Social stress, therapeutics and drug abuse: preclinical models of escalated and depressed intake

Divergent effects of chronic continuous and intermittent social defeat stress on emotional behaviors: Impact on phosphorylated CREB and BDNF protein levels in the rat hippocampus

Chronic psychosocial stress stands as a significant heterogeneous risk factor for psychiatric disorders. The brain's physiological response to such stress varies based on the frequency and intensity of stress episodes. However, whether stress episodes divergently could affect hippocampal cyclic AMP response element-binding protein (CREB)-brain-derived neurotrophic factor (BDNF) signaling remains unclear, a key regulator of psychiatric symptoms. We aimed to assess how two distinct patterns of social defeat stress exposure impact anxiety- and depression-like behaviors, fear, and hippocampal CREB-BDNF signaling in adult male rats. To explore this, adult male Sprague-Dawley rats were subjected to psychosocial stress using a Resident/Intruder paradigm for ten consecutive days (continuous social defeat stress: [CS]) or ten social defeat stress over the course of 21 days (intermittent social defeat stress [IS]). Behavioral tests (including novelty-suppressed feeding test, forced swimming test, and contextually conditioned fear) were conducted. Protein expression levels of phosphorylated CREB and BDNF in the dorsal and ventral hippocampi were examined. CS led to heightened anxiety-like behavior, fear, and increased levels of phosphorylated CREB in both the dorsal and ventral hippocampi. Conversely, IS resulted in increased anxiety-like behavior and behavioral despair alongside decreased levels of phosphorylated CREB and BDNF, particularly in the dorsal hippocampus. These findings indicate that chronic psychosocial stress divergently affects hippocampal CREB-BDNF signaling and emotional regulation depending on the stress episode. Such insights could enhance our understanding of the molecular basis of the heterogeneity of psychiatric disorders and facilitate the development of innovative treatment approaches to patients with psychiatric disorders.

Choice impulsivity after repeated social stress is associated with increased perineuronal nets in the medial prefrontal cortex

Repeated stress can predispose to substance abuse. However, behavioral and neurobiological adaptations that link stress to substance abuse remain unclear. This study investigates whether intermittent social defeat (ISD), a stress protocol that promotes drug-seeking behavior, alters intertemporal decision-making and cortical inhibitory function in the medial prefrontal cortex (mPFC). Male long evans rats were trained in a delay discounting task (DDT) where rats make a choice between a fast (1 s) small reward (1 sugar pellet) and a large reward (3 sugar pellets) that comes with a time delay (10 s or 20 s). A decreased preference for delayed rewards was used as an index of choice impulsivity. Rats were exposed to ISD and tested in the DDT 24 h after each stress episode, and one- and two-weeks after the last stress episode. Immunohistochemistry was performed in rat's brains to evaluate perineuronal nets (PNNs) and parvalbumin GABA interneurons (PV) labeling as markers of inhibitory function in mPFC. ISD significantly decreased the preference for delayed large rewards in low impulsive, but not high impulsive, animals. ISD also increased the density of PNNs in the mPFC. These results suggest that increased choice impulsivity and cortical inhibition predispose animals to seek out rewards after stress.

Gestational environmental enrichment prevents chronic social stress induced anxiety- and ethanol-related behaviors in offspring

Epidemiological surveys have shown a strong relationship between maternal stress and offspring's mood disorders. Growing evidence suggested that environmental enrichment (EE) improves cognitive function in models of psychiatric and neurological disorders. However, the potential protective effects of gestational EE on social stress-elicited mood disorders in offspring have not been studied. Knowing that the undeveloped brain is more sensitive to gestational environmental stimuli, we hypothesized that initiating cognitive stimulation, during gestation, would protect against social stress-induced behavioral alterations in adulthood. Therefore, the present study aimed to investigate the effects of gestational EE on social stress-elicited anxiety- and ethanol-related behaviors in adult offspring. EE consisted of free access, of dams, to tubular devices of different shapes, colors, and sizes that were changed regularly. After birth and weaning, young adult offspring were exposed to 19 days of social stress and anxiety-like behavior was evaluated by elevated plus maze, open field, and marbles burying tests. The two-bottle choice (TBC) drinking paradigm was used to assess stress-induced ethanol intake. Results showed that gestational EE prevented social stress-elicited anxiogenic-like effects with no differences in spontaneous locomotor activity. Moreover, in the TBC paradigm, mice pre-exposed to EE consistently showed a significantly decreased consumption and preference for ethanol with no effects on tastants' intakes. Interestingly, gestational EE increased serum BDNF levels, which showed a correlation with measures of anxiety- and ethanol-related behaviors. These findings indicate that some neurodevelopmental changes associated with prenatal EE may counteract adult social stress-induced behavioral alterations through a BDNF mechanism. Therefore, we propose that gestational EE has significant protective and beneficial effects on social stress-induced cognitive impairment. It can also alleviate anxiety-like behavior and subsequent excessive alcohol consumption.

Viral-mediated expression of Erk2 in the nucleus accumbens regulates responses to rewarding and aversive stimuli

Second-messenger signaling within the mesolimbic reward circuit is involved in both the long-lived effects of stress and in the underlying mechanisms that promote drug abuse liability. To determine the direct role of kinase signaling within the nucleus accumbens, specifically mitogen-activated protein kinase 1 (ERK2), in mood- and drug-related behavior, we used a herpes-simplex virus to up- or down-regulate ERK2 in adult male rats. We then exposed rats to a battery of behavioral tasks including the elevated plus-maze, open field test, forced-swim test, conditioned place preference, and finally cocaine self-administration. Herein, we show that viral overexpression or knockdown of ERK2 in the nucleus accumbens induces distinct behavioral phenotypes. Specifically, over expression of ERK2 facilitated depression- and anxiety-like behavior while also increasing sensitivity to cocaine. Conversely, down-regulation of ERK2 attenuated behavioral deficits, while blunting sensitivity to cocaine. Taken together, these data implicate ERK2 signaling, within the nucleus accumbens, in the regulation of affective behaviors and modulating sensitivity to the rewarding properties of cocaine.

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.

Emotional- and cognitive-like responses induced by social defeat stress in male mice are modulated by the BNST, amygdala, and hippocampus

Introduction

Chronic exposure to social defeat stress (SDS) has been used to investigate the neurobiology of depressive- and anxiety-like responses and mnemonic processes. We hypothesized that these affective, emotional, and cognitive consequences induced by SDS are regulated via glutamatergic neurons located in the bed nucleus of the stria terminalis (BNST), amygdaloid complex, and hippocampus in mice.

Methods

Here, we investigated the influence of chronic SDS on (i) the avoidance behavior assessed in the social interaction test, (ii) the anxiety-like behavior (e.g., elevated plus-maze, and open field tests) (iii) depressive-like behaviors (e.g., coat state, sucrose splash, nesting building, and novel object exploration tests), (iv) the short-term memory (object recognition test), (v) ΔFosB, CaMKII as well as ΔFosB + CaMKII labeling in neurons located in the BNST, amygdaloid complex, dorsal (dHPC) and the ventral (vHPC) hippocampus.

Results

The main results showed that the exposure of mice to SDS (a) increased defensive and anxiety-like behaviors and led to memory impairment without eliciting clear depressive-like or anhedonic effects; (b) increased ΔFosB + CaMKII labeling in BNST and amygdala, suggesting that both areas are strongly involved in the modulation of this type of stress; and produced opposite effects on neuronal activation in the vHPC and dHPC, i.e., increasing and decreasing, respectively, ΔFosB labeling. The effects of SDS on the hippocampus suggest that the vHPC is likely related to the increase of defensive- and anxiety-related behaviors, whereas the dHPC seems to modulate the memory impairment.

