Progesterone Implants Delay Age-Related Declines in Regular Estrous Cyclicity and the Ovarian Follicular Reserve in Long-Evans Rats¹

^a Departments of Obstetrics/Gynecology and Neurobiology, and the Laboratory of Neuroendocrinology of the Brain Research Institute, University of California, Los Angeles School of Medicine, Los Angeles, California 90095

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 age-related loss of ovarian follicles.

¹ This study was supported by NIH Grants AG04810 and AG01512, awarded by the National Institute on Aging.

² Correspondence: John K.H. Lu, Department of Obstetrics & Gynecology, Rm. 22–177 CHS, UCLA School of Medicine, Los Angeles, CA 90095–1740. FAX: (310) 206–6531.

³ Current address: Department of Biology & Microbiology, California State University Los Angeles, Los Angeles, CA 90032.

⁴ Current address: Department of Obstetrics/Gynecology, Medical College of Virginia, Richmond, VA 23298.

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