Social dominance predicts hippocampal glucocorticoid receptor recruitment and resilience to prenatal adversity

Colon impairments and inflammation driven by an altered gut microbiota leads to social behavior deficits rescued by hyaluronic acid and celecoxib

BACKGROUND

The exact mechanisms linking the gut microbiota and social behavior are still under investigation. We aimed to explore the role of the gut microbiota in shaping social behavior deficits using selectively bred mice possessing dominant (Dom) or submissive (Sub) behavior features. Sub mice exhibit asocial, depressive- and anxiety-like behaviors, as well as systemic inflammation, all of which are shaped by their impaired gut microbiota composition.

METHODS

An age-dependent comparative analysis of the gut microbiota composition of Dom and Sub mice was performed using 16S rRNA sequencing, from early infancy to adulthood. Dom and Sub gastrointestinal (GI) tract anatomy, function, and immune profiling analyses were performed using histology, RT-PCR, flow cytometry, cytokine array, and dextran-FITC permeability assays. Short chain fatty acids (SCFA) levels in the colons of Dom and Sub mice were quantified using targeted metabolomics. To support our findings, adult Sub mice were orally treated with hyaluronic acid (HA) (30 mg/kg) or with the non-steroidal anti-inflammatory agent celecoxib (16 mg/kg).

RESULTS

We demonstrate that from early infancy the Sub mouse gut microbiota lacks essential bacteria for immune maturation, including Lactobacillus and Bifidobacterium genera. Furthermore, from birth, Sub mice possess a thicker colon mucin layer, and from early adulthood, they exhibit shorter colonic length, altered colon integrity with increased gut permeability, reduced SCFA levels and decreased regulatory T-cells, compared to Dom mice. Therapeutic intervention in adult Sub mice treated with HA, celecoxib, or both agents, rescued Sub mice phenotypes. HA treatment reduced Sub mouse gut permeability, increased colon length, and improved mouse social behavior deficits. Treatment with celecoxib increased sociability, reduced depressive- and anxiety-like behaviors, and increased colon length, and a combined treatment resulted in similar effects as celecoxib administered as a single agent.

CONCLUSIONS

Overall, our data suggest that treating colon inflammation and decreasing gut permeability can restore gut physiology and prevent social deficits later in life. These findings provide critical insights into the importance of early life gut microbiota in shaping gut immunity, functionality, and social behavior, and may be beneficial for the development of future therapeutic strategies.

Metabolic, neuroendocrine and behavioral effects of social defeat in male and female mice using the chronic non-discriminatory social defeat stress model

Stress-related disorders predominately affect females, yet preclinical models of chronic stress exclusively use males especially in models where social stressors are studied. Here, we implemented a 21-day novel social defeat paradigm in which a female and male C57 intruder are simultaneously placed in the cage of a territorial, resident CD-1 male mouse, and the resident proceeds to attack both intruders. Mice were given access to a regular laboratory diet, high in carbohydrates, and a palatable diet, high in fat. Chronic social defeat stress using this paradigm resulted in increased caloric intake in male and female mice, with the effects being more pronounced in females. We observed sex differences in high fat diet intake in response to stress, which was correlated with higher levels of plasma ghrelin observed in female mice but not male mice. Furthermore, females exposed to chronic stress displayed changes in growth hormone secretatogue receptor (ghsr) and neuropeptide-y (npy) expression in the arcuate nucleus of the hypothalamus, potentially increasing ghrelin sensitivity and inducing changes in diet choice and caloric intake. Behavioral results show that females tended to spend more time interacting during the social interaction test, compared to males who displayed higher vigilance towards the stranger mouse. Overall, our results highlight unique neurometabolic alterations in female mice in response to stress that is not present in male mice and may be important for coping with chronic stress and sustaining reproductive function.

