Effects of Estrogen and Progesterone on Luteinizing Hormone Release in Ovariectomized Rats1

Publisher Website
Google Scholar
Abstract
Plasma LH [luteinizing hormone] was measured by radioimmunoassay in blood withdrawn through indwelling atrial cannulas in long-term ovariectomized (OVX) rats. S.c. injection of 50 .mu.g of estradiol benzoate (EB) in oil at 1100 h (day 0) caused an LH surge in plasma during the afternoon on days 2-6 in the majority of the rats. The pattern of the surge on day 2 was similar to that measured in 4-day cyclic rats. The time of onset and the magnitude of the surge was more variable than that in 4-day cyclic rats. S.c. administration of 25 mg of progesterone (P) in oil at 1100 h in rats treated with EB 2 days previously did not advance the time of onset of the LH surge that day, but injection of P at 1100 h in proestrous rats did advance the spontaneous LH surge. In contrast, injection of EB and P on day 0 in additional OVX rats caused an LH surge of reduced magnitude during the afternoon of day 1. P can advance the LH surge by 24 h in EB-treated OVX rats and in cyclic rats. Administration of pentobarbital or phenobarbital showed that a potential activation period for LH release exists in the EB-treated OVX rats as in cyclic rats and that the barbiturates are, in general, less effective in blocking LH release in the EB-treated OVX rats. Since anterior deafferentation of the medial basal hypothalamus blocked the EB-induced LH surge and administration of LHRH [luteinizing hormone releasing factor] was equally effective in releasing LH during the morning and afternoon hours, estrogen perhaps must exert a primary positive feedback action on the brain to release LHRH in amounts which exceed basal LHRH release to initiate the LH surge.