Dose-response relationship of tryptophan with large neutral amino acids, and its impact on physiological responses in the chick model

Tryptophan (Trp) has been associated with the regulation of several behavioral and physiological processes, through stimulation of serotonergic activity. Tryptophan utilization at the metabolic level is influenced by the competitive carrier system it shares with large neutral amino acids (LNAA). This study was carried out using meat-type chicken as a model, to investigate the dose response effects of Trp/LNAA on fear response (tonic immobility; TI) and hormonal responses, including corticosterone (CORT), serotonin (5-HT), triiodothyronine (T3) and thyroxine (T4). A total of 12 cages (48 birds) were assigned to each of the six experimental groups at 29-42 days of age. Experimental diets were formulated to have incremental levels of Trp/LNAA (0.025, 0.030, 0.035, 0.040, 0.045, and 0.050). The results revealed that, Trp/NAA had no significant effect on growth performance and TI of the birds. However, elevation of Trp/LNAA was associated concurred with a linear reduction in CORT (P < 0.0001, r2 =0.819) and linear increases in 5-HT (P < 0.0001, r2 =0.945), T3 (P = 0.0003, r2 =0.403) and T4 (P < 0.0001, r2 =0.937) levels. In conclusion, the results from the current study demonstrated that, although incremental levels of Trp/LNAA did not affect bird growth performance or fearfulness, it increased 5-HT, T3 and T4, and decreased CORT levels in a linear dose-dependent manner. Manipulation of Trp feeding levels could be applied to manage stressful conditions in birds.