Modulation of Delta Subunit-Containing GABAA Receptors by Etomidate Depends on Subunit Arrangement

Background: GABAA receptors are allosterically modulated by a variety of compounds including etomidate and tetrahydrodeoxycorticosterone (THDOC). Extrasynaptic αβδ GABAA receptors mediate tonic inhibition, and may be more sensitive to modulators than synaptic αβγ receptors. Studies of concatenated subunit assemblies suggests that the subunit arrangement of αβδ may vary (1). We compared the modulation by etomidate and THDOC on α1β3δ receptors with different subunit arrangements, and to α1β3γ2L receptors. Methods: Plasmids encoding concatenated GABAA receptor subunit assemblies and free subunits were obtained from Prof. Erwin Sigel (Bern, Switzerland). Xenopus oocytes were used to express β3-α1-δ/β3-α1 and β3-α1-δ/α1-β3 receptors as well as free subunit mixtures. Effects of etomidate (3 μM) or THDOC (1 μM) on maximal GABA-activated receptor currents were measured using two-microelectrode voltage-clamp electrophysiology. Etomidate enhancement of β3-α1-δ/β3-α1 was assessed over a broad [GABA] range, and analyzed with an allosteric model.Results: High GABA elicited small currents in oocytes expressing β3-α1-δ/β3-α1 and β3-α1-δ/α1-β3 receptors. THDOC produced similar large current enhancements in β3-α1-δ/β3-α1 and β3-α1-δ/α1-β3 receptors. However, etomidate produced a greater current enhancement in β3-α1-δ/α1-β3 receptors than in β3-α1-δ/β3-α1. Etomidate increased the maximal response of β3-α1-δ/β3-α1 receptors to GABA and produced a small leftward shift in GABA EC50.Discussion: using concatenated receptor constructs resulted in evidence of δ subunit incorporation that was not seen using free subunit mRNAs. GABA is a weak partial agonist in α1β3δ receptors, while etomidate and THDOC modulate via different sites. Etomidate efficacy in α1β3δ is similar to that in α1β3γ receptors with homologous subunit arrangement, and interactions at β3/β3 interfaces may be more efficacious than those at β3/α1 interfaces.Support: NIH (R01GM89745 and P01GM58448)Reference:1) Kaur KH et al, J. Biol Chem 2009;284:7889-96