Nurture outpaces nature: fostering with an attentive mother alters social dominance in a mouse model of stress sensitivity

Maternal care is critical for epigenetic programming during postnatal brain development. Stress is recognized as a critical factor that may affect maternal behavior, yet owing to high heterogeneity in stress response, its impact varies among individuals. We aimed here to understand the connection between inborn stress vulnerability, maternal care, and early epigenetic programming using mouse populations that exhibit opposite poles of the behavioral spectrum (social dominance [Dom] and submissiveness [Sub]) and differential response to stress. In contrast to stress-resilient Dom dams, stress-vulnerable Sub dams exhibit significantly lower maternal attachment, serum oxytocin, and colonic Lactobacillus reuteri populations. Sub offspring showed a reduced hippocampal expression of key methylation genes at postnatal day (PND) 7 and a lack of developmentally-dependent increase in 5-methylcytosine (5-mC) at PND 21. In addition, Sub pups exhibit significant hypermethylation of gene promoters connected with glutamatergic synapses and behavioral responses. We were able to reverse the submissive endophenotype through cross-fostering Sub pups with Dom dams (Sub/D). Thus, Sub/D pups exhibited elevated hippocampal expression of DNMT3A at PND 7 and increased 5-mC levels at PND 21. Furthermore, adult Sub/D offspring exhibited increased sociability, social dominance, and hippocampal glutamate and monoamine levels resembling the neurochemical profile of Dom mice. We postulate that maternal inborn stress vulnerability governs epigenetic patterning sculpted by maternal care and intestinal microbiome diversity during early developmental stages and shapes the array of gene expression patterns that may dictate neuronal architecture with a long-lasting impact on stress sensitivity and the social behavior of offspring.

Endocannabinoid basis of personality-Insights from animal model of social behavior

Rationale: The endocannabinoid system is known to be involved in learning, memory, emotional processing and regulation of personality patterns. Here we assessed the endocannabinoid profile in the brains of mice with strong characteristics of social dominance and submissiveness. Methods: A lipidomics approach was employed to assess the endocannabinoidome in the brains of Dominant (Dom) and Submissive (Sub) mice. The endocannabinoid showing the greatest difference in concentration in the brain between the groups, docosatetraenoyl ethanolamine (DEA), was synthesized, and its effects on the physiological and behavioral responses of Dom and Sub mice were evaluated. mRNA expression of the endocannabinoid receptors and enzymes involved in PUFA biosynthesis was assessed using qRT-PCR. Results: Targeted LC/MS analysis revealed that long-chain polyunsaturated ethanolamides including arachidonoyl ethanolamide (AEA), DEA, docosatrienoyl ethanolamide (DTEA), eicosatrienoyl ethanolamide (ETEA), eicosapentaenoyl ethanolamide (EPEA) and docosahexaenoyl ethanolamide (DHEA) were higher in the Sub compared with the Dom mice. Untargeted LC/MS analysis showed that the parent fatty acids, docosatetraenoic (DA) and eicosapentaenoic (EPA), were higher in Sub vs. Dom. Gene expression analysis revealed increased mRNA expression of genes encoding the desaturase FADS2 and the elongase ELOVL5 in Sub mice compared with Dom mice. Acute DEA administration at the dose of 15 mg/kg produced antinociceptive and locomotion-inducing effects in Sub mice, but not in Dom mice. Subchronic treatment with DEA at the dose of 5 mg/kg augmented dominant behavior in wild-type ICR and Dom mice but not in Sub mice. Conclusion: This study suggests that the endocannabinoid system may play a role in the regulation of dominance and submissiveness, functional elements of social behavior and personality. While currently we have only scratched the surface, understanding the role of the endocannabinoid system in personality may help in revealing the mechanisms underlying the etiopathology of psychiatric disorders.

Prenatal SAMe Treatment Changes via Epigenetic Mechanism/s USVs in Young Mice and Hippocampal Monoamines Turnover at Adulthood in a Mouse Model of Social Hierarchy and Depression

