Melatonin (GPCR-A9) is considered an important hormonal output of the circadian system mediating the entrainment of the circadian rhythms of several biological functions. Besides, it is involved in numerous physiological processes including blood pressure regulation, oncogenesis, retinal physiology, seasonal reproduction, ovarian physiology, immune function and most recently in inducing osteoblast differentiation. It interacts at either of the two melatonin receptor subtypes (MT1 and MT2) present in humans and other mammals. The MT1 subtype is present in high concentrations in the pituitary gland and the suprachiasmatic nuclei (SCN) of the hypothalamus, whereas the MT2 subtype is mainly present in the retina.
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