Reinstatement of cocaine seeking in rats by the pharmacological stressors, corticotropin-releasing factor and yohimbine: role for D1/5 dopamine receptors

Impaired extinction of cocaine seeking in HIV-infected mice is accompanied by peripheral and central immune dysregulation

Substance use disorders (SUDs) are highly comorbid with HIV infection, necessitating an understanding of the interactive effects of drug exposure and HIV. The relationship between HIV infection and cocaine use disorder is likely bidirectional, with cocaine use directly impacting immune function while HIV infection alters addiction-related behavior. To better characterize the neurobehavioral and immune consequences of HIV infection and cocaine exposure, this study utilizes a humanized mouse model to investigate the outcomes of HIV-1 infection on cocaine-related behaviors in a conditioned place preference (CPP) model, and the interactive effects of cocaine and HIV infection on peripheral and central nervous system inflammation. HIV infection selectively impairs cocaine CPP extinction without effecting reinstatement or cocaine seeking under conflict. Behavioral alterations are accompanied by immune changes in HIV infected mice, including increased prefrontal cortex astrocyte immunoreactivity and brain-region specific effects on microglia number and reactivity. Peripheral immune system changes are observed in human cytokines, including HIV-induced reductions in human TNFα, and cocaine and HIV interactions on GM-CSF levels. Together these data provide new insights into the unique neurobehavioral outcomes of HIV infection and cocaine exposure and how they interact to effect immune responses.

Impaired extinction of cocaine seeking in HIV-infected mice is accompanied by peripheral and central immune dysregulation

Substance use disorders (SUDs) are highly comorbid with HIV infection, necessitating an understanding of the interactive effects of drug exposure and HIV. The relationship between HIV infection and cocaine use disorder is likely bidirectional, with cocaine use directly impacting immune function while HIV infection alters addiction-related behavior. To better characterize the neurobehavioral and immune consequences of HIV infection and cocaine exposure, this study utilized a humanized mouse model to investigate the outcomes of HIV-1 infection on cocaine-related behaviors in a conditioned place preference (CPP) model, and the interactive effects of cocaine and HIV infection on peripheral and central nervous system inflammation. HIV infection selectively impaired cocaine CPP extinction without effecting reinstatement or cocaine seeking under conflict were observed. Behavioral alterations were accompanied by immune changes in HIV infected mice, including increased prefrontal cortex astrocyte immunoreactivity and brain-region specific effects on microglia number and reactivity. Peripheral immune system changes were observed in both mouse and human cytokines, including HIV-induced reductions in mouse IL-1α and G-CSF and human TNFα, and cocaine induced alterations in mouse GM-CSF. Together these data provide new insights into the unique neurobehavioral outcomes of HIV infection and cocaine exposure and how they interact to effect immune responses.

Impaired extinction of cocaine seeking in HIV-infected mice is accompanied by peripheral and central immune dysregulation

Substance use disorders (SUDs) are highly comorbid with HIV infection, necessitating an understanding of the interactive effects of drug exposure and HIV. The relationship between progressive HIV infection and cocaine use disorder is likely bidirectional, with cocaine use having direct effects on immune function while HIV infection can alter addiction-related behavior. To better characterized the neurobehavioral and immune consequences of HIV infection and cocaine exposure, this study utilized a humanized mouse model to investigate the outcomes of progressive HIV infection on cocaine-related behaviors in a cocaine conditioned place preference (CPP) model, and the interactive effects of cocaine and HIV infection on peripheral and central nervous system inflammation. HIV infection did not impact the formation of a cocaine CPP, but did result in resistance to extinction of the CPP. No effects of HIV on yohimbine-primed reinstatement or cocaine seeking under conflict were observed. These behavioral alterations were accompanied by immune changes in HIV infected mice, including increased prefrontal cortex astrocyte immunoreactivity and brain-region specific effects on microglia number and reactivity. Peripheral immune system changes were observed in both mouse and human markers. Among other targets, this included HIV-induced reductions in mouse IL-1α and G-CSF and human TNFα and cocaine-induced alterations in human TNFα and mouse GM-CSF such that cocaine exposure increases both cytokines only in the absence of HIV infection. Together these data provide new insights into the unique neurobehavioral processes underlying HIV infection and cocaine use disorders, and further how they interact to effect immune responses.

Reinstatement of Pavlovian responses to alcohol cues by stress

RATIONALE

Stress may contribute to relapse to alcohol use in part by enhancing reactivity to cues previously paired with alcohol. Yet, standard models of stress-induced reinstatement generally use contingent presentations of alcohol-paired cues to reinforce instrumental behaviors, making it difficult to isolate the ability of cues to invigorate alcohol-seeking.

