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-beta-hydroxamate (L-AbetaH). 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-AbetaH. 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 beta-N-oxalyl-L-alpha,beta-diaminopropionic acid, beta-L-ODAP) may trigger neurotoxicity.
Chase
L A
LA
Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, 321 Church Street SE, Minneapolis, MN 55455, USA.
Roon
R J
RJ
Wellman
L
L
Beitz
A J
AJ
Koerner
J F
JF
D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D002352:Carrier Proteins / Q000187:drug effects / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D003553:Cystine / Q000378:metabolism* / Q000493:pharmacokinetics; D004347:Drug Interactions / Q000502:physiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D018698:Glutamic Acid / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D007150:Immunohistochemistry; D007395:Interneurons / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009498:Neurotoxins / Q000378:metabolism; D009924:Organ Culture Techniques; D010599:Pharmacokinetics; D017966:Pyramidal Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D016318:Quisqualic Acid / Q000493:pharmacokinetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013462:Sulfur Radioisotopes / Q000493:pharmacokinetics; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D002352:Carrier Proteins; D004791:Enzyme Inhibitors; D009498:Neurotoxins; D013462:Sulfur Radioisotopes; D018698:Glutamic Acid; D003553:Cystine; D005680:gamma-Aminobutyric Acid; D016318:Quisqualic Acid