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

Adolescent environmental enrichment induces social resilience and alters neural gene expression in a selectively bred rodent model with anxious phenotype

Stress is a major influence on mental health status; the ways that individuals respond to or copes with stressors determine whether they are negatively affected in the future. Stress responses are established by an interplay between genetics, environment, and life experiences. Psychosocial stress is particularly impactful during adolescence, a critical period for the development of mood disorders. In this study we compared two established, selectively-bred Sprague Dawley rat lines, the "internalizing" bred Low Responder (bLR) line versus the "externalizing" bred High Responder (bHR) line, to investigate how genetic temperament and adolescent environment impact future responses to social interactions and psychosocial stress, and how these determinants of stress response interact. Male bLR and bHR rats were exposed to social and environmental enrichment in adolescence prior to experiencing social defeat and were then assessed for social interaction and anxiety-like behavior. Adolescent enrichment caused rats to display more social interaction, as well as nominally less social avoidance, less submission during defeat, and resilience to the effects of social stress on corticosterone, in a manner that seemed more notable in bLRs. For bHRs, enrichment also caused greater aggression during a neutral social encounter and nominally during defeat, and decreased anxiety-like behavior. To explore the neurobiology underlying the development of social resilience in the anxious phenotype bLRs, RNA-seq was conducted on the hippocampus and nucleus accumbens, two brain regions that mediate stress regulation and social behavior. Gene sets previously associated with stress, social behavior, aggression and exploratory activity were enriched with differential expression in both regions, with a particularly large effect on gene sets that regulate social behaviors. Our findings provide further evidence that adolescent enrichment can serve as an inoculating experience against future stressors. The ability to induce social resilience in a usually anxious line of animals by manipulating their environment has translational implications, as it underscores the feasibility of intervention strategies targeted at genetically vulnerable adolescent populations.

Environmental enrichment improves social isolation-induced memory impairment: The possible role of ITSN1-Reelin-AMPA receptor signaling pathway

Environmental enrichment (EE) through combination of social and non-biological stimuli enhances activity-dependent synaptic plasticity and improves behavioural performance. Our earlier studies have suggested that EE resilience the stress induced depression/ anxiety-like behaviour in Indian field mice Mus booduga. This study was designed to test whether EE reverses the social isolation (SI) induced effect and improve memory. Field-caught mice M. booduga were subjected to behaviour test (Direct wild, DW), remaining animals were housed under SI for ten days and then housed for short-term at standard condition (STSC)/ long-term at standard condition (LTSC) or as group in EE cage. Subsequently, we have examined reference, working memory and expression of genes associated with synaptic plasticity. Our analysis have shown that EE reversed SI induced impairment in reference, working memory and other accompanied changes i.e. increased level of Intersectin 1 (ITSN1), Huntingtin (Htt), Synaptotagmin -IV (SYT4), variants of brain-derived neurotrophic factor (Bdnf - III), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (GluR1) expression, and decreased variants of Bdnf (IV), BDNF, Reelin, Apolipoprotein E receptor 2 (ApoER2), very low-density lipoprotein receptor (VLDLR), Src family tyrosine kinase (SFKs), Disabled protein (Dab)-1, Protein kinase B (PKB/Akt), GluR2, Mitogen-activated protein kinase (MAPK) and Extracellular signal-regulated kinase (ERK1/2) expression. In addition, SI induced reduction in BDNF expressing neurons in dentate gyrus of hippocampus reversed by EE. Further, we found that SI decreases small neuro-active molecules such as Benzenedicarboxylic acid, and increases 2-Pregnene in the hippocampus and feces reversed by EE. Overall, this study demonstrated that EE is effectively reversed the SI induced memory impairment by potentially regulating the molecules associated with the ITSN1-Reelin-AMPA receptor pathway to increase synaptic plasticity.

