Fig. 4. Midazolam induces similar left shifts in γ-aminobutyric acid (GABA) concentration-responses of both wild-type α¹β²γ^2Land mutant α¹L264Tβ²γ^2LGABA type A receptors. (  A ) Normalized current measurements from two representative oocytes—one expressing wild-type receptors (  open symbols ) and one expressing mutant receptors (  solid symbols )—are plotted against GABA concentration. In each oocyte, GABA responses were measured both in the absence (  squares ) and presence (  circles ) of 10 nm midazolam. Maximal currents in this wild-type oocyte (and others) were slightly enhanced (approximately 5%) by midazolam.  Lines drawn through data represent logistic fits with equation 1 (Materials and Methods). Wild-type control (  open squares ): EC⁵⁰⁰= 52 ± 1.6 μm, n = 1.4 ± 0.06. Wild-type plus midazolam (  open circles ): EC^50MDZ= 40 ± 1.7 μm, n = 1.2 ± 0.05. Mutant control (  solid squares ): EC⁵⁰⁰= 1.1 ± 0.17 μm, n = 0.7 ± 0.14. Mutant plus midazolam (  solid circles ): EC^50MDZ= 0.85 ± 0.12 μm, n = 0.8 ± 0.13. The EC^50MDZ/EC⁵⁰⁰ratios for wild-type and mutant data from these oocytes are both 0.77. (  B ) Estimated P^openvalues were calculated (equation 2, Materials and Methods) from the data in  A and redrawn on identical axes for comparison. In the data for the oocyte expressing mutant receptors (  solid symbols ), both the basal activity (P⁰= 0.105) and direct activation by midazolam (P^0,MDZ= 0.16) are evident. Logistic fits (equation 3, Materials and Methods) to estimated P^openvalues derive EC⁵⁰s and Hill slopes almost identical to those from  A . Wild-type control (  open squares ): EC⁵⁰⁰= 52 ± 1.6 μm, n = 1.4 ± 0.06. Wild-type plus midazolam (  open circles ): EC^50MDZ= 40 ± 1.7 μm, n = 1.2 ± 0.05. Mutant control (  solid squares ): EC⁵⁰⁰= 1.1 ± 0.37 μm, n = 0.7 ± 0.14. Mutant plus midazolam (  solid circles ): EC^50MDZ= 0.86 ± 0.19 μm, n = 0.8 ± 0.13.