Background: Anesthetic contact residues in γ-aminobutyric acid type A (GABAA) receptors have been identified using photolabels, including two propofol derivatives. O-propofol diazirine labels H267 in β3 and α1β3 receptors, whereas m-azi-propofol labels other residues in intersubunit clefts of α1β3. Neither label has been studied in αβγ receptors, the most common isoform in mammalian brain. In αβγ receptors, other anesthetic derivatives photolabel m-azi-propofol-labeled residues, but not βH267. The authors’ structural homology model of α1β3γ2L receptors suggests that β3H267 may abut some of these sites.
Methods: Substituted cysteine modification–protection was used to test β3H267C interactions with four potent anesthetics: propofol, etomidate, alphaxalone, and R-5-allyl-1-methyl-5-(m-trifluoromethyl-diazirinylphenyl) barbituric acid (mTFD-MPAB). The authors expressed α1β3γ2L or α1β3H267Cγ2L GABAA receptors in Xenopus oocytes. The authors used voltage clamp electrophysiology to assess receptor sensitivity to γ-aminobutyric acid (GABA) and anesthetics and to compare p-chloromercuribenzenesulfonate modification rates with GABA versus GABA plus anesthetics.
Results: Enhancement of low GABA (eliciting 5% of maximum) responses by equihypnotic concentrations of all four anesthetics was similar in α1β3γ2L and α1β3H267Cγ2L receptors (n > 3). Direct activation of α1β3H267Cγ2L receptors, but not α1β3γ2L, by mTFD-MPAB and propofol was significantly greater than the other anesthetics. Modification of β3H267C by p-chloromercuribenzenesulfonate (n > 4) was rapid and accelerated by GABA. Only mTFD-MPAB slowed β3H267C modification (approximately twofold; P = 0.011).
Conclusions: β3H267 in α1β3γ2L GABAA receptors contacts mTFD-MPAB, but not propofol. The study results suggest that β3H267 is near the periphery of one or both transmembrane intersubunit (α+/β− and γ+/β−) pockets where both mTFD-MPAB and propofol bind.
Functional analysis and chemical modification–protection studies in a common brain γ-aminobutyric acid type A receptor revealed differences between potent anesthetics. Only the barbiturate protected β3H267C from modification; this mutation also enhanced agonism by propofol, indicating that β3H267 contributes to binding sites for barbiturates and propofol, but not for etomidate and alphaxalone.
Enhanced neuronal inhibition by γ-aminobutyric acid (GABA) type A receptors, GABA-gated heteropentameric ion channels, is a primary mechanism of action for the IV anesthetics propofol, etomidate, alphaxalone, and barbiturates.
Photolabeling with a propofol derivative suggested anesthetic sites near β3H267 in GABA type A receptors. However, the role of β3H267 in common heteromeric receptors remains uncertain.
Functional analysis and chemical modification–protection studies in a common brain γ-aminobutyric acid type A receptor revealed differences between potent anesthetics.
Only the barbiturate protected β3H267C from modification; this mutation also enhanced agonism by propofol, indicating that β3H267 contributes to binding sites for barbiturates and propofol, but not for etomidate and alphaxalone.
Alex T. Stern, Stuart A. Forman; A Cysteine Substitution Probes β3H267 Interactions with Propofol and Other Potent Anesthetics in α1β3γ2L γ-Aminobutyric Acid Type A Receptors. Anesthesiology 2016;124(1):89-100. doi: https://doi.org/10.1097/ALN.0000000000000934.
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