Maternal aggression is reduced in neuronal nitric oxide synthase-deficient mice

Lactating females express rapid extremes in behavior, ranging from gentle nurturance toward offspring to fiercely protective aggression against intruders. Although males often behave aggressively against intruders, female rodents usually express aggression only when rearing and protecting pups. Nitric oxide (NO) inhibits male aggression; however, its role in maternal aggression is unknown. In the present study, female mice with targeted disruption of the neuronal nitric oxide synthase gene (nNOS-/-) displayed significant deficits in maternal aggression relative to wild-type (WT) mice in terms of percentage displaying aggression, the average number of attacks against a male intruder, and the total time spent attacking the male intruder. The nNOS-/- mice displayed normal pup retrieval behavior. Because the specific deficits in maternal aggression in the nNOS-/- mice suggested a possible role for NO in maternal aggression, we combined behavioral testing of WT mice with immunohistochemistry for citrulline, an indirect marker of NO synthesis, to examine indirectly NO synthesis during maternal aggression. A significant increase in the number of citrulline-positive cells was identified in the medial preoptic nucleus, the suprachiasmatic nucleus, and the subparaventricular zone regions of the hypothalamus in aggressive lactating females relative to control mice. In other regions of the brain, no changes in the number of citrulline-positive cells were observed across either groups or treatments. These results provide two indirect lines of evidence that NO release is associated with maternal aggression.