Distinct mechanisms regulate GABA A receptor and gephyrin clustering at perisomatic and axo-axonic synapses on CA1 pyramidal cells

Abstract

Pyramidal cells express various GABA A receptor (GABA AR) subtypes, possibly to match inputs from functionally distinct interneurons targeting specific subcellular domains. Postsynaptic anchoring of GABA ARs is ensured by a complex interplay between the scaffolding protein gephyrin, neuroligin-2 and collybistin. Direct interactions between these proteins and GABA AR subunits might contribute to synapse-specific distribution of GABA AR subtypes. In addition, the dystrophin-glycoprotein complex, mainly localized at perisomatic synapses, regulates GABA AR postsynaptic clustering at these sites. Here, we investigated how the functional and molecular organization of GABAergic synapses in CA1 pyramidal neurons is altered in mice lacking the GABA AR α2 subunit (α2-KO). We report a marked, layer-specific loss of postsynaptic gephyrin and neuroligin-2 clusters, without changes in GABAergic presynaptic terminals. Whole-cell voltage-clamp recordings in slices from α2-KO mice show a 40% decrease in GABAergic mIPSC frequency, with unchanged amplitude and kinetics. Applying low/high concentrations of zolpidem to discriminate between α1- and α2/α3-GABA ARs demonstrates that residual mIPSCs in α2-KO mice are mediated by α1-GABA ARs. Immunofluorescence analysis reveals maintenance of α1-GABA AR and neuroligin-2 clusters, but not gephyrin clusters, in perisomatic synapses of mutant mice, along with a complete loss of these three markers on the axon initial segment. This striking subcellular difference correlates with the preservation of dystrophin clusters, colocalized with neuroligin-2 and α1-GABA ARs on pyramidal cell bodies of mutant mice. Dystrophin was not detected on the axon initial segment in either genotype. Collectively, these findings reveal synapse-specific anchoring of GABA ARs at postsynaptic sites and suggest that the dystrophin-glycoprotein complex contributes to stabilize α1-GABA AR and neuroligin-2, but not gephyrin, in perisomatic postsynaptic densities. © 2011 The Authors. Journal compilation © 2011 The Physiological Society.