Plasma FSH, inhibin A and inhibin isoforms containing pro- and -alphaC during winter anoestrus, spring transition and the breeding season in mares.

This study examines how the reproductive hormones FSH, inhibin A, and inhibin isoforms containing pro- and -alphaC are related to the ovarian cycles of mares during winter anoestrus, spring transition, and breeding seasons. The results suggest that FSH regulates the growth of anovulatory and preovulatory follicles in the spring and that Inhibins likely contribute to controlling the release of FSH from the pituitary gland during the transitional and breeding periods in mares.

Objective of the Study

• The primary objective of this research was to examine the connection between the resumption of ovarian cyclicity in mares during spring and the circulating concentrations of FSH, inhibin A, and inhibin isoforms containing pro- and -alphaC.
• Additionally, the researchers sought to study and compare these hormone levels during winter anoestrus (a period of sexual inactivity), spring transition, and the breeding season in mares.

Methodology

• Ten mares were studied from February (a period of winter anoestrus) to their second ovulation in the breeding season.
• Moreover, four additional mares were monitored during one estrous cycle for comparative analysis.
• The researchers utilized transrectal ultrasonography to monitor the growth and regression of ovarian follicles in the mares.
• The frequency of blood samples taken ranged from thrice a week to once daily depending on follicular activity.

Findings

• The concentrations of FSH, oestradiol, inhibin A, and inhibin isoforms were found to be low during deep winter anoestrus, a time of minimal follicular activity in the ovaries.
• During spring transition, an increase in FSH concentration preceded each wave of follicular emergence.
• Concentrations of the inhibin hormones were higher during growth of anovulatory follicles in spring transition than during winter anoestrus.
• Plasma concentrations of oestradiol and inhibin A were significantly higher during the growth of preovulatory follicles than during the growth of transitional anovulatory follicles, yet the concentrations of inhibin pro-alphaC isoforms did not differ between the two types of follicles.
• Inhibin pro-alphaC isoforms, but not inhibin A, reached a peak on the day of ovulation.

Conclusions

• The results of the study clearly point towards FSH being a regulator in the growth of spring anovulatory and preovulatory follicles.
• The inhibins appear to be key players in the negative feedback on the release of FSH from the pituitary gland during the transitional period and the breeding season in mares.