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

Abstract

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. Stimulation of cultured forebrain neurones with 100 μm 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. 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. 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. 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.

Effects of Oxidants and Glutamate Receptor Activation on Mitochondrial Membrane Potential in Rat Forebrain Neurons

“…In addition, mitochondria are involved in buffering increases in . [Ca2~I 1 induced by kainate (Hoyt et al, 1998). Thus, it is interesting that kainate does not depolarize mitochondriain our neurons (data not shown).…”

Prolonged Positive Modulation of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors Induces Calpain-Mediated PSD-95/Dlg/ZO-1 Protein Degradation and AMPA Receptor Down-Regulation in Cultured Hippocampal Slices

“…However, AMPArs are Na ϩ channels and are, by and large, impermeable to Ca 2ϩ ions (Lomeli et al, 1994;Jensen et al, 1998;Carlson et al, 2000;Iizuka et al, 2000;Krampfl et al, 2002;Kumar et al, 2002). Interestingly, elevated intracellular Na ϩ levels can increase Ca 2ϩ release from intracellular stores (Hoyt et al, 1998;Zhang and Lipton 1999), thus CX614-induced increases in AMPAr function and the resulting depolarization may contribute to increased intracellular levels of Ca 2ϩ . Pertinent to this, ryanodine receptors, which regulate intracellular Ca 2ϩ release, are also targets for calpain proteolysis (Shoshan-Barmatz et al, 1994).…”

Spontaneous Changes in Mitochondrial Membrane Potential in Cultured Neurons

“…First, we treated neurons with kainic acid, an agonist of the AMPA-kainate subtype of glutamate receptors. Activation of glutamate receptors with kainic acid results in increased intracellular sodium and calcium but does not lead to mitochondrial depolarization (Courtney et al, 1995, Hoyt et al, 1998. Accumulation of calcium within mitochondria is also significantly lower using kainic acid than it is with NMDA (Budd and Nicholls, 1996;Stout et al, 1998).…”

“…11 B). The differences between glutamate and kainate on ⌬⌿ m fluctuations are not surprising in light of the observation that their ability to activate glutamate receptors is not equivalent qualitatively or quantitatively (Hoyt et al, 1998;Sattler et al, 1998;Brocard et al, 2001). …”