Clustered and non-clustered GABAA receptors in cultured hippocampal neurons

In cultured hippocampal neurons, γ2 subunit-containing GABAARs form large postsynaptic clusters at GABAergic synapses and small clusters outside GABAergic synapses. We now show that a pool of non-clustered γ2 subunit-containing GABAARs are also present at the cell surface. We also demonstrate that myc- or EGFP-tagged γ2, α2, β3 or α1 subunits expressed in these neurons assemble with endogenous subunits, forming GABAARs that target large postsynaptic clusters, small clusters outside GABAergic synapses or a pool of non-clustered surface GABAARs. In contrast, myc- or EGFP-tagged δ subunits only form non-clustered GABAARs, which can be induced to form clusters by antibody capping. A myc-tagged chimeric γ2 subunit possessing the large intracellular loop (IL) of the δ-subunit IL (mycγ2S/δ-IL) assembled into GABAARs, but it did not form clusters, therefore behaving like the δ subunit. Thus, the large intracellular loops of γ2 and δ play an important role in determining the synaptic clustering/non-clustering capacity of the GABAARs.