Sex differences in neurotransmission parameters in response to repeated mild restraint stress exposures in intact male, female and ovariectomised female rats

Acute stress evokes sexually dimorphic, stressor-specific patterns of neural activation across multiple limbic brain regions in adult rats

Stress enhances the risk for psychiatric disorders such as anxiety and depression. Stress responses vary across sex and may underlie the heightened vulnerability to psychopathology in females. Here, we examined the influence of acute immobilization stress (AIS) and a two-day short-term forced swim stress (FS) on neural activation in multiple cortical and subcortical brain regions, implicated as targets of stress and in the regulation of neuroendocrine stress responses, in male and female rats using Fos as a neural activity marker. AIS evoked a sex-dependent pattern of neural activation within the cingulate and infralimbic subdivisions of the medial prefrontal cortex (mPFC), lateral septum (LS), habenula, and hippocampal subfields. The degree of neural activation in the mPFC, LS, and habenula was higher in males. Female rats exhibited reduced Fos positive cell numbers in the dentate gyrus hippocampal subfield, an effect not observed in males. We addressed whether the sexually dimorphic neural activation pattern noted following AIS was also observed with the short-term stress of FS. In the paraventricular nucleus of the hypothalamus and the amygdala, FS similar to AIS resulted in robust increases in neural activation in both sexes. The pattern of neural activation evoked by FS was distinct across sexes, with a heightened neural activation noted in the prelimbic mPFC subdivision and hippocampal subfields in females and differed from the pattern noted with AIS. This indicates that the sex differences in neural activation patterns observed within stress-responsive brain regions are dependent on the nature of stressor experience.

Sex-specific effects of stress on metabolic and cardiovascular disease: are women at higher risk?

Cardiovascular disease (CVD) has traditionally been viewed as a male disease; however, the relative risk for obesity and hypertension morbidity and mortality, major risk factors for CVD, is higher for women in the United States. Emerging epidemiological data strongly support stressful experiences as a modifiable risk factor for obesity, insulin resistance, and heart disease in women at all ages. Therefore, primary prevention of these diseases may be associated with both identifying and increasing the knowledge regarding the sex differences in emotional functioning associated with physiological responses to stress. The purpose of this review is to highlight the growing body of clinical and experimental studies showing that stress, obesity-associated metabolic disturbances, and CVD comorbidities are more prevalent in females. Overall, this review reveals the need for investigations to decipher the early origins of these comorbidities. Targeting the sources of behavioral/emotional stress through the trajectory of life has the potential to reduce the alarming projected rates for chronic disease in women.

CRF2 Receptor Deficiency Eliminates the Long-Lasting Vulnerability of Motivational States Induced by Opiate Withdrawal

Vulnerability to stressful life events is a hallmark of drug dependence that may persist long after cessation of drug intake and dramatically fuel key clinical features, such as deregulated up-shifted motivational states and craving. However, to date, no effective therapy is available for reducing vulnerability to stressful events in former drug users and drug-dependent patients, mostly because of poor knowledge of the mechanisms underlying it. In this study, we report that genetic inactivation of the stress-responsive corticotropin-releasing factor receptor-2 (CRF2-/-) completely eliminates the reemergence of increased nonrewarded nose-pokes, reflecting up-shifted motivational states, triggered by ethological environmental stressors long after cessation of morphine administration in mice. Accordingly, CRF2 receptor deficiency completely abolishes the increase in biomarkers of synthesis of major brain motivational substrates, such as ventral tegmental area (VTA) dopamine (DA) and amygdala γ-aminobutyric acid (GABA) systems, associated with the stress-induced reemergence of up-shifted motivational states long after opiate withdrawal. Nevertheless, neither CRF2 receptor deficiency nor long-term opiate withdrawal affects amygdala CRF or hypothalamus CRF expression, indicating preserved brain stress-coping systems. Moreover, CRF2 receptor deficiency does not influence the locomotor or the anxiety-like effect of long-term opiate withdrawal. Thus, the present results reveal an essential and specific role for the CRF2 receptor in the stress-induced reemergence of up-shifted motivational states and related alterations in brain motivational systems long after opiate withdrawal. These findings suggest new strategies for the treatment of the severe and long-lasting vulnerability that inexorably follows drug withdrawal and hinder drug abstinence.

