MPTP noun1 •1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine: a byproduct of synthetic heroin, or MPPP, which causes parkinsonian symptoms in users; and a neurotoxin that degrades into MPP+ after crossing the blood-brain barrier, which causes a degradation of the dopaminergic neurons within the substantia nigra.The neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine MPTP provides an excellent opportunity to study repair and response to injury in the basal ganglia. — 2005