It turns out that the majority of the 5-HT receptors (and 5-HT itself) are found in the gastrointestinal tract, not in the brain. https://www.sciencedirect.com

It turns out that the majority of the 5-HT receptors (and 5-HT itself) are found in the gastrointestinal tract, not in the brain. https://www.sciencedirect.com/science/article/pii/0163725894900094 https://www.sciencedirect.com/science/article/pii/0163725894... https://pubs.acs.org/doi/10.1021/acschemneuro.6b00414 https://pubs.acs.org/doi/10.1021/acschemneuro.6b00414 Maybe that is why bowel movements are fairly common during the come-up of a trip.