The selectivity of zolpidem and alpidem for the α₁-subunit of the GABA_A receptor

Abstract

The pharmacological profile of recombinant α₁β₂γ₂ and α₅β₂γ₂ GABA_A receptors was investigated using ³H-flumazenil as a probe and compared to that exhibited by native GABA_A receptors present in rat cerebellum and spinal cord. Diazepam, zopiclone and triazolam did not differentiate between these various GABA_A receptors. In contrast, the imidazopyridines zolpidem and alpidem showed high affinity for the cerebellar GABA_A receptor, intermediate affinity for the spinal cord receptor, but no affinity for the α₅β₂γ₂ recombinant form of the receptor.

Two subpopulations of omega (ω) or benzodiazepine binding sites ω₁ and ω₂ that are allosterically coupled to the GABA recognition site of the GABA_A receptor were initially proposed on the basis of their pharmacological profile and distinctive distribution in the brain (Langer and Arbilla, 1988). The ω₁ modulatory site was distinguished by its high affinity for drugs such as CL 218,872, β-CCE and the imidazopyridines, alpidem and zolpidem and its enrichment in the substantia nigra, cerebellum molecular layer and cortex layer IV. In contrast, the ω₂ modulatory site was defined as the binding site that did not exhibit high affinity for ω₁ selective compounds and which was notably enriched in the spinal cord. Available evidence now suggests more extensive GABA_A receptor heterogeneity. The recent cloning of cDNAs encoding GABA_A receptor subunits has revealed an extensive family of closely-related polypeptides which can be divided into five subunit types, α, β, γ, δ and ϱ, based upon their deduced amino acid sequences (for review, see Burt and Kamatchi, 1991). Interestingly, within each subunit class, different isoforms exist e.g. α₁, α₂ … α₆, and functional expression of GABA_A subunit genes in αβγ recombinants reveals that a wide diversity in ω site pharmacology is conferred by these different α-subunit isoforms.

For the present report, we have studied using ³H-flumazenil as a probe the interaction of the imidazopyridines, alpidem and zolpidem, the cyclopyrrolone, zopiclone, the triazolobenzodiazepine, triazolam and the benzodiazepine diazepam with recombinant αβγ forms of rat GABA_A receptors (notably α₁β₂γ₂ and α₅β₂γ₂) transiently-expressed in human embryonic kidney 293 cells. These findings are related to the affinities of these compounds for the previously-defined ω₁ and ω₂ binding sites associated with native GABA_A1 and GABA_A2 receptor subtypes of rat cerebellum and spinal cord, respectively.