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

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.