**Author details**

134 Prolactin

**5. Summary** 

the concentration of melatonin on a long day.

concentration of melatonin during a short day, but it caused a significant growth (*P*< 0.05) of

During a lengthening day the total PRL concentration in the plasma of sheep from the control group was markedly higher (*P*< 0.001) as contrasted with shortening days (132.28 19.87 vs. 44.41 8.27 ng/ml). Intraventricular infusions of exogenous leptin reduced the concentration of prolactin during SD (*P*< 0.001) as compared to the values observed in ewes from the control group. The average concentration of endogenous orexin was higher (P< 0.01) during short days (0.59 0.05 ng/ml) as opposed to LD (0.39 0.01 ng/ml) in the control group of sheep. Exogenous orexin caused a growth (*P*< 0.05) of the plasma concentration of endogenous orexin respectively on a long and short day (0.62 0.01 ng/ml and 0.71 0.03 ng/ml), whereas the effects of leptin reduced (*P*< 0.05) the concentration of orexin during LD and SD. Based on the conducted experiments it was concluded that orexin B and anorectic hormone – leptin directly interact closely with each other, regulating not only the processes of metabolism but influencing jointly release of melatonin and prolactin,

So far it has been believed that milk yields in mammals are determined by genetic and environmental factors. In recent years, though, a special focus has been placed on light, being the modulator of the prolactin level. In farm animals changes of the light day play a very important role as they determine their yields. The length of the light day, and in particular the melatonin profile, is of special importance in sheep as they determine reproductive processes, in which lactation is the last stage of reproductive physiology. Experiments carried out on sheep demonstrated that both the melatonin profile and prolactin profile retain features of a seasonal rhythm depending on the day length. The synthesis of melatonin by the pineal gland is a biochemical signal informing the organism about the break of the day or night. This hormone regulates activities of numerous organs. Until now the activity of melatonin has been associated with the impact on the reproductive system. Experiments carried out in recent years have shown that melatonin can modulate the level of prolactin. Under natural conditions the maximum concentration of prolactin in sheep blood is observed in the long-day period, and at this time the level of melatonin drops. The lowest concentration of prolactin is recorded during short days, when the level of melatonin is the highest. Shortening of the day length or long-term injection of melatonin in the period of physiologically increased concentration of prolactin leads to a reduced secretion of this hormone. Lactation in sheep involves a number of hormones, and for that reason in recent years a special focus has been placed on the role of orexins in the regulation of the prolactin secretion. Experiments conducted demonstrated that orexin A exerts a greater stimulating effect on the level of the PRL secretion in the summer season than in the winter season. The weaker reaction of sheep's pituitaries to orexin during short days as compared to the response during long days is a phenomenon of resistance of lactotroph cells to the orexin signal. Such a reaction of endocrine cells can be explained by the seasonal rhythm of biosynthesis and secretion of orexin in sheep, regulated by the photoperiod.

and the interactions additionally depend on the prevalent photoperiod.

Edyta Molik and Dorota A. Zieba *Agricultural University in Krakow, Poland* 

Tomasz Misztal *The Kielanowski Institute of Animal Physiology and Nutrition, Polish Academy of Sciences, Poland* 
