Duodenum is not a consistent source of melatonin in rats.

Markedly increased melatonin levels in plasma have been observed in response to tryptophan administration. This post-tryptophan melatonin increase has been attributed to the duodenum. Because extra-pineal sources of melatonin may be important in interpreting the meaning of altered melatonin production observed in patient populations, this work was undertaken to confirm whether melatonin is produced in the duodenum and to know whether the duodenum need be considered when investigating the circadian control of melatonin production. We measured melatonin in rat duodenum by HPLC both under basal conditions and following tryptophan load. No melatonin was observed in duodenum under conditions of 2.5 ng/g measurement limits. Neither was there any evidence found for the melatonin precursor N-acetylserotonin. Treatment with N-acetylserotonin resulted in increased melatonin content in the pineal gland, but no evidence for melatonin in the duodenum. In vitro incubation of duodenum tissue with 5-hydroxytryptophan, 5-methoxytryptophan, or N-acetylserotonin revealed no detectable melatonin synthesis, and incubation with melatonin revealed no detectable melatonin degradation. The lack of confirmation of melatonin content and the lack of either synthetic or degradative enzyme activity in duodenum tissue suggest that melatonin production from duodenum need not be considered in human or animal studies of melatonin production.