OBJECTIVE

Here we sought to test the impact of stress on behavioral responses to alcohol-paired cues, using a model of stress-induced reinstatement of Pavlovian conditioned approach, inspired by Nadia Chaudhri's work on context-induced reinstatement.

METHODS

Long Evans rats were trained to associate one auditory cue with delivery of alcohol or sucrose and an alternative auditory cue with no reward. Following extinction training, rats were exposed to a stressor prior to being re-exposed to the cues under extinction conditions. We assessed the effects of yohimbine, intermittent footshock and olfactory cues paired with social defeat on responses to alcohol-paired cues and the effects of yohimbine on responses to sucrose-paired cues.

RESULTS

The pharmacological stressor, yohimbine, enhanced alcohol seeking in a Pavlovian setting, but not in a cue-selective manner. Intermittent footshock and social defeat cues did not enhance alcohol seeking in this paradigm.

CONCLUSIONS

While yohimbine elicited reinstatement of reward-seeking in a Pavlovian setting, these effects may be unrelated to activation of stress systems or to interactions with specific cues.

NOP receptor antagonism attenuates reinstatement of alcohol-seeking through modulation of the mesolimbic circuitry in male and female alcohol-preferring rats

In patients suffering from alcohol use disorder (AUD), stress and environmental stimuli associated with alcohol availability are important triggers of relapse. Activation of the nociceptin opioid peptide (NOP) receptor by its endogenous ligand Nociceptin/Orphanin FQ (N/OFQ) attenuates alcohol drinking and relapse in rodents, suggesting that NOP agonists may be efficacious in treating AUD. Intriguingly, recent data demonstrated that also blockade of NOP receptor reduced alcohol drinking in rodents. To explore further the potential of NOP antagonism, we investigated its effects on the reinstatement of alcohol-seeking elicited by administration of the α2 antagonist yohimbine (1.25 mg/kg, i.p.) or by environmental conditioning factors in male and female genetically selected alcohol-preferring Marchigian Sardinian (msP) rats. The selective NOP receptor antagonist LY2817412 (0.0, 3.0, 10.0, and 30.0 mg/kg) was first tested following oral (p.o.) administration. We then investigated the effects of LY2817412 (1.0, 3.0, 6.0 μg/μl/rat) microinjected into three candidate mesolimbic brain regions: the ventral tegmental area (VTA), the central nucleus of the amygdala (CeA), and the nucleus accumbens (NAc). We found that relapse to alcohol seeking was generally stronger in female than in male rats and oral administration of LY2817412 reduced yohimbine- and cue-induced reinstatement in both sexes. Following site-specific microinjections, LY2817412 reduced yohimbine-induced reinstatement of alcohol-seeking when administered into the VTA and the CeA, but not in the NAc. Cue-induced reinstatement was suppressed only when LY2817412 was microinjected into the VTA. Infusions of LY2817412 into the VTA and the CeA did not alter saccharin self-administration. These results demonstrate that NOP receptor blockade prevents the reinstatement of alcohol-seeking through modulation of mesolimbic system circuitry, providing further evidence of the therapeutic potential of NOP receptor antagonism in AUD.

Activation of peroxisome proliferator-activated receptor γ reduces alcohol drinking and seeking by modulating multiple mesocorticolimbic regions in rats

Peroxisome proliferator-activated receptor γ (PPARγ) is an intracellular transcription factor whose signaling activation by the selective agonist pioglitazone reduces alcohol drinking and alcohol-seeking behavior in rats. The present study utilized the two-bottle choice and operant self-administration procedures to investigate neuroanatomical substrates that mediate the effects of PPARγ agonism on alcohol drinking and seeking in msP rats. Bilateral infusions of pioglitazone (0, 5, and 10 μg/μl) in the rostromedial tegmental nucleus (RMTg) decreased voluntary alcohol drinking and alcohol self-administration. Microinjections of pioglitazone in the ventral tegmental area (VTA), central amygdala (CeA), and nucleus accumbens (NAc) shell had no such effect. Notably, water, food, and saccharin consumption was unaltered by either treatment. The yohimbine-induced reinstatement of alcohol seeking was prevented by infusions of pioglitazone (0, 2.5, 5, and 10 μg/μl) in the CeA, VTA, and RMTg but not in the NAc shell. These results emphasize the involvement of mesocorticolimbic circuitries in mediating the effects of PPARγ agonists on alcohol drinking and seeking. These results will facilitate future studies that investigate the pathophysiological role of PPARγ in alcohol use disorder and help clarify the mechanisms by which the activation of this receptor decreases the motivation for drinking.

