TIME-COURSE AND MORPHOLOGY OF DOPAMINERGIC NEURONAL DEATH CAUSED BY THE NEUROTOXIN 1-METHYL-4-PHENYL-1,2,3,6-TETRAHYDROPYRIDINE

Mechanisms responsible for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridin e (MPTP)-induced dopamine (DA) neuronal death remain unknown and in mi ce it is even unclear whether neuronal death does occur. In vitro stud ies suggest that 1-methyl-4-phenylpyridinium ion (MPP(+)), the active metabolite of MPTP, kills neurons by apoptosis. Herein, we investigate d whether MPTP induces DA neuronal death in vivo in mice and whether t he mechanism is that of apoptosis. C57/bl Mice received different dose s of MPTP administered in four intraperitoneal injections every 2 hour s and were sacrificed at different time points for analyses of tyrosin e hydroxylase (TH) immunohistochemistry, silver staining, and Nissl st aining within the mesencephalon. We found that MPTP induces neuronal d estruction in the substantia nigra pars compacta (SNpc) and the ventra l tegmental area (VTA). The active phase of degeneration began at 12 h postinjection and continued up to 4 days. During this period, there w as a greater decrease in TH-defined neurons than in Nissl-stained neur ons suggesting that MPTP can cause a loss in TH without necessarily de stroying the neuron. Thereafter, neuronal counts by both techniques eq ualized and there was no further loss of DA neurons. Dying neurons sho wed shrunken eosinophilic cytoplasm and shrunken darkly stained nuclei . Double staining revealed degenerating neurons solely among TH positi ve neurons of SNpc and VTA. At no time point and at no dose of MPTP wa s apoptosis observed. In addition, in situ labelling revealed no evide nce of DNA fragmentation. This study demonstrates that the MPTP mouse model replicates several key features of neurodegeneration of DA neuro ns in PD and provides no in vivo evidence that, using this specific pa radigm of injection, MPTP kills DA neurons by apoptosis. (C) 1995 Acad emic Press Limited