Steep decrease in the specific membrane resistance in the apical dendrites of hippocampal CA1 pyramidal neurons

Specific membrane resistance (R_m), distributed non-uniformly over the dendrite, has a substantial effect on neuronal information processing, since it is a major determinant in subthreshold-synaptic integration. From experimental data of dendritic excitatory postsynaptic potential (EPSP) spread, we previously reported that non-uniform R_m distribution in hippocampal CA1 pyramidal neurons could be expressed as a step function. However, it remains unclear how steeply R_m decreases. Here, we estimated the R_m distribution using sigmoid function to evaluate the steepness of decrease in R_m. Simulations were performed to find the distribution which reproduced experimental voltage responses to extracellular electric field applied to CA1 slices, in contrast to the EPSP spread. Distribution estimated from the responses to electric field was a steep-sigmoid function, similar to that from the EPSP spread. R_m in distal dendrite was estimated to be ≲ 10^3.5 Ω c m² whereas that in proximal dendrite/soma was ≳ 10^4.5 Ω c m². Our results not only supported previous studies, but, surprisingly, implied that R_m decreases at a location more distal, and that distal dendrite was leakier, than previous estimates by other groups. Simulations satisfactorily reproduced the responses to two distinct perturbations, suggesting that steep decrease in R_m is reliable. Our study suggests that the non-uniform R_m distribution plays an important role in information processing for spatially segregated synaptic inputs.