2025 Theses Doctoral

Hippocampal Representations of Partner and Novel Individuals in Monogamous California Mice

Hernandez Palacios, Kimberly

For many species, having strong social bonds is essential for survival. Social bonds allow species to accomplish common goals, such as building a home and rearing offspring. However, it is not well understood what role social memory has in establishing and maintaining these bonds. Prior neurobiological studies of social memory have largely focused on Mus musculus, a promiscuous species that does not form lifelong pair bonds. Thus, it is unknown how monogamous species encode and recall social memory. The hippocampal region CA2 is critical for social memory, discriminating if a conspecific is novel or familiar, but its role in social bonding of monogamous species is unknown. In the rodent genus Peromyscus, closely related species display monogamous or promiscuous mating systems, making it an advantageous system to study the pair-bond. I sought to understand if monogamous versus promiscuous species store and retrieve memories differently.

To identify if mating systems impact social memory, I first compared social memory behaviors of promiscuous Peromyscus maniculatus with monogamous Peromyscus californicus and monogamous Peromyscus polionotus. To understand the social behavior of these mice, I employed four different behavioral assays: sociability, social novelty, five-trial social recognition, and a single stimulus pair-bond assay. I find that P. maniculatus and P. polionotus have no preference for either novel or littermate conspecifics but do demonstrate social memory in the five-trial social recognition tasks. In contrast, monogamous P. californicus has a strong preference for a littermate over a novel animal. P. polionotus and P. californicus both demonstrated selective pair-bond behaviors, suggesting that both monogamous species were able to recognize a pair-bonded partner. Results indicate species-specific and potentially sex-dependent mechanisms underlying the retrieval of established social memories.

To examine further how social memories are encoded, I sought to understand if CA2 participates in the memorization and recognition of a pair-bonded mate and how it computes this recognition in P. californicus (California mouse). To test this, I utilized 64-channel silicon probes to record from regions CA1 and CA2 of the hippocampus of freely behaving animals as they engaged in a social recognition task. Test mice were given the option to explore either its opposite-sex partner or an opposite-sex novel conspecific. Neural activity patterns differed across regions between social conditions as the pair-bond matured, with distinct populations of cells showing selective responses to partner versus novel conspecifics. At the completion of social recognition task, California mice were tested in a pair-bond task to assess pair-bond formation. When presented with a partner or novel mouse in a tether pair-bond option task, test mice prefer to display affiliative behavior towards partner mouse over novel. These findings help provide new insights into how pair-bond formation impacts the neural encoding of social information in the hippocampus.