Exposure to chronic variable social stress during adolescence alters affect-related behaviors and adrenocortical activity in adult male and female inbred mice

Chronic Adolescent Restraint Stress Downregulates miRNA-200a Expression in Male and Female C57BL/6J and BALB/cJ Mice

Adolescence is a critical developmental period when the brain is plastic, and stress exposure can have lasting physiological consequences. One mechanism through which adolescent stress may have lasting effects is by altering microRNAs (miRNAs), leading to wide-scale gene expression changes. Three prior independent studies used unbiased approaches (RNA sequencing or microarray) to identify miRNAs differentially expressed by chronic variable stress in male rodents. In all three studies, miRNA-200a was differentially expressed in areas of the brain associated with emotion regulation. The current study extends this research to determine if chronic non-variable adolescent stress downregulates miRNA-200a expression by looking at two strains (BALB/cJ and C57BL/6J) of male and female mice. We utilized a 14-day (2 h/day) restraint stress protocol and verified stress effects on adolescent body weight gain and circulating corticosterone concentrations relative to non-restraint controls. Mice were then left undisturbed until they were euthanized in adulthood, at which time brains were collected to measure miRNA-200a in the ventral hippocampus. Three weeks after adolescent stress ended, differences in body weight between groups were no longer significant; however, animals exposed to stress had less miRNA-200a expression in the ventral hippocampus than control animals. These data implicate miRNA-200a expression as a potential mechanism by which adolescent stress can have persistent impacts on multiple outcomes in both male and female mice.

Enduring sex-dependent implications of pubertal stress on the gut-brain axis and mental health

The gut-brain axis (GBA) is a network responsible for the bidirectional communication between the central nervous system and the gastrointestinal tract. This multifaceted system is comprised of a complex microbiota, which may be altered by both intrinsic and extrinsic factors. During critical periods of development, these intrinsic and extrinsic factors can cause long-lasting sex-dependent changes in the GBA, which can affect brain structure and function. However, there is limited understanding of how the GBA is altered by stress and how it may be linked to the onset of mental illness during puberty. This article reviews current literature on the relationships between the GBA, the effects of stress during puberty, and the implications for mental health.

Sex- and age-dependent effects of chronic corticosterone exposure on depressive-like, anxiety-like, and fear-related behavior: Role of amygdala glutamate receptors in the rat

Persistent glucocorticoid elevation consistent with chronic stress exposure can lead to psychopathology, including mood and anxiety disorders. Women and stress-exposed adolescents are more likely to be diagnosed with mood disorders, suggesting that sex and age are important factors in determining vulnerability, though much remains to be determined regarding the mechanisms underlying this risk. Thus, the aim of the present experiments was to use the chronic corticosterone (CORT) exposure paradigm, a model of depression-like behavior that has previously been established primarily in adult males, to determine the mood-related effects of CORT in female and adolescent rats. Depression- and anxiety-like effects in adulthood were determined using the sucrose preference (SPT), the forced swim test (FST), the elevated plus maze, and fear conditioning. Basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) glutamate receptor subunit levels were then measured. In a subsequent experiment, adult male and female rats were tested for the effects of pharmacological activation (via AMPA) or inhibition (via NBQX) of AMPA receptors in the BLA on behavior in the FST. Overall, females showed reduced anxiety- and depressive-like behaviors relative to males. However, females treated with CORT in adolescence, but not adulthood, had increased immobility in the FST, indicative of depression-like behavior. In contrast, CORT did not alter behavior in adolescent-treated males, though the previously reported depression-like effect of adult CORT exposure was observed. Control females had higher expression of the AMPA receptor subunits GluA1 and GluA2/3 selectively in the BLA relative to males. Adolescent CORT treatment, however, decreased BLA GluA1 and GluA2/3 expression in females, but increased expression in males, consistent with the direction of depression-like behavioral effects. Male and female rats also demonstrated opposing patterns of response to BLA AMPA receptor modulation in the FST, with AMPA infusion magnifying the sex difference of decreased immobility in females. Overall, these experiments show that increased glutamate receptor function in the BLA may decrease the risk of developing depressive-like behavior, further supporting efforts to target glutamatergic receptors for the treatment of stress-related psychiatric disorders. These findings also support further focus on sex as a biological variable in neuropsychiatric research.

