Neuroendocrine and neurochemical impact of aggressive social interactions in submissive and dominant mice: implications for stress-related disorders

Environmental Enrichment Differentially Activates Neural Circuits in FVB/N Mice, Inducing Social Interaction in Females but Agonistic Behavior in Males

Environmental enrichment induces behavioral and structural modifications in rodents and influences the capability of mice to cope with stress. However, little is understood about hippocampal neurogenesis and the appearance of social/agonistic (aggressive) behavior upon activation of different neuronal circuits in FVB/N mice. Thus, in this study we hypothesized that environmental enrichment differentially regulates neurogenesis, neural circuit activation and social/agonistic behavior in male and female FVB/N mice. We explored the (1) neurogenic process as an indicative of neuroplasticity, (2) neuronal activation in the limbic system, and (3) social behavior using the resident-intruder test. On postnatal day 23 (PD23), mice were assigned to one of two groups: Standard Housing or Environmental Enrichment. At PD53, rodents underwent the resident-intruder test to evaluate social behaviors. Results revealed that environmental enrichment increased neurogenesis and social interaction in females. In males, environmental enrichment increased neurogenesis and agonistic behavior. Enriched male mice expressed higher levels of agonistic-related behavior than female mice housed under the same conditions. Neural circuit analysis showed lower activation in the amygdala of enriched males and higher activation in enriched females than their respective controls. Enriched females also showed higher activation in the frontal cortex without differences in male groups. Moreover, the insular cortex was less activated in females than in males. Thus, our results indicate that environmental enrichment has different effects on neuroplasticity and social/agonistic behavior in FVB/N mice, suggesting the relevance of sexual dimorphism in response to environmental stimuli.

Targeted Ubiquitin-Proteasomal Proteolysis Pathway in Chronic Social Defeat Stress

Stressful events promote psychopathogenic changes that might contribute to the development of mental illnesses. Some individuals tend to recover from the stress response, while some do not. However, the molecular mechanisms of stress resilience during stress are not well-characterized. Here, we identify proteomic changes in the hippocampus using proteomic technique to examine mice following chronic social defeat stress. We showed that small ubiquitin-like modifier (SUMO)-1 expression was significantly decreased in susceptible mice following chronic social defeat stress. We also examined a protein inhibitor of activated signal transducer of transcription (PIAS)1 levels, an E3 SUMO-protein ligase protein inhibitor of activated STAT1, which is known to interact with SUMO-1. PIAS1 was shown to be profoundly decreased and monoamine oxidase (MAO)-A increased in the hippocampus of susceptible mice following chronic social defeat stress. Furthermore, the manipulated PIAS1 expression in the hippocampus also has an influence on glucocorticoid receptor (GR) translocation. We also found that knockdown of PIAS1 expression in the hippocampus then subject to submaximal stress increased GR to glucocorticoid response element (GRE)-binding site on the MAO-A promoter. The present study raises the possibility of different levels of PIAS1 between individuals in response to chronic social defeat stress and that such differences may contribute to the susceptibility to stress.

Cytokine variations and mood disorders: influence of social stressors and social support

Stressful events have been implicated in the evolution of mood disorders. In addition to brain neurotransmitters and growth factors, the view has been offered that these disorders might be provoked by the activation of the inflammatory immune system as well as by de novo changes of inflammatory cytokines within the brain. The present review describes the impact of social stressors in animals and in humans on behavioral changes reminiscent of depressive states as well as on cytokine functioning. Social stressors increase pro-inflammatory cytokines in circulation as well as in brain regions that have been associated with depression, varying with the animal's social status and/or behavioral methods used to contend with social challenges. Likewise, in humans, social stressors that favor the development of depression are accompanied by elevated circulating cytokine levels and conversely, conditions that limit the cytokine elevations correlated with symptom attenuation or reversal. The implications of these findings are discussed in relation to the potentially powerful effects of social support, social identity, and connectedness in maintaining well-being and in diminishing symptoms of depression.

