-trauma morphological changes in the mPFC is associated with susceptibility or resilience to trauma-related symptoms.

Local GABAA Receptor-Mediated Suppression of Dopamine Release within the Nucleus Accumbens

Benzodiazepines make up a class of psychoactive drugs that act as allosteric co-activators of the inhibitory GABA_A receptor. These drugs are useful for the treatment of several psychiatric disorders but also hold considerable abuse liability. Despite the common use and misuse of benzodiazepines, the mechanisms through which these drugs exert their reinforcing effects remain incompletely understood. Transient phasic increases in dopamine levels are believed to play an important role in defining the reinforcing properties of drugs of abuse, and we recently demonstrated that systemic administration of benzodiazepines increased the frequency of these events but concomitantly reduced their amplitude. This observation provides insight into the pharmacological effects of benzodiazepines on dopamine signaling, but the processes through which benzodiazepines drive changes in phasic dopamine signals remain unclear. In these studies, we investigated the mechanisms through which benzodiazepines may reduce the phasic dopamine transient amplitude. We tested the effect of the benzodiazepine diazepam and the GABA_A agonist muscimol on evoked dopamine release from nucleus accumbens brain slices using fast scan cyclic voltammetry. We found that both diazepam and muscimol reduce dopamine release and that reductions in dopamine release following GABA_A receptor activation can be blocked by co-application of a GABA_B receptor antagonist. These results suggest that activation of GABA_A receptors in the nucleus accumbens decreases dopamine release by disinhibition of local GABA signaling and subsequent activation of GABA_B receptors. Overall, this work provides a putative mechanism through which benzodiazepines reduce the amplitude of phasic dopamine release in vivo.

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.

Hypocretin receptor 1 involvement in cocaine-associated behavior: Therapeutic potential and novel mechanistic insights

Since its discovery in 1998, the hypocretin/orexin system has been identified as a critical modulator of behavior. Through interactions with dopamine neurons of the ventral tegmental area, this system is poised to regulate motivation for drug rewards by impacting dopamine neurotransmission in target structures including the nucleus accumbens. Across numerous experiments, we and others have identified a critical influence of hypocretin receptor 1 in mediating the behavioral and physiological effects of cocaine which positions this receptor as a potential target for the treatment of cocaine addiction. Here we discuss evidence for hypocretin receptor 1 involvement in driving cocaine-associated behavior and how hypocretin receptor 1 in the ventral tegmental area are critical for supporting dopamine neuron activity and dopamine neurotransmission. We then present new data supporting the novel hypothesis that in addition to exerting acute actions on dopamine systems, pharmacological hypocretin manipulations also produce lasting adaptations to dopamine terminals that impact sensitivity to cocaine, and ultimately, future behavior.