Electrophysiological characterization of remote chemical synapses.

I. The degree of electrotonic coupling between a given synapse located on a dendrite and the spike trigger zone is a major determinant of the relative influence of that synapse on neuronal firing. The degree of electrotonic coupling between a synapse and the recording site also influences the measurement of the biophysical properties of that synapse. We present theoretical justification for a practical method for determining the electrotonic coupling between any given remote synaptic input and the site of intracellular measurements. We also derive useful equations for the electrophysiological characterization of that synaptic input. The approach is independent of many of the assumptions of the equivalent cable model (39). 2. The neuron is depicted as a linear twoport electrical network, with the site of potential recording and current injection representing one port and the synaptic input the other. Derivations are presented for three parameters of the network: k12, k2], and k2. 3. k12 is the electrotonic coupling coefficient between the recording site (usually the soma) and the subsynaptic membrane. It defines the degree of steady-state voltage decay from soma to synapse. It also defines the charge transfer from synapse to soma and the steady-state current decay from synapse to soma when a voltage clamp is applied to the soma. k12 can be determined empirically by measuring the change in the reversal potential as a function of ionic manipulations in the extracellular medium. Given k12, the true synaptic equilibrium potential and