Effects of Social Isolation on the Development of Anxiety and Depression-Like Behavior in Model Experiments in Animals

This review describes the role of social isolation in the development of anxiety and depression-like behavior in rodents. The duration of social isolation, age from onset of social isolation, sex, species, and strain of animals, the nature of the model used, and other factors have been shown to have influences. The molecular-cellular mechanisms of development of anxiety and depression-like behavior under the influence of social isolation and the roles of the HHAS, oxidative and nitrosative stress, neuroinflammation, BDNF, neurogenesis, synaptic plasticity, as well as monoamines in these mechanisms are discussed. This review presents data on sex differences in the effects of social isolation, along with the effects of interactions with other types of stress, and the roles of an enriched environment and other factors in ameliorating the negative sequelae of social isolation.

An ecological animal model of subthreshold depression in adolescence: behavioral and resting state 18F-FDG PET imaging characterization

The different depressive disorders that exist can take root at adolescence. For instance, some functional and structural changes in several brain regions have been observed from adolescence in subjects that display either high vulnerability to depressive symptoms or subthreshold depression. For instance, adolescents with depressive disorder have been shown to exhibit hyperactivity in hippocampus, amygdala and prefrontal cortex as well as volume reductions in hippocampus and amygdala (prefrontal cortex showing more variable results). However, no animal model of adolescent subthreshold depression has been developed so far. Our objective was to design an animal model of adolescent subthreshold depression and to characterize the neural changes associated to this phenotype. For this purpose, we used adolescent Swiss mice that were evaluated on 4 tests assessing cognitive abilities (Morris water maze), anhedonia (sucrose preference), anxiety (open-field) and stress-coping strategies (forced swim test) at postnatal day (PND) 28-35. In order to identify neural alterations associated to behavioral profiles, we assessed brain resting state metabolic activity in vivo using ¹⁸F-FDG PET imaging at PND 37. We selected three profiles of mice distinguished in a composite Z-score computed from performances in the behavioral tests: High, Intermediate and Low Depressive Risk (HDR, IDR and LDR). Compared to both IDR and LDR, HDR mice were characterized by passive stress-coping behaviors, low cognition and high anhedonia and anxiety and were associated with significant changes of ¹⁸F-FDG uptakes in several cortical and subcortical areas including prelimbic cortex, infralimbic cortex, nucleus accumbens, amygdala, periaqueductal gray and superior colliculus, all displaying higher metabolic activity, while only the thalamus was associated with lower metabolic activity (compared to IDR). LDR displayed an opposing behavioral phenotype and were associated with significant changes of ¹⁸F-FDG uptakes in the dorsal striatum and thalamus that both exhibited markedly lower metabolic activity in LDR. In conclusion, our study revealed changes in metabolic activities that can represent neural signatures for behavioral profiles predicting subthreshold depression at adolescence in a mouse model.

The anxiogenic effects of adolescent psychological stress in male and female mice

Adolescence is a period of transition during which there is extensive development of the brain and the hypothalamic-pituitary-adrenal axis. However, the term adolescence is broad and covers a number of important developmental periods ranging from pre-pubescence to sexual maturity. Using a predator stress model, we investigated the effects of chronic psychological stress on anxiety-like, depression-like, and social behaviours in male and female mice during early adolescence, when mice are pre-pubertal, and late adolescence, when mice are sexually mature. All stressed mice showed hyperactivity and increased anxiety-like behaviours. The anxiogenic effects were generally more pronounced in mice exposed to late, rather than early adolescent stress, but were clearly evident when stress was experienced at either timepoint. Risk assessment behaviours were also affected by the stress treatments, but the direction of these changes were sometimes sex- and age-specific. Surprisingly, mice stressed during adolescence showed no depressive-like behaviours as adults. This study provides evidence that adolescent psychological stress has pronounced long-term anxiogenic effects but that the precise behavioural phenotype differs based on sex and the sub-stage of adolescence during which the individual is exposed.

