This study tested the role of melatonin in the regulation of seasonal physiological change in the pony stallion. Four 3-year-old, Welsh Mountain pony stallions were housed initially under the prevailing short-day photoperiod in December (8 of light [L]:16 h of darkness [D]) before being transferred to long days (16L:8D) on 13 January for the remaining 22 weeks of the study. On Day 76 (11 weeks later) the stallions began an 11-week period of daily melatonin treatment (20 mg orally, 8 h after lights on). Marked changes in mean plasma testosterone, beta-endorphin and cortisol concentrations occurred in response to long days and to subsequent melatonin treatment. Photostimulation produced a sharp rise in overall mean daily testosterone to a peak of 6.74 nmol/litre by Day 30. Values then fell to a nadir (3.17 nmol/litre) by Day 85, suggesting a role for melatonin in the termination of breeding activity in the horse. Cortisol and beta-endorphin values remained low throughout the first 11 weeks, but by Day 105 (Day 30 of melatonin treatment) concentrations had risen sharply, attaining a peak on Day 125 (510 pg beta-endorphin/ml, 50 ng cortisol/ml). Concentrations of both hormones had fallen by Day 77 of melatonin treatment (Day 152), perhaps as a result of refractoriness. Parallelism between beta-endorphin and cortisol suggests a pituitary origin for peripheral beta-endorphin. Diurnal variation in cortisol was observed under long days but no change in beta-endorphin was detected. Long days and melatonin treatment stimulated shedding of the winter and summer coats respectively, whereas growth rate was increased (2.03 kg/week) during the period of melatonin treatment relative to that of long days only (0.37 kg/week). The study provides evidence that the diurnal pattern of melatonin secretion mediates the reproductive and non-sexual responses to photoperiodic change in pony stallions.
|Number of pages||11|
|Journal||Journal of reproduction and fertility. Supplement|
|Publication status||Print publication - 1991|