The effects of tryptophan depletion and loading on laboratory aggression in men: time course and a food-restricted control

Some studies have shown that sharp reduction of L-tryptophan (Trp) concentr ation in plasma results in increases in laboratory-measured aggression. Con versely, raising plasma Trp has blunted aggression. These effects are presu mably due to impaired or enhanced serotonin synthesis and neurotransmission in the brain. In this study, the laboratory-measured aggressive behavior o f eight men under both Trp depletion (T-) and Trp loading (T+) conditions w as compared to their aggressive behavior under food-restricted control cond itions (overnight fast without an amino acid beverage). Subjects were provo ked by periodic subtraction of money which was attributed to a fictitious o ther participant, and aggression was defined as the number of retaliatory r esponses the subject made ostensibly to reduce the earnings of the (fictiti ous) other participant. Following ingestion of the T- beverage. aggressive responding was significantly elevated relative to the food-restricted contr ol condition. and this increased aggressive behavior became more pronounced across behavioral testing sessions on a time-course which paralleled previ ously documented decreases in plasma Trp concentrations. In contrast, no ch anges were observed in aggressive responding under T+ conditions relative t o food-restricted conditions. These within-subject behavioral changes under depleted plasma Trp conditions support earlier indications of a role of se rotonin in regulating aggression.