Effects of repeated social defeat on adolescent mice on cocaine-induced CPP and self-administration in adulthood: integrity of the blood-brain barrier

A gut (microbiome) feeling about addiction: Interactions with stress and social systems

In recent years, an increasing attention has given to the intricate and diverse connection of microorganisms residing in our gut and their impact on brain health and central nervous system disease. There has been a shift in mindset to understand that drug addiction is not merely a condition that affects the brain, it is now being recognized as a disorder that also involves external factors such as the intestinal microbiota, which could influence vulnerability and the development of addictive behaviors. Furthermore, stress and social interactions, which are closely linked to the intestinal microbiota, are powerful modulators of addiction. This review delves into the mechanisms through which the microbiota-stress-immune axis may shape drug addiction and social behaviors. This work integrates preclinical and clinical evidence that demonstrate the bidirectional communication between stress, social behaviors, substance use disorders and the gut microbiota, suggesting that gut microbes might modulate social stress having a significance in drug addiction.

Gains and Losses: Resilience to Social Defeat Stress in Adolescent Female Mice

BACKGROUND

Adolescence is a unique period of psychosocial growth during which social adversity can negatively influence mental health trajectories. Understanding how adolescent social stress impacts males and females and why some individuals are particularly affected is becoming increasingly urgent. Social defeat stress models for adolescent male mice have been effective in reproducing some physical/psychological aspects of bullying. Designing a model suitable for females has proven challenging.

METHODS

We report a version of the adolescent male accelerated social defeat stress (AcSD) paradigm adapted for females. Early adolescent C57BL/6J female mice (N = 107) were exposed to our modified AcSD procedure twice a day for 4 days and categorized as resilient or susceptible based on a social interaction test 24 hours later. Mice were then assessed for changes in Netrin-1/DCC guidance cue expression in dopamine systems, for inhibitory control in adulthood using the Go/No-Go task, or for alterations in dopamine connectivity organization in the matured prefrontal cortex.

RESULTS

Most adolescent females showed protection against stress-induced social avoidance, but in adulthood, these resilient females developed inhibitory control deficits and showed diminution of prefrontal cortex presynaptic dopamine sites. Female mice classified as susceptible were protected against cognitive and dopaminergic alterations. AcSD did not alter Netrin-1/DCC in early adolescent females, contrary to previous findings with males.

CONCLUSIONS

Preserving prosocial behavior in adolescent females may be important for survival advantage but seems to come at the price of developing persistent cognitive and dopamine deficiencies. The female AcSD paradigm produced findings comparable to those found in males, allowing mechanistic investigation in both sexes.

Dopamine D3 Receptor Modulates Akt/mTOR and ERK1/2 Pathways Differently during the Reinstatement of Cocaine-Seeking Behavior Induced by Psychological versus Physiological Stress

Stress triggers relapses in cocaine use that engage the activity of memory-related nuclei, such as the basolateral amygdala (BLA) and dentate gyrus (DG). Preclinical research suggests that D3 receptor (D3R) antagonists may be a promising means to attenuate cocaine reward and relapse. As D3R regulates the activity of the Akt/mTOR and MEK/ERK_1/2 pathways, we assessed the effects of SB-277011-A, a D3R antagonist, on the activity of these kinases during the reinstatement of cocaine-induced conditioned place preference (CPP) induced by psychological (restraint) and physiological (tail pinch) stress. Both stimuli reactivated an extinguished cocaine-CPP, but only restrained animals decreased their locomotor activity during reinstatement. Cocaine-seeking behavior reactivation was correlated with decreased p-Akt, p-mTOR, and p-ERK_1/2 activation in both nuclei of restrained animals. While a D3R blockade prevented stress-induced CPP reinstatement and plasma corticosterone enhancement, SB-277011-A distinctly modulated Akt, mTOR, and ERK_1/2 activation depending on the stressor and the dose used. Our data support the involvement of corticosterone in the SB-277011-A effects in restrained animals. Additionally, the ratios p-mTOR/mTOR and/or p-ERK_1/2 /ERK_1/2 in the BLA during stress-induced relapse seem to be related to the locomotor activity of animals receiving 48 mg/kg of the antagonist. Hence, our study indicates the D3R antagonist's efficacy to prevent stress-induced relapses in drug use through distinct modulation of Akt/mTOR and MEK/ERK_1/2 pathways in memory-processing nuclei.

Oleoylethanolamide restores stress-induced prepulse inhibition deficits and modulates inflammatory signaling in a sex-dependent manner

RATIONALE

Social stress contributes to the development of depressive and anxiety symptomatology and promotes pro-inflammatory signaling in the central nervous system. In this study, we explored the effects of a lipid messenger with anti-inflammatory properties - oleoylethanolamide (OEA) - on the behavioral deficits caused by social stress in both male and female mice.

METHODS

Adult mice were assigned to an experimental group according to the stress condition (control or stress) and treatment (vehicle or OEA, 10 mg/kg, i.p.). Male mice in the stress condition underwent a protocol consisting of four social defeat (SD) encounters. In the case of female mice, we employed a procedure of vicarious SD. After the stress protocol resumed, anxiety, depressive-like behavior, social interaction, and prepulse inhibition (PPI) were assessed. In addition, we characterized the stress-induced inflammatory profile by measuring IL-6 and CX3CL1 levels in the striatum and hippocampus.

RESULTS

Our results showed that both SD and VSD induced behavioral alterations. We found that OEA treatment restored PPI deficits in socially defeated mice. Also, OEA affected differently stress-induced anxiety and depressive-like behavior in male and female mice. Biochemical analyses showed that both male and female stressed mice showed increased levels of IL-6 in the striatum compared to control mice. Similarly, VSD female mice exhibited increased striatal CX3CL1 levels. These neuroinflammation-associated signals were not affected by OEA treatment.

