ABSTRACT
ABSTRACT Melatonin is an important hormone of the neuroendocrine system having a wide range of functions, from being an antioxidant to the regulation of the circadian and rhythmic annual seasonal cycles. Melatonin levels in terms of dynamic secretions from the pineal gland vary between day and night, providing information about the environmental day length for the hypothalamic-pituitary-gonadal (HPG) axis. The endogenous circadian rhythm of melatonin is generated in the suprachiasmatic nucleus (SCN) of the hypothalamus. The mammalian pineal gland has a rich network of noradrenergic innervations. Noradrenergic stimulations as outputs from the supracervical ganglion increase severalfold during the nighttime. The nocturnal regulation of melatonin secretion is based prominently on the activation of protein kinase A, which thereby brings about the phosphorylation of the cAMP response-binding protein (CREB). MT1 and MT2 are the two prominent G-coupled melatonin receptors, having seven transmembrane domains, although both receptors differ in their binding afnities to melatonin. The seasonal control of reproduction in animals is an adaptive strategy to prevent the birth of offspring at times of inclement weather and reduced availability of nutrients. Seasonality in domesticated animals is a serious impediment to maximizing production. Hormones such as progestins, eCG, hCG, and PGF2alpha analog have been used to try to evade seasonality in addition to utilizing the ram effect. Recently, dopamine antagonists have been successfully used to bring about cyclicity in anestrus sheep in nonbreeding season. The development of a marker for out-of-season breeding in sheep is a current area of research, especially in relation to SNP markers of the MTNR1A gene.
