Abstract

It is apparent that pituitary gonadotropin release changes quantitatively during various endocrine states, such as those found during ovarian cyclicity, lactation, aging, and ovariectomy. In the present work, we have used a radioligand prepared from a nondegradable, superagonist of gonadotropin-releasing hormone (GnRH) to determine the number and binding affinity of GnRH receptors during these endocrine states. While receptor affinity was unaltered (range 1.6-2.7 x 1O¹⁰ M^-1), marked differences were observed in the receptor number throughout the estrous cycle [maximal in late diestrus II and proestrus immediately preceding the luteinizing hormone (LH) spike]. Lactating animals and old animals also had diminished concentrations (number per milligram of protein) of GnRH receptors as compared with young precycling females. Ovariectomy increased the number of receptors, and injections of estradiol benzoate to ovariectomized animals reversed this increase within 3 hr. Throughout the study, elevated receptor numbers were generally associated with elevated LH levels, although this alone did not appear to be sufficient for increased LH secretion. The present results suggest that regulation of the GnRH receptor may be one mechanism through which gonadotrope sensitivity is regulated.