Tryptophan Metabolism: A Link Between the Gut Microbiota and Brain.

The gut-brain axis (GBA) is a bilateral communication network between the gastrointestinal (GI) tract and the central nervous system. The essential amino acid tryptophan contributes to the normal growth and health of both animals and humans and, importantly, exerts modulatory functions at multiple levels of the GBA. Tryptophan is the sole precursor of serotonin, which is a key monoamine neurotransmitter participating in the modulation of central neurotransmission and enteric physiological function. In addition, tryptophan can be metabolized into kynurenine, tryptamine, and indole, thereby modulating neuroendocrine and intestinal immune responses. The gut microbial influence on tryptophan metabolism emerges as an important driving force in modulating tryptophan metabolism. Here, we focus on the potential role of tryptophan metabolism in the modulation of brain function by the gut microbiota. We start by outlining existing knowledge on tryptophan metabolism, including serotonin synthesis and degradation pathways of the host, and summarize recent advances in demonstrating the influence of the gut microbiota on tryptophan metabolism. The latest evidence revealing those mechanisms by which the gut microbiota modulates tryptophan metabolism, with subsequent effects on brain function, is reviewed. Finally, the potential modulation of intestinal tryptophan metabolism as a therapeutic option for brain and GI functional disorders is also discussed.

Increasing carbohydrate availability in the hindgut promotes hypothalamic neurotransmitter synthesis: aromatic amino acids linking the microbiota-brain axis.

The gut microbiota is increasingly recognized to modulate brain function by recent studies demonstrating the central effects of various gut microbial manipulation strategies. Our previous study demonstrated that antibiotic-induced alterations of hindgut microbiota are associated with changes in aromatic amino acid (AAA) metabolism and hypothalamic neurochemistry, while the underlying mechanistic insight is limited. Given that the microbial AAA metabolism can be affected by luminal carbohydrate availability, here we hypothesize that increasing hindgut carbohydrate availability affects the expression of neurotransmitters in the porcine hypothalamus. A hindgut microbiota-targeted strategy was adopted by increasing hindgut carbohydrate availability in a cecal-cannulated piglet model. Mechanistic involvement of AAAs along the gut microbiota-brain axis was further investigated in mice and neuronal cells. Increasing carbohydrate availability by cecal starch infusion led to a decrease in hindgut AAA metabolism, and an increase in systemic AAA availability, central AAA-derived neurotransmitters (5-HT, dopamine), and neurotrophin BDNF in piglets, indicating that hindgut microbiota affect hypothalamic neurochemistry in an AAA-dependent manner. Single AAA i.p. injection in mice revealed that an increase in circulating tryptophan and tyrosine elevated their concentrations in brain and finally promoted the expressions of 5-HT, dopamine, and BDNF in a time-dependent manner. Neuronal cells treated with single AAAs in vitro further demonstrated that tryptophan and tyrosine enhanced 5-HT and dopamine synthesis, respectively, and promoted BDNF expression partly through the 5-HT1A/DRD1-CREB pathway. Our study reveals that increasing hindgut carbohydrate availability promotes hypothalamic neurotransmitter synthesis and that AAAs act as potential mediators between hindgut microbiota and brain neurochemistry.

Antibiotics-induced modulation of large intestinal microbiota altered aromatic amino acid profile and expression of neurotransmitters in the hypothalamus of piglets.

