Alteration of GABAergic synapses and gephyrin clusters in the thalamic reticular nucleus of GABAA receptor α3 subunit-null mice

Multiple GABA(A)-receptor subtypes are assembled from alpha, beta and gamma subunit variants. GABA(A) receptors containing the alpha 3 subunit represent a minor population with a restricted distribution in the CNS. In addition, they predominate in monoaminergic neurons and in the nucleus reticularis thalami (nRT), suggesting a role in the regulation of cortical function and sleep. Mice with a targeted deletion of the alpha 3 subunit gene (alpha 3(0/0)) are viable and exhibit a subtle behavioural phenotype possibly related to dopaminergic hyperfunction. Here, we investigated immunohistochemically the consequences of the loss of alpha 3 subunit for maturation of GABA(A) receptors and formation of GABAergic synapses in the nRT. Throughout postnatal development, the regional distribution of the alpha 1, alpha 2, or alpha 5 subunit was unaltered in alpha 3(0/0) mice and the prominent alpha 3 subunit staining of nRT neurons in wildtype mice was not replaced. Subcellularly, as seen by double immunofluorescence, the alpha 3 and gamma 2 subunit were clustered at postsynaptic sites in the nRT of adult wildtype mice along with the scaffolding protein gephyrin. In alpha 3(0/0) mice, gamma 2 subunit clustering was disrupted and gephyrin formed large aggregates localized at the cell surface, but unrelated to postsynaptic sites, indicating that nRT neurons lack postsynaptic GABA(A) receptors in mutant mice. Furthermore, GABAergic terminals were enlarged and reduced in number, suggesting a partial deficit of GABAergic synapses. Therefore, GABA(A) receptors are required for gephyrin clustering and long-term synapse maintenance. The absence of GABA(A)-mediated transmission in the nRT may have a significant impact on the function of the thalamo-cortical loop of alpha 3(0/0) mice.