Is increased D2 receptor availability associated with response to stimulant medication in ADHD

The purpose of this study was to estimate striatal dopamine (D2) receptor a vailability in non-drug treated children with attention-deficit-hyperactivi ty disorder (ADHD) before and after methylphenidate therapy, and to examine correlations between severity of symptoms and response rates to stimulant medication with levels of striatal D2 receptor binding. Nine children (six males, three females; mean age 9.8 years, SD 2.3 years) with ADHD participa ted. Ali underwent iodobenzamide (123I IBZM) brain SPECT within 2 hours fol lowing intravenous injection of 123I IBZM before and 3 months after methylp henidate therapy. A semiquantitative approach was used to generate indices of specific D2 receptor binding in the basal ganglia. Specific binding rati os at baseline were higher than the previously reported specific binding va lues obtained in studies using young healthy adults. D2 availability reduce d significantly (paired t-test, p <0.05) as a function of methylphenidate t herapy in patients with ADHD in all four regions of the striatum. When the relation between therapy response and D2 availability was investigated, we observed that the higher the baseline D2 levels were, the higher the respon se rate was (detected as the percentage reduction of hyperactivity scores a nd Conners Teacher Rating Scale scores), while no such trend was observed b etween the initial D2 binding levels and the response in attention-deficit scores. Results indicate that in non-drug treated children with ADHD, highe r D2 receptor availability is observed at baseline which is down-regulated back to reported near-normal values after methylphenidate therapy. The effe ct of methylphenidate on D2 receptor levels in patients with ADHD is simila r to that observed in healthy adults; a down-regulation phenomenon within 0 to 30%. In addition, initially higher values of D2 availability seem to in dicate better response to methylphenidate therapy in ADHD.