ESTIMATED CONDUCTANCE OF GLUTAMATE-RECEPTOR CHANNELS ACTIVATED DURINGEPSCS AT THE CEREBELLAR MOSSY FIBER-GRANULE CELL SYNAPSE

We have analyzed the variance associated with the decay of the non-NMD A receptor component of synaptic currents, recorded from mossy fiber-g ranule cell synapses in cerebellar slices, to obtain a conductance est imate for the synaptic channel. Current fluctuations arising from the random channel gating properties were separated from those arising fro m the fluctuations in the population of channels by subtracting the me an excitatory postsynaptic current (EPSC) waveform scaled to the EPSC peak amplitude. A weighted mean single-channel conductance of approxim ately 20 pS was determined from the relationship between the mean curr ent and the variance around the mean during the decay of evoked and sp ontaneous synaptic currents. This result suggests that high conductanc e non-NMDA channels, such as the 10-30 pS glutamate receptor channel p reviously characterized in granule cells, carry the majority of the fa st component of the EPSC at this synapse. In addition, our data are co nsistent with the activation of surprisingly few (approximately 10) no n-NMDA channels by a single packet of transmitter.