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 (P4) treatments on estrous cyclicity and the loss of ovarian follicles during aging. Young rats received repeated treatments with P4 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 P4, estradiol (E2), LH, and FSH in control animals, P4-implanted rats exhibited elevated serum P4 but low E2, LH, and FSH levels. After implant treatments, the follicular reserve was significantly (p < 0.05) larger in P4-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 P4 implants on the follicular reserve were associated with a subsequently higher incidence of regular estrous cycles after P4 treatment. These results demonstrate that P4 prevents cyclic increases in E2 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 P4 on the agerelated loss of ovarian follicles.