Increased depression-like behaviors with altered brain dopamine metabolisms in male mice housed in large cages are alleviated by bupropion

Psycho-environmental stress-based animal models of anxiety and depression are useful for investigating pathological mechanisms and drug development. Although several rodent-based studies have reported the beneficial effects of environmental enrichment (EE) on brain plasticity and anxiety- and depression-like behaviors, other studies have reported inverse effects. Here, we found that housing male mice in EE involving large cages and other EE materials increased anxiety- and depression-like behaviors in open field and tail suspension tests (TST). We further confirmed that housing in large cages was sufficient to induce increased depression-like behaviors in the TST and reduce the saccharine preference percentage, a sign of anhedonia, in male mice. In these experiments, the number of animals per cage was equivalent to that in standard cage housing, suggesting that low density in large cages may be a determining factor for behavioral alteration. In mice housed in large cages, sex-specific dysregulation of brain monoamine systems was observed; dopamine turnover to homovanillic acid or norepinephrine in the prefrontal cortex was elevated in males, while serotonin turnover to 5-hydroxyindoleacetic acid in the amygdala was increased in females. Finally, we demonstrated that daily intraperitoneal injections of bupropion, a dopamine and norepinephrine reuptake inhibitor, counteracted large-cage housing-induced changes in depression- and anhedonia-like behaviors in male mice. Our results suggest that housing in large cages with a low density of mice is a novel paradigm to clarify the mechanisms of environmental stress-induced emotional dysregulation and to identify drugs or food factors to alleviate the dysregulation.

Environmental Enrichment Increased Bdnf Transcripts in the Prefrontal Cortex: Implications for an Epigenetically Controlled Mechanism

Environmental enrichment (EE) is a condition characterized by its complexity regarding social contact, exposure to novelty, tactile stimuli and voluntary exercise, also is considered as a eustress model. The impact of EE on brain physiology and behavioral outcomes may be at least partly underpinned by mechanisms involving the modulation of the brain-derived neurotrophic factor (BDNF), but the connection between specific Bdnf exon expression and their epigenetic regulation remain poorly understood. This study aimed to dissect the transcriptional and epigenetic regulatory effect of 54-day exposure to EE on BDNF by analysing individual BDNF exons mRNA expression and the DNA methylation profile of a key transcriptional regulator of the Bdnf gene, exon IV, in the prefrontal cortex (PFC) of C57BL/6 male mice (sample size = 33). Bdnf exons II, IV, VI and IX mRNA expression were upregulated and methylation levels at two CpG sites of exon IV were reduced in the PFC of EE mice. As deficit in exon IV expression has also been causally implicated in stress-related psychopathologies, we also assessed anxiety-like behavior and plasma corticosterone levels in these mice to determine any potential correlation. However, no changes were observed in EE mice. The findings may suggest an EE-induced epigenetic control of BDNF exon expression via a mechanism involving exon IV methylation. The findings of this study contribute to the current literature by dissecting the Bdnf gene topology in the PFC where transcriptional and epigenetic regulatory effect of EE takes place.

The influence of long-term housing in enriched environment on behavior of normal rats and subjected to neonatal pro-inflammatory challenge

It is well known that neonatal pro-inflammatory challenge (NPC) acquire a predisposition to the development of a number of neuropsychiatric diseases: depression, anxiety disorders, autism, attention deficit hyperactivity disorder. Symptoms of these diseases can manifest themselves in adulthood and adolescent after repeated exposure to negative influences. Preventing the development of the negative consequences of NPC is one of the main tasks for researchers. The exposure to an enriched environment (EE) was shown to have anxiolytic, anti-depressive, and pro-cognitive effects. The present work was aimed to investigate the effects of the long-term EE on anxious-depressive and conditioned fear behavior in normal male and female rats and subjected to NPC. The NPC was induced by subcutaneous administration of lipopolysaccharide (LPS, 50 μg/kg) on 3d and 5th PNDs. The control animals received saline (SAL). The rats were placed in the EE from 25 to 120 PND. Animals housed in the standard conditions (STAND) served as controls. In adult female and male rats of the STAND groups, LPS did not affect the anxiety, depressive-like behavior and conditioned fear. The EE increased motor and search activity in males and females. In the open field, the EE reduced anxiety in males of the SAL and LPS groups and in females of SAL groups compared to the STAND housed animals. In the elevated plus maze, the EE decreased anxiety only in males of the SAL group. In the sucrose preference test, the EE did not change sucrose consumption in males and females of SAL and LPS groups, while, in the forced swimming test, the EE reduced depressive-like behavior in females of both SAL and LPS groups. The enrichment decreased the contextual conditioned fear in male and female of SAL groups, but not of the LPS group, and did not affect the cue conditioned fear. The corticosterone reactivity to the forced swimming stress increased in males of the EE groups. The basal level of IL-1beta in blood serum decreased in males of the SAL-EE group. Thus, the EE reduced anxiety in males, depressive-like behavior in females, and contextual conditioned fear in males and females compared to the STAND housed animals. Although the NPC did not affect these behaviors in the STAND groups, LPS prevented the beneficial EE effects on anxiety and conditioned fear. The opposing effects of LPS were dependent on sex and type of testing.

