Brainstem hemisection decreases corticotropin-releasing hormone mRNA in the paraventricular nucleus but not in the central amygdaloid nucleus.

Corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) of the hypothalamus and in the central nucleus of the amygdala (ACE) participate in neurohumoral and behavioral responses to stress. To understand better the central regulation of CRH, the present study assessed the effects of ipsilateral surgical hemisection of the brainstem on expression of CRH mRNA in the PVN and the ACE. In situ hybridization was used to demonstrate PVN CRH mRNA expression in hemisected, sham-operated or intact rats before and after 3 h of immobilization (IMMO). In addition, hypothalamic-pituitary-adrenocortical (HPA) axis activity at baseline and during IMMO was assessed by measurements of plasma concentrations of ACTH and corticosterone. IMMO markedly increased CRH mRNA expression in the PVN in all experimental groups. Rats with brainstem hemisections had lower PVN CRH mRNA expression ipsilateral to the lesion and markedly blunted responses after IMMO, compared to values in sham-operated rats. In contrast, neither hemisection nor IMMO affected CRH mRNA expression in the ACE. Lesioned and SHAM-operated groups did not differ in baseline or IMMO-induced increases in plasma ACTH or corticosterone levels. The present results indicate that baseline levels and IMMO-induced increments in CRH mRNA expression in the PVN, but not in the ACE, depend on ipsilaterally ascending medullary tracts and that IMMO-induced HPA activation does not depend on these pathways.