Effect of Background Synaptic Activity on Excitatory-Postsynaptic Potential-Spike Coupling

Neurons receive large amount of synaptic inputs in vivo, which may impact the coupling between EPSPs and spikes. We mimicked the in vivo synaptic activity of the cell with the dynamic clamp system. We recorded from pyramidal cells in neocortical slices in vitro to investigate how timing and probability of spike generation in response to an EPSP is affected by background synaptic conductance under these conditions. We found that near threshold, background synaptic conductance improved the precision of spike timing by reducing the depolarization-related prolongation of the EPSP. In cells with ongoing spike activity and background synaptic conductances, an EPSP rapidly increased the probability of firing. The time window of the spike probability increase was comparable to the EPSP rise time and was followed by a long period of reduced firing. We found that the net synaptic gain was determined not only by the amplitude of the EPSP, but also by the firing frequency of the cell. In addition, a background fluctuating conductance reduced the time window of perturbation of spike patterns generated by EPSP related spikes. Taken together, these results indicate that in vivo, the level of the background synaptic activity may regulate spike-timing precision and affect synaptic gain