CONCLUSIONS

In summary, our results confirm that SD and VSD induced behavioral deficits together with inflammatory signaling in the striatum and hippocampus. We observed that OEA treatment reverses stress-induced PPI alterations in male and female mice. These data suggest that OEA can exert a buffering effect on stress-related sensorimotor gating behavioral processing.

Resilience to the short- and long-term behavioral effects of intermittent repeated social defeat in adolescent male mice

BACKGROUND

Exposure to intermittent repeated social defeat (IRSD) increases the sensitivity of mice to the rewarding effects of cocaine in the conditioned place preference (CPP) paradigm. Some animals are resilient to this effect of IRSD, though research exploring this inconsistency in adolescent mice is scarce. Thus, our aim was to characterize the behavioral profile of mice exposed to IRSD during early adolescence and to explore a potential association with resilience to the short- and long-term effects of IRSD.

METHODS

Thirty-six male C57BL/6 mice were exposed to IRSD during early adolescence (PND 27, 30, 33 and 36), while another 10 male mice did not undergo stress (controls). Defeated mice and controls then carried out the following battery of behavioral tests; the Elevated Plus Maze, Hole-Board and Social Interaction Test on PND 37, and the Tail Suspension and Splash tests on PND 38. Three weeks later, all the mice were submitted to the CPP paradigm with a low dose of cocaine (1.5 mg/kg).

RESULTS

IRSD during early adolescence induced depressive-like behavior in the Social Interaction and Splash tests and increased the rewarding effects of cocaine. Mice with low levels of submissive behavior during episodes of defeat were resilient to the short- and long-term effects of IRSD. In addition, resilience to the short-term effects of IRSD on social interaction and grooming behavior predicted resilience to the long-term effects of IRSD on cocaine reward.

CONCLUSION

Our findings help to characterize the nature of resilience to the effects of social stress during adolescence.

Interaction between maternal immune activation and peripubertal stress in rats: impact on cocaine addiction-like behaviour, morphofunctional brain parameters and striatal transcriptome

Substance use disorders are more prevalent in schizophrenia, but the causal links between both conditions remain unclear. Maternal immune activation (MIA) is associated with schizophrenia which may be triggered by stressful experiences during adolescence. Therefore, we used a double-hit rat model, combining MIA and peripubertal stress (PUS), to study cocaine addiction and the underlying neurobehavioural alterations. We injected lipopolysaccharide or saline on gestational days 15 and 16 to Sprague-Dawley dams. Their male offspring underwent five episodes of unpredictable stress every other day from postnatal day 28 to 38. When animals reached adulthood, we studied cocaine addiction-like behaviour, impulsivity, Pavlovian and instrumental conditioning, and several aspects of brain structure and function by MRI, PET and RNAseq. MIA facilitated the acquisition of cocaine self-administration and increased the motivation for the drug; however, PUS reduced cocaine intake, an effect that was reversed in MIA + PUS rats. We found concomitant brain alterations: MIA + PUS altered the structure and function of the dorsal striatum, increasing its volume and interfering with glutamatergic dynamics (PUS decreased the levels of NAA + NAAG but only in LPS animals) and modulated specific genes that could account for the restoration of cocaine intake such as the pentraxin family. On its own, PUS reduced hippocampal volume and hyperactivated the dorsal subiculum, also having a profound effect on the dorsal striatal transcriptome. However, these effects were obliterated when PUS occurred in animals with MIA experience. Our results describe an unprecedented interplay between MIA and stress on neurodevelopment and the susceptibility to cocaine addiction.

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.

Stress- and drug-induced neuroimmune signaling as a therapeutic target for comorbid anxiety and substance use disorders

Stress and substance use disorders remain two of the most highly prevalent psychiatric conditions and are often comorbid. While individually these conditions have a debilitating impact on the patient and a high cost to society, the symptomology and treatment outcomes are further exacerbated when they occur together. As such, there are few effective treatment options for these patients, and recent investigation has sought to determine the neural processes underlying the co-occurrence of these disorders to identify novel treatment targets. One such mechanism that has been linked to stress- and addiction-related conditions is neuroimmune signaling. Increases in inflammatory factors across the brain have been heavily implicated in the etiology of these disorders, and this review seeks to determine the nature of this relationship. According to the "dual-hit" hypothesis, also referred to as neuroimmune priming, prior exposure to either stress or drugs of abuse can sensitize the neuroimmune system to be hyperresponsive when exposed to these insults in the future. This review completes an examination of the literature surrounding stress-induced increases in inflammation across clinical and preclinical studies along with a summarization of the evidence regarding drug-induced alterations in inflammatory factors. These changes in neuroimmune profiles are also discussed within the context of their impact on the neural circuitry responsible for stress responsiveness and addictive behaviors. Further, this review explores the connection between neuroimmune signaling and susceptibility to these conditions and highlights the anti-inflammatory pharmacotherapies that may be used for the treatment of stress and substance use disorders.

Intermittent voluntary wheel running promotes resilience to the negative consequences of repeated social defeat in mice

A novel approach to reduce the incidence of substance use disorders is to promote resilience to stress using environmental resources such as physical exercise. In the present study we test the hypothesis that Voluntary Wheel Running (VWR) during adolescence blocks the negative consequences of stress induced by intermittent repeated social defeat (IRSD). Four groups of adolescent male C57BL/6 mice were employed in the experiment; two groups were exposed to VWR (1 h, 3 days/week) from postnatal day (PND) 21 until the first social defeat (PND 47), while the remaining two groups did not have access to activity wheels (controls). On PND 47, 50, 53 and 56 mice, who had performed VWR, were exposed to an episode of social defeat by a resident aggressive mouse (VWR+IRSD group) or allowed to explore an empty cage (VWR+EXPL group). The same procedure was performed with control mice that had not undergone VWR (CONTROL+IRSD and CONTROL+EXPL groups). On PND 57, all the mice performed the Elevated Plus Maze (EPM), Hole-Board, Social Interaction, Tail Suspension and Splash tests. After an interval of 3 weeks, all mice underwent a conditioned place preference (CPP) procedure with 1 mg/kg of cocaine. Exposure to VWR prevented the negative consequences of social stress in the EPM, splash test and CPP, since the VWR+IRSD group did not display anxiety- or depression-like effects or the potentiation of cocaine reward observed in the Control+IRSD group. Our results support the idea that physical exercise promotes resilience to stress and represents an excellent target in drug abuse prevention.

