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The control of domestic mammal reproduction by light and melatonin |
Cahiers Agricultures. Volume 2, Number 2, 81-92, Mars-Avril 1993, Synthèse
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 Résumé
 Summary
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Author(s) : Philippe Chemineau, Xavier Berthelot, Benoît Malpaux, Yvon Guérin, Daniel Guillaume, Jean Pelletier, INRA, Physiologie de la reproduction, 37380 Nouzilly, France, Département des productions animales, ENV, 31076 Toulouse Cedex, France.. |
Summary : Day length variations are transmitted to the pineal gland which secretes melatonin into the blood during the dark phase of the night/day cycle. This molecule is responsible for the interpretation of day length. In the bovine and equine species, long days (LD) are stimulatory and short days (SD) inhibitory of sexual activity \; in swine sheep and goats, it is the reverse, LD are inhibitory and SD stimulatory. These effects are more marked in small ruminants of temperate regions where the photoperiod is considered as the principal external cue behind seasonal reproductive activity.
Breeds of sheep and goats from temperate latitudes present seasonal variations of breeding activity which limit their productivity. These variations are controlled by annual photoperiodic changes. Short days stimulate sexual activity, but, if used for a long time, cause the appearance of refractoriness which stops reproductive activity. Refractoriness can be broken by exposing the animals to the opposite photoperiod (LD), leading to the principle that alternations between LD and SD are essential for the photoperiodic control of seasonal reproduction. The interruption of night by light can mimic a long day (« LD ») and melatonin treatments can mimic a short day (« SD »). All these observations are important for practical applications. For out-of-season control of sexual activity, treatments using the « LD » -decreasing days or « LD »-melatonin successions were demonstrated to be very efficient in advancing puberty in young rams. They caused a dramatic increase in sperm production allowing earlier use of these animals in progeny tests for artificial insemination (AI). In adult rams, such treatments also caused a substantial increase in testicular weight (significantly different from control rams for 100 days in the spring). In the female goat, the « LD »-melatonin succession is very efficient in inducing and maintaining œstrous and ovulatory activities during the spring, leading to unusually high fertility after natural mating during this period of the year. In seasonal breeds of ewes from Northern Europe, such treatment has so far appeared less efficient than it has in she-goats. However, melatonin alone can be used after the end of May to advance the sexual season and increase fecundity. Induction of permanent reproductive activity in rams and he-goats was made possible after the observation that monthly alternations between LD and SD abolished seasonality of behavioural and spermato-genetic activities. This treatment greatly improved quantitative and qualitative sperm production in both species. The males could be used all year around to produce AI doses with no variations in sperm quality and no alteration of fertility. Short light cycles can be used in open barns by alternating « LD » and melatonin which abolishes seasonal testicular weight variations. This last result, obtained in Ile-de-France rams, is the first record of inducing a permanently high sexual activity in normally seasonal males kept under conditions of normal husbandry. On the other hand, these short light cycles were unable to abolish seasonality of ovulatory activity in the ewe. Knowledge of the various effects of photoperiod on the neurœndocrine pathways and reproductive activity in sheep and goats has therefore allowed the successful testing of light treatments in controlling seasonal reproductive activity in the field and in males raised in AI centres. In horses, LD treatment induces an earlier onset of ovulatory season after winter anœstrus, and melatonin inhibits this resumption. In other species, manipulation of light and melatonin appears less efficient in controlling reproduction. |
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