Coding of spatial information by soma and dendrite of pyramidal cells in the hippocampal CA1 of behaving rats

The soma and dendrite of a single neuron differ markedly in their anatomical and chemical organization. However, the difference between the neuronal codes by the soma and dendrite in the brain of behaving animals remains unknown. Here, we show that in the hippocampal CA1 of behaving rats, the soma and dendrite of pyramidal cells code distinct spatial information. To detect these neuronal codes, we used a unique extracellular multiunit recording technique with special electrodes (dodecatrodes) and a novel spike-sorting system with an independent component analysis (ICSort). First, we examined whether ICSort could separate extracellular signals from the soma and those from the dendrite of a single cell, in comparison with the separation obtained by a conventional spike-sorting technique. The results suggest that ICSort could distinguish extracellular signals originating from the soma and dendrite. Second, we examined spatial information coded by signals from the soma and dendrite of hippocampal pyramidal cells when the rats were moving in a familiar open environment. The results indicate that the somatic units had single place fields, and showed higher spatial specificity, lower sparsity and lower firing rates than the dendritic units. Therefore, we conclude that a hippocampal pyramidal cell has the ability to transform redundant spatial information received from upstream neurons via the dendrite into more place-specific information along the dendrosomatic axis and transmit this information to downstream neurons via the soma.