Maternal and mating-induced aggression is associated with elevated citrulline immunoreactivity in the paraventricular...

Lactating female rodents are fiercely aggressive against intruders when they are rearing and protecting pups. In monogamous prairie voles, Microtus ochrogaster, males are parental and exhibit a dramatic increase in aggression, termed mating-induced aggression, in association with reproduction. In mice, the gas, nitric oxide (NO), inhibits male aggression, but may have an excitatory role in the production of maternal aggression. In this study, we combined aggressive behavioral testing of female and male prairie voles with immunohistochemistry for citrulline, a marker of NO synthesis, to examine NO synthesis indirectly during maternal and mating-induced aggression. A significant increase in the number of citrulline-positive cells was identified in the paraventricular nucleus (PVN) of the hypothalamus in aggressive lactating females compared with unstimulated lactating females. A significant increase in the number of citrulline-positive cells was also observed in the PVN of aggressive mated males compared with nonaggressive unmated males and unstimulated mated males. Both nonaggressive unmated males and unstimulated mated males show similar levels of citrulline immunoreactivity in the PVN. In other regions of the brain, no changes in the number of citrulline-positive cells were observed. These results suggest that NO is released specifically in the PVN during both maternal and mating-induced aggression in prairie voles.