>>1200653990-95% of the serotonin in the body is present in the GI system. But as
>>12006536 said, serotonin is used to signal many many different things in the body and brain, and considering it a molecule that makes you happy is just completely dead wrong.
Dopamine is also much more complicated than popsci would have you believe, but a bit simpler and more predictable than serotonin, I think. (You can generally fairly accurately predict the neurological and subjective psychological effects of a dopaminergic drug. Not necessarily true with a serotonergic drug.)
