Changes in behavior and plasma metabolites after tryptophan supplementation in steers

Tryptophan (Trp) is an amino acid used as precursor for several molecules in the body, including serotonin. Serotonin is involved in a broad range of physiological and behavioral functions, including stress response. Because serotonin cannot cross the blood-brain barrier, supply of Trp to the brain could be used to elevate serotonin synthesis and release. Evidence in monogastric species indicates that Trp supplementation may increase circulating serotonin and reduces stress response, but there is conflicting evidence in the literature about degradation of Trp in the rumen. Some authors indicated that Trp is degraded slowly in the rumen and that it may remain available to be absorbed, whereas others observed that it is quickly degraded by rumen bacteria. Thus, the goal of the experiment was to test if supplementation of free L-Trp in steers may increase plasma Trp and serotonin, thus changing behavior and reducing circulating concentrations of hormones that are related to stress response. Four hybrid steers were used in a Latin square design with supplementation of L-Trp at 0, 50, 100, and 150 mg/kg BW/d for one week and three weeks as washout period. Blood samples were analyzed for amino acids, adrenocorticotropic hormone, adrenaline, cortisol, serotonin, and glucose. Videos were used to evaluate posture and activities every 10 minutes. Results indicated no effect of diet or diet by day interaction for most of the evaluated variables and activities. The only difference detected was a diet effect on time spent inactive. Steers receiving 150 mg Trp/kg BW spent more time inactive than steers in the control (69.51 ± 2.01 vs. 60.9 ± 1.92; P = 0.0305). We concluded that it was not possible to raise plasma Trp in steers using up to 150 mg/kg BW/d free Trp and that higher doses might be needed to increase plasma Trp and Trp:large neutral amino acid ratio, and to induce meaningful behavioral changes in steers.