Functional characterization of Tet-AMPA [tetrazolyl-2-amino-3-(3-hydroxy-5-methyl- 4-isoxazolyl)propionic acid] analogues at ionotropic glutamate receptors GluR1-GluR4. The molecular basis for the functional selectivity profile of 2-Bn-Tet-AMPA.

Four 2-substituted Tet-AMPA [Tet = tetrazolyl, AMPA = 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid] analogues were characterized functionally at the homomeric AMPA receptors GluR1i, GluR2Qi, GluR3i, and GluR4i in a Fluo-4/Ca2+ assay. Whereas 2-Et-Tet-AMPA, 2-Pr-Tet-AMPA, and 2-iPr-Tet-AMPA were nonselective GluR agonists, 2-Bn-Tet-AMPA exhibited a 40-fold higher potency at GluR4i than at GluR1i. Examination of homology models of the S1-S2 domains of GluR1 and GluR4 containing 2-Bn-Tet-AMPA suggested four nonconserved residues in a region adjacent to the orthosteric site as possible determinants of the GluR4i/GluR1i selectivity. In a mutagenesis study, doubly mutating M686V/I687A in GluR1i in combination with either D399S or E683A increased both the potency and the maximal response of 2-Bn-Tet-AMPA at this receptor to levels similar to those elicited by the agonist at GluR4i. The dependence of the novel selectivity profile of 2-Bn-Tet-AMPA upon residues located outside of the orthosteric site underlines the potential for developing GluR subtype selective ligands by designing compounds with substituents that protrude beyond the (S)-Glu binding pocket.