The role of hippocampal monoamines and their related genes in the etiology and pathogenesis of depression-like behavior, particularly in impaired sociability traits and the meaning of changes in USVs emitted by pups, remains unknown. We assessed the effects of prenatal administration of S-adenosyl-methionine (SAMe) in Sub mice that exhibit depressive-like behavior on serotonergic, dopaminergic and noradrenergic metabolism and the activity of related genes in the hippocampus (HPC) in adulthood in comparison to saline-treated control Sub mice. During postnatal days 4 and 8, we recorded and analyzed the stress-induced USVs emitted by the pups and tried to understand how the changes in the USVs' calls may be related to the changes in the monoamines and the activity of related genes. The recordings of the USVs showed that SAMe induced a reduction in the emitted flat and one-frequency step-up call numbers in PND4 pups, whereas step-down type calls were significantly increased by SAMe in PND8 pups. The reduction in the number of calls induced by SAMe following separation from the mothers implies a reduction in anxiety, which is an additional sign of decreased depressive-like behavior. Prenatal SAMe increased the concentrations of serotonin in the HPC in both male and female mice without any change in the levels of 5HIAA. It also decreased the level of the dopamine metabolite DOPAC in females. There were no changes in the levels of norepinephrine and metabolites. Several changes in the expression of genes associated with monoamine metabolism were also induced by prenatal SAMe. The molecular and biochemical data obtained from the HPC studies are generally in accordance with our previously obtained data from the prefrontal cortex of similarly treated Sub mice on postnatal day 90. The changes in both monoamines and their gene expression observed 2-3 months after SAMe treatment are associated with the previously recorded behavioral improvement and seem to demonstrate that SAMe is effective via an epigenetic mechanism.

Influence of gut microbiota on resilience and its possible mechanisms

Excessive stress leads to disruptions of the central nervous system. Individuals' responses to stress and trauma differ from person to person. Some may develop various neuropsychiatric disorders, such as post-traumatic stress disorder, major depression, and anxiety disorders, while others may successfully adapt to the same stressful events. These two neural phenotypes are called susceptibility and resilience. Previous studies have suggested resilience/susceptibility as a complex, non-specific systemic response involving central and peripheral systems. Emerging research of mechanisms underlying resilience is mostly focussing on the physiological adaptation of specific brain circuits, neurovascular impairment of the blood-brain barrier, the role of innate and adaptive factors of the immune system, and the dysbiosis of gut microbiota. In accordance with the microbiota-gut-brain axis hypothesis, the gut microbiome directly influences the interface between the brain and the periphery to affect neuronal function. This review explored several up-to-date studies on the role of gut microbiota implicated in stressful events-related resilience/susceptibility. We mainly focus on the changes in behavior and neuroimaging characteristics, involved brain regions and circuits, the blood-brain barrier, the immune system, and epigenetic modifications, which contribute to stress-induced resilience and susceptibility. The perspective of the gut-brain axis could help to understand the mechanisms underlying resilience and the discovery of biomarkers may lead to new research directions and therapeutic interventions for stress-induced neuropsychiatric disorders.

The bed nucleus of the stria terminalis in threat detection: task choice and rodent experience

Behavioural reactivity to potential threat is used to experimentally refine models of anxiety symptoms in rodents. We present a short review of the literature tying the most commonly used tasks to model anxiety symptoms to functional recruitment of bed nucleus of the stria terminalis circuits (BNST). Using a review of studies that investigated the role of the BNST in anxiety-like behaviour in rodents, we flag the certain challenges for the field. These stem from inconsistent methods of reporting the neuroanatomical BNST subregions and the interpretations of specific behaviour across a wide variety of tasks as 'anxiety-like'. Finally, to assist in interpretation of the findings, we discuss the potential interactions between typically used 'anxiety' tasks of innate behaviour that are potentially modulated by the social and individual experience of the animal.

Prenatal SAMe Treatment Induces Changes in Brain Monoamines and in the Expression of Genes Related to Monoamine Metabolism in a Mouse Model of Social Hierarchy and Depression, Probably via an Epigenetic Mechanism

