Spatial modulation along the proximodistal axis of the subiculum.
1. Ctr for the Biol of Memory, NTNU, Trondheim, Norway
2. NSMA, Univ Arizona, Tucson AZ , USA.
The subiculum is seen as a main cortical output structure of the hippocampus but the structure also receives extensive input from the entorhinal cortex (EC). The medial entorhinal cortex (MEC) projects to the distal part of the subiculum, while the lateral entorhinal cortex (LEC) projects to the proximal part. The present experiment was designed to ask whether the segregation between medial and lateral entorhinal inputs along the proximodistal axis of the subiculum is associated with stronger spatial modulation in the distal than in the proximal part, considering that cells with spatially confined firing fields (‘grid cells’) have been observed only in the medial subdivision of EC. Multiple recording tetrodes were implanted aimed for the full transverse axis of the dorsal subiculum in rats. Recordings were made while rats chased food in a 2 m wide cylinder and in two similarly shaped interconnected boxes where the rat could shuttle freely between the compartments. The majority of the units from the subiculum had characteristics expected of pyramidal cells in this area, such as broad waveforms and average firing rates of approximately 5-10 Hz. There was no detectable change in spatial modulation between the proximal and the distal part of the subiculum in the pyramidal-cell population. In both proximal and distal regions, cells fired in all parts of the cylinder but with consistently higher rates in some locations than others. In the interconnected boxes, the distribution of firing fields was frequently uncorrelated between the two boxes, suggesting that firing was more determined by path-integrator inputs than landmarks and geometrical boundaries. A small subpopulation of units with narrow spike waveforms, presumably from axons, showed firing patterns reminiscent of grid cells and head-direction cells in EC and other parahippocampal cortices. The results suggest that the subiculum operates as an integrated structure, with spatial inputs reaching both proximal and distal regions of the brain area.