Discrimination by added ions of ligands at ionotropic excitatory amino acid receptors insensitive to N-methyl-D-aspartate in rat brain using membrane binding techniques.

The addition of potassium thiocyanate almost quadrupled binding of [3H]DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) to an AMPA-sensitive subclass of brain excitatory amino acid receptors in rat brain synaptic membranes, treated with Triton X-100. Among several ligands tested, quisqualic acid (QA) was the most potent displacer of [3H]AMPA binding in the absence of added SCN- ions, followed by AMPA, 6,7-dinitroquinoxaline-2,3-dione (DNQX), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), glutamic (Glu) and kainic (KA) acids in a rank order of decreasing potency. The addition of SCN- ions was effective in significantly reducing the potencies of antagonists such as DNQX and CNQX, without affecting those of agonists including QA, AMPA, Glu and KA. On the other, the addition of Ca2+ ions significantly inhibited [3H]KA binding in a concentration-dependent manner at concentrations of above 2.5 mM. Calcium ions were also effective in significantly potentiating potencies to displace [3H]KA binding of antagonists such as DNQX and CNQX, with concomitant reduction of those of agonists including KA, QA and Glu. However, N-methyl-D-aspartic acid (NMDA) did not affect binding of both radioligands at concentrations of below 0.1 mM. These results suggest that both SCN- and Ca2+ ions may be useful to discriminate agonists and antagonists among a variety of displacers of ligand binding to the non-NMDA receptors in the brain.