Follicular growth and regulation of circulating concentrations of FSH and LH during the ovulatory and anovulatory seasons in mares

The effects of follicles and season on circulating gonadotropins were studied in mares (n = 42). Temporal relationships indicated that the decline in circulating FSH during ovulatory follicular waves was induced by inhibin from the follicles ≥13 mm. After the beginning of diameter deviation (largest follicle, 22 mm), the largest follicle alone produced increased inhibin and estradiol and accounted for the continued suppression of FSH. Based on temporal relationships, the largest follicle of a wave initially inhibited the circulating concentrations of the preovulatory LH surge, an effect mediated, at least partially, by estradiol. The follicular effect was removed three days before ovulation resulting in a final, more pronounced increase in the LH surge concentrations. A mid-anovulatory period and a transitional period were identified during the second half of the anovulatory season and were delineated by an increase in the diameter of the largest follicle to >21 mm a mean of 52 days before ovulation. Follicular waves developed during the two periods and were temporally associated with circulating FSH surges. Temporal associations indicated that the declining portion of the FSH surge was induced by follicular inhibin. The follicles ≥22 mm during the transitional period and early ovulatory season produced increased inhibin and this increase was counteracted by a season-mediated stimulation of circulating FSH. This resulted in a similar wave-associated decline in circulating FSH throughout the second half of the anovulatory season and early ovulatory season. Circulating LH was reduced during the anovulatory season and increased during the ovulatory season by mediation of seasonal effects. It was concluded that the circulating FSH concentrations during follicular waves result from negative follicular effects, mediated by inhibin and estradiol, and positive seasonal effects. The balance between negative and positive influences is maintained across the anovulatory and ovulatory seasons, resulting in similar concentrations of circulating FSH during the two seasons. In addition, circulating LH during follicular waves is regulated by positive seasonal effects and by negative follicular effects that are mediated, at least partly, by estradiol. These effects result in preovulatory surges in circulating LH during the ovulatory season.