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. Author manuscript; available in PMC: 2016 May 1.
Published in final edited form as: Clin Ther. 2015 May 1;37(5):984–995. doi: 10.1016/j.clinthera.2015.04.002

Table 2.

Pathways Involved in Bidirectional Communication Between Gut Microbiota, the Brain and the Immune System

  Afferent arm
  Pathways Effect
Change of the gut microbiota due to usage of antibiotics/infectious agents/probiotic bacteria Alteration in the circulating levels of pro/anti-inflammatory cytokines that affect brain function
Modulation of various host metabolic reactions Production of essential metabolites (bile acids, choline, short-chain fatty acids)
Generation of neurotransmitters or neuromodulators in the intestinal lumen Induction of epithelial cell release of molecules that stimulate afferent axons
Changes in tryptophan metabolism Effects on behavior
Activation of sensory vagal fibers Conveyance of information about the state of intestine to the CNS
  Efferent arm
  Pathways Effect
HPA axis activation Regulation of immune cells locally in the gut and systematically affecting gut permeability, motility, secretion, barrier function and gut microbiota composition
Anti-inflammatory cholinergic reflex and/or sympathetic activation Release of neurotransmitters that may affect gut microbiota composition, intestinal permeability and local immunity
Activation of CNS regulatory areas of satiety Impact on nutrient availability to intestinal microbiota and their composition