Localization and anatomical identification of theta and complex spike cells in dorsal hippocampal formation of rats

Microelectrodes were passed through the dorsal hippocampal formation of unrestrained rats, recording for at least 5 min each 35.3 μm. At each site the amplitude and duration of action potential spikes, frequency of firing, relation to slow wave theta rhythm, and presence of complex spikes or theta cells was recorded. One thousand and fourteen neurons were recorded from. (When recording from many neurons simultaneously, the "number" of the neurons was "counted" in an arbitrary and approximate way.) Of 949 nontheta cells greater than 80 μV amplitude, only one was not in the hilus of fascia dentata or in a layer of cells which overlapped stratum pyramidale and stratum granulosum. These are the locations of the cell bodies of projection cells (pyramidal cells and granule cells). However, this layer is, up to 400 μm thicker than stratum pyramidale. Theta cells were seen in sites of cell bodies of projection cells and also in stratum oriens of CA1, suprapyramidal layers of CA3, and dorsal part of the hilus of fascia dentata. The frequency of occurrence in these locations corresponded to the distribution of cell bodies of interneurons. We conclude that the class of projection cells and the class of nontheta cells have a very large overlap, and that the class of interneurons and the class of theta cells have a very large overlap.