SPONTANEOUS OPENINGS OF NMDA RECEPTOR CHANNELS IN CULTURED RAT HIPPOCAMPAL-NEURONS

Spontaneous and N-methyl-D-aspartate (NMDA)-evoked single-channel curr ents were studied in outside-out patches isolated from cultured rat hi ppocampal neurons. Both spontaneous and NMDA-evoked single-channel cur rents reversed at potentials close to 0 mV and exhibited multiple ampl itude levels of similar amplitude. Both spontaneous and NMDA-evoked si ngle-channel currents were inhibited by Mg2+ in a voltage-dependent ma nner and by 7-chlorokynurenic acid. The activity of spontaneous single -channel currents was reduced by the competitive NMDA receptor antagon ists, but by one to three orders of magnitude less than expected assum ing that the spontaneous activity is due to an ambient NMDA receptor a gonist present in the extracellular solution. Our results suggest that , similar to other ligand-gated ion channels, NMDA receptor channels h ave a dual mode of activation-spontaneous and agonist induced.