Role of kainate receptor activation and desensitization on the [Ca2+](i) changes in cultured rat hippocampal neurons

We investigated the role of kainate (KA) receptor activation and desensitiz ation in inducing the increase in the intracellular free Ca2+ concentration ([Ca2+](i)) in individual cultured rat hippocampal neurons. The rat hippoc ampal neurons in the cultures were shown to express kainate receptor subuni ts, KA2 and GluR6/7, either by immunocytochemistry or by immunoblot analysi s. The effect of LY303070, an alpha -amino-3-hydroxy-5-methyl-isoxazole-4-p ropionate (AMPA) receptor antagonist, on the alterations in the [Ca2+](i) c aused by kainate showed cell-to-cell variability. The [Ca2+](i) increase ca used by kainate was mostly mediated by the activation of AMPA receptors bec ause LY303070 inhibited the response to kainate in a high percentage of neu rons. The response to kainate was potentiated by concanavalin A (Con A), wh ich inhibits kainate receptor desensitization, in 82.1 % of the neurons, an d this potentiation was not reversed by LY303070 in about 38% of the neuron s. Also, upon stimulation of the cells with 4-methylglutamate (MGA), a sele ctive kainate receptor agonist, in the presence of Con A, it was possible t o observe [Ca2+](i) changes induced by kainate receptor activation, because LY303070 did not inhibit the response in all neurons analyzed. In toxicity studies, cultured rat hippocampal neurons were exposed to the drugs for 30 min, and the cell viability was evaluated at 24 hr using the 3-(4,5-dimeth ylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The selective activation of kainate receptors with MGA, in the presence of Con A, induced a toxic effect, which was not prevented by LY303070, revealing a contribut ion of a small subpopulation of neurons expressing kainate receptors that i ndependently mediate cytotoxicity. Taken together, these results indicate t hat cultured hippocampal neurons express not only AMPA receptors, but also kainate receptors, which can modulate the [Ca2+](i) and toxicity. (C) 2001 Wiley-Liss, Inc.