Melatonin and the gastrointestinal tract.

Melatonin has been detected in the digestive tract of several species and appears to be synthesized by the enterochromaffin cells of the gut. Although a diurnal rhythm of melatonin release has been observed in gastrointestinal tissues of birds, in other species it seems to depend on the presence of nutritional elements. Melatonin binding sites have been identified in the digestive tract of several species and it has been suggested that melatonin could act as a protecting agent of the gastrointestinal mucosa. Moreover, the potential role of melatonin as a local regulator of gastrointestinal motility has been investigated. It seems that at least some effects of melatonin on gut activity depend on an antagonistic relationship between serotonin and melatonin. The effects of melatonin on segmental gut transit both in the fasting and in the fed state, as well as on the electromyographic pattern of the small intestine in rats, are described.