The sites of action of estradiol and phentolamine in the inhibition of the pulsatile, circhoral discharges of LH in the rhesus monkey (Macaca mulatta).

Pulsatile LH secretion was re-established in ovariectomized monkeys bearing hypothalamic lesions by an intermittent infusion of LHRH. The administration of estradiol to such animals resulted in a prompt cessation of these pulsatile discharges of LH and a resultant decline in the mean plasma concentration of the gonadotropin. The time course of this inhibition of LH secretion was indistinguishable from that observed after estrogen administration to ovariectomized animals with intact nervous systems. In contrast, phentolamine did not interrupt the pulsatile LH discharges occasioned by the hourly administration of exogenous LHRH to the lesioned animals. These results are consistent with the conclusion that the acute negative feedback action of estradiol on circhoral LH release in the monkey is at the level of the pituitary gland, whereas the inhibitory action of phentolamine on this mode of LH secretion is at a neural site.