[Schatze]

Actually it was synthetic heroin that caused Parkinsonian symptoms in users in California in the 90's. The dude accidentally synthesized a toxin that attacked neurons in the nigrastriatal pathway. Funnily enough, several of them were cured by micropipette injection of aborted fetuses dopaminergic neurons into the regions of the brain damaged. Went from totally immobile to being able to ride bikes. High success rate.

That's why the Parkinson's crowd is so hot to trot on stem cells; not only is a cure possible, we know how to do it. Ethical restrictions on the use of aborted fetuses killed that program, but if they can create differentiated dopaminergic neurons and inject them into the substantia nigra and surrounding areas it almost certainly will cure/substantially improve the outcomes of people with Parkinsons. There are a few trials using undifferentiated stem cells that have shown success too. Michael J. Fox really does have very, very good reason to advocate stem cell research.

But MDMA DOES damage your serotonin circuits, with long term heavy use (years). That's a fact. What we don't know is why some of these individuals show no clinical symptoms while others show severe symptoms consistent with the disruption of the role these circuits play.

Again, like alcohol. Alcoholics suffer severe brain damage; their brain shrivels, the sulci shrink, the atria enlarge. Some go on to lead normal lives with no cognitive or clinical problems, while others become dry drunks, get pre-senile dementia, Wernicke's-Korsakoff etc.

But this is PROLONGED, CHRONIC, FREQUENT use. Minor insults like introducing psychoactive chemicals to your brain get shrugged off pretty easy except in extreme cases. That hangover from last night's carouse? A large portion of it is the disrupted homeostasis of your brain; within a few hours or a day, you're fine as a a fiddle. The brain rocks, except when it gets fucked up :/

edit: P.S. My pharmacology texts all, very respectfully, make fun of the gateway theory of marijuana. What you'll get in a drugs and behaviour or pharmacology course is worlds apart from the crap that's spewed by mass media. And usually, from what I've seen, in regards to drugs and behaviours text, they even include graphs in the sections on marijuana rating drugs based upon physiological damage, criminality, and addiction potential. Marijuana is right near the bottom on every one. It's not scientists that make laws, but politicians pandering to the public. Nicotine is usually in the top 4, and alcohol within the top 7 (depends on metrics, but I've seen 5-8 rankings).