Effects of inhalational general anaesthetics on native glycine receptors in rat medullary neurones and recombinant glycine receptors in Xenopus oocytes

1 Glycine responses were studied under voltage clamp in Xenopus oocytes injected with cDNA encoding mammalian glycine receptor subunits and in rat medullary neurones. Bath application of glycine gave strychnine-sensitive currents which reversed close to the expected equilibrium potentials for chloride ions. The peak currents for the receptors expressed in oocytes fitted a Hill equation with EC(50)=215+/-5 mu M and Hill coefficient n(H)=1.70+/-0.05 (means+/-s.e.means). The peak currents from the receptors in medullary neurones fitted a Hill equation with EC(50)=30+/-1 mu M and Hill coefficient n(H)=1.76+/-0.08. The current-voltage relationship for the receptors expressed in oocytes showed strong outward rectification (with V-rev=-21+/-2 mV), while that for the glycine responses from the medullary neurones in symmetrical Cl- was linear (with V-rev=3.2+/-0.6 mV). 2 Inhalational general anaesthetics, at concentrations close to their human minimum alveolar concentrations (MACs), potentiated responses to low concentrations of glycine. The potentiation observed with the recombinant receptors (between 60-220%) was approximately twice that found with the medullary neurones (between 40-80%). For both the recombinant receptors and the receptors in medullary neurones, the degree of potentiation increased in the order of methoxyflurane approximate to sevoflurane < halothane approximate to isoflurane approximate to enflurane. There was no significant difference between the potentiations observed for the two optical isomers of isoflurane. 3 For both the recombinant and native receptors, isoflurane potentiated the currents in a dose-dependent manner at low concentrations of glycine, although at high glycine concentrations the anaesthetic had no significant effect on the glycine-activated responses. The major effect of isoflurane was to cause a parallel leftward shift in the glycine concentration-response curves. The glycine EC(50) concentration for the recombinant receptors decreased from a control value of 215+/-5 mu M to 84+/-7 mu M glycine at 610 mu M isoflurane, while that for the medullary neurones decreased from a control value of 30+/-1 mu M to 18 +/- 2 mu M glycine at the same concentration of isoflurane. The potentiation was independent of membrane potential. 4 Isoflurane also potentiated responses to taurine, a partial agonist at the glycine receptor. This was observed for receptors expressed in oocytes at both low and saturating concentrations of taurine. The EC(50) concentration decreased from a control value of 1.6+/-0.2 to 0.9+/-0.1 mM taurine in the presence of 305 mu M isoflurane, while the maximum response to taurine increased from 47+/-2 to 59+/-2% of the maximum response to glycine. 5 Glycine receptors, like other members of the fast ligand-gated receptor superfamily, are sensitive to clinically relevant concentrations of inhalational general anaesthetics. Effects at these receptors may, therefore, play some role in the maintenance of the anaesthetic state.