In reaction to acute stress, prepubertal (25–28 days of age) animals demonstrate a prolonged adrenocorticotropic hormone (ACTH) and corticosterone response compared to adults (>65 days of age), while after chronic stress, prepubertal animals show a higher peak ACTH and corticosterone response, but a faster return to baseline compared to adults. Differential activation of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) of prepubertal and adult animals have been suggested to mediate these changes in stress responsiveness. The purpose of the present set of experiments was to further elucidate possible differences in PVN structure and function in prepubertal (28 days of age) and adult (77 days of age) male rats. The results indicate that PVN volume and somal size and cell number are similar in the parvocellular and magnocellular subdivision of the PVN before and after pubertal development. Furthermore, after a peripheral injection of the retrograde tracer Fluoro-Gold (FG), prepubertal and adult males demonstrate similar numbers of anterior pituitary projecting neurosecretory neurons in the parvocellular region of the PVN. Finally, using in situ hybridization we show that in response to acute stress, CRH mRNA in the PVN was affected by both age and stress such that prepubertal males have greater CRH expression than adults and both prepubertal and adult males show significant stress-induced increases in CRH mRNA. Interestingly, in response to repeated restraint, neither age nor stress significantly influence CRH expression. Together, these data indicate that both age and experience with stress interact to modulate CRH expression in the PVN.

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