Depression by sodium ions of calcium uptake mediated by non-N-methyl-D-aspartate receptors in cultured cerebellar neurons and correlation with evoked D-[3H]aspartate release.
In a previous study we noted that the release of D-[3H]aspartate evoked by non-N-methyl-D-aspartate (non-NMDA) receptor agonists in cultured rat cerebellar granule cells was enhanced in the absence of extracellular Na+. To explain this apparent paradox, we tried in the present investigation to correlate the effect of Na+ removal on the kainate (KA)- and quisqualate (QA)-induced D-[3H]aspartate release with that on KA- and QA-induced 45Ca2+ accumulation. The releasing activity of KA, which was only partially Ca2+ dependent in the presence of Na+, became totally Ca2+ dependent in its absence. Moreover, the releasing activity of QA, which was Ca2+ independent in the presence of Na+, became 50% Ca2+ dependent in the absence of the monovalent cation. The releasing action of both agonists was in all cases antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and that induced by KA was also sensitive to kynurenic acid. When glutamate was tested as an agonist in the presence of Na+, it was found that its D-[3H]aspartate releasing action was Ca2+ independent and was largely due to heteroexchange. The evoked release was Ca2+ independent, scarcely sensitive to CNQX, and insensitive to NMDA antagonists. In Na(+)-free medium, the glutamate-evoked D-[3H]aspartate release was lower (due to the abolishment of heteroexchange), but was totally Ca2+ dependent and antagonized by CNQX and kynurenate. KA (30 microM-1 mM) stimulated the accumulation of 45Ca2+ in a dose-dependent and CNQX-sensitive way, the effect being progressively higher as the Na+ concentration in the medium was decreased. Li+ affected KA-induced 45Ca2+ accumulation in a way similar to Na+, although 45Ca2+ uptake was somewhat lower in Li(+)-containing medium. The voltage-activated calcium channel antagonists La3+ and (-)-202-791 caused only a limited inhibition of the KA-induced 45Ca2+ influx both in the presence and in the absence of Na+. Under all the conditions tested [presence and absence of Na+ and of (-)-202-791], the kainate-induced 45Ca2+ uptake was scarcely sensitive to the NMDA antagonist 2-amino-5-phosphonovalerate. QA and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid also stimulated 45Ca2+ influx in a CNQX-sensitive way, the effect being enhanced in Na(+)-free media. These agonists were, however, less effective than KA.(ABSTRACT TRUNCATED AT 400 WORDS)
Section of Neurobiology, Istituto Superiore di Sanità, Rome, Italy.
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Gallo
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Giovannini
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Levi
D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism*; D002118:Calcium / Q000493:pharmacokinetics*; D002478:Cells, Cultured; D002531:Cerebellum / Q000166:cytology / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism / Q000494:pharmacology; D018698:Glutamic Acid; D007477:Ions; D007608:Kainic Acid / Q000494:pharmacology; D009474:Neurons / Q000378:metabolism; D016318:Quisqualic Acid / Q000494:pharmacology; D017470:Receptors, Glutamate; D016194:Receptors, N-Methyl-D-Aspartate / Q000502:physiology; D017981:Receptors, Neurotransmitter / Q000502:physiology*; D012964:Sodium / Q000494:pharmacology*; D014316:Tritium
D005971:Glutamates; D007477:Ions; D017470:Receptors, Glutamate; D016194:Receptors, N-Methyl-D-Aspartate; D017981:Receptors, Neurotransmitter; D014316:Tritium; D001224:Aspartic Acid; D018698:Glutamic Acid; D016318:Quisqualic Acid; D012964:Sodium; D007608:Kainic Acid; D002118:Calcium