Melatonin has received enormous attention from the news media because of the possible role of this hormone in controlling normal sleep rhythms, as a treatment for jet-lag and as a means to slow the aging process. The list of possible roles and potential uses of melatonin does not stop there, but at this time much of our understanding of melatonin in most biological processes is very limited.
However, one exception is the role of melatonin in timing the onset and cessation of the breeding season in mammals. In this process, melatonin is not a timekeeper or clock, but acts to synchronize inherent rhythms to the correct time of year. Secretion of melatonin is confined to the nighttime hours and for this reason it is referred to as the hormone of darkness. Information regarding the time of year is therefore measured by the length of the night and the duration of increased melatonin secretion.
Although domesticated by man, the horse has retained a seasonal pattern of reproductive activity confined to the spring and summer months. The importance of day length in controlling the timing of the breeding season was recognized nearly 60 years ago, and today many broodmare farms employ artificial lighting--beginning in early December--as a practical method to advance the onset of the breeding season.
While most mares cease reproductive activity during the winter months, California veterinary researchers reported more than two decades ago that during the winter a small proportion of mares continue to exhibit estrous cycles. However, mares that display activity in one winter may become reproductively inactive the succeeding winter, suggesting it is not an all-or-none phenomenon. It is also of interest that while reproductive rhythm may be disturbed in some mares, other seasonal rhythms are not. It would appear the occurrence of estrous cycles during the winter stems not from a failure to recognize the change in day length, but rather from misinterpretation of the signal provided by melatonin.
Since melatonin synchronizes the annual rhythm of reproductive activity, it may provide a convenient alternative to lighting programs. The concept of using melatonin as "time in a bottle" for application to the horse breeding industry is hampered by the lack of a method to reduce melatonin secretion to simulate the long days and short nights of the summer months. An alternative approach might be to alter the response of the reproductive system to melatonin. The strategy would be to induce refractoriness to melatonin such that the animal no longer shows a response to the hormone.
Research in France has demonstrated that administration of melatonin for six months, beginning in the summer, resulted in an earlier onset of the breeding season in the subsequent year, despite the fact that mares did not receive supplemental lighting. The apparent escape of the reproductive system to constant exposure to melatonin is referred to as refractoriness.
Treatment with melatonin may offer many advantages to conventional lighting programs because of the ease of administration and cost. If incorporated into a biodegradable matrix it may be possible to reduce administration of melatonin to 2-3 month intervals, thereby providing enormous flexibility in the management of the broodmare.
Additional applications for melatonin include the suppression of estrous cycles and, conceivably, timing the occurrence of parturition so that foals are born predominantly during the daytime rather than at night. Future research may show "time in a bottle" to be a realistic tool for the equine practitioner.