Cable properties of dendrites in hippocampal neurons of the rat mapped by a voltage-sensitive dye

Dendrites of pyramidal neurons from embryonic rat hippocampus are investigated in culture using a voltage-sensitive fluorescent dye. The electrical response to somatic stimulation is observed as a time-resolved map with a resolution of 0.9 mu m at a time constant of 0.4 ms without signal averaging. The data are interpreted in terms of a tapering cable with Hodgkin-Huxley parametrization. The spread of short hyperpolarizing transients is damped by capacitive shunting. The invasion of an action potential is boosted by voltage-gated conductances of a low density. No irregularity is observed at a bifurcation. The passive cable parameters of internal resistance and membrane resistance at resting voltage are R-i = 300 Omega cm and R-m = 40 k Omega cm(2) respectively, at a maximum sodium conductance of similar to 4.4 mS/cm(2). The electrotonic length constant and the dynamic length constant at 1 kHz are 580 and 90 mu m respectively. These results are compatible with electrophysiological data of dendrites in slices of adult hippocampus and with optical data of narrow processes of leech neurons in culture. The functional implications of boosting an action potential by voltage-gated channels of low density are considered.