Activation of the hypothalamic-pituitary axis in adrenalectomised rats: potentiation by chronic stress

The influence of chronic stress on the status of the hypothalamo-pituitary-adrenal (HPA) axis of sham-operated and adrenalectomised rats was assessed. Animals underwent bilateral adrenalectomy (ADX) and 3 days later they were either left undisturbed or subjected daily to immobilization for 2 h each morning for 14 days (chronic IMO). In situ hybridization histochemistry revealed that ADX increased corticotropin-releasing factor (CRF) mRNA levels in the paraventricular nucleus of the hypothalamus (PVN) and proopiomelanocortin (POMC) mRNA levels in the anterior pituitary, in both control and chronically stressed rats as measured on the day following the last exposure to stress. Chronic IMO increased CRF mRNA levels in the PVN and POMC mRNA levels in the anterior pituitary of sham-operated rats, as measured on the day following the last exposure to stress. Chronic IMO potentiated the increase in CRF mRNA in the PVN following ADX and resulted in further increases in CRF mRNA above levels seen in adrenal-intact animals. Finally, chronic stress, while not altering basal ACTH levels of ADX rats, reduced the ACTH response of these animals to a novel stressor (tail-shock for 30 min). These results suggest that chronic stress exerts a stimulatory influence at the hypothalamic level that is partially restrained by daily stress-induced glucocorticoid release. Despite the potentiation by chronic stress of CRF mRNA content in the PVN of ADX rats, a blunted circulating ACTH response to an acute short-term stressor was apparent in ADX-chronically stressed rats, suggesting that chronic stress might also alter POMC processing and/or ACTH secretory patterns in the anterior pituitary in ADX animals.