Discussion

Present findings add to a growing body of evidence indicating the involvement of glutamatergic neurotransmission in the circuits that modulate emotional and cognitive consequences induced by social defeat stress.

Oleoylethanolamide attenuates the stress-mediated potentiation of rewarding properties of cocaine associated with an increased TLR4 proinflammatory response

The lipid-derived messenger oleoylethanolamide (OEA) has been involved in multiple physiological functions including metabolism and the immune response. More recently, OEA has been observed to affect reward-related behavior. Stress is a major risk factor for drug use and a predictor of drug relapse. In the laboratory, social stress has been largely studied using the social defeat (SD) model. Here, we explored the effects of different OEA administration schedules on the increased rewarding properties of cocaine induced by SD. In addition, we evaluated the anti-inflammatory action of OEA pretreatment in TLR4 expression caused by SD in the cerebellum, a novel brain structure that has been involved in the development of cocaine addiction. Adult OF1 mice were assigned to an experimental group according to the stress condition (exploration or SD) and treatment (OEA before SD, OEA before conditioning or subchronic OEA treatment). Mice were administered with OEA i.p (10 mg/kg) 10 min previously to the corresponding event. Three weeks after the last SD encounter, conditioned place preference (CPP) was induced by a subthreshold cocaine dose (1 mg/kg). As expected, socially defeated mice presented greater vulnerability to the cocaine reinforcing effects and expressed CPP. Conversely, this effect was not observed under a non-stressed condition. Most importantly, we observed that OEA pretreatment before SD or before conditioning prevented cocaine CPP in defeated mice. Biochemical analysis showed that OEA administration before SD decreased proinflammatory TLR4 upregulation in the cerebellum caused by social stress. In summary, our results suggest that OEA may have a protective effect on stress-induced increased cocaine sensitivity by exerting an anti-inflammatory action.

The Influence of the COVID-19 Pandemic Emergency on Alcohol Use: A Focus on a Cohort of Sicilian Workers

The period between the beginning and the end of the COVID-19 pandemic emergency generated a general state of stress, affecting both the mental state and physical well-being of the general population. Stress is the body's reaction to events or stimuli perceived as potentially harmful or distressing. Particularly when prolonged over time, it can promote the consumption of different psychotropic substances such as alcohol, and thus the genesis of various pathologies. Therefore, our research aimed to evaluate the differences in alcohol consumption in a cohort of 640 video workers who carried out activities in smart working, subjects particularly exposed to stressful situations due to the stringent rules of protection and prevention implemented during the pandemic. Furthermore, based on the results obtained from the administration of the AUDIT-C, we wanted to analyse the different modes of alcohol consumption (low, moderate, high, severe) to understand whether there is a difference in the amount of alcohol consumed that could predispose individuals to health problems. To this end, we administered the AUDIT-C questionnaire in two periods (T₀ and T₁), coinciding with annual occupational health specialist visits. The results of the present research showed an increase in the number of subjects consuming alcohol (p = 0.0005) and in their AUDIT-C scores (p < 0.0001) over the period considered. A significant decrease in subgroups who drink in a low-risk (p = 0.0049) mode and an increase in those with high (p = 0.00012) and severe risk (p = 0.0002) were also detected. In addition, comparing the male and female populations, it emerged that males have drinking patterns that lead to a higher (p = 0.0067) health risk of experiencing alcohol-related diseases than female drinking patterns. Although this study provides further evidence of the negative impact of the stress generated by the pandemic emergency on alcohol consumption, the influence of many other factors cannot be ruled out. Further research is needed to better understand the relationship between the pandemic and alcohol consumption, including the underlying factors and mechanisms driving changes in drinking behaviour, as well as potential interventions and support strategies to address alcohol-related harm during and after the pandemic.

Vicarious Social Defeat Increases Conditioned Rewarding Effects of Cocaine and Ethanol Intake in Female Mice

Stress is a critical factor in the development of mood and drug use disorders. The social defeat model is not appropriate for female rodents due to their low level of aggression. Therefore, a robust female model of social stress needs to be developed and validated. The aim of the present study was to unravel the long-lasting effects of vicarious social defeat (VSD) on the conditioned rewarding effects of cocaine and ethanol intake in female mice. Although VSD seems to be a good model for inducing behavioral and physiologic endophenotypes induced by stress, there are no studies to date that characterize the effect of VSD on cocaine or alcohol use. The results confirm that VSD females showed an increase in corticosterone levels after a vicarious experience while also displaying an increase in anxiety- and anhedonic-like behaviors. Three weeks after the last VSD, vicariously defeated female mice showed an increased developed preference for a non-effective dose of cocaine in the conditioned place preference (CPP) paradigm and showed an increase in ethanol intake. Our results suggest that female mice vicariously experience a state of distress through the social observation of others suffering from adverse events, confirming the use of VSD as a valid model to study the response to social stress in females. The fact that VSD in females induced a comparable behavioral phenotype to that observed in physically defeated males could indicate a relationship with the higher rate of psychopathologies observed in women. Notwithstanding, more studies are needed to dissect the neurobiological and behavioral peculiarities of the female response to social stress.

Context-dependent effects of the CB1 receptor antagonist rimonabant on morphine-induced behavioral sensitization in female mice

Introduction: The endocannabinoid system has been implicated in the neurobiology of opioid use disorder. While the CB1 receptor antagonist rimonabant has been shown to block some of the behavioral effects of opioids, studies suggest that the treatment environment (i.e., receiving treatment in the drug-associated environment, and/or novelty) can influence its effects. In the present study, we investigated the role of the treatment environment in the effects of rimonabant on the expression of morphine-induced behavioral sensitization. Methods: Adult female Swiss mice were submitted to a behavioral sensitization protocol, during which they received morphine (20 mg/kg, i.p.) in the open-field apparatus, and were subsequently treated with vehicle or rimonabant (1 or 10 mg/kg, i.p.) either in the open-field, in the home-cage or in an activity box (novel environment). The expression of conditioned locomotion (increased locomotor activity in the open-field apparatus in the absence of morphine) and of morphine-induced behavioral sensitization (increased locomotor activity in animals sensitized to morphine) was evaluated during asubsequent saline and morphine challenge, respectively. Results: Animals treated with morphine expressed behavioral sensitization, showing a significant increase in locomotor activity over time. Animals sensitized to morphine and treated with vehicle in the home-cage expressed conditioned locomotion, an effect that was blocked by home-cage treatment with rimonabant. During a saline challenge, only animals sensitized to morphine and treated with saline in the home-cage expressed morphine-induced conditioned locomotion. All morphine-treated animals that received saline during the treatment phase (control groups) expressed behavioral sensitization during the morphine challenge. Treatment with rimonabant in the open-field and in the activity box, but not in the home-cage, blocked the expression of morphine-induced behavioral sensitization. Discussion: Our findings suggest that CB1 receptor antagonism can modulate conditioned responses to morphine even when administered in the home-cage. However, exposure to the drug-associated environment or to a novel environment is necessary for the expression of rimonabant's effects on morphine-induced behavioral sensitization during a morphine challenge.