The impact of adolescent stress on nicotine use and affective disorders in rodent models

Recent findings indicate that stress exposure during adolescence contributes to the development of both nicotine use and affective disorders, suggesting a potential shared biological pathway. One key system that may mediate the association between adolescent stress and nicotine or affective outcomes is the hypothalamic-pituitary-adrenal (HPA) axis. Here we reviewed evidence regarding the effects of adolescent stress on nicotine responses and affective phenotypes and the role of the HPA-axis in these relationships. Literature indicates that stress, possibly via HPA-axis dysfunction, is a risk factor for both nicotine use and affective disorders. In rodent models, adolescent stress modulates behavioural responses to nicotine and increases the likelihood of affective disorders. The exact role that the HPA-axis plays in altering nicotine sensitivity and affective disorder development after adolescent stress remains unclear. However, it appears likely that adolescent stress-induced nicotine use and affective disorders are precipitated by repetitive activation of a hyperactive HPA-axis. Together, these preclinical studies indicate that adolescent stress is a risk factor for nicotine use and anxiety/depression phenotypes. The findings summarized here suggest that the HPA-axis mediates this relationship. Future studies that pharmacologically manipulate the HPA-axis during and after adolescent stress are critical to elucidate the exact role that the HPA-axis plays in the development of nicotine use and affective disorders following adolescent stress.

Adolescent Stress Reduces Adult Morphine-Induced Behavioral Sensitization in C57BL/6J Mice

Deaths related to opioid use have skyrocketed in the United States, leading to a public health epidemic. Research has shown that both biological (genes) and environmental (stress) precursors are linked to opioid use. In particular, stress during adolescence-a critical period of frontal lobe development-influences the likelihood of abusing drugs. However, little is known about the biological mechanisms through which adolescent stress leads to long-term risk of opioid use, or whether genetic background moderates this response. Male and female C57BL/6J and BALB/cJ mice were exposed to chronic variable social stress (CVSS) or control conditions throughout adolescence and then tested for morphine locomotor sensitization or morphine consumption in adulthood. To examine possible mechanisms that underlie stress-induced changes in morphine behaviors, we assessed physiological changes in response to acute stress exposure and prefrontal cortex (PFC) miRNA gene expression. Adolescent stress did not influence morphine sensitization or consumption in BALB/cJ animals, and there was limited evidence of stress effects in female C57BL/6J mice. In contrast, male C57BL/6J mice exposed to adolescent CVSS had blunted morphine sensitization compared to control animals; no differences were observed in the acute locomotor response to morphine administration or morphine consumption. Physiologically, C57BL/6J mice exposed to CVSS had an attenuated corticosterone recovery following an acute stressor and downregulation of twelve miRNA in the PFC compared to control mice. The specificity of the effects for C57BL/6J vs. BALB/cJ mice provides evidence of a gene-environment interaction influencing opioid behaviors. However, this conclusion is dampened by limited locomotor sensitization observed in BALB/cJ mice. It remains possible that results may differ to other doses of morphine or other behavioral responses. Long-term differences in stress reactivity or miRNA expression in C57BL/6J mice suggests two possible biological mechanisms to evaluate in future research.

Developmental stress has sex-specific effects on contextual and cued fear conditioning in adulthood

Stress-induced deviations in central nervous system development has long-term effects on adult mental health. Previous research in humans demonstrates that prenatal or adolescent stress increases the risk for psychiatric disorders. Animal models investigating the effects of stress during prenatal or adolescent development produces behavioral outcomes analogous to those observed in humans. However, whether adolescent stress exposure potentiates the effects of prenatal stress is currently unknown. Thus, the current study tested whether adolescent stress increases the impact of prenatal stress on contextual and cued fear memory in adulthood. Male and female Sprague Dawley rats were exposed to a chronic variable stress schedule during the last week of gestation, during adolescence, or during both developmental periods before undergoing fear conditioning training in adulthood. Our hypothesis predicted that the combined effects of prenatal and adolescent stress on contextual and cued fear memory would be greater than the effects of stress during either time period alone. In contrast to our hypothesis, however, we found independent effects of prenatal and adolescent stress on contextual and cued fear memory in both sexes, with no additional combined impact of stress exposure during both developmental phases. In males, developmental stress increased freezing behavior during contextual and cued testing regardless of whether stress exposure was prenatal, adolescent, or combined prenatal and adolescent stress exposure. In contrast, the effects of developmental stress in females were both test- and ovarian hormone status-dependent. During cued testing, nonstressed female freezing behavior depended on estrous cycle phase, whereas freezing behavior in stressed females did not, suggesting that developmental stress interferes with hormone-dependent cued fear memory. No effects of developmental stress or estrous cycle phase were observed for contextual fear memory in females. The results of the current study suggest that the effects of prenatal and adolescent stress on contextual and cued fear memory are not cumulative, but the effects of developmental stress on associative memory differ between males and females.

