GABAA receptor: Positive and negative allosteric modulators

Abstract

gamma-Aminobutyric acid (GABA)-mediated inhibitory neurotransmission and the gene products involved were discovered during the mid-twentieth century. Historically, myriad existing nervous system drugs act as positive and negative allosteric modulators of these proteins, making GABA a major component of modern neuropharmacology, and suggesting that many potential drugs will be found that share these targets. Although some of these drugs act on proteins involved in synthesis, degradation, and membrane transport of GABA, the GABA receptors Type A (GABA_AR) and Type B (GABA_BR) are the targets of the great majority of GABAergic drugs. This discovery is due in no small part to Professor Norman Bowery. Whereas the topic of GABA_BR is appropriately emphasized in this special issue, Norman Bowery also made many insights into GABA_AR pharmacology, the topic of this article. GABA_AR are members of the ligand-gated ion channel receptor superfamily, a chloride channel family of a dozen or more heteropentameric subtypes containing 19 possible different subunits. These subtypes show different brain regional and subcellular localization, age-dependent expression, and potential for plastic changes with experience including drug exposure. Not only are GABA_AR the targets of agonist depressants and antagonist convulsants, but most GABA_AR drugs act at other (allosteric) binding sites on the GABA_AR proteins. Some anxiolytic and sedative drugs, like benzodiazepine and related drugs, act on GABA_AR subtype-dependent extracellular domain sites. General anesthetics including alcohols and neurosteroids act at GABA_AR subunit-interface trans-membrane sites. Ethanol at high anesthetic doses acts on GABA_AR subtype-dependent trans-membrane domain sites. Ethanol at low intoxicating doses acts at GABA_AR subtype-dependent extracellular domain sites. Thus GABA_AR subtypes possess pharmacologically specific receptor binding sites for a large group of different chemical classes of clinically important neuropharmacological agents. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Keywords: Alphaxalone, 104845; Anxiolytics/sedatives/general anesthetics; Bicuculline, 10237; CNS depressant drug target; Diazepam, 3016; Ethanol; Etomidate, 667484; GABA: 119; Inhibitory neurotransmission; Muscimol, 4266; Pentobarbital, 4737; Picrotoxinin, 442292; Ro15-4513, 5081; TBPS, 104781; γ-Aminobutyric acid type A receptors.

FIGURE 1

GABAAR ligand sites at subunit interfaces identified by mutagenesis and/or affinity labeling. The left panel shows the transmembrane binding sites, while the right panel shows the ECD binding sites. The protein structures are taken from the X-ray crystallography-derived structure of the recombinant mammalian homomeric β3 GABA_AR (Miller and Aricescu, 2014), on which is displayed an homologous native GABA_AR comprised of an α–β–α–β–γ/δ heteropentamer (actual subunits arbitrary, no specific sequence implied although they are all homologous to β3). The protein is viewed looking from the extracellular face, perpendicular to the cell membrane/synapse. Thus the ECD pentamer on right would actually be positioned directly on top of the TMD pentamer at left. Both portions indicate locations of ligand binding sites found in the β3 homomer structure. The two α subunits are indicated by the green (or dark gray) shaded oval, the two β subunits by the pink (or light gray) shaded ovals, and the one γ/δ subunit indicated by the clear (white) oval. An example C-terminus is indicated by a small red circled “C” at the bottom of the TMD of the γ/δ subunit; the M1,2,3,4 domains are also labeled in this example subunit, and the N-terminus of the TMD of each subunit would attach to its ECD at the position indicated by the small blue (or black) oval “ECD”. Ligand binding sites for the compounds listed (in shorthand) are indicated by arrows (Cf. Table 1). Note that the heteropentameric proteins show several different but homologous subunit interfaces (Table 1) so the pharmacological specificity varies with GABAAR subtype. The ligands named are BZ (benzodiazepines) and GABA, EtOH, and Pyr (pyrazoloquinoliines, see Table 1) in the ECD. In the TMD, Eto (etomidate), Pro (propofol), octanol, volatiles (volatile anesthetics), and barbs (barbiturates) binding sites are located.