Tryptophan metabolism: role in modulating the function of the "brain–gut axis"

The "brain–gut axis" is a bidirectional network of information exchange between the gut and the brain, in which tryptophan metabolism plays a central role, which is directly and indirectly regulated by the gut microbiota. Modulation of the gut microbiota composition is a promising therapeutic strategy for diseases associated with dysfunction of the "brain-gut axis". The aim of the study was to summarize the available literature data on the role of tryptophan metabolism in modulating the function of the "brain–gut axis". The current national and international scientific literature on the role of tryptophan metabolism in modulating the brain–gut axis was analyzed. To search for literature sources, the databases Scopus, PubMed, ResearchGate, Wiley Online Library, Google Scholar for 2018–2024 were studied, a total of 33 sources. The study described the mechanisms of serotonin and kynurenine synthesis. The impact of the gut microbiota on tryptophan metabolism in the gastrointestinal tract is considered. The role and place of serotonin, kynurenine and microbial tryptophan metabolites in the functioning of the "brain–gut axis" are described. We concluded that the gut microbiota modulates the function of the "brain–gut axis" through the interaction between the immune system, bacterial metabolites, and changes in tryptophan metabolism. Due to the fact that the composition of the gut microbiota of animals and humans is different, it is not possible to extrapolate the results of animal studies on the pathogenesis, pathophysiology and treatment of "brain–gut axis" disorders to the human population. There is a need for further human studies to explore the possibility of using tryptophan and its metabolites as biomarkers for diagnosis and development of new therapeutic strategies for diseases associated with "brain–gut axis" dysfunction. One of the options for such treatment may be methods of intestinal microbiota rebiosis that modulate tryptophan availability. Keywords: serotonin, kynurenine, central nervous system, gut, gut microbiota.