Peri-adolescent asthma: Acute impacts on innate immune response, corticosterone, and microglia in mice

Asthma is highly comorbid with anxiety in youth. We investigated the hypothalamic-pituitary-adrenal (HPA) axis and microglia as mechanisms underlying asthma and anxiety comorbidity. We induced asthma symptoms in developing BALB/cJ mice with house dust mite (HDM) for airway inflammation and methacholine (MCH) for bronchoconstriction. On the last day of exposure, we analyzed samples at six timepoints. Lung IL-5 and IL-1β expression peaked 4 h after final HDM exposure. Circulating corticosterone was blunted in a sex- and treatment-specific temporal pattern. Hippocampal IL-1β expression and microglial area were marginally increased 24 h after MCH exposure. These results provide a foundation for further work investigating asthma-anxiety mechanisms.

Morphine improved stress-induced amnesia and anxiety through interacting with the ventral hippocampal endocannabinoid system in rats

The present study aimed to investigate the possible role of the ventral hippocampal (VH) cannabinoid CB1 receptors in the improving effect of morphine on stress-induced memory formation impairment and anxiety. A step-through type passive avoidance task and a hole-board test were used to measure memory formation and anxiety-like exploratory behavior, respectively. The results showed that the exposure to 10-min stress immediately after the successful training phase impaired memory formation and also produced anxiogenic-like exploratory behaviour in adult male Wistar rats. Moreover, morphine administration before stress exposure improved the adverse effects of stress on memory formation and exploratory behaviour. After training, intra-VH microinjection of cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (0.01-0.05 μg/rat) enhanced the response of an ineffective dose of morphine (0.5 mg/kg for memory; 5 mg/kg for anxiety, i.p.) on memory impairment and anxiogenic-like exploratory behaviour induced by acute stress. Intra-VH microinjection of the higher dose of WIN 55,212-2 alone impaired memory formation. Post-training microinjection of a cannabinoid CB1 receptor antagonist/inverse agonist, AM-251 (100-150 ng/rat) into the VH attenuated the response of an effective dose of morphine (5 mg/kg for memory; 6 mg/kg for anxiety, i.p.) in stress-exposed rats. Taken together, the present results showed that morphine administration could improve stress-induced memory impairment and anxiety in the rats exposed to the inescapable acute stress. Interestingly, the improving effect of morphine on the adverse effect of stress on memory formation and anxiety-like exploratory behaviour may be mediated through the VH endocannabinoid CB1/CB2 receptors mechanism.

Alcohol Binge Drinking and Anxiety-Like Behavior in Socialized Versus Isolated C57BL/6J Mice

BACKGROUND

Binge alcohol drinking has been characterized as a key feature of alcoholism. The drinking-in-the-dark (DID) preclinical model, a procedure that promotes high levels of ethanol (EtOH) intake in short periods of time, has been extensively used to investigate neuropharmacological and genetic determinants of binge-like EtOH consumption. Using DID methodology, alcohol-preferring strains of mice such as C57BL/6J (B6) mice consume enough EtOH to achieve blood concentrations (≥1.0 mg/ml) associated with behavioral intoxication (i.e., motor incoordination). DID procedures typically involve the use of socially isolated animals (single-housed prior to and during the experiment). Previous research indicates that stress associated with social isolation can induce anxiety-like behavior and promote increases in EtOH intake. The present study investigates the role of housing conditions in anxiety-like behavior and binge-like EtOH intake using a DID procedure.

METHODS

Male and female B6 mice were isolated or pair-housed for a period of 6 weeks prior to evaluation of anxiety-like (elevated plus maze, light and dark box, open field) and drinking (water, 10% sucrose, 10 to 30% EtOH) behavior. In order to measure intake, a variation of the standard DID procedure using a removable, transparent, and perforated plastic barrier strip (designed to temporarily divide the cage in 2) was introduced. This allowed for individual intake records (2-hour test) of isolated and socially housed animals.

