Role of Melatonin and the Biological Clock in Regulating Lactation in Seasonal Sheep

Impact of light on animal behavior has been known for a long time—from 1925, Rowan [30] showed that lighting conditions influence gonad activity in birds and the related processes are controlled not only by means of intraorganic signals. Studies carried out in subsequent years have established that, also in mammals, the gland reacting to changes in light conditions is the pineal gland, producing a substance called melatonin. Biosynthesis of melatonin in most animals studied to date occurs at a rhythm dependent on the photocycle. The highest concentrations of this hormone—often called “the hormone of darkness”—are recorded at night. Seasonal changes in melatonin secretion conditioned by activity of the biological clock, known also as “biochemical calendar”, are the key signals in the annual reproductive cycles of animals exhibiting seasonality of reproduction. Seasonality in sheep refers not only to the reproduction itself but also to lactation. One of the main hormones conditioning initiation and maintenance of lactation, synthesis of milk proteins, fat and immunoglobulins is prolactin (PRL), secreted primarily by lactotrophic cells in the adenohypophysis. Prolactin is also produced locally by the mammary gland—the hormone of this origin is identical to prolactin secreted by the pituitary gland. Until now, it was considered that the level of milk production in mammals is determined by both genetic and environmental factors. However, in recent years, many studies focused on the role of light as a modulator of prolactin levels. In livestock, changes in light-period length play a very important role as this determines their productivity and milk yield. Photoperiod is particularly important in short-day breeder animals (sheep), for which the length of light period is associated with changes in melatonin level. The modulating effect of melatonin on secretion of prolactin may take place via two different mechanisms. One is associated with the circadian rhythm, wherein—directly or through the medium of a factor popularly termed “tuberalin”—melatonin stimulates the release of prolactin. However, this effect is short-lived and is most likely applicable only to prolactin stored in lactotrophic cells of the pituitary. The second mechanism regulating the secretion of melatonin and prolactin is associated with the annual rhythms of secretion—melatonin, due to its lipophilic characteristics, has a direct effect on the secretion of prolactin. Under natural conditions, the maximum concentration of prolactin in the blood of sheep is observed over the long-day period, during which the melatonin level decreases. The lowest prolactin concentration is observed over the short-day period, where melatonin levels are at their highest. Changes in secretion of prolactin during lactation in sheep undoubtedly affect the amount of milk produced