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J Neurosci. 2000 Oct 1;20(19):7208-19.

Mitochondrial membrane potential and glutamate excitotoxicity in cultured cerebellar granule cells.

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Neurosciences Institute, Department of Pharmacology and Neuroscience, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom.


The relationship between changes in mitochondrial membrane potential (Deltapsi(m)) and the failure of cytoplasmic Ca(2+) homeostasis, delayed Ca(2+)deregulation (DCD), is investigated for cultured rat cerebellar granule cells exposed to glutamate. To interpret the single-cell fluorescence response of cells loaded with tetramethylrhodamine methyl ester (TMRM(+)) or rhodamine-123, we devised and validated a mathematical simulation with well characterized effectors of Deltapsi(m) and plasma membrane potential (Deltapsi(P)). Glutamate usually caused an immediate decrease in Deltapsi(m) of <10 mV, attributable to Ca(2+) accumulation rather than enhanced ATP demand, and these cells continued to generate ATP by oxidative phosphorylation until DCD. Cells for which the mitochondria showed a larger initial depolarization deregulated more rapidly. The mitochondria in a subpopulation of glutamate-exposed cells that failed to extrude Ca(2+) that was released from the matrix after protonophore addition were bioenergetically competent. The onset of DCD during continuous glutamate exposure in the presence or absence of oligomycin was associated with a slowly developing mitochondrial depolarization, but cause and effect could not be established readily. In contrast, the slowly developing mitochondrial depolarization after transient NMDA receptor activation occurs before cytoplasmic free Ca(2+) ([Ca(2+)](c)) has risen to the set point at which mitochondria retain Ca(2+). In the presence of oligomycin no increase in [Ca(2+)](c) occurs during this depolarization. We conclude that transient Ca(2+) loading of mitochondria as a consequence of NMDA receptor activation initiates oxidative damage to both plasma membrane Ca(2+) extrusion pathways and the inhibition of mitochondrial respiration. Depending on experimental conditions, one of these factors becomes rate-limiting and precipitates DCD.

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