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MOLECULAR PHARMACOLOGY 52:380-388 (1997).

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

Riikka Mäkelä, Mikko Uusi-Oukari, Gregg E. Homanics, Joseph J. Quinlan, Leonard L. Firestone, William Wisden, and Esa R. Korpi

Department of Mental Health and Alcohol Research, National Public Health Institute, Helsinki, Finland (R.M., E.R.K.), Tampere Brain Research Center, University of Tampere Medical School, Tampere, Finland (R.M.), Department of Anesthesiology/Critical Care Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261 (G.E.H., J.L.Q., L.L.F.), Medical Research Council Laboratory of Molecular Biology, Medical Research Council Centre, Cambridge CB2 2QH, UK (W.W.), and Department of Pharmacology and Clinical Pharmacology, University of Turku, Turku, Finland (M.U.-O., E.R.K.)

The vast molecular heterogeneity of brain -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 6 subunit gene. Cerebellar granule cells of 6^/ 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 in tert-[³⁵S]butylbicyclophosphorothionate binding to picrotoxin-sensitive convulsant sites. These observations agree with the known specific properties of these sites on recombinant 62/32 receptors. In the presence of GABA concentrations that fail to activate 1 subunit-containing receptors, methyl-6,7-dimethoxy-4-ethyl--carboline (30 µM), allopregnanolone (100 nM), and Zn²⁺ (10 µM) are less efficacious in altering tert-[³⁵S]butylbicyclophosphorothionate binding in the granule cell layer of the 6^/ than 6^+/+ animals. These data concur with the deficiency of the cerebellar 6 and  subunit-containing receptors in the 6^/ animals and could also account for the decreased affinity of [³H]muscimol binding to 6^/ cerebellar membranes. Predicted additional alterations in the cerebellar receptors of the mutant mice may explain a surplus of methyl-6,7-dimethoxy-4-ethyl--carboline-insensitive receptors in the 6^/ 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 ( and ) from granule cells.