Synaptic GABAA Receptor Clustering without the γ2 Subunit

Rapid activation of postsynaptic GABA(A) receptors (GABA(A)Rs) is crucial in many neuronal functions, including the synchronization of neuronal ensembles and controlling the precise timing of action potentials. Although the gamma 2 subunit is believed to be essential for the postsynaptic clustering of GABAARs, synaptic currents have been detected in neurons obtained from gamma 2(-/-) mice. To determine the role of the gamma 2 subunit in synaptic GABA(A)R enrichment, we performed a spatially and temporally controlled gamma 2 subunit deletion by injecting Cre-expressing viral vectors into the neocortex of GABA(A)R gamma 2(77I)lox mice. Whole-cell recordings revealed the presence of miniature IPSCs in Cre(+) layer 2/3 pyramidal cells (PCs) with unchanged amplitudes and rise times, but significantly prolonged decays. Such slowly decaying currents could be evoked in PCs by action potentials in presynaptic fast-spiking interneurons. Freeze-fracture replica immunogold labeling revealed the presence of the alpha 1 and beta 3 subunits in perisomatic synapses of cells that lack the gamma 2 subunit. Miniature IPSCs in Cre(+) PCs were insensitive to low concentrations of flurazepam, providing a pharmacological confirmation of the lack of the gamma 2 subunit. Receptors assembled from only alpha beta subunits were unlikely because Zn2+ did not block the synaptic currents. Pharmacological experiments indicated that the alpha beta gamma 3 receptor, rather than the alpha beta delta, alpha beta epsilon, or alpha beta gamma 1 receptors, was responsible for the slowly decaying IPSCs. Our data demonstrate the presence of IPSCs and the synaptic enrichment of the alpha 1 and beta 3 subunits and suggest that the gamma 3 subunit is the most likely candidate for clustering GABAARs at synapses in the absence of the gamma 2 subunit.