Interactions of follicular factors and season in the regulation of circulating concentrations of gonadotrophins in mares.

This research discusses the effects of follicular changes and seasonal factors on the circulating concentrations of reproductive hormones in mares. Using a sample of 13 mares and a process of follicle ablation, the study shows the direct relationship between follicle size, hormone concentrations, and ovulation periods.

Research Methodology

• The study involved 13 mares and used a method known as follicle ablation to manipulate follicular function.
• Over four 13-day periods, follicle growth and hormone concentrations were measured at different times relative to the first ovulation: mid-anovulatory, early-transitional, late-transitional, and ovulatory periods.
• All follicles larger than 5mm in diameter were removed at the start of each period. Then, in one group, no further ablations were performed (all-follicle group). In another group, all follicles larger than 5mm were removed whenever a new follicle grew larger than 10mm (0-follicle group).

Results

• New follicular waves emerged between 1.5 to 2.5 days in all periods for the all-follicle group.
• Follicle activity and growth increased between the mid-anovulatory and transitional periods. However, the concentration of Follicle-stimulating hormone (FSH) – related to the stimulation of follicle waves – didn’t show any difference throughout the study.
• Total inhibin, a hormone that inhibits FSH, increased between day 3 and day 6 in the all-follicle groups but not in the 0-follicle groups and was significantly higher during the late-transitional and ovulatory periods.
• Meanwhile, FSH decreased in all-follicle groups between day 3 and day 6, while in the 0-follicle groups, FSH did not decrease.
• Luteinizing hormone (LH), which stimulates ovulation, was higher during the late-transitional and ovulatory periods and showed no differences between the all-follicle and 0-follicle groups.

Conclusion

• The researchers concluded that follicles during a wave produce inhibin throughout all periods, which suppresses circulating FSH levels.
• Increased circulating inhibin induced by the growth of large follicles (>21mm) during late-transitional and ovulatory periods counteracts a seasonally-driven stimulatory effect on FSH.