The Effect of Tryptophan Depletion on Inhibitory Brain Function in Asperger's Syndrome

Background: The serotonergic system, involved in aggression and impulse control, has been implicated in Autism Spectrum Disorder (ASD), and has been linked to some typical symptoms, such as repetitive behaviours. Although a number of studies have found differences between ASD and controls on biological measures of serotonin, little is known about the effect of these differences in the serotonergic system on brain function of people with ASD during cognitive tasks.
Objectives: This study investigated the differential effects of acute tryptophan depletion (ATD) on the brain function of adult males, 12 with and 12 without Asperger’s syndrome, during the performance of an inhibitory task.
Methods: Participants performed an event-related go/no-go task in two functional magnetic resonance imaging (fMRI) scanning sessions in a 1.5T scanner. In a double-blind, counterbalanced design, participants consumed either a tryptophan-free or a balanced amino acid drink at each session. The task required subjects to selectively execute or inhibit a motor response.
Results: Levels of neural activation in dorsolateral pre-frontal cortex and right temporal regions were differentially affected by tryptophan depletion in the two groups; after ATD, activation increased in these regions in controls, but decreased in the Asperger’s group.
Conclusions: These findings provide neuro-functional evidence of differential modulatory effects of serotonin on inhibitory neuronetworks in Asperger’s syndrome.