Interaction between the posterior pituitary and LHRH in the control of LH secretion

Abstract

We have recently reported that the posterior lobe of the pituitary differentially inhibits the secretion of prolactin (PRL) and luteinizing hormone (LH), but not follicle stimulating hormone (FSH) throughout the estrous cycle. Removal of the posterior pituitary (posterior pituitary lobectomy) results in elevations of plasma LH on all days of the cycle except on diestrus-day-2. In the present study we examined: 1. whether the control of LH release involves an interaction between the posterior pituitary and hypothalamic luteinizing hormone-releasing hormone (LHRH), and 2. whether the elevation of LH seen following posterior lobectomy is due to the removal of a posterior pituitary substance(s) which alters anterior pituitary sensitivity to LHRH. In order to block the action of hypothalamic LHRH, a potent LHRH inhibitory analog (50 μg) was injected SC two hours prior to removal of the posterior pituitary in estrous rats. Administration of the inhibitory analog completely eliminated the elevation of plasma LH seen following posterior lobectomy, but did not alter the posterior lobectomy-induced rise of plasma PRL, or plasma FSH concentrations. In order to test whether anterior pituitary sensitivity to LHRH is altered by posterior lobectomy, a moderate dose of LHRH (15 ng) was administered to both posterior lobectomized and sham lobectomized estrous rats. The time-course and magnitude of the LH response to LHRH was similar in both groups. The results are consistent with the hypothesis that LH secretion is controlled by an interaction between hypothalamic LHRH and the posterior lobe of the pituitary, but this interaction does not appear to involve lobectomy-induced changes in anterior pituitary responsiveness to LHRH. It remains to be determined whether the posterior pituitary contains a substance(s) which is capable of altering hypothalamic release of LHRH.