Modulation of aggression by social novelty recognition memory in the hippocampal CA2 region

Social aggression, a fundamental motivated behavior, is driven by external stimuli and internal states. Since early ethological studies, social novelty has been recognized as a key external trigger, with strangers eliciting increased aggression compared with familiar conspecifics. While progress has been made in identifying the neural bases of aggression and social novelty recognition (SNR) memory, the mechanisms linking social novelty detection to aggression remain unknown. Here, we report that the dorsal CA2 (dCA2) region of the hippocampus, previously implicated in both SNR memory and social aggression, plays a critical role in heightened aggression to novel conspecifics. Using calcium imaging during the resident-intruder test, we find that dCA2 neurons encode social behaviors-exploration, dominance, and aggression-with enhanced accuracy during interactions with novel compared with familiar conspecifics. Moreover, chemogenetic silencing of dCA2 pyramidal neurons preferentially suppresses aggression toward novel conspecifics. These results show a unified hippocampal mechanism; whereby social novelty tunes the representations of behavioral states to adaptively modulate aggression.