SEROTONIN IN THE GUT
There are 2 genes coding for the Tph(Tryptophan 5 hydroxylase) enzyme: Tph1 and Tph2. Tph1 is expressed mostly in enterochromaffin cells of the gut and is responsible for most of the serotonin present in the blood.
On the other hand, the gene Tph2 is expressed exclusively in serotonergic neurons of the brainstem(Raphe nuclei) and is responsible for the production of serotonin in the brain.
These two pools of serotonin, one in the blood and the other in the brain, never cross over; therefore these should be viewed from a functional point of view as two distinct molecules.
Brain-derived serotonin (BDS) acts as a neurotransmitter, while gut-derived serotonin (GDS) acts as a hormone and regulates a wide variety of processes.