In an effort to clarify the mechanism of action of isoflurane, we studied the effect of flumazenil on mice chronically treated with isoflurane or diazepam. Mice were pretreated with diazepam, isoflurane, or saline, with and without flumazenil. After 2 wk, responses to isoflurane and diazepam were assessed, and central benzodiazepine receptor (CBR) binding characteristics were assayed. Mice pretreated with isoflurane failed the horizontal wire test at a larger isoflurane concentration (0.5%) compared with saline-pretreated mice (0.4%) (P < 0.05). These differences did not occur when flumazenil was added to the pretreatment. After the administration of diazepam, 20% of diazepam- and 11% of isoflurane-pretreated mice failed the horizontal wire test, versus 50% and 44% when flumazenil was added to either drug (P < 0.002) and 80% and 100% in the saline and saline plus flumazenil-treated mice. The increased CBR density due to flumazenil was attenuated by the coadministration of isoflurane or diazepam. Flumazenil attenuated the development of tolerance to diazepam after chronic treatment with diazepam or isoflurane and attenuated the development of tolerance to isoflurane. Isoflurane, like diazepam, attenuated the effect of flumazenil on CBR ligand binding. These findings suggest that isoflurane shares a mechanism of action with diazepam, probably via the gamma-aminobutyric acid system, most probably the CBR.
Implications: Flumazenil attenuates the development of tolerance to isoflurane and diazepam after chronic isoflurane pretreatment. Isoflurane, like diazepam, attenuates the increase in central benzodiazepine receptor (CBR) density caused by flumazenil. These findings suggest that isoflurane and diazepam share a mechanism of action, most probably via the gamma-aminobutyric acid system and the CBR.