Escalation of cocaine self-administration in adulthood after social defeat of adolescent rats: role of social experience and adaptive coping behavior

Chronic social defeat stress in prairie voles (Microtus ochrogaster): A preclinical model for the study of depression-related phenotypes

BACKGROUND

Stress-induced illnesses, like major depression, are among the leading causes of disability across the world. Consequently, there is a dire need for the validation of translationally-suited animal models incorporating social stress to uncover the etiology of depression. Prairie voles (Microtus ochrogaster) are more translationally relevant than many other rodent models as they display monogamous social and bi-parental behaviors. Therefore, we evaluated whether a novel social defeat stress (SDS) model in male prairie voles induces depression-relevant behavioral outcomes.

METHODS

Adult sexually-naïve male prairie voles experienced SDS bouts from a conspecific pair-bonded male aggressor, 10 min per day for 10 consecutive days. Non-stressed controls (same-sex siblings) were housed in similar conditions but never experienced physical stress. Twenty-four h later, voles were evaluated in social interaction, sucrose preference, and Morris water maze tests - behavioral endpoints validated to assess social withdrawal, anhedonia-related behavior, and spatial memory performance, respectively.

RESULTS

SDS-exposed voles displayed lower sociability and body weight, decreased preference for a sucrose solution, and impairment of spatial memory retrieval. Importantly, no differences in general locomotor activity were observed as a function of SDS exposure.

LIMITATIONS

This study does not include female voles in the experimental design.

CONCLUSIONS

We found that repeated SDS exposure, in male prairie voles, results in a depression-relevant phenotype resembling an anhedonia-like outcome (per reductions in sucrose preference) along with social withdrawal and spatial memory impairment - highlighting that the prairie vole is a valuable model with potential to study the neurobiology of social stress-induced depression-related outcomes.

Stress inoculation during adolescence attenuates social stress-induced increase in ethanol intake in adult male mice

Social stress exposure heightens the risk of substance abuse disorder development, especially when endured during adolescence, influencing long-term mental health. This study investigates early-life stress's potential to confer resilience against later-life stressors. To investigate this hypothesis, we examined the impact of a single social defeat (SD) incident during adolescent mice's lives on subsequent voluntary ethanol consumption following repeated adult social stress exposure. Half of the adolescent mice experienced SD at postnatal day 28. Three weeks later (postnatal day 49), defeated groups encountered four confrontations with aggressive residents every 72 h, while control groups were exposed to non-resident exploration. A day after the last SD, defeated mice were classified as resilient or susceptible based on their response to a social interaction test (SIT), a model for depressive behavior. To assess ethanol consumption during young adulthood, researchers used the 'drinking in the dark' and oral ethanol self-administration paradigms. Stress inoculation (IS) slightly increased resilient animals in the SIT. In mice without IS exposure during adolescence, susceptible defeated mice displayed higher ethanol consumption and motivation than control and resilient mice. IS in adolescence effectively counteracted this effect, as IS-SD groups, whether resilient or susceptible, showed no increase in ethanol intake. These groups also exhibited similar motivation to control, measured by the progressive ratio. Notably, elevated IL-6 levels seen in SD-S mice were absent in IS-exposed mice. Additionally, IS-exposed groups had lower prefrontal cortex IL-6 and CX3CL1 levels. These findings support the hypothesis that IS, induced by moderate-intensity stress during adolescence, can enhance resilience to more severe stressors in adulthood.

Paternal morphine exposure in rats reduces social play in adolescent male progeny without affecting drug-taking behavior in juvenile males or female offspring

The ongoing opioid addiction crisis necessitates the identification of novel risk factors to improve prevention and treatment of opioid use disorder. Parental opioid exposure has recently emerged as a potential regulator of offspring vulnerability to opioid misuse, in addition to heritable genetic liability. An understudied aspect of this "missing heritability" is the developmental presentation of these cross-generational phenotypes. This is an especially relevant question in the context of inherited addiction-related phenotypes, given the prominent role of developmental processes in the etiology of psychiatric disorders. Paternal morphine self-administration was previously shown to alter the sensitivity to the reinforcing and antinociceptive properties of opioids in the next generation. Here, phenotyping was expanded to include the adolescent period, with a focus on endophenotypes related to opioid use disorders and pain. Paternal morphine exposure did not alter heroin or cocaine self-administration in male and female juvenile progeny. Further, baseline sensory reflexes related to pain were unaltered in morphine-sired adolescent rats of either sex. However, morphine-sired adolescent males exhibited a reduction in social play behavior. Our findings suggest that, in morphine-sired male offspring, paternal opioid exposure does not affect opioid intake during adolescence, suggesting that this phenotype does not emerge until later in life. Altered social behaviors in male morphine-sired adolescents indicate that the changes in drug-taking behavior in adults sired by morphine-exposed sires may be due to more complex factors not yet fully assessed.

Social stress under binge-like alcohol withdrawal in adolescence: evidence of cannabidiol effect on maladaptive plasticity in rats

BACKGROUND

Alcohol binge drinking may compromise the functioning of the nucleus accumbens (NAc), i.e. the neural hub for processing reward and aversive responses.

METHODS

As socially stressful events pose particular challenges at developmental stages, this research applied the resident-intruder paradigm as a model of social stress, to highlight behavioural neuroendocrine and molecular maladaptive plasticity in rats at withdrawal from binge-like alcohol exposure in adolescence. In search of a rescue agent, cannabidiol (CBD) was selected due to its favourable effects on alcohol- and stress-related harms.

RESULTS

Binge-like alcohol exposed intruder rats displayed a compromised defensive behaviour against the resident and a blunted response of the stress system, in addition to indexes of abnormal dopamine (DA)/glutamate plasticity and dysfunctional spine dynamics in the NAc. CBD administration (60 mg/kg) was able to: (1) increase social exploration in the binge-like alcohol exposed intruder rats, at the expenses of freezing time, and in control rats, which received less aggressive attacks from the resident; (2) reduce corticosterone levels independently on alcohol previous exposure; (3) restore DA transmission and (4) facilitate excitatory postsynaptic strength and remodelling.

CONCLUSIONS

Overall, the maladaptive behavioural and synaptic plasticity promoted by the intersection between binge-like alcohol withdrawal and exposure to adverse social stress can be rescued by a CBD détente effect that results in a successful defensive strategy, supported by a functional endocrine and synaptic plasticity. The current data highlight CBD's relevant therapeutic potential in alcohol- and stress-related harms, and prompt further investigation on its molecular targets.

Adolescent binge-like alcohol exposure dysregulates NPY and CGRP in rats: Behavioural and immunochemical evidence

Alcohol binge drinking during adolescence impacts affective behaviour, possibly impinging on developing neural substrates processing affective states, including calcitonin gene-related peptide (CGRP) and neuropeptide Y (NPY). Here, we modelled binge-like alcohol exposure in adolescence, by administering 3.5 g/kg alcohol per os, within 1 h, to male adolescent rats every other day, from postnatal day 35 to 54. The effects on positive and negative affective behaviour during abstinence were explored including: consummatory behaviour and weight gain; social behaviour in the modified social interaction test; thermal nociception in the tail-flick test; psychosocial stress coping in the resident-intruder paradigm. Moreover, CGRP and NPY levels were evaluated in functionally relevant brain regions. Our data shows that binge-like intermittent alcohol administration during adolescence decreased weight gain, social preference and motivation, nociception, and active psychosocial stress coping during abstinence. In addition, intermittent alcohol-exposed rats displayed increased expression of CGRP and NPY in the prefrontal cortex and nucleus accumbens; decreased NPY levels in the amygdala; opposite changes in CGRP levels in the hypothalamus and brainstem. Overall, our data shows that adolescent binge-like alcohol exposure, through the allostatic load of alternate intoxication and withdrawal, produces long-term consequences in sensory and affective processes and dysregulated complementary neuropeptidergic systems. Thus, neuropeptide-targeted interventions hold promising potential for addressing negative affect during prolonged withdrawal in young subjects.

