A hypothalamic–hippocampal circuit-driven plasticity in fasciola cinereum mediates social memory

The ability to recognize conspecifics requires social memory, a process that is mediated by distinct hippocampal subregions. However, the contributions of fasciola cinereum (FC), a cytoarchitecturally independent subregion in the hippocampus, remain unknown. Here, we identify FC as a critical hub for social memory, driven by inputs from the hypothalamic supramammillary nucleus (SuM). A neuroanatomically segregated SuM subpopulation projects monosynaptically to FC. Silencing SuM–FC projections impaired memory formation, while silencing FC recipient neurons disrupted both memory formation and recall. In vivo electrophysiology in freely moving mice revealed that familiarity suppressed firing in SuM→FC projection neurons but potentiated firing in FC neurons. Social experience enhanced postsynaptic excitability in FC neurons and induced input-specific potentiation at SuM–FC excitatory synapses. Crucially, optogenetic induction of long-term depression at SuM–FC synapses abolished established social memory. Therefore, these findings establish FC as a previously unrecognized hippocampal subregion for social memory, mediated by a SuM input-dependent, experience-tuned plasticity mechanism.