**1. Introduction**

120 Prolactin

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Light, being an environmental factor, has a significant effect on reproductive functions of animals exhibiting sensitivity to changes of the day length [1]. Among mammals there are many species displaying seasonality of reproduction, and given that, two models of seasonal sensitivity were distinguished. The first one refers to long-day animals (horses), in which reproductive processes are induced by lengthening days, i.e. in the spring. The other model concerns short-day animals, which include sheep, goats and deer; in these animals the reproduction system is stimulated and estrus takes place in the autumn and winter period [2]. In sheep the phenomenon of seasonality relates not only to reproduction but also to lactation. Following the process of mammogenesis in mammals, a mammary gland is developed, which is a complex cutaneous acinotubular gland [3]. The endocrine mechanism of entering and maintenance of lactation in sheep involves a number of hormones, which proves that the process relies basically on the activity of hypothalamus and pituitary gland [4,5,6]. One of the principal hormones conditioning both triggering and maintenance of lactation, synthesis of milk proteins, fat and immunoglobulins is prolactin (PRL), which is secreted chiefly by lactotroph cells of the anterior pituitary gland [7]. Prolactin is also produced locally by the mammary gland of mammals and does not differ immunologically from prolactin produced by the pituitary gland [8]. An important role in the process of mammogenesis and lactogenesis is assigned also to glycocorticoids, insulin and growth hormone and estrogens [9].

The fundamental feature of all living organisms is the ability to receive and process information about changes in the environment. Succession of physiological changes is synchronized with changes of environmental conditions and conditioned by the activity of

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the biological clock [10,11]. It is confirmed via seasonal changes of the activity of the hypothalamic-pituitary axis in animals kept under permanent light conditions. Thanks to constant and cyclical factors physiological processes can be synchronized with a relevant season of the year. The synthesis of melatonin is a biochemical signal informing the organism about changing light conditions [12,13,14]. Numerous studies go to show the existence of a molecular mechanism involved in deciphering of the melatonin signal which is found in the SCN (Suprachiasmatic Nucleus) and in the PT (Pars Tuberalis). Both in the SCN and PT there are several dozens of genes of the biological clock such as *Per1, Per2* or *Cry 1, Cry2*, which are associated with each other [15,16]. The melatonin profile changing in a 24-hour cycle affects the rhythmical changes in the expression of the clock genes, which is reflected via their different amounts of mRNA in the PT and SCN. The maximum expression of the *Cry1* gene occurs during the dusk period parallel to the growth of the melatonin concentration, whereas the expression of the *Per1* gene is induced at dawn [17,18]. The Effect of Physiological and Environmental Factors on the Prolactin Profile in Seasonally Breeding Animals 123

seasonal rhythm of these two hormones. The experiments demonstrated a definite influence of the day length on the parameters of ewes' milk yields. Mothers entering lactation in the period of shortening days yielded 50% less milk as compared to ewes milked in the longday period. The drop in milk yields in the shortening photoperiod resulted from the change in the prolactin secretion. The highest PRL concentration in sheep milked in the long-day period was identified in May 312.6 ± 45.2 ng/ml, at this time the concentration of melatonin amounted to 33.5± 11.2 ng/ml. As lactation progressed and days became shorter, the

Months May June July August September

pg/ml 133.5 11.3 77.8 18.9 73.3 15.1 124.7 21.6 91.3 22.2

ng/ml 312.6 45.2 185.7 34.7 247.0 48.9 151.6 33.9 43.9 10.1

As regards sheep lambed and milked in the short-day period and kept under the natural photoperiod conditions the highest level of prolactin was observed in August, i.e. 124.0 ± 48.8 ng/ml. In the first month of milking the concentration of prolactin in sheep

Months August September October November

pg/ml 87.8 15.5 82.3 15.0 77.5 16.1 93.2 17.4

ng/ml 124.6 48.8 60.5 11.1 30.8 9.7 16.8 4.1

**Table 2.** Changes in the concentration of melatonin and prolactin in sheep milked in the short-day period

As the light day shortened, the secretion of PRL declined as the level of the hormone in September was lower by 15% as compared to the concentration observed in August. A distinct drop in the prolactin level was observed in the period of the last two months of lactation, i.e. in October and November, and amounted respectively to 30.8 ± 9.7 ng/ml and 16.8 ± 4.1 ng/ml. The low concentration of prolactin already in the first month of milking and systematic growth of the melatonin secretion in the period of shortening days exerted an impact on the parameters of sheep milk yields, causing a drop of ewes' milk yields. In the

*x* SE *x* SE *x* SE *x* SE

**Table 1.** Changes in the concentration of melatonin and prolactin in sheep milked in the long-day

corresponded to its seasonal rhythm and declined in subsequent months (table 2).

*x* SE *x* SE *x* SE *x* SE *x* SE

concentration of prolactin declined, and that of melatonin increased (table 1).

Melatonin

Prolactin

Melatonin

Prolactin

period