Impact of Mephedrone on Fear Memory in Adolescent Rats: Involvement of Matrix Metalloproteinase-9 (MMP-9) and N-Methyl-D-aspartate (NMDA) Receptor

Treatment of Post-Traumatic Stress Disorder (PTSD) is complicated by the presence of drug use disorder comorbidity. Here, we examine whether conditioned fear (PTSD model) modifies the rewarding effect of mephedrone and if repeated mephedrone injections have impact on trauma-related behaviors (fear sensitization, extinction, and recall of the fear reaction). We also analyzed whether these trauma-induced changes were associated with exacerbation in metalloproteinase-9 (MMP-9) and the GluN2A and GluN2B subunits of N-methyl-D-aspartate (NMDA) glutamate receptor expression in such brain structures as the hippocampus and basolateral amygdala. Male adolescent rats underwent trauma exposure (1.5 mA footshock), followed 7 days later by a conditioned place preference training with mephedrone. Next, the post-conditioning test was performed. Fear sensitization, conditioned fear, anxiety-like behavior, extinction acquisition and relapse were then assessed to evaluate behavioral changes. MMP-9, GluN2A and GluN2B were subsequently measured. Trauma-exposed rats subjected to mephedrone treatment acquired a strong place preference and exhibited impairment in fear extinction and reinstatement. Mephedrone had no effect on trauma-induced MMP-9 level in the basolateral amygdala, but decreased it in the hippocampus. GluN2B expression was decreased in the hippocampus, but increased in the basolateral amygdala of mephedrone-treated stressed rats. These data suggest that the modification of the hippocampus and basolateral amygdala due to mephedrone use can induce fear memory impairment and drug seeking behavior in adolescent male rats.

Short-Term Consequences of Single Social Defeat on Accumbal Dopamine and Behaviors in Rats

The present study aimed to explore the consequences of a single exposure to a social defeat on dopamine release in the rat nucleus accumbens measured with a fast-scan cyclic voltammetry. We found that 24 h after a social defeat, accumbal dopamine responses, evoked by a high frequency electrical stimulation of the ventral tegmental area, were more profound in socially defeated rats in comparison with non-defeated control animals. The enhanced dopamine release was associated with the prolonged immobility time in the forced swim test. The use of the dopamine depletion protocol revealed no alteration in the reduction and recovery of the amplitude of dopamine release following social defeat stress. However, administration of dopamine D2 receptor antagonist, raclopride (2 mg/kg, i.p.), resulted in significant increase of the electrically evoked dopamine release in both groups of animals, nevertheless exhibiting less manifested effect in the defeated rats comparing to control animals. Taken together, our data demonstrated profound alterations in the dopamine transmission in the association with depressive-like behavior following a single exposure to stressful environment. These voltammetric findings pointed to a promising path for the identification of neurobiological mechanisms underlying stress-promoted behavioral abnormalities.

The Protective Effect of Social Reward on Opioid and Psychostimulant Reward and Relapse: Behavior, Pharmacology, and Brain Regions

Until recently, most modern neuroscience research on addiction using animal models did not incorporate manipulations of social factors. Social factors play a critical role in human addiction: social isolation and exclusion can promote drug use and relapse, while social connections and inclusion tend to be protective. Here, we discuss the state of the literature on social factors in animal models of opioid and psychostimulant preference, self-administration, and relapse. We first summarize results from rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of traditional experimenter-controlled social interaction procedures on opioid and psychostimulant conditioned place preference, self-administration, and relapse. Next, we summarize behavioral and brain-mechanism results from studies using newer operant social-interaction procedures that inhibit opioid and psychostimulant self-administration and relapse. We conclude by discussing how the reviewed studies point to future directions for the addiction field and other neuroscience and psychiatric fields, and their implications for mechanistic understanding of addiction and development of new treatments.SIGNIFICANCE STATEMENT In this review, we propose that incorporating social factors into modern neuroscience research on addiction could improve mechanistic accounts of addiction and help close gaps in translating discovery to treatment. We first summarize rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of both traditional experimenter-controlled and newer operant social-interaction procedures. We then discuss potential future directions and clinical implications.

Excessive alcohol consumption after exposure to two types of chronic social stress: intermittent episodes vs. continuous exposure in C57BL/6J mice with a history of drinking

RATIONALE

The attraction to alcohol can be greatly increased when it is consumed in a social context. While pro-social interactions can potentiate voluntary alcohol drinking under some conditions, aversive social experience (i.e., social stress) can similarly intensify alcohol consumption.

OBJECTIVE

We sought to determine how exposure to different types of chronic social stress (i.e., intermittent episodes of social defeat or continuous social stress) influences alcohol consumption and the reinforcing effects of alcohol in mice with a history of drinking.

METHODS

Separate cohorts of male C57BL/6J mice were exposed to either 10 days of continuous or intermittent social defeat stress. In experiment 1, mice were assigned to 20% w/v alcohol consumption in a two-bottle choice protocol both prior to and after exposure to social defeat stress. In a second experiment, mice engaged in an operant response sequence to gain access to alcohol wherein completion of a fixed interval (FI; 5 min) schedule was reinforced with continuous access to alcohol (fixed ratio; FR1) for up to 1.8 g/kg. Alcohol-reinforced responding and subsequent alcohol consumption were assessed daily for 4 weeks prior to the 10-day social stress exposure and for 6-week post-stress. Machine learning was implemented to standardize the analysis of defeat behaviors exhibited by the intruder mouse during confrontation with an attacking resident.

RESULTS

In mice with a prior history of alcohol drinking, intermittent episodes of social defeat stress produced a significant increase in 20% EtOH consumption in preference over concurrently available water. This increased intake persisted for at least 6 weeks after the final social stress experience. Intermittently stressed mice also accelerated their anticipatory responding during the fixed interval component of the operant response chain that was reinforced by alcohol. Neither unstressed controls nor mice exposed to continuous social stress exhibited significant increases in alcohol consumption and alcohol reinforcement.

DISCUSSION

Episodic social defeat stress promotes the seeking and consumption of alcohol, extending earlier work to alcohol-experienced mice. We hypothesize that intermittent access to alcohol and intermittent episodes of social stress are additive and share common sensitizing neural mechanisms that engender excessive alcohol consumption.

Can we enhance the clinical efficacy of cognitive and psychological approaches to treat substance use disorders through understanding their neurobiological mechanisms?

Despite decades of research in the field of addiction, relapse rates for substance use disorders remain high. Consequently, there has been growing focus on providing evidence-based treatments for substance use disorders, resulting in the increased development and use of cognitive and psychological interventions. Such treatment approaches, including contingency management, community-reinforcement approach, and cognitive bias modification, have shown promising clinical efficacy in reducing substance use and promoting abstinence during treatment. However, these interventions are still somewhat limited in achieving sustained periods of abstinence post-treatment. The neurobiological mechanisms underpinning these treatment approaches remain largely unknown and under-studied, in part, due to a lack of translational animal models. The adoption of a reverse translational approach may assist in development of more representative models that can facilitate elucidation of the mechanisms behind these clinically relevant interventions. This review examines our current understanding of addiction neurobiology from clinical, preclinical research and existing animal models, and considers how the efficacy of such behavioral-oriented interventions alone, or in combination with pharmacotherapy, may be enhanced to improve treatment outcomes.

The role of social isolation stress in escalated aggression in rodent models

Anti-social behavior and violence are major public health concerns. Globally, violence contributes to more than 1.6 million deaths each year. Previous studies have reported that social rejection or neglect exacerbates aggression. In rodent models, social isolation stress is used to demonstrate the adverse effects of social deprivation on physiological, endocrinological, immunological, and behavioral parameters, including aggressive behavior. This review summarizes recent rodent studies on the effect of social isolation stress during different developmental periods on aggressive behavior and the underlying neural mechanisms. Social isolation during adulthood affects the levels of neurosteroids and neuropeptides and increases aggressive behavior. These changes are ethologically relevant for the adaptation to changes in local environmental conditions in the natural habitats. Chronic deprivation of social interaction after weaning, especially during the juvenile to adolescent periods, leads to the disruption of the development of appropriate social behavior and the maladaptive escalation of aggressive behavior. The understanding of neurobiological mechanisms underlying social isolation-induced escalated aggression will aid in the development of therapeutic interventions for escalated aggression.

