A direct lateral entorhinal cortex to hippocampal CA2 circuit conveys social information required for social memory

The storage of information by the hippocampus in long-term memory is thought to involve two distinct but related processes. First, the hippocampus determines whether a given stimulus is novel or familiar; next, the hippocampus stores the novel information in long-term memory. To date, the neural circuits that detect novelty and their relation to the circuits that store information of a specific memory are poorly understood. Here we address this question by examining the circuits by which the CA2 region of the hippocampus, which is essential for social memory, both detects social novelty and stores social memory. CA2, like the more thoroughly studied CA1 region, receives its major excitatory input from the entorhinal cortex through both a direct monosynaptic and indirect trisynaptic pathway. We find that the direct inputs to CA2 from the lateral entorhinal cortex, but not the indirect trisynaptic inputs, provide social information that is required for social memory. However, these direct inputs fail to discriminate a novel from a familiar animal. Thus, social novelty and social identity signals are likely conveyed through separate pathways, with the entorhinal cortex providing specific multisensory information about an animals identity and novelty detection requiring a local computation within CA2.