Hypothalamic LH may play a role in control of pituitary LH release.

We have recently reported immunoassayable luteinizing hormone (LH) in several areas of the rat brain and conspicuously present in the hypothalamus. In this report, we focus on the presence of LH in the hypothalamus and its potential role in regulation of pituitary LH release. In adult female rats, examined during the course of the estrous cycle, a significant fall in hypothalamic LH coincides with the surge in pituitary and serum LH at the time of proestrus, signaling ovulation. Ovariectomized adult rats show no change in hypothalamic LH at a time when there is a dramatic rise in both anterior pituitary and serum LH. These data support the concept that hypothalamic LH is not of pituitary origin and that it may play a role in the short-loop negative feedback system controlling the surge in anterior pituitary LH release, and event initiating ovulation.