The role of the GABA A receptor α1 subunit n-terminal extracellular domain in propofol potentiation of chloride current

Abstract

Propofol (2,6-diisopropylphenol), an intravenous general anesthetic in active clinical use today, potentiates the action of γ-aminobutyric acid (GABA) at the type-A receptor and also directly induces current in the absence of GABA. We expressed different combinations of murine GABA A receptor α1, β3 and γ2 subunits in Xenopus oocytes to investigate the subunit dependence of propofol potentiation of pentobarbital-induced current. Pentobarbital induces current in all β3-subunit-containing receptors, whereas current gating by GABA requires the presence of both α1 and β3 subunits. Therefore, pentobarbital rather than GABA was used to induce current in order to separate the subunit dependence of current gating from the subunit dependence of potentiating action of propofol. α1β3γ2, α1β3, β3γ2, or β3 subunit combinations all responded to pentobarbital in a dose-dependent manner. True potentiation was defined as the current magnitude to simultaneous application of pentobarbital and propofol exceeding the additive responses to individual drug applications. A dose-dependent propofol potentiation of pentobarbital-induced current was observed in oocytes injected with α1⨿3 or α1β3γ2 but not in β3γ2 or β3 subunits, suggesting that the α1 subunit was necessary for this modulatory action of propofol. Further examination of the propofol potentiation in chimeras between the α1 and β3 subunits showed that the extracellular amino-terminal half of the α1 subunit was sufficient to support propofol potentiation. The different requirements of the receptor structure for the agonistic (gating) and the potentiating actions suggest that these two actions of propofol are distinct processes mediated through its action at distinct sites.