THE ROLE OF STEROID HORMONES IN THE REGULATION OF GONADOTROPIN SECRETION

Abstract

An acute rise in serum gonadotropins occurs in the immature rat on castration indicating the role of gonadal steroids in gonadotropin regulation. In the castrated male rat testosterone brings about restoration of gonadotropins to intact levels at doses that are “physiological” as judged by seminal vesicle and ventral prostate weights. The suppression of LH takes place at lower doses than FSH. Estradiol prevents the post-castration rise of gonadotropins in the male rat at a dose level of 0·3% of the testosterone dose. Using a variety of techniques, it has been demonstrated that receptors for estradiol are found in the hypothalamus and pituitary of both male and female rats in similar concentrations. No such high affinity receptors are found for androgens in either sex although androgens appear to interfere with estrogen binding. The regulation of gonadotropin secretion in the male thus may be mediated by conversion of androgens to estrogens and the interference manifested by them in estrogen binding. The conversion to estrogens appears to be of importance in the control of FSH because androgens not convertible to estrogens such as 5α-dihydrotestosterone and 5α-androstane-3α, 17β-diol are even less effective than testosterone in suppressing FSH. However, for the suppression of LH, such a conversion does not appear necessary. Conclusive evidence for a positive feedback effect of estradiol on the pituitary leading to greater sensitivity to LHRH in the release of LH has been provided by studies in the cyclic rat and a pituitary stalk-sectioned rat model. Progesterone in the absence of estrogens does not have any effect on gonadotropins. In the presence of estrogens the effect may be stimulatory or inhibitory depending on dose.