DIURNAL AND NOCTURNAL SERUM CONCENTRATIONS OF MELATONIN IN SHEEP ACCORDING TO THE REPRODUCTION SEASON

The sheep breeds in temperate regions manifest sexual behaviour (estrus) in the months close to the winter solstice and non-sexual behaviour in the months of the summer solstice. This rhythm can be altered by changes of the photoperiod or rather, the changing of the annual photoperiod rhythm determines the change of the reproductive rhythm. The experiment seeks to identify the photo-induced variations of blood concentrations of melatonin and their correlation to the "reproductive" reactivity of sheep in the seasonal non-estrus period and in the reproduction season. The experiments took place during the mating season (July-September 2008) and the seasonal non-estrus period in 2 groups of 10 sheep each (Merinos of Palas) with normal reproductive cycle. The blood samples were collected during daytime but also at night and were kept frozen at -18 degrees C till processing using RIDASCREEN kits. The dosing of the melatonin concentration in the 40 samples of blood serum was accomplished by ELISA. As it was expected, the highest level of melatonin concentration was recorded at night; during the night, the highest average value was recorded in the sheep in estrus (130 pg/ml). During daytime, the lowest serum concentration of melatonin was recorded in the sheep in seasonal non-estrus, when the daylight period is longer (31.5 pg/ml).