Gut–Brain Signaling in Parkinson’s Disease: A Narrative Review

The formulation of the gut-brain-microbiota axis (GBA) theory has led to new research directions that have expanded our understanding of the pathogenesis, phenotypic variability, and clinical course of Parkinson's disease (PD). Models of PD pathogenesis, based on the Braak hypothesis, suggest a subtype of the disease in which pathological changes begin in the gut many years before the onset of brain pathology and the manifestation of motor symptoms. Gut microbiota may influence nervous system function along the GBA by influencing intestinal permeability, chronic inflammation, and α-synuclein aggregation. Accumulating evidence suggests that the gut microbiota may also regulate the synthesis and metabolism of neurotransmitters, including dopamine (DA), serotonin (5-HT), acetylcholine (ACh) and γ-aminobutyric acid (GABA), both in the gut and brain, and indirectly stimulate central nervous system activity via the vagus nerve, which receives signals from the enteric nervous system. Research on the effects of microbiota on GBA has paved the way for the identification of novel treatment strategies, including probiotics, prebiotics, synbiotics, postbiotics, antibiotics, and fecal microbiota transplantation (FMT), aimed at not only symptomatic but also disease-modifying treatment of PD. In this article, we propose a novel approach to GBA as a link between gut microbiota and gut and brain neurotransmitter metabolism in PD. We review the latest research on the gut epithelial barrier. We analyze and summarize the potential of therapeutic interventions targeting gut microbiota and their impact on neurotransmitter regulation in PD.