Estradiol as a gonadotropin releasing hormone in the rhesus monkey

The temporal relationship between cessation of GnRH delivery to the pituitary gland and the loss of responsiveness to the stimulatory action of estradiol (E2) was examined in 4 ovariectomized rhesus monkeys whose endogenous GnRH production had been abolished by hypothalamic lesions. Gonadotropin secretion was re-established by the intermittent administration of GnRH. The GnRH-replacement regimen was then discontinued and estradiol benzoate (EB) injected 24, 48, 72 and 96 hours later. Unambiguous gonadotropin discharges were induced when EB was administered 24 or 48 hours after discontinuation of GnRH replacement. We conclude that E2 can initiate gonadotropin discharges in the absence of circulating GnRH. E2 may, therefore, be viewed as a gonadotropin releasing hormone.