Mapping the contribution of beta3-containing GABAA receptors to volatile and intravenous general anesthetic actions.

Background: Agents belonging to diverse chemical classes are used clinically as general anesthetics. The molecular targets mediating their actions are however still only poorly defined. Both chemical diversity and substantial differences in the clinical actions of general anesthetics suggest that general anesthetic agents may have distinct pharmacological targets. It was demonstrated previously that the immobilizing action of etomidate and propofol is completely, and the immobilizing action of isoflurane partly mediated, by beta3-containing GABAA receptors. This was determined by using the beta3(N265M) mice, which carry a point mutation known to decrease the actions of general anesthetics at recombinant GABAA receptors. In this communication, we analyzed the contribution of beta3-containing GABAA receptors to the pharmacological actions of isoflurane, etomidate and propofol by means of beta3(N265M) mice.

Results: Isoflurane decreased core body temperature and heart rate to a smaller degree in beta3(N265M) mice than in wild type mice, indicating a minor but significant role of beta3-containing GABAA receptors in these actions. Prolonged time intervals in the ECG and increased heart rate variability were indistinguishable between genotypes, suggesting no involvement of beta3-containing GABAA receptors. The anterograde amnesic action of propofol was indistinguishable in beta3(N265M) and wild type mice, suggesting that it is independent of beta3-containing GABAA receptors. The increase of heart rate variability and prolongation of ECG intervals by etomidate and propofol were also less pronounced in beta3(N265M) mice than in wild type mice, pointing to a limited involvement of beta3-containing GABAA receptors in these actions. The lack of etomidate- and propofol-induced immobilization in beta3(N265M) mice was also observed in congenic 129X1/SvJ and C57BL/6J backgrounds, indicating that this phenotype is stable across different backgrounds.

Conclusion: Our results provide evidence for a defined role of beta3-containing GABAA receptors in mediating some, but not all, of the actions of general anesthetics, and confirm the multisite model of general anesthetic action. This pharmacological separation of anesthetic endpoints also suggests that subtype-selective substances with an improved side-effect profile may be developed.