1. Actions of two GABAA receptor benzodiazepine-site ligands that are mediated via non-γ2-dependent modulation.
- Author
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Leppä E, Linden AM, Rabe H, Vekovischeva OY, Wulff P, Lüddens H, Wisden W, and Korpi ER
- Subjects
- Animals, Autoradiography, Azides pharmacology, Behavior, Animal drug effects, Benzodiazepines pharmacology, Binding Sites drug effects, Brain drug effects, Brain metabolism, Convulsants metabolism, Convulsants pharmacology, Female, HEK293 Cells, Humans, Hypnotics and Sedatives metabolism, Hypnotics and Sedatives pharmacology, Ligands, Male, Mice, Mice, Inbred C57BL, Protein Binding drug effects, Protein Subunits chemistry, Protein Subunits metabolism, Zolpidem, Benzodiazepines metabolism, Carbolines metabolism, Carbolines pharmacology, Pyridines metabolism, Pyridines pharmacology, Receptors, GABA-A chemistry, Receptors, GABA-A metabolism
- Abstract
The potent sedative-hypnotic zolpidem and the convulsant methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate (DMCM) act primarily by binding to the benzodiazepine site of the main inhibitory neurotransmitter receptor, the pentameric γ-aminobutyric acid type A receptor (GABA(A)). This binding depends critically on the wild-type F77 residue of the GABA(A) receptor γ2 subunit. Mice with γ2 subunit F77I point mutation (γ2I77 mouse line) lose the high-affinity nanomolar binding of these ligands as well as their most robust behavioral actions at low doses. Interestingly, the γ2I77 mice offer a tool to study the actions of these substances mediated via other possible binding sites of the GABA(A) receptor. In ligand autoradiographic experiments, we discovered in γ2I77 mouse brain sections a significant amount of residual non-γ2 subunit-dependent benzodiazepine site binding enriched to the striatum and septum. Zolpidem only weakly affected this residual binding at micromolar concentrations, and only a high zolpidem dose (≥ 40 mg/kg) caused sedation and deficits in motor coordination in γ2I77 mice. DMCM had an agonistic action through a secondary, low-affinity non-benzodiazepine binding site of the GABA(A) receptor in the forebrain of γ2I77 mice, and this drug also fully displaced the residual benzodiazepine-site labeling. In behavioral tests, a high dose (20mg/kg) of DMCM was sedative and modulated fear learning. DMCM, but not zolpidem, acted as an agonist in recombinant GABA(A) α1/6β3 receptors studied using ligand binding and electrophysiological assays. Our results highlight the less well-known actions of high doses of DMCM and zolpidem that are not mediated via the γ2 subunit-containing benzodiazepine site of the GABA(A) receptor., (Copyright © 2011 Elsevier B.V. All rights reserved.)
- Published
- 2011
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