Pituitary response to bilateral adrenalectomy, metyrapone treatment and ether stress in the newborn rat

ACTH level, determined by radioimmunoassay, has been used as a reliable test to investigate pituitary response to bilateral adrenalectomy, metyrapone treatment and ether stress in 8-day-old rat newborns. The pituitary gland able to enhance ACTH release significantly, first in response to the decrease in plasma corticosteroid levels induced either by bilateral adrenalectomy or by metyrapone inhibition of the corticosteroidogenesis, and second in response to 2 min of ether exposure. This latter response was greater in females than in males but smaller in neonates than in adults. These data suggest that the steroid-feedback mechanism was operating fully at the 8th postnatal day and that central mechanisms partially responded to external signals of stress. However, a relative 'stress-non-responsive period' was evident by the 8th postnatal day. The newborn rat's hypothalamic-pituitary axis has partially matured to be responsive to stress by the end of the first week after birth.