L-Quisqualic acid transport into hippocampal neurons by a cystine-sensitive carrier is required for the induction of quisqualate sensitization

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 (K_M = 0.54 mM) for a high capacity (V_max = 0.9 nmol (mg protein)^-1 min^-1) Na⁺-dependent transport system(s) and a high-affinity substrate (K_M = 0.033 mM) for a low-capacity (V_max = 0.051 nmol (mg protein)^-1 min^-1) transporter with properties similar to the cystine/glutamate exchange carrier, System x_c^-. We present evidence that suggests that System x_c^- participates in quisqualate sensitization. First, simultaneous application of L-quisqualate and inhibitors of System x_c^-, 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 x_c^-, including cystine. Third, immunocytochemical analysis of L-quisqualate uptake demonstrates that only inhibitors of System x_c^- 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 x_c^-. 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.