Alterations in the expression of GABAA receptor subunits in cerebellar granule cells after the disruption of the α6 subunit gene

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Abstract
Any given subunit of the heteromultimeric type‐A γ‐aminobutyric acid (GABA) GABAA receptor may be present in several receptor subtypes expressed by individual neurons. Changes in the expression of a subunit may result in differential changes in the expression of other subunits depending on the subunit composition of the receptor subtype, leading to alterations in neuronal responsiveness to GABA. We used the targeted disruption of the α6 subunit gene to test for changes in the expression of other GABAA receptor subunits. Immunoprecipitation and ligand binding experiments indicated that GABAA receptors were reduced by ≈ 50% in the cerebellum of α6 –/– mice. Western blot experiments indicated that the α6 subunit protein completely disappeared from the cerebellum of α6 –/– mice, which resulted in the disappearance of the δ subunit from the plasma membrane of granule cells. The amount of β2, β3 and γ2 subunits was reduced by ≈ 50%, 20% and 40%, respectively, in the cerebella of α6 –/– mice. A comparison of the reduction in the level of α1, β2, β3, γ2, or δ‐subunit‐containing receptors in α6 –/– cerebellum with those observed after removal of α6‐subunit‐containing receptors from the cerebella of α6 +/+ mice by immuno‐affinity chromatography demonstrated the presence of a significantly higher than expected proportion of receptors containing β3 subunits in α6 –/– mice. The receptors containing α1, β2, β3 and γ2 subunits were present in the plasma membrane of granule cells of α6 –/– mice at both synaptic and extrasynaptic sites, as shown by electron microscopic immunocytochemistry. Despite the changes, the α1 subunit content of Golgi‐cell‐to‐granule‐cell synapses in α6 –/– animals remained unaltered, as did the frequency of α1 immunopositive synapses in the glomeruli. Furthermore, no change was apparent in the expression of the α1, β2 and γ2 subunits in Purkinje cells and interneurons of the molecular layer.These results demonstrate that in α6 –/– mice, the cerebellum expresses only half of the number of GABAA receptors present in wild‐type animals. Since these animals have no gross motor deficits, synaptic integration in granule cells is apparently maintained by α1‐subunit‐containing receptors with an altered overall subunit composition, and/or by changes in the expression of other ligand and voltage gated channels.