Circadian Rhythms of Pineal Function in Rats

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Abstract
I. Pathway for Melatonin Synthesis in Rat Pineal Glands (Fig. 1) MELATONIN is believed to be synthesized in the pineal gland from serotonin by a two-step pathway. N-Acetyltransferase (NAT) catalyzes the acetylation of serotonin (5HT) by acetyl coenzyme A (AcCoA) to form N-acetylserotonin (NAS); hydroxyindole-O-methyltransferase (HIOMT) catalyzes the methylation of NAS by S-adenyosyl methionine (SAM) to make melatonin (MEL, MT). The activities of the enzymes are easily measured with radiochemical assays (1–3). Serotonin is found in pinealocytes (70%) and in the sympathetic nerve terminals (30%) in the pineal gland. It is thought that pinealocytes take up tryptophan from the blood but that the presence of serotonin in the nerve terminals represents uptake by the terminals (4). Tryptophan-5-hydroxylase and aromatic L-amino acid decarboxylase convert the tryptophan to serotonin (4). There are several aspects of pineal metabolism which make it unique. First, the pineal gland appears to be the principal source of rhythmically produced melatonin and the pineal gland was the tissue in which the indole hormone, melatonin, was discovered (5).

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