Hippocampal cells encode places by forming small anatomical clusters.

The hippocampus has been hypothesized to function as a "spatial" or "cognitive" map, however, the functional cellular organization of the spatial map remains a mystery. The majority of electrophysiological studies, thus far, have supported the view of a random-type organization in the hippocampus. However, using immediate early genes (IEGs) as an indicator of neuronal activity, we recently observed a cluster-type organization of hippocampal principal cells, whereby a small number ( approximately 4) of nearby cells were activated in rats exposed to a restricted part of an environment. To determine the fine structure of these clusters and to provide a 3D image of active hippocampal cells that encode for different parts of an environment, we established a functional mapping of IEGs zif268 and Homer1a, using in situ hybridization and 3D-reconstruction imaging methods. We found that, in rats exposed to the same location twice, there were significantly more double IEG-expressing cells, and the clusters of nearby cells were more "tightly" formed, in comparison to rats exposed to two different locations. We propose that spatial encoding recruits specific cell ensembles in the hippocampus and that with repeated exposure to the same place the ensembles become better organized to more accurately represent the "spatial map."