The pharmacological stressor yohimbine, but not U50,488, increases responding for conditioned reinforcers paired with ethanol or sucrose

RATIONALE

Environmental stimuli paired with alcohol can function as conditioned reinforcers (CRfs) and trigger relapse to alcohol-seeking. In animal models, pharmacological stressors can enhance alcohol consumption and reinstate alcohol-seeking, but it is unknown whether stress can potentiate the conditioned reinforcing properties of alcohol-paired stimuli.

OBJECTIVES

We examined whether the pharmacological stressors, the α-2 adrenoreceptor antagonist yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) and the K-opioid receptor agonist U50,488 (vehicle, 1.25, 2.5 mg/kg; SC), increase responding for conditioned reinforcement, and if their effects interact with the nature of the reward (alcohol vs. sucrose). We subsequently examined the effects of yohimbine (vehicle, 1.25, 2.5 mg/kg; IP) on responding for sensory reinforcement.

METHODS

Male Long-Evans underwent Pavlovian conditioning, wherein a tone-light conditioned stimulus (CS) was repeatedly paired with 12% EtOH or 21.7% sucrose. Next, tests of responding for a CRf were conducted. Responding on the CRf lever delivered the CS, whereas responding on the other lever had no consequences. In a separate cohort of rats, the effects of yohimbine on responding just for the sensory reinforcer were examined.

RESULTS

Both doses of yohimbine, but not U50,488, increased responding for conditioned reinforcement. This enhancement occurred independently of the nature of the reward used during Pavlovian conditioning. Yohimbine-enhanced responding for a CRf was reproducible and stable over five tests; it even persisted when the CS was omitted. Both doses of yohimbine also increased responding for sensory reinforcement.

CONCLUSIONS

Yohimbine increases operant responding for a variety of sensory and conditioned reinforcers. This enhancement may be independent of its stress-like effects.

Anti-stress neuropharmacological mechanisms and targets for addiction treatment: A translational framework

Stress-related substance use is a major challenge for treating substance use disorders. This selective review focuses on emerging pharmacotherapies with potential for reducing stress-potentiated seeking and consumption of nicotine, alcohol, marijuana, cocaine, and opioids (i.e., key phenotypes for the most commonly abused substances). I evaluate neuropharmacological mechanisms in experimental models of drug-maintenance and relapse, which translate more readily to individuals presenting for treatment (who have initiated and progressed). An affective/motivational systems model (three dimensions: valence, arousal, control) is mapped onto a systems biology of addiction approach for addressing this problem. Based on quality of evidence to date, promising first-tier neurochemical receptor targets include: noradrenergic (α1 and β antagonist, α2 agonist), kappa-opioid antagonist, nociceptin antagonist, orexin-1 antagonist, and endocannabinoid modulation (e.g., cannabidiol, FAAH inhibition); second-tier candidates may include corticotropin releasing factor-1 antagonists, serotonergic agents (e.g., 5-HT reuptake inhibitors, 5-HT3 antagonists), glutamatergic agents (e.g., mGluR2/3 agonist/positive allosteric modulator, mGluR5 antagonist/negative allosteric modulator), GABA-promoters (e.g., pregabalin, tiagabine), vasopressin 1b antagonist, NK-1 antagonist, and PPAR-γ agonist (e.g., pioglitazone). To address affective/motivational mechanisms of stress-related substance use, it may be advisable to combine agents with actions at complementary targets for greater efficacy but systematic studies are lacking except for interactions with the noradrenergic system. I note clinically-relevant factors that could mediate/moderate the efficacy of anti-stress therapeutics and identify research gaps that should be pursued. Finally, progress in developing anti-stress medications will depend on use of reliable CNS biomarkers to validate exposure-response relationships.

Social defeat stress: Mechanisms underlying the increase in rewarding effects of drugs of abuse

Social interaction is known to be the main source of stress in human beings, which explains the translational importance of this research in animals. Evidence reported over the last decade has revealed that, when exposed to social defeat experiences (brief episodes of social confrontations during adolescence and adulthood), the rodent brain undergoes remodeling and functional modifications, which in turn lead to an increase in the rewarding and reinstating effects of different drugs of abuse. The mechanisms by which social stress cause changes in the brain and behavior are unknown, and so the objective of this review is to contemplate how social defeat stress induces long-lasting consequences that modify the reward system. First of all, we will describe the most characteristic results of the short- and long-term consequences of social defeat stress on the rewarding effects of drugs of abuse such as psychostimulants and alcohol. Secondly, and throughout the review, we will carefully assess the neurobiological mechanisms underlying these effects, including changes in the dopaminergic system, corticotrophin releasing factor signaling, epigenetic modifications and the neuroinflammatory response. To conclude, we will consider the advantages and disadvantages and the translational value of the social defeat stress model, and will discuss challenges and future directions.

