SEROTONIN-BACTERIA INTERFACE. “The vast majority of serotonin, an estimated 95%, is found within the GI tract. Here, 90% of serotonin is localized within epithelial enterochromaffin cells (ECs) and the remaining 10% remains within the ENS. And while serotonin biosynthesis in the brain remains largely independent of the GI tract, serotonin biosynthesis within the GI tract involves specific microbiota–gut interactions.”
http://onlinelibrary.wiley.com/doi/10.1111/cmi.12585/full