Resilience to social defeat stress in adolescent male mice

Adverse social experiences during adolescence are associated with the appearance of mental illness in adulthood. Social defeat (SD) is an ethologically valid murine model to study the consequences of social stress. In adolescent mice, SD induces depressive-like behaviors, increased anxiety and potentiates the reinforcing effects of cocaine and alcohol. However, not all mice exposed to SD will be susceptible to these effects. Adult mice resilient to the effects of SD show a consistent phenotype being resilient to depressive-like behaviors and to the increase in cocaine and alcohol consumption. The aim of the present study was to characterize the resilient phenotype to depressive-like behaviors and increase cocaine and ethanol rewarding effects of mice socially defeated during adolescence. To that end, adolescent mice were exposed to repeated SD, and 24 h after the last encounter, they underwent a social interaction test (SIT) in order to evaluate depressive-like behaviors. Cocaine-induced reward conditioning and ethanol intake was evaluated in two different sets of mice 3 weeks after the last SD using cocaine-induced conditioned place preference (CPP) and oral ethanol self-administration (SA). The neuroinflammation response was measured at the end of the experimental procedure by measuring striatal and cortical levels of IL-6 and CX3CL1. The results confirmed that a comparable percentage of adolescent mice develop resilience to depressive-like behaviors to that observed in adult mice. However, increased anxiety was more severe in resilient mice. Likewise, an increased preference for an ineffective dose of cocaine and an increased ethanol consumption was observed in resilient mice compared to controls. The increase in IL-6 and CX3CL1 was mainly observed in the striatum of susceptible mice compared to that of control mice. Our results confirm that, contrary to prior assumptions in adults, responses to SD stress are more complex and singular in adolescents, and caution should be taken for the correct interpretation and translation of those phenotypes.

Chronic Physical and Vicarious Psychosocial Stress Alter Fentanyl Consumption and Nucleus Accumbens Rho GTPases in Male and Female C57BL/6 Mice

Chronic stress can increase the risk of developing a substance use disorder in vulnerable individuals. Numerous models have been developed to probe the underlying neurobiological mechanisms, however, most prior work has been restricted to male rodents, conducted only in rats, or introduces physical injury that can complicate opioid studies. Here we sought to establish how chronic psychosocial stress influences fentanyl consumption in male and female C57BL/6 mice. We used chronic social defeat stress (CSDS), or the modified vicarious chronic witness defeat stress (CWDS), and used social interaction to stratify mice as stress-susceptible or resilient. We then subjected mice to a 15 days fentanyl drinking paradigm in the home cage that consisted of alternating forced and choice periods with increasing fentanyl concentrations. Male mice susceptible to either CWDS or CSDS consumed more fentanyl relative to unstressed mice. CWDS-susceptible female mice did not differ from unstressed mice during the forced periods, but showed increased preference for fentanyl over time. We also found decreased expression of nucleus accumbens Rho GTPases in male, but not female mice following stress and fentanyl drinking. We also compare fentanyl drinking behavior in mice that had free access to plain water throughout. Our results indicate that stress-sensitized fentanyl consumption is dependent on both sex and behavioral outcomes to stress.

Friend of the Devil: Negative Social Influences Driving Substance Use Disorders

Substance use disorders in humans have significant social influences, both positive and negative. While prosocial behaviors promote group cooperation and are naturally rewarding, distressing social encounters, such as aggression exhibited by a conspecific, are aversive and can enhance the sensitivity to rewarding substances, promote the acquisition of drug-taking, and reinstate drug-seeking. On the other hand, withdrawal and prolonged abstinence from drugs of abuse can promote social avoidance and suppress social motivation, accentuating drug cravings and facilitating relapse. Understanding how complex social states and experiences modulate drug-seeking behaviors as well as the underlying circuit dynamics, such as those interacting with mesolimbic reward systems, will greatly facilitate progress on understanding triggers of drug use, drug relapse and the chronicity of substance use disorders. Here we discuss some of the common circuit mechanisms underlying social and addictive behaviors that may underlie their antagonistic functions. We also highlight key neurochemicals involved in social influences over addiction that are frequently identified in comorbid psychiatric conditions. Finally, we integrate these data with recent findings on (±)3,4-methylenedioxymethamphetamine (MDMA) that suggest functional segregation and convergence of social and reward circuits that may be relevant to substance use disorder treatment through the competitive nature of these two types of reward. More studies focused on the relationship between social behavior and addictive behavior we hope will spur the development of treatment strategies aimed at breaking vicious addiction cycles.

Methods and Challenges in Investigating Sex-Specific Consequences of Social Stressors in Adolescence in Rats: Is It the Stress or the Social or the Stage of Development?

Adolescence is a time of social learning and social restructuring that is accompanied by changes in both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal (HPA) axis. The activation of these axes by puberty and stressors, respectively, shapes adolescent development. Models of social stress in rats are used to understand the consequences of perturbations of the social environment for ongoing brain development. This paper reviews the challenges in investigating the sex-specific consequences of social stressors, sex differences in the models of social stress used in rats and the sex-specific effects on behaviour and provides an overview of sex differences in HPA responding to stressors, the variability in pubertal development and in strains of rats that require consideration in conducting such research, and directions for future research.

Effect of early life social adversity on drug abuse vulnerability: Focus on corticotropin-releasing factor and oxytocin

Early life adversity can set the trajectory for later psychiatric disorders, including substance use disorders. There are a host of neurobiological factors that may play a role in the negative trajectory. The current review examines preclinical evidence suggesting that early life adversity specifically involving social factors (maternal separation, adolescent social isolation and adolescent social defeat) may influence drug abuse vulnerability by strengthening corticotropin-releasing factor (CRF) systems and weakening oxytocin (OT) systems. In adulthood, pharmacological and genetic evidence indicates that both CRF and OT systems are directly involved in drug reward processes. With early life adversity, numerous studies show an increase in drug abuse vulnerability measured in adulthood, along a concomitant strengthening of CRF systems and a weakening of OT systems. Mechanistic studies, while relatively few in number, are generally consistent with the theme that strengthened CRF systems and weakened OT systems mediate, at least in part, the link between early life adversity and drug abuse vulnerability. Establishing a direct role of CRF and OT in mediating the relation between early life social stressors and drug abuse vulnerability will inform clinical researchers and practitioners toward the development of intervention strategies to reduce risk among those suffering from early life adversities. This article is part of the special issue on 'Vulnerabilities to Substance Abuse'.

