Effect of tryptophan administration on circulating melatonin levels in chicks and rats: Evidence for stimulation of melatonin synthesis and release in the gastrointestinal tract

The administration of L-tryptophan (Trp, 150-300 mg/kg) to rats and chicks causes a rapid and dose-dependent elevation of circulating melatonin. The elevation of serum melatonin was greater after oral compared to the intraperitoneal route of administration of the same dose of Trp (150 mg/kg). The Trp-induced increase of circulating melatonin was unaffected by prior pinealectomy but was almost abolished by a partial ligature of the portal vein. The Trp-induced increase of melatonin in the portal blood preceded that in the systemic circulation. The gut contains considerable amounts of melatonin and the Trp-induced elevation of melatonin was greater in the duodenum compared to the pineal or the blood. The enterochromaffin cells of the gastrointestinal tract appear to be the major source of the Trp-induced increment of circulating melatonin. The possibility is discussed that the sedating, sleep inducing effects of Trp are mediated by the Trp-induced elevation of circulating melatonin.