One day away from mum has lifelong consequences on brain and behaviour

This chapter presents a brief overview of attachment theory and discusses the importance of the neonatal period in shaping an individual's physiological and behavioural responses to stress later in life, with a focus on the role of the parent-infant relationship, particularly in rodents. In rodents, the role of maternal behaviours goes far beyond nutrition, thermoregulation and excretion, acting as hidden regulators of the pup's physiology and development. In this review, we will discuss the inhibitory role of specific maternal behaviours on the ACTH and corticosterone (CORT) stress response. The interest of our group to explore the long-term consequences of maternal deprivation for 24 h (DEP) at different ages (3 days and 11 days) in rats was sparked by its opposite effects on ACTH and CORT levels. In early adulthood, DEP3 animals (males and females alike) show greater negative impact on affective behaviours and stress related parameters than DEP11, indicating that the latter is more resilient in tests of anxiety-like behaviour. These findings create an opportunity to explore the neurobiological underpinnings of vulnerability and resilience to stress-related disorders. The chapter also provides a brief historical overview and highlights the relevance of attachment theory, and how DEP helps to understand the effects of childhood parental loss as a risk factor for depression, schizophrenia, and PTSD in both childhood and adulthood. Furthermore, we present the concept of environmental enrichment (EE), its effects on stress responses and related behavioural changes and its benefits for rats previously subjected to DEP, along with the clinical implications of DEP and EE.

Individualized Housing Modifies the Immune–Endocrine System in CD1 Adult Male Mice

In the last years, different research groups have made considerable efforts to improve the care and use of animals in research. Mice (Mus musculus) are the most widely used animal species in research in the European Union and are sociable and hierarchical creatures. During experiments, researchers tend to individualize males, but no consideration is given to whether this social isolation causes them stress. The aim of this study was, therefore, to explore whether 4 weeks of social isolation could induce changes in different physiological parameters in adult Crl:CD1(ICR) (CD1) males, which may interfere with experimental results. Body weight, blood cells, and fecal corticosterone metabolites levels were the analyzed parameters. Blood and fecal samples were collected at weeks 1 and 4 of the experimental procedure. Four weeks of single housing produced a significant time-dependent decrease in monocytes and granulocytes. Fecal corticosterone metabolite levels were higher in single-housed mice after 1 week and then normalized after 4 weeks of isolation. Body weight, red blood cells, and platelets remained unchanged in both groups during this period. We can, therefore, conclude that social isolation affects some immune and endocrine parameters, and that this should be taken into account in the interpretation of research data.

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

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

Aggression in Group-Housed Male Mice: A Systematic Review

Aggression among group-housed male mice is a major animal welfare concern often observed at animal facilities. Studies designed to understand the causes of male mice aggression have used different methodological approaches and have been heterogeneous, using different strains, environmental enrichments, housing conditions, group formations and durations. By conducting a systematic literature review based on 198 observed conclusions from 90 articles, we showed that the methodological approach used to study aggression was relevant for the outcome and suggested that home cage observations were better when studying home cage aggression than tests provoking aggression outside the home cage. The study further revealed that aggression is a complex problem; one solution will not be appropriate for all animal facilities and all research projects. Recommendations were provided on promising tools to minimize aggression, based on the results, which included what type of environmental enrichments could be appropriate and which strains of male mice were less likely to be aggressive.

