Halothane-induced desensitization of α1β2γ2S and α6β2γ2S GABA_A receptors expressed in HEK293 cells

Background: Inahalational general anesthetics are known to exert a dual action on the GABA_A receptor, potentiation and suppression. Whereas potentiation is regarded as one of the most important mechanisms of anesthesia, the role of GABA_A receptor suppression remains to be seen. Methods: GABA-induced currents were recorded by the whole-cell patch clamp technique from HEK cells expressing the α1β2γ2S and α6β2γ2S subunits, and the effects of halothane were examined. Results: The EC₅₀ and the Hill coefficient (n_H) were estimated to be 10.5 μM and 1.40, respectively, for the α1β2γ2S subunits, and 1.07 μM and 1.63, respectively, for the α6β2γ2S subunits. Preperfusion of halothane for 3 min suppressed or desensitized currents induced by a high concentration of GABA (3 × EC₅₀) with the IC₅₀s and n_Hs of 0.84 mM and 1.61, respectively, for the α1β2γ2S, and 0.93 mM and 2.57, respectively, for the α6β2γ2S. Halothane 0.9 mM genarated a small current in the α6β2γ2S subunit. The time courses of halothane inhibition and recovery after washout resemble the time course of GABA-induced desensitization, GABA-induced receptor desensitization was further enhanced by halothane due to a shift of GABA desensitization curve in the direction of low concentrations of GABA, and the shift was more pronounced in α1β2γ2S than α6β2γ2S. Conclusions: Halothane-induced desensitization was observed when it was preperfused and tested by high concentrations of GABA, conditions that colosely resemble those in vivo since halothane is continuously administered to the patient and the concentration of GABA released from nerve terminals is very high (≥ 500 μM) at the synaptic cleft. Since high concentrations of halothane is used during induction and also since desensitization of GABA_A receptors could lead to disinhibition, halothane desensitization may play an important role in excitement of the patient observed before surgical anesthesia sets in.