Dopamine Reward Pathways Found Abnormal in ADHD

Article excerpt

People with attention-deficit/hyperactivity disorder show dopamine abnormalities in the midbrain and accumbens--brain regions that are key to reward and motivation pathways, a report shows.

Shown on PET imaging, deficits in dopamine synaptic markers found in these brain regions "could underlie the clinical evidence of abnormal responses to reward in ADHD," which include "failure to delay gratification, impaired response to partial schedules of reinforcement, and preference for small immediate rewards over larger delayed rewards," said Dr. Nora D. Volkow of the National Institute on Drug Abuse, Bethesda, Md., and her associates.

The dopamine deficits in the accumbens also correlated in this study with deficient attention, as opposed to correlating with hyperactivity or impulsivity, the two other major traits in the triad that characterize ADHD (JAMA 2009;302:1084-91).

The findings could explain why attentional deficits in ADHD "are most evident in tasks that are considered boring, repetitive, and uninteresting (i.e., tasks or assignments that are not intrinsically rewarding)," they noted.

The investigators used PET imaging to study these brain locations because "the mesoaccumbens dopamine pathway, which projects from the ventral tegmental area in the midbrain to the nucleus accumbens ... has been hypothesized to underlie the reward and motivational deficits observed in ADHD."

They assessed the availability of dopamine presynaptic and postsynaptic markers in the brains of 53 adults with ADHD who had never been medicated for the disorder and 44 healthy control subjects. To minimize confounding from drug exposures or comorbidities, they excluded subjects who had ever received psychotropic medications, had psychiatric diagnoses, or had medical conditions that could alter cerebral function. Obese participants were excluded.

The imaging showed lower availability of both types of markers in the ADHD group than in the control group in the accumbens and midbrain, and also in the caudate. Previous imaging studies have demonstrated "smaller caudate volumes and caudate functional un-deractivation in ADHD participants compared with controls," Dr. …