Increase of the intracellular Ca2+ concentration mediated by transport of glutamate into rat hippocampal synaptosomes: characterization of the activated voltage sensitive Ca2+ channels

Abstract

The changes in the intracellular free Ca2+ concentration, [Ca2+]i, mediated by glutamate and D-aspartate into rat hippocampal synaptosomes was studied. Glutamate increased the [Ca2+]i in a dose-dependent manner with an EC50 of 1.87 [mu]M and a maximal increase of 31.5±0.9 nM. We also observed that stimulation of the synaptosomes with 100 [mu]M [alpha]-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), 100 [mu]M kainate, or 100 [mu]M D-aspartate increased the synaptosomal [Ca2+]i. The effect of either of these non-NMDA receptor agonists and of D-aspartate was additive, suggesting the activation of two different components (the ionotropic non-NMDA receptors or the glutamate transporters). Stimulation of synaptosomes with 100 [mu]M glutamate increased the [Ca2+]i and prevented the effect of either non-NMDA receptor agonists and the effect of D-aspartate. We also observed that incubation of the synaptosomes with D-aspartate induced the Ca2+-independent release of glutamate, possibly through the reversal of the glutamate carrier. The aim of incubating the synaptosomes with D-aspartate was to avoid undesirable secondary activation of glutamate receptors. After incubating the synaptosomes with 100 [mu]M D-aspartate (10 min at 37°C), the subsequent stimulation with D-aspartate increased the [Ca2+]i due to glutamate transport. this increase in [Ca2+]i induced by 100 [mu]M D-aspartate was insensitive to 1 [mu]M nitrendipine, but was inhibited by about 50% by the presence of both 500 nM [omega]-CgTx GVIA and 100 nM [omega]-Aga IVA or by 500 nM [omega]-CgTx MVIIC. We clearly identified two different processes by which glutamate increased the [Ca2+]i in rat hippocampal synaptosomes: activation of non-NMDA receptors and activation of the glutamate transporters. We also characterized the voltage sensitive Ca2+ channels (VSCC) activated as a consequence of the glutamate transport, and determined that class B (N-type) and class A (P or Q-type) Ca2+ channels were responsible for about 50% of the signal.