Reduction in the levels of monoamines, such as serotonin and dopamine in the brain, were reported in patients and animals with depression. SAMe, a universal methyl donor and an epigenetic modulator, is successfully used as an adjunct treatment of depression. We previously found that prenatal treatment with SAMe of Submissive (Sub) mice that serve as a model for depression alleviated many of the behavioral depressive symptoms. In the present study, we treated pregnant Sub mice with 20 mg/kg of SAMe on days 12–15 of gestation and studied the levels of monoamines and the expression of genes related to monoamines metabolism in their prefrontal cortex (PFC) at the age of 3 months. The data were compared to normal saline-treated Sub mice that exhibit depressive-like symptoms. SAMe increased the levels of serotonin in the PFC of female Sub mice but not in males. The levels of 5-HIAA were not changed. SAMe increased the levels of dopamine and of DOPAC in males and females but increased the levels of HVA only in females. The levels of norepinephrine and its metabolite MHPG were unchanged. SAMe treatment changed the expression of several genes involved in the metabolism of these monoamines, also in a sex-related manner. The increase in several monoamines induced by SAMe in the PFC may explain the alleviation of depressive-like symptoms. Moreover, these changes in gene expression more than 3 months after treatment probably reflect the beneficial effects of SAMe as an epigenetic modulator in the treatment of depression.

Impact of Chronic Prenatal Stress on Maternal Neuroendocrine Function and Embryo and Placenta Development During Early-to-Mid-Pregnancy in Mice

Psychological stress, both leading up to and during pregnancy, is associated with increased risk for negative pregnancy outcomes. Although the neuroendocrine circuits that link the stress response to reduced sexual motivation and mating are well-described, the specific pathways by which stress negatively impacts gestational outcomes remain unclear. Using a mouse model of chronic psychological stress during pregnancy, we investigated 1) how chronic exposure to stress during gestation impacts maternal reproductive neuroendocrine circuitry, and 2) whether stress alters developmental outcomes for the fetus or placenta by mid-pregnancy. Focusing on the stress-responsive neuropeptide RFRP-3, we identified novel contacts between RFRP-3-immunoreactive (RFRP-3-ir) cells and tuberoinfundibular dopaminergic neurons in the arcuate nucleus, thus providing a potential pathway linking the neuroendocrine stress response directly to pituitary prolactin production and release. However, neither of these cell populations nor circulating levels of pituitary hormones were affected by chronic stress. Conversely, circulating levels of steroid hormones relevant to gestational outcomes (progesterone and corticosterone) were altered in chronically-stressed dams across gestation, and those dams were qualitatively more likely to experience delays in fetal development. Together, these findings suggest that, up until at least mid-pregnancy, mothers appear to be relatively resilient to the effects of elevated glucocorticoids on reproductive neuroendocrine system function. We conclude that understanding how chronic psychological stress impacts reproductive outcomes will require understanding individual susceptibility and identifying reliable neuroendocrine changes resulting from gestational stress.

Proteostasis and resilience: on the interphase between individual's and intracellular stress

A long proportion of the population is resilient to the negative consequences of stress. Glucocorticoids resulting from endocrine responses to stress are essential adaptive mediators, but also drive alterations to brain function, negatively impacting neuronal connectivity, synaptic plasticity, and memory-related processes. Recent evidence has indicated that organelle function and cellular stress responses are relevant determinant of vulnerability and resistance to environmental stress. At the molecular level, a fundamental mechanism of cellular stress adaptation is the maintenance of proteostasis, which also have key roles in sustaining basal neuronal function. Here, we discuss recent evidence suggesting that proteostasis unbalance at the level of the endoplasmic reticulum, the main site for protein folding in the cell, represents a possible mechanistic link between individuals and cellular stress.

Antidepressant-like effects of a chlorogenic acid- and cynarine-enriched fraction from Dittrichia viscosa root extract

Dittrichia viscosa is a perennial Mediterranean plant used in traditional medicine for “calming purposes”, pointing at a possible antidepressant activity of the plant. We conducted chromatographic and bioassay-guided fractionation of D. viscosa root extract to isolate a specific fraction (fraction “K”) with antidepressant-like characteristics in vivo and strong antioxidant properties in vitro. A single dose of “K” reduced immobility time in the forced swim test with a mouse model possessing a depressive-like phenotype. Neurochemical profiling for 5-hydroxytryptamine (5-HT) and its primary metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in prefrontal cortex and hippocampus of “K”-treated mice showed reduction in 5-HIAA, indicative of either serotonin uptake transporter or monoamine oxidase-A inhibition, as well as slight increases in 5-HT content. These neurochemical alterations, as well as the behavioral changes observed, were comparable to the effects of paroxetine. “K” also protected PC12 cells in a H₂O₂ cytotoxicity assay, thus demonstrating antioxidant properties, yet paroxetine augmented oxidative damage and cell death. Identification of the main compounds in “K” by high-performance liquid chromatography-tandem mass spectrometry (HPLC–MS/MS) indicated that chlorogenic acid and cynarine comprised 87% of the total components. D. viscosa root extract appears to produce antidepressant and cytoprotective effects and may serve as an attractive alternative to standard therapies for depression.

