Changes in the hypothalamic contents of LHRH-I and -II and in pituitary responsiveness to synthetic chicken LHRH-I and -II during the progesterone-induced surge of LH in the laying hen

Abstract

ABSTRACT The contents of LHRH-I and -II in the anterior hypothalamus and posterior hypothalamus (including the mediobasal hypothalamus and median eminence) were measured at 90, 180 and 360 min after the i.m. injection of laying hens with progesterone. Whilst no changes were observed in the content of LHRH-I in the anterior hypothalamus, LHRH-I in the posterior hypothalamus tended to fall at 90 and 180 min after injection of progesterone in hens maintained on 16 h light:8 h darkness (16L:8D) and 8L:16D respectively. Pretreatment of laying hens with tamoxifen significantly increased the hypothalamic contents of LHRH-I and -II, raised the basal plasma concentration of LH and modified the LH response to progesterone injection. In hens in which tamoxifen prevented an increase in the plasma concentration of LH after progesterone injection, the content of LHRH-I in the posterior hypothalamus remained unchanged. In contrast, in hens in which progesterone stimulated a steep increase in LH within 90 min, there was a pronounced and significant fall in LHRH-I content of the posterior hypothalamus. No change in the hypothalamic content of LHRH-II was observed during the progesterone-induced surge of LH until plasma concentrations had attained maximal values or started to decline. Then, in hens maintained on 16L:8D, a significant fall in the content of LHRH-II in the anterior hypothalamus was found at both 180 and 360 min after injection with progesterone. Tests in vitro and in vivo of the responsiveness of the pituitary gland to synthetic LHRH-I and -II revealed no change at 90 min after injection of laying hens with progesterone, when plasma concentrations of LH were increasing, but a pronounced reduction when plasma LH concentrations were maximal or falling. These results suggest that LHRH-I mediates in the progesterone-induced increase in the plasma concentration of LH. Although the subsequent decline in plasma LH was associated with a reduced responsiveness of the pituitary gland to LHRH, a significant correlation between the contents of LHRH-I and -II in the anterior hypothalamus and a fall in the hypothalamic content of LHRH-II when plasma LH was maximal or declining allows the possibility of an involvement of this peptide in the neuroendocrine events preceding ovulation. Journal of Endocrinology (1990) 127, 487–496