Christel Westenbroek

Sex differences in stress responses: Focus on ovarian hormones

Women in the reproductive age are more vulnerable to develop affective disorders than men. This difference may attribute to anatomical differences, hormonal influences and environmental factors such as stress. However, the higher prevalence in women normalizes once menopause is established, suggesting that ovarian hormones may play an important role in the development of depression in women. Ovarian hormones such as estrogen can pass the brain-blood barrier and bind to cytoplasmatic estrogen receptor (ER)-alpha and ER-beta in different areas of the limbic system. During stress, estrogen can modulate the behavioral and neurobiological response depending on the concentrations of estrogen. In this review we present evidence for disparate effects of chronic stress on neuroplasticity and brain activity in male and female rats. Furthermore, we will demonstrate that effects of social support on coping with stress can be mimicked by social housing of rats and that this model can be used for identification of underlying neurobiological mechanisms, including behavior, phosphorylation of CREB and ERK1/2, and brain activity changes as measured with fos expression. Using cyclic administration of estrogen in ovariectomized female rats we could specifically address effects of different plasma estrogen levels and antidepressants on stress-induced neuroplasticity and activity changes. In this model we also studied effects of estrogen on recovery after chronic stress. We conclude that the female brain has a different innate strategy to handle stress than the male brain and that female animal models are necessary for studying the underlying mechanisms and options for treatment.

Sex Differences in the Effects of Acute and Chronic Stress and Recovery after Long-Term Stress on Stress-Related Brain Regions of Rats

Studies show that sex plays a role in stress-related depression, with women experiencing a higher vulnerability to its effect. Two major targets of antidepressants are brain-derived neurotrophic factor (BDNF) and cyclic adenosine monophosphate response element–binding protein (CREB). The aim of this study was to investigate the levels of CREB, phosphorylation of CREB (pCREB), and BDNF in stress-related brain regions of male and female rats after stress and recovery. CREB and pCREB levels were examined in CA1, CA2, CA3, paraventricular nucleus of the thalamus (PVT), amygdala, anterior cingulate area, dorsal part (ACAd), and infralimbic area of prefrontal cortex (PFC), whereas dentate gyrus (DG) and prelimbic area (PL) of PFC were examined for BDNF levels. Our results demonstrate that levels of CREB and pCREB in male CA1, CA2 and CA3, PVT, amygdala, and ACAd were reduced by stress, whereas the same brain regions of female rats exhibited no change. BDNF levels were decreased by chronic stress in female PL but were increased by acute stress in female DG. BDNF levels in male DG and PL were found not to undergo change in response to stress. Abnormalities in morphology occurred after chronic stress in males but not in females. In all cases, the levels of CREB, pCREB, and BDNF in recovery animals were comparable to the levels of these proteins in control animals. These findings demonstrate a sexual dimorphism in the molecular response to stress and suggest that these differences may have important implications for potential therapeutic treatment of depression.

Unique patterns of FOS, phospho-CREB and BrdU immunoreactivity in the female rat brain following chronic stress and citalopram treatment

Affective disorders are common psychiatric illnesses characterized by marked gender-related prevalence. Recent evidence links chronic stress and dysregulation of neurotrophin signaling with the development of depression, while novel theories suggest that antidepressants may act by promoting intracellular adaptations linked to neuroplasticity. Although selective serotonin reuptake inhibitors (SSRIs) efficaciously improve a variety of dysfunctions in males, their neuroendocrine effects and intracellular signaling patterns in females are not well determined. Here we show that chronic footshock stress (21 days) promotes HPA axis hyperactivity (as seen by the increased FOS-ir in the paraventricular hypothalamic nucleus (PVN), plasma corticosterone and adrenal hypertrophy), reduces hippocampal BrdU immunoreactivity and suppresses cortical-limbic CREB phosphorylation in female rats. Long-term citalopram treatment, in contrast, attenuates stress-induced elevation of corticosterone levels and adrenal hypertrophy, although it does not reverse footshock-mediated induction of FOS-ir in the PVN, inhibition of CREB phosphorylation and reduction of hippocampal BrdU-labeling. Moreover, citalopram administration was also associated with significant hypophagic effects and inhibition of CREB phosphorylation. These data suggest that, in female rats, normalization of chronic stress-induced HPA axis abnormalities may represent an initial phase of citalopram-mediated therapeutic actions and despite this SSRIs apparent lack of effects on neuroplasticity, we cannot exclude the possibility that some neurochemical adaptations occur in a later stage which may require more than 3 weeks of treatment to manifest.

Gender-specific effects of social housing on chronic stress-induced limbic Fos expression.

Stress plays an important role in the development of affective disorders. Women show a higher prevalence for these disorders then men. The course of a depressive episode is thought to be positively influenced by social support. We have used a chronic mild stress model in which rats received footshocks daily for 3 weeks. Since rats are social animals we hypothesised that social housing, as a possible model for human social support, might reduce the adverse effects of chronic stress. Brain activity after chronic stress was measured in several limbic brain areas with the neuronal activation marker c-fos. High behavioural activity due to housing rats under reversed light-dark conditions could be responsible for the observed high within group variability in some limbic regions. FOS- (ir) in the paraventricular nucleus of the hypothalamus (PVN) was increased in all stress-exposed groups, except for the socially housed females who showed increased FOS-ir in control condition. Individually housed males and socially housed females showed increased FOS-ir in the dorsal raphe (DRN). Amygdala nuclei were differentially affected by stress, gender and housing conditions. Also the mesolimbic dopaminergic system showed gender specific responses to stress and housing conditions. These results indicate that social support can enhance stress coping in female rats, whereas in males rats, group housing appears to increase the adverse effects of chronic stress, although the neurobiological mechanism is not simply a reduction or enhancement of stress-induced brain activation.