Brief Maternal Separation Inoculates Against the Effects of Social Stress on Depression-Like Behavior and Cocaine Reward in Mice

Exposure to intermittent repeated social defeat (IRSD) increases the vulnerability of mice to the rewarding effects of cocaine in the conditioned place preference (CPP) paradigm. According to the "inoculation of stress" hypothesis, a brief period of maternal separation (MS) can provide protection against the negative effects of IRSD. The aim of the present study was to assess whether exposure to a brief episode of MS prevents the subsequent short-term effects of IRSD on depression- and anxiety-like behaviors and to explore its long-term effects on cocaine CPP in mice. Four groups of male C57BL/6 mice were employed; two groups were separated from their mother [6 h on postnatal day (PND) 9], while the other two groups were not (controls). On PND 47, 50, 53 and 56, mice that had experienced MS were exposed to social defeat in the cage of an aggressive resident mouse (MS + IRSD group) or were allowed to explore an empty cage (MS + EXPL group). The same procedure was performed with control mice that had not experienced MS (CONTROL + IRSD and CONTROL + EXPL groups). On PND57-58, all the mice performed the elevated plus maze and the hole-board, social interaction and splash tests. Three weeks after the last episode of defeat, all the mice underwent the CPP procedure with cocaine (1 mg/kg). Irrespective of whether or not MS had taken place, a reduction in open arms measures, dips, and social interaction was observed in mice that experienced IRSD. A higher latency of grooming and acquisition of cocaine-induced CPP were observed only in mice exposed to IRSD alone (CONTROL + IRSD). These results suggest that exposure to a brief episode of stress early in life increases the subsequent resilience of animals to the effects of social stress on vulnerability to cocaine.

How Stress Influences the Dynamic Plasticity of the Brain’s Extracellular Matrix

Diffuse and structured extracellular matrix (ECM) comprise ∼20% of the brain’s volume and play important roles in development and adult plasticity. Perineuronal nets (PNNs), specialized ECM structures that surround certain types of neurons in the brain, emerge during the postnatal period, making their development and maintenance potentially sensitive to experience. Recent studies have shown that stress affects diffuse ECM as well as PNNs, and that such effects are dependent on life stage and brain region. Given that the ECM participates in synaptic plasticity, the generation of neuronal oscillations, and synchronous firing across brain regions, all of which have been linked to cognition and emotional regulation, ECM components may be candidate therapeutic targets for stress-induced neuropsychiatric disease. This review considers the influence of stress over diffuse and structured ECM during postnatal life with a focus on functional outcomes and the potential for translational relevance.

Ethanol intake in male mice exposed to social defeat: Environmental enrichment potentiates resilience

Large preclinical evidence shows that exposure to social defeat (SD) increases vulnerability to drug abuse, increasing the consumption of ethanol. However, not all subjects are equally affected by the changes induced by stress. Previous reports have evidenced that the resilient phenotype to depressive-like behaviors after SD is associated with the resistant phenotype to cocaine-increased rewarding effects and the smaller neuroinflammatory response. The aim of the present study was to further clarify whether the resilient profile to depressive-like behavior also predicts a protection against the increase in ethanol intake induced by SD. The neuroinflammatory profile was studied after the end of the oral ethanol self-administration (SA) procedure, measuring levels of the pro-inflammatory cytokine IL-6 and the chemokine CX3CL1 or fractalkine in the striatum and prefrontal cortex. Previous studies have shown that environmental enrichment (EE) is an effective mechanism to dimish the detrimental effects of social stress. In a second study, we aimed to evaluate if EE housing before exposure to SD could potentiate resilience. Our results showed that mice with a phenotype susceptible to SD-induced depressive-like behaviors showed increased ethanol consumption and increased neuroinflammatory signaling. In contrast, despite the lack of effect on depressive-like behaviors, defeated mice previously housed under EE conditions did not show an increase in ethanol SA or an increase in immune response. To sum up, the resilient phenotype to SD develops at different levels, such as depressive-like behaviors, ethanol consumption and the neuroinflammatory response. Our results also point to the protective role of EE in potentiating resilience to SD effects.

Unravelling the Neuroinflammatory Mechanisms Underlying the Effects of Social Defeat Stress on Use of Drugs of Abuse

The immune system provides the first line of the organism's defenses, working to maintain homeostasis against external threats and respond also to internal danger signals. There is much evidence to suggest that modifications of inflammatory parameters are related to vulnerability to develop mental illnesses, such as depression, autism, schizophrenia, and substance use disorders. In addition, not only are inflammatory parameters related to these disorders, but stress also induces the activation of the immune system, as recent preclinical research demonstrates. Social stress activates the immune response in the central nervous system through a number of mechanisms; for example, by promoting microglial stimulation, modifying peripheral and brain cytokine levels, and altering the blood brain barrier, which allows monocytes to traffic into the brain. In this chapter, we will first deal with the most important short- and long-term consequences of social defeat (SD) stress on the neuroinflammatory response. SD experiences (brief episodes of social confrontations during adolescence and adulthood) induce functional modifications in the brain, which are accompanied by an increase in proinflammatory markers. Most importantly, inflammatory mechanisms play a significant role in mediating the process of adaptation in the face of adversity (resilience vs susceptibility), allowing us to understand individual differences in stress responses. Secondly, we will address the role of the immune system in the vulnerability and enhanced sensitivity to drugs of abuse after social stress. We will explore in depth the effects seen in the inflammatory system in response to social stress and how they enhance the rewarding effects of drugs such as alcohol or cocaine. To conclude, we will consider pharmacological and environmental interventions that seek to influence the inflammatory response to social stress and diminish increased drug intake, as well as the translational potential and future directions of this exciting new field of research.

