Title: The Effects of Tryptophan Depletion and Supplementation on Serotonergic Functioning and Aggression in High and Low Aggressive Subjects
Name: Donald M. Dougherty
University of Texas
Year: 1997, 1998
Type: Research Grant

Aggression is a complex behavior with complex underlying biological mechanisms that may contribute to elevated aggressive behavior in some individuals. Dysfunction of the serotonin (5-HT) neurotransmitter has been related to a number of psychological disturbances: impulsive/violent criminal behaviors, alcohol abuse/dependence, bulimia, suicide attempts, and in aggressive children.

The production of brain 5-HT depends on availability of the essential amino acid tryptophan (Trp). Changes in dietary Trp can temporarily increase or decrease 5-HT in the brain. Specifically, concentrations of 5-HT are decreased by a Trp deficient amino acid drink (T-). Alternatively, a Trp loaded amino acid drink (T+) increases plasma Trp concentrations. Therefore, experimenters can temporarily manipulate brain 5-HT function to measure the temporary effects on laboratory aggressive responding.

The studies resulting from this grant investigated whether: (1) T+ and T- beverages produced changes in laboratory-measured aggressive behavior of eight men, each experiencing both experimental conditions plus a control (no-drink) condition, (2) aggression self-report scores on the Buss-Perry Aggression Questionnaire and the Lifetime History of Aggression were related to changes in laboratory-measured behavior following either the T+ or T- amino acid beverages, and (3) hostile and non-hostile groups of men (twelve per group) responded differently to low and high provocation conditions after the T+ or T- beverages.

In the first study of eight men, we found that aggressive responding was increased after T-, but remained unchanged following T+. This established that a temporary reduction in 5-HT synthesis resulted in elevated aggression. In a subsequent analysis, we divided this group of eight men into high- and low-aggression groups, which showed that aggressive responding was specific to the high-aggression group.

Since this first study was small (only four men in both the high- and low-aggression groups), we needed to replicate these results in a second study comparing two groups, high vs. low aggressive history, with twelve men each. In this study each of the 24 men experienced all of the experimental conditions on nonconsecutive days and we compared their aggressive responding after the T+ and T- beverages compared to the no-drink control day. These results showed that aggression was increased under T- conditions relative to T+ in the aggressive men, but the reverse was true in nonaggressive men, aggression was reduced under T+ compared to T. Additionally, analysis of plasma Trp at 7.25 hours following the amino acid drinks showed that the nonaggressive men maintained significantly higher plasma Trp on the T+ (loading) day compared to the aggressive men.

Collectively, these experiments revealed significantly different effects of plasma Trp manipulations on aggressive behavior, particularly for men with high trait-aggression compared to those with low trait-aggression. Additionally, aggressive men metabolized the Trp loading (T+) more quickly than the non-aggressive men, which points to a possible difference in metabolizing plasma Trp peripherally (prior to entry into the central nervous system) and this may trigger increased aggression in men with lower brain 5-HT function.