Tryptophan-supplemented diet modulates the metabolic response of European seabass (Dicentrarchus labrax) juveniles reared under space-confined conditions and submitted to acute inflammation

The energetic costs of being in stressful conditions require the involvement of hormones associated with metabolic support, which may also influence immune function. The present work aimed to explore the links between tryptophan nutrition and metabolic responses in European seabass (Dicentrarchus labrax) held under space-confined conditions, and subsequently submitted to an immune challenge. To study that, two dietary treatments were evaluated, i.e. control diet (CTRL) and CTRL-based diet supplemented with tryptophan (0.3%; TRP) to fish under space-confinement conditions (10 kg/m³) or not (5 kg/m³). Dietary treatments were offered for 15 days after which fish were intraperitoneally injected (i.p.) with Photobacterium damselae piscicida. Liver was sampled before the immune challenge (at end of the nutritional trial, 0 h) and at 4, 24, 48 and 72 h post-injection. Fish-fed TRP exhibited distinct metabolic profiles compared to those fed CTRL diets, particularly in energy metabolism and stress response. CTRL-fed fish in space-confined conditions showed a gradual reduction of lipid oxidative enzyme activity post-injection, pointing to acute stress-induced lipid catabolism feedback. In contrast, stressed fish fed TRP under same rearing conditions presented reduced glucose levels and cortisol production but unchanged 3-hydroxiacil-CoA dehydrogenase (EC 1.1.1.35, HOAD) activity patterns, suggesting an inhibitory and modulatory role of tryptophan in stress response. Discriminant analysis revealed that fish fed TRP under space-confined conditions resembled fish CTRL-fed under non-stressful conditions. These findings suggest that tryptophan dietary supplementation for stressed fish modulates their metabolic responses and potentially mitigates the negative effects of rearing stressful conditions after subsequent acute stress induced by an immune challenge.