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. 2018 Mar 2;5:53. doi: 10.3389/fmed.2018.00053

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Copyright © 2018 Calvani, Picca, Lo Monaco, Landi, Bernabei and Marzetti.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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A gut–brain axis supports the interactions between gut microbiota and the CNS through direct and indirect pathways involving vagal nerve activation, cytokine production, and release of neuropeptide/neurotransmitters and SCFAs. These mediators can pass the BBB and control the maturation and activation of brain immune cells (microglia). Following its activation, microglia modulates immune surveillance, synaptic pruning, and clearance of debris. On the other side, the HPA axis can suppress microglia activation, as well as influence cytokine release and trafficking of monocytes from the periphery to the brain. Abbreviations: BBB, blood–brain barrier; BDNF, brain-derived neurotrophic factor; GABA, gamma-aminobutyric acid; HPA axis, hypothalamic–pituitary–adrenal axis; LPS, lipopolysaccharide; MAMPs, microbe-associated molecular patterns; SCFAs, short-chain fatty acids; CNS, central nervous system.