Table 1.
Summary of the functional properties of the constructs studied as defined by whole-cell recordings
| GABA gating | Pentobarbital gating | Pentobarbital block | |||||||
|---|---|---|---|---|---|---|---|---|---|
| Ihold (pA) | EC50 (μm) | nH | EC50 (mm) | nH | IC50 (mm) | nH | Potentiation | Relative gating | |
| α1β3 | 101 ± 19 (16) | 6·2 ± 0·9 | 0·9 ± 0·05 | 0·6 ± 0·04 | 2·3 ± 0·3 | 0·58 ± 0·06 | 1·7 ± 0·1 | 9·5 ± 2·7 (3) | 2·0 ± 0·9 (3) |
| α1c7 | 44 ± 10 (33) | 9·2 ± 3·0 | 0·6 ± 0·1 | 1·1 ± 0·1 | 2·3 ± 0·6 | 1·23 ± 0·13 | 1·9 ± 0·3 | 4·9 ± 1·6 (3) | 2·8 ± 0·6 (4) |
| α1c1 | 142 ± 28 (43) | 5·3 ± 0·7 | 1·0 ± 0·1 | 1·1 ± 0·1 | 1·9 ± 0·4 | 0·47 ± 0·01 | 1·7 ± 0·05 | 1·8 ± 0·4 (3) | 0·3 ± 0·06 (4) |
| α1c2 | 617 ± 69 (26) | 3·1 ± 0·8 | 0·9 ± 0·05 | NR | * | (> 1) | * | 1·01 ± 0·0 (3) | (0) |
| p1 | 82 ± 9 (140) | 7·3 ± 0·4 | 1·6 ± 0·3 | NR | * | (> 1) | * | 1·0 | (0) |
| β3 | 241 ± 71 (23) | NR | * | (<0·05, > 1) | * | * | * | * | * |
| c7 | 110 ± 38 (6) | NR | * | (> 1) | * | * | * | * | * |
| c1 | 214 ± 71 (7) | NR | * | (∼0·7) | * | * | * | * | * |
| c2 | 720 ± 91 (8) | NR | * | NR | * | * | * | * | * |
Data are shown for the heteromeric receptors (upper 4 rows) and homomeric receptors (lower 5 rows) examined. The first column gives the composition of the subunits transfected. The second column gives the initial holding current at -60 mV in the absence of applied drugs (means ±s.e.m., with numbers of cells in parentheses). The third and fourth columns give values obtained from fitting the Hill equation to the concentration-response data for activation by GABA (best fit parameter value ± 95% confidence limit on the fit). The fifth and sixth columns give similar fit parameters for activation by pentobarbital. The values in parentheses for the EC50(β3, c1, c7) are estimates from the concentration-response curves (Fig. 3), since in the presence of spontaneous activity and channel block by pentobarbital no effort was made to fit the data. The concentration-response curve for β3 showed two apparent components. The response in c7 and in the low affinity component of β3 is not saturated at 10 mm pentobarbital and therefore it is not possible to define the EC50. The seventh and eighth columns give parameters for block by pentobarbital. No estimate was made for the IC50 for β3, c1 or c7 receptors, due to spontaneous activity, and block of α1c2 and p1 receptors was not fully characterized. The ninth column gives the potentiation by 100 μm pentobarbital, expressed as the ratio of the response produced by co-application of 1 μm GABA plus 100 μm pentobarbital to that produced by application of 1 μm GABA alone. The tenth column gives the relative maximal response to pentobarbital, expressed as the ratio of the response to 10 mm pentobarbital to the response in the same cell to 100 μm GABA. NR indicates no activation, (0) indicates that since gating was not observed the relative gating was set to 0, and* indicates that the parameter was not estimated.