Habituation of hypothalamic-pituitary-adrenocortical axis hormones to repeated homotypic stress and subsequent heterotypic stressor exposure in male and female rats

Understanding potential sex differences in repeated stress-induced hypothalamic-pituitary-adrenocortical (HPA) axis habituation could provide insight into the sex-biased prevalence of certain affective disorders such as anxiety and depression. Therefore in these studies, male and female rats were exposed to 30 min of either audiogenic or restraint stress daily for 10 days in order to determine whether sex regulates the extent to which HPA axis hormone release is attenuated upon repeated homotypic stressor presentation. In response to the initial exposure, both stressors robustly increased plasma concentrations of both adrenocorticotropic hormone (ACTH) and corticosterone (CORT) in both sexes. Acutely, females displayed higher ACTH and CORT concentrations following restraint stress, whereas males exhibited higher hormone concentrations following loud noise stress. HPA axis hormone responses to both stressors decreased incrementally over successive days of exposure to each respective stressor. Despite the differential effect of sex on acute hormone responses, the extent to which HPA axis hormone response was attenuated did not differ between male and female animals following either stressor. Furthermore, ACTH and CORT responses to a novel environment were not affected by prior exposure to stress of either modality in either male or female rats. These experiments demonstrate that despite the acute stress response, male and female rats exhibit similar habituation of HPA axis hormones upon repeated homotypic stressor presentations, and that exposure to repeated stress does not produce exaggerated HPA axis hormone responses to a novel environment in either female or male rats.

Aquaporin-4 knockout exacerbates corticosterone-induced depression by inhibiting astrocyte function and hippocampal neurogenesis

AIMS

The predominant expression of aquaporin-4 (AQP4) in the brain implies that this water channel may be involved in a range of brain disorders. This study was designed to investigate the role of AQP4 in the pathogenesis of depression, and related possible biological mechanism.

METHODS AND RESULTS

Wild-type (AQP4(+/+) ) and AQP4 knockout (AQP4(-/-) ) mice were given daily subcutaneous injections of corticosterone (20 mg/kg) for consecutive 21 days. Forced swimming test (FST) and tail suspension test (TST) showed longer immobility times in corticosterone-treated AQP4(-/-) genotype, indicating AQP4 knockout exacerbated depressive-like behaviors in mice. Using immunohistological staining, western blot, and enzyme-linked immunosorbent assay (ELISA), we found a significant loss of astrocytes, aggravated downregulation of excitatory amino acid transporter 2 (EAAT2), synapsin-1, and glial cell line-derived neurotrophic factor (GDNF) in the hippocampus of AQP4(-/-) mice. Moreover, even less hippocampal neurogenesis was identified in corticosterone-treated AQP4(-/-) mice in vivo and hippocampus-derived adult neural stem cells (ANSCs) in vitro.

CONCLUSIONS

The present findings suggest AQP4 involves the pathogenesis of depression by modulating astrocytic function and adult neurogenesis, highlighting a novel profile of AQP4 as a potential target for the treatment for depression.

Expression of cFos and brain-derived neurotrophic factor in cortex and hippocampus of ethanol-withdrawn male and female rats

Objective:

To map areas of brain activation (cFos) alongside changes in levels of brain-derived neurotrophic factor (BDNF) to provide insights into neuronal mechanisms contributing to previously observed sex differences in behavioral measures of ethanol withdrawal (EW).

Materials and Methods:

Immunohistochemical analysis of cFos and BDNF levels using protein-specific antibodies and visualization with nickel-enhanced DAB staining in 3 cortical and 4 hippocampal regions was used to assess EW-induced expression of these proteins.

Results:

EW male and female rats showed significantly higher levels of cFos expression compared to controls in the hippocampal regions whereas EW OVX rats showed higher levels compared to controls only at 1 day EW in the dentate gyrus. Males expressed higher basal levels of cFos in the CA1 subfield of the hippocampus and in the motor cortex than either intact or OVX female rats. BDNF immunoreactivity was also significantly higher in EW rats compared to that in controls, varying with sex and brain region at 1 and 3 days EW.

Conclusions:

Sex-and brain region-specific changes in expression of cFos and BDNF occurring during 1 and 3-day EW, suggest that differential activation and expression of neurotrophins may influence the observed sex differences and support the suggestion that EW is a chronic stressor, eliciting sequential neuronal activation and neurotrophin regulation.

The Effects of Sex and Chronic Restraint on Instrumental Learning in Rats

Chronic stress has been shown to impact learning, but studies have been sparse or nonexistent examining sex or task differences. We examined the effects of sex and chronic stress on instrumental learning in adult rats. Rats were tested in an aversive paradigm with or without prior appetitive experience, and daily body weight data was collected as an index of stress. Relative to control animals, reduced body weight was maintained across the stress period for males (−7%, P ≤ .05) and females (−5%, P ≤ .05). For males, there were within-subject day-by-day differences after asymptotic transition, and all restrained males were delayed in reaching asymptotic performance. In contrast, stressed females were facilitated in appetitive and aversive-only instrumental learning but impaired during acquisition of the aversive transfer task. Males were faster than females in reaching the appetitive shaping criterion, but females were more efficient in reaching the appetitive tone-signaled criterion. Finally, an effect of task showed that while females reached aversive shaping criterion at a faster rate when they had prior appetitive learning, they were impaired in tone-signaled avoidance learning only when they had prior appetitive learning. These tasks reveal important nuances on the effect of stress and sex differences on goal-directed behavior.

