Tryptophan-induced transcriptomic changes in the European Seabass are highly dependent on neuroendocrine-immune conditions

In European seabass (Dicentrarchus labrax), dietary tryptophan (TRP) surplus has a notable modulatory effect on the hypothalamic-pituitary-interrenal axis under chronic stress and acute inflammation, affecting cortisol levels and neuroendocrine- and immune-related gene expression. A transcriptomic approach (RNA-seq) was applied to head-kidney samples of fish submitted to confinement stress and/or acute inflammation to uncover the biological mechanisms behind these effects. Undisturbed seabass fed dietary TRP supplementation showed an up-regulation of various innate immune functions, contrasting previous studies which indicated mainly a TRP regulatory role. Upon bacterial injection, TRP-fed fish showed a transcriptomic profile similar to their counterparts fed on control diet, indicating TRP's inability to modulate immune mechanisms under bacterial challenge. Under confinement stress, TRP-fed fish exhibited a molecular profile similar to unstressed control fish, highlighting TRP's role in mitigating stress. However, combining dietary TRP supplementation with confinement stress and immune stimulation by bacterial inoculation resulted in a unique molecular profile. Stressed fish fed TRP did not show the restorative effect of immune stimulation on carbohydrate metabolism and showed downregulated genes related to glycolysis and glycogenolysis. Additionally, transcription upregulation in these fish after bacterial injection included terms related to serine and steroid metabolism (carboxyl ester lipase 2), indicating tryptophan-induced changes in lipid mobilization in the head-kidney, potentially affecting cortisol synthesis and other hormones.