Pharmacological investigations of a yohimbine-impulsivity interaction in rats

Both impulsivity and stress are risk factors for substance abuse, but it is not clear how these two processes interact to alter susceptibility for the disorder. The aim of this project was to examine the pharmacology of a stress-impulsivity interaction in rats. To do so, we tested the effects of yohimbine on impulsive action and then assessed whether behavioural changes could be reduced by antagonists at different receptor subtypes. Male Long-Evans rats were injected with various doses of yohimbine (0-5.0 mg/kg) before testing in the response-inhibition task. In subsequent experiments, yohimbine (2.5 mg/kg) was injected following pretreatment with the following receptor antagonists: corticotropin-releasing factor receptor 1, antalarmin (0-20 mg/kg); glucocorticoid, mifepristone (0-30 mg/kg); noradrenergic (NA) α1, prazosin (0-2 mg/kg); NA α2, guanfacine (0-0.5 mg/kg); NA β2, propranolol (0.5-2.0 mg/kg); dopamine D1/5, SCH 39166 (0-0.0625 mg/kg); μ opioid, naloxone (0-2 mg/kg); or 5-HT2A, M100907 (0.005-0.05 mg/kg). In all experiments, impulsive action was measured as increased premature responding. Yohimbine dose dependently increased impulsive action, but the effect was not reversed by antagonist pretreatment. None of the drugs altered any other behavioural measure. We conclude that stress-impulsivity interactions are likely mediated by a synergy of multiple neurotransmitter systems.

Dissociable Role of Corticotropin Releasing Hormone Receptor Subtype 1 on Dopaminergic and D1 Dopaminoceptive Neurons in Cocaine Seeking Behavior

The ability of many drugs of abuse, including cocaine, to mediate reinforcement and drug-seeking behaviors is in part mediated by the corticotropin-releasing hormone (CRH) system, in which CRH exerts its effects partly via the CRH receptor subtype 1 (CRHR1) in extra-hypothalamic areas. In fact, CRHR1 expressed in regions of the mesolimbic dopamine (DA) system have been demonstrated to modify cocaine-induced DA release and alter cocaine-mediated behaviors. Here we examined the role of neuronal selectivity of CRHR1 within the mesolimbic system on cocaine-induced behaviors. First we used a transgenic mouse line expressing GFP under the control of the Crhr1 promoter for double fluorescence immunohistochemistry to demonstrate the cellular location of CRHR1 in both dopaminergic and D1 dopaminoceptive neurons. We then studied cocaine sensitization, self-administration, and reinstatement in inducible CRHR1 knockouts using the CreERT2/loxP in either dopamine transporter (DAT)-containing neurons (DAT-Crhr1) or dopamine receptor 1 (D1)-containing neurons (D1-Crhr1). For sensitization testing, mice received five daily injections of cocaine (15 mg/kg IP). For self-administration, mice received eight daily 2 h cocaine (0.5 mg/kg per infusion) self-administration sessions followed by extinction and reinstatement testing. There were no differences in the acute or sensitized locomotor response to cocaine in DAT-Crhr1 or D1-Crhr1 mice and their respective controls. Furthermore, both DAT-Crhr1 and D1-Crhr1 mice reliably self-administered cocaine at the level of controls. However, DAT-Crhr1 mice demonstrated a significant increase in cue-induced reinstatement relative to controls, whereas D1-Crhr1 mice demonstrated a significant decrease in cue-induced reinstatement relative to controls. These data demonstrate the involvement of CRHR1 in cue-induced reinstatement following cocaine self-administration, and implicate a bi-directional role of CRHR1 for cocaine craving.

