An unpaired neuroendocrine and highly vascularized organ, the pineal gland is located in the middle of the brain nearly in all vertebrates. Its main function is to synthesize the melatonin hormone (N-acetyl-5-methoxytryptamine) in the pinealocytes, which converts light and dark information to complete body physiology. Light inhibits melatonin production whereas darkness stimulates it. Melatonin is synthesized from amino acid tryptophan. Dietary amino acid tryptophan is converted into melatonin in four consecutive biochemical steps.This conversion process includes: conversion of amino acid tryptophan to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase 1 (TPH1); synthesis of 5-hydroxytryptamine (5-HT or serotonin) by aromatic amino acid decarboxylase; formation of N-acetylserotonin (NAS) by arylalkylamine N-acetyltransferase (AANAT), and production of melatonin by hydroxyindole-O-methyltransferase (HIOMT) (also termed N-acetylserotonin methyltransferase or ASMT). The pineal gland in rat exhibits circadian rhythms of release for all four tryptophan enzymatic derivatives, with larger levels at night. 5-HTP and serotonin are both released at quite high levels during the day and increase much more at night, in contrast to NAS and melatonin, which are hardly detectable during the day.