Differential expression of c-fos mRNA within neurocircuits of male hamsters exposed to acute or chronic defeat

Chronic exposure to stress has been implicated in physical and mental illne ss, and such experiences can produce alterations in the connectivity and nu mber of neurones within the brain to variations in the expression of specif ic genes. The purpose of this study was to determine how repeated exposure to social defeat affects neuronal activation patterns within the male Syria n hamster brain. Toward this end, the levels c-fos mRNA were compared among three groups: (1) handled controls (HC); (2) acutely defeated males (AD); and (3) chronically defeated males (15 min aggression daily, 7 days) expose d to an acute challenge (CD), Plasma glucocorticoids were also measured and compared among groups as an index of neuroendocrine activity, The results show a selective pattern of habituation of immediate early gene expression within the brains of chronically defeated males, In particular, c-fos mRNA levels were significantly decreased within the paraventricular nucleus of t he hypothalamus (PVN), supraoptic nucleus of the hypothalamus, septohypotha lamic nucleus, intermediate subdivision of the lateral septum, central amyg daloid nucleus, and the amygdalohippocampal area in the CD group exposed to an acute challenge when compared to males defeated only once, In contrast, c-fos expression within the anterior and ventromedial nuclei of the hypoth alamus, dorsal periaqueductal grey, dorsal raphe, cuneiform nucleus, and lo cus coeruleus did not differ between AD and CD groups, Similarly, plasma le vels of cortisol and corticosterone in CD group were equivalent to those ob served after a single defeat experience, We discuss the possibility that de creased expression of c-fos mRNA within the PVN and other brain regions of defeated animals-in the presence of elevated adrenal steroids-may reflect a state of molecular plasticity that could alter neurotransmission within th e limbic-hypothalamo-pituitary-adrenocortical axis. In contrast, brain area s that maintain relatively high levels of c-fos mRNA following repeated def eat may reflect processes less likely to adapt such as defensive behaviour.