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.