β3-containing gamma-aminobutyric acid_A receptors are not major targets for the amnesic and immobilizing actions of isoflurane

Mice bearing an N265M point mutation in the gamma-aminobutyric acid (GABA)_A receptor β3 subunit resist various anesthetic effects of propofol and etomidate. They also require a 16% larger concentration of enflurane and a 21% larger concentration of halothane to abolish the withdrawal reflex than do wild-type mice. Using a Pavlovian test, we measured whether this mutation increased the concentration of isoflurane required to impair learning and memory relative to wild-type mice. We found that the concentration was not significantly increased. We also measured MAC (the minimum alveolar concentration required to eliminate movement in response to noxious stimulation in 50% of subjects). Isoflurane MAC for mutant mice (1.93% ± 0.0.03%; mean ± SE; n = 14) was 17.0% larger than MAC for wild-type mice (1.65 ± 0.04; n = 14; P < 0.001). Similarly, the cyclopropane MAC for mutant mice (27.6% ± 0.55%; n = 16) was 13.6% larger than MAC for wild-type mice (24.3 ± 0.46; n = 8; P < 0.01). The increase in MAC for cyclopropane was unexpected, because published reports find only minimal actions at α1β2γ2 GABA_A receptors whereas isoflurane provides a large enhancement. Consistent with previous work on α1β2γ2 GABA_A receptors, we found in Xenopus oocytes that 5 MAC cyclopropane enhanced the effect of GABA on α1β2γ2 GABA_A receptors by only 76%, and by a nearly identical enhancement in α1β3γ2, and α6β3γ2 receptors. In contrast, a much smaller concentration of isoflurane (1 MAC) produced a 160% to 310% enhancement in these receptors. If, relative to isoflurane, cyclopropane minimally increases GABA-induced chloride currents at any GABA_A receptor subtype, the present data for MAC are consistent with the notion that GABA_A receptors do not mediate the immobility produced by inhaled anesthetics.