Apical dendritic depolarizations and field interactions evoked by stimulation of afferent inputs to rat hippocampal CA1 pyramidal cells.

The relationship between orthodromic extracellular field potentials and intradendritic depolarizations in apical dendrites of CA1 pyramidal neurons was investigated using the in vitro slice preparation of rat hippocampus. Orthodromic synaptic field potentials evoked by stimulation of afferent inputs in stratum radiatum or stratum oriens were used to measure extracellular voltage gradients generated over the pyramidal cell axis. Extracellular gradients were of opposite polarity over the region of pyramidal cell apical dendrites in stratum radiatum. The stratum radiatum-evoked gradient was negative towards the apical dendrites and the stratum oriens-evoked gradient negative towards the cell body layer, with gradients reaching values of up to 50 mV/mm over the apical dendritic axis. Intradendritic recordings obtained greater than 150 microns from stratum pyramidale directly measured the subthreshold apical dendritic excitatory postsynaptic potentials evoked by stratum radiatum or stratum oriens stimulation. These ground-referenced recordings were then compared to the transmembrane potential calculated by subtraction of the corresponding extradendritic field potential. Both stratum radiatum and stratum oriens stimulation evoked graded excitatory postsynaptic potentials that could be recorded in apical dendritic impalements up to 265 microns from stratum pyramidale. The calculated transmembrane potential of the stratum radiatum-evoked excitatory postsynaptic potential had a significantly greater rate of rise, peak amplitude, and rate of decay than that of the ground-referenced excitatory postsynaptic potential. In contrast, the rates of rise and decay of the transmembrane potential of the stratum oriens-evoked excitatory postsynaptic potential were reduced with respect to the ground-referenced recording. The peak amplitude of the stratum oriens-evoked transmembrane potential, however, varied according to the polarity of the corresponding extradendritic population spike response recorded in stratum radiatum. These data reveal that synaptic activation of either basal or apical dendrites of CA1 pyramidal cells evokes a depolarization that can be recorded over a substantial region of the apical dendritic arbor. Furthermore, extradendritic field potentials evoked by stimulation of these inputs produce opposite effects on the transmembrane potential of apical dendrites. The magnitude of the accompanying extracellular voltage gradients suggest that these shifts in transmembrane potential reflect ephaptic interactions at the apical dendritic level of pyramidal cells.