Social isolation at adolescence: a systematic review on behaviour related to cocaine, amphetamine and nicotine use in rats and mice

Adolescence is known for its high level of risk-taking, and neurobiological alterations during this period may predispose to psychoactive drug initiation and progression into more severe use patterns. Stress is a risk factor for drug consumption, and post-weaning social isolation increases drug self-administration in rodents. This review aimed to provide an overview of the effects of adolescent social isolation on cocaine, amphetamine and nicotine use-related behaviours, highlighting the specific period when animals were submitted to stress and these drugs. We wondered if there was a specific period during adolescence that isolation stress would increase drug use vulnerability. A total of 323 publications from the Scopus, Web of Science and PubMed (Medline) electronic databases were identified using the words "social isolation" and "adolescence" and "drug" or "cocaine" or "amphetamine" or "nicotine", resulting in 24 articles after analyses criteria following the PRISMA statement. The main points raised were social isolation during adolescence increased cocaine self-administration, amphetamine and nicotine locomotor activity. We did not observe a pattern of a specific moment during the adolescent period that could lead to an increased vulnerability to drug use. The precise conditions under which adolescent social stress alters drug use parameters are complex and likely depend on several factors.

Housing conditions during self-administration determine motivation for cocaine in mice following chronic social defeat stress

RATIONALE

Stress exposure has a lasting impact on motivated behavior and can exacerbate existing vulnerabilities for developing a substance use disorder. Several models have been developed to examine how stressful experiences shape drug reward. These range from locomotor sensitization and conditioned place preference to the propensity for drug self-administration or responding to drug-predictive cues. While self-administration studies are considered to have more translational relevance, most of the studies to date have been conducted in rats. Further, many self-administration studies are conducted in single-housed animals, adding the additional stressor of social isolation.

OBJECTIVES

We sought to establish how chronic social defeat stress (CSDS) and social housing conditions impact cocaine self-administration and cocaine-seeking behaviors in C57BL/6 mice.

METHODS

We assessed self-administration behavior (cocaine or saline, 0.5 mg/kg/infusion) in C57BL/6 mice subjected to 10-day CSDS or in unstressed controls. Mice were housed either in pairs or in isolation during self-administration. We compared the effect of housing on acquisition of self-administration, seeking, extinction, drug-induced reinstatement, and after re-exposure to the social stressor.

RESULTS

Pair-housing during self-administration revealed increased social avoidance after CSDS is associated with decreased cocaine intake. In contrast, single-housing revealed stress-sensitive cocaine intake, with increased social avoidance after CSDS associated with increased early cocaine intake. Pair-, but not single-housed mice are susceptible to drug-induced reinstatement independent of CSDS history. Stress re-exposure sensitized cocaine-seeking in stressed single-housed mice.

CONCLUSIONS

The social context surrounding cocaine intake can bidirectionally influence cocaine-related behaviors after psychosocial stress and should be considered when studying stress and drug cross-sensitization.

Methionine mediates resilience to chronic social defeat stress by epigenetic regulation of NMDA receptor subunit expression

RATIONALE

Previous studies suggested that methionine (Met) levels are decreased in depressed patients. However, whether the decrease in this amino acid is important for phenotypic behaviors associated with depression has not been deciphered.

OBJECTIVE

The response of individuals to chronic stress is variable, with some individuals developing depression and others becoming resilient to stress. In this study, our objective was to examine the effect of Met on susceptibility to stress.

METHODS

Male C57BL/6J mice were subjected to daily defeat sessions by a CD1 aggressor, for 10 days. On day 11, the behavior of mice was assessed using social interaction and open-field tests. Mice received Met 4 h before each defeat session. Epigenetic targets were assessed either through real-rime RTPCR or through Western Blots.

RESULTS

Met did not modulate anxiety-like behaviors, but rather promoted resilience to chronic stress, rescued social avoidance behaviors and reversed the increase in the cortical expression levels of N-methyl-D-aspartate receptor (NMDAR) subunits. Activating NMDAR activity abolished the ability of Met to promote resilience to stress and to rescue social avoidance behavior, whereas inhibiting NMDAR did not show any synergistic or additive protective effects. Indeed, Met increased the cortical levels of the histone methyltransferase SETDB1, and in turn, the levels of the repressive histone H3 lysine (K9) trimethylation (me3).

CONCLUSIONS

Our data indicate that Met rescues susceptibility to stress by inactivating cortical NMDAR activity through an epigenetic mechanism involving histone methylation.

Mid-adolescent stress differentially affects binge-like intake of sucrose across estrous cycles in female rats✰

Binge eating disorder (BED), characterized by excessive food consumption within a discrete period of time, is the most prevalent of all eating disorders, with higher rates in women than men. Chronic stress, particularly during adolescence, is a significant risk factor for BED in women, but the mechanism underlying this relationship remains elusive. We investigated the phenomenon by testing the impact of mid-adolescent intermittent physical stress (IPS) on binge-like intake of sucrose in adult female rats, assessing how the behavior changed across reproductive cycles. One hundred and nineteen Long-Evans rats were exposed to IPS (n = 59) or no stress (NS; n = 60) for 12 days during mid-adolescence (PD35-46). Binge-like eating was induced in adult animals using an intermittent access protocol: animals were provided with 12 h or 24 h access to sucrose, 12 h access to saccharin, or 12 h access to food over 28 days. After 1- or 28-day abstinence, compulsive responding for sucrose was measured using a conditioned suppression paradigm. Rats given 12 h access to sucrose developed binge-like intake, measured as increased consumption during the first hour; the effect was magnified in IPS animals and most pronounced during proestrous. Solution intake in IPS rats was predicted by open arm entries in the elevated plus maze, suggesting that increased risk-taking behavior is associated with greater binge-like eating. IPS blocked conditioned suppression after 28 days of abstinence, pointing to a role of mid-adolescent stress in compulsivity. Collectively, these findings emphasize the impact of stress on the emergence of binge eating in females and suggest that intervention programs for women with a history of adolescent adversity should be investigated as a means to reduce risk for BED.

Role of adolescent-formed, context-drug-associations on reinstatement of drug-seeking behavior in rats

RATIONALE

Drug use during adolescence results in a lifelong risk to develop substance-use disorders. Adolescent rats are less reactive to cocaine-associated cues compared with adults; however, the contribution of adolescent-formed, context-drug-associations to elicit relapse-like behavior is underexplored. Although it is known that social isolation can impact drug-seeking behavior, the effects of housing conditions on context-induced, cocaine-seeking during adolescence vs adulthood are unknown.

OBJECTIVES

The present study compared the effect of adolescent vs adult-formed context-drug associations under different housing conditions (pair vs single) on cocaine-seeking behavior during adolescence or adulthood. This objective was accomplished using operant cocaine self-administration (Coc-SA) under a standard, non-abbreviated (Non-ABRV) or modified abbreviated (ABRV) paradigm.

METHODS

In experiment 1, adolescent and adult rats received Non-ABRV Coc-SA in a distinct context (2 h, 1×/day, 10 days), and extinction training (EXT) in a second context (1 h, 1×/day, 8 days) with reinstatement test (TEST) during adulthood in the cocaine-paired context. In experiments 2 and 3, rats received all behavioral phases during adolescence or adulthood: ABRV Coc-SA (2 h, 2×/day, 5 days), EXT (1 h, 4×/day, 2 days) with TEST in a cocaine-paired or novel, unpaired context. All experiments included pair and single-housing conditions.

RESULTS AND CONCLUSIONS

Age at cocaine exposure did not influence behavior in Non-ABRV or ABRV paradigms. Under Non-ABRV conditions, adolescent and adult single-housed rats had higher seeking behavior than pair housed. These data suggest that social isolation influences context-induced, cocaine-seeking regardless of age at drug exposure and provides a condensed, ABRV paradigm to investigate context-induced, cocaine-seeking behavior during adolescence.