Immobility responses between mouse strains correlate with distinct hippocampal serotonin transporter protein expression and function

Mouse strain differences in immobility and in sensitivity to antidepressants have been observed in the forced swimming test (FST) and the tail suspension test (TST). However, the neurotransmitter systems and neural substrates that contribute to these differences remain unknown. To investigate the role of the hippocampal serotonin transporter (5-HTT), we measured baseline immobility and the immobility responses to fluoxetine (FLX) in the FST and the TST in male CD-1, C57BL/6, DBA and BALB/c mice. We observed strain differences in baseline immobility time, with CD-1 mice showing the longest and DBA mice showing the shortest. In contrast, DBA and BALB/c mice showed the highest sensitivity to FLX, whereas CD-1 and C57BL/6 mice showed the lowest sensitivity. Also we found strain differences in both the total 5-HTT protein level and the membrane-bound 5-HTT level (estimated by V max) as follows: DBA>BALB/c>CD-1=C57BL/6. The uptake efficiency of the membrane-bound 5-HTT (estimated by 1/K m) was highest in DBA and BALB/c mice and lowest in CD-1 and C57BL/6 mice. A correlation analysis of subregions within the hippocampus revealed that immobility time was negatively correlated with V max and positively correlated with K m in the hippocampus. Therefore a higher uptake capacity of the membrane-bound 5-HTT in the hippocampus was associated with lower baseline immobility and greater sensitivity to FLX. These results suggest that alterations in hippocampal 5-HTT activity may contribute to mouse strain differences in the FST and the TST.

Nonaggressive and adapted social cognition is controlled by the interplay between noradrenergic and nicotinic receptor mechanisms in the prefrontal cortex

Social animals establish flexible behaviors and integrated decision-making processes to adapt to social environments. Such behaviors are impaired in all major neuropsychiatric disorders and depend on the prefrontal cortex (PFC). We previously showed that nicotinic acetylcholine receptors (nAChRs) and norepinephrine (NE) in the PFC are necessary for mice to show adapted social cognition. Here, we investigated how the cholinergic and NE systems converge within the PFC to modulate social behavior. We used a social interaction task (SIT) in C57BL/6 mice and mice lacking β2*nAChRs (β2(-/-) mice), making use of dedicated software to analyze >20 social sequences and pinpoint social decisions. We performed specific PFC NE depletions before SIT and measured monoamines and acetylcholine (ACh) levels in limbic corticostriatal circuitry. After PFC-NE depletion, C57BL/6 mice exhibited impoverished and more rigid social behavior and were 6-fold more aggressive than sham-lesioned animals, whereas β2(-/-) mice showed unimpaired social behavior. Our biochemical measures suggest a critical involvement of DA in SIT. In addition, we show that the balance between basal levels of monoamines and of ACh modulates aggressiveness and this modulation requires functional β2*nAChRs. These findings demonstrate the critical interplay between prefrontal NE and nAChRs for the development of adapted and nonaggressive social cognition.

Environmental enrichment influences brain cytokine variations elicited by social defeat in mice

Environmental enrichment may protect against some of the adverse behavioural and biological effects of stressors. However, unlike the effects seen in some species, among male mice housed in groups, enrichment may alter social stability, encourage competition and aggression, and thus promote the establishment of a stressful environment. A potent psychosocial stressor such as social defeat in mice promotes brain neurochemical changes as well as pro-inflammatory cytokine variations in the prefrontal cortex (PFC) and hippocampus. The present investigation demonstrated that enrichment in group-housed male mice, even in the relatively nonaggressive, although highly anxious BALB/cByJ strain encouraged the effects of a repeated social defeat stressor experienced 4 weeks later, especially with respect to corticosterone as well as hippocampal corticotropin-releasing hormone (CRH) and interleukin (IL)-6 variations. Moreover, within the hippocampus, enrichment itself was accompanied by modest reductions in the expression of the IL-1β receptor (IL-1r1). Thus, it seems that living in an enriched environment among group-housed male mice might promote a stressful environment that enhances basal hippocampal CRH and cytokine variations and increased vulnerability to further changes upon subsequent exposure to a social stressor.

