Newly Published
Perioperative Medicine  |   June 2020
Competitive Antagonism of Etomidate Action by Diazepam: In Vitro GABAA Receptor and In Vivo Zebrafish Studies
Author Notes
  • From the Department of Anesthesia, Critical Care, and Pain Medicine, Massachusetts General Hospital (M.M., H.H., A.P., X.Z., S.A.F., K.W.M., D.E.R.) and Department of Neurobiology, Harvard Medical School (S.S.J., J.B.C.), Boston, Massachusetts.
  • Supplemental Digital Content is available for this article. Direct URL citations appear in the printed text and are available in both the HTML and PDF versions of this article. Links to the digital files are provided in the HTML text of this article on the Journal’s Web site (www.anesthesiology.org).
    Supplemental Digital Content is available for this article. Direct URL citations appear in the printed text and are available in both the HTML and PDF versions of this article. Links to the digital files are provided in the HTML text of this article on the Journal’s Web site (www.anesthesiology.org).×
  • Submitted for publication November 13, 2019. Accepted for publication May 8, 2020.
    Submitted for publication November 13, 2019. Accepted for publication May 8, 2020.×
  • Correspondence: Address correspondence to Dr. Raines: Fruit Street, GRB444, Boston, Massachusetts 02114. draines@partners.org. Information on purchasing reprints may be found at www.anesthesiology.org or on the masthead page at the beginning of this issue. Anesthesiology’s articles are made freely accessible to all readers, for personal use only, 6 months from the cover date of the issue.
Article Information
Perioperative Medicine / Pharmacology
Perioperative Medicine   |   June 2020
Competitive Antagonism of Etomidate Action by Diazepam: In Vitro GABAA Receptor and In Vivo Zebrafish Studies
Anesthesiology Newly Published on June 10, 2020. doi:https://doi.org/10.1097/ALN.0000000000003403
Anesthesiology Newly Published on June 10, 2020. doi:https://doi.org/10.1097/ALN.0000000000003403
Abstract

Background: Recent cryo-electron microscopic imaging studies have shown that in addition to binding to the classical extracellular benzodiazepine binding site of the α1β3γ2L γ-aminobutyric acid type A (GABAA) receptor, diazepam also binds to etomidate binding sites located in the transmembrane receptor domain. Because such binding is characterized by low modulatory efficacy, the authors hypothesized that diazepam would act in vitro and in vivo as a competitive etomidate antagonist.

Methods: The concentration-dependent actions of diazepam on 20 µM etomidate-activated and 6 µM GABA-activated currents were defined (in the absence and presence of flumazenil) in oocyte-expressed α1β3γ2L GABAA receptors using voltage clamp electrophysiology. The ability of diazepam to inhibit receptor labeling of purified α1β3γ2L GABAA receptors by 3[H]azietomidate was assessed in photoaffinity labeling protection studies. The impact of diazepam (in the absence and presence of flumazenil) on the anesthetic potencies of etomidate and ketamine was compared in a zebrafish model.

Results: At nanomolar concentrations, diazepam comparably potentiated etomidate-activated and GABA-activated GABAA receptor peak current amplitudes in a flumazenil-reversible manner. The half-maximal potentiating concentrations were 39 nM (95% CI, 27 to 55 nM) and 26 nM (95% CI, 16 to 41 nM), respectively. However, at micromolar concentrations, diazepam reduced etomidate-activated, but not GABA-activated, GABAA receptor peak current amplitudes in a concentration-dependent manner with a half-maximal inhibitory concentration of 9.6 µM (95% CI, 7.6 to 12 µM). Diazepam (12.5 to 50 µM) also right-shifted the etomidate-concentration response curve for direct activation without reducing the maximal response and inhibited receptor photoaffinity labeling by 3[H]azietomidate. When administered with flumazenil, 50 µM diazepam shifted the etomidate (but not the ketamine) concentration–response curve for anesthesia rightward, increasing the etomidate EC50 by 18-fold.

Conclusions: At micromolar concentrations and in the presence of flumazenil to inhibit allosteric modulation via the classical benzodiazepine binding site of the GABAA receptor, diazepam acts as an in vitro and in vivo competitive etomidate antagonist.

Editor’s Perspective:

What We Already Know about This Topic:

  • Diazepam binds to the γ-aminobutyric acid type A (GABAA) receptor high-affinity extracellular benzodiazepine site

  • Diazepam can also bind to the GABAA receptor transmembrane etomidate site

  • It is unknown whether diazepam or similar compounds can antagonize etomidate

What This Article Tells Us That Is New:

  • In vitro and in vivo zebrafish studies show that diazepam and other like compounds can competitively antagonize etomidate at the GABAA receptor etomidate binding site

  • This provides proof-of-concept for development of competitive anesthetic antagonists