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High Ih channel density in the distal apical dendrite of layer V
pyramidal cells increases bidirectional attenuation of EPSPs. J Neurophysiol
85: 855−868, 2001. Despite the wealth of recent research on
active signal propagation along the dendrites of layer V neocortical
pyramidal neurons, there is still little known regarding the traffic of
subthreshold synaptic signals. We present a study using three simultaneous
whole cell recordings on the apical dendrites of these cells in
acute rat brain slices to examine the spread and attenuation of spontaneous
excitatory postsynaptic potentials (sEPSPs). Equal current
injections at each of a pair of sites separated by ;500 mm on the
apical dendrite resulted in equal voltage transients at the other site
("reciprocity"), thus disclosing linear behavior of the neuron. The
mean apparent "length constants" of the apical dendrite were 273 and
446 mm for somatopetal and somatofugal sEPSPs, respectively.
Trains of artificial EPSPs did not show temporal summation. Blockade
of the hyperpolarization−activated cation current (Ih) resulted in
less attenuation by 17% for somatopetal and by 47% for somatofugal
sEPSPs. A pronounced location−dependent temporal summation of
EPSP trains was seen. The subcellular distribution and biophysical
properties of Ih were studied in cell−attached patches. Within less than
;400 mm of the soma, a low density of ;3 pA/mm2 was found, which
increased to ;40 pA/mm2 in the apical distal dendrite. Ih showed
activation and deactivation kinetics with time constants faster than 40
ms and half−maximal activation at 295 mV. These findings suggest
that integration of synaptic input to the apical tuft and the basal
dendrites occurs spatially independently. This is due to a high Ih
channel density in the apical tuft that increases the electrotonic distance
between these two compartments in comparison to a passive
dendrite