Social agonistic distress in male and female mice: changes of behavior and brain monoamine functioning in relation to acute and chronic challenges

Stressful events promote several neuroendocrine and neurotransmitter changes that might contribute to the provocation of psychological and physical pathologies. Perhaps, because of its apparent ecological validity and its simple application, there has been increasing use of social defeat (resident-intruder) paradigms as a stressor. The frequency of stress-related psychopathology is much greater in females than in males, but the typical resident-intruder paradigm is less useful in assessing stressor effects in females. An alternative, but infrequently used procedure in females involves exposing a mouse to a lactating dam, resulting in threatening gestures being expressed by the resident. In the present investigation we demonstrated the utility of this paradigm, showing that the standard resident-intruder paradigm in males and the modified version in females promoted elevated anxiety in a plus-maze test. The behavioral effects that reflected anxiety were more pronounced 2 weeks after the stressor treatment than they were 2 hr afterward, possibly reflecting the abatement of the stress-related of hyper-arousal. These treatments, like a stressor comprising physical restraint, increased plasma corticosterone and elicited variations of norepinephrine and serotonin levels and turnover within the prefrontal cortex, hippocampus and central amygdala. Moreover, the stressor effects were exaggerated among mice that had been exposed to a chronic or subchronic-intermittent regimen of unpredictable stressors. Indeed, some of the monoamine changes were more pronounced in females than in males, although it is less certain whether this represented compensatory changes to deal with chronic stressors that could result in excessive strain on biological systems (allostatic overload).

The differential impact of social defeat on mice living in isolation or groups in an enriched environment: plasma corticosterone and monoamine variations

Social defeat in mice is a potent stressor that promotes the development of depressive- and anxiety-like behaviours, as well as variations of neuroendocrine and brain neurotransmitter activity. Although environmental enrichment may protect against some of the adverse behavioural and biological effects of social defeat, it seems that, among male group-housed mice maintained in an enriched environment (EE), aggressive behaviours may be more readily instigated, thus promoting distress and exacerbating psychopathological features. Thus, although an EE can potentially have numerous beneficial effects, these may depend on the general conditions in which mice were raised. It was observed in the current investigations that EE group-housed BALB/cByJ mice displayed increased anxiety-like behaviours compared to their counterparts maintained in a standard environment (SE). Furthermore, in response to social defeat, EE group-housed male mice exhibited decreased weight gain, exaggerated corticosterone elevations and altered hippocampal norepinephrine utilization compared to their SE counterparts. These effects were not apparent in the individually housed EE mice and, in fact, enrichment among these mice appeared to buffer against serotonin changes induced by social defeat. It is possible that some potentially beneficial effects of enrichment were precluded among group-housed mice, possibly owing to social disturbances that might occur in these conditions. In fact, even if social interaction is an essential feature of enrichment, it seems that some of the positive effects of this housing condition might be optimal when mice are housed individually, particularly with regard to buffering the effects of social defeat.

Synergistic and antagonistic actions of acute or chronic social stressors and an endotoxin challenge vary over time following the challenge

Acute stressor exposure and immunogenic challenges can synergistically increase behavioral, endocrine and neuroinflammatory responses, but much less is known about how chronic stressors influence the actions of immune challenges. In the present investigation we assessed the influence of bacterial endotoxin, lipopolysaccharide (LPS), administered on an acute chronic stressors backdrop, on sickness behavior, changes of circulating corticosterone and cytokine levels, and cytokine mRNA expression in the prefrontal cortex (PFC) and hippocampus. In this regard, it was of particular interest to determine whether the stressors would alter the temporal biological effects (onset and normalization) of LPS. There was a leftward shift in the temporal curve, in that sickness behavior, corticosterone and plasma IL-6 elevations among stressed mice appeared sooner after LPS treatment, but 3h after treatment corticosterone and IL-6 were lower than in nonstressed mice. In contrast, the stressor, especially when applied chronically, diminished the effects of LPS on TNF-α over the course of measurement, whereas effects on IL-10 were enhanced. In contrast to these peripheral effects, central cytokine mRNA expression, especially IL-1β and TNF-α, were diminished 1.5h following stressor and LPS administration, but were then synergistically enhanced at 3h compared to non-stressed controls. Although acute and chronic stressors provoked similar behavioral and neuroendocrine responses when combined with LPS, the effects of chronic stressors and LPS on brain cytokines were generally diminished, particularly in the PFC. The implications of the temporal changes related to stressors and immune activation are discussed, and several possible mechanisms for these effects are suggested.

