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

Abstract

The influence of prolactin treatment (100 .mu.g/100 g body wt s.c. 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 5.alpha.-reductase activity were also observed. Secretion of DHB [5.alpha.-dihydrocorticosterone] in adrenal venous blood was decreased as well, without changes in corticosterone or THB [3.beta., 5.alpha. tetrahydrocorticosterone] secretion rates. Combined treatment with prolactin and ACTH produced greater increments in the secretion rates of corticosterone than those obtained with prolactin alone. Prolactin treatment to ovariectomized rats appears to have a dual effect: adrenal responsiveness to ACTH is enhanced by its effects on adrenal 5.alpha.-reductase activity, and pituitary-adrenal response to stress is dampered by prolactin treatment. The effects of prolactin on adrenal 5.alpha.-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.