The gut-brain axis: interactions between microbiota and nervous systems

Abstract

Humans coexist in a mutualistic relationship with the intestinal microbiota, a complex microbial ecosystem that resides largely in the distal bowel. The lower gastrointestinal tract contains almost 100 trillion microorganisms, most of which are bacteria. More than 1,000 bacterial species have been identified in this microbiota. The intestinal microbiota lives in a symbiotic relationship with the host. A bidirectional neurohumoral communication system, known as the gut–brain axis, integrates the host gut and brain activities (Mayer et al. 2015). Communication between the brain and gut occurs along a network of pathways collectively termed the brain-gut axis. The brain-gut axis encompass the CNS, ENS, sympathetic and parasympathetic branches of the autonomic nervous system, neuroendocrine and neuroimmune pathways, and the gut microbiota (Colins et al. 2012). The gut microbiota can signal to the brain via a number of pathways which include: regulating immune activity and the production of roinflammatory cytokines that can either stimulate the HPA axis to produce CRH, ACTH and cortisol, or directly impact on CNS immune activity; through the production of SCFAs such as propionate, butyrate, and acetate; the production of neurotransmitters which may enter circulation and cross the blood brain barrier; by modulating tryptophan metabolism and downstream metabolites, serotonin, kynurenic acid and quinolinic acid. Neuronal and spinal pathways, particularly afferent signaling pathways of the vagus nerve, are critical in mediating the effect of the gut microbiota on brain function and behavior. Microbial produced SCFAs and indole also impact on EC cells of the enteric nervous system (Romijn et al. 2008; Cani et al. 2013). The purpose of this presentation was to summarize our current knowledge regarding the role of microbiota in bottom-up pathways of communication in the gutbrain axis.