Decreased kynurenine pathway potentiate resilience to social defeat effect on cocaine reward

The kynurenine (KYN) pathway of tryptophan (TRP) degradation is activated by stress and inflammatory factors. It is now well established that social stress induces the activation of the immune system, with central inflammation and KYN metabolism being two of the main factors linking stress with depression. The aim of the present study was to evaluate the long-lasting changes in the KYN pathway induced by social defeat (SD) associated with the resilience or susceptibility to an increase in the conditioned rewarding effects of cocaine. Mice were exposed to repeated SD and 3 weeks later, a conditioned place preference (CPP) induced by a subthreshold dose of cocaine (1.5 mg/kg) was developed. KYN levels in plasma, cerebellum, hippocampus, striatum and limbic forebrain were studied at the end of the CPP procedure. Changes in the KYN pathway after exposure to pharmacological (oxytocin and indomethacin) and environmental interventions (environmental enrichment) were also evaluated. Our results showed that defeated susceptible (SD-S) mice had higher conditioning scores than resilient mice (SD-R). In addition, although KYN concentration was elevated in all defeated mice, SD-R mice showed smaller increases in KYN concentration in the cerebellum than SD-S mice. Oxytocin or Indomethacin treatment before SD normalized cocaine-induced CPP, although the increase in the KYN pathway was maintained. However, environmental enrichment before SD normalized cocaine-induced CPP and prevented the increase in the KYN pathway. The present study highlights the role of the KYN pathway and anti-inflammatory drugs acting on TRP metabolism as pharmacological targets to potentiate resilience to social stress effects.

Pathophysiology of Pediatric Traumatic Brain Injury

The national incidence of traumatic brain injury (TBI) exceeds that of any other disease in the pediatric population. In the United States the Centers for Disease Control and Prevention (CDC) reports 697,347 annual TBIs in children ages 0–19 that result in emergency room visits, hospitalization or deaths. There is a bimodal distribution within the pediatric TBI population, with peaks in both toddlers and adolescents. Preclinical TBI research provides evidence for age differences in acute pathophysiology that likely contribute to long-term outcome differences between age groups. This review will examine the timecourse of acute pathophysiological processes during cerebral maturation, including calcium accumulation, glucose metabolism and cerebral blood flow. Consequences of pediatric TBI are complicated by the ongoing maturational changes allowing for substantial plasticity and windows of vulnerabilities. This review will also examine the timecourse of later outcomes after mild, repeat mild and more severe TBI to establish developmental windows of susceptibility and altered maturational trajectories. Research progress for pediatric TBI is critically important to reveal age-associated mechanisms and to determine knowledge gaps for future studies.

Role of acute social stress in the rewarding effects of MDMA in adolescent mice

Drug use among adolescents is a serious problem in our society, as some individuals develop dependence and addiction. MDMA/Esctasy is one of the most typically used substances by this age group. It is well known that environmental factors can alter the rewarding properties of drugs and the propensity to drug-related disorders. In this sense, exposure to social stress induces long-term effects in mice, enhancing the rewarding effects of MDMA in the conditioned place preference (CPP) paradigm. On the other hand, previous research has not provided conclusive results regarding the short-term effects of social defeat on MDMA reward in adolescent animals, probably due to the use of very low or very high doses. Thus, in the present study, we set out to evaluate whether exposure to social defeat immediately before each conditioning session with an intermediate dose of MDMA (2.25 mg/kg) modulates the rewarding effect of this drug in adolescent animals. Our results indicate that both control and socially defeated mice acquired CPP, but only stressed mice showed reinstatement. These findings indicate that social defeat induces an increase in the rewarding effect of MDMA, suggesting that this type of stress is a potential factor in the development of MDMA addiction.

The Role of Social Stress in the Development of Inhibitory Control Deficit: A Systematic Review in Preclinical Models

Inhibitory control deficit and impulsivity and compulsivity behaviours are present in different psychopathological disorders such as addiction, obsessive-compulsive disorders and schizophrenia, among others. Social relationships in humans and animals are governed by social organization rules, which modulate inhibitory control and coping strategies against stress. Social stress is associated with compulsive alcohol and drug use, pointing towards a determining factor in an increased vulnerability to inhibitory control deficit. The goal of the present review is to assess the implication of social stress and dominance on the vulnerability to develop impulsive and/or compulsive spectrum disorders, with the aid of the information provided by animal models. A systematic search strategy was carried out on the PubMed and Web of Science databases, and the most relevant information was structured in the text and tables. A total of 34 studies were recruited in the qualitative synthesis. The results show the role of social stress and dominance in increased drug and alcohol use, aggressive and impulsive behaviour. Moreover, the revised studies support the role of Dopaminergic (DA) activity and the alterations in the dopaminergic D1/D2 receptors as key factors in the development of inhibitory control deficit by social stress.

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

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

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'.

