If most of the body's serotonin is made in the gut, why does it not reach the brain? · Grey Matter

About 90 % of the body's serotonin is made by enterochromaffin cells in the lining of the gut, where it regulates motility and is carried in the blood inside platelets.

If most of the body's serotonin is made in the gut, why does it not reach the brain?

About 90 % of the body's serotonin is made by enterochromaffin cells in the lining of the gut, where it regulates motility and is carried in the blood inside platelets. Serotonin is a charged molecule and does not cross the blood-brain barrier, so this peripheral pool and the brain's pool stay separate. The brain makes its own serotonin in the raphe nuclei from tryptophan, an amino acid that does cross the barrier on a transporter. The gut can still influence the brain through the vagus nerve, immune signals and the supply of tryptophan, which are indirect routes.