WHOLE-CELL CURRENTS ACTIVATED BY N-METHYL-D-ASPARTATE IN FRESHLY ISOLATED NEURONS FROM THE OLFACTORY-BULB OF THE RAT

The whole-cell patch clamp technique was used to study currents that were induced by exposure of freshly isolated interneurones from the rat olfactory bulb to N-methyl-D-aspartate (NMDA). At negative holding potentials application of N-methyl-D-aspartate produced inward currents that were accompanied by an increase in current variance provided mu-M concentrations of glycine was added and nominally Mg2+-free solutions were used. At concentrations > 50 mu-M NMDA induced desensitizing currents. Spectral analysis of current "noise" gave power spectra that were best fitted by the sum of two Lorentzian functions with corner frequencies of 14 +/- 2 and 84 +/- 8 Hz. The apparent single channel conductance for the main event estimated from fitted power spectra was 48.8 +/- 3.7 pS. Recording of channel openings in the whole-cell mode indicated a main event of about 55 pS. All cells that responded to NMDA were also sensitive to kainate and/or (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). The apparent single channel conductances obtained from spectral analysis of the increase in current produced by kainate and AMPA were lower than the conductance of NMDA gated channels. The present findings are discussed with regard to the efficacy of intrinsic feedback circuits.