Double trouble: Prenatal immune activation in stress sensitive offspring

Viral infections during pregnancy are associated with increased incidence of psychiatric disorders in offspring. The pathological outcomes of viral infection appear to be caused by the deleterious effects of innate immune response-associated factors on development of the fetus, which predispose the offspring to pathological conditions in adulthood. The negative impact of viral infections varies substantially between pregnancies. Here, we explored whether differential stress sensitivity underlies the high heterogeneity of immune reactivity and whether this may influence the pathological consequences of maternal immune activation. Using mouse models of social dominance (Dom) and submissiveness (Sub), which possess innate features of stress resilience and vulnerability, respectively, we identified differential immune reactivity to the synthetic analogue of viral double-stranded RNA, Poly(I:C), in Sub and Dom nulliparous and pregnant females. More specifically, we found that Sub females showed an exacerbated pro- and anti-inflammatory cytokine response to Poly(I:C) as compared with Dom females. Sub offspring born to Sub mothers (stress sensitive offspring) showed enhanced locomotory response to the non-competitive NMDA antagonist, MK-801, which was potentiated by prenatal Poly(I:C) exposure. Our findings suggest that inherited stress sensitivity may lead to functional changes in glutamatergic signaling, which in turn is further exacerbated by prenatal exposure to viral-like infection. The maternal immunome seems to play a crucial role in these observed phenomena.

Multidimensional nature of dominant behavior: Insights from behavioral neuroscience

Social interactions for many species of animals are critical for survival, wellbeing, and reproduction. Optimal navigation of a social system increases chances for survival and reproduction, therefore there is strong incentive to fit into social structures. Social animals rely heavily on dominant-submissive behaviors in establishment of stable social hierarchies. There is a link between extreme manifestation of dominance/submissiveness and behavioral deviations. To understand neural substrates affiliated with a specific hierarchical rank, there is a real need for reliable animal behavioral models. Different paradigms have been consolidated over time to study the neurobiology of social rank behavior in a standardized manner using rodent models to unravel the neural pathways and substrates involved in normal and abnormal intraspecific social interactions. This review summarizes and discusses the commonly used behavioral tests and new directions for the assessment of dominance in rodents. We discuss the hierarchy inheritable nature and other critical issues regarding hierarchical rank manifestation which may help in designing social-rank-related studies that serve as promising pre-clinical tools in behavioral psychiatry.

Rethinking stress resilience

Resilience to stressful life events has received considerable attention in both clinical and preclinical studies. A number of neural substrates have been identified as putatively mediating resilience to stress. However, there remains considerable diversity in how resilience is defined and studied. This article aims to examine how resilience is defined and conceptualized in social psychology, public health, and related fields, to better inform the understanding of stress resilience in the neurobiological context, and to differentiate resilience from other patterns of response to stressful experiences. An understanding of resilience through the lens of clinical and applied sciences is likely to lead to the identification of more robust and reproducible neural substrates, though many challenges remain.

Reproduction-Associated Hormones and Adult Hippocampal Neurogenesis

The levels of reproduction-associated hormones in females, such as estrogen, progesterone, prolactin, and oxytocin, change dramatically during pregnancy and postpartum. Reproduction-associated hormones can affect adult hippocampal neurogenesis (AHN), thereby regulating mothers' behavior after delivery. In this review, we first briefly introduce the overall functional significance of AHN and the methods commonly used to explore this front. Then, we attempt to reconcile the changes of reproduction-associated hormones during pregnancy. We further update the findings on how reproduction-related hormones influence adult hippocampal neurogenesis. This review is aimed at emphasizing a potential role of AHN in reproduction-related brain plasticity and its neurobiological relevance to motherhood behavior.