Sex-Specific Brain Transcriptional Signatures in Human MDD and Their Correlates in Mouse Models of Depression

Major depressive disorder (MDD) is amongst the most devastating psychiatric conditions affecting several millions of people worldwide every year. Despite the importance of this disease and its impact on modern societies, still very little is known about the etiological mechanisms. Treatment strategies have stagnated over the last decades and very little progress has been made to improve the efficiency of current therapeutic approaches. In order to better understand the disease, it is necessary for researchers to use appropriate animal models that reproduce specific aspects of the complex clinical manifestations at the behavioral and molecular levels. Here, we review the current literature describing the use of mouse models to reproduce specific aspects of MDD and anxiety in males and females. We first describe some of the most commonly used mouse models and their capacity to display unique but also shared features relevant to MDD. We then transition toward an integral description, combined with genome-wide transcriptional strategies. The use of these models reveals crucial insights into the molecular programs underlying the expression of stress susceptibility and resilience in a sex-specific fashion. These studies performed on human and mouse tissues establish correlates into the mechanisms mediating the impact of stress and the extent to which different mouse models of chronic stress recapitulate the molecular changes observed in depressed humans. The focus of this review is specifically to highlight the sex differences revealed from different stress paradigms and transcriptional analyses both in human and animal models.

Cholinergic and dopaminergic-mediated motivated behavior in healthy states and in substance use and mood disorders

Acetylcholine is an important neuromodulator of the mesolimbic dopamine (DA) system, which itself is a mediator of motivated behavior. Motivated behavior can be described by two primary components, termed directional and activational motivation, both of which can be examined and dissociated using effort-choice tasks. The directional component refers to motivated behavior directed towards reinforcing stimuli and away from aversive stimuli. Behaviors characterized by increased vigor, persistence, and work output are considered to reflect activational components of motivation. Disruption of DA signaling has been shown to decrease activational components of motivation, while leaving directional features intact. Facilitation of DA release promotes the activational aspects of motivated behavior. In this review, we discuss cholinergic and DA regulation of motivated behaviors. We place emphasis on effort-choice processes and the ability of effort-choice tasks to examine and dissociate changes of motivated behavior in the context of substance use and mood disorders. Furthermore, we consider how altered cholinergic transmission impacts motivated behavior across disease states, and the possible role of cholinergic dysregulation in the etiology of these illnesses. Finally, we suggest that treatments targeting cholinergic activity may be useful in ameliorating motivational disruptions associated with substance use and comorbid substance use and mood disorders.

Peer presence and familiarity as key factors to reduce cocaine intake in both rats and humans: an effect mediated by the subthalamic nucleus

RATIONALE

Stimulant use, including cocaine, often occurs in a social context whose influence is important to understand to decrease intake and reduce associated harms. Although the importance of social influence in the context of drug addiction is known, there is a need for studies assessing its neurobiological substrate and for translational research.

OBJECTIVES

Here, we explored the influence of peer presence and familiarity on cocaine intake and its neurobiological basis. Given the regulatory role of the subthalamic nucleus (STN) on cocaine intake and emotions, we investigated its role on such influence of social context on cocaine intake.

METHODS

We first compared cocaine consumption in various conditions (with no peer present or with peers with different characteristics: abstinent peer or drug-taking peer, familiar or not, cocaine-naive or not, dominant or subordinate) in rats (n = 90). Then, with a translational approach, we assessed the influence of the social context (alone, in the group, in a dyad with familiar or non-familiar peers) on drug intake in human drug users (n = 77).

RESULTS

The drug consumption was reduced when a peer was present, abstinent, or drug-taking as well, and further diminished when the peer was non-familiar. The presence of a non-familiar and drug-naive peer represents key conditions to diminish cocaine intake. The STN lesion by itself reduced cocaine intake to the level reached in presence of a non-familiar naive peer and affected social cognition, positioning the STN as one neurobiological substrate of social influence on drug intake. Then, the human study confirmed the beneficial effect of social presence, especially of non-familiar peers.

CONCLUSION

Our results indirectly support the use of social interventions and harm reduction strategies and position the STN as a key cerebral structure to mediate these effects.

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.

Effects of moderate-intensity exercise on social health and physical and mental health of methamphetamine-dependent individuals: A randomized controlled trial

OBJECTIVE

Methamphetamine (MA)-dependent individuals' health problems are widespread and need to be solved urgently. Exercise is considered a potential treatment for MA dependents. The study aimed to determine the effects of a 12-week aerobic exercise on the social, physical, and mental health of MA-dependent individuals.

MATERIALS AND METHODS

Sixty MA-dependent individuals were randomly assigned into two groups. Subjects in the exercise group (n = 30) received an exercise intervention five days a week for 60 min each for 12 weeks. Subjects in the control group (n = 30) received regular corrective rehabilitation without exercise in the same setting. Outcome measures, including questionnaires [quality of life scale for drug addiction (QOL-DA), self-rating anxiety scale (SAS), self-rating depression scale (SDS), and Pittsburgh sleep quality index (PSQI)] and physical fitness, were arranged the day before the start of the intervention and the day after the end of the intervention. Two-factor repeated measures ANOVA was used to compare the treatment differences between the two groups.

RESULTS

After 12 weeks of the intervention period, social health was significantly improved in the exercise group (P < 0.01), and there was a statistically significant difference in mental health scores between exercise group and control group, with a greater impact in exercise group.(Psychology: P < 0.01; SAS: P < 0.01; SDS: P < 0.01; PSQI: P < 0.01), physical health improved in the exercise group, physiology (P < 0.01), symptom (P < 0.01), heart rate (P < 0.01), systolic blood pressure (P < 0.01), systolic blood pressure (P < 0.01), vital capacity (P < 0.05), grip (P < 0.01), vertical jump (P < 0.001), sit and reach (P < 0.01), 50-meter run (P < 0.01), and reaction time (P < 0.01).

CONCLUSION

Aerobic exercise intervention is an effective treatment for MA-dependent individuals, and the 12-week intervention improved the social, physical, and mental health of MA-dependent individuals. We recommend that future studies focus more on drug-dependent individuals' overall health status rather than just relapse.Clinical trial registration: [https://www.chictr.org.cn/hvshowproject.aspx?id=131048], identifier [ChiCTR2200055348].

Neurobiological Bases of Alcohol Consumption After Social Stress

The urge to seek and consume excessive alcohol is intensified by prior experiences with social stress, and this cascade can be modeled under systematically controlled laboratory conditions in rodents and non-human primates. Adaptive coping with intermittent episodes of social defeat stress often transitions to maladaptive responses to traumatic continuous stress, and alcohol consumption may become part of coping responses. At the circuit level, the neural pathways subserving stress coping intersect with those for alcohol consumption. Increasingly discrete regions and connections within the prefrontal cortex, the ventral and dorsal striatum, thalamic and hypothalamic nuclei, tegmental areas as well as brain stem structures begin to be identified as critical for reacting to and coping with social stress while seeking and consuming alcohol. Several candidate molecules that modulate signals within these neural connections have been targeted in order to reduce excessive drinking and relapse. In spite of some early clinical failures, neuropeptides such as CRF, opioids, or oxytocin continue to be examined for their role in attenuating stress-escalated drinking. Recent work has focused on neural sites of action for peptides and steroids, most likely in neuroinflammatory processes as a result of interactive effects of episodic social stress and excessive alcohol seeking and drinking.

