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

Individual differences in behavioral responses to predator odor predict subsequent stress reactivity in female rats

Stress-induced neuropsychiatric disorders are among the most prevalent medical conditions and have widespread effects on both patients and society. Females experience over twice the rates of stress-related anxiety and depression when compared to males and often exhibit worse symptomatology and treatment outcomes. However, preclinical experiments exploring the neurobiological mechanisms of stress susceptibility in females have been traditionally understudied. Previous data from our lab has determined that females are selectively vulnerable to the consequences of vicarious witness stress, and these experiments were designed to determine specific behavioral and physiological factors that could predict which groups would be more susceptible to the effects of stress. Adult, female, Sprague-Dawley rats were first exposed to a ferret predator odor to determine baseline individual differences in behavioral responses. Rats were stratified by the duration of freezing behavior exhibited in response to the ferret odor and equally balanced into non-stressed controls and vicarious witness stress exposed groups. These female rats were then assessed on a battery of behavioral tasks including sucrose preference, elevated plus maze, acoustic startle, and the ferret odor and witness stress cue exposures to determine if baseline differences in stress responding can predict the behavioral response to future stress and stress cues. High freezing in response to the ferret odor was associated with behavioral sensitization to witness stress and hypervigilant responses to stress cues that was accompanied by exaggerated neuroimmune responses. These experiments establish a powerful behavioral predictor of stress susceptibility in females and begin to address neurobiological correlates that underlie this response.

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.

An automated group-housed oral fentanyl self-administration method in mice

RATIONALE AND OBJECTIVES

Social factors play a critical role in human drug addiction, and humans often consume drugs together with their peers. In contrast, in traditional animal models of addiction, rodents consume or self-administer the drug in their homecage or operant self-administration chambers while isolated from their peers. Here, we describe HOMECAGE ("Home-cage Observation and Measurement for Experimental Control and Analysis in a Group-housed Environment"), a translationally relevant method for studying oral opioid self-administration in mice. This setting reduces experimental confounds introduced by social isolation or interaction with the experimenter.

METHODS

We have developed HOMECAGE, a method in which mice are group-housed and individually monitored for their consumption of a drug vs. a reference liquid.

RESULTS

Mice in HOMECAGE preserve naturalistic aspects of behavior, including social interactions and circadian activity. The mice showed a preference for fentanyl and escalated their fentanyl intake over time. Mice preferred to consume fentanyl in bouts during the dark cycle. Mice entrained to the reinforcement schedule of the task, optimizing their pokes to obtain fentanyl rewards, and maintained responding for fentanyl under a progressive ratio schedule. HOMECAGE also enabled the detection of cage-specific and individual-specific behavior patterns and allowed the identification of differences in fentanyl consumption between co-housed control and experimental mice.

CONCLUSIONS

HOMECAGE serves as a valuable procedure for translationally relevant studies on oral opioid intake under conditions that more closely mimic the human condition. The method enables naturalistic investigation of factors contributing to opioid addiction-related behaviors and can be used to identify novel treatments.

Divergent changes in social stress-induced motivation in male and female mice

Exposure to stressors has been shown to dysregulate motivated behaviors in a bidirectional manner over time. The relationship between stress and motivation is relevant to psychological disorders, including depression, binge eating, and substance use disorder; however, this relationship is not well characterized, especially in females, despite their increased risk of these disorders. Social defeat stress is a common model to study stress-induced motivation changes, however, historically this model excluded females due to lack of female-to-female aggression and unreliable male-to-female aggression. Additionally, changes in motivation are often assessed well after stress exposure ends, potentially missing or occluding changes to motivation during stress. Recently, the chronic non-discriminatory social defeat stress (CNSDS) model has demonstrated social defeat of male and female C57BL/6J mice by simultaneously exposing both mice to an aggressive male CD-1 mouse. Here we use this model to directly compare changes in the motivated behavior of male and female mice during and following chronic stress. We hypothesized that motivated behavioral responses would be dysregulated during stress and that the effects would worsen as the stress exposure continued. To monitor motivated behavior, mice had access to a Feeding Experimental Device.3 (FED3), a home cage device for operant responding. Operant responding was monitored prior to, during, and after stress by measuring nose pokes for sucrose pellets on a modified progressive ratio schedule of reinforcement. Our results demonstrated divergent behavioral outcomes between males and female mice in response to stress; where male mice increased motivated behavior during stress only, whereas female mice exhibited a decrease in motivation during and after stress. This study highlights the need to investigate the effects of stress-induced motivation over time, as well as the increased need to understand differences in the stress response in females.

