STEREOSELECTIVE AND NON-STEREOSELECTIVE ACTIONS OF ISOFLURANE ON THE GABA(A) RECEPTOR

1 Acutely dissociated cerebellar Purkinje neurones from 8-14 day old rats were studied under voltage clamp in the whole-cell patch-clamp configuration. Cl- currents induced by bath application of gamma-aminobutyric acid (GABA) were measured (using symmetrical Cl- solutions) at both low (2 mu M) non-desensitizing and high (300 mu M) desensitizing concentrations of GABA. 2 At 2 mu M GABA, the bicuculline-sensitive Cl- currents were potentiated by racemic isoflurane and both of its optical isomers. Isoflurane had no effect on membrane current in the absence of GABA. The dose-response data for potentiation by racemic isoflurane could be fitted with a Hill equation with an EC(50) = 320 +/- 20 mu M isoflurane and a Hill coefficient of h = 2.7 +/- 0.4 (means +/- s.e.mean). 3 The potentiations produced by the optical isomers of isoflurane at 2 mu M GABA were stereoselective at moderate and high anaesthetic concentrations. The maximum stereoselectivity, about two fold, occurred at the EC(50) concentration for general anaesthesia (310 mu M isoflurane), with S(+)-isoflurane being more effective than R(-)-isoflurane. At sub-anaesthetic concentrations, the stereoselectivity was less marked and vanished at the lowest concentration used (77 mu M isoflurane). 4 The sustained residual current remaining after exposure of neurones to a desensitizing concentration of GABA (300 mu M) was inhibited non-stereoselectively, but only at high concentrations of isoflurane. The ratio of inhibitions by S(+)- and R(-)-isoflurane (mean + s.e.mean) was 1.14 +/- 0.21 at 770 mu M isoflurane. At the EC(50) concentration for general anaesthesia, however, the inhibition was barely significant. 5 The above results are discussed in relation to the possible role of the GABA(A), receptor channel in general anaesthesia.