Abstract: 【Objective】The objectives of this experiment were to evaluate the effects of dietary tryptophan on behavior, concentrations of relative hormones and expression of 5-HT2A mRNA in piglets after mixing. 【Method】A total of 144 weaning piglets at 28 d of age with initial average weight at (8.00±0.25) kg were assigned to three groups randomly. Each group included 6 replicates with 15 piglets each. The control group was fed with the basal diet including 0.21% tryptophan, the trentment groups (treatment groupsⅠ and Ⅱ) with basal diets plus 0.1% and 0.2% tryptophan, respectively. Piglets were mixed after diet introduction for 28 d. The aggressive and daily behavior were recorded after mixing and the levels of salivary cortisol , hypothalamic 5-HT and 5-HT2A mRNA were determined. 【Result】After mixing, piglets fed with supplemental tryptophan had less time and number of fights and spent more time lying and less time standing (P＜0.05) than pigs fed with the control diet. There were no significant differences in food gathering and sitting behavior among groups (P＞0.05). As the levels of dietary tryptophan increased, the concentrations of salivary cortisol decreased significantly (P＜0.05), whereas the concentrations of hypothalamic 5-HT increased significantly (P＜0.05). The expression of hypothalamic 5-HT2A in piglets decreased gradually as the levels of dietary tryptophan increased, however, the difference was only significant between the control group and the treatment group Ⅱ (P＜0.05).【Conclusion】Surplus 0.1% and 0.2% tryptophan that exceeded the standard requirement for piglets can increase the concentrations of hypothalamic 5-HT concentrations, modify the expression of hypothalamic 5-HT2A mRNA, decrease the concentrations of salivary cortisol, and also alleviatesusceptibility to stress in piglets after mixing.

Key words: tryptophan, piglet, behavior, cortisol, 5-HT