Constance S. Harrell

Stress-induced sex differences: Adaptations mediated by the glucocorticoid receptor

Clinical evidence has indicated that women are more susceptible to stress-related and autoimmune disorders than men. Although females may be more susceptible to some disease states, males do not escape unscathed and are more susceptible to metabolic dysfunction. The hypothalamic-pituitary-axis plays a pivotal role in the sexually dimorphic effects of chronic stress through alterations in negative feedback. Recent evidence has implicated the glucocorticoid receptor and its co-chaperones in the etiology of psychiatric and somatic diseases. Gonadal hormones heavily interact with both glucocorticoid receptor expression and glucocorticoid receptor action either through direct or indirect effects on proteins in the chaperone and co-chaperone complex. Diverse systems including the hypothalamic-pituitary-axis, the immune system, and metabolism are affected differently in males and females, possibly through the glucocorticoid receptor system. New considerations of glucocorticoid regulation through the co-chaperone complex in the brain will be vital to the development of treatment strategies for men and women afflicted by neuropsychiatric and somatic disorders.

Sex differences in the effects of adolescent stress on adult brain inflammatory markers in rats.

Both basic and clinical research indicates that females are more susceptible to stress-related affective disorders than males. One of the mechanisms by which stress induces depression is via inflammatory signaling in the brain. Stress during adolescence, in particular, can also disrupt the activation and continued development of both the hypothalamic-pituitary-adrenal (HPA) and -gonadal (HPG) axes, both of which modulate inflammatory pathways and brain regions involved in affective behavior. Therefore, we tested the hypothesis that adolescent stress differentially alters brain inflammatory mechanisms associated with affective-like behavior into adulthood based on sex. Male and female Wistar rats underwent mixed-modality stress during adolescence (PND 37-48) and were challenged with lipopolysaccharide (LPS; 250μg/kg, i.p.) or saline 4.5weeks later (in adulthood). Hippocampal inflammatory marker gene expression and circulating HPA and HPG axes hormone concentrations were then determined. Despite previous studies indicating that adolescent stress induces affective-like behaviors in female rats only, this study demonstrated that adolescent stress increased hippocampal inflammatory responses to LPS in males only, suggesting that differences in neuroinflammatory signaling do not drive the divergent affective-like behaviors. The sex differences in inflammatory markers were not associated with differences in corticosterone. In females that experienced adolescent stress, LPS increased circulating estradiol. Estradiol positively correlated with hippocampal microglial gene expression in control female rats, whereas adolescent stress negated this relationship. Thus, estradiol in females may potentially protect against stress-induced increases in neuroinflammation.

Estradiol activates epithelial sodium channels in rat alveolar cells through the G protein-coupled estrogen receptor

Female sex predisposes individuals to poorer outcomes during respiratory disorders like cystic fibrosis and influenza-associated pneumonia. A common link between these disorders is dysregulation of alveolar fluid clearance via disruption of epithelial sodium channel (ENaC) activity. Recent evidence suggests that female sex hormones directly regulate expression and activity of alveolar ENaC. In our study, we identified the mechanism by which estradiol (E2) or progesterone (P4) independently regulates alveolar ENaC. Using cell-attached patch clamp, we measured ENaC single-channel activity in a rat alveolar cell line (L2) in response to overnight exposure to either E2 or P4. In contrast to P4, E2 increased ENaC channel activity (NPo) through an increase in channel open probability (Po) and an increased number of patches with observable channel activity. Apical plasma membrane abundance of the ENaC α-subunit (αENaC) more than doubled in response to E2 as determined by cell surface biotinylation. αENaC membrane abundance was approximately threefold greater in lungs from female rats in proestrus, when serum E2 is greatest, compared with diestrus, when it is lowest. Our results also revealed a significant role for the G protein-coupled estrogen receptor (Gper) to mediate E2s effects on ENaC. Overall, our results demonstrate that E2 signaling through Gper selectively activates alveolar ENaC through an effect on channel gating and channel density, the latter via greater trafficking of channels to the plasma membrane. The results presented herein implicate E2-mediated regulation of alveolar sodium channels in the sex differences observed in the pathogenesis of several pulmonary diseases.

