Stable expression of recombinant AMPA receptor subunits: binding affinities and effects of allosteric modulators

Homomeric AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)-type glutamate receptors (GluRs) were stably expressed in kidney cells from cDNAs encoding GluR1 flop, GluR2 flip, GluR2 flop, and GluR3 flop subunits. The recombinant receptors were of the expected size and showed functional properties in whole-cell recording as previously reported. [3H]AMPA binding to all subunits was increased to a similar extent by the chaotropic ion thiocyanate (SCN-). Significant differences were found in the Scatchard plots, however, which were linear and of high affinity for GluR1 and -3 receptors (K(D) values of 33 and 52 nM, respectively) but showed curvature for GluR2 receptors, indicating the presence of two components with distinct affinities. As with brain AMPA receptors, solubilization of GluR2 receptors reduced the number of lower-affinity sites and correspondingly increased the number of higher-affinity sites. The sulfhydryl reagent p-chloromercuriphenylsulfonic acid, which increases binding to brain receptors, produced only minor changes except in the case of GluR2 flip. These results indicate that GluR2, among the subunits examined here, most closely resembles the native AMPA receptors in brain membranes. [3H]AMPA binding was inhibited in a noncompetitive manner by two drugs that change the desensitization kinetics of the AMPA receptor. In agreement with physiological observations, the apparent affinity of cyclothiazide for GluR2 flip (EC50 = 7 microM) was higher than that for receptors made of flop subunits (49-130 microM). In contrast, BDP-37, a member of the benzamide family of drugs, exhibited a lower potency for GluR2 flip (58 microM) than for any of the flop isoforms (18-40 microM). These results predict that the action of centrally active AMPA-receptor modulators varies across brain regions depending on their flip/flop composition.