In addition to its role in blood vessel1,2and macrophage3,4function, nitric oxide (NO) is a neurotransmitter5found in high densities in emotion-regulating brain regions6–8. Mke with targeted disruption of neuronal NO synthase (nNOS) display grossly normal appearance, locomotor activity, breeding9, long-term potentiation10and long-term depression11. The nNOS−mice are resistant to neural stroke damage following middle cerebral artery ligation12. Although CO2-induced cerebral vasodilatation in wild-type mice is NO−dependent, in nNOS" mice this vasodilation is unaffected by NOS inhibitors13. Establishing a behavioural role for NO has, until now, not been feasible, as NOS inhibitor drugs can only be administered acutely and because their pronounced effects on blood pressure and other body functions obfuscate behavioural interpretations. We now report a large increase in aggressive behaviour and excess, inappropriate sexual behaviour in nNOS−mice.
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