Complex influence of the 1-type calcium-channel agonist BayK8644(±) on N-methyl-D-aspartate responses and neuronal survival

Past studies have implicated calcium influx through the N-methyl-D-aspartate class of ionotropic glutamate receptors as a key factor in excitotoxicity. Here, primary cultures of hippocampal neurons were exposed to N-methyl-D-aspartate with or without the L-type calcium channel agonist BayK8644(+/-). Calcium influxes were monitored with Fura-2 microfluorescent imaging and Ca-45 measurements, and survival was assayed through cell counts. While 100 mu M BayK8644 alone evoked a moderate elevation of intraneuronal calcium concentrations ([Ca2+](i)), it dramatically attenuated the larger calcium influxes triggered by 500 mu M N-methyl-D-arpartate. This attenuation was non-competitive and reversible; it was not inhibited by charybdotoxin or cyclosporin A. In spite of this: attenuation of [Ca2+](i) responses, 5-min exposures to BayK8644 produced much greater neurotoxicity 24 h later than did doses of N-methyl-D-aspartate evoking larger [Ca2+](i) increases. This neurotoxicity was not observed with potassium-mediated depolarization or cobalt; indeed, both reversed the neurotoxicity of BayK8644. The relevant conclusions are two-fold: BayK8644 inhibits influx of calcium through a ligand-gated glutamate receptor, and BayK8644 exhibits considerable neurotoxicity. The former effect does not appear to depend upon the major metabolic pathways that modulate N-methyl-D-aspartate channels and thus map involve a direct allosteric interaction with the N-methyl-D-aspartate receptor. The toxicity of BayK8644 depends, at least partially, upon its activation of voltage-gated (cobalt-sensitive) calcium channels. However, the reversal of this toxicity by depolarization suggests that depolarization can be beneficial to neuronal survival through mechanisms other than calcium influx through voltage-gated calcium channels. (C) 1998 IBRO. Published by Elsevier Science Ltd.