Inhibition by prolactin of post-castration rise in LH

Castration of male and female rats resulted in a marked rise in serum LH. The rise in serum LH was partially or completely prevented by injection of prolactin (Prl), by implantation of a small amount of Prl in the median eminence (ME), by grafting 2 anterior pituitaries (APs) underneath the kidney capsule, or by transplantation of a Prl-secreting pituitary tumor underneath the skin. The larger pituitary tumor transplants secreted more Prl and were more effective in reducing LH release than the smaller tumors which secreted less Prl. Suppression of LH release generally was greater during the earlier than in the later phases of the different treatments. The pituitary LH response to synthetic LH-RH was the same in ovariectomized rats with or without pituitary grafts, and the decrease in hypothalamic LH-RH after orchidectomy was prevented by pituitary grafts. These results indicate that Prl can depress LH release after castration and that these effects are mediated via the hypothalamus.