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 hours 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. In addition, our results uncovered a positive association between AVPR expression levels in the vACC and male-female nesting distance, a novel measure of affiliative behavior. 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. Plain English SummarySome mammals, such as the monogamous California mouse, establish a long term partner with which they defend territories, forage, and raise young. This is a large behavioral shift, wherein these animals form a cooperative dyad. In order to behaviorally adjust to this social change, we expected to see neural changes that are indicative of this adjustment, such as changes in neurons that make them more flexible or responsive to social stimuli. While we detected an increase in neural plasticity across the pair bond for both sexes, we also noted important sex differences in cortical regions that were not expected. The anterior cingulate cortex, which is conserved across mammals, is a brain area associated with social decision making and empathy. Sex differences in both receptors and plasticity were detected in this area, indicating that the sexes may use different neural mechanisms to adjust to large social changes. Further exploration into how the sexes use different modes of neural plasticity to appropriately adjust behavior is needed to explain this novel finding. HighlightsO_LICalifornia mice exhibited a graded shift from male-female aggression to greater affiliation across pair bond establishment. C_LIO_LIAggrecan mRNA expression, an indicator of perineuronal net (PNN) abundance, decreased in the ventral anterior cingulate cortex (vACC), but not the lateral septum (LS), across pair bond establishment. C_LIO_LIFemales displayed greater aggrecan mRNA in the vACC relative to males. C_LIO_LIThe vACC displayed greater oxytocin receptor (OXTR) mRNA expression in females and greater vasopressin receptor (AVPR) mRNA expression in males. C_LI