Effects of short-term treatment of a gonadotropin-releasing hormone agonist on the follicular development and gonadotropin secretion in the rat.

The gonadotropin-releasing hormone (GnRH) controls pituitary gonadotropin biosynthesis and secretion and therefore it is indispensable in regulating reproductive function. We have studied the effects of a short term treatment (7-days) of a GnRH-agonist (GnRH-Ag) in vivo (a) on reproductive cyclicity and follicular development and (b) on peripheral gonadotropin secretion of normal cycling rat. GnRH-Ag (0.2, 1 or 5 micrograms/day), administered continuously through an osmotic minipump for 7-days, had a varied effect on ovarian cyclicity as is evident by vaginal cytology and it also decreased ovarian weight. A dose of 1 as well as 5 micrograms/day for 7-days of GnRH-Ag caused the complete demise of early and late antral follicles, whereas a dose of 0.2 microgram/day of GnRH-Ag for 7-days caused a significant decrease in the number of late antral follicles. There was a remarkable increase in the number of atretic follicles in the ovary of rats that were treated with 5 micrograms/day of GnRH-Ag for 7-days. Ovarian histology showed the predominance of corpora lutea in rats treated with 1 or 5 micrograms/day of GnRH-Ag and the interstitial cells in general appeared pycnotic. GnRH-Ag treatment significantly enhanced the serum LH secretion but FSH secretion remained unaffected. Serum PRL concentration diminished in rats that were treated with the highest dose (5 micrograms/day) of GnRH-Ag. Results from this study clearly demonstrate that GnRH-Ag treatment interferes with normal cyclicity of rats and it profoundly affects the follicular development. Therefore, it acts as an atretognic factor in the ovary.