Effects of melatonin and thyrotropin releasing hormone on mares during the nonbreeding season.

The study investigates the impact of two hormones, melatonin and thyrotropin-releasing hormone (TRH), on horsess’ ovarian activity and estrus, particularly during the non-breeding season. However, it concludes that neither melatonin nor TRH directly stimulated the mares’ ovarian activity.

Study Methodology

• The experiment included 15 mares that were functionally anestrous (not ovulating).
• Among these, five were administered with intravaginal sponges containing 0.75 g of melatonin consistently from December 16 to February 3, and were given daily injections of saline.
• Five other mares received daily injections of 100 micrograms of thyrotropin-releasing hormone (TRH) and control sponges.
• The remaining five mares were the control group, receiving control injections and control sponges.

Observations and Findings

• Mares that received intravaginal sponges with melatonin exhibited increased concentrations of melatonin in their systemic plasma that lasted for a minimum of seven days, with levels at least ten times higher than normal nighttime hours.
• The TRH injections significantly increased the thyroid-stimulating hormone concentrations within 60 minutes after injection, but there was no detectable increase in the prolactin hormone.
• All three groups of mares showed increases in their ovarian sizes shortly after the commencement of treatment.
• Concentration surges of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were noted in plasma that coincided with the insertion of sponges or injections of TRH or saline in all mare groups.

Conclusions

• The researchers hypothesize that the placement of intravaginal sponges may have caused the release of LH and FSH, possibly through a neuroendocrine reflex, which could have triggered the observed ovarian response.
• Alternatively, they considered the possibility of an unknown environmental factor stimulating the ovarian activity, which was not examined in the study.
• Crucially, the study concluded that neither melatonin nor TRH directly augmented or inhibited this nonspecific response, contrary to the initial hypothesis.