Sex differences in the neural circuitry of aggression in Syrian hamsters.

The social behavior neural network (SBNN) is a circuit composed of reciprocally connected limbic structures that regulate social behaviors, including aggression. Although both males and females of many species display a range of agonistic behaviors in response to social encounters, studies of the neural circuitry underlying these behaviors have focused almost exclusively on males. In the present study, we investigated sex differences in the neural circuitry activated in response to social interactions in Syrian hamsters (Mesocricetus auratus). We employed c-Fos immunohistochemistry to quantify neuronal activation following agonistic encounters between same-sex male and female dyads during a resident-intruder test. Animals were tested in their home cage either alone (n = 7 per sex) or with a same-sex, non-aggressive intruder (n = 7 per sex) for 10 min. Social interactions were characterized almost exclusively by aggressive interactions with aggression occurring more frequently in females than in males. Our data revealed substantial sex differences in the neuronal activation of the SBNN in response to these encounters. In some regions, neuronal activity changed in opposite directions in males and females compared to controls (e.g., posterior lateral septum), while in others, there was a change in neuronal activation in only one sex (e.g., medial amygdala). These findings support the hypothesis that the neural circuitry activated in response to social interactions including aggression exhibit marked sexual differentiation.