The difference between mechanisms of kainate and glutamate excitotoxicity in vitro: Osmotic lesion versus mitochondrial depolarization

The hypothesis that a destabilization of mitochondrial function during neur onal exposure to excitatory amino acids may be involved in the mechanism of neuronal death was examined. The mitochondrial membrane potential (Delta P si(m)) and the cytoplasmic Ca2+ concentration ([Ca2+](c)) were monitored si multaneously in single cultured rat cerebellar granule cells (CGCs) loaded with tetramethylrhodamine methyl eater (TMR) and fura-2; CGCs were depolari zed with K+, or exposed to excitotoxic doses of glutamate or kainate, and v iability of the same neurons was studied for 24-30 h. This approach made it possible to single out the neurons that died, and to describe the changes in Delta Psi(m) and [Ca2+](c) that were characteristic for these neurons. E xposure to glutamate caused an increase in [Ca2+](c) that was associated wi th a decrease in the mitochondrial TMR fluorescence, which indicates a decr ease in Delta Psi(m). The neurons that failed to restore Delta Psi(m) follo wing glutamate withdrawal, also failed to restore low [Ca2+](c), and later died. Although a similar number of neurons died following kainate exposure as did after glutamate exposure, the kainate-elicited neuronal death result ed not from the collapse of Delta Psi(m) but from an excessive neuronal swe lling, which led to rupture of the plasma membrane. Depolarization with Kwas not neurotoxic and caused only a minor decrease in TMR fluorescence. Th ese results indicate that in vitro glutamate and kainate destroy neurons by different mechanisms: glutamate by a failure to restore Delta Psi(m) follo wing the exposure, and kainate by an osmotic lesion of the plasma membrane.