Distinct Regulation of Dopamine D3 Receptor in the Basolateral Amygdala and Dentate Gyrus during the Reinstatement of Cocaine CPP Induced by Drug Priming and Social Stress

Relapse in the seeking and intake of cocaine is one of the main challenges when treating its addiction. Among the triggering factors for the recurrence of cocaine use are the re-exposure to the drug and stressful events. Cocaine relapse engages the activity of memory-related nuclei, such as the basolateral amygdala (BLA) and the hippocampal dentate gyrus (DG), which are responsible for emotional and episodic memories. Moreover, D3 receptor (D3R) antagonists have recently arisen as a potential treatment for preventing drug relapse. Thus, we have assessed the impact of D3R blockade in the expression of some dopaminergic markers and the activity of the mTOR pathway, which is modulated by D3R, in the BLA and DG during the reinstatement of cocaine-induced conditioned place preference (CPP) evoked by drug priming and social stress. Reinstatement of cocaine CPP paralleled an increasing trend in D3R and dopamine transporter (DAT) levels in the BLA. Social stress, but not drug-induced reactivation of cocaine memories, was prevented by systemic administration of SB-277011-A (a selective D3R antagonist), which was able, however, to impede D3R and DAT up-regulation in the BLA during CPP reinstatement evoked by both stress and cocaine. Concomitant with cocaine CPP reactivation, a diminution in mTOR phosphorylation (activation) in the BLA and DG occurred, which was inhibited by D3R blockade in both nuclei before the social stress episode and only in the BLA when CPP reinstatement was provoked by a cocaine prime. Our data, while supporting a main role for D3R signalling in the BLA in the reactivation of cocaine memories evoked by social stress, indicate that different neural circuits and signalling mechanisms might mediate in the reinstatement of cocaine-seeking behaviours depending upon the triggering stimuli.

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

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

Oxytocin Signaling as a Target to Block Social Defeat-Induced Increases in Drug Abuse Reward

There is huge scientific interest in the neuropeptide oxytocin (OXT) due to its putative capacity to modulate a wide spectrum of physiological and cognitive processes including motivation, learning, emotion, and the stress response. The present review seeks to increase the understanding of the role of OXT in an individual’s vulnerability or resilience with regard to developing a substance use disorder. It places specific attention on the role of social stress as a risk factor of addiction, and explores the hypothesis that OXT constitutes a homeostatic response to stress that buffers against its negative impact. For this purpose, the review summarizes preclinical and clinical literature regarding the effects of OXT in different stages of the addiction cycle. The current literature affirms that a well-functioning oxytocinergic system has protective effects such as the modulation of the initial response to drugs of abuse, the attenuation of the development of dependence, the blunting of drug reinstatement and a general anti-stress effect. However, this system is dysregulated if there is continuous drug use or chronic exposure to stress. In this context, OXT is emerging as a promising pharmacotherapy to restore its natural beneficial effects in the organism and to help rebalance the functions of the addicted brain.

Oxytocin reverses ethanol consumption and neuroinflammation induced by social defeat in male mice

Oxytocin (OXT) modulates social interactions, attenuates stressful responses and can decrease drug-seeking and taking behaviors. In previous studies, we observed that social defeat (SD) induced a long-lasting increase in ethanol intake and neuroinflammation in male mice. We also know that OXT blocks the increase in cocaine reward induced by SD. Therefore, in the present study we aimed to evaluate the effect of 1 mg/kg of OXT administered 30 min before each episode of SD on ethanol consumption and the neuroinflammatory response in adult male mice. Three weeks after the last SD, mice underwent oral ethanol self-administration (SA) procedure, and striatal levels of the two chemokines CX3CL1 and CXCL12 were measured after the last SD and at the end of the ethanol SA. OXT administration blocked the increase in voluntary ethanol consumption observed in defeated mice, although it did not affect motivation for ethanol. An increase in the striatal levels of CX3CL1 and CXCL12 was observed in defeated animals immediately after the last defeat and after the ethanol SA. However, defeated mice treated with OXT did not show this increase in the neuroinflammatory response. In conclusion, OXT treatment can be a powerful therapeutic target to reduce the negative effects of social stress on ethanol consumption and the neuroinflammatory process.

Hippocampal-Dependent Inhibitory Learning and Memory Processes in the Control of Eating and Drug Taking

As manifestations of excessive and uncontrolled intake, obesity and drug addiction have generated much research aimed at identifying common neuroadaptations that could underlie both disorders. Much work has focused on changes in brain reward and motivational circuitry that can overexcite eating and drug-taking behaviors. We suggest that the regulation of both behaviors depends on balancing excitation produced by stimuli associated with food and drug rewards with the behavioral inhibition produced by physiological "satiety" and other stimuli that signal when those rewards are unavailable. Our main hypothesis is that dysregulated eating and drug use are consequences of diet- and drug-induced degradations in this inhibitory power. We first outline a learning and memory mechanism that could underlie the inhibition of both food and drug-intake, and we describe data that identifies the hippocampus as a brain substrate for this mechanism. We then present evidence that obesitypromoting western diets (WD) impair the operation of this process and generate pathophysiologies that disrupt hippocampal functioning. Next, we present parallel evidence that drugs of abuse also impair this same learning and memory process and generate similar hippocampal pathophysiologies. We also describe recent findings that prior WD intake elevates drug self-administration, and the implications of using drugs (i.e., glucagon-like peptide- 1 agonists) that enhance hippocampal functioning to treat both obesity and addiction are also considered. We conclude with a description of how both WD and drugs of abuse could initiate a "vicious-cycle" of hippocampal pathophysiology and impaired hippocampal-dependent behavioral inhibition.

Blockade of D3 receptor prevents changes in DAT and D3R expression in the mesolimbic dopaminergic circuit produced by social stress- and cocaine prime-induced reinstatement of cocaine-CPP

BACKGROUND

Cocaine may cause persistent changes in the brain, which are more apparent in DA transporter (DAT) and DA receptor availability within the nucleus accumbens (NAc). On the other hand, the DA D3 receptor (D3R) has emerged as a promising pharmacotherapeutic target for substance use disorders.

AIMS

This study aims to assess the impact of selective D3R antagonism on DAT and D3R after reinstatement of cocaine preference (CPP) induced by an acute session of social defeat stress (SDS) and a cocaine prime in mice after a period of abstinence.

METHODS

Male mice were conditioned with 25 mg/kg of cocaine for 4 days. After 60 days of extinction training mice were pretreated with the selective D3R antagonist SB-277011A before the re-exposure to a priming dose of cocaine or to a single SDS session. CPP scores were determined and levels of DAT, D3R, phospho Akt (pAkt) and phospho mTOR (pmTOR) were assessed in the NAc shell.

