Aggression--interactions of serotonin and testosterone in healthy men and women.

Serotonin (5-HT) and testosterone (T) have both been implicated in the regulation of aggression. Findings in humans however are very inconclusive, with respect to main effects of either system. Animal models implicate T to modulate 5-HT system activity, and furthermore have shown behaviorally relevant interactions of T and 5-HT with respect to aggression. We tested for associations between habitual T-level and 5-HT system activity, as well as behaviorally relevant interactions of T and 5-HT with respect to trait aggression in 48 healthy male and female subjects. 5-HT activity was measured by means of neuroendocrine challenge paradigm with S-citalopram. T-levels were measured in saliva samples. Trait aggression was assessed by self-report measures. T-levels were not associated with indices of central 5-HT activity. Results showed significant interaction effects between 5-HT and T for trait aggression in men only (p<0.05). Trait aggression was significantly higher in the combinations "high T+high cortisol responses" (indicating decreased 5-HT availability), and "low T+low cortisol responses" (indicating increased 5-HT availability), after S-citalopram. Results support the notion of behaviorally relevant interactions between T and 5-HT, with respect to aggression in humans, but also indicate the need for further studies.