Estimating the Time Course of the Excitatory Synaptic Conductance in Neocortical Pyramidal Cells Using a Novel Voltage Jump Method

We introduce a method that permits faithful extraction of the decay time course of the synaptic conductance independent of dendritic geometry and the electrotonic location of the synapse. The method is based on the experimental procedure of Pearce (1993), consisting of a series of identical somatic voltage jumps repeated at various times relative to the onset of the synaptic conductance. The progression of synaptic charge recovered by successive jumps has a characteristic shape, which can be described by an analytical function consisting of sums of exponentials. The voltage jump method was tested with simulations using simple equivalent cylinder cable models as well as detailed compartmental models of pyramidal cells. The decay time course of the synaptic conductance could be estimated with high accuracy, even with high series resistances, low membrane resistances, and electrotonically remote, distributed synapses. The method also provides the time course of the voltage change at the synapse in response to a somatic voltage-clamp step and thus may be useful for constraining compartmental models and estimating the relative electrotonic distance of synapses. In conjunction with an estimate of the attenuation of synaptic charge, the method also permits recovery of the amplitude of the synaptic conductance. We use the method experimentally to determine the decay time course of excitatory synaptic conductances in neocortical pyramidal cells. The relatively rapid decay time constant we have estimated (tau approximately 1.7 msec at 35 degrees C) has important consequences for dendritic integration of synaptic input by these neurons.