Abstract: A high-protein diet readily leads to anxiety and depression, and autonomous behaviors are an important avenue to judge anxiety and depression. In this study, we investigated the intervention effect of different doses of L-theanine (100, 200 and 400 mg/(kg mb·d)) (intragastric administration for 40 days) on behavioral changes in SD rats (SPF grade, male, 6 weeks old) fed five experimental diets with different protein-to-energy (P:E) ratios of 20%, 30%, 40% and 50% through open field and light/dark box tests and physiological and biochemical characterization, in order to provide a scientific basis for multi-nutritional intervention in high-protein diet fed animals and in-depth utilization of L-theanine. The results showed that the feed intake and body mass of rats in the 50% P:E ratio group were significantly reduced when compared with the ordinary diet group with 20% P:E ratio (P < 0.05). The residence time in the light box was significantly shortened (P < 0.05), and the number of times of entering the light box was significantly decreased (P < 0.05). The contents of dopamine, 5-hydroxytryptamine and norepinephrine were decreased but not statistically significantly, suggesting that high dietary protein levels can induce depressive behaviors in rats. Low-dose L-theanine treatment significantly increased the body mass of rats in the high-protein diet group, and prolonged the residence time in the light box (P < 0.05). Low- and medium-dose L-theanine intervention significantly increased the number of crossing and rearing in the open field test, the number of times of entering the light box, and serum norepinephrine and dopamine levels (P < 0.05), and low-dose L-theanine intervention significantly increased the concentration of 5-hydroxytryptamine in the serum of rats (P < 0.05). In conclusion, L-theanine can improve the autonomic behavior of SD rats with anxiety or depression caused by high-protein diet through a mechanism related to the metabolism of monoamine transmitters.

Key words: L-theanine; high-protein diet; spontaneous behavior; nutritional intervention; antidepressant