Structural Determinants for High-Affinity Zolpidem Binding to GABA-A receptors (original) (raw)

2006, Molecular Pharmacology

The imidazopyridine zolpidem (Ambien) is one of the most commonly prescribed sleep aids in the United States . Similar to classic benzodiazepines (BZDs), zolpidem binds at the extracellular N-terminal ␣/␥ subunit interface of the GABA-A receptor (GABAR). However, zolpidem differs significantly from classic BZDs in chemical structure and neuropharmacological properties. Thus, classic BZDs and zolpidem are likely to have different requirements for high-affinity binding to GABARs. To date, three residues-␥2Met57, ␥2Phe77, and ␥2Met130 -have been identified as necessary for high-affinity zolpidem binding (Proc Natl Acad Sci USA 94:8824 -8829, 1997; Mol Pharmacol 52: 874 -881, 1997). In this study, we used radioligand binding techniques, ␥2/␣1 chimeric subunits (), site-directed mutagenesis, and molecular modeling to identify additional ␥2 subunit residues important for high-affinity zolpidem binding. Whereas ␣1␤2 receptors containing only the first 161 amino-terminal residues of the ␥2 subunit bind the classic BZD flunitrazepam with wild-type affinity, zolpidem affinity is decreased ϳ8-fold. By incrementally restoring ␥2 subunit sequence, we identified a seven-amino acid stretch in the ␥2 subunit loop F region (amino acids 186 -192) that is required to confer high-affinity zolpidem binding to GABARs. When mapped to a homology model, these seven amino acids make up part of loop F located at the ␣/␥ interface. Based on in silico zolpidem docking, three residues within loop F, ␥2Glu189, ␥2Thr193, and ␥2Arg194, emerge as being important for stabilizing zolpidem in the BZD binding pocket and probably interact with other loop F residues to maintain the structural integrity of the BZD binding site.