Changes in plasma cortisol concentrations during the ovulatory cycle of the mare.

The research article investigates and discusses changes in the concentration of cortisol in the blood of mares during their ovulatory cycle. It indicates that cortisol levels fluctuate, with a peak during the non-estrus phase (dioestrus) and a decrease during the fertile phase (estrus), suggesting that such changes might facilitate ovulation in horses.

Methods and Key Findings

• The researchers took daily blood samples from four mares throughout eight ovulatory cycles to monitor cortisol levels. This hormone, produced by the adrenal glands, is responsible for various physiological responses in the body such as mounting a stress response.
• The researchers found that the mean cortisol concentrations on specific days before ovulation during the non-fertile phase (dioestrus) were notably higher than the days leading up to ovulation (estrus). According to their findings, day -10 from ovulation had the highest cortisol level measurement.
• Data showed that a distinctive spike in cortisol levels occurred in five out of eight cycles, specifically on a day in the late dioestrus phase. This denotes that the adrenals secrete cortisol more actively during dioestrus than during oestrus.
• To ensure the accuracy of cortisol readings, the researchers applied extraction of plasma samples with petroleum ether or chromatography before the assay. This process helped to eliminate potential interference from progesterone and its metabolites. Despite this, the pattern of cortisol concentration remained consistent with higher levels during the non-estrus phase and lower levels during the fertile phase.

Implications

• Interestingly, the study noticed a negative correlation between the highest follicular diameter at ovulation and the cortisol concentration for that cycle. This implies that a drop in cortisol values during the fertile phase may potentially favor full follicular growth and ovulation.
• Such findings might add essential knowledge to the understanding of how hormones regulate the ovulatory cycle in horses. Additionally, it could provide insight that can potentially be extended to further studies on fertility and conception in other mammals.