Abstract

Comparison of the kinetics of the inward Ca²⁺ ion flux to (S)-α-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid [(S)-AMPA] in cerebrocortical homogenates and that of the previously reported transmembrane Na⁺ ion influx mediated by an AMPA receptor in hippocampal homogenates established that the agonist-induced opening of the AMPA receptor channels occurs in two kinetically distinguishable phases. Here we report that the 2-methyl-4-oxo-3H-quinazoline-3-acetic acid (Q1) inhibits the major slow-phase response specifically, whereas the acetyl piperidine derivative (Q5) is a more potent inhibitor of the fast-phase response. Both the quinazolone-3-propionic acid (Q2) and the quinazolone-3-acetic acid methyl ester (Q3) enhanced the slow-phase response to (S)-AMPA. The information provided by docking different Q1, Q2, and Q5 models at the ligand-binding core of iGluRs were used to define agonistic and antagonistic modes of interactions. Based on the effects of quinazolone-3-alkyl-carboxylic acid derivatives on specific [³H]Glu binding and kinetically distinguishable Ca²⁺ ion permeability responses to (S)-AMPA and molecular modeling, the fast- and the slow-phase (S)-AMPA-elicited Ca²⁺ ion fluxes were corresponded to different subunit compositions and degrees of S1S2 bridging interaction relative to substitution of kainate thereupon. Substitutions of agonists and antagonists into the iGluR2 S1S2 ligand binding core induced different modes of domain-domain bridging.