A detailed study of hormonal profiles in mares at luteolysis.

This research observes the hormonal fluctuations in mares during their oestrous cycle, focusing particularly around luteolysis, the breakdown of the corpus luteum in the ovaries. The findings indicate a connection between the hormonal release patterns of PGFM and oxytocin in mares, a pattern similar to that in ruminants, and suggest that the hormone prolactin may play a role in follicle maturation.

Study Methodology

• Jugular blood samples were taken from six mares at 4-hour intervals during a single oestrous cycle—specifically from day 10 post-ovulation until day 3 of oestrus. These samples were tested for various hormones: prostaglandin metabolite, oxytocin, prolactin, progesterone, and oestrogen conjugates.
• A separate test was conducted to assess the response of oxytocin and prolactin in mid-dioestrous mares given doses of PGF2alpha.

Findings

• A number of significant pulses of PGFM, oxytocin, and prolactin were detected in the mares during the three-day period starting on day 13. Large pulses of PGFM and oxytocin coincided with a decrease in progesterone concentrations, indicating a correlation between these hormones.
• The first large prolactin pulse occurred as progesterone concentrations were at their lowest and preceded a rise in oestrogen conjugate levels by approximately 2 days. This suggests that prolactin might have a role in the maturation of follicles rather than in luteolysis.
• Both doses of PGF2alpha significantly boosted prolactin concentrations, while only the larger dose increased oxytocin concentrations.

Conclusions

• The results indicate that in mare oestrous cycles, there are patterns of hormonal secretion consistent with the ruminant model of luteolysis initiation, which posits that pulses of the hormone oxytocin trigger sporadic release of uterine PGF2alpha.
• The researchers also speculate that prolactin may play a role in the maturation of ovarian follicles based on the timing of large prolactin pulses and changes in progesterone and oestrogen conjugates.