β3-containing gamma-aminobutyric acidA 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 beta 3 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 NIAC 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 alpha 1 beta 2 gamma 2 GABA, receptors whereas isoflurane provides a large enhancement. Consistent with previous work on alpha 1 beta 2 gamma 2 GABA, receptors, we found in Xenopus oocytes that 5 MAC cyclopropane enhanced the effect of GABA on alpha 1 beta 2 gamma 2 GABA(A) receptors by only 76%, and by a nearly identical enhancement in alpha 1 beta 3 gamma 2, and alpha 6 beta 3 gamma 2 receptors. In contrast, a much smaller concentration of isoflurane (I 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.