Chronic Mild Stressors and Diet Affect Gene Expression Differently in Male and Female Rats

Sex differences in the chronic mild stress model of depression

A large volume of clinical and experimental evidence documents sex differences in brain anatomy, chemistry, and function, as well as in stress and drug responses. The chronic mild stress model (CMS) is one of the most extensively investigated animal models of chronic stress. However, only a limited number of studies have been conducted in female rodents despite the markedly higher prevalence of major depression among women. Herein, we review CMS studies conducted in rats and mice of both sexes and further discuss intriguing sex-dependent behavioral and neurobiological findings. The PubMed literature search engine was used to find and collect all relevant articles analyzed in this review. Specifically, a multitermed search was performed with 'chronic mild stress', 'chronic unpredictable stress' and 'chronic variable stress' as base terms and 'sex', 'gender', 'females' and 'depression' as secondary terms in various combinations. Male and female rodents appear to be differentially affected by CMS application, depending on the behavioral, physiological, and neurobiological indices that are being measured. Importantly, the CMS paradigm, despite its limitations, has been successfully used to assess a constellation of interdisciplinary research questions in the sex differences field and has served as a 'silver bullet' in assessing the role of sex in the neurobiology of major depression.

Sex-specific effects of isolation stress and consumption of palatable diet during the prepubertal period on metabolic parameters

OBJECTIVES

Social isolation during the prepubertal period may have long-term effects on metabolism. The exposure to stressful events is associated with increased palatable food intake, constituting reward-based eating. However, palatable food consumption in early life may lead to metabolic alterations later in life. We investigated whether isolation stress during early life can lead to metabolic alterations in male and female rats with or without exposure to a palatable diet.

METHODS

Animals were stressed by isolation during one week after weaning, with or without exposure to a palatable diet.

RESULTS

Stress and palatable diet induced increased caloric consumption. In females, there was a potentiation of consumption in animals exposed to stress and palatable diet, reflected by increased weight gain and triacylglycerol levels in juveniles, as well as increased adiponectin levels. Most of the effects had disappeared in the adults. Different effects were observed in males: in juveniles, stress increased unacylated ghrelin levels, and hypothalamic neuropeptide Y (NPY). Subsequently, adult males that were exposed to a palatable diet during prepuberty showed increased body weight and retroperitoneal fat deposition, increased glycemia, and decreased plasma adiponectin and hypothalamic NPY. Exposure to stress during prepuberty led to increased adrenals during adulthood, decreased LDL-cholesterol and increased triacylglycerol levels.

CONCLUSION

Isolation stress and consumption of palatable diet changes metabolism in a sex-specific manner. Prepuberty female rats were more prone to stress effects on food consumption, while males showed more long-lasting effects, being more susceptible to a metabolic programming after the consumption of a palatable diet.

Peptidergic Edinger-Westphal neurons and the energy-dependent stress response

The continuously changing environment demands for adequate stress responses to maintain the internal dynamic equilibrium of body and mind. A successful stress response requires energy, in an amount matching the severity of the stressor and the type of response ('fight, flight or freeze'). The stress response is generated by the central nervous system, which needs to be informed about both the threatening stressor and the availability of energy. In this review, evidence is considered for a role of the midbrain Edinger-Westphal centrally projecting neuron population (EWcp; synonym: non-preganglionic Edinger-Westphal nucleus) in the energy-dependent stress adaptation response. It deals with studies on the neurochemical organization of the EWcp with particular reference to the neuropeptides urocortin-1 and cocaine- and amphetamine-regulated transcript peptide, on the EWcp responses to different types of stressor (e.g., acute and chronic) and a changed energy state (e.g., fasting and leptin change), and on the sex-specificity of these responses. Finally, a model is presented for the way the EWcp might contribute to the coordination of the energy-dependent stress adaptation response.

