Follicle-stimulating hormone pulse amplitude decreases with the onset of the breeding season in the mare.

This research study investigates how the hormone FSH (Follicle-Stimulating Hormone) changes in female horses as they transit into the breeding season. The results show that the intensity of FSH pulses decrease during the transition, and these changes could potentially be influenced by the development of follicles or changes in brain signaling.

Research Methodology

• The study involved five mature female horses that were not in heat (anestrous).
• The mares were exposed to a natural increase in day length.
• Blood sampling took place daily from February 1 until the first ovulation of the breeding season (April 14 +/- 3.7 days).
• Every two weeks, more frequent blood sampling occurred, consisting of samples every 15 minutes over a 12-hour period.
• Blood samples were either collected through a jugular venipuncture or a jugular cannula.

Findings

• Mean daily concentrations of FSH in the blood, as determined by RIA (Radioimmunoassay), decreased during the transition into the breeding season.
• Patterns of FSH levels detected through frequent sampling were pulsatile, meaning the hormone is released in pulses rather than steadily.
• FSH pulse amplitude (the size of the pulses) decreased during the seasonal transition.
• The decrease in the size of FSH pulses was associated with the overall decrease in mean FSH concentrations in the blood.

Possible Interpretations

• The research suggests that the decrease in FSH pulse amplitude may reflect either the involvement of a follicular product from developing follicles or a change in the stimulation of pituitary FSH release from the hypothalamus in the brain.
• This indicates that the mares’ bodies are adjusting hormone levels in response to the transition into the breeding season, potentially to help optimize the chances of successful reproduction.