Calcium Sequestering Ability of Mitochondria Modulates Influx of Calcium through Glutamate Receptor Channel

The excitotoxicity of glutamate is believed to be mediated by sustained increase in the cytosolic Ca2+ concentration. Mitochondria play a vital role in buffering the cytosolic calcium overload in stimulated neurons. Here we have studied the glutamate induced Ca2+ signals in cortical brain slices under physiological conditions and the conditions that modify the mitochondrial functions. Exposure of slices to glutamate caused a rapid increase in [Ca2+]i followed by a slow and persistently rising phase. The rapid increase in [Ca2+]i was mainly due to influx of Ca2+ through the N-methyl-D-aspartate (NMDA) receptor channels. Glutamate stimulation in the absence of Ca2+ in the extracellular medium elicited a small transient rise in [Ca2+]i which can be attributed to the mobilization of Ca2+ from IP3 sensitive endoplasmic reticulum pools consequent to activation of metabotropic glutamate receptors. The glutamate induced Ca2+ influx was accompanied by depolarization of the mitochondrial membrane, which was inhibited by ruthenium red, the blocker of mitochondrial Ca2+ uniporter. These results imply that mitochondria sequester the Ca2+ loaded into the cytosol by glutamate stimulation. Persistent depolarization of mitochondrial membrane observed in presence of extracellular Ca2+ caused permeability transition and released the sequestered Ca2+ which is manifested as slow rise in [Ca2+]i. Protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) depolarized the mitochondrial membrane and enhanced the glutamate induced [Ca2+]i response. Contrary to this, treatment of slices with mitochondrial inhibitor oligomycin or ruthenium red markedly reduced the [Ca2+]i response. Combined treatment with oligomycin and rotenone further diminished the [Ca2+]i response and also abolished the CCCP mediated rise in [Ca2+]i. However, rotenone alone had no effect on glutamate induced [Ca2+]i response. These changes in glutamate-induced [Ca2+]i response could not be explained on the basis of deficient mitochondrial Ca2+ sequestration or ATP dependent Ca2+ buffering. The mitochondrial inhibitors reduced the cellular ATP/ADP ratio, however, this would have restrained the ATP dependent Ca2+ buffering processes leading to elevation of [Ca2+]i. In contrast our results showed repression of Ca2+ signal except in case of CCCP which drastically reduced the ATP/ADP ratio. It was inferred that, under the conditions that hamper the Ca2+ sequestering ability of mitochondria, the glutamate induced Ca2+ influx could be impeded. To validate this, influx of Mn2+ through ionotropic glutamate receptor channel was monitored by measuring the quenching of Fura-2 fluorescence. Treatment of slices with oligomycin and rotenone prior to glutamate exposure conspicuously reduced the rate of glutamate induced fluorescence quenching as compared to untreated slices. Thus our data establish that the functional status of mitochondria can modify the activity of ionotropic glutamate receptor and suggest that blockade of mitochondrial Ca2+ sequestration may desensitize the NMDA receptor operated channel.