Stereoselective interaction of thiopentone enantiomers with the GABA(A) receptor.
1. As pharmacokinetic differences between the thiopentone enantiomers seem insufficient to explain the approximately 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 alpha1beta2gamma2 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 microM GABA, the EC50 values were (-)-S-thiopentone (mean 26.0+/-s.e.mean 3.2 microM, n=9 cells) >rac-thiopentone (35.9+/-4.2 microM, n=6, P=0.1) >(+)-R-thiopentone (52.5+/-5.0 microM, n=8, P<0.02) >rac-pentobarbitone (97.0+/-11.2 microM, 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 microM (-)-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 microM, n=8) <(+)-R-thiopentone (36.2+/-3.2 microM, n=9, P<0.005). 5 (-)-S-thiopentone was found to be approximately 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.
Cordato
D J
DJ
Department of Neurology, University of Sydney at Royal North Shore Hospital, St. Leonards, N.S.W. 2065, Australia.
Chebib
M
M
Mather
L E
LE
Herkes
G K
GK
Johnston
G A
GA
D000818:Animals; D001665:Binding Sites; D002614:Chelating Agents / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000378:metabolism; D004553:Electric Conductivity; D005260:Female; D058785:GABA-A Receptor Agonists; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010424:Pentobarbital / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D011963:Receptors, GABA-A / Q000235:genetics / Q000378:metabolism*; D013237:Stereoisomerism; D013379:Substrate Specificity; D013874:Thiopental / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D002614:Chelating Agents; D058785:GABA-A Receptor Agonists; D011963:Receptors, GABA-A; D004533:Egtazic Acid; D005680:gamma-Aminobutyric Acid; D010424:Pentobarbital; D013874:Thiopental; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid