Pituitary Gonadotropin-Releasing Hormone (GnRH) Receptor Responses to GnRH in Hypothalamus-Lesioned Rats: Inhibition of Responses by Hyperprolactinemia and Evidence that Testosterone and Estradiol Modulate Gonadotropin Secretion at Postreceptor Sites*

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Abstract
Increased hypothalamic gonadotropin-releasing hormone [GnRH] secretion appears to influence positively the number of pituitary GnRH receptors (GnRH-R). GnRH-R increase after castration in male rats and this rise can be prevented by testosterone (T), anti-GnRH sera or hypothalamic lesions. GnRH also increases serum LH and GnRH-R in hypothalamus-lesioned rats, and these animals injected with exogenous GnRH are, therefore, a good model in which to study the site of steroid feedback at the pituitary level. Adult male and female rats were gonadectomized and radiofrequency lesions were placed in the hypothalamus. Males received T implants and females received estradiol implants at the time of surgery. Empty capsules were placed in the control animals. Beginning 3-5 days later, animals in each group were injected every 8 h with vehicle (bovine serum albumin [BSA]) or GnRH (0.002-200 .mu.g/day) for 2 days. After these GnRH injections, all rats received 6.6 .mu.g GnRH, s.c. 1 h before decapitation to determine acute luteinizing hormone [LH] and FSH responses. GnRH-R were determined by saturation analysis using 125I-D-Ala6-GnRH ethylamide as ligand. In males, GnRH injections increased GnRH-R. T inhibited acute LH and FSH responses to GnRH in all groups, but had little effect on GnRH-R, indicating that T inhibits gonadotropin secretion at a post-GnRH receptor site. In females, the GnRH-R response to GnRH was less marked and only the 200 .mu.g/day dose of GnRH increased GnRH-R, indicating that the positive feedback effects of estradiol at the pituitary level are also exerted at a site distal to the GnRH receptor. There was no positive correlation between the number of GnRH-R and GnRH-stimulated gonadotropin release in males or females. Female rats with hypothalamic lesions had markedly elevated serum prolactin [PRL] levels (> 300 ng/ml). Suppression of PRL secretion by bromocryptine resulted in augmented GnRH-R responses to GnRH, and GnRH-R concentrations rose to the same values induced in males. Hyperprolactinemia inhibits GnRH-R responses to GnRH in females by a direct action on the pituitary gonadotroph.

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