Maternal Separation Stress Affects Voluntary Ethanol Intake in a Sex Dependent Manner

Maternal separation (MS) stress is a predictive animal model for evaluating the effects of early stress exposure on alcohol use disorders (AUD). The extended amygdala (AMY) is a complex circuit involved in both stress- and ethanol-related responses. We hypothesized that MS stress may increase ethanol consumption in adulthood, as well as augment neuronal activity in extended AMY, in a sex-dependent manner. We aimed to investigate the influence of MS stress on the ethanol consumption of male and female mice, and the involvement of extended amygdala sub-nuclei in this process. The C57BL/6J pups were subjected to 180min of MS, from postnatal day (PND) 1 to 14. The control group was left undisturbed. On PND 45, mice (n=28) in cages were exposed to a bottle containing 20% ethanol (w/v) for 4h during the dark period of the light-dark cycle, for 3weeks. Afterward, mice underwent ethanol self-administration training in operant chambers under fixed ratio (FR) schedule. Then, subjects were tested under 2h sessions of a progressive-ratio (PR) schedule of reinforcement (the last ratio achieved was considered the breaking point), and at the end, a 4h session of FR schedule (binge-intake). An immunohistochemistry assay for Fos protein was performed in Nucleus Accumbens (NAcc), Bed Nucleus of Stria Terminalis (BNST), and AMY. Our results showed that in the third week of training, the female MS group consumed more ethanol than the respective control group. The MS group presented increased breakpoint parameters. Female control group and male MS group were more resistant to bitter quinine taste. Increased Fos-immunoreactive neurons (Fos-IR) were observed in the central nucleus of AMY, but not in NAcc nor BNST in male maternal-separated mice. Maternal separation stress may influence ethanol intake in adulthood, and it is dependent on the sex and reinforcement protocol.

Factors modulating the incubation of drug and non-drug craving and their clinical implications

It was suggested in 1986 that cue-induced cocaine craving increases progressively during early abstinence and remains high during extended periods of time. Clinical evidence now supports this hypothesis and that this increase is not specific to cocaine but rather generalize across several drugs of abuse. Investigators have identified an analogous incubation phenomenon in rodents, in which time-dependent increases in cue-induced drug seeking are observed after abstinence from intravenous drug or palatable food self-administration. Incubation of craving is susceptible to variation in magnitude as a function of biological and/or the environmental circumstances surrounding the individual. During the last decade, the neurobiological correlates of the modulatory role of biological (sex, age, genetic factors) and environmental factors (environmental enrichment and physical exercise, sleep architecture, acute and chronic stress, abstinence reinforcement procedures) on incubation of drug craving has been investigated. In this review, we summarized the behavioral procedures adopted, the key underlying neurobiological correlates and clinical implications of these studies.

Early life stress and susceptibility to addiction in adolescence

Early life stress (ELS) is a risk factor for developing a host of psychiatric disorders. Adolescence is a particularly vulnerable period for the onset of these disorders and substance use disorders (SUDs). Here we discuss ELS and its effects in adolescence, especially SUDs, and their correlates with molecular changes to signaling systems in reward and stress neurocircuits. Using a maternal separation (MS) model of neonatal ELS, we studied a range of behaviors that comprise a "drug-seeking" phenotype. We then investigated potential mechanisms underlying the development of this phenotype. Corticotropin releasing factor (CRF) and serotonin (5-HT) are widely believed to be involved in "stress-induced" disorders, including addiction. Here, we show that ELS leads to the development of a drug-seeking phenotype indicative of increased susceptibility to addiction and concomitant sex-dependent upregulation of CRF and 5-HT system components throughout extended brain reward/stress neurocircuits.

Stress-induced vulnerability to develop cocaine addiction depends on cofilin modulation

Actin dynamics in dendritic spines can be associated with the neurobiological mechanisms supporting the comorbidity between stress exposure and cocaine increase rewards. The actin cytoskeleton remodeling in the nucleus accumbens (NA) has been implicated in the expression of stress-induced cross-sensitization with cocaine. The present study evaluates the involvement of cofilin, a direct regulator of actin dynamics, in the impact of stress on vulnerability to cocaine addiction. We assess whether the neurobiological mechanisms that modulate repeated-cocaine administration also occur in a chronic restraint stress-induced cocaine self-administration model. We also determine if chronic stress induces alterations in dendritic spines through dysregulation of cofilin activity in the NA core. Here, we show that the inhibition of cofilin expression in the NA core using viral short-hairpin RNA is sufficient to prevent the cocaine sensitization induced by chronic stress. The reduced cofilin levels also impede a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor surface expression enhancement and promote the reduction of head diameter in animals pre-exposed to stress after a cocaine challenge in the NA core. Moreover, downregulation of cofilin expression prevents facilitation of the acquisition of cocaine self-administration (SA) in male rats pre-exposed to chronic stress without modifying performance in sucrose SA. These findings reveal a novel, crucial role for cofilin in the neurobiological mechanisms underpinning the comorbidity between stress exposure and addiction-related disorders.

Hypoactive Thalamic Crh+ Cells in a Female Mouse Model of Alcohol Drinking After Social Trauma

BACKGROUND

Comorbid stress-induced mood and alcohol use disorders are increasingly prevalent among female patients. Stress exposure can disrupt salience processing and goal-directed decision making, contributing to persistent maladaptive behavioral patterns; these and other stress-sensitive cognitive and behavioral processes rely on dynamic and coordinated signaling by midline and intralaminar thalamic nuclei. Considering the role of social trauma in the trajectory of these debilitating psychopathologies, identifying vulnerable thalamic cells may provide guidance for targeting persistent stress-induced symptoms.

METHODS

A novel behavioral protocol traced the progression from social trauma to the development of social defensiveness and chronically escalated alcohol consumption in female mice. Recent cell activation-measured as cFos-was quantified in thalamic cells after safe social interactions, revealing stress-sensitive corticotropin-releasing hormone-expressing (Crh+) anterior central medial thalamic (aCMT) cells. These cells were optogenetically stimulated during stress-induced social defensiveness and abstinence-escalated binge drinking.

RESULTS

Crh+ aCMT neurons exhibited substantial activation after social interactions in stress-naïve but not in stressed female mice. Photoactivating Crh+ aCMT cells dampened stress-induced social deficits, whereas inhibiting these cells increased social defensiveness in stress-naïve mice. Optogenetically activating Crh+ aCMT cells diminished abstinence-escalated binge alcohol drinking in female mice, regardless of stress history.

CONCLUSIONS

This work uncovers a role for Crh+ aCMT neurons in maladaptive stress-induced social interactions and in binge drinking after forced abstinence in female mice. This molecularly defined thalamic cell population may serve as a critical stress-sensitive hub for social deficits caused by exposure to social trauma and for patterns of excessive alcohol drinking in female populations.

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.

Development of Mixed Anxiety/Depression-Like State as a Consequence of Chronic Anxiety: Review of Experimental Data

The review presents experimental data considered from the point of view of dynamic changes in the brain neurochemistry, physiology, and behavior of animals during the development of mixed anxiety/depression-like disorder caused by chronic social stress from norm to severe psychopathology. Evidences are presented to support the hypothesis that chronic anxiety rather than social defeat stress is an etiological factor in depression. The consequences of chronic anxiety for human health and social life are discussed.

