Effect of estradiol on hypothalamic GnRH and pituitary and serum LH and FSH in ovariectomized pigs.

Two experiments were conducted to measure pituitary gonadotropins, hypothalamic-gonadotropin releasing hormone (GnRH) and pituitary response to GnRH during periods when serum luteinizing hormone (LH) was suppressed by estradiol-17 beta (e2) in ovariectomized pigs. In the first experiment, 10 ovariectomized gilts were assigned to two groups of five each according to time of slaughter (24 or 36 h after injection). Within each group, gilts were given corn oil (n = 2) or 400 micrograms E2 (n = 3). Neither serum nor anterior pituitary (AP) concentrations of follicle-stimulating hormone (FSH) were affected by E2. Serum LH was suppressed from 12 to 26 h after E2. Concentrations of LH in AP were unchanged at 24 h, but increased at 36 h after E2 injection. Concentrations of GnRH in medial basal hypothalamus (MBH), stalk-median eminence (SME) and hypophyseal portal area (HPA) were lower at 24 h after E2 than in oil-treated gilts. At 36 h after E2, suppressive effects of E2 on LH in serum had subsided and concentrations of LH in AP and GnRH in MBH and SME were greater than in oil-treated controls. The observation that E2 suppressed LH in serum without a detectable suppression of LH in AP led to the hypothesis that E2 had caused the suppression of serum LH by suppression of GnRH release. In a second experiment, 12 ovariectomized gilts were assigned to receive corn oil (n = 4), 400 micrograms E2 (n = 4) or 400 micrograms E2 plus GnRH (1.5 micrograms/h; n = 4). Patterns of LH in sera of E2-treated animals were similar to those in the first experiment, with serum LH in E2-treated gilts suppressed from 4 to 32 h after treatment. However, in gilts receiving GnRH in addition to E2, serum LH concentrations during 20 to 32 h after treatment were intermediate between gilts receiving E2 alone and controls. Thus the pituitary of the pig is capable of responding to GnRH when LH is normally suppressed by E2. These experiments provide two lines of evidence that suppression of serum LH by E2 is due at least in part to suppression of GnRH. These experiments also establish the hypothalamus as a site for negative feedback of E2 in the female pig.