PROGESTERONE IMPLANTS DELAY AGE-RELATED DECLINES IN REGULAR ESTROUS CYCLICITY AND THE OVARIAN FOLLICULAR RESERVE IN LONG-EVANS RATS

The present study examined the effects of progesterone (P-4) treatment s on estrous cyclicity and the loss of ovarian follicles during aging. Young rats received repeated treatments with P-4 or empty implants be tween 3.5 and 8 mo of age. At 8 mo, ovaries were obtained from some an imals to determine the numbers of resting follicles, and estrous cycle patterns and hormone levels were determined from other groups of trea ted females. In contrast to the cyclic increases in P-4, estradiol (E- 2), LH, and FSH in control animals, P-4-implanted rats exhibited eleva ted serum P-4 but low E-2, LH, and FSH levels. After implant treatment s, the follicular reserve was significantly (p < 0.05) larger in P-4-t reated females (2012 +/- 297 resting follicles per ovary, n = 5 rats p er group) than in regularly cyclic control rats (713 +/- 226 follicles per ovary, n = 7). The effects of P-4 implants on the follicular rese rve were associated with a subsequently higher incidence of regular es trous cycles after P-4 treatment. These results demonstrate that P-4 p revents cyclic increases in E-2 secretion and is associated with a con servation of the ovarian follicular reserve and the maintenance of reg ular estrous cycle patterns, indicating a protective effect of P-4 on the age-related loss of ovarian follicles.