FIGURE 3.

Structure and functional expression in the Xenopus oocytes of the GABA_A receptors investigated. The code for the subunits is given at the top line of the table (read the subunit sequence of concatenated receptors anti-clockwise). The figure shows subunit composition and current amplitude (nA) evoked by 1 mm GABA in the absence and presence of 1 μm THDOC, the EC₅₀ for GABA, and the current elicited by 100 μm THIP of non-concatenated and concatenated receptors. Please note the considerable variability of current expression by δ subunit-containing receptors (see “Experimental Procedures”). Unless indicated otherwise, oocytes were injected with 50 nl of water containing 50 nm RNA coding for each subunit. At this high concentration, some of the multisubunit constructs resulted in the expression of small currents, for reasons that are poorly understood. Therefore, we consider here a current amplitude of >25 nA in the absence and >80 nA in the presence of THDOC as evidence for functional expression of a receptor type. α₁/α₆/β₃/δ receptors (line 1) were formed by injection of 10 nm RNA each coding for α₁, α₆, and β₃ and 50 nm RNA coding for δ. The concatenated β₃-α₁ subunit was injected at 50, 25, and 10 nm as indicated. Mean ± S.E. for each subunit combination is shown. n, number of oocytes from at least two batches; −, not analyzed. Footnotes a and b, a better fit was obtained if two components reflecting two sites for GABA were assumed. For R12, the respective EC₅₀ values were 0.36 ± 0.06 and 65 ± 24 μm, the first component accounting for about 72% and the second accounting for about 28%; for R23, the respective EC₅₀ values were 10.4 ± 4.6 and 357 ± 126 μm, the two components amounting to 24 and 76%. Footnote c, the experiments were carried out in the presence of 1 μm THDOC.