This is actually quite interesting. I'd love to see some hard data on the actual impact this use had on the serotonergic system; for as large a part as it plays in the brain, and all of the diseases that we believe are related to it (everything from mental illnesses like depression and schizophrenia to migraines and cluster headaches), we really know very little about it. Our current drugs acting on serotonin receptors are brute-force hacks -- we need to know a whole, whole lot more.
Picking a nit, but SSRIs don't actually act on serotonin receptors. Although the action of SSRIs isn't completely understood, they generally work by increasing the level of serotonin in the brain by reducing reuptake.
So does the medical establishment at large - it's well acceptd that while SSRI's seem to work, there is no way, other than trying them out, to know which one will work for person X.
That said - given the possibility of serotonin syndrom and the mad bender this guy was allegedely on, it's a wonder he's alive.
