Post-weaning Environmental Enrichment in Male CD-1 Mice: Impact on Social Behaviors, Corticosterone Levels and Prefrontal Cytokine Expression in Adulthood.

Environmental enrichment is typically associated with enhanced well-being, improved cognitive function and stress resilience. However, in some instances grouping adult male mice in enriched conditions promoted a stressful environment, which resulted in elevated endocrine, monoamine and inflammatory outcomes in response to subsequent stressor exposure. The current investigation examined whether raising male mice in an enriched environment (EE) would modulate social and anxiety-like behaviors in early adulthood and influence brain expression of pro-inflammatory cytokines and brain-derived neurotrophic factor (BDNF). Immediately after weaning (postnatal day [PD] 21), CD-1 male mice were housed with their siblings (3/cage) for 6 weeks in an EE or a standard (SE) environment. Body weights and aggressive interactions were monitored weekly. Social avoidance behaviors in the social interaction test and anxiety-like behaviors in the elevated-plus maze were examined in early adulthood. Ninety minutes following the behavioral tests, mice were sacrificed and a blood sample and the prefrontal cortex (PFC) were collected for the determination of plasma corticosterone levels as well as cytokine and BDNF mRNA expression. Mice raised in an EE exhibited more wounds and gained less weight than mice housed in a SE. Enriched mice also spent a greater amount of time in proximity of a social target in the social interaction test and made fewer transitions into the closed arms of the elevated-plus maze. Interestingly, the elevated plasma corticosterone and upregulated prefrontal interleukin (IL)-1ß expression observed after the social interaction test among the SE mice were not apparent among those housed in an EE. Enrichment also increased prefrontal BDNF expression, especially among mice that experienced the social interaction test. These results suggest that although raising male mice in an EE may elicit aggressive interactions between sibling cage-mates (as indicated by a high number of wounds), this environment also enhances social behaviors and limits the corticosterone and cytokine impacts of mild social stressors encountered in early adulthood.

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.

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-1ß 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.

Short- and long-term effects of juvenile stressor exposure on the expression of GABAA receptor subunits in rats.

During the juvenile period rodents are particularly sensitive to stressors. Aversive events encountered during this period may have enduring effects that are not evident among animals initially stressed as adults. Interestingly, experiencing stressor during juvenile period was found to elicit a biphasic behavioral pattern over the course of development. During the juvenile period, the expression of several GABAA receptor subunits is subject to elevated plasticity, rendering the GABAergic system sensitive to stressors. In the present investigation, animals were exposed to a juvenile variable stressor regimen (JUV-S) at 27-29 postnatal days (PND): 27 PND-acute swim stress (10 min), 28 PND-elevated platform stress (3 sessions×30 min each), and 29 PND-restraint (2 h). One hour following the last exposure to stressor or in adulthood (60 PND), anxiety-related behaviors were assessed in a 5-min elevated plus maze test. The western blotting technique was used to evaluate whether the juvenile stress induced behavioral pattern will be accompanied by respective changes in GABAA α1, α2, and α3 protein expression in male rats. Our findings further established that juvenile stressor elicits hyper-reactivity when rats were tested as juveniles, whereas rats exhibited reduced activity and increased anxiety when tested as adults. Additionally, the effects of juvenile stressor on α1, α2, and α3 were more pronounced among juvenile stressed rats that were challenged as adults compared with rats that were only challenged as juveniles. Interestingly, the stress-induced modulation of the subunits was particularly evident in the amygdala, a brain region closely associated with anxiety. Thus, age- and region-specific alterations of the α subunits may contribute to the age-specific behavioral alterations observed following juvenile stress exposure.

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)-1ß 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-1ß 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.

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.

Differential impact of juvenile stress and corticosterone in juvenility and in adulthood, in male and female rats.

