Site Independence of EPSP Time Course Is Mediated by DendriticI h in Neocortical Pyramidal Neurons

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Abstract
Neocortical layer 5 pyramidal neurons possess long apical dendrites that receive a significant portion of the neurons excitatory synaptic input. Passive neuronal models indicate that the time course of excitatory postsynaptic potentials (EPSPs) generated in the apical dendrite will be prolonged as they propagate toward the soma. EPSP propagation may, however, be influenced by the recruitment of dendritic voltage-activated channels. Here we investigate the properties and distribution of Ih channels in the axon, soma, and apical dendrites of neocortical layer 5 pyramidal neurons, and their effect on EPSP time course. We find a linear increase (9 pA/100 μm) in the density of dendritic Ihchannels with distance from soma. This nonuniform distribution ofIh channels generates site independence of EPSP time course, such that the half-width at the soma of distally generated EPSPs (up to 435 μm from soma) was similar to somatically generated EPSPs. As a corollary, a normalization of temporal summation of EPSPs was observed. The site independence of somatic EPSP time course was found to collapse after pharmacological blockade ofIh channels, revealing pronounced temporal summation of distally generated EPSPs, which could be further enhanced by TTX-sensitive sodium channels. These data indicate that an increasing density of apical dendritic Ihchannels mitigates the influence of cable filtering on somatic EPSP time course and temporal summation in neocortical layer 5 pyramidal neurons.