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) treatments on estrous cyclicity and the loss of ovarian follicles during aging. Young rats received repeated treatments with P-4 or empty implants between 3.5 and 8 mo of age. At 8 mo, ovaries were obtained from some animals to determine the numbers of resting follicles, and estrous cycle patterns and hormone levels were determined from other groups of treated females. In contrast to the cyclic increases in P-4, estradiol (E-2), LH, and FSH in control animals, P-4-implanted rats exhibited elevated serum P-4 but low E-2, LH, and FSH levels. After implant treatments, the follicular reserve was significantly (p < 0.05) larger in P-4-treated females (2012 +/- 297 resting follicles per ovary, n = 5 rats per group) than in regularly cyclic control rats (713 +/- 226 follicles per ovary, n = 7). The effects of P-4 implants on the follicular reserve were associated with a subsequently higher incidence of regular estrous cycles after P-4 treatment. These results demonstrate that P-4 prevents cyclic increases in E-2 secretion and is associated with a conservation of the ovarian follicular reserve and the maintenance of regular estrous cycle patterns, indicating a protective effect of P-4 on the age-related loss of ovarian follicles.