SLC6A3 and Parkinson disease: Mitochondria were first implicated in PD when it was found that the metabolite 1-methyl-4-phenylpyridinium (MPP+) of 1-methyl 4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP), a mitochondrial neurotoxin, enters dopaminergic neurons through dopamine transporters and inhibits complex I of the mitochondrial electron transport chain, causing parkinsonism in designer-drug abusers [22].