TIAM1-mediated synaptic plasticity underlies comorbid depression-like and ketamine antidepressant-like actions in chronic pain

Chronic pain often leads to depression, increasing patient suffering and worsening prognosis. While hyperactivity of the anterior cingulate cortex (ACC) appears to be critically involved, the molecular mechanisms underlying comorbid depressive symptoms in chronic pain remain elusive. T cell lymphoma invasion and metastasis 1 (Tiam1) is a Rac1 guanine nucleotide exchange factor (GEF) that promotes dendrite, spine, and synapse development during brain development. Here, we show that Tiam1 orchestrates synaptic structural and functional plasticity in ACC neurons via actin cytoskeleton reorganization and synaptic N-methyl-d-aspartate receptor (NMDAR) stabilization. This Tiam1-coordinated synaptic plasticity underpins ACC hyperactivity and drives chronic pain-induced depressive-like behaviors. Notably, administration of low-dose ketamine, an NMDAR antagonist emerging as a promising treatment for chronic pain and depression, induces sustained antidepressant-like effects in mouse models of chronic pain by blocking Tiam1-mediated maladaptive synaptic plasticity in ACC neurons. Our results reveal Tiam1 as a critical factor in the pathophysiology of chronic pain-induced depressive-like behaviors and the sustained antidepressant-like effects of ketamine.

The Influence of Environmental Enrichment on Affective and Neural Consequences of Social Isolation Across Development

Social stress is associated with depression and anxiety, physiological disruptions, and altered brain morphology in central stress circuitry across development. Environmental enrichment strategies may improve responses to social stress. Socially monogamous prairie voles exhibit analogous social and emotion-related behaviors to humans, with potential translational insight into interactions of social stress, age, and environmental enrichment. This study explored the effects of social isolation and environmental enrichment on behaviors related to depression and anxiety, physiological indicators of stress, and dendritic structural changes in amygdala and hippocampal subregions in young adult and aging prairie voles. Forty-nine male prairie voles were assigned to one of six groups divided by age (young adult vs. aging), social structure (paired vs. isolated), and housing environment (enriched vs. non-enriched). Following 4 weeks of these conditions, behaviors related to depression and anxiety were investigated in the forced swim test and elevated plus maze, body and adrenal weights were evaluated, and dendritic morphology analyses were conducted in hippocampus and amygdala subregions. Environmental enrichment decreased immobility duration in the forced swim test, increased open arm exploration in the elevated plus maze, and reduced adrenal/body weight ratio in aging and young adult prairie voles. Age and social isolation influenced dendritic morphology in the basolateral amygdala. Age, but not social isolation, influenced dendritic morphology in the hippocampal dentate gyrus. Environmental enrichment did not influence dendritic morphology in either brain region. These data may inform interventions to reduce the effects of social stressors and age-related central changes associated with affective behavioral consequences in humans.

Genomic modules and intramodular network concordance in susceptible and resilient male mice across models of stress

The multifactorial etiology of stress-related disorders necessitates a constant interrogation of the molecular convergences in preclinical models of stress that use disparate paradigms as stressors spanning from environmental challenges to genetic predisposition to hormonal signaling. Using RNA-sequencing, we investigated the genomic signatures in the ventral hippocampus common to mouse models of stress. Chronic oral corticosterone (CORT) induced increased anxiety- and depression-like behavior in wild-type male mice and male mice heterozygous for the gene coding for brain-derived neurotrophic factor Val66Met, a variant associated with genetic susceptibility to stress. In a separate set of male mice, chronic social defeat stress (CSDS) led to a susceptible or a resilient population, whose proportion was dependent on housing conditions, namely standard housing or enriched environment. Rank-rank-hypergeometric overlap (RRHO), a threshold-free approach that ranks genes by their p value and effect size direction, was used to identify genes from a continuous gradient of significancy that were concordant across groups. In mice treated with CORT and in standard-housed susceptible mice, differentially expressed genes (DEGs) were concordant for gene networks involved in neurotransmission, cytoskeleton function, and vascularization. Weighted gene co-expression analysis generated 54 gene hub modules and revealed two modules in which both CORT and CSDS-induced enrichment in DEGs, whose function was concordant with the RRHO predictions, and correlated with behavioral resilience or susceptibility. These data showed transcriptional concordance across models in which the stress coping depends upon hormonal, environmental, or genetic factors revealing common genomic drivers that embody the multifaceted nature of 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.

