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

Abstract

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 EC₅₀ = 215 ± 5 μm 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 EC₅₀ = 30 ± 1 μm 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). 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 ≅ sevoflurane < halothane ≅ isoflurane ≅ enflurane. There was no significant difference between the potentiations observed for the two optical isomers of isoflurane. 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 EC₅₀ concentration for the recombinant receptors decreased from a control value of 215 ± 5 μm to 84 ± 7 μm glycine at 610 μm isoflurane, while that for the medullary neurones decreased from a control value of 30 ± 1 μm to 18 ± 2 μm glycine at the same concentration of isoflurane. The potentiation was independent of membrane potential. 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 EC₅₀ concentration decreased from a control value of 1.6 ± 0.2 to 0.9 ± 0.1 mM taurine in the presence of 305 μm isoflurane, while the maximum response to taurine increased from 47 ± 2 to 59 ± 2% of the maximum response to glycine. 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.