One detail of the story that I still remember: the dealers of the MPTP heroin realized they had a bad batch and cut the price to get rid of it.
That said, I have often thought there was a link between cocaine use and early parkinsons. Its interesting to see that bad opiates can also cause similar symptoms.
The scary thing about this drug is that it doesn't really have anything to do with the fact that they were trying to make opiates, it just happens that if you mess up the reaction you get this really toxic byproduct. As governments keep banning new drugs, chemists (amateur and professional, but it's hard to know which is which since this is all unregulated) keep coming up with new not technically illegal ways to achieve the same effects, and it's really just a matter of time before you create something else toxic. And it's entirely possible that some designer drugs that seem safe at first can end up having bad complications later on, since they're not very well researched and understood. At least we know how toxic meth is - with research drugs, especially newly discovered ones, no one really knows what kind of damage they're doing to themselves.
In Parkinson's, a specific group of neurons in the Substantia Nigra (which do happen to be dopaminergic neurons), selectively die off. They no longer release dopamine to their downstream targets, causing the motor and other affects characteristic of PD.
The compensatory mechanism you described is unlikely to cause those specific neurons to die, unless something far more complicated is going on, which you can almost never rule out but doesn't tell us much.
edit for clarity
However, if you combine fewer neurons and fewer receptors there is presumably going to be a faster apparent disease progression.
In a way it's nice and calming to think, "oh, if I don't abuse illegal drugs I'm not affected by this. Most non drug chemicals don't cross the blood brain barrier anyway"
But then you might remember the NYT's attempt to do an expose on drinking water. http://www.nytimes.com/2009/12/17/us/17water.html
Chemicals that the EPA tests for stop being produced, industrial chemist tinker with them in the exact same way these drug chemists do, and then new chemicals are released into our environment that we don't have data on or testing to mitigate.
Fyi, writing it like this makes you sound like the kind of person who thinks "chemicals" are "not natural" but produced by industry and therefore dangerous and evil. Even though literally everything is made up of chemicals.
Do you think it's possible that long term moderate to heavy caffeine use could contribute to Parkinson's by the same method of action you have described?
As far as caffeine goes, I don't think it affects dopamine that much, so it probably wouldn't have that effect on its own. I can't imagine long term heavy caffeine use would be good for you though, I just don't think it would be enough to do the same damage that drugs that directly release dopamine do. Or at least, it would do different kinds of damage - my guess is memory issues due to lack of quality sleep and high blood pressure.
(sorry, one of my favourite songs growing up ..)
i.e. as opposed to some genetic problem.
Obviously, but they are pretty straightforward things.
This chemical that causes Parkinsons is quite different, it has a complicated method of actually becoming active, and seems to target a very specific part of the brain. Unlike those other chemicals, this one is not something you could predict by simply looking at it.
Which is my point - the straightforward, or easily predicted chemicals we already are cautious about. The issue is the complicated or unexpected ones, the ones with very delayed effects.