Electrophysiology of isolated hippocampal pyramidal dendrites

In order to study the electrical properties of dendritic membranes independent of the effects of somatic potentials, intracellular recordings in guinea pig hippocampal slices were obtained from the dendrites of CA1 pyramidal neurons (HPCs) which had been isolated from their somata by cuts made through the proximal stratum radiatum. Spikes and subthreshold membrane responses to intracellular current pulses were compared in intact and isolated dendrites and in the residual portions of neurons whose apical dendrites had been severed ("isolated somata"). Isolated dendrites generated both fast, QX-314-sensitive, sodium-mediated spikes, and slow higher threshold spikes which were QX-314-resistant and presumably mediated by Ca2+. Depolarization of "isolated somata" ordinarily evoked only fast (Na+) spikes, but presumed Ca2+ spikes could be elicited after exposure to QX-314 (a local anesthetic). Anomalous inward rectification was depressed by QX-314 in somata but not in dendrites, suggesting that the ionic basis for subthreshold as well as regenerative conductances was different at different sites on the neuron. The dendritic membrane in CA2 HPCs thus generates both Na+- and CA2+-mediated spike potentials and a subthreshold response which probably is mediated primarily by CA2+. Attempts to describe the integrative functions of these neurons must take into account the variety of conductances which are activated nonuniformly in somata and dendrites by changes in membrane potential.