Effect of oral melatonin treatment on the seasonal physiology of pony stallions.

This research investigates the role of melatonin in moderating seasonal physiological changes in pony stallions, specifically related to reproductive activities and growth patterns. The study found that altering light exposure periods and administering melatonin led to significant changes in hormone levels and physical development, pointing to melatonin’s influence in seasonal equine biology.

Research Context and Methodology

• The researchers conducted the study over 22 weeks with four 3-year-old Welsh Mountain pony stallions.
• The stallions were initially kept under short-day photoperiod conditions (8 hours of light and 16 hours of darkness) resembling December’s natural photoperiod.
• On the 13th of January, the ponies’ photoperiod was changed to long day conditions (16 hours of light and 8 hours of darkness).
• After 11 weeks into this photoperiod, the stallions began receiving daily oral melatonin treatment.

Physiological Responses and Observations

• Notable changes in plasma testosterone, beta-endorphin, and cortisol concentrations were observed.
• Under long-day conditions, the average daily testosterone level quickly rose, peaking by Day 30 of the study. By Day 85, these levels had dropped, indicating melatonin’s likely role in the end of breeding activity in horses.
• Cortisol and beta-endorphin levels remained low for the initial 11 weeks, but sharply increased by Day 105, the 30th day of melatonin treatment.
• While cortisol exhibited diurnal variation under long days, no such change was observed for beta-endorphin.
• Similarities in beta-endorphin and cortisol levels suggest that peripheral beta-endorphin may originate from the pituitary gland.

Impact on Coat Shedding and Growth

• Extend photoperiod and melatonin treatment spurred the shedding of winter and summer coats, respectively.
• Growth rate during the melatonin treatment period (2.03 kg/week) was higher than during the long days only period (0.37 kg/week).

Conclusions

• The research suggests that melatonin secretion’s diurnal pattern facilitates the reproductive and non-sexual adaptive responses to photoperiod change in pony stallions.