Neural mechanisms of aggression

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Abstract
Dysregulated aggressive behaviour has important negative consequences for human societies. A complicating factor is that aggression that is exhibited in different social contexts can be regulated by different neurobiological mechanisms. Neurobiological studies have identified a subset of hypothalamic and limbic brain areas that tend to facilitate aggressive behaviour in rodents and primates. In contrast, neural activity in the frontal cortex generally acts to inhibit aggressive behaviour. Aggressive behaviours in animal models and humans are known to be regulated by serotonin neurotransmission. Behaviour can be modified at several levels, including regulation of serotonin release, reuptake and sensitivity (via serotonin receptors). Dopaminergic function appears to be necessary for aggressive behaviour, possibly by regulating arousal, learning and memory. Neuronal nitric oxide (nNOS) synthase signalling (via nitric oxide gas) exerts inhibitory effects on male aggression in rodents. Several studies suggest that nNOS assists in the processing of salient social stimuli. Mutations in the monoamine oxidase A (MAOA) enzyme are associated with increased aggressive behaviours in humans. MAOA knockout mice show increased aggression. Steroid hormones have long been a focus of aggression research, but the relationship among androgens, oestrogens and behaviour is complex. These hormones do not function in isolation and their actions are affected by the environmental context. Gene–environment interactions have important effects on aggressive behaviours. Mutations or hormones that increase aggression in one environment have no effect (or decrease aggression) in different environments. Novel pharmacological treatments must target specific subtypes of aggression to have improved effectiveness. An appreciation of the contribution of environmental stressors to aggressive phenotypes is necessary for further advancements in the successful management of maladaptive aggression.