Electrophysiological characterization of remote chemical synapses.

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Abstract
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. A theoretical method for determining the electrotonic coupling between any given remote synaptic input and the site of intracellular measurements is presented. Equations were derived for the electrophysiological characterization of that synaptic input. The approach is independent of many of the assumptions of the equivalent cable model. The neuron is depicted as a linear 2-port electrical network, with the site of potential recording and current injection representing one port and the synaptic input the other. Derivations are presented for 3 parameters of the network: k12, k21 and k2. 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. k21 is the electrotonic coupling coefficient between the synapse and the soma. It defines the degree of steady-state voltage decay from synapse to soma and the charge transfer and current decay from soma to synapse. k2 is the product of k12 and k21. How k12, k21 and k2 can be determined empirically, and the assumptions inherent in the various derivations and present methods for reducing potential errors, are discussed.