Social Stress and Aggression in Murine Models

Throughout life, animals engage in a variety of social interactions ranging from the affiliative mother-offspring interaction and juvenile play to aggressive conflict. Deprivation of the appropriate social interaction during early development is stressful and disrupts the development of appropriate social behaviors and emotional responses later in life. Additionally, agonistic encounters can induce stress responses in both dominant and subordinate individuals. This review focuses on the social stress that escalates aggressive behavior of animals and discusses the known neurobiological and physiological mechanisms underlying the link between social stress and aggression. Social instigation, a brief exposure to a rival without physical contact, induces aggressive arousal in dominant animals and escalates aggressive behaviors in the following agonistic encounter. Furthermore, the experience of winning an aggressive encounter is known to be as rewarding as addictive drugs, and the experience of repeatedly winning induces addiction-like behavioral and neurobiological changes and leads to abnormal aggressive behaviors. Social isolation stress in early development from neonatal to juvenile and adolescent periods also affects aggressive behavior, but these effects largely depend on the strain, sex, and species as well as the stage of development in which isolation stress is experienced. In conclusion, understanding neurobiological mechanisms underlying the link between social stress and aggression will provide an important insight for the development of more effective and tolerable treatments for maladaptive aggression in humans.

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.

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.

Prolonged chronic social defeat stress promotes less resilience and higher uniformity in depression-like behaviors in adult male mice

Chronic social defeat stress (CSDS) is widely applied to study of depression in rodents. 10-day CSDS was a most commonly employed paradigm but with high resilience ratio (∼30%), producing potential variation in depression-like behavioral symptoms. Whether prolonged period (21 days) of CSDS would promote less resilience and reduce behavioral variability remains unknown. We applied 10-day and 21-day CSDS paradigms to induce mouse model of depression and compared their resilience ratio and behavioral phenotypes. Mice under 21-day CSDS had significantly lower resilience ratio and greater changes in behavioral indicators relative to mice under 10-day CSDS. Behavioral indicators from 21-day CSDS paradigm had higher correlations and better prediction for susceptibility which indicating higher uniformity in behavioral phenotypes. Furthermore, a subset of behavioral indicators in 21-day CSDS had high prediction efficacy and should be first applied to screen susceptibility of CSDS. Thus, our study demonstrates that 21-day CSDS is a more robust paradigm inducing reliable depression-like behaviors relative to 10-day CSDS, and should be preferentially used in rodent studies of depression.

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.

The neurochemistry of social reward during development: What have we learned from rodent models?

Social rewards are fundamental to survival and overall health. Several studies suggest that adequate social stimuli during early life are critical for developing appropriate socioemotional and cognitive skills, whereas adverse social experiences negatively affect the proper development of brain and behavior, by increasing the susceptibility to develop neuropsychiatric conditions. Therefore, a better understanding of the neural mechanisms underlying social interactions, and their rewarding components in particular, is an important challenge of current neuroscience research. In this context, preclinical research has a crucial role: Animal models allow to investigate the neurobiological aspects of social reward in order to shed light on possible neurochemical alterations causing aberrant social reward processing in neuropsychiatric diseases, and they allow to test the validity and safety of innovative therapeutic strategies. Here, we discuss preclinical research that has investigated the rewarding properties of two forms of social interaction that occur in different phases of the lifespan of mammals, that is, mother-infant interaction and social interactions with peers, by focusing on the main neurotransmitter systems mediating their rewarding components. Together, the research performed so far helped to elucidate the mechanisms of social reward and its psychobiological components throughout development, thus increasing our understanding of the neurobiological substrates sustaining social functioning in health conditions and social dysfunction in major psychiatric disorders.

Unique effects of social defeat stress in adolescent male mice on the Netrin-1/DCC pathway, prefrontal cortex dopamine and cognition (Social stress in adolescent vs. adult male mice)

For some individuals, social stress is a risk factor for psychiatric disorders characterised by adolescent onset, prefrontal cortex (PFC) dysfunction and cognitive impairments. Social stress may be particularly harmful during adolescence when dopamine (DA) axons are still growing to the PFC, rendering them sensitive to environmental influences. The guidance cue Netrin-1 and its receptor, DCC, coordinate to control mesocorticolimbic DA axon targeting and growth during this age. Here we adapted the accelerated social defeat (AcSD) paradigm to expose male mice to social stress in either adolescence or adulthood and categorised them as "resilient" or "susceptible" based on social avoidance behaviour. We examined whether stress would alter the expression of DCC and Netrin-1 in mesolimbic dopamine regions and would have enduring consequences on PFC dopamine connectivity and cognition. While in adolescence the majority of mice are resilient but exhibit risk-taking behaviour, AcSD in adulthood leads to a majority of susceptible mice without altering anxiety-like traits. In adolescent, but not adult mice, AcSD dysregulates DCC and Netrin-1 expression in mesolimbic DA regions. These molecular changes in adolescent mice are accompanied by changes in PFC DA connectivity. Following AcSD in adulthood, cognitive function remains unaffected, but all mice exposed to AcSD in adolescence show deficits in inhibitory control when they reach adulthood. These findings indicate that exposure to AcSD in adolescence vs. adulthood has substantially different effects on brain and behaviour and that stress-induced social avoidance in adolescence does not predict vulnerability to deficits in cognitive performance.Significance statement During adolescence, dopamine circuitries undergo maturational changes which may render them particularly vulnerable to social stress. While social stress can be detrimental to adolescents and adults, it may engage different mechanisms and impact different domains, depending on age. The accelerated social defeat (AcSD) model implemented here allows exposing adolescent and adult male mice to comparable social stress levels. AcSD in adulthood leads to a majority of socially avoidant mice. However, the predominance of AcSD-exposed adolescent mice does not develop social avoidance, and these resilient mice show risk-taking behaviour. Nonetheless, in adolescence only, AcSD dysregulates Netrin-1/DCC expression in mesolimbic dopamine regions, possibly disrupting mesocortical dopamine and cognition. The unique adolescent responsiveness to stress may explain increased psychopathology risk at this age.

Social withdrawal and gender differences: Clinical phenotypes and biological bases

Evidence from everyday life suggests that differences in social behaviors between males and females exist, both in animal and in humans. These differences can be related to socio-cultural determinants, but also to specialized portions of the brain (the social brain), from the neurotransmitter to the neural network level. The high vulnerability of this system is expressed by the wide range of neuropsychiatric disorders associated with social dysfunctions, particularly social withdrawal. The principal psychiatric disorders with prominent social withdrawal are described, including hikikomori-like syndromes, and anxiety, depressive, autistic, schizophrenic, and personality disorders. It is hypothesized that social withdrawal can be partially independent from other symptoms and likely reflect alterations in the social brain itself, leading to a similar, transdiagnostic social dysfunction, reflecting defects in the social brain across a variety of psychopathological conditions. An overview is provided of gender effects in the biological determinants of social behavior, including: the anatomical structures of the social brain; the dimorphic brain structures, and the modulation of their development by sex steroids; gender differences in "social" neurotransmitters (vasopressin and oxytocin), and in their response to social stress. A better comprehension of gender differences in the phenotypes of social disorders and in the neural bases of social behaviors may provide new insights for timely, focused, innovative, and gender-specific treatments.

Long-Term Impacts of Post-weaning Social Isolation on Nucleus Accumbens Function

Adolescence is a period of incredible change, especially within the brain's reward circuitry. Stress, including social isolation, during this time has profound effects on behaviors associated with reward and other neuropsychiatric disorders. Because the Nucleus Accumbens (NAc), is crucial to the integration of rewarding stimuli, the NAc is especially sensitive to disruptions by adolescent social isolation stress. This review highlights the long-term behavioral consequences of adolescent social isolation rearing on the NAc. It will discuss the cellular and molecular changes within the NAc that might underlie the long-term effects on behavior. When available sex-specific effects are discussed. Finally by mining publicly available data we identify, for the first time, key transcriptional profiles induced by adolescence social isolation in genes associated with dopamine receptor 1 and 2 medium spiny neurons and genes associated with cocaine self-administration. Together, this review provides a comprehensive discussion of the wide-ranging long-term impacts of adolescent social isolation on the dopaminergic system from molecules through behavior.

