Serotonin in the Microbiota–Gut–Brain Axis of Autism Spectrum Disorder: A Systematic Review of Preclinical and Clinical Evidence

This systematic review synthesizes preclinical and clinical evidence on serotonin's role within the microbiota–gut–brain axis in autism spectrum disorder (ASD). ASD is frequently accompanied by gastrointestinal symptoms and gut microbial alterations, with emerging research implicating the microbiota–gut–brain axis as a key pathway linking intestinal microbiota, immune-metabolic signaling, and neurobehavioral outcomes.