RESULTS

An increase in DAT and D3R expression was seen in the NAc after both a cocaine prime- and SDS-induced reinstatement of CPP. Pretreatment with SB-277011A blocked elevated DAT and D3R expression as well as SDS-induced reinstatement. By contrast, the blockade of D3R did not modified the cocaine prime-induced CPP. Changes in DAT and D3R expression do not seem to occur via the canonic pathway involving Akt/mTOR.

CONCLUSIONS

Our results suggest that the selective D3R antagonist ability to inhibit DAT and D3R up-regulation could represent a possible mechanism for its behavioral effects in cocaine-memories reinstatement induced by social stress.

Critical role of TLR4 in uncovering the increased rewarding effects of cocaine and ethanol induced by social defeat in male mice

BACKGROUND

Substance use disorders and social stress are currently associated with changes in the immune system response by which they induce a proinflammatory state in neurons and glial cells that eventually modulates the reward system.

AIMS

The aim of the present work was to assess the role of the immune TLR4 (Toll-like receptors 4) and its signaling response in the increased contextual reinforcing effects of cocaine and reinforcing effects of ethanol (EtOH) induced by social defeat (SD) stress.

METHODS

Adult male C57BL/6 J wild-type (WT) mice and mice deficient in TLR4 (TLR4-KO) were assigned to experimental groups according to stress condition (exploration or SD). Three weeks after the last SD, conditioned place preference (CPP) was induced by a subthreshold cocaine dose (1 mg/kg), while another set underwent EtOH 6% operant self-administration (SA). Several inflammatory molecules were analyzed in the hippocampus and the striatum.

RESULTS

SD induced higher vulnerability to the conditioned rewarding effects of cocaine only in defeated WT mice. Similarly, defeated WT mice exhibited higher 6% EtOH consumption, an effect that was not observed in the defeated TLR4-KO group. However, the motivation to obtain the drug was observed in both genotypes of defeated animals. Notably, a significant upregulation of the protein proinflammatory markers NFkBp-p65, IL-1β, IL-17 A and COX-2 were observed only in the defeated WT mice, but not in their defeated TLR4-KO counterparts.

CONCLUSIONS

These results suggest that TLR4 receptors mediate the neuroinflammatory response underlying the increase in the rewarding effects of cocaine and EtOH induced by social stress.

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.

Voluntary wheel running protects against the increase in ethanol consumption induced by social stress in mice

Previous studies have shown that exposure to social defeat (SD), a model of social stress, produces a long-term increase in the consumption of ethanol, most likely through an increase in the neuroinflammation response. The aim of the present study was to evaluate whether exposure to physical activity in the form of voluntary wheel running (VWR) could block the increase in ethanol consumption and the neuroinflammatory response induced by social stress. Mice were exposed to either 4 sessions of repeated social defeat (RSD) or a non-stressful experience. During the whole procedure, half of the mice were exposed to controlled physical activity, being allowed 1 h access to a low-profile running wheel three times a week. Three weeks after the last RSD, animals started the oral self-administration (SA) of ethanol (6% EtOH) procedure. Biological samples were taken 4 h after the first and the fourth RSD, 3 weeks after the last RSD, and after the SA procedure. Brain tissue (striatum) was used to determine protein levels of the chemokines fractalkine (CX3CL1) and SDF-1 (CXCL12). RSD induced an increase in ethanol consumption and caused greater motivation to obtain ethanol. The striatal levels of CX3CL1 and CXCL12 were also increased after the last RSD. VWR was able to reverse the increase in ethanol intake induced by social stress and the neuroinflammatory response. In conclusion, our results suggest that VWR could be a promising tool to prevent and reduce the detrimental effects induced by social stress.

The Nucleus Accumbens: A Common Target in the Comorbidity of Depression and Addiction

The comorbidity of depression and addiction has become a serious public health issue, and the relationship between these two disorders and their potential mechanisms has attracted extensive attention. Numerous studies have suggested that depression and addiction share common mechanisms and anatomical pathways. The nucleus accumbens (NAc) has long been considered a key brain region for regulating many behaviors, especially those related to depression and addiction. In this review article, we focus on the association between addiction and depression, highlighting the potential mediating role of the NAc in this comorbidity via the regulation of changes in the neural circuits and molecular signaling. To clarify the mechanisms underlying this association, we summarize evidence from overlapping reward neurocircuitry, the resemblance of cellular and molecular mechanisms, and common treatments. Understanding the interplay between these disorders should help guide clinical comorbidity prevention and the search for a new target for comorbidity treatment.

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.

Social defeat-induced increase in the conditioned rewarding effects of cocaine: Role of CX3CL1

Social stress is associated with higher vulnerability to drug use, as it enhances the reinforcing effects of psychostimulants in rodents. Furthermore, continued or severe stress induces a proinflammatory state of microglial activation and augmented cytokine production. The aim of the present work was to evaluate the role of fractalkine [C-X3-C motif ligand 1 (CX3CL1)], an inflammatory chemokine, in the increased conditioned rewarding effects of cocaine in animals exposed to social defeat stress. In addition, we measured the signaling cascade pathway of CX3CL1 in the hippocampus (HPC) (including p-ERK/ERK, p-p38/p38 MAPK, p-p65/p65 NFκB and p-CREB/CREB ratios). The glutamate receptor subunits NR1, NR2B and GluA1 were also assessed. A total of 102 adult male C57BL/6 J wild-type (WT) and Cx3cr1 knockout (KO) mice were divided into different experimental groups according to stress condition (exploration or social defeat). Three weeks after the last social defeat, conditioned place preference (CPP) was induced by a subthreshold dose of cocaine (1 mg/kg). Brain tissue samples were taken 24 h after the CPP procedure to determine the levels of the proteins and transcription factors. Our results showed that, in WT animals, repeated social defeat (RSD) decreased CX3CL1 striatal levels without producing changes in the HPC. In addition, RSD induced an increase in the conditioned rewarding effects of cocaine, regardless of the genotype. After CPP induced by cocaine, defeated Cx3cr1-deficient mice showed a decrease in the p-p65/p65 NFκB and pCREB/CREB ratio in the HPC, and an increase in the hippocampal levels of CX3CL1 and p-p38/p38 MAPK relation. In all defeated mice, there was a decrease in the ionotropic glutamate receptor subunit NR1. In conclusion, these results suggest that the lack of CX3CL1/Cx3cr1 signaling under stress conditions induces changes in protein and transcription factors, indicating that CX3CL1 is needed to shield the response to social defeat.

