MPP+ (1-methyl-4-phenylpyridine) is a neurotoxin to dopamine-, norepinephrine- and serotonin-containing neurons.

1-Methyl-4-phenylpyridine (MPP+) is an oxidative metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydroxypyridine (MPTP). MPP+ produced local cell death when injected directly into substantia nigra compacta, locus coeruleus, or dorsal and median raphe nuclei of rats. Corresponding significant decreases in dopamine, serotonin and norepinephrine levels were observed in the terminal areas. These observations indicate that MPP+ is a non-selective neurotoxin which causes lesions not only in dopaminergic neurons but also in noradrenergic and serotonergic systems following intracranial administration. Selective lesioning of these monoaminergic systems could only be achieved by a stereotaxic injection of MPP+ into specific brain regions containing monoamine neurons.