Two weeks of predatory stress induces anxiety-like behavior with co-morbid depressive-like behavior in adult male mice

Psychological stress can have devastating and lasting effects on a variety of behaviors, especially those associated with mental illnesses such as anxiety and depression. Animal models of chronic stress are frequently used to elucidate the mechanisms underlying the relationship between stress and mental health disorders and to develop improved treatment options. The current study expands upon a novel chronic stress paradigm for mice: predatory stress. The predatory stress model incorporates the natural predator-prey relationship that exists among rats and mice and allows for greater interaction between the animals, in turn increasing the extent of the stressful experience. In this study, we evaluated the behavioral effects of exposure to 15 days of predatory stress on an array of behavioral indices. Up to 2 weeks after the end of stress, adult male mice showed an increase of anxiety-like behaviors as measured by the open field and social interaction tests. Animals also expressed an increase in depressive-like behavior in the sucrose preference test. Notably, performance on the novel object recognition task, a memory test, improved after predatory stress. Taken as a whole, our results indicate that 15 exposures to this innovative predatory stress paradigm are sufficient to elicit robust anxiety-like behaviors with evidence of co-morbid depressive-like behavior, as well as changes in cognitive behavior in male mice.

Meloxicam Blocks Neuroinflammation, but Not Depressive-Like Behaviors, in HIV-1 Transgenic Female Rats

Adolescents living with human immunodeficiency virus (HIV) comprise approximately 12% of the HIV-positive population worldwide. HIV-positive adolescents experience a higher rate of clinical depression, a greater risk of sexual and drug abuse behaviors, and a decreased adherence to highly active antiretroviral therapies (HAART). Using adolescent HIV-1 transgenic rats (HIV-1 tg) that display related immune response alterations and pathologies, this study tested the hypothesis that developmental expression of HIV-1-related proteins induces a depressive-like phenotype that parallels a decrease in hippocampal cell proliferation and an increase in pro-inflammatory cytokine expression in the hippocampus. Consistent with this hypothesis, adolescent HIV-1 tg rats demonstrated a depressive-like behavioral phenotype, had decreased levels of cell proliferation, and exhibited elevated expression of monocyte chemotactic protein-1 (Mcp-1) in the hippocampus relative to controls. Subsequently, we tested the ability of meloxicam, a selective COX-2 inhibitor, to attenuate behavioral deficits via inflammatory mechanisms. Daily meloxicam treatments did not alter the behavioral profile despite effectively reducing hippocampal inflammatory gene expression. Together, these data support a biological basis for the co-morbid manifestation of depression in HIV-positive patients as early as in adolescence and suggest that modifications in behavior manifest independent of inflammatory activity in the hippocampus.

Chronic stress modulates regional cerebral glucose transporter expression in an age-specific and sexually-dimorphic manner

Facilitative glucose transporters (GLUT) mediate glucose uptake across the blood-brain-barrier into neurons and glia. Deficits in specific cerebral GLUT isoforms are linked to developmental and neurological dysfunction, but less is known about the range of variation in cerebral GLUT expression in normal conditions and the effects of environmental influences on cerebral GLUT expression. Knowing that puberty is a time of increased cerebral plasticity, metabolic demand, and shifts in hormonal balance for males and females, we first assessed gene expression of five GLUT subtypes in four brain regions in male and female adolescent and adult Wistar rats. The data indicated that sex differences in GLUT expression were most profound in the hypothalamus, and the transition from adolescence to adulthood had the most profound effect on GLUT expression in the hippocampus. Next, given the substantial energetic demands during adolescence and prior demonstrations of the adverse effects of adolescent stress, we determined the extent to which chronic stress altered GLUT expression in males and females in both adolescence and adulthood. Chronic stress significantly altered cerebral GLUT expression in males and females throughout both developmental stages but in a sexually dimorphic and brain region-specific manner. Collectively, our data demonstrate that cerebral GLUTs are expressed differentially based on brain region, sex, age, and stress exposure. These results suggest that developmental and environmental factors influence GLUT expression in multiple brain regions. Given the importance of appropriate metabolic balance within the brain, further assessment of the functional implications of life stage and environmentally-induced changes in GLUTs are warranted.

High-fructose diet during periadolescent development increases depressive-like behavior and remodels the hypothalamic transcriptome in male rats

Fructose consumption, which promotes insulin resistance, hypertension, and dyslipidemia, has increased by over 25% since the 1970s. In addition to metabolic dysregulation, fructose ingestion stimulates the hypothalamic-pituitary-adrenal (HPA) axis leading to elevations in glucocorticoids. Adolescents are the greatest consumers of fructose, and adolescence is a critical period for maturation of the HPA axis. Repeated consumption of high levels of fructose during adolescence has the potential to promote long-term dysregulation of the stress response. Therefore, we determined the extent to which consumption of a diet high in fructose affected behavior, serum corticosterone, and hypothalamic gene expression using a whole-transcriptomics approach. In addition, we examined the potential of a high-fructose diet to interact with exposure to chronic adolescent stress. Male Wistar rats fed the periadolescent high-fructose diet showed increased anxiety-like behavior in the elevated plus maze and depressive-like behavior in the forced swim test in adulthood, irrespective of stress history. Periadolescent fructose-fed rats also exhibited elevated basal corticosterone concentrations relative to their chow-fed peers. These behavioral and hormonal responses to the high-fructose diet did not occur in rats fed fructose during adulthood only. Finally, rats fed the high-fructose diet throughout development underwent marked hypothalamic transcript expression remodeling, with 966 genes (5.6%) significantly altered and a pronounced enrichment of significantly altered transcripts in several pathways relating to regulation of the HPA axis. Collectively, the data presented herein indicate that diet, specifically one high in fructose, has the potential to alter behavior, HPA axis function, and the hypothalamic transcriptome in male rats.

