Figure 3.

Summary of the steroid analogue concentration–effect relationships. The wild-type α1β2γ2L receptors were activated by 5 µM GABA (∼EC₂₅) in the absence and presence of the steroids 3α5α18nor17βCN, 3α5αCDNC12, 3α5α17βEt and 3α5α17H. The data points show mean ± SEM from four to eleven cells. The effect by 3α5α18nor17βCN was determined by exposing a cell to various concentrations of steroid for 4 s, separated by 30 s wash-out periods and control exposures to GABA alone. The effects by 3α5αCDNC12, 3α5α17βEt and 3α5α17H were determined by exposing a cell to GABA followed by an application of GABA + a single concentration of the steroid. No wash-out of the effect was observed following an up to 5 min exposure to bath solution. The curves were fitted to the Hill equation with an offset fixed at 100%. For 3α5α18nor17βCN, the best-fit parameters are: maximal potentiation = 365 ± 9%, EC₅₀= 148 ± 16 nM and n_H= 1.1 ± 0.1. For 3α5αCDNC12, the best-fit parameters are: maximal potentiation = 233 ± 31%, EC₅₀= 75 ± 62 nM and n_H= 1.3 ± 1.2. For 3α5α17βEt, the best-fit parameters are: maximal potentiation = 266 ± 48%, EC₅₀= 154 ± 188 nM and n_H= 1 (fixed). For 3α5α17H, the best-fit parameters are: maximal potentiation = 268 ± 32%, EC₅₀= 246 ± 173 nM and n_H= 1 (fixed). The dashed line corresponds to data in the presence of the neurosteroid 3α5αP. The best-fit parameters for 3α5αP are: maximal potentiation = 351%, EC₅₀= 41 nM and n_H= 1.2 (Akk et al., 2008).