Extracellular ATP potentiates function of both synaptic and extrasynaptic GABA
Rs in cultured hippocampal neurons. A-C, ATP increases the amplitude (but not the frequency) of mIPSCs mediated by synaptic GABAARs. Pharmacologically isolated GABAAR-mediated mIPSCs were recorded under whole-cell voltage clamp mode at a holding potential of -60 mV. A, Representative continuous recording traces taken 5 min before and after drug applications or washout showing dose-dependent effects of ATP on mIPSCs. ATP at a concentration of 2 mM, but not 100 μM, potentiated mIPSCs. The potentiated mIPSCs were fully blocked by additional application of bicuculline (10 μM) and returned to the control level after ATP washout. B, Cumulative amplitude and frequency distribution histograms of mIPSCs from the same cell shown in A before (black lines) and after (dotted lines) ATP application illustrating a specific increase in the mIPSC amplitude without alteration of its frequency. C, Histogram summarizing averaged changes in the amplitude and frequency of mIPSCs before (Control) and after application of ATP (2 mM) from 7 individual neurons. All values were presented as percentage of the controls. D, Extracellular ATP increases the tonic GABA current. Tonic GABA currents were revealed by blocking native GABAARs with addition of bicuculline (20 μM) in the absence (Control) and presence (ATP 2 mM) of ATP in the extracellular recording solution. Left, Representative tonic GABA current traces in the absence and presence of ATP taken from the same neuron 5 min before and after bath application of ATP (2 mM). Bar graphs on the Right summarizing data obtained from 6 individual neurons. ** P < 0.01.