Mitochondrial transmembrane potential and free radical production in excitotoxic neurodegeneration

Excitotoxic cell death is involved in many forms of acute and chronic neurodegeneration. We induced excitotoxic cell death in cultured rat hippocampal neurons by brief exposure to two selective glutamate receptor agonists with different neurotoxic potencies, N-methyl-D-aspartate (NMDA) and kainate (KA). Digital video imaging was performed during exposure to the agonists to monitor free radical production and changes in mitochondrial transmembrane potential, psi(m). Brief exposure to NMDA (10 min) induced significant cell death in the hippocampal neurons reaching a maximum at a concentration of 300 microM (57.2+/-2.6% cell death; P<0.001). In parallel imaging experiments we found that exposure to NMDA (300 microM, 10 min) induced a significant increase in superoxide production monitored with the oxidation-sensitive probe, hydroethidine (increase of 280+/-33% above baseline; P<0.001). Rhodamine-123-based imaging revealed a loss of psi(m) in 70.1+/-10.1 % of the hippocampal neurons during the exposure to NMDA. In contrast to NMDA, brief exposure to KA (10 min) produced limited neurotoxicity reaching a maximum at a concentration of 100 microM (10.2+/-4.0% cell death; P<0.05). Exposure to KA (100 microM, 10 min) also caused a significant increase in superoxide production. This increase, however, was significantly less pronounced than that produced by NMDA (increase of 94+/-17% above baseline; P<0.001 compared to controls or NMDA-exposed cultures). Moreover, rhodamine-123-based imaging revealed that KA (100 microM) caused a collapse of psi(m) in only 13.5+/-1.4% of the hippocampal neurons. In conclusion, the present study demonstrates that early changes in intracellular superoxide production and psi(m) relate to neuronal survival outcome in excitotoxic cell death.