Ethanol intake in male mice exposed to social defeat: Environmental enrichment potentiates resilience

Large preclinical evidence shows that exposure to social defeat (SD) increases vulnerability to drug abuse, increasing the consumption of ethanol. However, not all subjects are equally affected by the changes induced by stress. Previous reports have evidenced that the resilient phenotype to depressive-like behaviors after SD is associated with the resistant phenotype to cocaine-increased rewarding effects and the smaller neuroinflammatory response. The aim of the present study was to further clarify whether the resilient profile to depressive-like behavior also predicts a protection against the increase in ethanol intake induced by SD. The neuroinflammatory profile was studied after the end of the oral ethanol self-administration (SA) procedure, measuring levels of the pro-inflammatory cytokine IL-6 and the chemokine CX3CL1 or fractalkine in the striatum and prefrontal cortex. Previous studies have shown that environmental enrichment (EE) is an effective mechanism to dimish the detrimental effects of social stress. In a second study, we aimed to evaluate if EE housing before exposure to SD could potentiate resilience. Our results showed that mice with a phenotype susceptible to SD-induced depressive-like behaviors showed increased ethanol consumption and increased neuroinflammatory signaling. In contrast, despite the lack of effect on depressive-like behaviors, defeated mice previously housed under EE conditions did not show an increase in ethanol SA or an increase in immune response. To sum up, the resilient phenotype to SD develops at different levels, such as depressive-like behaviors, ethanol consumption and the neuroinflammatory response. Our results also point to the protective role of EE in potentiating resilience to SD effects.

Differential impact of stress and environmental enrichment on corticolimbic circuits

Stress exposure can produce profound changes in physiology and behavior that can impair health and well-being. Of note, stress exposure is linked to anxiety disorders and depression in humans. The widespread impact of these disorders warrants investigation into treatments to mitigate the harmful effects of stress. Pharmacological treatments fail to help many with these disorders, so recent work has focused on non-pharmacological alternatives. One of the most promising of these alternatives is environmental enrichment (EE). In rodents, EE includes social, physical, and cognitive stimulation for the animal, in the form of larger cages, running wheels, and toys. EE successfully reduces the maladaptive effects of various stressors, both as treatment and prophylaxis. While we know that EE can have beneficial effects under stress conditions, the morphological and molecular mechanisms underlying these behavioral effects are still not well understood. EE is known to alter neurogenesis, dendrite development, and expression of neurotrophic growth factors, effects that vary by type of enrichment, age, and sex. To add to this complexity, EE has differential effects in different brain regions. Understanding how EE exerts its protective effects on morphological and molecular levels could hold the key to developing more targeted pharmacological treatments. In this review, we summarize the literature on the morphological and molecular consequences of EE and stress in key emotional regulatory pathways in the brain, the hippocampus, prefrontal cortex, and amygdala. The similarities and differences among these regions provide some insight into stress-EE interaction that may be exploited in future efforts toward prevention of, and intervention in, stress-related diseases.

Effects of early rearing enrichments on modulation of brain monoamines and hypothalamic-pituitary-interrenal axis (HPI axis) of fish mahseer (Tor putitora)

