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 ti me course of the synaptic conductance independent of dendritic geometr y and the electrotonic location of the synapse. The method is based on the experimental procedure of Pearce (1993), consisting of a series o f identical somatic voltage jumps repeated at various times relative t o the onset of the synaptic conductance. The progression of synaptic c harge recovered by successive jumps has a characteristic shape, which can be described by an analytical function consisting of sums of expon entials. The voltage jump method was tested with simulations using sim ple equivalent cylinder cable models as well as detailed compartmental models of pyramidal cells. The decay time course of the synaptic cond uctance could be estimated with high accuracy, even with high series r esistances, low membrane resistances, and electrotonically remote, dis tributed synapses. The method also provides the time course of the vol tage change at the synapse in response to a somatic voltage-clamp step and thus may be useful for constraining compartmental models and esti mating the relative electrotonic distance of synapses. In conjunction with an estimate of the attenuation of synaptic charge, the method als o permits recovery of the amplitude of the synaptic conductance. We us e the method experimentally to determine the decay time course of exci tatory synaptic conductances in neocortical pyramidal cells. The relat ively rapid decay time constant we have estimated (tau similar to 1.7 msec at 35 degrees C) has important consequences for dendritic integra tion of synaptic input by these neurons.