Dissociable effects of systemic and orbitofrontal administration of adrenoceptor antagonists on yohimbine-induced motor impulsivity

The α₂-adrenoceptor antagonist, yohimbine, is commonly used as a pharmacological stressor. Its behavioural effects are typically attributed to elevated noradrenaline release via blockade of central, inhibitory autoreceptors. We have previously reported that yohimbine increases motor impulsivity in rats on the five-choice serial reaction time task (5CSRTT), a cognitive behavioural assessment which measures motor impulsivity and visuospatial attention. Furthermore, this effect depended on cyclic adenomonophosphate (cAMP) signalling via cAMP response element binding (CREB) protein in the orbitofrontal cortex (OFC). However, the role of specific adrenoceptors in this effect is not well-characterised. We therefore investigated whether the pro-impulsive effects of systemic yohimbine could be reproduced by direct administration into the OFC, or attenuated by intra-OFC or systemic administration of prazosin and propranolol-antagonists at the α₁- and β-adrenoceptor, respectively. Male Long-Evans rats were trained on the 5CSRTT and implanted with guide cannulae aimed at the OFC. Systemically administered α₁- or β-adrenoceptor antagonists attenuated yohimbine-induced increases in premature responding. In contrast, local infusion of yohimbine into the OFC reduced such impulsive responding, while blockade of α₁- or β-adrenoceptors within the OFC had no effect on either basal or yohimbine-stimulated motor impulsivity. Direct administration of selective antagonists at the α₁-, α₂- or β-adrenoceptor into the OFC therefore produce clearly dissociable effects from systemic administration. Collectively, these data suggest that the pro-impulsivity effect of yohimbine can be modulated by adrenergic signalling in brain areas outside of the OFC, in addition to non-adrenergic signalling pathways within the OFC.

Dopamine D2 receptors mediate the increase in reinstatement of the conditioned rewarding effects of cocaine induced by acute social defeat

Social stress modifies the activity of brain areas involved in the rewarding effects of psychostimulants, inducing neuroadaptations in the dopaminergic mesolimbic system and modifying the sensitivity of dopamine receptors. In the present study we evaluated the effect of the dopamine D₁- and D₂-like receptor antagonists (SCH23390 and raclopride, respectively) on the short-time effects of acute social defeat (ASD). Male OF1 mice were socially defeated before each conditioning session of the conditioned place preference (CPP) induced by 1mg/kg or 25mg/kg of cocaine plus the corresponding dopamine antagonist. A final experiment was designed to evaluate the effect of the dopamine antagonists on the CPP induced by 3mg/kg of cocaine with or without a stress experience. Mice exposed to ASD showed an increase in reinstatement of the conditioned reinforcing effects of cocaine that was blocked by all of the dopamine receptor antagonists. Blockade of dopamine D₂-like receptors with raclopride specifically prevented the effects of stress without affecting the rewarding properties of cocaine. However, SCH23390 inhibited cocaine-induced preference in the control groups and even induced aversion in defeated mice conditioned with the lower dose of cocaine. Moreover, the lowest dose of SCH23390 blocked the rewarding effects of 3mg/kg of cocaine-induced CPP. Our results confirm that the dopamine D₂ receptor is involved in the short-term effects of ASD on the rewarding effects of cocaine. The dopamine D₁ receptor is clearly involved in the rewarding effects of cocaine, but its role in the effects of ASD remains to be demonstrated.

[18F]-Fluorodeoxyglucose-Positron Emission Tomography in Rats with Prolonged Cocaine Self-Administration Suggests Potential Brain Biomarkers for Addictive Behavior

The DSM5-based dimensional diagnostic approach defines substance use disorders on a continuum from recreational drug use to habitual and ultimately addicted behavior. Biomarkers that are indicative of recreational drug use and addicted behavior are lacking. We performed a translational [18F]-fluorodeoxyglucose-positron emission tomography (FDG-PET) study in the multi-dimensional 0/3crit model of cocaine addiction. Addict-like (3crit) and non-addict-like (0crit) rats, which shared identical life conditions and levels of cocaine self-administration, were acquired for FDG-PET under baseline conditions and following cocaine and yohimbine challenges. Compared to cocaine-naïve control rats, 0crit animals showed higher glucose uptake in the caudate putamen (CPu) and medial prefrontal cortex (mPFC) respect to naïve controls. 3crit animals did not show this adaptive higher glucose utilization, but had lower uptake in several cortical areas. Both cocaine and yohimbine challenges affected glucose uptake in control rats in several brain sites, but not in 0crit and 3crit rats, indicating that impaired glucose mobilization in response to these challenges is not specifically associated with addictive behavior. Compared to 0crit, 3crit rats showed higher reinstatement responses, which were negatively associated with glucose uptake in the ventral tegmental area. Data indicate that cocaine non-addict- and addict-like phenotypes are associated with several potential biomarkers. Specifically, we propose that increased glucose uptake in the CPu and mPFC is a function of controlled drug use, whereas a loss of striatal and prefrontal metabolic activity and reduced uptake in cortical areas are indicative of addictive behavior.

