The ventral hippocampus and nucleus accumbens underlie long-term social memory about female conspecifics in male mice

For many social animals, including humans, the ability to remember and recognize conspecifics (i.e., having social memory), especially a mating partner, is essential for adaptive reproductive strategies. Our previous studies have shown that social memory about same-sex conspecifics, namely male-to-male social memory, is stored in hippocampal ventral CA1 (vCA1) neurons and that neural projections from the vCA1 to the nucleus accumbens (NAc) are crucial for social discriminatory behavior. However, how the memory of an opposite-sex conspecific, specifically male-to-female social memory, and how the social memory modulates sexual behavior in males remain unknown. Herein, we investigated the ultrasonic vocalizations (USVs) and socially approaching behavior of males toward a female before and after social memory formation. We found that the total duration of USVs was reduced when the male mice interacted with a familiar female. This reduction in USV emission was blocked by optogenetic inhibition of vCA1 neurons, as well as D1- and D2-dopamine receptor (D1R and D2R) expressing neurons in the NAc. Furthermore, fiber photometry recordings in the NAc revealed that the activities of both D1R and D2R neuronal populations were altered during social interaction with familiar or novel females after the process of familiarization. The response of D1R-neurons was reduced only when interacting with familiar females. In contrast, although responses of D2R-neurons were reduced toward both familiar and novel females after social memory formation, the reduction towards novel females was significantly more pronounced compared to that towards familiar females. Our findings suggested that the activities of both vCA1 neurons and D1R- and D2R-expressing NAc neurons in males were differentially modulated by the presence of social memory about females during adaptive reproductive strategies. Significance StatementLong-term social memory about females modulates male sexual behavior, specifically ultrasonic vocalization emissions, mediated by hippocampal ventral CA1 and dopamine receptor expressing neurons in the nucleus accumbens.