Stargazin Reduces Desensitization and Slows Deactivation of the AMPA-Type Glutamate Receptors

Stargazin/γ2 enhances surface expression and reduces desensitization of GluR1 in oocytes. A, Enhancement of GluR1 whole-cell currents by γ2 depends on the amount of cRNA injected and on the incubation time. GluR1 cRNA was injected at different amounts (0.1-5 ng/oocyte), as indicated, without (black bars) or with (gray bars; 1 ng/oocyte) γ2 cRNA, and whole-cell currents in response to 1 mm glutamate plus 0.1 mm CTZ were measured 1.5 and 3 d after injection. In each case, enhancement in whole-cell current after coexpression with γ2 is indicated above the gray bars (n = 14; ^**p < 0.01; ^***p < 0.001). Similar results were obtained for GluR1 (5 ng) injected with 5 or 10 ng of γ2, indicating that γ2 was not a limiting factor at these expression conditions. B, Surface biotinylation assay (performed 2 d after injection) on oocytes injected with GluR1 (0.5 ng) without (-) or with (+γ2; 1 ng) γ2 shows that γ2 mainly enhances GluR1 surface expression and moderately increases total expression. Blots were exposed with anti-GluR1 antibody. C, CTZ enhancement of GluR1 whole-cell currents in response to 1 mm glutamate is lower for oocytes coinjected with γ2. GluR1 (5 or 0.5 ng) was injected, as indicated, without (-) or with (+γ2) stargazin. Currents were measured after 1.5 d without (black bars) or with (gray bars) CTZ. CTZ-dependent current enhancement is indicated above the gray bars. ^***p < 0.001. D, Glutamate dose-response (with CTZ) measurements on oocytes injected with GluR1 (5 ng) alone (black circles) and GluR1 (0.5 ng) plus γ 2 (1 ng) (gray circles). The maximum responses (at 3 mm glutamate) and EC₅₀ values were 1090 ± 160 nA, 18.5 ± 0.5 μm (without γ2) and 1400 ± 120 nA, 5.0 ± 0.3 μm (with γ2), respectively (n = 6). At these conditions, responses to 3 mm glutamate of GluR1 (0.5 ng) injected alone were 160 ± 50 nA.