Statistical analysis of amplitude fluctuations in EPSCs evoked in rat CA1 pyramidal neurones in vitro.

1. EPSCs were evoked in CA1 pyramidal neurones of young rats in vitro by extracellular stimulation of axons in a restricted stratum radiatum field, and were recorded using the whole-cell technique. 2. Quantal fluctuations in EPSC amplitude could be demonstrated for nineteen of fifty EPSCs analysed. Quantal currents (at the soma) ranged from 2.6 to 9.5 pA (after correction for the access resistance) with a mean of 4.0 +/- 2.0 pA. 3. Quantal variance was negligible for the majority (13/19) of the EPSCs. However, a large quantal variance (with a coefficient of variation > 0.4) is one possible reason why a large number of the EPSCs (29/50) could not be shown to have quantal fluctuations. 4. The statistical pattern of fluctuations in the amplitude of the majority of the quantal EPSCs (18/19) could not be described by conventional models of transmitter release. 5. The time course of the EPSC and a compartmental model of CA1 pyramidal neurones were used to calculate synaptic location. The quantal current (at the soma) was independent of the electrotonic location of the synapse at which it was evoked. The peak quantal conductance generating each quantal current ranged from 0.5 to 6.8 nS (mean 1.3 +/- 1.4 nS), its magnitude increasing with distance from the soma. The mean peak conductance is likely to be generated by the opening of at least 60-160 AMPA channels.