Depressive and anxiety symptoms experienced by children have often been attributed to normative age-related mood disturbances. In the last decade, early-onset of affective illness has been recognized as a major public health problem. The current study utilized exposure to juvenile stress, which was previously shown to have long-term effects into adulthood in male rats, to compare early to adulthood consequences of juvenile stress on anxiety indices and exploratory behavior in the open field and elevated plus maze. Furthermore, both male and female rats were examined, and in addition, the role of corticosterone as a potential mediator of some of these effects in juvenility and in adulthood, was assessed. Experiments 1-2 examined in male and female rats immediate and long-term behavioral effects of juvenile stress. Experiment 3 assessed circulating corticosterone (CORT) levels, and in experiment 4 male and female rats were injected i.p. with CORT, in either juvenility or in adulthood, to examine whether CORT could mimic the effects of juvenile stress. As expected, in adulthood, juvenile stress resulted in reduced exploratory behavior and reduced exploration of high-risk areas of the mazes. A similar pattern was found in both males and females. In contrast, a reversed behavioral pattern was found in juvenility in both male and female rats, although detailed differences were found between sexes. The application of CORT induced a similar reversed behavioral pattern in juvenile and adult rats of both sexes. Thus, the juvenile stress model might serve as an animal model for studying early-onset of affective illness.

Juvenile stress-induced alteration of maturation of the GABAA receptor alpha subunit in the rat.

Profound evidence indicates that GABAA receptors are important in the control of physiological response to stress and anxiety. The alpha subunit type composition contributes significantly to the functional characterization of the GABAA receptors. The alpha2, alpha3, alpha5 subunits are predominately expressed in the brain during embryonic and early postnatal periods of normal rats, whilst alpha1 are most prominent during later developmental stages. In the present study, we examined the long-term effects of juvenile stress on GABA alpha subunit expression in adulthood in the amygdala and hippocampus. We applied the elevated platform stress paradigm at juvenility and used the open-field and startle response tests to assess anxiety level in adulthood. Juvenile stress effects without behavioural tests in adulthood were also examined since previous studies indicated that the mere exposure to these tests might be stressful for rats, enhancing the effects of the juvenile exposure to stress. In adulthood, we quantitatively determined the level of expression of alpha1, alpha2 and alpha3 in the hippocampus and amygdala. Our results indicate that subjecting juvenile stressed rats to additional challenges in adulthood results in an immature-like expression profile of these subunits. To test for potential functional implications of these alterations we examined the effects of the anxiolytic (diazepam) and the sedative (brotizolam) benzodiazepines on juvenile stressed and control rats following additional challenges in adulthood. Juvenile stressed rats were more sensitive to diazepam and less sensitive to brotizolam, suggesting that the alterations in GABA alpha subunit expression in these animals have functional consequences.

Different signal transduction cascades are activated simultaneously in the rat insular cortex and hippocampus following novel taste learning.

Novel taste learning is a robust one-trial incidental learning process, dependent on functional activity of the insular (taste) cortex. In contrast to that of the cortex, the role of the hippocampus in taste learning is controversial. We set out to identify the time courses of the activation of mitogen-associated protein kinase (MAPK), transcription factor cAMP-response element-binding protein (CREB) and Akt/PKB (protein kinase B) in the insular cortex and hippocampus of rats subsequent to novel taste learning. Following taste learning, an early response (20 min) occurred at the same time in the insular cortex and the hippocampus. However, whereas MAPK was activated specifically in the insular cortex, CREB and Akt were phosphorylated in the hippocampus but not in the cortex. In addition, the immediate early gene, CCAAT/enhancer binding protein (C/EBPbeta) was induced in both the hippocampus and the insular cortex 18 h following taste learning. The results demonstrate, for the first time, correlative activation and gene expression in the hippocampus following novel taste learning. Moreover, the results suggest that different signal transduction cascades necessary for taste learning are activated in concert in different brain structures, to enable taste learning and consolidation.

Behavioral interference and C/EBPbeta expression in the insular-cortex reveal a prolonged time period for taste memory consolidation.

Memory consolidation is defined as the time window during which the memory trace is susceptible to behavioral, electrical, or pharmacological interventions. Here, we presented rats with two novel tastes at consecutive time intervals. Clear interference was evident when a novel taste formed the second taste input whereby, surprisingly, the time window for interference was found to last more than 10 h. In addition, we detected an increase of C/EBPbeta protein expression in the gustatory cortex 18 h after novel taste learning. This modulation was attenuated by a subsequent novel taste. Our findings reveal temporal constraints and a lingering nature of taste memory consolidation.