Selective retention of MPP+ within the monoaminergic systems of the primate brain following MPTP administration: an in vivo autoradiographic study.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) selectively destroys dopaminergic neurons of the substantia nigra pars compacta in humans and other primates, producing a parkinsonian condition. MPTP is metabolized to the toxin 1-methyl-4-phenylpyridine (MPP+) which is taken up by dopamine terminals. The subsequent events culminating in cell death in the substantia nigra pars compacta are not understood. To examine these events we first produced a chronic hemiparkinsonian condition in monkeys by administering a toxic dose of MPTP via the right carotid artery. One year later, these monkeys were given a trace dose of [14C]MPTP intravenously and allowed to survive 1, 3, or 10 days. In two acute conditions, monkeys were either given the radiolabeled trace dose intravenously immediately following the toxic intracarotid dose, or were given a single toxic intracarotid radiolabeled dose, and allowed to survive 1, 3, or 10 days. We show by histology and autoradiography that the chronic hemiparkinsonian condition is characterized by selective unilateral loss of nigrostriatal dopamine neurons and absence of MPP+ retention in the caudate-putamen. In the acute conditions, MPP+ is accumulated and selectively retained in high concentrations in the caudate-putamen bilaterally and throughout the nigrostriatal pathway only on the side receiving the toxic dose. In the substantia nigra pars compacta. MPP+ is accumulated in very low concentrations in the dopamine cell bodies and is not selectively retained there. At 10 days survival, the caudate-putamen on the side receiving the toxic dose loses its ability to retain MPP+. The apparent degeneration of the dopamine axon terminals in the caudate-putamen and the development of Parkinson-like behavioral signs seen at 10 days survival were observed to precede the loss of cell bodies in the substantia nigra, which appeared normal by the criteria of Nissl staining and neuromelanin content at all time points in the acute conditions. Other areas of dense MPP+ retention in all cases include noradrenergic and serotonergic cell groups and noradrenergic pathways. MPP+ in the locus coeruleus and other caudal catecholaminergic cell groups is apparently retrogradely transported there after uptake in terminal regions, and although it is retained in high concentrations, no cell loss occurs. These findings suggest that experimentally induced Parkinsonism results from molecular events initiated in the neostriatum and selectively elaborated in the nigrostriatal pathway, ultimately resulting in the death of substantia nigra pars compacta dopamine neurons. They do not support a significant role for neuromelanin binding in the toxicity of MPP+.