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.

Toward Understanding the Sex Differences in the Biological Mechanism of Social Stress in Mouse Models

Significant sex differences in terms of prevalence, symptomatic profiles, severity, and comorbidities of psychiatric disorders are quite common. Women have been shown to be more vulnerable to stress and are nearly twice as likely as men to develop stress-related disorders such as depression and anxiety. Therefore, understanding sex differences with respect to the neurobiological mechanisms underlying stress-related disorders is important for developing more efficient pharmacological interventions for women. However, most preclinical studies on stress-related disorders have focused heavily on male rodents. Here, recent developments in the study of repeated social defeat stress models in female mice are summarized. Our findings suggest that a variety of factors need to be considered when employing this model.

Nicotine treatment buffers negative behavioral consequences induced by exposure to physical and emotional stress in adolescent male mice

Early life stress influences adult psychopathology and is associated with an increase in the propensity for drug use/seeking throughout the lifespan. Animal models corroborate that stress exposure exacerbates maladaptive reactivity to stressful stimuli while also shifting the rewarding properties of many drugs of abuse, including nicotine (NIC), a stimulant commonly misused by adolescents. Interestingly, NIC treatment can also normalize some stress-induced behavioral deficits in adult rodents; however, little is known about NIC's therapeutic efficacy following stress experienced during adolescence. The goal of the following experiments was to elucidate NIC's ability to buffer the negative consequences of stress exposure, and to further assess behavioral responsivity while on the drug. Given that stress often occurs in both physical and non-physical forms, we employed the vicarious social defeat stress (VSDS) model which allows for investigation of both physical (PS) and emotional stress (ES) exposure. After 10 days, exposure to PS and ES decreased interaction with a social target in the social interaction test (SIT), confirming social avoidance. Groups were further divided and given NIC (0.0 or 160 mg/L) in their drinking water. After 1 month of NIC consumption, the mice were exposed to the SIT, elevated plus maze (EPM), and the forced swim test (FST), respectively. NIC-treated mice showed a reversal of stress-induced deficits in the EPM and FST. Surprisingly, the mice did not show improvement in the SIT regardless of treatment condition. Together, these data confirm NIC's ability to normalize some stress-induced behavioral deficits; however, NIC's effects on social behavior need further investigation.

Can I Get a Witness? Using Vicarious Defeat Stress to Study Mood-Related Illnesses in Traditionally Understudied Populations

The chronic social defeat stress model has been instrumental in shaping our understanding of neurobiology relevant to affect-related illnesses, including major depressive disorder. However, the classic chronic social defeat stress procedure is limited by its exclusive application to adult male rodents. We have recently developed a novel vicarious social defeat stress procedure wherein one mouse witnesses the physical defeat bout of a conspecific from the safety of an adjacent compartment. This witness mouse develops a similar behavioral phenotype to that of the mouse that physically experiences social defeat stress, modeling multiple aspects of major depressive disorder. Importantly, this new procedure allows researchers to perform vicarious social defeat stress in males or females and in juvenile mice, which typically are excluded from classic social defeat experiments. Here we discuss several recent advances made using this procedure and how its application provides a new preclinical approach to study the neurobiology of psychological stress-induced phenotypes.

Naringin Confers Protection against Psychosocial Defeat Stress-Induced Neurobehavioral Deficits in Mice: Involvement of Glutamic Acid Decarboxylase Isoform-67, Oxido-Nitrergic Stress, and Neuroinflammatory Mechanisms

Psychosocial stress has been widely reported to contribute to psychiatric disturbances. Perturbations in the enzymes of GABAergic and cholinergic systems have been implicated as precursors in different stress-related neuropsychiatric diseases. Targeting glutamic acid decarboxylase-67 kDa (GAD67) and acetylcholinesterase (AChE) via oxidative, nitrergic, and neuroinflammatory mechanisms have been recognized as prospective strategies for the prevention of psychosocial stress-induced behavioral impairments. Naringin, a neuro-active flavonoid compound isolated from citrus fruits, has been shown to produce memory-enhancing, antiepileptic, antidepressant, and anti-inflammatory activities similarly to ginseng, a very potent adaptogen. In this communication, we assessed the effect of naringin on social-defeat stress (SDS)-induced behavioral, GABAergic, cholinergic, oxidative, nitrergic, and neuroinflammatory changes in mice using the resident-intruder paradigm. The intruder male mice were culled into six groups. Groups 1 and 2 (normal- and SDS-controls) received sterile saline, groups 3-5 were given naringin (25-100 mg/kg, i.p.) whereas group 6 had ginseng (50 mg/kg, i.p.) daily for 14 days, but followed by 10 min SDS (physical and psychological) exposure to groups 2-6 with aggressor-resident mice. Behavioral effects using Y-maze, elevated-plus maze, sociability, and tail-suspension tests were assessed on day 14. GAD67, AChE enzymes, and biomarkers of oxidative, nitrergic, and neuroinflammatory changes were assayed in the striatum, prefrontal cortex, and hippocampus. Naringin and ginseng reversed all SDS-induced behavioral impairments. Naringin increased the levels of GAD67 and decreased AChE activities in the striatum, prefrontal cortex, and hippocampus. Furthermore, naringin reduced pro-inflammatory cytokines (TNF-α, IL-6), malondialdehyde, nitrite concentrations, and increased glutathione levels in a region-dependent manner. Our study suggests that naringin attenuated SDS-induced behavioral endophenotypes of neuropsychiatric disease through increased GAD67 synthesis, inhibition of AChE activity, oxidative, nitrergic stress, and neuroinflammatory processes in stress-sensitive brain regions.

Involvement between social defeat stress and pain-related behavior in a rat lumbar disk herniation model

INTRODUCTION

Psychological and social factors are involved in the disability and chronicity of pain. Our study aim was to investigate whether social defeat stress (SDS) as a psychophysical stress affected mechanical withdrawal thresholds in the lumbar disk herniation (LDH) rat model. Changes in microglia and astrocytes, which play important roles in neuropathic pain states, were also investigated.

MATERIALS AND METHODS

For the LDH model, nucleus pulposus (NP) was applied to the L5 dorsal root ganglion (DRG) in adult female Sprague-Dawley rats. SDS was performed 15 min daily for 8 days. Mechanical withdrawal thresholds were measured, and immunoreactive cells of glial fibrillary acidic protein (GFAP) and ionized calcium-binding adaptor molecule-1 (Iba-1), which were used as markers of microglia, satellite glial cells, and astrocytes, were assessed in the DRG, spinal cord (SC), and ventrolateral periaqueductal gray matter (VLPAG).

RESULTS

Mechanical withdrawal thresholds decreased in the NP group for 21 days and for 35 days in the NP + SDS group. Expression of GFAP and Iba-1 in the DRG and SC increased up to day 21 in the NP and NP + SDS groups. In the sham + SDS and NP + SDS groups, expression of GFAP in the VLPAG decreased until day 35.

CONCLUSION

SDS prolongs mechanical allodynia induced by NP. Changes of GFAP expression in the VLPAG were associated with mechanical allodynia of the NP + SDS group during the late phase. These results suggest that psychological chronic stress might delay recovery from mechanical allodynia induced by the LDH model.