Enriching rearing environment is the strategy suggested for improving the post release survivorship of captive-reared animals. Here, an attempt has been made to evaluate the impact of early rearing enrichment on the hypothalamic-pituitary-interrenal axis (HPI axis), blood glucose, and brain dopaminergic and serotonergic systems of Tor putitora. Fifteen-day-old hatchlings of T. putitora were reared up to advanced fry stage in barren, semi-natural, and physically enriched environments and compared them with regard to pre-stress and post-stress levels of whole-body cortisol, blood glucose, brain serotonergic activity (5HIAA/5HT ratio), dopaminergic activity (DOPAC/DA and HVA/DA ratios) and norepinephrine (NE) levels. Significantly low basal whole-body cortisol, glucose and brain NE levels were observed in a physically enriched group of fish as compared to the other two groups. However, after acute stress, all rearing groups showed elevated levels of cortisol, blood glucose, brain 5HIAA/5HT, DOPAC/DA and HVA/DA ratios and NE levels but the magnitude of response was different among different rearing groups. The barren reared group showed a higher magnitude of response as compared to semi-natural and physically enriched groups. Similarly, the recovery rate of whole-body cortisol, blood glucose, and whole-brain monoamines were long-lasting in barren-reared mahseer. We illustrate that increased structural complexity (physical enrichment) during the early rearing significantly modulates various physiological and stress-coping mechanisms of mahseer.

Neuroplasticity-related correlates of environmental enrichment combined with physical activity differ between the sexes

Environmental enrichment (EE), comprising positive physical (exercise) and cognitive stimuli, influences neuronal structure and usually improves brain function. The promise of EE as a preventative strategy against neuropsychiatric disease is especially high during early postnatal development when the brain is still amenable to reorganization. Despite the fact that male and female brains differ in terms of connectivity and function that may reflect early life experiences, knowledge of the neural substrates and mechanisms by which such changes arise remains limited. This study compared the impact of EE combined with physical activity on neuroplasticity and its functional consequences in adult male and female rats; EE was provided during the first 3 months of life and our analysis focused on the hippocampus, an area implicated in cognitive behavior as well as the neuroendocrine response to stress. Both male and female rats reared in EE displayed better object recognition memory than their control counterparts. Interestingly, sex differences were revealed in the effects of EE on time spent exploring the objects during this test. Independently of sex, EE increased hippocampal turnover rates of dopamine and serotonin and reduced expression of 5-HT_1A receptors; in addition, EE upregulated expression of synaptophysin, a presynaptic protein, in the hippocampus. As compared to their respective controls, EE-exposed males exhibited parallel increases in phosphorylated Tau and the GluN2B receptor, whereas females responded to EE with reduced hippocampal levels of glutamate and GluN2B. Together, these observations provide further evidence on the differential effects of EE on markers of hippocampal neuroplasticity in males and females.

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)-1β 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.

An Improved Model of Physical and Emotional Social Defeat: Different Effects on Social Behavior and Body Weight of Adolescent Mice by Interaction With Social Support

Social stress is a prevalent etiological environmental factor that can affect health, especially during adolescence. Either experiencing or witnessing a traumatic event during adolescence can increase the risk of psychiatric disorders, such as PTSD. The present study attempted to establish an improved social stress model to better distinguish the effects of physical and emotional social stress on the behavior and physiology of adolescent mice. In addition, we investigated how social support affected these stress-induced changes in social behavior. On PND 28, male littermates were exposed to either physical stress (PS) or emotional stress (ES), afterwards, half of them were paired-housed and the others were singly housed. The PS exposed mice were directly confronted with a violent aggressor using the social defeat stress (SDS) paradigm for 15 min/trial (with the total of 10 trials randomly administered over a week), while the ES exposed mice were placed in a neighboring compartment to witness the PS procedure. Our results indicate that both stressors induced an effective stress response in adolescent mice, but PS and ES had differential influence in the context of relevant social anxiety/fear and social interaction with peers. Additionally, social support following stress exposure exerted beneficial effects on the social anxiety/fear in ES exposed mice, but not on PS exposed mice, suggesting that the type of stressor may affect the intervention efficacy of social support. These findings provide extensive evidence that physical and emotional stressors induce different effects. Moreover, ES exposed mice, rather than PS exposed mice, seemed to benefit from social support. In summary, the study suggests that this paradigm will be helpful in investigating the effects of psychological intervention for the treatment of stress-related psychiatric disorders.

