Effects of Applying Gamma-Aminobutyric AcidB Drugs into the Medial Basal Hypothalamus on Basal Luteinizing Hormone Concentrations and on Luteinizing Hormone Surges in the Female Sheep

Prior investigations have shown that localized infusion by microdialysis of gamma-aminobutyric acidB (GABAB) agonists into the medial basal hypothalamus of male sheep rapidly increases GnRH and LH pulse amplitude. The objectives of these studies were to determine if infusion of GABAB agonists SKF 97541 or baclofen into the medial basal hypothalamus of female sheep would affect basal LH secretion and if infusion of a potent antagonist would alter expression of LH surges induced by injection of estrogen. Infusion of either SKF 97541 (10 or 40 (mu)M) or baclofen (1 mM) into estrogen-treated ovariectomized ewes did not alter basal LH secretory patterns, whereas both drugs significantly elevated mean LH and LH pulse amplitude in ovariectomized ewes during the nonbreeding season. Infusion of the antagonist CGP 52432 (250 or 500 (mu)M) did not affect expression of estrogen-induced LH surges in ovariectomized ewes. These observations support the concept that GABAB receptors in the medial basal hypothalamus regulate basal LH secretion but do not regulate the surge mode of LH secretion in the female sheep.