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Pituitary Content of Gonadotropins and GnRH-Receptors in Pregnant, Postpartum and Steroid-Treated OVX Ewes¹

Colorado State University⁴, Fort Collins 80523

Abstract

Parturition in the ewe is followed by an anestrous period during which gonadotropin-releasing hormone- (GnRH) induced release of luteinizing hormone (LH) is depressed. Several possible underlying causes of this depressed responsiveness, including changes in anterior pituitary content of gonadotropins, concentration of pituitary GnRH-receptors and hypothalamic content of GnRH, were examined during late gestation and at d 1, 11, 22 and 35 after parturition in the ewe. The GnRH-induced release of LH and pituitary content of LH increased with time after parturition (P<.05) and were highly correlated (r = .976). Pituitary concentrations of follicle-stimulating hormone (FSH) and GnRH-induced release of FSH did not change after parturition. Numbers of receptors for GnRH in the pituitary declined during the postpartum period (P<.05), but concentrations of hypothalamic GnRH did not change. In a second experiment, ewes which had been ovariectomized (OVX) for at least 60 d were chronically treated with estradiol and progesterone via silastic implants to examine the feasibility of using this as an animal model for examining hypothalamo-hypophyseal changes common to pregnancy. Pituitary concentrations of LH and FSH, hypothalamic content of GnRH and the number of hypophyseal receptors for GnRH were measured after 0, 50, 100 and 150 d of steroid treatment. The quantity of LH released in response to GnRH was measured every 25 d throughout the 150-d treatment period. Concentrations of LH and FSH in the pituitary as well as the GnRH-induced release of LH were depressed (P<.05) by steroid treatment. Receptors for GnRH were higher after 100 d of steroid treatment (P<.005) than at any other time during the study. However, on d 150 of steroid treatment, the number of GnRH-receptors did not differ from that measured either in OVX ewes not subjected to steroid treatment or ewes during late pregnancy. Content of GnRH in the hypothalamus did not change during steroid treatment. After 150 d of treatment, steroid implants were removed from one group of steroid-treated OVX ewes. Subsequently, the release of LH in response to GnRH increased similar to that seen during the postpartum period in intact ewes. Based on these data, it appears that while decreased quantities of LH in the pituitary of the postpartum ewe may contribute to ovarian acyclicity after parturition, neither pituitary concentrations of FSH, GnRH-receptors nor the concentration of GnRH in the hypothalamus limit the ability of the postpartum ewe to ovulate. In addition, the OVX ewe treated with estradiol and progesterone for 150 d provides a suitable model for examining hypothalamo-hypophyseal interactions of pregnancy. Further, following the removal of steroid treatment in OVX ewes, changes in the hypothalamo-hypophyseal axis are similar to those seen during early postpartum anestrus.

¹ This research was supported by grants HD 07841 and HD 13568 and a grant from the Colorado Agr. Exp. Sta.

² Present address: Instituto di Fisiologia Veterinaria, Universita di Bologna, Bologna, Italy. Supported in part by CNR (Centro Navionale delle Ricerche).

³ Present address: Dept. of Anim. Sci., Purdue Univ., West Lafayette, IN.

⁴ Dept. of Physiol. and Biophys.