Presynaptic G Protein-Coupled Receptors: Gatekeepers of Addiction?

Drug abuse and addiction cause widespread social and public health problems, and the neurobiology underlying drug actions and drug use and abuse is an area of intensive research. Drugs of abuse alter synaptic transmission, and these actions contribute to acute intoxication as well as the chronic effects of abused substances. Transmission at most mammalian synapses involves neurotransmitter activation of two receptor subtypes, ligand-gated ion channels that mediate fast synaptic responses and G protein-coupled receptors (GPCRs) that have slower neuromodulatory actions. The GPCRs represent a large proportion of neurotransmitter receptors involved in almost all facets of nervous system function. In addition, these receptors are targets for many pharmacotherapeutic agents. Drugs of abuse directly or indirectly affect neuromodulation mediated by GPCRs, with important consequences for intoxication, drug taking and responses to prolonged drug exposure, withdrawal and addiction. Among the GPCRs are several subtypes involved in presynaptic inhibition, most of which are coupled to the Gi/o class of G protein. There is increasing evidence that these presynaptic Gi/o-coupled GPCRs have important roles in the actions of drugs of abuse, as well as behaviors related to these drugs. This topic will be reviewed, with particular emphasis on receptors for three neurotransmitters, Dopamine (DA; D1- and D2-like receptors), Endocannabinoids (eCBs; CB1 receptors) and glutamate (group II metabotropic glutamate (mGlu) receptors). The focus is on recent evidence from laboratory animal models (and some evidence in humans) implicating these receptors in the acute and chronic effects of numerous abused drugs, as well as in the control of drug seeking and taking. The ability of drugs targeting these receptors to modify drug seeking behavior has raised the possibility of using compounds targeting these receptors for addiction pharmacotherapy. This topic is also discussed, with emphasis on development of mGlu2 positive allosteric modulators (PAMs).

Role of dopamine neurotransmission in the long-term effects of repeated social defeat on the conditioned rewarding effects of cocaine

Numerous studies report that social defeat stress alters dopamine (DA) neurotransmission in several areas of the brain. Alterations of the mesolimbic dopaminergic pathway are believed to be responsible for the increased vulnerability to drug use observed as a result of social stress. In the present study, we evaluated the influence of DA receptors on the long-term effect of repeated social defeat (RSD) on the conditioned rewarding and reinstating effects of cocaine. For this purpose, the D1R antagonist SCH 23390 and the D1R antagonist raclopride were administered 30min before each social defeat and a cocaine-induced CPP procedure was initiated three weeks later. The expression of the D1R and D2R was also measured in the cortex and hippocampus throughout the entire procedure. Mice exposed to RSD showed an increase in the conditioned rewarding effects of cocaine that was blocked by both DA receptors antagonists when a subthreshold dose of cocaine was employed. However, while the vulnerability to reinstatement of the preference induced by 25mg/kg cocaine-induced CPP was abolished by the D1R antagonist, it was practically unaffected by raclopride. Increases in D2R receptor levels were observed in the cortex of defeated animals after the first and fourth social defeats and in the hippocampus 3weeks later. Nevertheless, D1R receptor levels in the hippocampus decreased only after the last social defeat. Our results confirm that RSD enhances the conditioned rewarding effects of cocaine and that both DA receptors are involved in this enduring effect of social stress.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

Long-term effects of chronic cocaine exposure throughout adolescence on anxiety and stress responsivity in a Wistar rat model

Adolescents display increased vulnerability to engage in drug experimentation. This is often considered a risk factor for later drug abuse. In this scenario, the permanent effects of cocaine exposure during adolescence on anxiety levels and stress responsivity, which may result in behavioral phenotypes prone to addiction, are now starting to be unveiled. Thus, the purpose of the present study was to evaluate the long-lasting effects of chronic cocaine administration during adolescence, on anxiety-like behavior and on stress response. Adolescent male Wistar rats were daily administered 45-mg cocaine/kg of body weight in three equal intraperitoneal doses with 1-h interval, from postnatal day (PND) 35 to 50. The effects of cocaine administration on anxiety levels, assessed in the Elevated Plus Maze (EPM), and on social stress response, assessed in the resident-intruder paradigm (R/I), were evaluated 10 days after withdrawal, when rats were reaching the adulthood. The underlying dopaminergic activity, and the corticosterone and testosterone levels were determined. Our results showed that cocaine induced long-lasting alterations in the hypothalamus-pituitary-adrenals (HPA) axis function and in testosterone levels. Such alterations resulted in significant and enduring changes in behavioral responses to environmental challenges, such as the EPM and R/I, including the evaluation of potential threats that may lead to high-risk behavior and low-benefit choices. This was further supported by an altered dopaminergic function in the amygdala and hippocampus. The present findings provide new insights into how the use of cocaine during adolescent development may modulate emotional behavior later in life. Compromised ability to recognize and deal with potential threats is an important risk factor to perpetuate compulsive drug seeking and relapse susceptibility.

