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

Pyramidal cells express various GABAA receptor (GABAAR) subtypes, possibly to match inputs from functionally distinct interneurons targeting specific subcellular domains. Postsynaptic anchoring of GABAAR is ensured by a complex interplay between the scaffolding protein gephyrin, neuroligin-2, and collybistin. Direct interactions between these proteins and GABAAR subunits might contribute to synapse-specific distribution of GABAAR subtypes. In addition, the dystrophin-glycoprotein complex, mainly localized at perisomatic synapses, regulates GABAAR 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 GABAAR ?2 subunit (?2-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 ?2-KO mice show a 40% decrease in GABAergic mIPSC frequency, with unchanged amplitude and kinetics. Applying low/high concentrations of zolpidem to discriminate between ?1- and ?2/?3-GABAAR demonstrates that residual mIPSCs in ?2-KO mice are mediated by ?1-GABAAR. Immunofluorescence analysis reveals maintenance of ?1-GABAAR 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 ?1-GABAAR 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 GABAAR at postsynaptic sites and suggest that the dystrophin-glycoprotein complex contributes to stabilize ?1-GABAAR and neuroligin-2, but not gephyrin, in perisomatic postsynaptic densities.