CEREBELLAR GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTORS - PHARMACOLOGICALSUBTYPES REVEALED BY MUTANT MOUSE LINES

The vast molecular heterogeneity of brain gamma-aminobutyric acid type A (GABA(A)) receptors forms the basis for receptor subtyping. Using a utoradiographic techniques, we established the characteristics of cere bellar granule cell GABA(A) receptors by comparing wild-type mice with those with a targeted disruption of the alpha 6 subunit gene. Cerebel lar granule cells of alpha 6(-/-) animals have severe deficits in high affinity [H-3]muscimol and [H-3]SR 95531 binding to GABA sites, in ag onist-insensitive [H-3]Ro 15-4513 binding to benzodiazepine sites, and in furosemide-induced increases in tert-[S-35]butylbicyclophosphoroth ionate binding to picrotoxin-sensitive convulsant sites. These observa tions agree with the known specific properties of these sites on recom binant alpha 6 beta 2/3 gamma 2 receptors. In the presence of GABA con centrations that fail to activate alpha 1 subunit-containing receptors , methyl-6,7-dimethoxy-4-ethyl-beta-carboline (30 mu M), allopregnanol one (100 nM), and Zn2+ (10 mu M) are less efficacious in altering tert -[S-35]butylbicyclophosphorothionate binding in the granule cell layer of the alpha 6(-/-) than alpha 6(+/+) animals. These data concur with the deficiency of the cerebellar alpha 6 and delta subunit-containing receptors in the alpha 6(-/-) animals and could also account for the decreased affinity of [H-3]muscimol binding to alpha 6(-/-) cerebellar membranes. Predicted additional alterations in the cerebellar recepto rs of the mutant mice may explain a surplus of l-6,7-dimethoxy-4-ethyl -beta-carboline-insensitive insensitive receptors in the alpha 6(-/-) granule cell layer and an increased diazepam-sensitivity in the molecu lar layer. These changes may be adaptive consequences of altered GABA( A) receptor subunit expression patterns in response to the loss of two subunits (alpha and delta) from granule cells.