Sex chromosome complement influences vulnerability to cocaine in mice

Women acquire cocaine habits faster and are more motivated to obtain drug than men. In general, female rodents acquire intravenous cocaine self-administration (SA) faster and show greater locomotor responses to cocaine than males. Sex differences are attributed to differences in circulating estradiol. We used the four core genotype (FCG) mouse to ask whether sex chromosome complement influences vulnerability to cocaine's reinforcing and/or locomotor-activating effects. The FCG cross produces ovary-bearing mice with XX or XY genotypes (XXF, XYF) and testes-bearing mice with XX or XY genotypes (XXM, XYM). A greater percentage of gonadal females acquired cocaine SA via infusions into jugular catheters as compared with XYM mice, but XXM mice were not significantly different than any other group. Discrimination of the active versus inactive nose poke holes and cocaine intake were in general greater in gonadal females than in gonadal males. Progressive ratio tests for motivation revealed an interaction between sex chromosomes and gonads: XYM mice were more motivated to self-administer cocaine taking more infusions than mice in any other group. Locomotor responses to cocaine exposure revealed effects of sex chromosomes. After acute exposure, activity was greater in XX than in XY mice and the reverse was true for behavioral sensitization. Mice with XY genotypes displayed more activity than XX mice when given cocaine after a 10-day drug-free period. Our data demonstrate that sex chromosome complement alone and/or interacting with gonadal status can modify cocaine's reinforcing and locomotor-activating effects. These data should inform current studies of sex differences in drug use.

Social interaction reward: A resilience approach to overcome vulnerability to drugs of abuse

Drug addiction is a multifactorial disorder resulting from the complex interaction between biological, environmental and drug-induced effects. Generally, stress is a well-known risk factor for the development of drug addiction and relapse. While most of the research focuses on risk factors that increase the vulnerability to drugs of abuse, recent studies are focusing on the areas of strength/positive coping approaches that can increase resistance to drugs of abuse. In this review, we concentrate on resilience, seen as a dynamic process, which can allow individuals to positively adapt within the context of a specific risk for psychiatric illness. Here, we discuss the effects of social stress in animal models on drug use, particularly cocaine. In contrast, we suggest social interaction reward when available as an alternative to drug use as an approach contracting negative stress effects and increasing resistance to drug use. Indeed, interventions, which aim at enhancing resilience to stress through the facilitation of social interaction and the enhancement of social support, could be particularly effective in helping people cope with stress and preventing drug use problems or relapse. Finally, understanding the neurobiological mechanisms underlying protective factors such as social interaction reward should provide the basis for future evidence-based interventions targeting substance abuse and stress-related pathologies.

Enduring effects of adolescent ketamine exposure on cocaine- and sucrose-induced reward in male and female C57BL/6 mice

Ketamine has shown promising antidepressant efficacy for adolescent treatment-resistant depression. However, the potential enduring consequences of ketamine exposure have not been thoroughly evaluated. Thus, we examined if juvenile ketamine treatment results in long-lasting changes for the rewarding properties of sucrose and cocaine in adulthood, across three separate experiments. In Experiment 1, adolescent male and female C57BL/6 mice received ketamine (20 mg/kg) for 15 consecutive days (Postnatal Day [PD] 35-49). Twenty-one days later (PD70; adulthood) we examined their behavioral responsivity to sucrose (1%) on a two-bottle choice design, or cocaine (0, 5, 10 mg/kg) using the conditioned place preference (CPP) test. We found that juvenile ketamine-pretreatment increased preference for sucrose and environments paired with cocaine in male, but not female, adult mice. This long-term outcome was not observed when male and female mice received ketamine as adults (PD70-84) and tested for sucrose and cocaine preference 21-days later (Experiment 2). Similarly, in Experiment 3, no long-lasting differences in these measures were observed when adolescent male mice were exposed to concomitant ketamine and social stressors (PD35-44), namely the social defeat or vicarious defeat stress paradigms-procedures that mediated a depression-related phenotype (along with a ketamine antidepressant-like response). Collectively, we demonstrate that in the absence of physical or psychological stress, adolescent ketamine exposure increases later life preference for the rewarding properties of sucrose and cocaine in a sex- and age-specific manner. As such, this preclinical work provides awareness for the potential long-term behavioral consequences associated with juvenile ketamine exposure.

Opposite regulation of conditioned place preference and intravenous drug self-administration in rodent models: Motivational and non-motivational examples

Although developed from a common antecedent, conditioned place preference (CPP) and intravenous drug self-administration (SA) represent different behavioral paradigms, each with strong face validity. The field has treated results from these studies largely interchangeably; however, there is considerable evidence of opposite modulation of CPP vs. SA. This review outlines four manipulations that differentially affect CPP and SA based on alterations of motivation. These examples are contrasted with one example of differential CPP and SA results that can be explained by simple parallel shifts in dose-response functions. The final two examples have yet to be classified as motivation-based or parallel shifts. Important aspects, including motivation, volitional control of drug administration, reward, and the role of cues are discussed. One major conclusion of this paper is that explanations for apparent discrepancies between CPP and SA require full dose effect functions and assessment of PR breakpoints. Overall, this manuscript offers a more nuanced insight into how CPP and SA can be used to study different aspects of substance use disorders.

Non-pharmacological factors that determine drug use and addiction

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

Ibogaine Administration Modifies GDNF and BDNF Expression in Brain Regions Involved in Mesocorticolimbic and Nigral Dopaminergic Circuits

Ibogaine is an atypical psychedelic alkaloid, which has been subject of research due to its reported ability to attenuate drug-seeking behavior. Recent work has suggested that ibogaine effects on alcohol self-administration in rats are related to the release of Glial cell Derived Neurotrophic Factor (GDNF) in the Ventral Tegmental Area (VTA), a mesencephalic region which hosts the soma of dopaminergic neurons. Although previous reports have shown ibogaine’s ability to induce GDNF expression in rat midbrain, there are no studies addressing its effect on the expression of GDNF and other neurotrophic factors (NFs) such as Brain Derived Neurotrophic Factor (BDNF) or Nerve Growth Factor (NGF) in distinct brain regions containing dopaminergic neurons. In this work, we examined the effect of ibogaine acute administration on the expression of these NFs in the VTA, Prefrontal Cortex (PFC), Nucleus Accumbens (NAcc) and the Substantia Nigra (SN). Rats were i.p. treated with ibogaine 20 mg/kg (I₂₀), 40 mg/kg (I₄₀) or vehicle, and NFs expression was analyzed after 3 and 24 h. At 24 h an increase of the expression of the NFs transcripts was observed in a site and dose dependent manner. Only for I₄₀, GDNF was selectively upregulated in the VTA and SN. Both doses elicited a large increase in the expression of BDNF transcripts in the NAcc, SN and PFC, while in the VTA a significant effect was found only for I₄₀. Finally, NGF mRNA was upregulated in all regions after I₄₀, while I₂₀ showed a selective upregulation in PFC and VTA. Regarding protein levels, an increase of GDNF was observed in the VTA only for I₄₀ but no significant increase for BDNF was found in all the studied areas. Interestingly, an increase of proBDNF was detected in the NAcc for both doses. These results show for the first time a selective increase of GDNF specifically in the VTA for I₄₀ but not for I₂₀ after 24 h of administration, which agrees with the effective dose found in previous self-administration studies in rodents. Further research is needed to understand the contribution of these changes to ibogaine’s ability to attenuate drug-seeking behavior.

