The Gut–Brain Axis and Psychosis: A Systematic Review of Microbial Metabolites and Their Impact on Dopaminergic Pathways in Schizophrenia

Schizophrenia is a psychiatric disorder that has psychotic, cognitive, and negative symptoms, with dopaminergic dysfunction. The gut–brain axis is identified as an important modulator for neurochemical function. Following PRISMA 2020 guidelines, this systematic review is conducted through searches (PubMed, Scopus, SpringerLink, Taylor & Francis, and Google Scholar). After removing duplicates and screening, six studies included three clinical studies, two translational studies involving fecal microbiota transplantation (FMT), and one rodent model. The findings highlighted three main themes:(1) the dysregulation of the tryptophan–kynurenine pathway, with elevated kynurenine and reduced kynurenic acid linked to disrupted dopaminergic and glutamatergic signaling; (2) changes of microbiota composition and metabolite imbalances, which included fewer butyrate producers and more succinate generators, which were associated with neurotransmitter dysregulations; and (3) relationships within clinical symptoms, in which metabolites correlated with comprehension of negative/cognitive deficits. Collectively, microbial dysbiosis and metabolite imbalance influence dopamine biosynthesis, receptor activity, immune, metabolic, and neurochemical signaling.