**2.4 Neurotransmitters**

The gut microbiota can produce and/or consume numerous neurotransmitters, including dopamine, norepinephrine, serotonin, or gamma-aminobutyric acid (GABA) [4, 31]. Microbiota-dependent effects on gut serotonin significantly impact host physiology. For example, it is known that the gut contains the bulk of the body's serotonin (more than 85 percent 5-hydroxytryptamine (5-HT)), but the mechanisms that control the metabolism of 5-HT obtained from the gut are still unclear. A mammalian experiment showed that indigenous spore-forming bacteria from mouse and human microbiota promote 5-HT biosynthesis from colonic enterochromaffin cells, which supply 5-HT to the mucosa, lumen, and circulating platelets [32].
