Effect of l-tryptophan and melatonin supplementation on the serotonin gastrointestinal content and digestive enzymatic activity for Salmo salar and Oncorhynchus kisutch

Abstract Several studies describe gastrointestinal tract (GIT) melatonin (MEL) synthesis from 5-HT, which itself derives from the essential amino acid l -tryptophan ( l -trp) in the intestine. Supplementing l -trp and MEL through diet has shown social-response effects and suppresses neuroendocrine stress in teleosts. In this study, the effects of a MEL and l -trp-supplemented diet on the endocrine intestinal function and enzymatic response activity of two salmonid species were examined. To assess the possible effect of l -trp and MEL on intestinal serotonin content and digestive enzyme activity, three l -trp-supplemented diets and three MEL-supplemented diets were orally administered to a group of Salmo salar and Oncorhynchus kisutch for seven days under normal density conditions. Plasma biochemistry (cortisol, l -trp, MEL) as well as enzyme activity (amylase, lipase, and total protease) and serotonin content were measured in the pyloric caeca, midgut, and hindgut. Plasma l -trp levels were found to be directly related to l -trp supplemented diet levels. Similarly, MEL supplementation increased plasma MEL levels, and the presence of MEL in both salmon species resulted in a significant interaction with cortisol concentrations in plasma, and the highest concentrations of l -trp caused an increased GIT content for 5-HT in S. salar . No differences were seen in the GIT content for 5-HT for the l -trp supplemented diets in O. kisutch . An inhibitory effect was found on digestive enzymes in the supplemented diets of both salmonid species. In general, the presence of MEL in the diet reduced cortisol levels; diets supplemented with l -trp and MEL had either a stimulatory or inhibitory effect on digestive enzyme activity, which seemed to be indirect and tissue dependent.