Pituitary-Adrenal Response in Neonatal Rats*

Abstract

Neonatal rats were, until recently, believed to be relatively nonresponsive to stress. With improved methods now available, rat pups as young as 1–2 days of age have been found to have basal and stimulated plasma corticosterone (B) values approaching those of adults but have very low values at 4–11 days which increase thereafter and reach adult values at about 21 days of age. We examined the responsiveness of neonatal rats to the administration of a crude preparation of corticotropinreleasing factor (CRF) and ACTH to determine the locus of the variation with age of the response. Plasma B increased in 1-dayold rats after ACTH or CRF. In 7-day-old rats, neither caused increases in plasma B. Some response after these stimuli was elicited in 14-day-old rats, and the responses of 21-day-old rats approached those in adults. Adrenal tissue from rats of the same ages showed a pattern of sensitivity to ACTH in vitro similar to that in vivo, except that the response of adrenals from 21-dayold rats was still feeble. Pretreatment of rat pups for the first 6 postnatal days with ACTH or corticosterone resulted in a moderate increase in plasma B after CRF or ACTH on day 7. The postnatal fall in sensitivity is partially explained by a decreased adrenal sensitivity to ACTH. (Endocrinology106: 991, 1980)