GABA(A) receptors containing the α4-subunit: Prevalence, distribution, pharmacology, and subunit architecture in situ

Abstract

Recombinant GABA(A) receptors, expressed from α-, β-, and γ2- subunits, are diazepam-insensitive when the α-subunit is either α4 or α6. In situ, diazepaminsensitive receptors containing the α6-subunit are almost exclusively expressed in the granule cell layer of the cerebellum. However, diazepam-insensitive receptors are also expressed in forebrain areas. Here, we report on the presence of diazepam-insensitive GABA(A) receptors in various brain areas containing the α4-subunit. GABA(A) receptors immunoprecipitated with a newly developed α4-subunit-specific antiserum displayed a drug binding profile that was indistinguishable from those of α4β2γ2-recombinant receptors and diazepam-insensitive [3H] Ro 15-4513 binding sites in rat brain membranes. In addition, α4-subunit containing receptors and forebrain diazepam-insensitive receptors are present at comparably low abundance in rat brain and exhibit virtually identical patterns of distribution. Analysis of the subunit architecture of α4- subunit containing receptors revealed that the α4-subunit contributes to several receptor subtypes. Depending on the brain region, the α4-subunit can be coassembled with a second type of α-subunit variant being α1, α2, or α3. The data demonstrate that native receptors containing the α4-subunit are structurally heterogeneous, expressed at very low abundance in the brain, and display the drug binding profile of diazepam-insensitive [3H]Ro 15-4513 binding sites. Pharmacologically, these receptors may contribute to the actions of nonclassical ligands such as Ro 15-4513 and bretazenil.