Serotonin: A mediator of the gut–brain axis in multiple sclerosis

BACKGROUND: The significance of the gut microbiome for the pathogenesis of multiple sclerosis (MS) has been established, although the underlying signaling mechanisms of this interaction have not been sufficiently explored. OBJECTIVES: We address this point and use serotonin (5-hydroxytryptamine (5-HT))—a microbial-modulated neurotransmitter (NT) as a showcase to demonstrate that NTs regulated by the gut microbiome are potent candidates for mediators of the gut–brain axis in demyelinating disorders. METHODS, RESULTS, AND CONCLUSION: Our comprehensive overview of literature provides evidence that 5-HT levels in the gut are controlled by the microbiome, both via secretion and through regulation of metabolites. In addition, we demonstrate that the gut microbiome can influence the formation of the serotonergic system (SS) in the brain. We also show that SS alterations have been related to MS directly—altered expression of 5-HT transporters in central nervous system (CNS) and indirectly—beneficial effects of 5-HT modulating drugs on the course of the disease and higher prevalence of depression in patients with MS. Finally, we discuss briefly the role of other microbiome-modulated NTs such as γ-aminobutyric acid and dopamine in MS to highlight a new direction for future research aiming to relate microbiome-regulated NTs to demyelinating disorders.