THE CONTRIBUTION OF EXTRAPINEAL SITES OF MELATONIN SYNTHESIS TO CIRCULATING MELATONIN LEVELS IN HIGHER VERTEBRATES

While the production of melatonin in higher vertebrates occurs in othe r organs and tissues besides the pineal, the contribution of extrapine al sites of melatonin synthesis such as the retina, the Harderian glan ds and the gut to circulating melatonin levels is still a matter of de bate. The amount of melatonin found in the gastrointestinal tract is m uch higher than in any other organ including the pineal and the gut ap pears to make a significant contribution to circulating melatonin at l east under certain conditions. The gut has been identified to be the m ajor source of the elevated plasma concentrations of melatonin seen af ter tryptophan administration and of the changes of circulating melato nin level induced by the feeding regime. Whereas the circadian and cir cannual fluctuations of the concentration of melatonin in the blood se em to be triggered by changes of the photoenvironment and its effect o f pineal melatonin formation, basal daytime melatonin levels and the e xtent of their elevation at nighttime appear to be additionally contro lled by nutritional factors, such as the amount and the composition of ingested food and therefore availability of tryptophan as a rate-limi ting precursor of melatonin formation by the enterochromaffin cells of the gastrointestinal tract.