Characterization of the variability of glutamatergic synaptic responses topresynaptic trains in rat hippocampal pyramidal neurons

Excitatory postsynaptic currents from CA3 hippocampal neurons, elicited by trains of presynaptic action potentials either in messy fibres or associati ve commissural fibres, have been analysed, by using a quantal analysis appr oach, in order to characterize their variability and the correlation among successive responses. As quantal parameters may change during the train acc ording to the previous release events, correlation within consecutive EPSCs is expected. We tested simple hypotheses on how quantal. parameters p and N may change on the basis of correlation detection in EPSCs. The statistica l significance of these tests has been evaluated. The tests showed that, al though simple binomial distributions can give a good description of synapti c responses at the level of single spikes, only stochastic chains can alway s account for correlations observed within the train. A systematic model fi tting procedure has been developed and applied to extract information on th e dynamics of synaptic transmission. As an application of this novel type o f analysis, a measure of transmitted information to be associated with syna ptic variability, a quantity that allows an estimate of the capability of t he synapse to transmit reliable information in time, is proposed. We showed that this transmitted information depends on short-term plasticity and tha t the change in the type of short-term plasticity from facilitating to depr essing obtained by increasing the extracellular calcium concentration resul ts in a change of the related transmitted information.