Influence of Ovarian Hormones on Prolactin Release In the Rhesus Monkey1,2

Abstract

Blood samples were collected from female rhesus monkeys by 2 different techniques and serum prolactin levels were determined by radioimmunoassay. Prolactin levels in samples from ketamine anesthetized monkeys varied widely and there was no apparent difference between those of intact (23.5 .+-. 4.2 ng/ml, mean .+-. SEM [standard error of the mean]) and ovariectomized (24.0 .+-. 5.1 ng/ml) monkeys. When blood samples were taken from unanesthetized monkeys in primate restraint chairs, prolactin levels were 8.2 .+-. 1.7 ng/ml in intact monkeys and were reduced to 2.5 .+-. 0.4 ng/ml in ovariectomized monkeys. In ovariectomized monkeys, the peak release of prolactin following injection of either thyrotropin releasing hormone (TRH, 24 ug, i.v.) or pimozide (0.2 mg/kg i.v.) were reduced by 72% and 68%, respectively. Replacement of estradiol-17.beta., by means of Silastic capsule implants for periods of 1-6 mo., restored the responsiveness of ovariectomized monkeys to TRH. Pituitary weight in 7 ovariectomized monkeys treated with estrogen for 2-5 mo. were 81.0 .+-. 3.1 mg as compared to 61.1 .+-. 2.3 mg for 5 ovariectomized controls (P < 0.001). Estrogen treatment also increased pituitary prolactin concentrations from 452 .+-. 76 ng/mg to 974 .+-. 105 ng/mg (P < 0.01). Apparently ovariectomy diminishes prolactin release under suitable experimental conditions. It appears that estrogens are involved in this effect, since estrogen replacement restores TRH-induced prolactin release and increases both pituitary weight and prolactin concentration of ovariectomized monkeys.