Social experience alters socially induced serotonergic fluctuations in the inferior colliculus

Past social experience and current social context shape the responses of animals to social signals. The serotonergic system is one potential mechanism by which both experiential and contextual factors could be conveyed to sensory systems, such as the auditory system, for multiple reasons. 1) Many features of the serotonergic system are sensitive to social experience. 2) Elevations in serotonergic activity are triggered by social partners, and variations in socially triggered serotonergic responses reflect behavioral differences among social encounters. 3) Serotonin is an auditory neuromodulator, altering how auditory neurons respond to sounds including conspecific vocalizations. In this study, we tested how social experience influences the socially triggered serotonergic response in the inferior colliculus, an auditory midbrain region with an important role in vocalization processing. We used carbon fiber voltammetry to measure serotonin during social interactions of male mice ( Mus musculus) from different social backgrounds: 4 weeks of grouped or individual housing. When paired with an unfamiliar male, both group-housed and individually housed males demonstrated elevations in serotonin; however, individually housed males exhibited socially triggered serotonergic responses with delayed time courses compared with the group-housed males. Furthermore, group-housed males displayed previously described correlations between the socially triggered serotonergic response and behaviors such as social investigation. In contrast, individually housed males did not show these serotonin-behavior relationships. These results suggest that social experience gained via social housing may shape the ability of the central serotonergic system to encode social context in sensory regions.

NEW & NOTEWORTHY We demonstrate that past social experience influences the fidelity with which the serotonergic system represents social context in an auditory region. Social experience altered the time course of socially triggered serotonergic responses and changed how the serotonergic system reflects behavioral variations among social encounters of the same context. These findings are significant to the study of communication, suggesting that centralized neuromodulatory systems potentially convey integrated information regarding past experience and current context to primary sensory regions.

Adolescent Social Defeat Induced Alterations in Social Behavior and Cognitive Flexibility in Adult Mice: Effects of Developmental Stage and Social Condition

Negative social experiences during adolescence increase the risk of psychiatric disorders in adulthood. Using "resident-intruder" stress, the present study aimed to investigate the effects of adolescent social defeat on emotional and cognitive symptoms associated with psychiatric disorders during adulthood and the effects of the developmental stage and social condition on this process. In Experiment 1, animals were exposed to social defeat or manipulation for 10 days during early adolescence (EA, postnatal days [PND] 28-37), late adolescence (LA, PND 38-47), and adulthood (ADULT, PND 70-79) and then singly housed until the behavioral tests. Behaviors, including social avoidance of the defeat context and cortically mediated cognitive flexibility in an attentional set-shifting task (AST), were assessed during the week following stress or after 6 weeks during adulthood. We determined that social defeat induced significant and continuous social avoidance across age groups at both time points. The mice that experienced social defeat during adulthood exhibited short-term impairments in reversal learning (RL) on the AST that dissipated after 6 weeks. In contrast, social defeat during EA but not LA induced a delayed deficit in extra-dimensional set-shifting (EDS) in adulthood but not during adolescence. In Experiment 2, we further examined the effects of social condition (isolation or social housing after stress) on the alterations induced by social defeat during EA in adult mice. The adult mice that had experienced stress during EA exhibited social avoidance similar to the avoidance identified in Experiment 1 regardless of the isolation or social housing after the stress. However, social housing after the stress ameliorated the cognitive flexibility deficits induced by early adolescent social defeat in the adult mice, and the social condition had no effect on cognitive function. These findings suggest that the effects of social defeat on emotion and cognitive function are differentially affected by the developmental stage and social condition. EA may comprise a particularly sensitive developmental period in which social defeat may produce a delayed impairment in cognitive flexibility during adulthood, and the social condition following stress appears to play an important intermediary role in the development of these cognitive deficits.

The anterior cingulate cortex is a critical hub for pain-induced depression

BACKGROUND

Besides chronic stress, chronic pain is a prevalent determinant for depression. Changes induced in specific brain regions by sustained pain may alter the processing of affective information, thus resulting in anxiodepressive disorders. Here, we compared the role of the anterior cingulate cortex (ACC) and the posterior insular cortex in the anxiodepressive, sensory, and affective aspects of chronic pain.

