Dendrosomatic voltage and charge transfer in rat neocortical pyramidal cells in vitro

Most excitatory synapses on neocortical pyramidal cells are located on dend rites, which are endowed with a variety of active conductances. The main or igin for action potentials is thought to be at the initial segment of the a xon, although local regenerative activity can be initiated in the dendrites . The transfer characteristics of synaptic voltage and charge along the den drite to the soma remains largely unknown, although this is an essential de terminant of neural input-output transformations. Here we perform dual whol e-cell recordings from layer V pyramidal cells in slices from somatosensory cortex of juvenile rats. Steady-state and sinusoidal current injections ar e applied to characterize the voltage transfer characteristics of the apica l dendrite under resting conditions. Furthermore, dendrosomatic charge and voltage transfer are determined by mimicking synapses via dynamic current-c lamping. 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 lineari ty, transfer resistances are reciprocal in opposite directions, and the cen troids of the synaptic time course are on the order of the membrane time co nstant. Transfer of excitatory postsynaptic potential (EPSP) charge, but no t peak amplitude, is positively correlated with membrane potential. We conc lude that the integrative properties of dendrites in infragranular neocorti cal pyramidal cells appear to be linear near resting membrane potential. Ho wever, at polarized potentials charge transferred is voltage-dependent with a loss of charge at hyperpolarized and a gain of charge at depolarized pot entials.