To determine what role pituitary responsiveness plays in the suppression of gonadotropin secretion during lactation in the rat, three parameters of pituitary responsiveness to LHRH were compared during lactation and the estrous cycle: 1) the magnitude of the response, 2) the duration of the response, and 3) the dose response. Doses of 25,100, or 500 ng of LHRH were administered to animals on diestrus-1 (D-l), diestrus- 2 (D-2), and on days 5, 10, 15, and 20 postpartum. Following each of the three doses of LHRH, dose-dependent peak levels of LH were reached which did not differ in cycling animals and in lactating females nursing either two or eight pups. In all groups of animals, the magnitude of the response following 25 ng LHRH was approximately 50% of that following 500 ng of LHRH. Therefore, pituitary responsiveness to LHRH during lactation is similar to that during diestrus of the cycle, as judged by peak levels of LH reached. Furthermore, since 500 ng of LHRH released significantly more LH in ovariectomized females nursing eight pups than in intact lactating females, ovarian steroids may be the primary factor determining the magnitude of the response to LHRH during lactation. The duration of the response following 25 ng LHRH was similar in females nursing two pups and in D-2 animals, and was somewhat greater than the duration in females nursing eight pups and in D-l animals. The duration of the response following 500 ng LHRH was significantly greater on D-l and D-2 than during lactation with either two or eight pups suckling. Even though pituitary responsiveness was not greatly different in females nursing eight or two pups, serum and pituitary LH concentrations and hypothalamic LHRH content were significantly less in females nursing eight pups. Therefore, the suppression of gonadotropin secretion during lactation is primarily the result of suppressed hypothalamic stimulation of gonadotropin secretion plus a small decrease in pituitary responsiveness.
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