DOMESTIC ANIMAL ENDOCRINOLOGY Vol. 4(2):123-132, 1987 PITUITARY RECEPTORS FOR GnRH AND ESTRADIOL, AND PITUITARY CONTENT OF GONADOTROPINS IN BEEF COWS. I. CHANGES DURING THE ESTROUS CYCLE 1 T.M. Nett, D. Cermak, T. Braden, J. Manns2 and G. Niswender Department of Physiology Colorado State University Fort Collins, Colorado 80523 Received November 11, 1986 ABSTRACT To further characterize the endocrinological changes in the hypothalamo-hypophyseal axis thoughout the bovine estrous cycle, cycling beef heifers (n = 24) were randomly assigned to six groups. These heifers were slaughtered 6, 12, 18, 19, 20 or 21 days following their previous estrus (day 0). Anterior pituitaries and hypothalami were collected. Hypothalami were divided into the preoptic area and medial l~asal hypo- thalamus, and content of gonadotropin-releasing hormone (GnRH) was quantified by radioimmunoassay. Contents of luteinizing hormone (LH) and follicle stimulating hor- mone (FSH) in the anterior pituitary gland were quantified by radioimmunoassay. Membrane receptors for GnRH were quantified by a standard curve technique and receptors for estradiol in anterior pituitary cytosol were quantified by saturation analysis. There was no significant change in content of GnRH in the hypothalamus or content of FSH in the anterior pituitary on any of the days examined; however, content of GnRH in the preoptic area was lower (P<.I) on day 19 postestrus. Cytosolic receptors for estradiol increased (P<.05) on day 18 post-estrus and returned to baseline by day 19. Content of LH and the number of receptors for GnRH in the anterior pituitary gland decreased (P<.01) on day 19 postestrus, and the number of receptors for GnRH remained low through day 21 postestrus. The reduction in anterior pituitary content of LH was transient indicating that synthesis of LH restores pituitary content to preovulatory levels before the number of receptors for GnRH returns to normal. INTRODUCTION The relationship between serum concentrations of gonadotropic hormones and ovarian function during the bovine estrous cycle has been described previously (1,2); however, regulation of the hypothalamo-hypophyseal axis is less clear. Pituitary content of gonadotropins (3), and sensitivity of the pituitary to exogenous gonadotropin-releasing hormone (GnRH; 4,5) during the bovine estrous cycle have been examined, but correlation of these factors and others such as hypothalamic content of GnRH and pituitary receptors for GnRH and estradiol, particularly around the time of the luteinizing hormone (LH) surge, has been lacking. Pituitary concentration of receptors for GnRH has been reported to increase on the day of proestrus in rats (6-8) and hamsters (9) as well as during the periovulatory period of the ewe (10). The current study was undertaken to quantify hypothalamic content of GnRH, pituitary content of LH and follicle- stimulating hormone (FSH) and to quantify the number of hypophyseal re- ceptors for GnRH and estradiol at different stages of the bovine estrous cycle with emphasis given to the period around the preovulatory surge of LH and ovulation. Copyright © 1987 by DOMENDO, INC. 123 0739-7240/87/$3.00