Electrophysiological Properties of AMPA Receptors Are Differentially Modulated Depending on the Associated Member of the TARP Family

Increase in agonist-induced currents of AMPA receptors during coexpression of TARPs. Glutamate-induced (300 μm; black columns) and kainate-induced (150 μm; gray columns) responses of AMPA receptors alone and in combination with each of the four TARPs were recorded from Xenopus oocytes in magnesium Ringer's solution. In the top row, GluR1 (A, B) and GluR2 (C, D) are depicted compared with GluR3 (E, F) and GluR4 (G, H) in the bottom row. For quantification, agonist-induced currents of all eight AMPA receptors in coexpression with a TARP were normalized to the responses mediated by the respective AMPA receptor subunit expressed without TARP. Data are shown ± SEM (n = 5–12). *p < 0.05; **p < 0.01; ***p < 0.005 (Student's t test) compared with the respective AMPA receptor subunit expressed without TARP.

Patch-clamp analysis of desensitization of GluR3(Q)flop and GluR4(Q)flop. A, Glutamate-induced whole-cell responses of GluR3(Q)flop in the presence and absence of γ2 were recorded from HEK293 cells. The application of agonist (3 mm glutamate) is indicated by black bars. B, Quantification of the extent of desensitization [100% − (I_{steady state}/I_peak) × 100%] of GluR3(Q)flop and GluR4(Q)flop in both the presence and absence of γ2. Data are shown ± SEM (n = 3–6).

Comparison of the current amplitude-modulating effects of TARPs on GluR2(R)flip and several heteromeric AMPA receptors. A–D, Identical amounts of GluR2(R)flip (A), GluR1(Q)flop/GluR2(R)flip (B), GluR3(Q)flop/GluR2(R)flip (C), and GluR1(Q)flip/GluR4(Q)flip (D) cRNAs were coinjected with each TARP, and agonist-induced currents were recorded 4–6 d after injection. Glutamate-induced (300 μm; black columns) and kainate-induced (150 μm; gray columns) responses of AMPA receptors alone and in combination with each of the four TARPs were recorded from Xenopus oocytes in magnesium Ringer's solution. Normalization was performed as described in Figure 2 (*p < 0.05; **p < 0.01; ***p < 0.005). E, Glutamate-induced (300 μm; black columns) and kainate-induced (150 μm; gray columns) responses of heteromeric GluR1(Q)/GluR2(R) combinations in the absence and presence of γ2 were recorded from Xenopus oocytes. Normalization was performed as described in Figure 2 (*p < 0.05; **p < 0.01; ***p < 0.005 compared with the respective heteromeric AMPA receptor expressed without γ2). The given combination of contributing splice variants is indicated by the abbreviations listed below the x-axis: o/o, GluR1(Q)flop/GluR2(R)flop; i/o, GluR1(Q)flip/GluR2(R)flop.

Influence of TARPs on the desensitization properties of AMPA receptors. A, Xenopus laevis oocytes injected with 2 ng of GluR1(Q)flip–L479Y cRNA, and 0.2 ng of TARP cRNA were recorded in magnesium Ringer's solution 4–5 d after injection. Glutamate-induced (black columns) and kainate-induced (gray columns) responses of GluR1(Q)flip–L479Y alone and in combination with each of the four TARPs were recorded. To quantify the increase in agonist-induced currents, agonist-induced currents without coexpression of a TARP were set to 1 for GluR1(Q)flip–L479Y (mean absolute current responses, 4270 ± 644 nA for glutamate and 341 ± 53 nA for kainate). Data are shown ± SEM (n = 5–7). B, Ratios of kainate- to glutamate-induced currents were calculated for five to seven oocytes and averaged. Data are shown ± SEM. C, Dose–response curves for the agonist l-glutamate were determined from oocytes expressing GluR1(Q)flip or GluR1(Q)flip-L479Y in the presence or absence of γ2. Values are means ± SEM (n = 4–6), normalized to the maximal responses. The EC₅₀ values are given in Results.