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

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

Pharmacological modulation of the behavioral effects of social defeat in memory and learning in male mice

RATIONALE

Previous studies have demonstrated that repeated social defeat (RSD) stress only induces cognitive deficits when experienced during adulthood. However, RSD increases cocaine-rewarding effects in adult and adolescent mice, inducing different expressions of proBDNF in the ventral tegmental area.

OBJECTIVE

The aim of the present study was to evaluate the effect of cocaine administration in socially defeated adult or adolescent mice on learning, memory, and anxiety. Additionally, the role of BDNF was also studied.

METHODS

Adolescent and young adult mice were exposed to four episodes of social defeat or exploration (control), being treated with a daily injection of four doses of saline or 1 mg/kg of cocaine 3 weeks after the last social defeat. Other groups were treated with the TrkB receptor antagonist ANA-12 during this 21-day period. After this treatment, their cognitive and anxiogenic profiles were evaluated, along with the expression of BDNF, pCREB, and pERK1/2 in the dentate gyrus (DG) and basolateral amygdala (BLA).

RESULTS

Cocaine induced an increased expression of pCREB and BDNF in the DG and BLA only in defeated animals. Although RSD did not affect memory, the administration of cocaine induced memory impairments only in defeated animals. Defeated adult mice needed more time to complete the mazes, and this effect was counteracted by cocaine administration. RSD induced anxiogenic effects only when experienced during adulthood and cocaine induced a general anxiolytic effect. Blockade of Trkb decreased memory retention without affecting spatial learning and modified anxiety on non-stressed mice depending on their age.

CONCLUSION

Our results demonstrate that the long-lasting effects of social defeat on anxiety and cognition are modulated by cocaine administration. Our results highlight that the BDNF signaling pathway could be a target to counteract the effects of cocaine on socially stressed subjects.

Social Housing Conditions Modulate the Long-Lasting Increase in Cocaine Reward Induced by Intermittent Social Defeat

Social defeat is considered the most representative animal model for studying the consequences of social stress. Intermittent social defeat (ISD) has proved to enhance the response to cocaine hedonic properties. In the present research, we evaluated if different social housing conditions, as housing with a familiar conspecific or with a female, exert a protective effect modulating the negative consequences of ISD as the increased sensitivity to cocaine and the induction of anxiety-like behavior. To achieve this objective, non-stressed or ISD OF1 male mice were divided into five different experimental groups according to their social environment: standard housing (four adult males per cage); male adolescent or adult in pairs (two males per cage); and adult males housed with a female for a short or long period (3 days vs. the whole duration of the study). Anxiety-like behavior was evaluated 19 days after the last episode of ISD using an elevated plus maze (EPM), and 24 h later the animals underwent a conditioned place preference paradigm (CPP) induced by a sub-threshold dose of cocaine (1 mg/kg). Following CPP, biological samples were taken to measure striatal levels of interleukin 6 (IL-6) and plasmatic levels of oxytocin (OT). Our results confirmed that ISD animals housed in standard condition displayed an anxious phenotype, developed CPP and had increased levels of IL-6 in the striatum. However, animals housed with a female or with a familiar male since adolescence did not develop CPP and were protected against the anxiogenic and neuroinflammatory potential of ISD stress. In the group of animals paired with a female throughout the experimental procedure, an increase in OT levels may have underlain this buffering effect, while the protective effect of being housed with a familiar male mouse seems to be related with a better resolution of the stress response. The present results expand our knowledge of the neurobiology of vulnerability to drug addiction and highlight the benefit of social support for recovery from the adverse effects of social stress.

Persistent increase of I.V. cocaine self-administration in a subgroup of C57BL/6J male mice after social defeat stress

RATIONALE

Stressful life experiences can persistently increase the motivation for, and consumption of, intensely rewarding stimuli, like cocaine, over time. In rodents, intermittent versus continuous exposure to social stress engenders opposing changes to reward-related behavior, as measured by consumption of sucrose and cocaine.

OBJECTIVE

The present study examines if the effects of intermittent versus continuous social stress on cocaine self-administration in mice parallel those seen in rats.

METHODS

Both forms of social stress involve a brief daily physical confrontation with an aggressive resident for 10 consecutive days. Continuous social stress involves constant visual and olfactory exposure to an aggressive resident via habitation in a protected portion of the resident's home cage, while exposure to an aggressive resident during intermittent social stress is limited to a single, physical encounter per day. Implementing a femoral vein catheterization method for the first time in mice, we determined divergent changes to intravenous cocaine self-administration.

RESULTS

Modestly increased cocaine self-administration after intermittent social stress was confirmed. In a subset of animals, continuous social stress in mice substantially increased cocaine self-administration and sucrose intake. By stark contrast, another subpopulation had substantial attenuation of cocaine self-administration and sucrose intake after continuous social stress.

CONCLUSIONS

Bimodal divergence in responding for rewarding stimuli including cocaine after social stress experience likely reflects two opposing forms of coping to continuous social stress that promote either a sensitization or attenuation of reward-seeking.

