DISCRIMINATION BY ADDED IONS OF LIGANDS AT IONOTROPIC EXCITATORY AMINO-ACID RECEPTORS INSENSITIVE TO N-METHYLY-D-ASPARTATE IN RAT-BRAIN USING MEMBRANE-BINDING TECHNIQUES

The addition of potassium thiocyanate almost quadrupled binding of lph a-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) to an AMPA -sensitive subclass of brain excitatory amino acid receptors in rat br ain synaptic membranes treated with Triton X-100. Among several ligand s tested, quisqualic acid (QA) was the most potent displacer of [H-3]A MPA binding in the absence of added SCN- ions, followed by AMPA, 6,7-d initroquinoxaline-2,3-dione (DNQX), 6-cyano-7-nitroquinoxaline-2,3-dio ne (CNQX), glutamic (Glu) and kainic (KA) acids in a rank order of dec reasing potency. The addition of SCN- ions was effective in significan tly reducing the potencies of antagonists such as DNQX and CNQX, witho ut affecting those of agonists including QA, AMPA, Glu and KA. On the other hand, the addition of Ca2+ ions significantly inhibited [H-3]KA binding in a concentration-dependent manner at concentrations of above 2.5 mM. Calcium ions were also effective in significantly potentiatin g potencies to displace [H-3]KA binding of antagonists such as DNQX an d CNQX, with concomitant reduction of those of agonists including KA, QA and Glu. However, N-methyl-D-aspartic acid (NMDA) did not affect bi nding of both radioligands at concentrations of below 0.1 mM. These re sults suggest that both SCN- and Ca2+ ions may be useful to discrimina te agonists and antagonists among a variety of displacers of ligand bi nding to the non-NMDA receptors in the brain.