Effects of Long-Term Stress and Recovery on the Prefrontal Cortex and Dentate Gyrus in Male and Female Rats

Women show a higher prevalence for depression than men. However, the biological basis of gender differences in stress response and recovery still remain poorly understood. Therefore, the aim of the study was to assess the gender differences in response to acute stress, chronic stress and recovery in rats. Our results showed that stress decreased male body weight but had no effect on female rats. Open field test demonstrated behavioral changes in grooming and velocity after chronic stress and recovery. Increased activity of hypothalamic-pituitary-adrenocortical axis was reflected by adrenal hypertrophy and increase of plasma corticosterone levels except in the socially housed female rats after stress. Gender and brain region differences in response to stress and recovery were found in the expression of cAMP response element-binding protein (CREB) and phosphorylated CREB (pCREB). On the whole, expression of CREB and pCREB in male dentate gyrus (DG) and prefrontal cortex (PFC) was sensitive but in female DG and PFC it was resistant to acute and chronic stress. Interestingly, recovery restored the measured parameters to the normal level in male rats but not in female rats. In conclusion, these results suggest that male and female rats responded to stress and recovery in a different way.

Increased stress vulnerability after a prefrontal cortex lesion in female rats

Neuroimaging studies in patients suffering from affective disorders have shown decreased volume and reduced regional cerebral blood flow in multiple areas of the prefrontal cortex, including the medial prefrontal cortex and the orbitofrontal cortex. This aberrant brain activity is among other things attributed to chronic stress. Affective disorders occur more often in women than in men. In the current experiment, female mPFC-lesioned and non-lesioned rats were subjected to 3 weeks of chronic unpredictable stress in order to determine the role of the mPFC in dealing with chronic stress, and the consequences of mPFC damage for coping with consecutive stressful events. mPFC damage in female rats intensified the stress-induced activation of the dorsomedial nucleus of the hypothalamus and the paraventricular nucleus of the hypothalamus as measured with Fos expression changes and markedly increased plasma catecholamine levels after 3 weeks of unpredictable stress. Additionally, an mPFC lesion significantly reduced the time of appearance of stress-induced behavioral changes in the open field. Altogether, mPFC dysfunction affects the way female rats react to chronic stress, it not only increased the activation of brain regions involved in neuroendocrine and autonomic responses to stress but it also significantly reduced the time of onset of behavioral changes.

Increased limbic phosphorylated extracellular-regulated kinase 1 and 2 expression after chronic stress is reduced by cyclic 17β-estradiol administration

Chronic stress induced neuronal changes that may have consequences for subsequent stress responses. For example, chronic stress in rats rearranges dendritic branching patterns and disturbs the phosphorylation of extracellular-regulated kinase 1 and 2 (ERK) 1/2 throughout the limbic system. Stress-induced psychopathology occurs more often in women, however, most of studies have been done in male rats. Therefore, we studied the effect of stress in female rats. Other studies show that estradiol can modulate neuronal plasticity and might protect against stress-induced aberrations. To investigate the role of estradiol in stress responses we manipulated the hormone levels. Ovariectomized rats were cyclically treated with vehicle or with 17beta-estradiol-benzoate (1x in 4 days, 10 microg/250 g, s.c.) and subjected to either acute (3 days) or chronic (22 days) stress. In ovariectomized rats, the number of c-Fos positive cells in the infralimbic and prelimbic cortex of the prefrontal cortex and in the medial and basolateral amygdala was increased after acute stress. Moreover, acute stress reduced the number of phosphorylated ERK1/2 positive neurons in the prefrontal cortex of ovariectomized rats. Chronic stress, on the other hand, abolished normal patterns of c-Fos immunoreactivity in the prefrontal cortex and amygdala and increased the prefrontocortical phosphorylation of ERK1/2 in ovariectomized rats. Cyclic estradiol treatment preserved the neuronal reactivity in the infralimbic cortex after chronic stress and prevented sustained accumulation of phosphorylated ERK1/2. Therefore, cyclic estradiol administration apparently preserves the integrity of signal transduction cascades in limbic structures, which may protect against the harmful consequences of recurrent stress.

Stressverwerking door het mannelijke en het vrouwelijke brein

Vrouwen lijden vaker aan angststoornissen en depressie dan mannen. Zijn vrouwen stressgevoeliger, of zijn er verschillen in de wijze waarop mannen en vrouwen omgaan met chronische stress? De anatomie en functie van het mannelijke en vrouwelijke brein zijn verschillend en dierexperimenteel onderzoek laat zien dat er principiële verschillen zijn in de wijze waarop het mannelijke en vrouwelijke brein omgaan met stress. Huisvesting blijkt voor proefdieren een belangrijke omgevingsfactor voor het omgaan met stress en staat model voor sociale steun, een factor die vooral bij vrouwen een positieve invloed heeft op het voorkomen van een terugkeer van affectieve stoornissen. Wij gaan in op de neurobiologie van sociale steun en beschrijven geslachtsverschillen in intracellulaire signaaltransductiecascades die betrokken zijn bij de handhaving van de plasticiteit van het brein.