Hippocampal CA2 modulates trace fear conditioning through circuit-specific tuning of CA1 activity

Forming associations between temporally separated events depends on pathways linking the CA1, the subiculum (SUB), and the entorhinal cortex. The degree to which this process requires the CA2, which shares extensive connectivity with these regions, remains unknown. Using trace fear conditioning (TFC), in which mice learn to associate a tone and shock separated by a temporal gap, we found that chronic blockade of dorsal CA2 (dCA2) output reduced cue-associated freezing without comparable effects following delay fear conditioning or during contextual fear recall. In contrast, acute inhibition of dCA2→dCA1 projections during either conditioning or recall increased cue-test freezing, whereas inhibition of dCA2→dSUB projections produced no detectable behavioral effect. These manipulations were associated with distinct patterns of hippocampal cFos expression. Fiber photometry showed that dCA2→dCA1 calcium activity exhibited tone-onset modulation during conditioning and late-trace modulation near the time at which shock had occurred during conditioning. Moreover, acute CA2 inhibition during conditioning was associated with persistent tone-onset CA1 modulation during subsequent drug-free recall. Together, these findings identify dCA2 as a contributor to TFC and link dCA2→dCA1 signaling to fear expression and the temporal organization of CA1 activity across learning and recall.