RESULTS

Increased anxiety-like behavior and reduced sucrose consumption were found in isolated mice. The effects of housing conditions on EtOH intake were sex- and concentration-dependent. In male mice, isolation increased 20 and 30% EtOH intake. In females, however, an increased intake of EtOH (30%) was found in socialized animals. No effects of housing or sex were found at EtOH 10%.

CONCLUSIONS

Together with previous literature, the present study suggests that social isolation can promote anxiety-associated behavior and produce sex-dependent changes in binge-like EtOH consumption.

Chronic psychological stress during adolescence induces sex-dependent adulthood inflammation, increased adiposity, and abnormal behaviors that are ameliorated by selective inhibition of soluble tumor necrosis factor with XPro1595

Physical and psychosocial maltreatment experienced before the age of 18, termed early life adversity (ELA), affects an estimated 39% of the world's population, and has long-term detrimental health and psychological outcomes. While adult phenotypes vary following ELA, inflammation and altered stress responsivity are pervasive. Cytokines, most notably tumor necrosis factor (TNF), are elevated in adults with a history of ELA. While soluble TNF (solTNF) drives chronic inflammatory disease, transmembrane TNF facilitates innate immunity. Here, we test whether solTNF mediates the behavioral and molecular outcomes of adolescent psychological stress by administering a brain permeable, selective inhibitor of solTNF, XPro1595. Male and female C57BL/6 mice were exposed to an aggressive rat through a perforated translucent ball ('predatory stress') or transported to an empty room for 30 min for 30 days starting on postnatal day 34. Mice were given XPro1595 or vehicle treatment across the last 15 days. Social interaction, sucrose preference, and plasma inflammation were measured at 2 and 4 weeks, and open field behavior, adiposity, and neuroinflammation were measured at 4 weeks. Chronic adolescent stress resulted in increased peripheral inflammation and dysregulated neuroinflammation in adulthood in a sex-specific manner. Abnormal social and open field behavior, fat pad weight, and fecal boli deposition were noted after 30 days; solTNF antagonism ameliorated the effects of stress. Together, these data support our hypothesis, and suggest that targeting solTNF with XPro1595 may improve quality of life for individuals with a history of adolescent stress.

Influence of Social Isolation During Prolonged Simulated Weightlessness by Hindlimb Unloading

The hindlimb unloading (HU) model has been used extensively to simulate the cephalad fluid shift and musculoskeletal disuse observed in spaceflight with its application expanding to study immune, cardiovascular and central nervous system responses, among others. Most HU studies are performed with singly housed animals, although social isolation also can substantially impact behavior and physiology, and therefore may confound HU experimental results. Other HU variants that allow for paired housing have been developed although no systematic assessment has been made to understand the effects of social isolation on HU outcomes. Hence, we aimed to determine the contribution of social isolation to tissue responses to HU. To accomplish this, we developed a refinement to the traditional NASA Ames single housing HU system to accommodate social housing in pairs, retaining desirable features of the original design. We conducted a 30-day HU experiment with adult, female mice that were either singly or socially housed. HU animals in both single and social housing displayed expected musculoskeletal deficits versus housing matched, normally loaded (NL) controls. However, select immune and hypothalamic-pituitary-adrenal (HPA) axis responses were differentially impacted by the HU social environment relative to matched NL controls. HU led to a reduction in % CD4⁺ T cells in singly housed, but not in socially housed mice. Unexpectedly, HU increased adrenal gland mass in socially housed but not singly housed mice, while social isolation increased adrenal gland mass in NL controls. HU also led to elevated plasma corticosterone levels at day 30 in both singly and socially housed mice. Thus, musculoskeletal responses to simulated weightlessness are similar regardless of social environment with a few differences in adrenal and immune responses. Our findings show that combined stressors can mask, not only exacerbate, select responses to HU. These findings further expand the utility of the HU model for studying possible combined effects of spaceflight stressors.

