Proximodistal organization of the CA2 hippocampal area

The proximodistal axis is considered a major organizational principle of the hippocampus. Interfacing between the hippocampus and other brain systems, the CA2 region apparently breaks this rule. Apart from its specific role in social memory, CA2 has been involved in temporal and contextual memory but mechanisms remain elusive. Here, we used intracellular and extracellular recordings followed by neurochemical identification of single-cells to evaluate CA2 and surrounding areas in the rat. We found marked proximodistal trends of synaptic activity, as well as in subthreshold membrane potentials and phase-locked firing coupled to theta and gamma oscillations. Opposite proximodistal correlations between membrane potential fluctuations and theta sinks and sources at different layers revealed influences from up to three different generators. CA2 memory engrams established after a social memory task reflected these trends. We suggest that the structure and function of CA2 is segregated along the proximodistal hippocampal axis.