Elsevier

Brain Research

Volume 1051, Issues 1–2, 27 July 2005, Pages 90-99
Brain Research

Research Report
Expression of c-fos and BDNF mRNA in subregions of the prefrontal cortex of male and female rats after acute uncontrollable stress

https://doi.org/10.1016/j.brainres.2005.05.065Get rights and content

Abstract

Women exhibit higher lifetime prevalences of stress-related disorders than men. These disorders have been associated with changes in prefrontal cortex structure and function. Here, we examine the effects of acute inescapable stress, an animal model of behavioral depression and post-traumatic stress disorder, on plasma corticosterone (CORT) and on c-fos mRNA and brain-derived neurotrophic factor (BDNF) mRNA in regions of the prefrontal and frontal cortex in male and cycling female rats. Inescapable stress consisted of 100 1 mA tailshocks, and no-stress controls remained in their home cages. Rats were sacrificed immediately (0 min) or 60 min after termination of the stressor. CORT levels were increased at both 0 and 60 min post-stress termination relative to controls, and the increase was greater in females at both time points. c-fos mRNA expression increased at 0 min in prefrontal cortical regions, but this increase was greater in males than estrus and proestrus females. At 60 min, c-fos mRNA levels were lower than at 0 min in males but not females. No correlations between CORT and c-fos mRNA levels in prefrontal regions were observed in females in the stress groups, but significant correlations were observed in males in several prefrontal regions. BDNF mRNA expression was greater in control females than control males. Inescapable stress increased BDNF mRNA expression at 0 but not 60 min in males, but there was no effect of inescapable stress on BDNF mRNA in females. These results reveal sex differences in inescapable stress-induced gene expression that may have implications for differences in vulnerability to stress-related disorders.

Introduction

Women exhibit higher lifetime prevalences than men for stress-related disorders such as major depressive disorder (MDD) and post-traumatic stress disorder (PTSD) [4], [5], [6]. The etiology of MDD and PTSD is often argued to involve the experience of stressful events [28], [51], and preclinical studies have revealed sex differences in responses to acute [33] and chronic [2] stress, yet much remains unknown about the neurobiology of the acute response to stress in females relative to males. Because even a single very stressful event can precipitate the onset of these disorders in vulnerable individuals [51], it is of interest to examine sex differences in responding to an acute intense stressor.

The medial prefrontal cortex (mPFC) is known to be highly sensitive to stressors and modulates many behavioral and physiological responses to such events (see [44] for review). Stress can also cause lasting changes in the mPFC, such as decreased dendritic arborization [11]. These changes may affect the ability of the mPFC to modulate subsequent responses to stressors. In support of this idea, stress-related disorders have been associated with differences in both the structure and function of the mPFC [3], [12], [13], [30]. Functional MRI studies show hypofunction of the mPFC in both MDD [12] and PTSD [3] patients, and cell loss in the mPFC has been observed in post-mortem studies of patients with these disorders [13], [30]. It is possible that differential responding of the mPFC during a precipitating stressful event may contribute to the vulnerability of females to stress-related disorders. Ovarian hormones have been shown to impact mPFC function. For example, estrogen exacerbates stress-induced deficits on a PFC-dependent task in rats [31], while progesterone can enhance performance on a PFC-dependent task in humans [38].

In the present study, male and estrous cycling female rats were exposed to a single session of uncontrollable tailshock to examine stress-induced changes in the PFC. This stressor produces long-lasting changes in brain and behavior that closely resemble symptoms of depression [48] and PTSD [18], [43]. Uncontrollable tailshock has been shown to produce sexually dimorphic behavioral deficits [39], [40], and effects of the estrous cycle on vulnerability to the effects of this stressor have also been reported [22].

Females have long been known to exhibit higher stress-induced levels of the adrenal hormone corticosterone (CORT) [24], especially during proestrus [46], but the effect of uncontrollable tailshock on CORT in females is unknown. Thus, we measured plasma levels of CORT to assess hypothalamic–pituitary–adrenal (HPA) axis activation. To assess neuronal activation of the mPFC after uncontrollable stress, we measured mRNA expression of the immediate early gene c-fos. c-fos is a proto-oncogene that is induced in response to diverse stimuli [32] and has been widely used as a marker of neuronal activation resulting from stress exposure [7], [10]. We also assessed mRNA expression of brain-derived neurotrophic factor (BDNF), which is known to be regulated by stress, although this has been best characterized in the hippocampus [37]. BDNF is involved in neuroprotection as well as plasticity and is of special interest because of its proposed role in depression [14], [37]. Although it has been reported that acute stress does not affect BDNF mRNA expression in the mPFC of females [9], it has also been reported that acute stress increases BDNF mRNA expression in the mPFC of males [26]. To our knowledge, a direct comparison between sexes has not been previously reported, and the effects of an intense stressor on BDNF mRNA expression are unknown. The mPFC is a heterogeneous structure, comprising of distinct anatomical and functional subdivisions [41], [42], [45]. Because of this diversity, we examined the anterior cingulate (AC), infralimbic (IL), prelimbic (PL), and ventral orbital (VO) subdivisions of the PFC. We also examined primary motor cortex (M1) as a comparison region for general cortical activity. To measure the expression of c-fos and BDNF mRNA, we used in situ hybridization both immediately and 60 min after termination of the stressor as well as in unstressed (no stress, NS) controls.

Section snippets

Subjects

Adult male (n = 18) and female (n = 43) Harlan Sprague–Dawley rats were used. They were purchased at 75 days old and thus were approximately 90 days old at the onset of vaginal lavage or handling. Rats were housed 2 per cage on a 12 h light–dark cycle (on 07:00 h:off 19:00 h) with food and water freely available. Males and females were housed in separate rooms. Rats were allowed to acclimate to the colony for 2 weeks. After this, daily vaginal lavage (females) or handling (males) occurred

CORT

Inescapable stress produced large increases in plasma CORT at both 0 min and 60 min post-stress termination; these increases were greater in females than in males (Fig. 2). ANOVA revealed a significant sex by stress interaction on plasma CORT [F(2,56) = 3.52, P < .05]. Post hoc Fisher's LSD tests (alpha set at .05) indicated that plasma CORT increased significantly in both male and female rats at 0 min post-stress relative to NS controls, and this increase was greater in females than in males.

Discussion

The present results reveal a pattern of sexually differentiated responses to intense inescapable stress in plasma CORT, as well as in expression of c-fos and BDNF mRNA in regions of the prefrontal cortex immediately or 60 min after termination of the stressor relative to unstressed controls. Primary motor cortex (M1) showed a similar pattern, but the responses were smaller in this region. Inescapable stress resulted in a greater increase in plasma CORT in females than in males. Conversely,

Acknowledgments

We would like to thank Dr. Milena Girotti for excellent technical advice. Supported by NIH grants DA016004 (NRSA post-doc fellowship, S.T.B) and DA13159 (S.F.M).

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