Abstract

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype of ionotropic glutamate receptors consists of rapidly gating ion channels. Positive modulation of channel gating may slow gating kinetics through at least two distinct mechanisms, evidenced by the predominant slowing of either the rate of receptor desensitization or the rate of offset after agonist withdrawal (deactivation). This study compares the actions of two positive allosteric modulators [cyclothiazide, which modulates desensitization, and 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546), which modulates deactivation] in a mutant shown previously to impede modulation by cyclothiazide. These experiments test the hypothesis that the point mutation, GluR1(S493T), would also cause a loss of modulation by CX546. Wild-type GluR1 through -4 receptors were modulated by CX546, as assayed by the potentiation of steady-state currents in the Xenopus laevis oocyte expression system. CX546 potentiated steady-state currents of both splice isoforms of GluR1. Modulation by CX546 was completely abolished in GluR1(S493T) and its homolog, GluR2(S497T), although this mutation did not affect apparent agonist affinity in the absence of CX546. Thus, the GluR1(S493T) mutation has a similar impairment of modulation by either cyclothiazide or CX546, indicating that some residues at the subunit interface of glutamate receptors play an important role in channel deactivation and desensitization.