Stereoselective interaction of thiopentone enantiomers with the GABAA receptor

1 As pharmacokinetic differences between the thiopentone enantiomers seem insufficient to explain the similar to 2 fold greater potency for CNS effects of (-)-S- over (+)-R-thiopentone, this study was performed to determine any enantioselectivity of thiopentone at the GABA(A) receptor, the primary receptor for barbiturate hypnotic effects. 2 Two electrode voltage clamp recording was performed on Xenopus laevis oocytes expressing human GABA(A) receptor subtype alpha(1)beta(2)gamma(2) to determine relative differences in potentiation of the GABA response by rac-, (+)-R- and (-)-S-thiopentone, and rac-pentobarbitone. Changes in the cellular environment pH and in GABA concentrations were also evaluated. 3 With 3 mu M GABA, the EC50 values were (-)-S-thiopentone (mean 26.0 +/- s.e.mean 3.2 mu M, n = 9 cells) >rac-thiopentone (35.9 +/- 4.2 mu M, n = 6, P = 0.1) >(+)-R-thiopentone (52.5 +/- 5.0 mu M, n=8, P < 0.02) > rac-pentobarbitone (97.0 +/- 11.2 mu M, n = 11, P < 0.01). Adjustment of environment pH to 7.0 or 8.0 did not alter the EC50 values for (+)-R- or (-)-S-thiopentone. 4 Uninjected oocytes responded to > 100 mu M (-)-S- and R-thiopentone. This direct response was abolished by intracellular oocyte injection of 1,2-bis(2-aminophenoxy)ethane-N,N,N1,N1-tetraacetic acid (BAPTA), a Ca2+ chelating agent. With BAPTA, the EC50 values were (-)-S-thiopentone (20.6 +/- 3.2 mu M, n = 8) <(+)-R-thiopentone (36.2 +/- 3.2 mu M, n = 9, P < 0.005). 5 (-)-S-thiopentone was found to be similar to 2 fold more potent than (+)-R-thiopentone in the potentiation of GABA at GABA(A) receptors expressed on Xenopus oocytes. This is consistent with the differences in potency for CNS depressant effects found in vivo.