Corticotropin-releasing factor receptor signaling and modulation: implications for stress response and resilience

Introduction In addition to their role in regulation of the hypothalamic-pituitary-adrenal-axis, corticotropin-releasing factor (CRF) and its related peptides, the urocortins, are important mediators of physiological and pathophysiological processes of the central nervous, cardiovascular, gastrointestinal, immune, endocrine, reproductive, and skin systems. Altered regulation of CRF-mediated adaptive responses to various stressful stimuli disrupts healthy function and might confer vulnerability to several disorders, including depression and anxiety. Methodology This narrative review was conducted through search and analysis of studies retrieved from online databases using a snowball method. Results This review covers aspects beginning with the discovery of CRF, CRF binding protein and their actions via interaction with CRF receptors type 1 and type 2. These are surface plasma membrane receptors, activation of which is associated with conformational changes and interaction with a variety of G-proteins and signaling pathways. We also reviewed the pharmacology and mechanisms of the receptor signaling modulatory activity of these receptors. Conclusion This review compiles and presents knowledge regarding the CRFergic system, including CRF related peptides, CRF binding protein, and CRF receptors, as well as some evidence that is potentially indicative of the biological roles of these entities in several physiological and pathophysiological processes.

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.

Enhanced psychostimulant response, but not social avoidance, depends on GluA1 AMPA receptors in VTA dopamine neurons following intermittent social defeat stress in rats

Evidence from both human and animal studies demonstrates the importance of social stress in the development of addiction‐related behaviour. In rats, intermittent social defeat stress causes long‐lasting psychostimulant cross‐sensitization. Our recent data reveal heightened expression of AMPA receptor (AMPAR) GluA1 subunit in rat ventral tegmental area (VTA), which occurs concurrently with social stress‐induced amphetamine (AMPH) cross‐sensitization. In addition, social stress in rats induced social avoidance behaviour. The present study evaluated the effects of intermittent social defeat stress on GluA1 expression in VTA dopamine (DA) neurons, then utilized Cre‐dependent virus‐mediated gene transfer to determine the functional role of homomeric GluA1‐AMPARs in these neurons. Social defeat stress exposure induced GluA1 expression in VTA DA neurons, as demonstrated by a greater density of GluA1/tyrosine hydroxylase (TH) double‐labelling in VTA neurons in stressed rats. Additionally, functional inactivation of VTA GluA1 AMPARs in DA neurons prevented stress‐induced cross‐sensitization, or augmented locomotor response to low dose AMPH challenge (1.0 mg/kg, i.p.), but had no effect on social stress‐induced social avoidance behaviour. Furthermore, wild‐type overexpression of GluA1 in VTA DA neurons had the opposite effect; locomotor‐activating effects of AMPH were significantly augmented, even in the absence of stress. Taken together, these results suggest that stress‐induced GluA1 expression in VTA DA neurons is necessary for psychostimulant cross‐sensitization, but not for social avoidance. This differential effect suggests that different neural pathways are implicated in these behaviours. These findings could lead to novel pharmacotherapies to help prevent stress‐induced susceptibility to substance abuse.

Chronic harmine treatment has a delayed effect on mobility in control and socially defeated rats

INTRODUCTION

Depression is characterized by behavioral, cognitive and physiological changes, imposing a major burden on the overall wellbeing of the patient. Some evidence indicates that social stress, changes in growth factors (e.g., brain-derived neurotrophic factor (BDNF)), and neuroinflammation are involved in the development and progression of the disease. The monoamine oxidase A inhibitor drug harmine was suggested to have both antidepressant and anti-inflammatory properties and may, therefore, be a potential candidate for treatment of depression.

AIM

The goal of this study was to assess the effects of harmine on behavior, brain BDNF levels, and microglia activation in control rats and a rat model of social stress.

MATERIAL AND METHODS

Rats were submitted to 5 consecutive days of repeated social defeat (RSD) or control conditions. Animals were treated daily with harmine (15 mg/kg) or vehicle from day 3 until the end of the experiment. To assess the effects of harmine treatment on behavior, the sucrose preference test (SPT) was performed on days 1, 6, and 15, the open field test (OFT) on days 6 and 14, and the novel object recognition test (NOR) on day 16. Brain microgliosis was assessed using [11C]PBR-28 PET on day 17. Animals were terminated on day 17, and BDNF protein concentrations in the hippocampus and frontal cortex were analyzed using ELISA.

RESULTS

RSD significantly decreased bodyweight and increased anxiety and anhedonia-related parameters in the OFT and SPT on day 6, but these behavioral effects were not observed anymore on day 14/15. Harmine treatment caused a significant reduction in bodyweight gain in both groups, induced anhedonia in the SPT on day 6, and significantly reduced the mobility and exploratory behavior of the animals in the OFT mainly on day 14. PET imaging and the NOR test did not show any significant effects on microglia activation and memory, respectively. BDNF protein concentrations in the hippocampus and frontal cortex were not significantly affected by either RSD or harmine treatment.

DISCUSSION

Harmine was not able to reverse the acute effects of RSD on anxiety and anhedonia and even aggravated the effect of RSD on bodyweight loss. Moreover, harmine treatment caused unexpected side effects on general locomotion, both in RSD and control animals, but did not influence glial activation status and BDNF concentrations in the brain. In this model, RSD-induced stress was not strong enough to induce long-term effects on the behavior, neuroinflammation, or BDNF protein concentration. Thus, the efficacy of harmine treatment on these delayed parameters needs to be further evaluated in more severe models of chronic stress.

Repeated psychosocial stress causes glutamic acid decarboxylase isoform-67, oxidative-Nox-2 changes and neuroinflammation in mice: Prevention by treatment with a neuroactive flavonoid, morin

Psychosocial stress and biological predispositions are linked to mood and personality disorders related to psychiatric behaviors. Targeting neuroinflammation and oxidative stress has been recognized as a potential strategy for the prevention of psychosocial stress-induced psychiatric disorders. Morin, a bioactive compound isolated from mulberry leaf has been shown to produce antiamnesic, antipsychotic and anti-inflammatory effects relative to ginseng, a well-known adaptogen. Hence, the present study investigated the effect of morin on social-defeat stress (SDS)-induced behavioral, neurochemical, neuroimmune and neurooxidative changes in mice using intruder-resident paradigm. The intruder male mice were distributed into 6 groups (n = 10). Groups 1 (normal-control) and 2 (SDS-control) received normal saline, groups 3-5 had morin (25-100 mg/kg) while group 6 received ginseng (50 mg/kg) intraperitoneally daily for 14 days. Thirty minutes after treatment from days 7-14 onwards, mice in groups 2-6 were exposed to SDS for 10 min physical and psychological confrontations respectively with aggressive-resident mice. Neurobehavioral effects (locomotor activity, cognitive performance, anxiety- and depressive-like behavior) were assessed on day 14. Biomarkers of oxidative/nitrergic stress and neuroinflammation; acetylcholinesterase (AChE) and glutamic-acid decarboxylase-67 (GAD₆₇) were measured in the striatum, prefrontal-cortex and hippocampus. Behavioral deficits induced by SDS were attenuated by morin and ginseng. Both morin and ginseng decreasedmalondialdehyde, nitrite levels and increased glutathione concentrations in the brain regions. They also reduced inflammatory mediators (TNF-α, IL-6, COX-2 and NF-κB), AChE activity and Nox-2 expression in the specific brain regions. However, morin increased the levels of GAD₆₇ in the striatum, prefrontal-cortex and hippocampus in contrast to ginseng. Our results suggest that morin mitigates SDS-induced neurobehavioral deficits through enhancement of GAD₆₇, inhibition of AChE activity, oxidative stress, Nox-2 and neuroinflammatory pathways.