METHODS

Neuropathic pain was induced by cuffing the right sciatic nerve of C57BL/6J mice. Lesions were performed by local injection of ibotenic acid and chronic activation of the ACC by optogenetic stimulation. Anxiodepressive-related behaviors were evaluated through the novelty suppressed feeding, marble burying, splash, and forced swimming tests. Mechanical thresholds were determined using von Frey filaments, and the relief of spontaneous pain was determined by using place conditioning.

RESULTS

The ACC lesion prevented the anxiodepressive consequences of chronic pain without affecting the sensory mechanical allodynia. Conversely, the tonic or spontaneous pain and the anxiodepressive consequences of pain remained present after posterior insular cortex lesion, even though the mechanical allodynia was suppressed. Furthermore, optogenetic stimulation of the ACC was sufficient to induce anxiety and depressive-like behaviors in naïve animals.

CONCLUSIONS

Our results show that, at cortical level, the sensory component of chronic pain remains functionally segregated from its affective and anxiodepressive components. Spontaneous tonic pain and evoked allodynia can be experimentally dissociated. Furthermore, the ACC appears as a critical hub for mood disorders, including for the anxiodepressive consequences of chronic pain, and thus constitutes an important target for divulging the underlying mechanism.

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.

The effects of environmental enrichment on depressive and anxiety-relevant behaviors in socially isolated prairie voles

OBJECTIVES

Social isolation is associated with depression, anxiety, and negative health outcomes. Environmental enrichment, including environmental and cognitive stimulation with inanimate objects and opportunities for physical exercise, may be an effective strategy to include in treatment paradigms for affective disorders as a function of social isolation. In a rodent model-the socially monogamous prairie vole-we investigated the hypothesis that depression- and anxiety-related behaviors after social isolation would be prevented and remediated with environmental enrichment.

METHODS

Experiment 1 investigated the preventive effects of environmental enrichment on negative affective behaviors when administered concurrently with social isolation. Experiment 2 investigated the remediating effects of enrichment on negative affective behaviors when administered after a period of isolation. Behaviors were measured in three operational tests: open field, forced swim test (FST), and elevated plus maze.

RESULTS

In isolated prairie voles, enrichment prevented depression-relevant (immobility in FST, group × housing interaction, p = .049) and anxiety-relevant behaviors (exploration in open field, group × housing interaction, p = .036; exploration in elevated plus maze, group × housing interaction, p = .049). Delayed enrichment also remediated these behaviors in isolated animals (immobility in FST, main effect of housing, p = .001; exploration in open field, main effect of housing, p = .047; exploration in elevated plus maze, main effect of housing, p = .001) and was slightly more effective than physical exercise alone in remediating anxiety-relevant behaviors.

CONCLUSIONS

These findings provide insight into the beneficial effects of an enriched environment on depression- and anxiety-relevant behaviors using a translational rodent model of social isolation.

Environmental enrichment improves novel object recognition and enhances agonistic behavior in male mice

Environmental enrichment (EE) is an experimental paradigm in which rodents are housed in complex environments containing objects that provide stimulation, the effects of which are expected to improve the welfare of these subjects. EE has been shown to considerably improve learning and memory in rodents. However, knowledge about the effects of EE on social interaction is generally limited and rather controversial. Thus, our aim was to evaluate both novel object recognition and agonistic behavior in NMRI mice receiving EE, hypothesizing enhanced cognition and slightly enhanced agonistic interaction upon EE rearing. During a 4-week period half the mice (n = 16) were exposed to EE and the other half (n = 16) remained in a standard environment (SE). On PND 56-57, animals performed the object recognition test, in which recognition memory was measured using a discrimination index. The social interaction test consisted of an encounter between an experimental animal and a standard opponent. Results indicated that EE mice explored the new object for longer periods than SE animals (P < .05). During social encounters, EE mice devoted more time to sociability and agonistic behavior (P < .05) than their non-EE counterparts. In conclusion, EE has been shown to improve object recognition and increase agonistic behavior in adolescent/early adulthood mice. In the future we intend to extend this study on a longitudinal basis in order to assess in more depth the effect of EE and the consistency of the above-mentioned observations in NMRI mice.

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).