Privileged access to mitochondria of calcium influx through N-methyl-D-aspartate receptors

Mitochondrial Ca2+ uptake responds dynamically and sensitively to changes in cytosolic Ca2+ levels and plays a crucial role in sequestering the large Ca2+ load induced by N-methyl-D-aspartate (NMDA) receptor activation. However, the precise interrelationships between NMDA receptor activation, cytosolic Ca2+ increase, and mitochondrial Ca2+ uptake remain obscure. To reliably, independently, and simultaneously detect cytosolic and mitochondrial Ca2+ concentration changes in the same cell, we loaded primary striatal neurons with two Ca2+ indicators, calcium green 1N and rhod-2, and visualized the fluorescence signals from single neurons with laser scanning confocal fluorescence microscopy. In kinetic data analysis, only calcium green signals from predefined cytosolic areas and rhod-2 signals from predefined mitochondrial regions were used, and attention was focused on the initial rapid rising phase of the responses. When neurons were treated with 100 microM NMDA, increases of cytosolic and mitochondrial Ca2+ showed similar time courses and rates of change, and seemed to be time-locked. In contrast, when neurons were treated with 100 microM kainate, 50 mM KCl, or 0.3 microM ionomycin, mitochondrial Ca2+ increases lagged behind cytosolic Ca2+ increases. These data suggest that mitochondrial Ca2+ uptake in response to an increase of cytosolic Ca2+ is faster and more tightly coupled during NMDA receptor activation than during non-NMDA receptor or voltage-dependent Ca2+ channel activation. This proficient mitochondrial Ca2+ uptake may avert a large rise in cytosolic Ca2+ concentration in response to NMDA receptor activation. Yet, it may lead to excessive Ca2+ accumulation inside mitochondria and render mitochondria susceptible to Ca2+ mediated injury.