A Role for Signal Propagation Through the Hippocampal CA2 Field in Memory Formation

Memory is very vivid and long-lasting, particularly when we have strong emotions, such as pleasure, love, fun, surprise, fear, anxiety and anger. This fact leads us to imagine that memory storage in the hippocampus can be switched according to the emotion-related brain state. In this paper, we first review the two classical concepts for the hippocampal network. One is the trisynaptic circuit connecting the entorhinal cortex, the dentate gyrus, the CA3 field and the CA1 field. The other is the lamellar hypothesis which proposes that the lamellar organization of the trisynaptic circuit. We then summarize our studies highlighting a role of the hippocampal CA2 field in memory formation. Finally we propose a novel circuit in the hippocampal network, which contains the CA3-CA2-CA1 pathway in which the CA2 field functions as a gate controlled by the activity of the supramammillary nucleus to control signal propagation between hippocampal lamellar organizations.

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