Gut microbiota determines the social behavior of mice and induces metabolic and inflammatory changes in their adipose tissue

The link between the gut microbiota and social behavior has been demonstrated, however the translational impact of a certain microbiota composition on stable behavioral patterns is yet to be elucidated. Here we employed an established social behavior mouse model of dominance (Dom) or submissiveness (Sub). A comprehensive 16S rRNA gene sequence analysis of Dom and Sub mice revealed a significantly different gut microbiota composition that clearly distinguishes between the two behavioral modes. Sub mice gut microbiota is significantly less diverse than that of Dom mice, and their taxa composition uniquely comprised the genera Mycoplasma and Anaeroplasma of the Tenericutes phylum, in addition to the Rikenellaceae and Clostridiaceae families. Conversely, the gut microbiota of Dom mice includes the genus Prevotella of the Bacteriodetes phylum, significantly less abundant in Sub mice. In addition, Sub mice show lower body weight from the age of 2 weeks and throughout their life span, accompanied with lower epididymis white adipose tissue (eWAT) mass and smaller adipocytes together with substantially elevated expression of inflammation and metabolic-related eWAT adipokines. Finally, fecal microbiota transplantation into germ-free mice show that Sub-transplanted mice acquired Sub microbiota and adopted their behavioral and physiological features, including depressive-like and anti-social behaviors alongside reduced eWAT mass, smaller adipocytes, and a Sub-like eWAT adipokine profile. Our findings demonstrate the critical role of the gut microbiome in determining dominance vs. submissiveness and suggest an association between gut microbiota, the eWAT metabolic and inflammatory profile, and the social behavior mode.

Link between temperament traits, brain neurochemistry and response to SSRI: insights from animal model of social behavior

BACKGROUND

Dominant-submissive relationships depend upon functionality of the neural circuits involving monoaminergic neurotransmission. Behavioral profiles of selectively bred dominant (Dom) and submissive (Sub) mice have been proposed to mimic hyperthymic- or depressive-like temperaments observed in patients with affective disorders. These mice differentially respond to psychotropic agents and stressful stimuli, however, the mechanisms underlying these differences remain unclear. To address these mechanisms, we analyzed the brain monoamine content and responses to paroxetine (PXT) in Dom and Sub mice.

METHODS

The behavioral effects of PXT (3 mg/kg, single injection) were assessed with the Elevated Plus Maze (EPM) and Forced Swim Test (FST). Monoamine tissue content was analyzed by HPLC-ECD.

RESULTS

Compared to Dom, Sub mice had decreased levels of serotonin (5-HT) in the brainstem (BS), reduced levels of norepinephrine (NE) in the prefrontal cortex (PFC), hippocampus (HPC), and striatum (STR) and elevated levels of dopamine (DA) in PFC, HPC, STR and BS. In EPM, PXT administration increased locomotion and exploration in Dom mice, with no effect in Sub mice. In FST, PXT disrupted immobility in Dom mice only. The PXT-produced differences in regional monoamine content were strain-dependent and consistent with the behavioral alterations.

LIMITATIONS

Chronic PXT treatment, in vivo monoamine assays and sex-dependent analysis were out of the scope of this study and will be performed in the future in order to provide an in-depth evaluation of the neurochemical mechanisms underlying temperament-dependent responses to SSRIs.

CONCLUSIONS

Our findings suggest neurochemical mechanisms that underlie temperament-based response to antidepressant treatment.

Animal Models of Depression: What Can They Teach Us about the Human Disease?

Depression is apparently the most common psychiatric disease among the mood disorders affecting about 10% of the adult population. The etiology and pathogenesis of depression are still poorly understood. Hence, as for most human diseases, animal models can help us understand the pathogenesis of depression and, more importantly, may facilitate the search for therapy. In this review we first describe the more common tests used for the evaluation of depressive-like symptoms in rodents. Then we describe different models of depression and discuss their strengths and weaknesses. These models can be divided into several categories: genetic models, models induced by mental acute and chronic stressful situations caused by environmental manipulations (i.e., learned helplessness in rats/mice), models induced by changes in brain neuro-transmitters or by specific brain injuries and models induced by pharmacological tools. In spite of the fact that none of the models completely resembles human depression, most animal models are relevant since they mimic many of the features observed in the human situation and may serve as a powerful tool for the study of the etiology, pathogenesis and treatment of depression, especially since only few patients respond to acute treatment. Relevance increases by the fact that human depression also has different facets and many possible etiologies and therapies.

