Hippocampal area CA2: properties and contribution to hippocampal function

This review focuses on area CA2 of the hippocampus, as recent results have revealed the unique properties and surprising role of this region in encoding social, temporal and contextual aspects of memory. Originally identified and described by Lorente de No, in 1934, this region of the hippocampus has unique intra-and extra-hippocampal connectivity, sending and receiving input to septal and hypothalamic regions. Recent in vivo studies have indicated that CA2 pyramidal neurons encode spatial information during immobility and play an important role in the generation of sharp-wave ripples. Furthermore, CA2 neurons act to control overall excitability in the hippocampal network and have been found to be consistently altered in psychiatric diseases, indicating that normal function of this region is necessary for normal cognition. With its unique role, area CA2 has a unique molecular profile, interneuron density and composition. Furthermore, this region has an unusual manifestation of synaptic plasticity that does not occur post-synaptically at pyramidal neuron dendrities but through the local network of inhibitory neurons. While much progress has recently been made in understanding the large contribution of area CA2 to social memory formation, much still needs to be learned.