Postnatal down-regulation of the GABA_A Receptor γ₂ subunit in neocortical NG2 cells accompanies synaptic-to-extrasynaptic switch in the GABAergic transmission mode

NG2 cells, a main pool of glial progenitors, express γ-aminobutyric acid A (GABA_A) receptors (GABA_ARs), the functional and molecular properties of which are largely unknown. We recently reported that transmission between GABAergic interneurons and NG2 cells drastically changes during development of the somatosensory cortex, switching from synaptic to extrasynaptic communication. Since synaptic and extrasynaptic GABA_ARs of neurons differ in their subunit composition, we hypothesize that GABA_ARs of NG2 cells undergo molecular changes during cortical development accompanying the switch of transmission modes. Single-cell RT-PCR and the effects of zolpidem and α5IA on evoked GABAergic currents reveal the predominance of functional α1- and α5-containing GABA_ARs at interneuron-NG2 cell synapses in the second postnatal week, while the α5 expression declines later in development when responses are exclusively extrasynaptic. Importantly, pharmacological and molecular analyses demonstrate that γ2, a subunit contributing to the clustering of GABA_ARs at postsynaptic sites in neurons, is down-regulated in NG2 cells in a cell type-specific manner in concomitance with the decline of synaptic activity and the switch of transmission mode. In keeping with the synaptic nature of γ2 in neurons, the down-regulation of this subunit is an important molecular hallmark of the change of transmission modes between interneurons and NG2 cells during development.