Aggression and brain serotonergic responsivity: response to slides in male macaques.

The association between central serotonergic responsivity (measured by prolactin response to acute administration of fenfluramine hydrochloride) and aggressivity was examined in 40 adult male cynomolgus monkeys (Macaca fascicularis). Prolactin response to fenfluramine was distributed bimodally with 24 monkeys displaying a "low" prolactin response and 15 showing a "high" prolactin response to the fenfluramine challenge. Behavioral responsivity was assessed by placing the monkeys individually in an open-field enclosure and presenting a series of photographic slides depicting both threatening and nonthreatening images. Monkeys that were low prolactin responders displayed significantly more aggressive gestures in response to a threatening slide of a human being than did the high responders (p < 0.05). Insofar as fenfluramine-stimulated prolactin release assesses serotonergic responsivity, these data support related findings in people and nonhuman primates linking reduced serotonergic activity and aggression.