PRIVILEGED ACCESS TO MITOCHONDRIA OF CALCIUM INFLUX THROUGH N-METHYL-D-ASPARTATE RECEPTORS

Mitochondrial Ca2+ uptake responds dynamically and sensitively to chan ges in cytosolic Ca2+ levels and plays a crucial role in sequestering the large Ca2+ load induced by N-methyl-D-aspartate (NMDA) receptor ac tivation. However, the precise interrelationships between NMDA recepto r activation, cytosolic Ca2+ increase, and mitochondrial Ca2+ uptake r emain obscure. To reliably, independently, and simultaneously detect c ytosolic 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 sin gle neurons with laser scanning confocal fluorescence microscopy. In k inetic data analysis, only calcium green signals from predefined cytos olic areas and rhod-2 signals from predefined mitochondrial regions we re used, and attention was focused on the initial rapid rising phase o f the responses. When neurons were treated with 100 mu M NMDA, increas es of cytosolic and mitochondrial Ca2+ showed similar time courses and rates of change, and seemed to be time-locked. In contrast, when neur ons were treated with 100 mu M kainate, 50 mM KCl, or 0.3 mu M ionomyc in, mitochondrial Ca2+ increases lagged behind cytosolic Ca2+ increase s. 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-depe ndent Ca2+ channel activation. This proficient mitochondrial Ca2+ upta ke may avert a large rise in cytosolic Ca2+ concentration in response to NMDA receptor activation. Yet, it may lead to excessive Ca2+ accumu lation inside mitochondria and render mitochondria susceptible to Ca2 mediated injury.