Evidence for a seasonal variation in the ability of exogenous melatonin to suppress prolactin secretion in the mare.

The research paper investigates the seasonal variation in the ability of melatonin, a hormone primarily released by the pineal gland, to suppress prolactin secretion in horses. The study also explores the role of melatonin in providing photoperiodic information to the reproductive system of these animals.

Methodology

• The researchers performed four experiments to study the effects of constant melatonin administration on prolactin secretion in horses.
• The time of treatment initiation varies in each experiment, beginning before the summer solstice in May (Experiment 1), at the autumnal equinox in September (Experiment 2), or the winter solstice in December (Experiment 3).
• In Experiment 4, melatonin was administered as a timed daily injection at 5 PM for six months, beginning at the summer solstice in June.

Findings

• The study found that constantly elevated physiological concentrations of melatonin during Experiments 1-3 and an extended nighttime elevation of melatonin during Experiment 4 suppressed prolactin concentrations only during the spring and early summer months (from April to August).
• During the rest of the year, prolactin concentrations in the treated horses were similar to those in untreated mares.
• The circannual rhythm of prolactin secretion wasn’t disturbed in the presence of a continuous melatonin implant.

Conclusion

• The research concluded that the prolactin axis of the mare is sensitive to an inhibitory melatonin signal during a specific period and that at other times it tends to be refractory or resistant to this signal.
• The findings indicate a seasonal variation in the response to melatonin treatment when it comes to inhibiting prolactin secretion, suggesting a more complex relationship between melatonin, photoperiodic information, and the reproductive and prolactin axes in these seasonally-breeding animals.