Inactivation of the nucleus accumbens core or medial shell attenuates reinstatement of sugar-seeking behavior following sugar priming or exposure to food-associated cues

Re-exposure to either palatable food or to conditioned stimuli associated with food is known to reinstate food-seeking after periods of abstinence. The nucleus accumbens core and shell are important for reinstatement in both food- and drug-seeking paradigms, although their potential differential roles have been difficult to delineate due to methodological differences in paradigms across laboratories. The present studies assessed the effects of temporary inactivation of the core or shell on priming- and cue-induced reinstatement of food-seeking in identically-trained rats. Inactivation of either the nucleus accumbens core (Experiment 1A; N = 10) or medial shell (Experiment 1B; N = 12) blocked priming-induced reinstatement in an equivalent manner. Similarly, inactivation of the core or medial shell (Experiments 2A & 2B; N = 11 each) also blocked cue-induced reinstatement, although there was also a significant treatment day X brain region X drug order interaction. Specifically, rats with core inactivation reinstated lever-pressing on the vehicle injection day regardless of whether that was their first or second test, whereas rats that had medial shell inactivation on the first day did not significantly reinstate lever-pressing on the second day of testing (when they received vehicle). Yohimbine, while a reportedly robust pharmacological stressor, was ineffective at inducing reinstatement in the current stress-induced reinstatement procedure. These data suggest that both the nucleus accumbens core and shell serve important roles in reinstatement of food-seeking in response to priming and cues.

The form of a conditioned stimulus can influence the degree to which it acquires incentive motivational properties

There is considerable individual variation in the extent to which food- and drug-associated cues (conditioned stimuli, CSs) acquire incentive salience, as indicated by whether they elicit approach towards them, and/or act as conditioned reinforcers. Here we asked whether this variation is influenced by properties of the CS itself. In rats, we assessed both the attractiveness and conditioned reinforcing properties of two CSs: a manipulable lever CS versus an auditory (tone) CS. There was considerable individual variation in the extent to which a lever CS acquired incentive motivational properties, as indicated by whether it became attractive (evoked a sign-tracking or goal-tracking conditioned response) or acted as a conditioned reinforcer. However, with a tone CS all rats learned a goal-tracking response, and the tone CS was an equally effective conditioned reinforcer in sign-trackers and goal-trackers. Even when presented in compound (a lever-tone CS), the two elements of the compound differentially acquired motivational properties. In contrast, amphetamine and stress potentiated the conditioned reinforcing properties of both visual and auditory CSs similarly in rats that primarily sign-tracked or goal-tracked. We conclude that variation in the to the ability of CSs to acquire incentive salience, and thus their ability to act as incentive stimuli capable of motivating behavior, is determined in part by properties of the CS itself.

Corticotropin releasing factor: a key role in the neurobiology of addiction

Drug addiction is a chronically relapsing disorder characterized by loss of control over intake and dysregulation of stress-related brain emotional systems. Since the discovery by Wylie Vale and his colleagues of corticotropin-releasing factor (CRF) and the structurally-related urocortins, CRF systems have emerged as mediators of the body's response to stress. Relatedly, CRF systems have a prominent role in driving addiction via actions in the central extended amygdala, producing anxiety-like behavior, reward deficits, excessive, compulsive-like drug self-administration and stress-induced reinstatement of drug seeking. CRF neuron activation in the medial prefrontal cortex may also contribute to the loss of control. Polymorphisms in CRF system molecules are associated with drug use phenotypes in humans, often in interaction with stress history. Drug discovery efforts have yielded brain-penetrant CRF1 antagonists with activity in preclinical models of addiction. The results support the hypothesis that brain CRF-CRF1 systems contribute to the etiology and maintenance of addiction.

