Heightened aggressive behavior in mice with lifelong versus postweaning knockout of the oxytocin receptor

Previous work implicating the neuropeptide oxytocin (Oxt) in the neural regulation of aggression in males has been limited. However, there are reports of heightened aggression in Oxt knockout and Oxt receptor (Oxtr) knockout male mice when they are born to null mutant mothers; suggesting that intrauterine exposure to Oxt may be important to normal aggression in adulthood. To explore this, we examined aggression in two lines of Oxtr mice, a total knockout (Oxtr-/-), in which the Oxtr gene is absent from the time of conception, and a predominantly forebrain specific knockout (Oxtr FB/FB), in which the Oxtr gene is not excised until approximately 21-28. days postnatally. Aggression was measured in males from both lines, as well as control littermates, using a resident-intruder behavioral test. Consistent with previous reports, male Oxtr-/- mice had elevated levels of aggression relative to controls. Oxtr FB/FB mice on the other hand displayed levels of aggression similar to control animals. In addition, following a resident-intruder test, Oxtr+/+ mice that displayed aggression had less c-fos immunoreactivity in the ventral portion of the lateral septum than those that did not. Further, Oxtr-/- mice had increased c-fos immunoreactivity in the medial amygdala relative to controls. These data suggest that Oxt may play an important role during development in the organization of the neural circuits that underlie aggressive behavior in adulthood, with its absence resulting in heightened aggression.