The vast molecular heterogeneity of brain gamma-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 alpha6 subunit gene. Cerebellar
granule cells of alpha6(-/-) 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 alpha6beta2/3gamma2 receptors.
In the presence of GABA concentrations that fail to activate alpha1
subunit-containing receptors,
methyl-6,7-dimethoxy-4-ethyl-beta-carboline (30 microM),
allopregnanolone (100 nM), and Zn²⁺ (10 microM) are less efficacious in
altering tert-[³⁵S]butylbicyclophosphorothionate binding in the granule
cell layer of the alpha6(-/-) than alpha6(+/+) animals. These data
concur with the deficiency of the cerebellar alpha6 and delta
subunit-containing receptors in the alpha6(-/-) animals and could also
account for the decreased affinity of [³H]muscimol binding to
alpha6(-/-) cerebellar membranes. Predicted additional alterations in
the cerebellar receptors of the mutant mice may explain a surplus of
methyl-6,7-dimethoxy-4-ethyl-beta-carboline-insensitive receptors in the
alpha6(-/-) 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 (alpha and delta) from granule cells.