Dietary melatonin and L-tryptophan supplementation counteracts the effects of acute stress in Salmo salar

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It has been suggested that supplementing fish diets with L-tryptophan (L-Trp) and melatonin (MEL) may attenuate stress; however, little clarity exists regarding the physiological effects of both additives and the associated mechanisms in attenuating the stress response. We investigated how MEL and L-Trp in the diet affects the physiology and acute stress responses in post-smolt Salmo salar. We supplemented in the fish diet with MEL and L-Trp for seven days and then used a chasing protocol to induce an acute stress response. After 0 (no stress), 5 and 15 min of chasing, we took plasma, gills, liver, and intestine samples. Our results suggest that MEL and L-Trp affect the physiology of S. salar differently. Plasma cortisol and lactate levels, in addition to branchial cortisol in fish fed with MEL decreased compared to controls; however, we did not observe any such differences in fish fed L-Trp. We detected significant increases in plasma MEL and L-Trp concentrations in fish fed MEL and L-Trp, respectively, but did not observe any changes in plasma MEL concentrations in fish fed L-Trp, while in the intestine we found that fish fed L-Trp increased MEL levels in pyloric caeca and hindgut, under stress and non-stress conditions, respectively. Serotonin (5-HT) levels in the intestine increased in fish fed MEL, while fish fed L-Trp increased their 5-HT levels in pyloric caeca and midgut under non-stress conditions. At the metabolic level, MEL generated hyperglycemia in plasma with greater glucose uptake in the gills, while L-Trp increased the levels of triglycerides, lactate and plasma proteins in non-stress conditions, in addition to a greater metabolism of amino acids in the liver. The modulation of glucocorticoid receptor transcription was similar in gills for MEL and L-Trp, while in liver, fish fed MEL had higher transcription of glucocorticoid receptors compared to fish fed L-Trp. In conclusion, dietary supplementation with MEL and L-Trp modulates the physiology of S. salar, but only MEL supplementation attenuates their acute stress response.