Potential Antidepressant Role of Neurotransmitter CART: Implications for Mental Disorders

Depression is one of the most prevalent and debilitating public health concerns. Although no single cause of depression has been identified, it appears that interaction among genetic, epigenetic, biochemical, environmental, and psychosocial factors may explain its etiology. Further, only a fraction of depressed patients show full remission while using current antidepressants. Therefore, identifying common pathways of the disorder and using that knowledge to develop more effective pharmacological treatments are two primary targets of research in this field. Brain-enriched neurotransmitter CART (cocaine- and amphetamine-regulated transcript) has multiple functions related to emotions. It is a potential neurotrophic factor and is involved in the regulation of hypothalamic-pituitary-adrenal axis and stress response as well as in energy homeostasis. CART is also highly expressed in limbic system, which is considered to have an important role in regulating mood. Notably, adolescents carrying a missense mutation in the CART gene exhibit increased depression and anxiety. Hence, CART peptide may be a novel promising antidepressant agent. In this paper, we summarize recent progress in depression and CART. In particular, we emphasize a new antidepressant function for CART.

Stress-related changes in the activity of cocaine- and amphetamine-regulated transcript and nesfatin neurons in the midbrain non-preganglionic Edinger-Westphal nucleus in the rat

Cocaine- and amphetamine-regulated transcript (CART) and nesfatin-1/nucleobindin 2 (NUCB2) are assumed to play a role in feeding and adaptation to stress. Both peptides are highly expressed in the midbrain non-preganglionic Edinger-Westphal nucleus (npEW), a center implicated in the regulation of stress adaptation and in the pathogenesis of stress-induced brain disorders, in a sex-specific manner. The present study was undertaken to test whether CART and nesfatin are involved in these actions of the npEW in the rat. Acute restraint and chronic variable mild stress were used. Following stress, physiological parameters (serum corticosterone levels, body, adrenal and thymus weights) were determined, CART and nesfatin-like immunoreactivity (LI) as well as mRNA expression were analyzed in the npEW nucleus. Our results depict the following changes: (1) Acute stress resulted in an increase in serum corticosterone levels that was higher in females; (2) In males, data on corticosterone and body weight gain and in females, data on body weight gain revealed an effect of chronic stress; (3) Both acute and chronic stress activated npEW neurons expressing CART and nesfatin-LI, as shown by increased cFos immunoreactivity; (4) Chronic, but not acute stress increased the amount of CART and nesfatin-LI in both males and females; (5) Neither acute nor chronic stress had an effect on CART and NUCB2 mRNA contents of npEW neurons in either sex. Taken together, our data suggest that CART and nesfatin are involved in the response of npEW neurons to chronic stress.

Meal patterns and hypothalamic NPY expression during chronic social stress and recovery

In the present study, we examined meal patterns during and after exposure to the visible burrow system (VBS), a rodent model of chronic social stress, to determine how the microstructure of food intake relates to the metabolic consequences of social subordination. Male Long-Evans rats were housed in mixed-sex VBS colonies (4 male, 2 female) for 2 wk, during which time a dominance hierarchy formed [1 dominant male (DOM) and 3 subordinate males (SUB)], and then male rats were individually housed for a 3-wk recovery period. Controls were individually housed with females during the 2-wk VBS period and had no changes in ingestive behavior compared with a habituation period. During the hierarchy-formation phase of VBS housing, DOM and SUB had a reduced meal frequency, whereas SUB also had a reduced meal size. However, during the hierarchy-maintenance phase of VBS housing, DOM meal patterns did not differ from controls, whereas SUB continued to display a reduced food intake via less frequent meals. During recovery, DOM had comparable meal patterns to controls, whereas SUB had an increased meal size. Hypothalamic neuropeptide Y (NPY) mRNA levels were not different between these groups during the experimental period. Together, the results suggest that exposure to chronic social stress alters ingestive behavior both acutely and in the long term, which may influence the metabolic changes that accompany bouts of stress and recovery; however, these differences in meal patterns do not appear to be mediated by hypothalamic NPY.