Important precursors of monoaminergic neurotransmitters, dietary tryptophan (TRP), tyrosine, and phenylalanine (all referred to as TTP), play crucial roles in a wide range of behavioral and emotional functions. In the current study, we investigated whether diets devoid of TTP or diets deficient in TRP alone can affect body weight, behavioral characteristics, and gut microbiota, by comparing mice fed on these amino acids-depleted diets to mice fed on diets containing regular levels of amino acids. Both dietary TTP- and TRP-deprived animals showed a reduction in food intake and body weight. In behavioral analyses, the mice fed TTP-deprived diets were more active than mice fed diets containing regular levels of amino acids. The TRP-deprived group exhibited a reduction in serum TRP levels, concomitant with a decrease in serotonin and 5-hydroxyindoleacetic acid levels in some regions of the brain. The TTP-deprived group showed a reduction in TTP levels in the serum, concomitant with decreases in both phenylalanine and tyrosine levels in the hippocampus, as well as serotonin, norepinephrine, and dopamine concentrations in some regions of the brain. Regarding the effects of TRP or TTP deprivation on gut microbial ecology, the relative abundance of genus Roseburia was significantly reduced in the TTP-deprived group than in the dietary restriction control group. Interestingly, TTP was found even in the feces of mice fed TTP- and TRP-deficient diets, suggesting that TTP is produced by microbial or enzymatic digestion of the host-derived proteins. However, microbe generated TTP did not compensate for the systemic TTP deficiency induced by the lack of dietary TTP intake. Collectively, these results indicate that chronic dietary TTP deprivation induces decreased monoamines and their metabolites in a brain region-specific manner. The altered activities of the monoaminergic systems may contribute to increased locomotor activity.
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