Z. Nusser et al., Alterations in the expression of GABA(A) receptor subunits in cerebellar granule cells after the disruption of the alpha 6 subunit gene, EUR J NEURO, 11(5), 1999, pp. 1685-1697
Any given subunit of the heteromultimeric type-A gamma-aminobutyric acid (G
ABA) GABA(A) receptor may be present in several receptor subtypes expressed
by individual neurons. Changes in the expression of a subunit may result i
n differential changes in the expression of other subunits depending on the
subunit composition of the receptor subtype, leading to alterations in neu
ronal responsiveness to GABA. We used the targeted disruption of the alpha
6 subunit gene to test for changes in the expression of other GABA(A) recep
tor subunits. Immunoprecipitation and ligand binding experiments indicated
that GABA(A) receptors were reduced by approximate to 50% in the cerebellum
of alpha 6-/- mice. Western blot experiments indicated that the alpha 6 su
bunit protein completely disappeared from the cerebellum of alpha 6-/- mice
, which resulted in the disappearance of the delta subunit from the plasma
membrane of granule cells. The amount of beta 2, beta 3 and gamma 2 subunit
s was reduced by approximate to 50%, 20% and 40%, respectively, in the cere
bella of alpha 6-/- mice. A comparison of the reduction in the level of alp
ha 1, beta 2, beta 3, gamma 2, or delta-subunit-containing receptors in alp
ha 6-/- cerebellum with those observed after removal of alpha 6-subunit-con
taining receptors from the cerebella of alpha 6+/+ mice by immune-affinity
chromatography demonstrated the presence of a significantly higher than exp
ected proportion of receptors containing beta 3 subunits in alpha 6-/- mice
. The receptors containing alpha 1, beta 2, beta 3 and gamma 2 subunits wer
e present in the plasma membrane of granule cells of alpha 6-/- mice at bot
h synaptic and extrasynaptic sites, as shown by electron microscopic immuno
cytochemistry. Despite the changes, the alpha 1 subunit content of Golgi-ce
ll-to-granule-cell synapses in alpha 6-/- animals remained unaltered, as di
d the frequency of alpha 1 immunopositive synapses in the glomeruli. Furthe
rmore, no change was apparent in the expression of the alpha 1, beta 2 and
gamma 2 subunits in Purkinje cells and interneurons of the molecular layer.
These results demonstrate that in alpha 6-/- mice, the cerebellum expresses
only half of the number of GABA(A) receptors present in wild-type animals.
Since these animals have no gross motor deficits, synaptic integration in
granule cells is apparently maintained by alpha 1-subunit-containing recept
ors with an altered overall subunit composition, and/or by changes in the e
xpression of other ligand and voltage gated channels.