Sex differences in response to stress and expression of depressive-like behaviours in the rat

Women are more susceptible than men to certain stress-related psychiatric disorders, such as depression. Preclinical studies aim to understand these sex differences by studying male and female rats in stress models. In this chapter, we review sex differences in behavioural aspects, as well as neurochemical and neurobiological findings derived from acute, repeated and chronic stress models. In particular, we focus on sex differences in depressive-like symptomatology expressed in the forced swim test, the chronic mild stress (CMS) and the learned helplessness models, the Flinders Sensitive Line rats (FSL), which is a genetic model of depression and in the lipopolysaccharide (LPS)-induced sickness behaviour, a putative inflammatory model of depression. Also, sex differences in stress effects on learning and memory parameters are discussed, because cognitive alterations are often seen in sex-differentiated psychiatric disorders. The observed behavioural alterations are often linked with abnormalities in the endophenotype, such as in hormonal, neurochemical, immune and neuroplasticity indices. From these data, it is clear that all stress models have strengths and limitations that need to be recognized in order to use them effectively in the investigation of sex differences in affective disorders.

Neuropeptide Y (NPY)-induced reductions in alcohol intake during continuous access and following alcohol deprivation are not altered by restraint stress in alcohol-preferring (P) rats

Administration of neuropeptide Y (NPY) reduces anxiety-like behavior and alcohol intake in alcohol-preferring rats. The present experiment examined whether the effects of NPY on alcohol drinking are modulated by stress exposure during continuous access or following ethanol deprivation. Female P rats underwent 6 weeks of continuous access to 15% v/v ethanol and water prior to intracerebroventricular (ICV) cannula implantation. Deprived rats underwent two cycles of 5 days of ethanol exposure followed by 2 days of ethanol deprivation, while non-deprived rats had uninterrupted access to ethanol. Stressed rats in both ethanol access groups were exposed to restraint stress for 1h 4-6h after ethanol was removed from the deprived group in both cycles. ICV infusions of 5.0 μg NPY or aCSF were administered 48 h following the deprivation/stress procedure, after which ethanol was returned. Rats showed increased ethanol intake following ethanol deprivation compared to non-deprived controls. Food and water intake were increased, while ethanol intake was decreased, in rats infused with NPY. Stress did not increase ethanol intake or alter the response to NPY. Although no stress effects were found, the present experiment replicates previous findings regarding the effectiveness of NPY in reducing ethanol consumption. Future studies aimed at determining the extent to which stress may affect relapse to ethanol drinking and response to NPY would benefit from implementing different stress paradigms and varying the pattern of ethanol access.

Neurochemical and behavioral alterations in an inflammatory model of depression: sex differences exposed

It is firmly established that women experience major depression (MD) at roughly twice the rate of men and that dysregulation of the immune system is associated with the appearance and course of this condition. In the present study, we sought to identify whether "sickness behavior", an inflammatory model of MD, is characterized by sexual dimorphism by focusing on both neurochemical and behavioral responses. Therefore, we investigated the serotonergic and dopaminergic activity of various brain regions implicated in the pathophysiology of affective disorders (hypothalamus, hippocampus, prefrontal cortex, amygdala and striatum) in response to a mild lipopolysaccharide (LPS) challenge, in rats of both sexes. According to our results, at 2 h post-LPS administration (100 microg/kg i.p.), the neurochemical substrate was primarily altered in female rats with the serotonergic function being markedly enhanced in all brain regions examined. Dopaminergic activation following immune system sensitization with LPS was not apparent in male rats and only modest in female rats with the exception of striatum. LPS administration also affected sickness-associated behaviors to a different extent in male and female rats, as assessed in the forced swim test (FST), the hot plate test (HPT) and the open-field arena. LPS-treated female rats coped better with the stressful FST procedure, as evidenced by an increase in swimming duration. The effects of LPS treatment appeared to be more robust in male rats, as far as suppression of locomotor activity is concerned, while the antinociceptive properties of LPS were evident in both sexes though showing sex-dependent kinetics. Moreover, when traditional measures of sickness (i.e. sucrose consumption, social exploration, food intake) were assessed, males and females appeared to be similarly affected, except for food intake. These data are the first to demonstrate that the serotonergic system is affected to a greater extent in female rats at 2 h post-LPS administration and further contribute to our understanding regarding sexual dimorphism upon sickness establishment.