Chronic stress produces enduring decreases in novel stress-evoked c-fos mRNA expression in discrete brain regions of the rat.

Chronic stress produces numerous adaptations within the hypothalamic-pituitary-adrenal (HPA) axis that persist well after cessation of chronic stress. We previously demonstrated profound attenuation of HPA axis responses to novel environment 4-7 days following chronic stress. The present study tests the hypothesis that this HPA axis hyporesponsivity is associated with reductions in stress-evoked c-fos mRNA expression, a marker of neuronal activation, in discrete brain regions. Adult male Sprague-Dawley rats underwent 1 week of chronic variable stress (CVS), with unhandled rats serving as controls. Independent groups of control and CVS rats were exposed to novel environment at 16 h, 4 days, 7 days, or 30 days after CVS. Marked reductions of c-fos mRNA expression in the CVS group persisted for at least 30 days within the paraventricular nucleus of the hypothalamus, and for at least 1 week in rostroventrolateral septum and lateral hypothalamus. Lower levels of c-fos mRNA expression were observed at 16 h recovery in the ventrolateral medial preoptic area, basolateral amygdala, anterior cingulate cortex, and prelimbic cortex. The results demonstrate long-term alterations in neuronal activation within neurocircuits critical for regulation of physiological and psychological responses to stressors.