Higher sensory processing sensitivity, introversion and ectomorphism: New biomarkers for human creativity in developing rural areas

The highly sensitive trait present in animals, has also been proposed as a human neurobiological trait. People having such trait can process larger amounts of sensory information than usual, making it an excellent attribute that allows to pick up subtle environmental details and cues. Furthermore, this trait correlates to some sort of giftedness such as higher perception, inventiveness, imagination and creativity. We present evidences that support the existance of key neural connectivity between the mentioned trait, higher sensory processing sensitivity, introversion, ectomorphism and creativity. The neurobiological and behavioral implications that these biomarkers have in people living in developing rural areas are discussed as well.

Environmental enrichment in male CD-1 mice promotes aggressive behaviors and elevated corticosterone and brain norepinephrine activity in response to a mild stressor

Housing rodents in an enriched environment (EE) has been typically considered to have positive effects on well-being and cognitive functioning of the animals. However, in some strains of mice, EEs have also been reported to elicit aggression and to promote stress-related outcomes. In the current investigation, we examined whether environmental enrichment would elicit aggression among CD-1 male mice and thus sensitize responses to a subsequent mild stressor. It was first observed that mice housed in an EE for 2 weeks displayed more aggressive behaviors than did mice that had been housed in a standard environment (SE). In the second experiment, it was noted that after 4 weeks of EE or SE housing, mice exhibited comparable plasma corticosterone concentrations as well as levels of brain norepinephrine and its metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), in the absence of a challenge. However, upon exposure to mild stressor (placement in a novel cage), relative to their SE counterparts, EE mice were more active and displayed higher plasma corticosterone concentrations and enhanced MHPG accumulation in the prefrontal cortex and hippocampus. It seems that enrichment in male CD-1 mice promotes aggression, and may sensitize biological processes, possibly increasing vulnerability to stressor-related outcomes.

Stressor exposure of male and female juvenile mice influences later responses to stressors: modulation of GABAA receptor subunit mRNA expression

Stressors encountered during the juvenile period may have persistent effects on later behavioral and neurochemical functioning and may influence later responses to stressors. In the current investigation, we evaluated the influence of stressor exposure applied during the juvenile period (26-28 days of age) on anxiety-related behavior, plasma corticosterone and on GABA(A) α2, α3, α5 and γ2 mRNA expression within the prefrontal cortex (PFC) and amygdala measured during adulthood. These changes were monitored in the absence of a further challenge, as well as in response to either a social or a non-social psychogenic stressor administered during adulthood. Exposure to an acute adult stressor elicited anxiety in females and was still more pronounced among females that had also experienced the juvenile stressor. Among males, arousal and impulsivity predominated so that anxiety responses were less notable. Furthermore, experiencing the stressor as a juvenile influenced adult GABA(A) subunit expression, as did the adult stressor experience. These changes were differentially expressed in males and females. Moreover, these subunit variations were further moderated among mice that stressed as juveniles and were again exposed to an adult stressor. Interestingly, under conditions in which the juvenile stressor increased the expression of a particular subunit, exposure to a further stressor in adulthood resulted in the γ-aminobutyric acid (GABA) subunit variations being attenuated in both sexes. The current results suggest that juvenile and adult stressor experiences elicit variations of GABA(A) receptor subunit expression that are region-specific as well as sexually-dimorphic. Stressful events during the juvenile period may have pronounced proactive effects on anxiety-related behaviors, but linking these to specific GABA(A) subunits is made difficult by the diversity of GABA changes that are evident as well as the dimorphic nature of these variations. Nevertheless, these GABA(A) sex-specific subunit variations may be tied to the differences in anxiety in males and females.

