Serum cortisol, glucose, thyroid hormones' and non-specific immune responses of Persian sturgeon, Acipenser persicus to exogenous tryptophan and acute stress

• The effects of exogenous tryptophan were investigated on Persian sturgeon.
• Tryptophan increased basal cortisol and extended cortisol response to stress.
• Tryptophan inhibited post-stress lysozyme and complement decrease.
• Tryptophan decreased serum thyroid hormones' levels before and after stress.

The effects of exogenous tryptophan on serum cortisol, glucose, lysozyme, alternative complement pathway (ACH50), TSH, T4 and T3 responses before and after an acute stress were investigated in Persian sturgeon, Acipenser persicus. The fish were divided into two groups: control and tryptophan groups. The control group was fed with basal diet (0.28% endogenous tryptophan), whereas the tryptophan group was fed with the basal diet supplemented with 0.5% exogenous tryptophan. Blood samples were taken after 0, 120, 240 and 360 h feeding with corresponding diets. Then the fish were subjected to an acute stress (0.5 h confinement) and further blood samples were taken after 0, 0.5, 3, 24 and 72 h post-stress. Results showed that exogenous tryptophan increased cortisol and decreased glucose after 240 and 360 h feeding. The tryptophan group had significantly higher serum cortisol 3 and 24 h post- stress, compared to the control group. There was no significant difference in post-stress serum glucose between the groups. Exogenous tryptophan had no significant effects on basal serum lysozyme and ACH50 activities, but it inhibited stress-induced decrease in these immune indicators. Tryptophan decreased serum thyroid hormones' levels at 240 and 360 h feeding, and 24 and 72 h post-stress compared to the control. In conclusion, circulating cortisol level and period of cortisol response to stress increase following 360 h feeding with tryptophan-supplemented diet. Tryptophan supplementation inhibits post-stress immunosuppression. Also, tryptophan decreases serum thyroid hormones' level both prior to and following an acute stress. This study encourages further researches on the effects of tryptophan on stress, thyroid and immune responses in sturgeons.Statement of relevanceData on the effects of dietary tryptophan on stress and immune response is controversial in different studies and there is no data on the effects of tryptophan on sturgeons. The present data provides new data on the effects of tryptophan on sturgeons and extends knowledge on the effects of tryptophan on stress and immune response in fish.Hoseini