Expression of the γ2-subunit distinguishes synaptic and extrasynaptic GABA_A receptors in NG2 cells of the hippocampus

NG2 cells are equipped with transmitter receptors and receive direct synaptic input from glutamatergic and GABAergic neurons. The functional impact of these neuron-glia synapses is still unclear. Here, we combined functional and molecular techniques to analyze properties of GABAA receptors in NG2 cells of the juvenile mouse hippocampus. GABA activated slowly desensitizing responses in NG2 cells, which were mimicked by muscimol and inhibited by bicuculline. To elucidate the subunit composition of the receptors we tested its pharmacological properties. Coapplication of pentobarbital, benzodiazepines, and zolpidem all significantly increased the GABA-evoked responses. The presence of small tonic currents indicated the presence of extrasynaptic GABAA receptors. To further analyze the subunit expression, single cell transcript analysis was performed subsequent to functional characterization of NG2 cells. The subunitsα1,α2,α3, γ1, and γ2 were most abundantly expressed, matching properties resulting from pharmacological characterization. Importantly, lack of the γ2-subunit conferred a highZn₂₊sensitivity to the GABA_A receptors ofNG2cells. Judging from the zolpidem sensitivity, postsynaptic GABA_A receptors in NG2 cells contain the γ 2-subunit, in contrast to extrasynaptic receptors, which were not modulated by zolpidem. To determine the effect of GABA_A receptor activation on membrane potential, perforated patch recordings were obtained from NG2 cells. In the current-clamp mode, GABAdepolarized the cells to approximately-30 mV, indicating a higher intracellular Cl^-concentration (~50mM) than previously reported. GABA-induced depolarization in NG2 cells might trigger Ca²⁺influx through voltage-activated Ca²⁺channels.