Effects of the lurcher mutation on GluR1 desensitization and activation kinetics

Previous studies of the lurcher mutation in GluR1 channels concluded that its main effect is to create constitutively active channels (Kohda et al., 2000; Taverna et al., 2000). GluR1Lc channels also exhibit slowed kinetics and a shift in their apparent affinity for glutamate ( Kohda et al., 2000; Taverna et al., 2000). Here, we have undertaken a kinetic analysis of GluR1Lc channels to quantify the effects of lurcher and to determine the relative contribution of these effects to the lurcher phenotype. Analysis of GluR1Lc leak current demonstrated that the 2,3-dioxo-6-nitro- 1,2,3,4-tetrahydro[f] quinoxaline-7-sulfonamide (NBQX)-sensitive portion of the leak current corresponded to a current generated by glutamate concentrations similar to the levels of contaminating glutamate measured in our normal external solutions. This result, and the small size of the leak current relative to the currents evoked by saturating glutamate, indicates that GluR1Lc channels exhibit little or no constitutive activity. Our results indicate that the primary effect of the lurcher mutation is to increase the affinity of GluR1 for glutamate and reduce the desensitization of GluR1 at nanomolar concentrations. We also found that the mutation makes both the rate and extent of GluR1Lc channel desensitization depend strongly on subunit occupancy. We conclude that the poor survival of GluR1Lc-transfected cells, and presumably cerebellar neurons in lurcher mice, results because channels carrying the lurcher mutation open and do not desensitize at ambient levels of glutamate.