a substance formed by the mammalian pineal gland, which appears to depress gonadal function in mammals and causes contraction of amphibian melanophores; a precursor is serotonin. Melatonin is rapidly metabolized and is taken up by all tissues. It is involved in circadian rhythms.
[melanophore + G. tonos, contraction, + -in]
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Examples: glitazone, GI cocktail, etc.