Characterization of the variability of glutamatergic synaptic responses to presynaptic trains in rat hippocampal pyramidal neurons.

Excitatory postsynaptic currents from CA3 hippocampal neurons, elicited by trains of presynaptic action potentials either in mossy fibres or associative commissural fibres, have been analysed, by using a quantal analysis approach, in order to characterize their variability and the correlation among successive responses. As quantal parameters may change during the train according 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 statistical significance of these tests has been evaluated. The tests showed that, although simple binomial distributions can give a good description of synaptic responses at the level of single spikes, only stochastic chains can always account for correlations observed within the train. A systematic model fitting procedure has been developed and applied to extract information on the dynamics of synaptic transmission. As an application of this novel type of analysis, a measure of transmitted information to be associated with synaptic variability, a quantity that allows an estimate of the capability of the synapse to transmit reliable information in time, is proposed. We showed that this transmitted information depends on short-term plasticity and that the change in the type of short-term plasticity from facilitating to depressing obtained by increasing the extracellular calcium concentration results in a change of the related transmitted information.