Sex differences in structural and receptor mRNA expression in the ventral anterior cingulate cortex and a potential role of perineuronal nets in monogamous pair bond establishment (Peromyscus californicus)

The monogamous California mouse (Peromyscus californicus) exhibits distinct behavioral changes during pair bond formation. Using a detailed temporal behavioral analysis over seven days, we found a rapid decrease in aggression within 24 h of pair introduction in this highly territorial species. After this aggression reduction, the gradual increase in affiliative behaviors varied by type of affiliative behavior and ranged from one to seven days. We then measured neurobiological changes at three time points during this transition to uncover mechanisms that might govern this shift from aggressive to affiliative behavior, revealing novel sex differences that add to current research on biological mechanisms of social bonding. Specifically, we examined plasticity through mRNA expression of two perineuronal net (PNN) associated proteins, HAPLN and ACAN, in two brain regions implicated in affiliation, aggression, and social cognition: the ventral anterior cingulate cortex (vACC) and lateral septum (LS). The vACC in females exhibited higher expression levels of both of these PNN components relative to males. Additionally, we observed a decrease in ACAN mRNA expression in the vACC over the course of pair bond establishment, but no such change in the LS. Furthermore, oxytocin receptor (OXTR) and vasopressin receptor (AVPR) plasticity exhibited sex-specific patterns in the vACC during pair bond formation. Females displayed higher OXTR mRNA expression across the bonding period, whereas males expressed higher AVPR mRNA levels. We discuss how a decrease in PNNs could allow for an increase in receptor plasticity in the vACC as the pair bond is established. Moreover, we suggest that structural plasticity across this social transition may differ between males and females due to factors such as pre-pair sociality and aggression/territoriality changes.