Regional Distribution of Hypothalamic Luteinizing Hormone-Releasing Hormone in Proestrous Rats: Effects of Ovariectomy and Estrogen Replacement*

Abstract

The effects of ovariectomy and estrogen treatment on hypothalamic LHRH [luteinizing hormone releasing hormone] and pituitary and serum LH [luteinizing hormone] were studied in female rats. The median eminence (ME) and 13 individual hypothalamic nuclei were removed by microdissection for determination of LHRH by RIA [radioimmunoassay]; anterior pituitary and serum LH were also measured by specific RIA. During the morning of proestrus, the highest LHRH concentration was present in the ME with high levels also present in organum vasculosum of the lamina terminalis (OVLT) and in the retrochiasmatic area. Two weeks after ovariectomy, LHRH in the ME was significantly reduced to 58% of control. Four weeks after ovariectomy, LHRH in the ME and in the arcuate nucleus was reduced to 36 and 30% of controls, respectively. This progressive reduction of LHRH after ovariectomy was accompanied by parallel increases in pituitary and serum LH. Daily injections of estradiol benzoate (20 .mu.g/day in oil) for 2 wk prevented these changes in ovariectomized rats. LHRH in the OVLT was unchanged by ovariectomy or estrogen treatment. Immunohistochemical staining corroborated the RIA findings in the ME and OVLT. LHRH in the ME is subject to profound influence by ovarian steroids. Ovariectomy induces an accelerated release of hypothalamic LHRH, particularly from the ME, with a resulting increase in pituitary LH secretion. LHRH in the ME and LHRH in the OVLT respond differently to ovarian steroids, implicating disparate regulation and anatomical organization of LHRH in these 2 regions.