Regulation of corticotropin-releasing hormone and pituitary-adrenocortical response during acute and repeated stress in the rat.

The hormonal response of the pituitary-adrenocortical system to acute or repeated immobilization (IMO) stress was studied in rats. Plasma ACTH response was markedly reduced in repeatedly stressed rats compared to acutely stressed ones, but no differences in plasma corticosterone responses were seen. Acute but not repeated IMO caused a marked reduction of the ACTH content in the anterior pituitary, but no changes of the ACTH content were found in the intermediate-posterior pituitary or in the median eminence (ME) in animals exposed to either acute or repeated stress. Acute IMO elicited a marked reduction of the content of corticotropin releasing hormone (CRF) in the ME. The basal concentration of CRF in the ME of repeatedly stressed rats did not differ from that in intact controls; subsequent exposure of repeatedly stressed rats to IMO caused an insignificant reduction of the CRF content in the ME. In response to peripheral injections of CRF (50 and 500 ng per rat) a dose dependent increase of plasma ACTH was observed in both control and repeatedly stressed rats, however, no differences in ACTH responses to CRF were seen between these two groups. Plasma corticosterone response to the high dose of peripherally administered CRF did not differ between control and repeatedly stressed rats, while plasma corticosterone response to the low dose of CRF was greater in repeatedly stressed rats than in controls.(ABSTRACT TRUNCATED AT 250 WORDS)