Feedback Effects of Ovarian Steroids on the Hypothalamic Hypophyseal Axis in the Rabbit" Biology of Reproduction 35

The feedback effects of two ovarian steroids, estradiol-1 7/3 (E2) and 200t-hydroxypregn-4-en-3-one (2OaOH), were examined in both intact (INT) and ovariectomized (0 VEX) does. We measured (1) steroid-induced altera-tions in endogenous gonad otropin-rel easing hormone (GnRH) from sequential 10-mm samples of hypothalamic perfusates, (2) simultaneous changes in peripheral plasma luteinizing hormone (LII) and follicle-stimulating hormone (FSH), and (3) the modification 0/pituitary responsiveness, i.e., increments in plasma LH ( LH) and plasma FSH (L FSH), after 50 ng, 250 ng, and 1 jig of exogenous GnRH in individual does of 6 treatment groups. The groups were: INT does, OVEX does, OVEX does receiving either one (1 E2) or two (2 E2) E2-filled Silastic capsules, OVEX does receiving a 2OctOH-filled capsule (2OaOH), and OVEX does receiving both capsules of E2 and 2OaOH (1 E2 + 2OaOH). Ovariectomy enhanced the pulsatile release 0/hypothalamic GnRH and pituitary LH and FSH, and increased the LH response (t LH) to exogenous GnRH (0 VEX vs. INT, p<0.O5). Replacement of E2 at the time of ovariectomy prevented the increased GnRH and gonadotropin secretion as well as the enhanced t LH that were observed in untreated OVEX does. The release 0/hypothalamic GnRH in the 2OcsOH group was lower (p<O.05) than that in the OVEX group and not different from that in the INT group. The release of pituitary LH an