Oxytocin prevents the increase of cocaine-related responses produced by social defeat

The neuropeptide oxytocin (OXT) plays a critical role in the regulation of social and emotional behaviors. OXT plays a role in stress response and in drug reward, but to date no studies have evaluated its implication in the long-lasting increase of the motivational effects of cocaine induced by repeated social defeat (RSD). During the social defeat procedure, 1 mg/kg of OXT was administered 30 min before each episode of RSD. Three weeks after the last defeat, the effects of cocaine on the conditioned place preference (CPP), locomotor sensitization and the self-administration (SA) paradigms were evaluated. The influence of OXT on the levels of BDNF in the prefrontal cortex (PFC), striatum and hippocampus was also measured. Our results confirm that raising the levels of OXT during social defeat stress can block the long-lasting effects of this type of stress. OXT counteracts the anxiety induced by social defeat and modifies BDNF levels in all the structures we have studied. Moreover, OXT prevents RSD-induced increases in the motivational effects of cocaine. Administration of OXT before each social defeat blocked the social defeat-induced increment in the conditioned rewarding effects of cocaine in the CPP, favored the extinction of cocaine-associated memories in both the CPP and SA, and decreased reinstatement of cocaine-seeking behavior in the SA. In conclusion, the long-lasting effects of RSD are counteracted by administering OXT prior to stress, and changes in BDNF expression may underlie these protective effects.

Social stress during adolescence activates long-term microglia inflammation insult in reward processing nuclei

The experience of social stress during adolescence is associated with higher vulnerability to drug use. Increases in the acquisition of cocaine self-administration, in the escalation of cocaine-seeking behavior, and in the conditioned rewarding effects of cocaine have been observed in rodents exposed to repeated social defeat (RSD). In addition, prolonged or severe stress induces a proinflammatory state with microglial activation and increased cytokine production. The aim of the present work was to describe the long-term effects induced by RSD during adolescence on the neuroinflammatory response and synaptic structure by evaluating different glial and neuronal markers. In addition to an increase in the conditioned rewarding effects of cocaine, our results showed that RSD in adolescence produced inflammatory reactivity in microglia that is prolonged into adulthood, affecting astrocytes and neurons of two reward-processing areas of the brain (the prelimbic cortex, and the nucleus accumbens core). Considered as a whole these results suggest that social stress experience modulates vulnerability to suffer a loss of glia-supporting functions and neuronal functional synaptic density due to drug consumption in later life.

Adolescent social isolation increases cocaine seeking in male and female mice

Childhood and adolescent adversity are associated with a wide range of psychiatric disorders, including an increased risk for substance abuse. Despite this, the mechanisms underlying the ability of chronic stress during adolescence to alter reward signaling remains largely unexplored. Understanding how adolescent stress increases addiction-like phenotypes could inform the development of targeted interventions both before and after drug use. The current study examined how prolonged isolation stress, beginning during adolescence, affected behavioral and neuronal underpinnings to the response to cocaine in male and female mice. Adolescent-onset social isolation did not alter the ability of mice to learn an operant response for food, nor influence food self-administration or motivation for food on a progressive ratio schedule. However, male and female social isolation mice exhibited an increase in motivation for cocaine and cocaine seeking during a cue-induced reinstatement session. Additionally, we demonstrated that adolescent-onset social isolation increased cocaine-induced neuronal activation, as assessed by c-Fos expression, within the nucleus accumbens core and shell, ventral pallidum, dorsal bed nucleus of the stria terminalis, lateral septum and basolateral amygdala. Taken together, the present studies demonstrate that social isolation stress during adolescence augments the behavioral responses to cocaine during adulthood and alters the responsiveness of reward-related brain circuitry.

Social defeat stress and escalation of cocaine and alcohol consumption: Focus on CRF

Both the ostensibly aversive effects of unpredictable episodes of social stress and the intensely rewarding effects of drugs of abuse activate the mesocorticolimbic dopamine systems. Significant neuroadaptations in interacting stress and reward neurocircuitry may underlie the striking connection between stress and substance use disorders. In rodent models, recurring intermittent exposure to social defeat stress appears to produce a distinct profile of neuroadaptations that translates most readily to the repercussions of social stress in humans. In the present review, preclinical rodent models of social defeat stress and subsequent alcohol, cocaine or opioid consumption are discussed with regard to: (1) the temporal pattern of social defeat stress, (2) male and female protocols of social stress-escalated drug consumption, and (3) the neuroplastic effects of social stress, which may contribute to escalated drug-taking. Neuroadaptations in corticotropin-releasing factor (CRF) and CRF modulation of monoamines in the ventral tegmental area and the bed nucleus of the stria terminalis are highlighted as potential mechanisms underlying stress-escalated drug consumption. However, the specific mechanisms that drive CRF-mediated increases in dopamine require additional investigation as do the stress-induced neuroadaptations that may contribute to the development of compulsive patterns of drug-taking.

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.

Animal Models of the Impact of Social Stress on Cocaine Use Disorders

Cocaine use disorders are strongly influenced by the social conditions prior, during, and after exposure to cocaine. In this chapter, we discuss how social factors such as early life stress, social rank stress, and environmental stress impact vulnerability and resilience to cocaine. The discussion of each animal model begins with a brief review of examples from the human literature, which provide the psychosocial background these models attempt to capture. We then discuss preclinical findings from use of each model, with emphasis on how social factors influence cocaine-related behaviors and how sex and age influence the behaviors and neurobiology. Models discussed include (1) early life social stress, such as maternal separation and neonatal isolation, (2) social defeat stress, (3) social hierarchies, and (4) social isolation and environmental enrichment. The cocaine-related behaviors reviewed for each of these animal models include cocaine-induced conditioned place preference, behavioral sensitization, and self-administration. Together, our review suggests that the degree of psychosocial stress experienced yields robust effects on cocaine-related behaviors and neurobiology, and these preclinical findings have translational impact for the future of cocaine use disorder treatment.

Negative consequences of early-life adversity on substance use as mediated by corticotropin-releasing factor modulation of serotonin activity

Early-life adversity is associated with increased risk for substance abuse in later life, with women more likely to report past and current stress as a mediating factor in their substance use and relapse as compared to men. Preclinical models of neonatal and peri-adolescent (early through late adolescence) stress all support a direct relationship between experiences of early-life adversity and adult substance-related behaviors, and provide valuable information regarding the underlying neurobiology. This review will provide an overview of these animal models and how these paradigms alter drug and alcohol consumption and/or seeking in male and female adults. An introduction to the corticotropin-releasing factor (CRF) and serotonin systems, their development and their interactions at the level of the dorsal raphe will be provided, illustrating how this particular stress system is sexually dimorphic, and is well positioned to be affected by stressors early in development and throughout maturation. A model for CRF-serotonin interactions in the dorsal raphe and how these influence dopaminergic activity within the nucleus accumbens and subsequent reward-associated behaviors will be provided, and alterations to the activity of this system following early-life adversity will be identified. Overall, converging findings suggest that early-life adversity has long-term effects on the functioning of the CRF-serotonin system, highlighting a potentially important and targetable mediator linking stress to addiction. Future work should focus on identifying the exact mechanisms that promote long-term changes to the expression and activity of CRF receptors in the dorsal raphe. Moreover, it is important to clarify whether similar neurobiological mechanisms exist for males and females, given the sexual dimorphism both in CRF receptors and serotonin indices in the dorsal raphe and in the behavioral outcomes of early-life adversity.

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Early life stress increases risk for substance abuse in adulthood.

