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.

The two enantiomeric pairs of erythro- and threo-amino-(3'-hydroxy-4',5'-dihydro-isoxazol-5'-yl)-acetic acids were synthesized via the 1,3-dipolar cycloaddition of bromonitrile oxide to ( R)- or ( S)-3-( tert-butoxycarbonyl)-2,2-dimethyl-4-vinyloxazolidine. The pharmacological profiles of the studied amino acids reflect the relationship between the activity/selectivity and the stereochemistry of the two stereogenic centers: while the (2 S,5' S) stereoisomer is an agonist at the AMPA and KA receptors, its (2 R,5' R) enantiomer interacts selectively with the NMDA receptors; the (2 S,5' R) stereoisomer is the only one capable to activate the mGluRs.