Reciprocal Day/Night Relationship between Serotonin Oxidation and N-Acetylation Products in the Rat Pineal Gland*

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Abstract
Pineal tryptophan, serotonin [5-hydroxytryptamine (5-HT)], N-acetylserotonin, melatonin, 5-hydroxyindoleacetic acid, 5-hydroxytryptophol, 5-methoxytryptophol and 5-methoxyindoleacetic acid were measured by high-pressure liquid chromatography with electrochemical detection. A complete analysis required less than the equivalent of 2 rat pineal glands. Samples were obtained at 8 time points. A reciprocal physiological relationship was found between oxidation and N-acetylation products of 5-HT. 5-HT and the oxidation products 5-hydroxyindoleacetic acid, 5-hydroxytryptophol, 5-methoxytryptophol and 5-methoxyindoleacetic acid decreased at night, when the N-acetylation products N-acetylserotonin and melatonin increased. These observations are consistent with the hypothesis that circadian changes in the N-acetylation of 5-HT by indoleamine N-acetyltransferase is the major factor controlling circadian changes in the amounts of 5-HT and 5-HT oxidation and N-acetylation products in the rat pineal gland.