Modulation of stress- and cocaine prime-induced reinstatement of conditioned place preference after memory extinction through dopamine D3 receptor

Accumulating evidence indicates that dopamine (DA) D3 receptor (DAD3R) antagonists appear highly promising in attenuating cocaine reward and relapse in preclinical models of addiction. In the present study, we investigated the effects of the selective DAD3R antagonist SB-277011-A on the reinstatement of cocaine-induced conditioned place preference (CPP) produced by a priming dose of cocaine, by social defeat stress and by two kinds of physiological stressors (restraint and tail pinch) in male adult mice. We also explored reinstatement-related plasma corticosterone levels (as marker of stress response) and the effects of blocking DAD3R. Administration of SB-277011-A (24 or 48 mg/kg i.p.) did not modify conditioned reinstatement of cocaine seeking triggered by cocaine prime. By contrast, we found that the vulnerability to reinstatement of the CPP of defeated animals that have undergone CPP extinction was abolished by the DAD3R antagonist (24 mg/kg) given 30 min before the test session. Reactivation of the CPP response produced by physiological stress stimuli was also attenuated by SB-277011-A (48 mg/kg i.p.). On the other hand, the blockade of DAD3R significantly prevented the increased corticosterone release during reinstatement of cocaine-induced CPP that was seen in social defeated animals, in mice suffering physiological stress and after cocaine prime. Present results demonstrate a modulation by DAD3R of the reactivation of the incentive value of cocaine-associated cues induced by social and physiological stress stimuli, which was associated to a glucocorticoid-dependent mechanism. Our results also point to a possible potential therapeutic use of selective DAD3R antagonists for the prevention of stress-induced cocaine-seeking and relapse.

Indomethacin blocks the increased conditioned rewarding effects of cocaine induced by repeated social defeat

It is well established that repeated social defeat stress can induce negative long-term consequences such as increased anxiety-like behavior and enhances the reinforcing effect of psychostimulants in rodents. In the current study, we evaluated how the immune system may play a role in these long-term effects of stress. A total of 148 OF1 mice were divided into different experimental groups according to stress condition (exploration or social defeat) and pre-treatment (saline, 5 or 10 mg/kg of the anti-inflammatory indomethacin) before each social defeat or exploration episode. Three weeks after the last social defeat, anxiety was evaluated using an elevated plus maze paradigm. After this test, conditioned place preference (CPP) was induced by a subthreshold dose of cocaine (1 mg/kg). Biological samples were taken four hours after the first and the fourth social defeat, 3 weeks after the last defeat episode, and after the CPP procedure. Plasma and brain tissue (prefrontal cortex, striatum and hippocampus) were used to determine the levels of the pro-inflammatory cytokine interleukin 6 (IL-6). Results showed an increase of peripheral and brain IL-6 levels after the first and fourth social defeat that was reverted three weeks later. Intraperitoneal administration of the anti-inflammatory drug indomethacin before each episode of stress prevented this enhancement of IL-6 levels and also reversed the increase in the rewarding effects of cocaine in defeated mice. Conversely, this protective effect was not observed with respect to the anxiogenic consequences of social stress. Our results confirm the hypothesis of a modulatory proinflammatory contribution to stress-induced vulnerability to drug abuse disorders and highlight anti-inflammatory interventions as a potential therapeutic tool to treat stress-related addiction disorders.

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: 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.

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.

Cocaine impairs serial-feature negative learning and blood-brain barrier integrity

Previous research has shown that diets high in fat and sugar [a.k.a., Western diets (WD)] can impair performance of rats on hippocampal-dependent learning and memory problems, an effect that is accompanied by selective increases in hippocampal blood brain barrier (BBB) permeability. Based on these types of findings, it has been proposed that overeating of a WD (and its resulting obesity) may be, in part, a consequence of impairments in these anatomical substrates and cognitive processes. Given that drug use (and addiction) represents another behavioral excess, the present experiments assessed if similar outcomes might occur with drug exposure by evaluating the effects of cocaine administration on hippocampal-dependent memory and on the integrity of the BBB. Experiment 1 of the present series of studies found that systemic cocaine administration in rats also appears to have disruptive effects on the same hippocampal-dependent learning and memory mechanism that has been proposed to underlie the inhibition of food intake. Experiment 2 demonstrated that the same regimen of cocaine exposure that produced disruptions in learning and memory in Experiment 1 also produced increased BBB permeability in the hippocampus, but not in the striatum. Although the predominant focus of previous research investigating the etiologies of substance use and abuse has been on the brain circuits that underlie the motivational properties of drugs, the current investigation implicates the possible involvement of hippocampal memory systems in such behaviors. It is important to note that these positions are not mutually exclusive and that neuroadaptations in these two circuits might occur in parallel that generate dysregulated drug use in a manner similar to that of excessive eating.

Maternal separation increases alcohol-drinking behaviour and reduces endocannabinoid levels in the mouse striatum and prefrontal cortex

Childhood adversity is associated with an increased risk of mood, anxiety and substance use disorders. Maternal separation is a reliable rodent model of early life adversity that leads to depression-like symptoms, which may increase the vulnerability to alcohol consumption during adolescence. However, the specific alterations in the pattern of alcohol consumption induced by maternal separation and the underlying molecular mechanisms are still unclear. The purpose of this study is to evaluate the long-term effects of maternal separation with early weaning (MSEW) on emotional and social behaviour, alcohol rewarding properties, and alcohol consumption, abstinence and relapse in adolescent male C57BL/6 mice. In addition, endocannabinoid and monoamine levels were analysed in discrete brain areas. Results showed that MSEW mice presented emotional alterations related to depressive-like behaviour and modified endocannabinoid levels in the striatum and the prefrontal cortex. MSEW mice also showed impairments in alcohol-induced conditioned place preference and higher alcohol intake in a model of binge drinking. Moreover, MSEW animals displayed a higher propensity to relapse in the two-bottle choice paradigm following a period of alcohol abstinence associated with reduced monoamine levels in the striatum. Such results indicate that exposure to early life stress increased the vulnerability to alcohol binge-drinking during adolescence, which may be partially explained by decreased sensitivity to alcohol rewarding properties and the ability to potentiate alcohol intake following a period of abstinence.

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.

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.