Translational reciprocity: Bridging the gap between preclinical studies and clinical treatment of stress effects on the adolescent brain

The genetic, biological, and environmental backgrounds of an organism fundamentally influence the balance between risk and resilience to stress. Sex, age, and environment transact with responses to trauma in ways that can mitigate or exacerbate the likelihood that post-traumatic stress disorder will develop. Translational approaches to modeling affective disorders in animals will ultimately provide novel treatments and a better understanding of the neurobiological underpinnings behind these debilitating disorders. The extant literature on trauma/stress has focused predominately on limbic and cortical structures that innervate the hypothalamic-pituitary-adrenal axis and influence glucocorticoid-mediated negative feedback. It is through these neuroendocrine pathways that a self-perpetuating fear memory can propagate the long-term effects of early life trauma. Recent work incorporating translational approaches has provided novel pathways that can be influenced by early life stress, such as the glucocorticoid receptor chaperones, including FKBP51. Animal models of stress have differing effects on behavior and endocrine pathways; however, complete models replicating clinical characteristics of risk and resilience have not been rigorously studied. This review discusses a four-factor model that considers the importance of studying both risk and resilience in understanding the developmental response to trauma/stress. Consideration of the multifactorial nature of clinical populations in the design of preclinical models and the application of preclinical findings to clinical treatment approaches comprise the core of translational reciprocity, which is discussed in the context of the four-factor model.

Not all depression is created equal: sex interacts with disease to precipitate depression

Depression is a common mental disorder that co-occurs in other neurological and somatic diseases. Further, sex differences exist in the prevalence rates of many of these diseases, as well as within non-disease associated depression. In this review, the case is made for needing a better recognition of the source of the symptoms of depression with respect to the sex of the individual; in that, some disease states, which includes the neuroendocrine and immune reactions to the underlying pathophysiology of the disease, may initiate depressive symptoms more often in one sex over the other. The diseases specifically addressed to make this argument are: epilepsy, Alzheimer’s disease, cancer, and cardiovascular disease. For each of these conditions, a review of the following are presented: prevalence rates of the conditions within each sex, prevalence rates of depressive symptoms within the conditions, identified relationships to gonadal hormones, and possible interactions between gonadal hormones, adrenal hormones, and immune signaling. Conclusions are drawn suggesting that an evaluation of the root causes for depressive symptoms in patients with these conditions is necessary, as the underlying mechanisms for eliciting the depressive symptoms may be qualitatively different across the four diseases discussed. This review attempts to identify and understand the mechanisms of depression associated with these diseases, in the context of the known sex differences in the disease prevalence and its age of onset. Hence, more extensive, sex-specific model systems are warranted that utilize these disease states to elicit depressive symptoms in order to create more focused, efficient, and sex-specific treatments for patients suffering from these diseases and concurrent depressive symptoms.

Sex and lineage interact to predict behavioral effects of chronic adolescent stress in rats

Neuropsychiatric disorders often derive from environmental influences that occur at important stages of development and interact with genetics. This study examined the effects of stress during adolescence in rats selectively bred for different behavioral responses to stress. The effects of chronic adolescent stress were compared between rats selected for susceptibility to reduced activity following acute stress (Swim-test Susceptible rats) and rats resistant to activity reduction after acute stress (Swim-test Resistant rats). Consistent with lineage, exposure to chronic adolescent stress increased swim-test activity of the Swim-test Resistant rats while tending to reduce activity of the Swim-test Susceptible rats. Consistent with the increased activity demonstrated post-stress in the swim test, chronic adolescent stress increased total activity in the open field for Swim-test Resistant rats. Indicative of anhedonia, chronic adolescent stress exposure decreased sucrose consumption in both male and female Swim-test Resistant rats but only in female Swim-test Susceptible rats. Although chronic stress induced changes in behavior across both breeding lines, the precise manifestation of the behavioral change was dependent on both breeding line and sex. Collectively, these data indicate that selective breeding interacts with chronic stress exposure during adolescence to dictate behavioral outcomes.