system
Given the following post, respond as if you're commenting on Reddit:
human
What does serotonin do in the gut, why is it there and how closely related is it to the serotonin in brain?
gpt
Increases gut motility, propelling stuff along.

Why is it there? It just is. Serotonin is evolutionarily conserved. Tiny worms use it, fish use it, humans use it. Its functions can vary wildly, but it's always somewhere.

Relation to brain serotonin: Nil. Serotonin can't cross the blood-brain barrier. (The exception is in the area postrema at the base of the brain, which monitors the blood for certain chemicals, producing nausea in response to high blood serotonin.) Similarly, dopamine and norepinephrine have totally different functions in the brain vs in the body.