SULFUR-CONTAINING EXCITATORY AMINO ACID-EVOKED CA2+-INDEPENDENT RELEASE OF D-[3H]ASPARTATE FROM CULTURED CEREBELLAR GRANULE CELLS - THE ROLE OF GLUTAMATE RECEPTOR ACTIVATION COUPLED TO REVERSAL OF THE ACIDIC AMINO-ACID PLASMA-MEMBRANE CARRIER

Sulphur-containing excitatory amino acid transmitter candidates (500-mu-M) stimulated the Ca2+-independent efflux of exogenously-supplied D-[H-3]aspartate from primary cultures of cerebellar granule cells superfused continuously with HEPES-buffered saline containing CoCl2 (1 MM) in place of CaCl2. The stimulated release of D-[H-3]aspartate was markedly attenuated by 200-mu-M 6,7-dinitroquinoxalinedione, a concentration at which the antagonist inhibits both non-N-methyl-D-aspartate and N-methyl-D-aspartate ionotropic excitatory amino acid receptors. The Ca2+-independent component of evoked release was also markedly attenuated and, in some cases, abolished by removing NaCl from the superfusion medium. Furthermore, when 700-mu-M dihydrokainate (demonstrated herein as a mixed/non-competitive inhibitor of the high-affinity dicarboxylic amino acid transporter in cultured granule cells) was included in the superfusion medium, stimulated efflux of D-[H-3]aspartate was reduced by between 15-78% of the control response; the extent of inhibition varying with the agonist employed. In contrast, agents which act as competitive inhibitors of the plasma membrane carrier in granule cells, e.g. beta-methylene-D,L-aspartate, potentiated the release of D-[H-3]aspartate in a synergistic manner. Taken together, these findings are consistent with a mechanism for the Ca2+-independent release of D-[H-3]aspartate that is mediated predominantly by activation of excitatory amino acid receptors resulting in a reversal of the high-affinity dicarboxylic amino acid transport system. Although the physiological relevance of such non-vesicular release from the cytosol remains obscure and is still a matter of some debate, this mode of release may be of pathological significance.