I think a past thread by Carolyn and my comment on it has some bearing on same topic. I have always had my funny theory that, for some, PD is caused by over use of Dopamine! For example if you are an anxious person, you over use Dopamine to deal with anxiety and keep it under control ! :
http://neurotalk.psychcentral.com/thread35150.html
Einstein researchers discover important clue to the cause of Parkinson's disease
Dopamine ! . Is It Our Saver And Our Killer !