Enriched rearing improves behavioral responses of an animal model for CNV-based autistic-like traits

Potocki-Lupski syndrome (PTLS; MIM #610883), characterized by neurobehavioral abnormalities, intellectual disability and congenital anomalies, is caused by a 3.7-Mb duplication in 17p11.2. Neurobehavioral studies determined that ∼70-90% of PTLS subjects tested positive for autism or autism spectrum disorder (ASD). We previously chromosomally engineered a mouse model for PTLS (Dp(11)17/+) with a duplication of a 2-Mb genomic interval syntenic to the PTLS region and identified consistent behavioral abnormalities in this mouse model. We now report extensive phenotyping with behavioral assays established to evaluate core and associated autistic-like traits, including tests for social abnormalities, ultrasonic vocalizations, perseverative and stereotypic behaviors, anxiety, learning and memory deficits and motor defects. Alterations were identified in both core and associated ASD-like traits. Rearing this animal model in an enriched environment mitigated some, and even rescued selected, neurobehavioral abnormalities, suggesting a role for gene-environment interactions in the determination of copy number variation-mediated autism severity.

The effects of allostatic load on neural systems subserving motivation, mood regulation, and social affiliation

The term allostasis, which is defined as stability through change, has been invoked repeatedly by developmental psychopathologists to describe long-lasting and in some cases permanent functional alterations in limbic–hypothalamic–pituitary–adrenal axis responding following recurrent and/or prolonged exposure to stress. Increasingly, allostatic load models have also been invoked to describe psychological sequelae of abuse, neglect, and other forms of maltreatment. In contrast, neural adaptations to stress, including those incurred by monoamine systems implicated in (a) mood and emotion regulation, (b) behavioral approach, and (c) social affiliation and attachment, are usually not included in models of allostasis. Rather, structural and functional alterations in these systems, which are exquisitely sensitive to prolonged stress exposure, are usually explained as stress mediators, neural plasticity, and/or programming effects. Considering these mechanisms as distinct from allostasis is somewhat artificial given overlapping functions and intricate coregulation of monoamines and the limbic–hypothalamic–pituitary–adrenal axis. It also fractionates literatures that should be mutually informative. In this article, we describe structural and functional alterations in serotonergic, dopaminergic, and noradrenergic neural systems following both acute and prolonged exposure to stress. Through increases in behavioral impulsivity, trait anxiety, mood and emotion dysregulation, and asociality, alterations in monoamine functioning have profound effects on personality, attachment relationships, and the emergence of psychopathology.

Social defeat promotes specific cytokine variations within the prefrontal cortex upon subsequent aggressive or endotoxin challenges

Stressful experiences typically have short-lived neuroendocrine and neurochemical effects, but the processes leading to these biological alterations may be sensitized so that later challenges promote exaggerated responses. As stressors and immunogenic insults have both been associated with inflammatory immune variations within the brain, we assessed whether a social defeat stressor would result in augmented corticosterone release and mRNA expression of pro-inflammatory cytokines within the prefrontal cortex (PFC) upon later social defeat (sensitization) or endotoxin (lipopolysaccharide: LPS) challenges (cross-sensitization). In the absence of a prior stressor experience, the social defeat challenge did not affect prefrontal interleukin (IL)-1β or tumor necrosis factor (TNF)-α mRNA expression, but increased that of IL-6, whereas LPS increased the expression of each cytokine. Among mice that had initially been repeatedly defeated, IL-1β and TNF-α expression was enhanced after the social defeat challenge, whereas this was not evident in response to the LPS challenge. In contrast, the initial social defeat stressor had protracted effects in that increase of IL-6 expression was limited upon subsequent challenge with either social defeat or LPS. Previous social stressor experiences also limited the corticosterone rise ordinarily elicited by either social defeat or LPS treatment. It seems that a powerful stressor, such as social defeat, may have persistent effects on later corticosterone and cytokine responses to different types of stressful insults (social versus systemic challenges), but the nature of the effects varies with the specific process assessed.

