Daily administration of gonadotrophin-releasing hormone increases pituitary gonadotroph number and pituitary gonadotrophin content, but not serum gonadotrophin levels, in female rats on day 1 of dioestrus.

Abstract

Gonadotrophin-releasing hormone (GnRH) has been shown to regulate the synthesis and release of gonadotrophins acutely, yet few studies have investigated the chronic effects of this agent on pituitary gonadotrophins. In the present study we determined the effect of chronic administration of GnRH on the female rat pituitary gland. Rats of 8 weeks of age were injected s.c. with various doses of GnRH daily for 30 days. After completion of the GnRH treatment, treated rats and age-matched controls were killed by decapitation at 09.00 h on the first day of dioestrus, as determined from vaginal smears. Treatment with 10 ng-10 micrograms GnRH/day increased pituitary contents of FSH and LH in a dose-dependent manner. The change in FSH content was much greater than that of LH content. The pituitary FSH content of rats treated with 40 micrograms GnRH was significantly less than that of rats treated with 10 micrograms GnRH. There was a marked increase in the number of cells which stained positively for FSH (266%) and LH (28%) in the anterior pituitary of rats given 10 micrograms GnRH, but there was no demonstrable change in the areas of single cells stained positively for FSH and LH. Serum levels of LH, FSH and oestradiol were not affected by the GnRH treatment. These data indicate that chronic administration of GnRH is capable of increasing the pituitary gonadotrophin content and numbers of FSH and/or LH-stained cells and that FSH cells are affected more than LH cells by the GnRH treatment. The increase in pituitary gonadotrophin content, however, does not necessarily produce an increase in circulating levels of gonadotrophins.