Social experience during adolescence in female rats increases 50 kHz ultrasonic vocalizations in adulthood, without affecting anxiety-like behavior

Adolescents are highly motivated to engage in social interactions, and researchers have hypothesized that positive social relationships during adolescence can have long term, beneficial effects on stress reactivity and mental well-being. Studies of laboratory rodents provide the opportunity to investigate the relationship between early social experiences and later behavioral and physiological responses to stressors. In this study, female Lister-hooded rats (N = 12 per group) were either (a) provided with short, daily encounters (10 min/day) with a novel partner during mid-adolescence (postnatal day 34-45; "social experience," SE, subjects) or (b) underwent the same protocol with a familiar cagemate during mid-adolescence ("control experience," CE, subjects), or (c) were left undisturbed in the home cage (non-handled "control," C, subjects). When tested in adulthood, the groups did not differ in behavioral responses to novel environments (elevated plus maze, open field, and light-dark box) or in behavioral and physiological (urinary corticosterone) responses to novel social partners. However, SE females emitted significantly more 50 kHz ultrasonic vocalizations than control subjects both before and after social separation from a familiar social partner, which is consistent with previous findings in male rats. Thus, enhanced adolescent social experience appears to have long-term effects on vocal communication and could potentially modulate adult social relationships.

Peripubertal stress of male, but not female rats increases morphine-induced conditioned place preference and locomotion in adulthood

Animal studies demonstrate that peripubertal social stress markedly increases the risk for subsequent substance use in adulthood. However, whether non-social stress has a similar long-term impact is not clear, and whether male and female animals show different sensitivity to peripubertal non-social stress has not been examined. In the present study, we addressed these issues by introducing two non-social stressors (elevated platform and predator odor 2,5-Dihydro-2,4,5-trimethylthiazoline) to male and female Wistar rats during adolescence (postnatal days 28-30, 34, 36, 40, and 42), then tested reward-related behaviors during adulthood, including morphine-induced conditioned place preference (CPP, 1 mg/kg morphine or 5 mg/kg morphine) and hyperlocomotor activity (5 mg/kg morphine). We found that adult male rats, but not females who were exposed to peripubertal non-social stressors showed enhanced morphine-induced CPP. Moreover, morphine-induced increase in locomotor activity was also significantly increased in adult male rats, but not in females. These results indicate that peripubertal exposure to repeated non-social stress may enhance sensitivity to the rewarding effects of opioids in adulthood in a sex-dependent manner, with males being even more sensitive than females in this regard.

Chronic stress dysregulates amygdalar output to the prefrontal cortex

Chronic stress contributes to the neuropathology of mental health disorders, including those associated with anxiety. The basolateral amygdala (BLA) coordinates emotional behavioral responses through glutamatergic outputs to downstream regions such as the prefrontal cortex (PFC), nucleus accumbens core (NAcc) and bed nucleus of the stria terminalis (BNST). We explored the effects of chronic stress on BLA outputs to the PFC, NAcc and BNST using slice electrophysiology combined with optogenetics in two inbred mouse strains with distinct stress-induced anxiety responses. We found that ten consecutive days of chronic restraint stress enhanced pre-synaptic glutamate release at BLA-to-PFC synapses in C57BL/6J mice, but reduced pre-synaptic glutamate release at these synapses in DBA/2J mice. To assess the behavioral relevance of enhanced glutamate output at BLA-to-PFC synapses, we approximated the effects of chronic stress on the BLA-PFC circuit using optogenetics. We found that photostimulation of the BLA-PFC circuit in unstressed C57BL/6J mice produced persistent (i.e., post-stimulation) increased anxiety-like behavior and hyperactivity in the elevated plus-maze - a profile consistent with prototypical behavioral responses of stressed C57BL/6J mice. These data demonstrate that chronic stress dysregulates the BLA-PFC circuit by altering pre-synaptic glutamate release from BLA outputs, and provide a mechanism by which chronic stress can lead to increased anxiety.

Adolescent social stress increases anxiety-like behavior and ethanol consumption in adult male and female C57BL/6J mice

Exposure to social stress is an important risk factor for comorbid affective disorders and problem alcohol use. To better understand mechanisms involved in social stress-induced affective disorder and alcohol use co-morbidity, we studied the effects of adolescent social stress on anxiety- and depression-like behaviors and binge-like ethanol consumption. Male and female C57BL/6J mice were exposed to chronic variable social stress (CVSS) or control conditions throughout adolescence (postnatal days, PND, 25-59) and then tested for anxiety-like behavior in the elevated plus maze and a novel open field environment, or depression-like behavior using the forced swim test on PND 64-66. Mice were then tested for binge-like ethanol consumption using the Drinking-in-the-Dark model. Male and female mice exposed to adolescent CVSS had increased adult anxiety-like behavior and increased locomotor adaptation to a novel environment. Further, CVSS mice consumed significantly more ethanol, but not saccharin, than controls. Despite group differences in both anxiety-like behavior and ethanol consumption, there was no relationship between these outcomes within individual mice. These data suggest that exposure to adolescent social stress is an important risk factor for later alcohol use and affective behaviors, but that social stress does not necessarily dictate co-morbidity of these outcomes.

