Distribution of [³H]zolpidem binding sites in relation to messenger RNA encoding the α1, β2 and γ2 subunits of GABA_A receptors in rat brain

Abstract

Localization of the messenger RNAs that encode the α1, β2 and γ2 subunits of GABA_A showed a distinct topographic pattern in rat brain which corresponded with [³H]zolpidem binding in most brain regions. The close topographic correspondence between the specific receptor subunits examined and the distribution of [³H]zolpidem binding sites provides support for the hypothesis that this benzodiazepine type 1 selective ligand binds to a GABA_A receptor that consists of α1, β2 and γ2 subunits in the rat brain. Brain regions with relatively high densities of α1, β2 and γ2 subunits of GABA_A and [³H]zolpidem binding included olfactory bulb, medial septum, ventral pallidum, diagonal band, inferior colliculus, substantia nigra pars reticulata and specific layers of the cortex. Two areas with low [³H]zolpidem binding and a virtual absence of these GABA_A receptor subunit messenger RNAs were the lateral septum and the striatum. In contrast to the discrete pattern observed for α1 and β2 subunit messenger RNAs, the γ2 subunit messenger RNA was distributed more diffusely in brain. Only the hippocampus, layer 2 of the piriform cortex and the cerebellum showed a strong concentration of the γ2 subunit messenger RNA. It was determined with a polymerase chain reaction assay that both long and short variants of the γ2 subunit messenger RNAs were present within several of the brain sites selected for examination. Sites with high densities of [³H]zolpidem binding sites had a greater relative abundance of the γ2 long splice variant, compared to the γ2 short variant. There were some regions that expressed high levels of α1, β2 and γ2S subunit messenger RNAs but low [³H]zolpidem binding, suggesting that γ2 splice variant expression may modulate high-affinity [³H]zolpidem binding. To determine relationships between in vitro [³H]zolpidem binding and functional sensitivity in vivo, interactions between zolpidem and GABA were assessed in brain regions that contained high and low densities of [³H]zolpidem binding sites. In the medial septum, a brain region with a high concentration of [³H]zolpidem binding sites, iontophoretic application of zolpidem enhanced the inhibitory effect of GABA responses on 70% of the neurons examined. In the lateral septum, which contains very low densities of [³H]zolpidem binding sites, neurons were not sensitive to zolpidem enhancement of GABA-induced inhibition. These electrophysiological results demonstrate a correspondence between the regional distribution of [³H]zolpidem binding in vitro and functional sensitivity to the drug in vivo.