The role of intracellular Na+ and mitochondria in buffering of kainate-induced intracellular free Ca2+ changes in rat forebrain neurones.

1. We have examined the mechanisms by which cultured central neurones from embryonic rat brain buffer intracellular Ca2+ loads following kainate receptor activation using fluorescent indicators of [Ca2+]i and [Na+]i. 2. Stimulation of cultured forebrain neurones with 100 microM kainate produced a rapid increase in [Ca2+]i that displayed a variable rate of recovery. Kainate also increased [Na+]i with a response that was slightly slower in onset and markedly slower in recovery. 3. The recovery of [Ca2+]i to baseline was not very sensitive to the [Na+]i. The magnitude of the increase in [Na+]i in response to kainate did not correlate well with the [Ca2+]i recovery time, and experimental manipulations that altered [Na+]i did not have a large impact on the rate of recovery of [Ca2+]i. 4. The recovery of [Ca2+]i to baseline was accelerated by the mitochondrial Na+-Ca2+ exchange inhibitor CGP-37157, suggesting that the recovery rate is influenced by release of Ca2+ from a mitochondrial pool and also that variation in the recovery rate is related to the extent of mitochondrial Ca2+ loading. Kainate did not alter the mitochondrial membrane potential. 5. These studies reveal that mitochondria have a central role in buffering neuronal [Ca2+]i changes mediated by non-N-methyl-D-aspartate (NMDA) glutamate receptors, and that the variation in recovery times following kainate receptor activation reflects a variable degree of mitochondrial Ca2+ loading. However, unlike NMDA receptor-mediated Ca2+ loads, kainate receptor activation has minimal effects on mitochondrial function.