Melatonin is a hormone and neurotransmitter that is involved in mostly regulating sleep. It is synthesized in the pineal gland from serotonin. Although a role in mood has also been suggested, based on the efficacy of a melatonin agonist and 5-HT2C antagonist, agomelatine. Melatonin acts on two receptor subtypes: MT1 and MT2. Melatonin increases GABA and serotonin concentrations in certain brain regions, especially those involved in sleep; it also enhances pyridoxal-kinase activity, hence accelerating the synthesis of GABA, dopamine and serotonin. It also inhibits gonadal development and is involved in protective skin colouration in response to sunlight.
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