Effects of Ketogenic Diet on Increased Ethanol Consumption Induced by Social Stress in Female Mice

Stress is a critical factor in the development of mental disorders such as addiction, underscoring the importance of stress resilience strategies. While the ketogenic diet (KD) has shown efficacy in reducing alcohol consumption in male mice without cognitive impairment, its impact on the stress response and addiction development, especially in females, remains unclear. This study examined the KD's effect on increasing ethanol intake due to vicarious social defeat (VSD) in female mice. Sixty-four female OF1 mice were divided into two dietary groups: standard diet (n = 32) and KD (n = 32). These were further split based on exposure to four VSD or exploration sessions, creating four groups: EXP-STD (n = 16), VSD-STD (n = 16), EXP-KD (n = 16), and VSD-KD (n = 16). KD-fed mice maintained ketosis from adolescence until the fourth VSD/EXP session, after which they switched to a standard diet. The Social Interaction Test was performed 24 h after the last VSD session. Three weeks post-VSD, the Drinking in the Dark test and Oral Ethanol Self-Administration assessed ethanol consumption. The results showed that the KD blocked the increase in ethanol consumption induced by VSD in females. Moreover, among other changes, the KD increased the expression of the ADORA1 and CNR1 genes, which are associated with mechanisms modulating neurotransmission. Our results point to the KD as a useful tool to increase resilience to social stress in female mice.

Region-Specific Gene Expression Changes Associated with Oleoylethanolamide-Induced Attenuation of Alcohol Self-Administration

Oleoylethanolamide (OEA) is a lipid with anti-inflammatory activity that modulates multiple reward-related behaviors. Previous studies have shown that OEA treatment reduces alcohol self-administration (SA) while inhibiting alcohol-induced inflammatory signaling. Nevertheless, the specific mechanisms that OEA targets to achieve these effects have not been widely explored. Here, we tested the effects of OEA treatment during alcohol SA, extinction or previous to cue-induced reinstatement of alcohol seeking. In addition, we measured gene expression changes in the striatum and hippocampus of relevant receptors for alcohol consumption (Drd1, Drd2, Cnr1, Oprm) as well as immune-related proteins (Il-6, Il-1β, Tlr4) and the brain-derived neurotrophic factor (Bdnf). Our results confirmed that when administered contingently, systemic OEA administration reduced alcohol SA and attenuated cue-induced reinstatement. Interestingly, we also observed that OEA treatment reduced the number of sessions needed for the extinction of alcohol seeking. Biochemical analyses showed that OEA induced gene expression changes in dopamine and cannabinoid receptors in the striatum and hippocampus. In addition, OEA treatment modulated the long-term immune response and increased Bdnf expression. These results suggest that boosting OEA levels may be an effective strategy for reducing alcohol SA and preventing relapse.

Behavioural traits related with resilience or vulnerability to the development of cocaine-induced conditioned place preference after exposure of female mice to vicarious social defeat

Exposure to stress induced by intermittent repeated social defeat (IRSD) increases vulnerability to the development of cocaine-induced conditioned place preference (CPP) among male mice; however, some defeated mice are resilient to these effects of stress. In the present study we evaluated the effects of vicarious IRSD (VIRSD) in female mice and explored behavioural traits that are potentially predictive of resilience. C57BL/6 female mice (n = 28) were exposed to VIRSD, which consisted of the animals witnessing a short experience of social defeat by a male mouse on postnatal day (PND) 47, 50, 53 and 56. The control group (n = 10) was not exposed to stress. Blood samples were collected on PND 47 and 56 for corticosterone and interleukin-6 determinations. On PND 57-58, female mice performed several behavioural tests (elevated plus maze, hole-board, object recognition, social interaction, TST and splash tests). Three weeks later, the effects of cocaine (1.5 mg/kg) on the CPP paradigm were assessed. VIRSD decreased corticosterone levels (on PND 56), increased interleukin-6 levels, enhanced novelty-seeking, improved recognition memory and induced anxiety- and depression-like symptoms. Control and VIRSD female mice did not acquire CPP, although some stressed individuals with certain behavioural traits - including a high novelty-seeking profile or the development of depression-like behaviour in the splash test shortly after VIRSD - acquired cocaine CPP. Our results confirm that some behavioural traits of female mice are associated with vulnerability or resilience to the long-term effects of social stress on cocaine reward, as previously observed in males.

Vicarious defeat stress induces increased alcohol consumption in female mice: Role of neurokinin-1 receptor and interleukin-6

There is a high frequency of comorbidity of alcohol use disorder (AUD) and depression in human populations. We have studied this relationship in our lab using the social defeat stress (SDS) model, which results in both depression-like behaviours and increased alcohol consumption in male mice. However, standard SDS procedures are difficult to use in female mice due to a lack of territorial aggression. In the experiments presented here, we used vicarious defeat stress (VDS) to assess social withdrawal and alcohol consumption in female C57BL6/J mice. We also assessed the expression of interleukin-6 (IL6), which is a proinflammatory cytokine that is associated with depression in humans and sensitivity to SDS in mice. In these experiments, C57BL/6 female mice underwent 10 days of VDS where they witnessed the physical defeat of a male conspecific by an aggressive CD1 mouse. After the end of VDS, mice were either given access to alcohol or sacrificed for the measurement of IL6 expression. We found that VDS increased alcohol consumption and IL6 expression in the frontal cortex and hippocampus. Given that the neurokinin-1 receptor (NK1R) can mediate both stress-induced alcohol consumption and IL6 expression, we tested the ability of NK1R antagonism to reduce VDS-induced alcohol consumption and found that this treatment reduced alcohol intake in both VDS-exposed mice and in unstressed controls. The observed increase in alcohol consumption suggests that VDS is a model that can be utilized to study stress-induced alcohol consumption in female mice, and that this is sensitive to NK1R antagonism.