The Journal of Neurophysiology Vol. 84 No. 3 September 2000, pp. 1445-1452
Copyright ©2000 by the American Physiological Society

Dendrosomatic Voltage and Charge Transfer in Rat Neocortical Pyramidal Cells In Vitro

Institute of Neuroinformatics, University of Zürich and Federal Institute of Technology, CH-8057 Zurich, Switzerland

Ulrich, Daniel and Christian Stricker. Dendrosomatic Voltage and Charge Transfer in Rat Neocortical Pyramidal Cells In Vitro. J. Neurophysiol. 84: 1445-1452, 2000. Most excitatory synapses on neocortical pyramidal cells are located on dendrites, which are endowed with a variety of active conductances. The main origin for action potentials is thought to be at the initial segment of the axon, although local regenerative activity can be initiated in the dendrites. The transfer characteristics of synaptic voltage and charge along the dendrite to the soma remains largely unknown, although this is an essential determinant of neural input-output transformations. Here we perform dual whole-cell recordings from layer V pyramidal cells in slices from somatosensory cortex of juvenile rats. Steady-state and sinusoidal current injections are applied to characterize the voltage transfer characteristics of the apical dendrite under resting conditions. Furthermore, dendrosomatic charge and voltage transfer are determined by mimicking synapses via dynamic current-clamping. We find that around rest, the dendrite behaves like a linear cable. The cutoff frequency for somatopetal current transfer is around 4 Hz, i.e., synaptic inputs are heavily low-pass filtered. In agreement with linearity, transfer resistances are reciprocal in opposite directions, and the centroids of the synaptic time course are on the order of the membrane time constant. Transfer of excitatory postsynaptic potential (EPSP) charge, but not peak amplitude, is positively correlated with membrane potential. We conclude that the integrative properties of dendrites in infragranular neocortical pyramidal cells appear to be linear near resting membrane potential. However, at polarized potentials charge transferred is voltage-dependent with a loss of charge at hyperpolarized and a gain of charge at depolarized potentials.