Role of prolactin in regulating gonadotropin secretion and gonad function in female rats

The role of prolactin in female reproduction has been studied extensively in two systems; in the regulation of corpus luteum function, and in the regulation of gonadotropin secretion. In the rat, prolactin is an essential component of the luteotropic complex since it is the hormone responsible for transforming a corpus luteum of the estrous cycle into a corpus luteum of pseudopregnancy or pregnancy, which allows progesterone secretion to be maintained. The biphasic surges of prolactin initiated by cervical stimulation at the time of ovulation continue to be important throughout pseudopregnancy to maintain elevated levels of progesterone. During pregnancy, the luteotropic requirements change so that prolactin is no longer required after midpregnancy. In addition to its luteotropic effects, prolactin also has antigonadotropic effects that contribute significantly to the suppression of gonadotropin secretion during lactation. Suckling-induced prolactin secretion contributes to the suppression of both basal gonadotropin secretion and the postcastration rise in gonadotropins during lactation. Prolactin may have some effect at the pituitary to suppress responsiveness to luteinizing hormone-releasing hormone (LHRH), but its primary site of action appears to be the hypothalamus. Prolactin may be acting at the hypothalamus either to cause a decrease in LHRH secretion or to cause the release of substances that inhibit pituitary gonadotropin secretion directly.