Reproductive hormone profiles in mares during the autumn transition as determined by collection of jugular blood at 6 h intervals throughout ovulatory and anovulatory cycles.

The research essentially studies the secretion pattern of follicle-stimulating hormone (FSH) in horses during two ovulatory cycles in autumn—it is the transition period before the last ovulation of the season, and this pattern is then compared with the profiles of other reproductive hormones and follicular growth. This comparison can help identify changes which may lead to stopping the follicular cycles.

Objective of the Research

• The main objective of this research was to clearly define the secretion pattern of the Follicle-Stimulating Hormone (FSH) in horses during two cycles of ovulation in the autumn season.
• The research aims to follow the FSH secretion pattern for 24 days post the last ovulation of the season, and compare it with the profiles of other reproductive hormones and follicular growth. This comparison is done to identify changes linked to the termination of follicular cycles.

Methodology of the Study

• Jugular blood samples were taken every 6 hours from ten light horse mares for a period of six weeks in the autumn season.
• These samples were then assayed for several hormones – FSH, Luteinising hormone (LH), prolactin, inhibin, oestrone conjugates, and progesterone.

Results and Findings

• The study found that luteolysis, the process of corpus luteum degradation, occurred earlier than expected. Also, the concentration of periovulatory oestrone was significantly lower in the last cycle compared to the penultimate one, but the levels of inhibin remained the same.
• In both ovulatory and anovulatory cycles, lower concentrations of FSH were noticed around the time of expected ovulation, and higher concentrations between days 9 and 11, which were usually after luteolysis.
• However, the FSH nadir during ovulation was elongated in the last cycle compared to the second last. There was also no significant difference found between peak and trough values in anovulatory cycles.
• Between day 5 and day 8, the FSH interpulse interval was approximately 2 days and did not vary in successive cycles.
• There were also progressive changes in the LH profile as horses entered acyclicity – the LH surge ended sooner in the last cycle than in the second last, and did not occur when expected during acyclicity.
• Some random prolactin pulses were noticed during luteolysis in similar proportions of ovulatory and anovulatory cycles.

Conclusions

• This study indicates that inadequate gonadotropin stimulation in early dioestrus may be a critical event leading to suboptimal follicular and luteal development, causing acyclicity. Furthermore, the timings of changes in pituitary and ovarian hormones and follicular growth become less synchronised during the autumn transition, which may contribute to the cessation of cyclicity.