GABAA alpha6-containing receptors are selectively compromised in cerebellar granule cells of the ataxic mouse, stargazer

Stargazer mice fail to express the gamma 2 isoform of transmembrane alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate ( AMPA) receptor regulatory proteins that has been shown to be absolutely required for the trafficking and synaptic targeting of excitatory AMPA receptors in adult murine cerebellar granule cells. Here we show that 30 +/- 6% fewer inhibitory gamma-aminobutyric acid, type A (GABA(A)), receptors were expressed in adult stargazer cerebellum compared with controls because of a specific loss of GABAA receptor expression in the cerebellar granule cell layer. Radioligand binding assays allied to in situ immunogold-EM analysis and furosemide-sensitive tonic current estimates revealed that expression of the extrasynaptic (alpha 6 beta x delta) alpha 6-containing GABA(A) receptor were markedly and selectively reduced in stargazer. These observations were compatible with a marked reduction in expression of GABAA receptor alpha 6,delta (mature cerebellar granule cell-specific proteins), and beta 3 subunit expression in stargazer. The subunit composition of the residual alpha 6-containing GABA(A) receptors was unaffected by the stargazer mutation. However, we did find evidence of an similar to 4-fold up-regulation of alpha 1 beta delta receptors that may compensate for the loss of alpha 6-containing GABA(A) receptors. PCR analysis identified a dramatic reduction in the steady-state level of alpha 6 mRNA, compatible with alpha 6 being the primary target of the stargazer mutation-mediated GABAA receptor abnormalities. We propose that some aspects of assembly, trafficking, targeting, and/or expression of extrasynaptic alpha 6-containing GABA(A) receptors in cerebellar granule cells are selectively regulated by AMPA receptor-mediated signaling.