EFFECTS OF PROLACTIN ON PITUITARY-ADRENAL FUNCTION IN INTACT AND OVARIECTOMIZED RATS

Abstract

ABSTRACT The influence of prolactin treatment (100 μg/100 g body wt. sc daily for 7 days) on plasma corticosterone levels, adrenal steroid production in vitro and in vivo and pituitary-adrenal responses to stress were studied in intact and castrated female rats. Prolactin enhanced plasma corticosterone levels and corticosterone production in vitro and in vivo in intact rats after stress. Differences were abolished with ACTH treatment. In contrast, prolactin administration to ovariectomized rats inhibited plasma corticosterone response to stress. Combined treatment with ACTH reversed these findings. A greater in vitro production of corticosterone by adrenal slices and adrenal homogenates associated with an effective inhibition of adrenal 5α-reductase activity were also observed. Secretion of DHB in adrenal venous blood was decreased as well, without changes in corticosterone or THB secretion rates. However, combined treatment with prolactin and ACTH produced greater increments in secretion rates of corticosterone than those obtained with prolactin alone. The data suggest that prolactin treatment to ovariectomized rats has a dual effect: a) adrenal responsiveness to ACTH is enhanced by its effects on adrenal 5α-reductase activity, and b) pituitary-adrenal response to stress is dampered by prolactin treatment. The effects of prolactin on adrenal 5α-reductase activity and corticosterone production in vitro were paralleled in vivo only after the exogenous administration of ACTH. The presence of the gonads apparently prevented the inhibitory effect of prolactin on ACTH secretion and in turn seemed to act synergistically with prolactin to facilitate pituitary-adrenal response to stress.