Impact of acute and chronic stressor experiences on heart atrial and brain natriuretic peptides in response to a subsequent stressor

The impact of stressful events on processes related to cardiovascular functioning might vary with previous stressor experiences, just as such sensitization effects have been detected with respect to several neurochemical and hormonal processes. The present investigation assessed the impact of a psychosocial stressor on factors directly or indirectly related to cardiovascular functioning among CD-1 mice that had previously experienced an acute or chronic stressor regimen. These factors included plasma variations of atrial and brain natriuretic peptides (ANP and BNP, respectively), inflammatory cytokines in plasma, mRNA expression of natriuretic peptides and inflammatory cytokines in the ventricles, and norepinephrine (NA) levels and utilization within the locus coeruleus, a brain region implicated in cardiac functioning. A social stressor (exposure to a dominant mouse) increased NE levels and utilization within the locus coeruleus, plasma corticosterone, cytokine and ANP levels. Among mice initially exposed to an acute stressor (restraint), NE utilization, ventricular ANP mRNA expression, and plasma interleukin-6 (IL-6) concentrations were markedly increased by the subsequent social stressor. In chronically stressed mice some of the effects of the social stressor were dampened, including changes of plasma corticosterone, locus coeruleus NE utilization, as well as plasma and ventricular IL-6 mRNA expression. Conversely, plasma ANP was markedly enhanced by the combined stressor events as was ventricular BNP and IL-1β mRNA expression. It seems that stressors may profoundly influence (sensitize or desensitize) on factors that could influence cardiovascular functioning. It remains to be determined whether these actions would be translated as pathophysiological outcomes.

Behavior and pro-inflammatory cytokine variations among submissive and dominant mice engaged in aggressive encounters: moderation by corticosterone reactivity

Psychosocial stressors contribute to the pathophysiology of affective disorders and variations of cytokine functioning have been implicated in this process. The present investigation demonstrated, in mice, the impact of stressful aggressive encounters on activity levels, plasma corticosterone and cytokine concentrations, and on cytokine mRNA expression within the prefrontal cortex (PFC) and hippocampus. As glucocorticoids have been tied to cytokine variations, mice were subdivided into low or high corticosterone responders, defined in terms of circulating hormone levels 75 min post-confrontation. Interestingly, stressor-induced effects among low and high responders varied as a function of whether mice were submissive or dominant during the aggressive bout. Agonistic encounters elicited subsequent hyperactivity, particularly among low corticosterone responders and among dominant mice. Plasma levels of corticosterone and interleukin (IL)-6 concomitantly increased after aggressive encounters and varied with dominance status and with the low versus high corticosterone response. Among the low responders corticosterone and IL-6 increases were modest and only apparent among submissive mice, whereas among high responders these elevations were more pronounced and comparable in submissive and dominant mice. Aggressive episodes also increased IL-1β and IL-6 mRNA brain expression. The IL-1β rise was greater in the PFC and hippocampus of submissive mice that were low responders. Among high responders IL-1β and IL-6 increased in both groups, although in the PFC this effect was specific to dominant mice. The data are discussed in terms of their relevance to the impact of aggressive encounters on affective behaviors, and to the role that cytokines might play in this regard.

Stressor experiences during the juvenile period increase stressor responsivity in adulthood: transmission of stressor experiences

Stressors experienced by rodents during the juvenile period may have repercussions on anxiety and impulsivity that extend into adulthood. In the present investigation we demonstrate that during social interactions stressed adults might transmit their responses to juveniles thereby affecting later behavioral responses in adulthood. In the present investigation adult mice exposed to a stressor, exhibited altered social exploration of a juvenile (26-28 day old) mouse that comprised reduced body contact but elevated anogenital and facial contact. The juvenile mice that encountered the stressed adult, in turn, exhibited greater impulsivity in an elevated plus maze test, as well as elevated corticosterone levels. In a second experiment, adult animals that had experienced a stressor during the juvenile period also exhibited reduced social exploration (of a juvenile), but upon exposure to a further social stressor (social defeat), social exploration was altered further. Furthermore, when tested in an elevated plus maze the juvenile mice that had encountered an adult that had itself been stressed as a juvenile, exhibited increased impulsivity. However, if they encountered an adult that had been stressed both as a juvenile and as an adult, the behavioral profile of the juveniles was altered yet again in that they exhibited greater impulsivity coupled with anxiety. It is suggested that the juvenile period represents one during which stressor sensitivity is high, so that transmission of stressor effects from adults occurs readily. Moreover, stressors experienced during the juvenile period may have persistent effects on social behaviors, thereby affecting conspecifics with which they interact.