Ectopic expression of the GABA sub(A) receptor alpha 6 subunit in hippocampal pyramidal neurons produces extrasynaptic receptors and an increased tonic inhibition

We generated transgenic (Thy1 alpha 6) mice in which the GABA sub(A) receptor alpha 6 subunit, whose expression is usually confined to granule cells of cerebellum and cochlear nuclei, is ectopically expressed under the control of the pan-neuronal Thy-1.2 promoter. Strong Thy1 alpha 6 subunit expression occurs, for example, in deep cerebellar nuclei, layer V iscocortical and hippocampal pyramidal cells and dentate granule cells. Ligand binding and protein biochemistry show that most forebrain alpha 6 subunits assemble as alpha 6 beta gamma 2-type receptors, and some as alpha 1 alpha 6 beta gamma 2 and alpha 3 alpha 6 beta gamma 2 receptors. Electron microscopic immunogold labeling shows that most Thy1-derived alpha 6 immunoreactivity is in the extrasynaptic plasma membrane of dendrites and spines in both layer V isocortical and CA1pyramidal cells. Synaptic immunolabeling is rare. Consistent with the alpha 6 subunits' extrasynaptic localization, Thy1 alpha 6 CA1 pyramidal neurons have a five-fold increased tonic GABA sub(A) receptor-mediated current compared with wild-type cells; however, the spontaneous IPSC frequency and the mIPSC amplitude in Thy1 alpha 6 mice decrease 37 and 30%, respectively compared with wild-type. Our results strengthen the idea that GABA sub(A) receptors containing alpha 6 subunits can function as extrasynaptic receptors responsible for tonic inhibition and further suggest that a homeostatic mechanism might operate, whereby increased tonic inhibition causes a compensatory decrease in synaptic GABA sub(A) receptor responses.