Differential behavioral effects of tryptophan and 5-hydroxytryptophan in vervet monkeys: influence of catecholaminergic systems.

In previous studies tryptophan and 5-hydroxy-tryptophan (5-HTP) treatments produced opposite effects on aggression and vigilance and differing effects on eating and locomoting in vervet monkeys. This study examined the effects of the serotonin reuptake inhibitor fluoxetine, and the catecholamine reuptake inhibitor desmethylimipramine (DMI) on tryptophan and 5-HTP induced behavioral changes. Thirty-two adult males from 16 different social groups were studied. Tryptophan (10, 20, and 40 mg/kg/day) produced dose-dependent reductions in aggression, vigilance, and locomotion and increases in eating. In contrast, 5-HTP (20, 40, and 80 mg/kg/day) increased aggression and vigilance and did not affect locomotion or eating. Fluoxetine (0.5, 1.0, and 2.0 mg/kg/day) produced effects identical to tryptophan while DMI (1.5, 3.0, and 6.0 mg/kg/day) resulted in dose-dependent increases in aggression, vigilance, and locomotion, and decrements in eating. When combined with tryptophan, fluoxetine augmented and DMI diminished the effects of tryptophan on all behaviors. Fluoxetine decreased and DMI increased the effects of 5-HTP on aggression and vigilance. Thus concurrent DMI enhanced and concurrent fluoxetine reduced the differences between 5-HTP and tryptophan. These results suggest that 5-HTP's effects on catecholaminergic systems may underlie the differing behavioral effects of tryptophan and 5-HTP on behavior in a species closely related to humans.