Excitotoxicity and mitochondria

Excitotoxicity is the process whereby a massive glutamate release in the central nervous system in response to ischaemia or related trauma leads to the delayed, predominantly necrotic death of neurons. Excitotoxicity is also implicated in a variety of slow neurodegenerative disorders. Mitochondria accumulate much of the post-ischaemic calcium entering the neurons via the chronically activated N-methyl-d-aspartate receptor. This calcium accumulation plays a key role in the subsequent death of the neuron. Cultured cerebellar granule cells demonstrate delayed calcium de-regulation (DCD) followed by necrosis upon exposure to glutamate. DCD is unaffected by the ATP synthase inhibitor oligomycin but is inhibited by the further addition of a respiratory chain inhibitor to depolarize the mitochondria and inhibit mitochondrial calcium accumulation without depleting ATP [Budd and Nicholls (1996) J. Neurochem. 67, 2282-2291]. Mitochondrial depolarization paradoxically decreases the cytoplasmic calcium elevation following glutamate addition, probably due to an enhanced calcium efflux from the cell. Cells undergo immediate calcium de-regulation in the presence of glutamate if the respiratory chain is inhibited; this is due to ATP depletion following ATP synthase reversal and can be reversed by oligomycin. In contrast, DCD is irreversible. Elevated cytoplasmic calcium is not excitotoxic as long as mitochondria are depolarized; alternative substrates do not rescue cells about to undergo DCD, suggesting that glycolytic failure is not involved. Mitochondria in situ remain sufficiently polarized during granule cell glutamate exposure to continue to generate ATP and show a classic mitochondrial state 3-state 4 hyperpolarization on inhibiting ATP synthesis; mitochondrial depolarization follows, and may be a consequence of rather than a cause of DCD. In addition, our studies show no evidence of the mitochondrial permeability transition prior to DCD. The mitochondrial generation of superoxide anions is enhanced during glutamate exposure and a working hypothesis is that DCD may be caused by oxidative damage to calcium extrusion pathways at the plasma membrane.