ESTIMATION OF SINGLE-CHANNEL CONDUCTANCE UNDERLYING SYNAPTIC TRANSMISSION BETWEEN PYRAMIDAL CELLS IN THE VISUAL-CORTEX

Axon collaterals originating from pyramidal cells are one of the most abundant presynaptic elements in the neocortical circuits.(3,5,15) To understand a quantitative aspect of synaptic transmission between pyra midal cells, we attempted to estimate single channel conductance by ap plying non-stationary noise analysis to unitary excitatory postsynapti c currents. Simultaneous recordings were carried out in two pyramidal cells of superficial layers in visual cortical slices. Unitary postsyn aptic currents, which mere evoked by action potentials of presynaptic cells impaled with conventional sharp electrodes, were recorded from p ostsynaptic cells with whole-cell patch clamp techniques. Estimated si ngle channel conductance was 12.8 +/- 3.8(S.D.) pS for kittens and 10. 4 +/- 1.5 pS for rats, Dividing these values by the conductance for un itary postsynaptic currents, we calculated the number of non-N-methyl- D-aspartate receptor channels activated during the postsynaptic curren ts. The obtained estimates were 52 (kittens) and 41 (rats), To further estimate the number of channels involved in each quantal event,,ve an alysed amplitude histograms of miniature and spike-evoked excitatory p ostsynaptic currents, The derived number of estimates from these two k inds of histograms agreed quite well; about 20 channels were required for individual quantal events. Assuming open probability of non-N-meth yl-D-aspartate receptor channels to be 0.7,(4) our results suggest tha t the number of channels available for synaptic transmission between i ndividual pyramidal cells would be 74 (kittens) and 59 (rats), We prop ose that at pyramidal-pyramidal synapses, the number of open channels is several times smaller than that previously reported for the synapse s between geniculo-cortical afferent and layer IV spiny stellate cells .(19) (C) 1998 IBRO. Published by Elsevier Science Ltd.