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Stress and drugs increase CRF which alters serotonin release in the brain.

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CRF₂ receptor expression in the dorsal raphe is altered by early life stress.

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Resultant changes to serotonin output facilitates dopamine in the accumbens.

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CRF₂-sertotonin-dopamine interactions may link early life stress with substance abuse.

Enhanced dopamine D2 autoreceptor function in the adult prefrontal cortex contributes to dopamine hypoactivity following adolescent social stress

Adult psychiatric disorders characterized by cognitive deficits reliant on prefrontal cortex (PFC) dopamine are promoted by teenage bullying. Similarly, male Sprague-Dawley rats exposed to social defeat in mid-adolescence (P35-39) show impaired working memory in adulthood (P56-70), along with decreased medial PFC (mPFC) dopamine activity that results in part from increased dopamine transporter-mediated clearance. Here, we determined if dopamine synthesis and D2 autoreceptor-mediated inhibition of dopamine release in the adult mPFC are also enhanced by adolescent defeat to contribute to later dopamine hypofunction. Control and previously defeated rats did not differ in either DOPA accumulation following amino acid decarboxylase inhibition (NSD-1015 100 mg/kg ip.) or total/phosphorylated tyrosine hydroxylase protein expression, suggesting dopamine synthesis in the adult mPFC is not altered by adolescent defeat. However, exposure to adolescent defeat caused greater decreases in extracellular dopamine release (measured using in vivo chronoamperometry) in the adult mPFC upon local infusion of the D2 receptor agonist quinpirole (3 nM), implying greater D2 autoreceptor function. Equally enhanced D2 autoreceptor-mediated inhibition of dopamine release is seen in the adolescent (P40 or P49) mPFC, which declines in control rats by adulthood. However, this developmental decrease in autoreceptor function is absent following adolescent defeat, suggesting retention of an adolescent-like phenotype into adulthood. Current and previous findings indicate adolescent defeat decreases extracellular dopamine availability in the adult mPFC via both enhanced inhibition of dopamine release and increased dopamine clearance, which may be viable targets for improving treatment of cognitive deficits seen in neuropsychiatric disorders promoted by adolescent stress.

Alcohol, psychomotor-stimulants and behaviour: methodological considerations in preclinical models of early-life stress

BACKGROUND

In order to assess the risk associated with early-life stress, there has been an increase in the amount of preclinical studies investigating early-life stress. There are many challenges associated with investigating early-life stress in animal models and ensuring that such models are appropriate and clinically relevant.

OBJECTIVES

The purpose of this review is to highlight the methodological considerations in the design of preclinical studies investigating the effects of early-life stress on alcohol and psychomotor-stimulant intake and behaviour.

METHODS

The protocols employed for exploring early-life stress were investigated and summarised. Experimental variables include animals, stress models, and endpoints employed.

RESULTS

The findings in this paper suggest that there is little consistency among these studies and so the interpretation of these results may not be as clinically relevant as previously thought.

CONCLUSION

The standardisation of these simple stress procedures means that results will be more comparable between studies and that results generated will give us a more robust understanding of what can and may be happening in the human and veterinary clinic.

A novel adolescent chronic social defeat model: reverse-Resident-Intruder Paradigm (rRIP) in male rats

Psychosocial stress is linked to the etiology of several neuropsychiatric disorders, including Major Depressive Disorder and Post-Traumatic-Stress-Disorder. Adolescence is a critical neurobehavioral developmental period wherein the maturing nervous system is sensitive to stress-related psychosocial events. The effects of social defeat stress, an animal model of psychosocial stress, on adolescent neurobehavioral phenomena are not well explored. Using the standard Resident-Intruder-Paradigm (RIP), adolescent Long-Evans (LE, residents, n = 100) and Sprague-Dawley (SD, intruders, n = 100) rats interacted for five days to invoke chronic social stress. Tests of depressive behavior (forced-swim-test (FST)), fear conditioning, and long-term synaptic plasticity are affected in various adult rodent chronic stress models, thus we hypothesized that these phenomena would be similarly affected in adolescent rats. Serendipitously, we observed the Intruders became the dominant rats and the Residents were the defeated/submissive rats. This robust and reliable role-reversal resulted in defeated LE-Residents showing a depressive-like state (increased time spent immobile in the FST), enhanced fear conditioning in both hippocampal-dependent and hippocampal-independent fear paradigms and altered hippocampal long-term synaptic plasticity, measured electrophysiologically in vitro in hippocampal slices. Importantly, SD-Intruders, SD and LE controls did not significantly differ from each other in any of these assessments. This reverse-Resident-Intruder-Paradigm (rRIP) represents a novel animal model to study the effects of stress on adolescent neurobehavioral phenomenon.

Antagonism of corticotropin-releasing factor CRF1 receptors blocks the enhanced response to cocaine after social stress

Numerous studies have shown that social defeat stress induces an increase in the rewarding effects of cocaine. In this study we have investigated the role played by the main hypothalamic stress hormone, corticotropin-releasing factor (CRF), in the effects that repeated social defeat (RSD) induces in the conditioned rewarding effects and locomotor sensitization induced by cocaine. A total of 220 OF1 mice were divided into experimental groups according to the treatment received before each social defeat: saline, 5 or 10 mg/kg of the nonpeptidic corticotropin-releasing factor CRF₁ receptor antagonist CP-154,526, or 15 or 30 µg/kg of the peptidic corticotropin-releasing factor CRF₂ receptor antagonist Astressin₂-B. Three weeks after the last defeat, conditioned place preference (CPP) induced by 1 mg/kg of cocaine was evaluated. Motor response to 10 mg/kg of cocaine was also studied after a sensitization induction. Blockade of corticotropin-releasing factor CRF₁ receptor reversed the increase in cocaine CPP induced by social defeat. Conversely, peripheral corticotropin-releasing factor CRF₂ receptor blockade produced similar effects to those observed in socially stressed animals. The effect of RSD on cocaine sensitization was again blocked by the corticotropin-releasing factor CRF₁ receptor antagonist, while peripheral CRF₂ receptor antagonist did not show effect. Acute administration of Astressin₂-B induced an anxiogenic response. Our results confirm that CRF modulates the effects of social stress on reinforcement and sensitization induced by cocaine in contrasting ways. These findings highlight CRF receptors as potential therapeutic targets to be explored by research about stress-related addiction problems.

Adolescent Corticosterone and TrkB Pharmaco-Manipulations Sex-Dependently Impact Instrumental Reversal Learning Later in Life

Early-life trauma can increase the risk for, and severity of, several psychiatric illnesses. These include drug use disorders, and some correlations appear to be stronger in women. Understanding the long-term consequences of developmental stressor or stress hormone exposure and possible sex differences is critically important. So-called “reversal learning” tasks are commonly used in rodents to model cognitive deficits in stress- and addiction-related illnesses in humans. Here, we exposed mice to the primary stress hormone corticosterone (CORT) during early adolescence (postnatal days 31–42), then tested behavioral flexibility in adulthood using an instrumental reversal learning task. CORT-exposed female, but not male, mice developed perseverative errors. Despite resilience to subchronic CORT exposure, males developed reversal performance impairments following exposure to physical stressors. Administration of a putative tyrosine kinase receptor B (trkB) agonist, 7,8-dihydroxyflavone (7,8-DHF), during adolescence blocked CORT-induced errors in females and improved performance in males. Conversely, blockade of trkB by ANA-12 impaired performance. These data suggest that trkB-based interventions could have certain protective benefits in the context of early-life stressor exposure. We consider the implications of our findings in an extended “Discussion” section.

