Gut microbiota & Serotonin (5-HT)

Over 90% of the Mammalian Host’s Serotonin Is Found in the Intestine.
Bacteria or body cells can convert tryptophan into the neurotransmitter serotonin, which plays a role in depression and other psychiatric disorders.

Intestinal serotonin secretion is affected by diet, and regulates intestinal movement, mood, appetite, sleep and cognitive functions. This dual role suggests that serotonin may link the intestine (including its microbiota) to host behavior.
Roshchina 2010 In: Lyte M, Fitzgerald P (eds). Microbial Endocrinology: Interkingdom Signaling in Infectious Disease and Health. New York: Springer 17–52
Neuman et al. 2015 FEMS Microbiol Rev 39: 509-21
Kumar et al. 2020c
Pennisi 2020 science 368: 570-573

Serotonin (5-HT) is an enteric nervous system and intestinal mucosa growth factor, and gastrointestinal motility depends on it
Gershon 2013 Curr. Opin. Endocrinol. Diabetes, Obes 20: 14-21

The plasma tryptophan concentrations in GF mice were 40% higher than those in normal mice, while plasma serotonin in normal mice was 280% higher than that in GF mice, which revealed that the gut microbiota is responsible for the crucial effects of turning peripheral tryptophan into serotonin
Wikoff et al. 2009 Proc Natl Acad Sci USA 106(10): 3698-3703

see also:
Serotonin &
Gut Microbiota & Hormones/Hormone System