Proefschrift_Holstein

Chapter 4

Abstract Attention-deficit/hyperactivity disorder (ADHD) is accompanied by impairments in cognitive control, such as task-switching deficits. We investigated whether such problems, and their remediation by medication, reflect abnormal reward motivation and associated striatal dopamine transmission in ADHD. We employed functional genetic neuroimaging to assess effects of dopaminergic medication and reward motivation on task switching and striatal BOLD signal in 23 adults with ADHD ON and OFF methylphenidate and 26 healthy controls. Critically, we took into account inter-individual variability in striatal dopamine by exploiting a common genetic polymorphism (3’-UTR VNTR) in the DAT1 gene coding for the dopamine transporter. Results revealed a highly significant group by genotype interaction in the striatum. This was due to a subgroup of patients with ADHD exhibiting greatly exaggerated effects of reward on striatal BOLD signal during task switching when they were OFF their dopaminergic medication. Specifically, patients carrying the 9R allele showed greater striatal signal than healthy controls carrying this allele, while no effect of diagnosis was observed in 10R homozygotes. Aberrant striatal responses were normalized when 9R-carrying patients with ADHD were ON medication. These pilot data demonstrate an important role for aberrant reward motivation, striatal dopamine and inter-individual genetic differences in cognitive processes in adult ADHD.

74