Thesis advisor: Alexa Veenema / Thesis advisor: Nicholas Worley / Oxytocin (OT) and vasopressin (AVP) often regulate social behaviors in sex-specific ways. We hypothesized that this could be mediated by sex differences in the OT receptor (OTR) and AVP V1a receptor (V1aR) in the brain. Here, we determined whether there are sex differences in OTR and V1aR binding densities in nodes of the social behavior neural network in the mouse brain. We also compared sex differences int he OTR and V1aR in the mouse brain with those found previously in the rat brain. Although mice and rats are closely related species, they also display differences in social behavior. Therefore, we predicted to find similar as well as unique sex differences in OTR and V1aR in mice compared to rats. Generally, we found that sex differences in OTR and V1aR binding densities are region-specific and species-specific. In detail, male mice showed higher OTR binding density than female mice in the medial amygdala, anterior lateral septum, and posterior bed nucleus of the stria terminalis. This is consistent with findings in rats. Furthermore, female mice displayed higher OTR binding density in the anteroventral periventricular nucleus and ventromedial hypothalamus. This is in contrast to rats, where males showed higher OTR binding densities in these regions. Lastly, females showed higher V1aR binding density in the anterior bed nucleus of the stria terminalis. However, this sex difference was not measured in rats due to low receptor expression in this region. Overall, these findings demonstrate the importance to determine sex differences in OTR and V1aR across species to gain a better understanding of the sex-specific behavioral functions of the OT and AVP systems. / Thesis (BS) — Boston College, 2017. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Biology.