Complex influence of the L-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 45Ca measurements, and survival was assayed through cell counts. While 100 microM BayK8644 alone evoked a moderate elevation of intraneuronal calcium concentrations ([Ca2+]i), it dramatically attenuated the larger calcium influxes triggered by 500 microM N-methyl-D-aspartate. 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 may 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.
Donald W. Reynolds Department of Geriatrics, University of Arkansas for Medical Sciences, McClellan Veterans Affairs Medical Center, Little Rock, AR 72205, USA.
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D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology*; D000818:Animals; D002120:Calcium Channel Agonists / Q000494:pharmacology*; D015220:Calcium Channels / Q000502:physiology*; D020746:Calcium Channels, L-Type; D002470:Cell Survival / Q000187:drug effects; D018999:Charybdotoxin / Q000494:pharmacology; D004558:Electric Stimulation; D004594:Electrophysiology; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology*; D005260:Female; D005456:Fluorescent Dyes; D016257:Fura-2; D006624:Hippocampus / Q000166:cytology; D015640:Ion Channel Gating / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D016202:N-Methylaspartate / Q000494:pharmacology*; D009474:Neurons / Q000737:chemistry* / Q000187:drug effects; D009498:Neurotoxins / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D011247:Pregnancy; D051381:Rats; D017207:Rats, Sprague-Dawley
D002120:Calcium Channel Agonists; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D018690:Excitatory Amino Acid Agonists; D005456:Fluorescent Dyes; D009498:Neurotoxins; D018999:Charybdotoxin; D016202:N-Methylaspartate; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D011188:Potassium; D016257:Fura-2