In vivo labeling of mitochondrial complex I (NADH : ubiquinone oxidoreductase) in rat brain using [H-3]dihydrorotenone

Defects in mitochondrial energy metabolism have been implicated in several neurodegenerative disorders. Defective complex I (NADH:ubiquinone oxidoredu ctase) activity plays a key role in Leber's hereditary optic neuropathy and , possibly, Parkinson's disease, but there is no way to assess this enzyme in the living brain. We previously described an in vitro quantitative autor adiographic assay using [H-3]dihydrorotenone ([H-3]DHR) binding to complex I. We have now developed an in vivo autoradiographic assay for complex I us ing [H-3]DHR binding after intravenous administration. In vivo [H-3]DHR bin ding was regionally heterogeneous, and brain uptake was rapid. Binding was enriched in neurons compared with glia, and white matter had the lowest lev els of binding. In vivo [H-3]DHR binding was markedly reduced by local and systemic infusion of rotenone and was enhanced by local NADH administration . There was an excellent correlation between regional levels of in vivo [H- 3]DHR binding and the in vitro activities of complex II (succinate dehydrog enase) and complex IV (cytochrome oxidase), suggesting that the stoichiomet ry of these components of the electron transport chain is relatively consta nt across brain regions. The ability to assay complex I in vivo should prov ide a valuable tool to investigate the status of this mitochondrial enzyme in the living brain and suggests potential imaging techniques for complex I in humans.