Adolescent Social Stress Increases Anxiety-like Behavior and Alters Synaptic Transmission, Without Influencing Nicotine Responses, in a Sex-Dependent Manner

Early-life stress is a risk factor for comorbid anxiety and nicotine use. Because little is known about the factors underlying this comorbidity, we investigated the effects of adolescent stress on anxiety-like behavior and nicotine responses within individual animals. Adolescent male and female C57BL/6J mice were exposed to chronic variable social stress (CVSS; repeated cycles of social isolation + social reorganization) or control conditions from postnatal days (PND) 25-59. Anxiety-like behavior and social avoidance were measured in the elevated plus-maze (PND 61-65) and social approach-avoidance test (Experiment 1: PND 140-144; Experiment 2: 95-97), respectively. Acute nicotine-induced locomotor, hypothermic, corticosterone responses, (Experiment 1: PND 56-59; Experiment 2: PND 65-70) and voluntary oral nicotine consumption (Experiment 1: PND 116-135; Experiment 2: 73-92) were also examined. Finally, we assessed prefrontal cortex (PFC) and nucleus accumbens (NAC) synaptic transmission (PND 64-80); brain regions that are implicated in anxiety and addiction. Mice exposed to adolescent CVSS displayed increased anxiety-like behavior relative to controls. Further, CVSS altered synaptic excitability in PFC and NAC neurons in a sex-specific manner. For males, CVSS decreased the amplitude and frequency of spontaneous excitatory postsynaptic currents in the PFC and NAC, respectively. In females, CVSS decreased the amplitude of spontaneous inhibitory postsynaptic currents in the NAC. Adolescent CVSS did not affect social avoidance or nicotine responses and anxiety-like behavior was not reliably associated with nicotine responses within individual animals. Taken together, complex interactions between PFC and NAC function may contribute to adolescent stress-induced anxiety-like behavior without influencing nicotine responses.

Adolescent chronic variable social stress influences exploratory behavior and nicotine responses in male, but not female, BALB/cJ mice

Anxiety disorders and nicotine use are significant contributors to global morbidity and mortality as independent and comorbid diseases. Early-life stress, potentially via stress-induced hypothalamic-pituitary-adrenal axis (HPA) dysregulation, can exacerbate both. However, little is known about the factors that predispose individuals to the development of both anxiety disorders and nicotine use. Here, we examined the relationship between anxiety-like behaviors and nicotine responses following adolescent stress. Adolescent male and female BALB/cJ mice were exposed to either chronic variable social stress (CVSS) or control conditions. CVSS consisted of repeated cycles of social isolation and social reorganization. In adulthood, anxiety-like behavior and social avoidance were measured using the elevated plus-maze (EPM) and social approach-avoidance test, respectively. Nicotine responses were assessed with acute effects on body temperature, corticosterone production, locomotor activity, and voluntary oral nicotine consumption. Adolescent stress had sex-dependent effects on nicotine responses and exploratory behavior, but did not affect anxiety-like behavior or social avoidance in males or females. Adult CVSS males exhibited less exploratory behavior, as indicated by reduced exploratory locomotion in the EPM and social approach-avoidance test, compared to controls. Adolescent stress did not affect nicotine-induced hypothermia in either sex, but CVSS males exhibited augmented nicotine-induced locomotion during late adolescence and voluntarily consumed less nicotine during adulthood. Stress effects on male nicotine-induced locomotion were associated with individual differences in exploratory locomotion in the EPM and social approach-avoidance test. Relative to controls, adult CVSS males and females also exhibited reduced corticosterone levels at baseline and adult male CVSS mice exhibited increased corticosterone levels following an acute nicotine injection. Results suggest that the altered nicotine responses observed in CVSS males may be associated with HPA dysregulation. Taken together, adolescent social stress influences later-life nicotine responses and exploratory behavior. However, there is little evidence of an association between nicotine responses and prototypical anxiety-like behavior or social avoidance in BALB/cJ mice.