Figure 1.

Schematic representation of the microbiota-gut-brain axis. The gut microbiota signals to the enteric nervous system (ENS) and to the CNS via different pathways, including endocrine, immune, metabolic and neuronal pathways as described throughout the text. In physiological conditions, the blood-brain barrier (BBB) allows the access of tryptophan metabolites and SCFAs into the CNS but blocks the passage of circulating neurotransmitters except for gamma aminobutyric acid (GABA). However, in pathological conditions, the disruption of BBB leads to an increased amount of circulating neurotransmitters in the brain.