Simulations to Derive Membrane Resistivity in Three Phenotypes of Guinea Pig Sympathetic Postganglionic Neuron

Abstract

The electrotonic behavior of three phenotypes of sympathetic postganglionic neuron has been analyzed to assess whether their distinct cell input capacitances simply reflect differences in morphology. Because the distribution of membrane properties over the soma and dendrites is unknown, compartmental models incorporating cell morphology were used to simulate hyperpolarizing responses to small current steps. Neurons were classified as phasic (Ph), tonic (T), or long-afterhyperpolarizing (LAH) by their discharge pattern to threshold depolarizing current steps and filled with biocytin to determine their morphology. Responses were simulated in models with the average morphology of each cell class using the program NEURON. Specific membrane resistivity, R m, was derived in each model. Fits were acceptable when specific membrane capacitance, C m, and specific resistivity of the axoplasm, R i, were varied within realistic limits and when underestimation of membrane area due to surface irregularities was accounted for. In all models with uniform R m, solutions for R m that were the same for all classes could not be found unless C m or R i were different for each class, which seems unrealistic. Incorporation of a small somatic shunt conductance yielded values for R m for each class close to those derived assuming isopotentiality ( R m approximately 40, 27, and 15 kΩcm2 for T, Ph, and LAH neurons, respectively). It is concluded that R mis distinct between neuron classes. Because Ph and LAH neurons relay selected preganglionic inputs directly, R m generally affects function only in T neurons that integrate multiple subthreshold inputs and are modulated by peptidergic transmitters.