DEPRESSION BY SODIUM-IONS OF CALCIUM-UPTAKE MEDIATED BY NON-N-METHYL-D-ASPARTATE RECEPTORS IN CULTURED CEREBELLAR NEURONS AND CORRELATION WITH EVOKED D-[H-3]ASPARTATE RELEASE

In a previous study we noted that the release of D-[H-3]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-[H-3]aspartate release with that on KA- and QA-induced Ca-45(2+) 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-[H-3]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-[H-3]aspartate release was lower (due to the abolishment of heteroexchange), but was totally Ca2+ dependent and antagonized by CNQX and kynurenate. KA (30-mu-M-1 mM) stimulated the accumulation of Ca-45(2+) 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 Ca-45(2+) accumulation in a way similar to Na+, although Ca-45(2+) 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 Ca-45(2+) 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 Ca-45(2+) 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 Ca-45(2+) influx in a CNQX-sensitive way, the effect being enhanced in Na+-free media. These agonists were, however, less effective than KA. It is concluded that in cultured cerebellar granule cells the influx of Ca2+ through non-NMDA receptor channels is inhibited by Na+ and similar monovalent cations, and that the potentiated KA- and QA-induced D-[H-3]aspartate release observed in Na+-free medium can be accounted for by the increased accumulation of Ca2+ occurring under this condition.