Autoradiographic Characterization and Localization of Quisqualate Binding Sites in Rat Brain Using the Antagonist [3H]6‐Cyano‐7‐Nitroquinoxaline‐2,3‐Dione: Comparison with (R,S)‐[3H]α‐Amino‐3‐Hydroxy‐5‐Methyl‐4‐Isoxazolepropionic Acid Binding Sites

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Abstract
Using quantitative autoradiography, we have investigated the binding sites for the potent competitive non‐N‐methyl‐D‐aspartate (non‐NMDA) glutamate receptor antagonist [3H]6‐cyano‐7‐nitro‐quinoxaline‐2,3‐dione ([3H]‐CNQX) in rat brain sections. [3H]CNQX binding was regionally distributed, with the highest levels of binding present in hippocampus in the stratum radiatum of CA1, stratum lucidum of CA3, and molecular layer of dentate gyrus. Scatchard analysis of [3H]CNQX binding in the cerebellar molecular layer revealed an apparent single binding site with a KD= 67 ± 9.0 nM and Bmax= 3.56 ± 0.34 pmol/mg protein. In displacement studies, quisqualate, L‐glutamate, and kainate also appeared to bind to a single class of sites. However, (R,S)‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) displacement of [3H]CNQX binding revealed two binding sites in the cerebellar molecular layer. Binding of [3H]AMPA to quisqualate receptors in the presence of potassium thiocyanate produced curvilinear Scatchard plots. The curves could be resolved into two binding sites with KD1= 9.0 ± 3.5 nM, Bmax= 0.15 ± 0.05 pmol/mg protein, KD2= 278 ± 50 nM, and Bmax= 1.54 ± 0.20 pmol/mg protein. The heterogeneous anatomical distribution of [3H]CNQX binding sites correlated to the binding of L‐[3H]glutamate to quisqualate receptors and to sites labeled with [3H]AMPA. These results suggest that the non‐NMDA glutamate receptor antagonist [3H]CNQX binds with equal affinity to two states of quisqualate receptors which have different affinities for the agonist [3H]AMPA.