Cerebellar γ-Aminobutyric Acid Type A Receptors: Pharmacological Subtypes Revealed by Mutant Mouse Lines

Abstract

The vast molecular heterogeneity of brain γ-aminobutyric acid type A (GABA_A) receptors forms the basis for receptor subtyping. Using autoradiographic techniques, we established the characteristics of cerebellar granule cell GABA_A receptors by comparing wild-type mice with those with a targeted disruption of the α6 subunit gene. Cerebellar granule cells of α6^−/− animals have severe deficits in high affinity [³H]muscimol and [³H]SR 95531 binding to GABA sites, in agonist-insensitive [³H]Ro 15–4513 binding to benzodiazepine sites, and in furosemide-induced increases intert-[³⁵S]butylbicyclophosphorothionate binding to picrotoxin-sensitive convulsant sites. These observations agree with the known specific properties of these sites on recombinant α6β2/3γ2 receptors. In the presence of GABA concentrations that fail to activate α1 subunit-containing receptors, methyl-6,7-dimethoxy-4-ethyl-β-carboline (30 μm), allopregnanolone (100 nm), and Zn²⁺ (10 μm) are less efficacious in alteringtert-[³⁵S]butylbicyclophosphorothionate binding in the granule cell layer of the α6^−/− than α6^+/+ animals. These data concur with the deficiency of the cerebellar α6 and δ subunit-containing receptors in the α6^−/− animals and could also account for the decreased affinity of [³H]muscimol binding to α6^−/−cerebellar membranes. Predicted additional alterations in the cerebellar receptors of the mutant mice may explain a surplus of methyl-6,7-dimethoxy-4-ethyl-β-carboline-insensitive receptors in the α6^−/− granule cell layer and an increased diazepam-sensitivity in the molecular layer. These changes may be adaptive consequences of altered GABA_A receptor subunit expression patterns in response to the loss of two subunits (α and δ) from granule cells.