Gut-brain connection in schizophrenia: A narrative review

Figure 1 Gut-brain axis and roles of the vagus nerve and the enteric nervous system in brain function and their relevance to schizophrenia. The vagus nerve regulates feeding and autonomic nervous system activity and transmits gut-derived signals to the brain, influencing mood and cognition. The enteric nervous system controls local gut function, modulates immune interactions, and communicates with the brain while mediating microbiota interactions. Disruptions in these pathways, including altered gut-brain signaling, immune activation, and microbiota imbalances, have been linked to neuropsychiatric disorders such as schizophrenia and autism spectrum disorder. GI: Gastrointestinal; ASD: Autism spectrum disorder.

Figure 2 Tryptophan metabolism. Tryptophan can be metabolized into serotonin or kynurenine (this process is influenced by gut microbiota and inflammation/stress). Inflammation or stress shifts tryptophan metabolism toward the kynurenine pathway. Serotonin helps in mood regulation, sleep cycle, and cognitive function. Quinolinic acid is a neurotoxin, while kynurenic acid is neuroprotective.