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  Discussion on Melatonin
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Posted on Wednesday, Mar 8, 2000 - 9:48 am:

Does anyone know if this is a safe supplement for horses? Someone suggested it for my horse that is very anxious and they thought it would help to calm her and let her relax when in the barn. Based on what it does for people I was wondering what if any the effects would be on horses, I understand that it is a hormone produced in our bodies, is it the same in theirs? Also if there would be any ill effects from giving it? And does anyone know what the recommended does of it would be...Some packages say not to exceed a certain amount of pills in a 24 hour period - so I take it is not given by weight. Any help would be appreciated.

Posted on Thursday, Mar 9, 2000 - 10:57 am:

Hello Karen,
I would not recommend it. Melatonin is one of the very basic hormones that has far reaching effects on a number of different endocrine and physiologic systems. There have been small studies of short time use but no studies on safety and effects of long term administration, see below.
DrO


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

J Reprod Fertil Suppl 1991;44:115-25
Argo CM, Cox JE, Gray JL
Department of Environmental and Evolutionary Biology, University of Liverpool, U.K.

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.
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