Rapid activation of postsynaptic GABAA receptors (GABAARs) is crucial in many neuronal
functions, including the synchronization of neuronal ensembles and controlling the
precise timing of action potentials. Although the gamma2 subunit is believed to be
essential for the postsynaptic clustering of GABAARs, synaptic currents have been
detected in neurons obtained from gamma2(-/-) mice. To determine the role of the gamma2
subunit in synaptic GABAAR enrichment, we performed a spatially and temporally controlled
gamma2 subunit deletion by injecting Cre-expressing viral vectors into the neocortex
of GABAARgamma2(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 alpha1 and beta3 subunits
in perisomatic synapses of cells that lack the gamma2 subunit. Miniature IPSCs in
Cre(+) PCs were insensitive to low concentrations of flurazepam, providing a pharmacological
confirmation of the lack of the gamma2 subunit. Receptors assembled from only alphabeta
subunits were unlikely because Zn(2+) did not block the synaptic currents. Pharmacological
experiments indicated that the alphabetagamma3 receptor, rather than the alphabetadelta,
alphabetaepsilon, or alphabetagamma1 receptors, was responsible for the slowly decaying
IPSCs. Our data demonstrate the presence of IPSCs and the synaptic enrichment of the
alpha1 and beta3 subunits and suggest that the gamma3 subunit is the most likely candidate
for clustering GABAARs at synapses in the absence of the gamma2 subunit.