Differential effect of orexin-1 and CRF-1 antagonism on stress circuits: a fMRI study in the rat with the pharmacological stressor Yohimbine

Translational approaches to study the neural substrates of stress and assess the mechanistic efficacy of novel anti-anxiety agents necessitate the use of stressors with a similar degree of saliency across species. The alpha-2 adrenoreceptor antagonist yohimbine represents an attractive experimental tool owing to its well-documented stress-inducing properties in humans and laboratory species. We investigated the neural substrates engaged by yohimbine in the rat brain by using functional magnetic resonance imaging and mapped their modulation by neurotransmitter systems involved in stress responses. Yohimbine elicited a composite pattern of brain activation, highlighting the recruitment of cortico-striato-thalamic regions and extra-hypothalamic stress neurocircuits. This effect was strongly attenuated by the α-2-adrenoceptor agonist medetomidine and by the dopamine (DA) D1 receptor antagonist SCH23390, thus revealing a primary contribution of both norepinephrine and DA on the neurofunctional cascade elicited by the drug. Pretreatment with the corticotrophin-releasing factor type-1 receptor (CRF1R) antagonist CP154,526 produced a region-dependent inhibition of yohimbine-induced activation in the amygdala, striatum, and cingulate cortex, while the orexin type-1 receptor (OX1R) antagonists GSK1059865 robustly inhibited the response in fronto-hippocampal regions as well as in several key components of the extended amygdala. CP154,526 and GSK1059865 did not prevent yohimbine-induced plasma corticosterone release, a finding that corroborates a central origin of the effects mapped. Our findings provide novel insight into the brain substrates and neurochemical mediators engaged by the stress-inducing agent yohimbine. The differential pattern of inhibition produced by CRF1R and OX1R antagonists suggests that these two neuropeptide systems can modulate the functional response to stress via distinct central neural pathways.

Inactivation of the lateral habenula reduces anxiogenic behavior and cocaine seeking under conditions of heightened stress

Recent anatomical and functional studies have renewed interest in the lateral habenula (LHb), a critical brain region that works in an opponent manner to modulate aversive and appetitive processes. In particular, increased LHb activation is believed to drive anxiogenic states during stressful conditions. Here, we reversibly inactivated the LHb with GABA receptor agonists (baclofen/muscimol) in rats prior to testing in an open field, elevated plus maze, and defensive burying task in the presence or absence of yohimbine, a noradrenergic α2-receptor antagonist that acts as an anxiogenic stressor. In a second set of experiments using a cocaine self-administration and reinstatement model, we inactivated the LHb during extinction responding and cue-induced reinstatement of cocaine seeking in the presence or absence of yohimbine pretreatment. Inactivation of the LHb after yohimbine treatment attenuated anxiogenic behavior by increasing time spent in the open arms and reducing the time spent burying. Inactivation of the LHb also reduced cocaine seeking when cue-induced reinstatement occurred in the presence of yohimbine, but did not affect extinction responding or cue-induced reinstatement by itself. These data demonstrate that the LHb critically regulates states of heightened anxiety during both unconditioned behavior and conditioned appetitive processes.

Antagonism of the dopamine D1-like receptor in mesocorticolimbic nuclei attenuates acute food deprivation-induced reinstatement of heroin seeking in rats

The neurotransmitter dopamine (DA) plays a critical role in both priming-and cue-induced reinstatement of extinguished drug-seeking behavior, but its role in stress-induced reinstatement is less clear. Our laboratory has recently demonstrated that systemic administration of the DA D1-like receptor antagonist, SCH 23390, attenuates acute food deprivation (FD) stress-induced reinstatement. The current study was designed to elucidate the brain regions critical to the effect of SCH 23390 on FD stress-induced reinstatement. Rats were trained to press a lever to self-administer heroin (0.1 mg/kg/inf) over a period of 10 days. Following training, heroin was removed leading to an extinction of lever pressing. Next, rats were tested for reinstatement twice, under extinction conditions: once following 21-48 h FD; and once under sated conditions. Prior to testing, SCH 23390 was administered into the nucleus accumbens (NAc) shell (0.0, 0.3, 0.6 μg/side), NAc core (0.0, 0.3, 0.6 μg/side), dorsomedial prefrontal cortex (dmPFC; 0.0, 0.2, 2.0 μg/side), ventromedial prefrontal cortex (vmPFC; 0.0, 2.0 μg/side) or basolateral amygdala (BLA; 0.0, 1.0, 2.0 μg/side). An attenuation of FD-induced reinstatement of heroin seeking was seen in rats injected with SCH 23390 into the NAc shell, dmPFC or BLA, but not into the NAc core or the vmPFC. These findings support the hypothesis that DA transmission through the DA D1-like receptors plays a critical role in stress-induced reinstatement of heroin seeking.