Figure 1.

The gut-brain axis. Gut microbiota communicates with the brain by different mechanisms, including (1) direct activation of the vagus nerve; (2) by producing molecules, such as short-chain fatty acids, that may cause gut epithelium to be “leaky” and then act on a range of other systems; (3) by directly producing or altering the synthesis of neurotransmitters, including serotonin, and; (4) activating immune signaling pathways, including the release of cytokines and other inflammatory molecules. Stress, as signaled via the hypothalamic-pituitary-adrenal (HPA) axis, is one top-down mechanism that may affect gut microbiota. [Based on data from Cryan and Dinan (10).]