LOW VERSUS HIGH PROLACTIN RESPONDERS TO FENFLURAMINE CHALLENGE - MARKER OF BEHAVIORAL-DIFFERENCES IN ADULT MALE CYNOMOLGUS MACAQUES

Prolactin response to acute administration of fenfluramine hydrochloride is considered an indirect assessment of ''net' central serotonergic activity. This study compared behavioral characteristics of adult, male cynomolgus macaques (Macaca fascicularis) having ''low'' or ''high'' prolactin responses to fenfluramine challenge. The subjects were 75 animals housed in five-member social groups for 28 months. In month 23, prolactin responses to fenfluramine challenge were evaluated. Observations of specific behaviors (aggressive, submissive, affiliative, and nonsocial) were made three times per week on animals in each social group. The dominance status of each animal within a social group was assessed at weekly intervals. Low prolactin responders had a significantly higher index of ''overt'' aggression (ratio of fights involving physical contact and chasing or lunging/all forms of aggressive behavior) compared to high prolactin responders (p < .03). There were no differences in the dominance status of low and high responders (p = .34). Furthermore, low responders were more socially withdrawn than high responders, as they spent significantly more time alone (passive or neutral state; p < .03) and less time in passive body contact with other animals than high responders (p < .05). These data support the hypothesis that reduced central serotonergic activity in nonhuman primates is associated with a high level of overt aggression and a low level of positive social interaction.