Impact of juvenile chronic stress on adult cortico-accumbal function: Implications for cognition and addiction

Repeated exposure to stress during childhood is associated with increased risk for neuropsychiatric illness, substance use disorders and other behavioral problems in adulthood. However, it is not clear how chronic childhood stress can lead to emergence of such a wide range of symptoms and disorders in later life. One possible explanation lies in stress-induced disruption to the development of specific brain regions associated with executive function and reward processing, deficits in which are common to the disorders promoted by childhood stress. Evidence of aberrations in prefrontal cortex and nucleus accumbens function following repeated exposure of juvenile (pre- and adolescent) organisms to a variety of different stressors would account not only for the similarity in symptoms across the wide range of childhood stress-associated mental illnesses, but also their persistence into adulthood in the absence of further stress. Therefore, the goal of this review is to evaluate the current knowledge regarding disruption to executive function and reward processing in adult animals or humans exposed to chronic stress over the juvenile period, and the underlying neurobiology, with particular emphasis on the prefrontal cortex and nucleus accumbens. First, the role of these brain regions in mediating executive function and reward processing is highlighted. Second, the neurobehavioral development of these systems is discussed to illustrate how juvenile stress may exert long-lasting effects on prefrontal cortex-accumbal activity and related behavioral functions. Finally, a critical review of current animal and human findings is presented, which strongly supports the supposition that exposure to chronic stress (particularly social aggression and isolation in animal studies) in the juvenile period produces impairments in executive function in adulthood, especially in working memory and inhibitory control. Chronic juvenile stress also results in aberrations to reward processing and seeking, with increased sensitivity to drugs of abuse particularly noted in animal models, which is in line with greater incidence of substance use disorders seen in clinical studies. These consequences are potentially mediated by monoamine and glutamatergic dysfunction in the prefrontal cortex and nucleus accumbens, providing translatable therapeutic targets. However, the predominant use of male subjects and social-based stressors in preclinical studies points to a clear need for determining how both sex differences and stressor heterogeneity may differentially contribute to stress-induced changes to substrates mediating executive function and reward processing, before the impact of chronic juvenile stress in promoting adult psychopathology can be fully understood.

Epigenetics: a link between addiction and social environment

The detrimental effects of drug abuse are apparently not limited to individuals but may also impact the vulnerability of their progenies to develop addictive behaviours. Epigenetic signatures, early life experience and environmental factors, converge to influence gene expression patterns in addiction phenotypes and consequently may serve as mediators of behavioural trait transmission between generations. The majority of studies investigating the role of epigenetics in addiction do not consider the influence of social interactions. This shortcoming in current experimental approaches necessitates developing social models that reflect the addictive behaviour in a free-living social environment. Furthermore, this review also reports on the advancement of interventions for drug addiction and takes into account the emerging roles of histone deacetylase (HDAC) inhibitors in the etiology of drug addiction and that HDAC may be a potential therapeutic target at nucleosomal level to improve treatment outcomes.

Repeated social defeat and the rewarding effects of cocaine in adult and adolescent mice: dopamine transcription factors, proBDNF signaling pathways, and the TrkB receptor in the mesolimbic system

RATIONALE

Repeated social defeat (RSD) increases the rewarding effects of cocaine in adolescent and adult rodents.

OBJECTIVE

The aim of the present study was to compare the long-term effects of RSD on the conditioned rewarding effects of cocaine and levels of the transcription factors Pitx3 and Nurr1 in the ventral tegmental area (VTA), the dopamine transporter (DAT), the D2 dopamine receptor (D2DR) and precursor of brain-derived neurotrophic factor (proBDNF) signaling pathways, and the tropomyosin-related kinase B (TrkB) receptor in the nucleus accumbens (NAc) in adult and adolescent mice.

METHODS

Male adolescent and young adult OF1 mice were exposed to four episodes of social defeat and were conditioned 3 weeks later with 1 mg/kg of cocaine. In a second set of mice, the expressions of the abovementioned dopaminergic and proBDNF and TrkB receptor were measured in VTA and NAc, respectively.

RESULTS

Adolescent mice experienced social defeats less intensely than their adult counterparts and produced lower levels of corticosterone. However, both adult and adolescent defeated mice developed conditioned place preference for the compartment associated with this low dose of cocaine. Furthermore, only adolescent defeated mice displayed diminished levels of the transcription factors Pitx3 in the VTA, without changes in the expression of DAT and D2DR in the NAc. In addition, stressed adult mice showed a decreased expression of proBDNF and the TrkB receptor, while stressed adolescent mice exhibited increased expression of latter without changes in the former.

CONCLUSION

Our findings suggest that dopaminergic pathways and proBDNF signaling and TrkB receptors play different roles in social defeat-stressed mice exposed to cocaine.

Modeling the development of drug addiction in male and female animals

An increasing emphasis has been placed on the development and use of animal models of addiction that capture defining features of human drug addiction, including escalation/binge drug use, enhanced motivation for the drug, preference for the drug over other reward options, use despite negative consequences, and enhanced drug-seeking/relapse vulnerability. The need to examine behavior in both males and females has also become apparent given evidence demonstrating that the addiction process occurs differently in males and females. This review discusses the procedures that are used to model features of addiction in animals, as well as factors that influence their development. Individual differences are also discussed, with a particular focus on sex differences. While no one procedure consistently produces all characteristics, different models have been developed to focus on certain characteristics. A history of escalating/binge patterns of use appears to be critical for producing other features characteristic of addiction, including an enhanced motivation for the drug, enhanced drug seeking, and use despite negative consequences. These characteristics tend to emerge over abstinence, and appear to increase rather than decrease in magnitude over time. In females, these characteristics develop sooner during abstinence and/or following less drug exposure as compared to males, and for psychostimulant addiction, may require estradiol. Although preference for the drug over other reward options has been demonstrated in non-human primates, it has been more difficult to establish in rats. Future research is needed to define the parameters that optimally induce each of these features of addiction in the majority of animals. Such models are essential for advancing our understanding of human drug addiction and its treatment in men and women.

Translational relevance of rodent models of hypothalamic-pituitary-adrenal function and stressors in adolescence

Elevations in glucocorticoids that result from environmental stressors can have programming effects on brain structure and function when the exposure occurs during sensitive periods that involve heightened neural development. In recent years, adolescence has gained increasing attention as another sensitive period of development, a period in which pubertal transitions may increase the vulnerability to stressors. There are similarities in physical and behavioural development between humans and rats, and rats have been used effectively as an animal model of adolescence and the unique plasticity of this period of ontogeny. This review focuses on benefits and challenges of rats as a model for translational research on hypothalamic-pituitary-adrenal (HPA) function and stressors in adolescence, highlighting important parallels and contrasts between adolescent rats and humans, and we review the main stress procedures that are used in investigating HPA stress responses and their consequences in adolescence in rats. We conclude that a greater focus on timing of puberty as a factor in research in adolescent rats may increase the translational relevance of the findings.

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.

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.

