Sheep is a breeding species depending on the season. However, they may show estrus in tropical regions throughout the year and periodically (Jainudeen ve Hafez, 1983). In the northern hemisphere, including Turkey, sheep have seasonal breeding properties. The incidence of estrus in ewes in Turkey is concentrated in the last period of the summer and autumn. The interactions between the hormones produced by the hypothalamus, anterior pituitary, and ovaries play a role in regulating the estrus cycle (Roche, 1996; Hunter et al., 2004). GnRH is secreted from the hypothalamus, FSH, LH, prolactin, and oxytocin secreted from the pituitary gland, estrogen secreted from ovaries, and PGF2α secreted by uterine endometrium. In addition, the relationship between the uterus and ovarium is essential because of the secretion of PGF2α, which causes the destruction of the corpus luteum and stops the synthesis of progesterone hormone (Bartlewski, 2001; Davies, 2005). The effects of photoperiodic on seasonal reproductive activity in sheep and these effects are mediated by the pineal gland and its hormone melatonin (Gökdal ve Baş, 1996). The pineal gland shows a rhythm of night and day. It also ensures the adaptation of the creature against light changes production and synthesis start with darkness and ends with light. Light stimuli cause fluctuations in the amount of melatonin. The prolongation of the light period or the sudden emergence of light stops melatonin production (Neils, 1997; Yilmaz, 1997).
For this reason, melatonin has been given a symbolic name as a biochemical descriptor of darkness (Neils, 1997). Melatonin hormone is a type of neuronal hormone synthesized from the pineal gland due to photoperiod. This mechanism takes place under the control of the nervous system. Melatonin released from the pineal gland is another hormone that effectively initiates the estrus cycle (Zhdanova, 2005). Light signals from the retina are also reduced due to reduced daylight. As a result, the level of melatonin released from the pineal gland begins to increase (Zarazaga et al., 2010). As a result of photoperiodic changes, the melatonin hormone is secreted by stimulating suprachiasmatic nuclei, light-sensitive cells. The melatonin hormone influences the hypothalamus and increases the gonadotropin-releasing hormone produced in the neurosecretory cells through the hypothalamus-pituitary portal veins and sinusoidal capillaries. Afterward, it stimulates gonadotropin cells and causes the secretion of FSH and LH, which enables the development of follicles (Senger, 1995).
Thus, seasonal reproductive activity begins. Starting with the hormone melatonin, GnRH, FSH, and LH stimulate the release of estrogen hormone, progesterone, and inhibin from the ovary. These secreted hormones control the release frequency of GnRH, FSH, and LH by positive and negative feedback mechanisms (Ekiz, 2005). There are synchronization practices that enable sheep to show estrus during and outside the breeding season. Progestagens, estrogens, PGF2α and its analogs, gonadotropins such as PMSG, GnRH, and hCG are widely used combinations in estrus synchronization practices (Akçapınar et al., 1998: Yaralı and Karaca, 2004). However, there are limited studies on estrus synchronization due to the change in plasma melatonin concentrations with the dark application, especially outside the breeding season. This study aimed to investigate the effect of the dark application on plasma melatonin concentration and reproductive performance in sheep's out-of-breeding season in Turkish native Akkaraman sheep breeds.