A brief exposure of hippocampal slices to L-quisqualic acid sensitizes CA1 pyramidal neurons 30-250-fold to depolarization by two classes of excitatory amino acid analogues: (1) those whose depolarizing effects are rapidly terminated following washout, e.g. L-2-amino-4-phosphonobutanoic acid (L-AP4) and L-2-amino-6-phosphonohexanoic acid (L-AP6) and (2) those whose depolarizing effects persist following washout, e.g. L-aspartate-β-hydroxamate (L-AβH). This process has been termed quisqualate sensitization. In this study we directly examine the role of amino acid transport systems in the induction of quisqualate sensitization. We report that L-quisqualate is a low-affinity substrate (KM = 0.54 mM) for a high capacity (Vmax = 0.9 nmol (mg protein)-1 min-1) Na+-dependent transport system(s) and a high-affinity substrate (KM = 0.033 mM) for a low-capacity (Vmax = 0.051 nmol (mg protein)-1 min-1) transporter with properties similar to the cystine/glutamate exchange carrier, System xc-. We present evidence that suggests that System xc- participates in quisqualate sensitization. First, simultaneous application of L-quisqualate and inhibitors of System xc-, but not inhibitors of Na+-dependent glutamate transporters, prevents the subsequent sensitization of hippocampal neurons to phosphonates or L-AβH. Second, L-quisqualic acid only sensitizes hippocampal neurons to other substrates of System xc-, including cystine. Third, immunocytochemical analysis of L-quisqualate uptake demonstrates that only inhibitors of System xc- inhibit the highly concentrative uptake of L-quisqualate into a widely dispersed group of GABAergic hippocampal interneurons. We conclude that quisqualate sensitization is a direct consequence of the unique interaction of various excitatory amino acids, namely L-quisqualate, cystine, and phosphonates, with the exchange carrier, System xc-. Therefore, the results of this study have important implications for the mechanism by which L-quisqualate, and other substrates of this transporter which are also excitatory amino acid agonists (such as glutamate and β-N-oxalyl-L-α,β-diaminopropionic acid, β-L-ODAP) may trigger neurotoxicity.
Bibliographical noteFunding Information:
Supported by National Institutes of Health Grant NS 35073. L.A.C. was also supported in part by NS 35608 and by Graduate School, Shevlin, Dosdall and Doctoral Dissertation Fellowships from the University of Minnesota .
- System x