Neurosteroids 3β, 20 (R/S)-pregnandiols decrease offset rate of the GABA-site activation at the recombinant GABA A receptor

Abstract

Neurosteroids directly modulate ligand gated ion channels such as GABA A receptors. Two such molecules, 3β-OH A-ring reduced pregnane steroids and pregnenolone sulfate (PS), inhibit recombinant GABA A receptor. Using a two-electrode voltage-clamp technique, we compared the effect of 5α-pregnan-3β,20(S)-diol (UC1019), 5β-pregnan-3β, 20(R)-diol (UC1020) and PS on the activation onset and offset times of the recombinant GABA A receptor ( rat α 1β 2γ 2L) in Xenopus oocytes. Rapid solution changes allowed the kinetic analysis of GABA-evoked currents. Steroids were co-applied with 30 µM GABA for 10 s, followed by a 80 s washout period. PS (≥ 0.3 µM) moderately increased the slow onset rate ( k on-S) of GABA-response. PS had no significant effects on the fast onset rate ( k on-F). UC1019 and UC1020 decreased the k on-S of the GABA-response in a concentration-dependent manner with no significant effects on the k on-F. Like PS, UC1019 and UC1020 decreased the slow offset rates ( k off-S). In addition, PS increased the fast offset rate ( k off-F) in a concentration-dependent manner, while UC1019 and UC1020 decreased k off-F. The EC 50 of PS to increase k off-F was calculated as 0.47 ± 0.1 µM. The corresponding IC 50 values of UC1019 and UC1020 to decrease k off-F were 5.0 ± 0.5 µM and 8.4 ± 0.9 µM, respectively. These results suggest differential actions of PS and 3β, 20(R/S)-pregnandiols on the offset time course of GABA-site activation.