`Up-regulation of histone acetylation induced by social defeat mediates the conditioned rewarding effects of cocaine

Social defeat (SD) induces a long-lasting increase in the rewarding effects of psychostimulants measured using the self-administration and conditioned place procedures (CPP). However, little is known about the epigenetic changes induced by social stress and about their role in the increased response to the rewarding effects of psychostimulants. Considering that histone acetylation regulates transcriptional activity and contributes to drug-induced behavioral changes, we addressed the hypothesis that SD induces transcriptional changes by histone modifications associated with the acquisition of place conditioning. After a fourth defeat, H3(K9) acetylation was decreased in the hippocampus, while there was an increase of HAT and a decrease of HDAC levels in the cortex. Three weeks after the last defeat, mice displayed an increase in histone H4(K12) acetylation and an upregulation of histone acetyl transferase (HAT) activity in the hippocampus. In addition, H3(K4)me3, which is closely associated with transcriptional initiation, was also augmented in the hippocampus three weeks after the last defeat. Inhibition of HAT by curcumin (100mg/kg) before each SD blocked the increase in the conditioned reinforcing effects of 1mg/kg of cocaine, while inhibition of HDAC by valproic acid (500mg/kg) before social stress potentiated cocaine-induced CPP. Preference was reinstated when animals received a priming dose of 0.5mg/kg of cocaine, an effect that was absent in untreated defeated mice. These results suggest that the experience of SD induces chromatin remodeling, alters histone acetylation and methylation, and modifies the effects of cocaine on place conditioning. They also point to epigenetic mechanisms as potential avenues leading to new treatments for the long-term effects of social stress on drug addiction.

CRF type 1 receptor antagonism in ventral tegmental area of adolescent rats during social defeat: prevention of escalated cocaine self-administration in adulthood and behavioral adaptations during adolescence

BACKGROUND

Activation of corticotropin-releasing factor type 1 receptors (CRF-R1) in the ventral tegmental area (VTA) represents a critical mechanism for social defeat to escalate cocaine self-administration in adult rats.

OBJECTIVE

We determined the acute effect of a CRF-R1 antagonist (CP376395) microinfusion into the VTA prior to each episode of social defeat in adolescent rats and determined whether this drug treatment could prevent later escalation of cocaine taking in early adulthood.

METHODS

Rats were implanted with bilateral cannulae aimed at the VTA 5 days before the first social defeat. Bilateral microinfusion of CP376395 (500 ng/side) or vehicle occurred 20 min before each episode of social defeat on postnatal days (P) 35, 38, 41, and 44. Behavior was quantified on P35 and P44. On P57, rats were implanted with intra-jugular catheters, and subsequent cocaine self-administration was analyzed.

RESULTS

CP376395-treated adolescent rats walked less and were attacked more slowly but were socially investigated more than vehicle-treated adolescents. Vehicle-treated rats showed increased social and decreased non-social exploration from P35 to P44, while CP376395-treated rats did not. Socially defeated, vehicle-treated adolescents took more cocaine during a 24-h unlimited access binge during adulthood. The latency to supine posture on P44 was inversely correlated with later cocaine self-administration during fixed and progressive ratio schedules of reinforcement and during the binge.

CONCLUSIONS

CP376395 treatment in adolescence blocked escalation of cocaine taking in adulthood. Episodes of social defeat stress engender neuroadaptation in CRF-R1s in the VTA that alter coping with social stress and that persist into adulthood.

Depression and substance use comorbidity: What we have learned from animal studies

Depression and substance use disorders are often comorbid, but the reasons for this are unclear. In human studies, it is difficult to determine how one disorder may affect predisposition to the other and what the underlying mechanisms might be. Instead, animal studies allow experimental induction of behaviors relevant to depression and drug-taking, and permit direct interrogation of changes to neural circuits and molecular pathways. While this field is still new, here we review animal studies that investigate whether depression-like states increase vulnerability to drug-taking behaviors. Since chronic psychosocial stress can precipitate or predispose to depression in humans, we review studies that use psychosocial stressors to produce depression-like phenotypes in animals. Specifically, we describe how postweaning isolation stress, repeated social defeat stress, and chronic mild (or unpredictable) stress affect behaviors relevant to substance abuse, especially operant self-administration. Potential brain changes mediating these effects are also discussed where available, with an emphasis on mesocorticolimbic dopamine circuits. Postweaning isolation stress and repeated social defeat generally increase acquisition or maintenance of drug self-administration, and alter dopamine sensitivity in various brain regions. However, the effects of chronic mild stress on drug-taking have been much less studied. Future studies should consider standardizing stress-induction protocols, including female subjects, and using multi-hit models (e.g. genetic vulnerabilities and environmental stress).

Adolescent Social Stress Produces an Enduring Activation of the Rat Locus Coeruleus and Alters its Coherence with the Prefrontal Cortex

Early life stress is associated with the development of psychiatric disorders. Because the locus coeruleus-norepinephrine (LC-NE) system is a major stress-response system that is implicated in psychopathology, developmental differences in the response of this system to stress may contribute to increased vulnerability. Here LC single unit and network activity were compared between adult and adolescent rats during resident-intruder stress. In some rats, LC and medial prefrontal cortex (mPFC) coherence was quantified. The initial stress tonically activated LC neurons and induced theta oscillations, while simultaneously decreasing LC auditory-evoked responses in both age groups. Stress increased LC-mPFC coherence within the theta range. With repeated exposures, adolescent LC neuronal and network activity remained elevated even in the absence of the stressor and were unresponsive to stressor presentation. In contrast, LC neurons of adult rats exposed to repeated social stress were relatively inhibited in the absence of the stressor and mounted robust responses upon stressor presentation. LC sensory-evoked responses were selectively blunted in adolescent rats exposed to repeated social stress. Finally, repeated stress decreased LC-mPFC coherence in the high frequency range (beta and gamma) while maintaining strong coherence in the theta range, selectively in adolescents. Together, these results suggest that adaptive mechanisms that promote stress recovery and maintain basal activity of the brain norepinephrine system in the absence of stress are not fully developed or are vulnerable stress-induced impairments in adolescence. The resulting sustained activation of the LC-NE system after repeated social stress may adversely impact cognition and future social behavior of adolescents.

Wheel running exercise attenuates vulnerability to self-administer nicotine in rats

BACKGROUND

Preventing or postponing tobacco use initiation could greatly reduce the number of tobacco-related deaths. While evidence suggests that exercise is a promising treatment for tobacco addiction, it is not clear whether exercise could prevent initial vulnerability to tobacco use. Thus, using an animal model, we examined whether exercise attenuates vulnerability to the use and reinforcing effects of nicotine, the primary addictive chemical in tobacco.

METHODS

Initial vulnerability was assessed using an acquisition procedure wherein exercising (unlocked running wheel, n=10) and sedentary (locked or no wheel, n=12) male adolescent rats had access to nicotine infusions (0.01-mg/kg) during daily 21.5-h sessions beginning on postnatal day 30. Exercise/sedentary sessions (2-h/day) were conducted prior to each of the acquisition sessions. The effects of exercise on nicotine's reinforcing effects were further assessed in separate groups of exercising (unlocked wheel, n=7) and sedentary (no wheel, n=5) rats responding for nicotine under a progressive-ratio schedule with exercise/sedentary sessions (2-h/day) conducted before the daily progressive-ratio sessions.

RESULTS

While high rates of acquisition of nicotine self-administration were observed among both groups of sedentary controls, acquisition was robustly attenuated in the exercise group with only 20% of exercising rats meeting the acquisition criterion within the 16-day testing period as compared to 67% of the sedentary controls. Exercise also decreased progressive-ratio responding for nicotine as compared to baseline and to sedentary controls.

CONCLUSIONS

Exercise may effectively prevent the initiation of nicotine use in adolescents by reducing the reinforcing effects of nicotine.