Etomidate and Etomidate Analog Binding and Positive Modulation of γ-Aminobutyric Acid Type A Receptors: Evidence for a State-dependent Cutoff Effect

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Editor’s PerspectiveWhat We Already Know about This TopicPositive γ-aminobutyric acid type A (GABAA) receptor modulators bind with higher affinity when the receptor is in the open state, increasing the fraction of open state receptors and, thereby, enhancing receptor functionAppending a phenyl ring substituent group onto the phenyl ring of etomidate abolished its conformational state selectivity and binding site selectivity, reducing the intrinsic efficacy of etomidate and turning it into an anesthetic-selective competitive GABAA receptor antagonistWhat This Article Tells Us That Is Newγ-Aminobutyric acid type A (GABAA) receptor positive modulatory activities of phenyl ring–substituted etomidate analogs decreased with increasing substituent group volume, reflecting decreases in both potencies and efficacies of the analogsTheir GABAA receptor positive modulatory activities were strongly correlated with their affinities for the two β+ – α transmembrane anesthetic binding sites of the GABAA receptorOpen-state binding affinity decreased progressively with increasing substituent group volumeBackground:Naphthalene-etomidate, an etomidate analog containing a bulky phenyl ring substituent group, possesses very low γ-aminobutyric acid type A (GABAA) receptor efficacy and acts as an anesthetic-selective competitive antagonist. Using etomidate analogs containing phenyl ring substituents groups that range in volume, we tested the hypothesis that this unusual pharmacology is caused by steric hindrance that reduces binding to the receptor’s open state.Methods:The positive modulatory potencies and efficacies of etomidate and phenyl ring–substituted etomidate analogs were electrophysiology defined in oocyte-expressed α1β3γ2L GABAA receptors. Their binding affinities to the GABAA receptor’s two classes of transmembrane anesthetic binding sites were assessed from their abilities to inhibit receptor labeling by the site-selective photolabels 3[H]azi-etomidate and tritiated R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid.Results:The positive modulatory activities of etomidate and phenyl ring–substituted etomidate analogs progressively decreased with substituent group volume, reflecting significant decreases in both potency (P = 0.005) and efficacy (P < 0.0001). Affinity for the GABAA receptor’s two β+ − α anesthetic binding sites similarly decreased with substituent group volume (P = 0.003), whereas affinity for the receptor’s α+ – β+ – β sites did not (P = 0.804). Introduction of the N265M mutation, which is located at the β+ − α binding sites and renders GABAA receptors etomidate-insensitive, completely abolished positive modulation by naphthalene-etomidate.Conclusions:Steric hindrance selectively reduces phenyl ring–substituted etomidate analog binding affinity to the two β+ − α anesthetic binding sites on the GABAA receptor’s open state, suggesting that the binding pocket where etomidate’s phenyl ring lies becomes smaller as the receptor isomerizes from closed to open.

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