Differential luteolytic function between the physiological breeding season, autumn transition and persistent winter cyclicity in the mare.

This study explores the increased occurrence of luteolytic failure in mares during autumn, which leads to prolonged function of the corpus luteum. The researchers examined potential seasonal changes in the levels of progesterone, PGF(2alpha), and prolactin during the estrous cycle. The findings suggest that the changes observed during autumn are not due to any alterations in the uterus’ capability to produce PGF(2alpha) or the corpus luteum’s sensitivity to PGF(2alpha), but rather, an alteration in the signal for PGF(2alpha) release.

Research methodology and scope

The researchers studied the estrous cycles of horses in summer and autumn, focusing on the natural and induced luteolysis process. They collected blood samples at regular intervals from untreated control mares and treated mares. They also performed ultrasonography for ovarian events and observed reproductive behavior by bringing a stallion close to the mares.

• The investigation involved testing a single estrous cycle of 12 mares in June and July, and comparing it to multiple estrous cycles of the same 12 mares plus 8 additional ones in September through December.
• For blood collection, they used different schedules for control and treated mares. In control mares, blood was collected every 6h from day 13 to 17 post-ovulation. In treated mares, blood collection took place at different time points for a total of 5 days following intrauterine saline infusion on day 7.

Testing and analysis

Progesterone and prolactin were measured in daily samples, while PGFM was determined in frequent plasma collections using the radioimmunoassay (RIA) technique. The researchers used analysis of variance (ANOVA) to compare the duration of ovarian luteal and follicular phases, progesterone, and prolactin levels, as well as PGFM secretion during luteolysis between treatments and seasons.

• They found that mean progesterone levels declined from June to December in all groups.
• Pulses of PGFM were identified in specific days in controls and saline-infused mares.
• The incidence of spontaneously prolonged corpus luteum function (SPCL) increased during autumn in the control group.

Findings and Conclusion

The study showed that circulating prolactin increased around natural or induced luteolysis. Also, the PGFM profiles in autumn were largely similar during spontaneous or saline-induced luteolysis compared to summer. The study concluded that the observed seasonal changes in luteolytic function are not caused by changes in the uterus’s production of PGF(2alpha) or the corpus luteum’s sensitivity to it. Instead, it’s caused by a modification in the signal for PGF(2alpha) release.