Effect of L-tryptophan supplemented diets on serotonergic system and plasma cortisol in Totoaba macdonaldi (Gilbert, 1890) juvenile exposed to acute stress by handling and hypoxia

Brain serotonin and cortisol play a central role in integrating the neuroendocrine response to stress. Previous studies in aquatic species show that dietary supplementation with the 5-HT precursor L-tryptophan amino acid is able to modulate this response. The aim of this study was to evaluate the effect of dietary tryptophan (Trp) on the serotonergic system and acute stress response in Totoaba macdonaldi. Four groups of fish were fed with one of four isonitrogenous and isocaloric diets formulated with different Trp content: 0.5% (1X TRP, control diet), 1.1% (2X TRP), 1.7% (3X TRP) and 2.3% (4X TRP). After a feeding period of 21 days, groups were exposed to acute stress by handling with a net or hypoxia. Feeding with Trp-enriched diets resulted in increased 5-HT content in telencephalon of undisturbed fish. Handling increased the 5-hydroxyindole acetic acid (5-HIAA) and serotonin turnover (5-HIAA/5-HT) in telencephalon of fish fed with 1X TRP and 2X TRP but was not significant for the fish fed 3X TRP and 4X TRP, and no significant increase was observed by hypoxia in both telencephalon and hypothalamus in any of the diets. An inverted U-shaped response was observed in plasma cortisol increasing with 2X TRP after both handling and hypoxia and decreasing with diets rich in Trp, 3X TRP and 4X TRP. A higher increase was observed in haematocrit, plasma glucose and lactate in hypoxia than the handling groups. Our results indicate that Trp supplementation modulated cortisol response and mediated negative feedback of 5-HT system in telencephalon after acute stress.