The evidence of gut microbiota-mediated modulation of brain function has been widely recognized from studies using germ-free rodents or animals with oral antibiotic-induced microbiota depletion. Since the large intestine harbors greater numbers and more diverse of microbes than in the small intestine, large intestinal microbiota may play a crucial role in the modulation of brain function. In this study, a large intestinal microbiota-targeted strategy was used to investigate the impact of large intestinal microbiota on brain function. Twelve piglets (12.08 ± 0.28 kg) fitted with a T-cannula at the distal ileum were fed a standard diet and randomly assigned to two groups (n = 6) for ileal infusion of either saline or antibiotics. After 25 days of infusion, ileal and fecal microbiota, serum amino acids and neurotransmitters, and hypothalamic transcriptomics were analyzed. While the antibiotic infusion did not change the proximal ileal microbial composition, it markedly altered the fecal microbial composition and increased aromatic amino acid (AAAs) metabolism (p < 0.05), suggesting the infusion specifically targeted large intestinal microbes. Concentrations of AAAs were likewise decreased in the blood and hypothalamus (p < 0.05) by antibiotic infusion. Antibiotic infusion further decreased concentrations of hypothalamic 5-hydroxytryptamine (5-HT) and dopamine, in line with AAAs being their precursors. An up-regulation in gene expressions of neurotransmitter transporters and synthetases was observed (q < 0.001). In conclusion, the distalileal-antibiotic infusion altered neurotransmitter expression in the porcine hypothalamus and this effect occurred simultaneously with changes in both the large intestinal microbiota, and AAAs in the large intestine, blood and hypothalamus. These findings indirectly indicate that large intestinal microbiota affects hypothalamic neurotransmitter expressions. Read the Editorial Highlight for this article on page 208.

Time-course responses of ileal and fecal microbiota and metabolite profiles to antibiotics in cannulated pigs.

We investigated the time-course effects of therapeutic antibiotics on intestinal microbial composition and metabolism in an ileal-cannulated pig model. Sixteen ileal-cannulated piglets (12 ± 0.5 kg) were assigned to two groups (n = 8) and fed standard diets with or without antibiotics. At 4 days before, and at days 2, 7, and 13 after antibiotic administration, ileal and fecal samples were collected for analysis of microbiota composition via 16S rRNA MiSeq sequencing and metabolites (short-chain fatty acids, biogenic amines, and indole). It was found that Lactobacillus and Bifidobacterium had decreased by an average 2.68-fold and 508-fold in ileum on days 2-13, and by an average 45.08-fold and 71.50-fold in feces on days 7-13 (P < 0.05). Escherichia/Shigella had increased by an average 265-fold in ileum on days 2-13, and by an average 36.70-fold in feces on days 7-13 (P < 0.05). Acetate concentration had decreased in ileum by an average 2.88-fold on days 2-13, and by 1.83-fold in feces on day 7 (P < 0.05). Cadaverine concentration had increased by an average 7.03-fold in ileum on days 2-13, and by an average 9.96-fold in feces on days 7-13 (P < 0.05), and fecal indole concentration had increased by an average 2.51-fold on days 7-13 (P < 0.05). Correlation analysis between significant microbes and metabolites indicated that the antibiotic-induced microbiota shift appeared to result in the changes of intestinal metabolism. In conclusion, antibiotic administration led to dynamic changes in microbial communities and metabolism in ileum and feces, with ileal microbiota being more prone to shift than fecal microbiota.

Bromochloromethane, a Methane Analogue, Affects the Microbiota and Metabolic Profiles of the Rat Gastrointestinal Tract.

Bromochloromethane (BCM), an inhibitor of methanogenesis, has been used in animal production. However, little is known about its impact on the intestinal microbiota and metabolic patterns. The present study aimed to investigate the effect of BCM on the colonic bacterial community and metabolism by establishing a Wistar rat model. Twenty male Wistar rats were randomly divided into two groups (control and treated with BCM) and raised for 6 weeks. Bacterial fermentation products in the cecum were determined, and colonic methanogens and sulfate-reducing bacteria (SRB) were quantified. The colonic microbiota was analyzed by pyrosequencing of the 16S rRNA genes, and metabolites were profiled by gas chromatography and mass spectrometry. The results showed that BCM did not affect body weight and feed intake, but it did significantly change the intestinal metabolic profiles. Cecal protein fermentation was enhanced by BCM, as methylamine, putrescine, phenylethylamine, tyramine, and skatole were significantly increased. Colonic fatty acid and carbohydrate concentrations were significantly decreased, indicating the perturbation of lipid and carbohydrate metabolism by BCM. BCM treatment decreased the abundance of methanogen populations, while SRB were increased in the colon. BCM did not affect the total colonic bacterial counts but significantly altered the bacterial community composition by decreasing the abundance of actinobacteria, acidobacteria, and proteobacteria. The results demonstrated that BCM treatment significantly altered the microbiotic and metabolite profiles in the intestines, which may provide further information on the use of BCM in animal production.