An important criterion for association of subunit isoforms into oligomeric native receptors is co-localization of the subunits. Immunocytochemical studies investigating the co-localization of subunits in GABAA receptor clusters on neuronal membranes (Fritschy et al., 1992; Bohlhalter et al., 1996), as well as electron microscopic studies (Nusser et al., 1995; Somogyi et al., 1996), indicate that the majority of GABAA receptors present in the brain are composed of α, β, and γ subunits. Receptors composed of α1, β2, and γ2 subunits are extensively co-localized among subsets of GABAergic interneurons in hippocampus and other brain regions, e.g. calretinin-, neuropeptideY-, somatostatin-positive cells, but not on calbindin-D28k-, cholecystokinin- and vasoactive intestinal peptide-containing cells (Gao and Fritschy, 1994), supporting the conclusion that α1β2γ2 receptors are the most abundant GABAA receptors in the brain (Pirker et al., 2000).
