Etomidate Effects on Desensitization and Deactivation of α4β3δ GABA_A Receptors Inducibly Expressed in HEK293 TetR Cells

Abstract

Central a4bd receptors are the most abundant isoform of d subunit-containing extrasynaptic GABA A receptors that mediate tonic inhibition. Although the amplitude of GABA-activated currents through a4bd receptors is modulated by multiple general anesthetics, the effects of general anesthetics on desensitization and deactivation of a4bd receptors remain unknown. In the current study, we investigated the effect of etomidate, a potent general anesthetic, on the kinetics and the pseudo steady-state current amplitude of a4b3d receptors inducibly expressed in human embryonic kidney 293 TetR cells. Etomidate directly activates a4b3d receptors in a concentrationdependent manner. Etomidate at a clinically relevant concentration (3.2 mM) enhances maximal response without altering the EC 50 of GABA concentration response. Etomidate also increases the extent of desensitization and prolongs the deactivation of a4b3d receptors in the presence of maximally activating concentrations of GABA (1 mM). To mimic the modulatory effect of etomidate on tonic currents, long pulses (30-60 seconds) of a low GABA concentration (1 mM) were applied to activate a4b3d receptors in the absence and presence of etomidate. Although etomidate increases the desensitization of a4b3d receptors, the pseudo steady-state current amplitude at 1 mM GABA is augmented by etomidate. Our data demonstrate that etomidate enhances the pseudo steady-state current of a4b3d receptors evoked by a GABA concentration comparable to an ambient GABA level, suggesting that a4b3d receptors may mediate etomidate's anesthetic effect in the brain.

GABAA Receptors in Astrocytes Are Targets for Commonly Used Intravenous and Inhalational General Anesthetic Drugs

“…Finally, the effects of anesthetic drugs on GABA A receptor kinetics are highly dependent on the subunit composition of the receptors. Thus, the subunit composition of the heterogeneous GABA A receptors influences the response to anesthetic drugs ( Uchida et al, 1995 ; Krasowski et al, 1998 ; Jenkins et al, 2001 ; Nishikawa and Harrison, 2003 ; Benkwitz et al, 2004 ; Zhong et al, 2008 ; Hoft et al, 2014 ; Woll et al, 2018 ; Liao et al, 2019 ).…”

The Influence of AA29504 on GABAA Receptor Ligand Binding Properties and Its Implications on Subtype Selectivity

“…Propofol and potentiating neurosteroids also bind between the same domains but at distinct binding pockets [ 54 ]. This inter-transmembrane binding site is in the vicinity of the physical desensitization gate at the intracellular end of the GABA A R channel [ 55 – 57 ] suggesting that the site could act as a target for modulating desensitization by AA29504, a mechanism of action already established for desensitization-modifying allosteric modulators (DAM) such as etomidate [ 58 , 59 ], propofol [ 60 , 61 ] and neurosteroids [ 62 , 63 ].…”

Steady-state activation of the high-affinity isoform of the α4β2δ GABAA receptor

“…Both properties support its presumed function to mediate tonic Cl − conductance in response to ambient GABA, and the concentration profile of ambient GABA in the brain. The α4βδ receptor is also activated by taurine, endogenous potentiating steroids, and various GABAergic sedative and anesthetic agents 3,14,20,30 . The overall goal of this study was to analyze the function of the α4β2δ receptor in the presence of one or more activators and modulators under steady-state conditions.…”

“…With few exceptions 3,4 , previous functional studies of the α4βδ receptor have concentrated on recording peak current responses, i.e., maximal responses to short-duration applications of one or more agonists. It may be argued that this approach does not accurately reflect native conditions, which can be characterized as essentially infinite-duration exposure to a low concentration of GABA with slowly developing changes in the concentrations of other endogenous agonists and modulators and, if so administered, GABAergic clinical agents.…”