Diminished serotonergic functioning in hostile children with ADHD:: Tryptophan depletion increases behavioural inhibition

Introduction: Serotonergic (5-HT) functioning has been shown to account for a variety of behavioural characteristics, in particular aggressive and impulsive behaviour. This study explored the effects of rapid tryptophan depletion (RTD) and the ensuing reduction of brain 5HT synthesis on behavioural inhibition in passive avoidance learning assessed in a computerized go/no-go task. Methods: 22 male patients with an ICD-10 diagnosis of ADHD were administered RTD within an amino acid drink lacking tryptophan, the natural precursor of 5-HT, thus lowering the central nervous 5-HT synthesis rate in a placebo-controlled double-blind within-subject cross-over-design. 4 hours after RTD/placebo intake the patients were subjected to a go/no-go task for assessment of behavioural inhibition. Results: Highly hostile aggressive patients showed increased inhibition errors under RTD compared to placebo. Low hostile aggressive patients showed lower rates of inhibition errors and thus better performance under RTD compared to placebo. Discussion: The data suggest that in ADHD levels of trait-aggressive characteristics influence the susceptibility to changed behavioural inhibition after an acute 5-HT dysfunction. The detected influence of 5-HT could also be relevant as regards behavioural inhibition being subject to a developmental change in 5-HT functioning.