Targeted Disruption of the GABA sub(A) Receptor delta Subunit Gene Leads to an Up-regulation of gamma sub(2) Subunit-containing Receptors in Cerebellar Granule Cells

GABA sub(A) receptors are chloride channels composed of five subunits. Cerebellar granule cells express abundantly six subunits belonging to four subunit classes. These are assembled into a number of distinct receptors, but the regulation of their relative proportions is yet unknown. Here, we studied the composition of cerebellar GABA sub(A) receptors after targeted disruption of the Delta subunit gene. In membranes and extracts of Delta -/- cerebellum, [ super(3)H]muscimol binding was not significantly changed, whereas [ super(3)H]Ro15-4513 binding was increased by 52% due to an increase in diazepam-insensitive binding. Immunocytochemical and Western blot analysis revealed no change in alpha sub(6) subunits but an increased expression of gamma sub(2) subunits in Delta -/- cerebellum. Immunoaffinity chromatography of cerebellar extracts indicated there was an increased coassembly of alpha sub(6) and gamma sub(2) subunits and that 24% of all receptors in Delta -/- cerebellum did not contain a gamma subunit. Because 97% of Delta subunits are coassembled with alpha sub(6) subunits in the cerebellum of wild-type mice, these results indicated that, in Delta -/- mice, alpha sub(6) beta gamma sub(2) and alpha beta receptors replaced Delta subunit- containing receptors. The availability of the Delta subunit, thus, influences the level of expression or the extent of assembly of the gamma sub(2) subunit, although these two subunits do not occur in the same receptor.