ABSTRACT
The specific mechanisms underlying general anesthesia are primarily unknown. The intravenous general anesthetic etomidate acts by potentiating GABA(A) receptors, with selectivity for beta2 and beta3 subunit-containing receptors determined by a single asparagine residue. We generated a genetically modified mouse containing an etomidate-insensitive beta2 subunit (beta2 N265S) to determine the role of beta2 and beta3 subunits in etomidate-induced anesthesia. Loss of pedal withdrawal reflex and burst suppression in the electroencephalogram were still observed in the mutant mouse, indicating that loss of consciousness can be mediated purely through beta3-containing receptors. The sedation produced by subanesthetic doses of etomidate and during recovery from anesthesia was present only in wild-type mice, indicating that the beta2 subunit mediates the sedative properties of anesthetics. These findings show that anesthesia and sedation are mediated by distinct GABA(A) receptor subtypes.
Subject(s)
Anesthetics/pharmacology , Etomidate/pharmacology , Hypnotics and Sedatives/pharmacology , Receptors, GABA-A/metabolism , Animals , Anticonvulsants/pharmacology , Behavior, Animal/drug effects , Binding, Competitive/drug effects , Bridged Bicyclo Compounds, Heterocyclic/pharmacokinetics , Cell Separation , Consciousness/drug effects , Dose-Response Relationship, Drug , Electroencephalography/drug effects , Gene Targeting , In Vitro Techniques , Male , Mice , Mice, Mutant Strains , Motor Activity/drug effects , Patch-Clamp Techniques , Protein Isoforms/genetics , Protein Isoforms/metabolism , Purkinje Cells/cytology , Purkinje Cells/drug effects , Purkinje Cells/physiology , Receptors, GABA-A/drug effects , Receptors, GABA-A/genetics , Recovery of Function/drug effects , Recovery of Function/genetics , Triazoles/pharmacologyABSTRACT
GPCRs are one of the most popular classes of therapeutic drug targets. It is therefore important to design specific assay formats to readily identify ligands at these receptors. CypHer 5 technology utilizes the general ability of GPCRs to be internalized into the endosomal pathway of a cell in response to agonist ligands. The CypHer 5 dye is fluorescent in acidic environments, but nonfluorescent at neutral pH. When CypHer 5 is bound to a receptor on the extracellular surface of the cell, it is essentially nonfluorescent. On internalization into a cell, it displays a significant increase in fluorescence. Here we demonstrate the detection of agonist activation of two GPCRs in stably transfected live cells using CypHer 5 technology. The G(q)-coupled TRHR-1 and the G(s)-coupled beta(2)-adrenoceptor were both N-terminally tagged with VSV-G. Following addition of CypHer 5-labeled anti-VSV-G antibodies to HEK 293 cells stably expressing the beta(2)-adrenoceptor or CHO-K1 cells stably expressing the TRHR-1, the cells were treated with agonists and then imaged on Amersham Biosciences' IN Cell Analyzer 3000. Data were quantified using a granularity analysis module. Concentration-response curves were obtained with signal-to-background ratios of 7:1 for both receptors. An EC(50) of 0.52 nM was observed on TRH stimulation of the TRHR-1, and an EC(50) of 30 nM was obtained on isoprenaline stimulation of the beta(2)-adrenoceptor. These results demonstrated that the CypHer technology was capable of measuring high-potency agonist responses. The beta(2)-adrenoceptor antagonist, alprenolol, competed for isoprenaline with an IC(50) of 30 nM, indicating that a high-potency antagonist inhibition curve could also be observed using CypHer. CypHer 5 provides a generic tool to measure GPCR activation in a live cell, homogeneous assay format, and may be equally suitable for detecting activation of other classes of cell surface receptors.