Link between personality and response to THC exposure

The effects of cannabis reported by users range from experiences of euphoria and anxiolytic effects to paranoia, anxiety, and increased risk of depression. Attempts to reconcile the apparent contradictions in user response have not been conclusive. Here, we utilized selectively-bred stress-resilient socially dominant (Dom) and stress-sensitive socially submissive (Sub) mice to elucidate this contradiction. Following short-term, repeated treatment with delta-9-tetrahydrocannabinol (THC) at two different doses (1.5 mg/kg and 15 mg/kg), Sub mice presented significant place-aversion in a Conditioned Place Preference paradigm at a high dose, whereas Dom mice displayed no place preference or aversion. Forced Swim test conducted after 6-week of washout period, revealed differential impact of the two THC doses depending upon behavioral pattern. Specifically, the low dose alleviated depressive-like behavior in Sub mice, while the high dose produced the opposite effect in Dom mice. Interestingly, corticosterone concentration in serum was elevated at the high dose regardless of the mice-population tested. We conclude here that differences in dominance behavior and stress vulnerability are involved in the regulation of cannabis response among users and should be considered when prescribing THC-containing medications to patients.

Innate sensitivity to stress facilitates inflammation, alters metabolism and shortens lifespan in a mouse model of social hierarchy

It is known that stress alters homeostasis and may lead to accelerated aging. However, little is known about the contribution of innate susceptibility to stress to the deterioration of physiological functions, acceleration of aging and developing of age-related diseases. By using socially-submissive stress susceptible (Sub) and socially-dominant stress resilient (Dom) selectively bred mouse model we observed a marked reduction in the lifespan of both male and female Sub mice. We found that innate susceptibility to stress correlates with chronic inflammation, development of splenomegaly and a significant increase in the levels of circulating pro-inflammatory cytokines IL-1β and IL-6. Furthermore, Sub mice showed a marked hypoglycemia, reduction of insulin levels, increase in GSK3 activity and elevation of IGF-1 serum levels, as well as low skin surface temperature and body weight. Interestingly, lifelong exposure of Sub mice to chronic mild stress did not further reduce their lifespan, indicating a high level of intrinsic stress. Taken together, our data reveal that social submissiveness coupled with innate stress sensitivity coincides with inflammation, leading to the deterioration of physiological functions and early aging independent of whether an individual is exposed to stress or not.

Social defeat-induced Cingulate gyrus immediate-early gene expression and anxiolytic-like effect depend upon social rank

Social hierarchy is considered to impart an adaptive advantage to the species by reducing long-term conflict between conspecifics. While social stratification is frequently established via stress-inducing stimuli, the subsequent integration of individuals into the hierarchy may attenuate anxiety. Presently, we hypothesized that repeated reinforcement of murine social hierarchy in the dominant-submissive relationship (DSR) food-competition test would engender divergent neuroplastic changes mediating both social and anxiety-like behavior among selectively-bred Dominant (Dom) and Submissive (Sub) mice. Two weeks of repeated respective social victory or defeat reduced serum corticosterone levels of both Dom and Sub mice, whereas socially-defeated Sub mice demonstrated markedly greater exploration of the open arms of the elevated plus maze (EPM). At the same time, social victory led to markedly greater expression of the immediate-early genes (IEGs) c-Jun and EGR-1 in the lateral septal nucleus (LSN) among Dom mice, in contrast with defeated Sub counterparts which demonstrated four-fold greater IEG expression in the cingulate gyrus (Cg). These findings point towards involvement of the Cg in the anxiety-like effect among Sub mice after repeated social defeat, and suggest stabilization of the social hierarchy to attenuate the stress-inducing nature of social interaction, particularly for subordinates. Further study of the potentially anxiolytic-like effects of Cg activity should shed light upon the functional significance of the Cg in social interaction, social hierarchical sorting and anxiety.