Stop signal response inhibition is not modulated by tryptophan depletion or the serotonin transporter polymorphism in healthy volunteers: implications for the 5-HT theory of impulsivity

Rationale: Reduced serotonin neurotransmission is implicated in disorders of impulse control, but the involvement of serotonin in inhibitory processes in healthy human subjects remains unclear. Objectives: To investigate the effects of an acute manipulation of serotonin and genotype at a functional polymorphism in a gene coding for the serotonin transporter (5-HTT) on an established measure of response inhibition. Methods: Serotonin function was reduced by the acute tryptophan depletion (ATD) procedure in a double-blind, crossover design in 42 healthy subjects. The Stop Signal Task (SST) was administered 57 h after drink administration. The influences of 5-HTT polymorphism, gender and trait impulsivity were investigated. Results: ATD was associated with significant depletion of plasma tryptophan levels but did not increase the stop signal reaction time in comparison to the balanced (placebo) amino acid mixture. Subjects possessing the short allele of the 5-HTT polymorphism were not more impulsive on the SST than subjects homozygous for the long allele under placebo conditions and were not disproportionately sensitive to the effects of ATD. There was no effect of gender or trait impulsivity on ATD-induced change. Conclusions: We find no support for the involvement of brain serotonin neurotransmission in this form of inhibitory control in healthy human subjects.