Quantal components of unitary EPSCs at the mossy fibre synapse on CA3 pyramidal cells of rat hippocampus (original) (raw)

1993, The Journal of Physiology

Ca2`and high Mg2+. Using the criteria of equidistance and the presence of peaks and dips in the autocorrelation function, five of nine EPSC peak amplitude distributions were judged to be quantal. From the likelihood ratio when fitting non-quantal and quantal model functions to the peak current data, the probability of wrongly rejecting the non-quantal models was estimated to be in the range < 0 001-9 4%. The apparent quantal conductance change was in the range 105-177 pS with a mean of 133 pS in different experiments. The coefficient of variation of a quantal event was estimated to be 22 %. 7. Spontaneously occurring miniature EPSCs were recorded at negative membrane potentials in the presence of 1 ,UM tetrodotoxin (TTX). Miniature EPSC 20-80% rise times varied between 0-2 and > 10 ms within each experiment. Peak amplitude distributions of the miniature EPSCs with rise times less than 0-8 ms (presumably arising from MF-CA3 synapses) were skewed. Mode and mean values of these distributions corresponded to apparent conductances of 106 + 19 and 251 + 22 pS, respectively (6 cells). 8. Fast application of 1-3 mm glutamate to outside-out patches isolated from the somata of CA3 pyramidal cells activated currents which were mediated by AMPA/kainate receptor channels. The elementary conductance of these channels estimated from non-stationary fluctuation analysis was 8-5 + 2-1 pS (9 patches), and the maximal open probability with 3 mm glutamate was 0-71 +0-06. Extracellular divalent cation concentrations had only small effects on the recorded glutamateactivated currents. 9. In the whole-cell recording configuration, responses to short current pulses and a biocytin fill were obtained from a CA3 pyramidal neurone. A compartmental model was made, based on the cell morphology as reconstructed using a light microscope. The electrical parameters of the model were adjusted until its short pulse response gave the best fit to the measured response of the neurone. This gave a specific membrane capacitance (Cm) of 0-683 ,tF cm-2, a specific membrane resistance (Rm) of 164000 Q cm2, and a cytoplasmic resistivity (Ri) of 294 Q cm, with zero somatic shunt conductance. 10. The most proximal and the most distal mossy fibre synaptic conductances were simulated, with the soma voltage clamped via different series resistances. Both the dendritic cable and the series resistance attenuated and slowed the EPSCs. With the plausible range of series resistances (1-5-10 MQ), the apparent peak conductance was reduced to 0 32-0{87 of the 'real' value, the 20-80 % rise time was increased by a factor of 1-2-4-5 and the effective decay time constant by a factor of 1-1-2-6. 11. The results indicate that unitary EPSCs of MF-CA3 synapses show a rapid rise and a fast decay, and that they are quantal in nature, at least in a subset of MF-CA3 synapses. We estimate that a typical unitary EPSC of the MF-CA3 synapse at a 'physiological' concentration of divalent cations has a quantal content between 2 and 16. Considering voltage clamp errors, a quantal event appears to be generated by the simultaneous opening of between fourteen and sixty-five glutamate receptor channels of the AMPA/kainate subtype.