A study done in patients with ADHD or attention deficit hyperactivity disorder provided the first concrete evidence that these patients have lower dopamine levels in areas of the brain which are mostly involved in the motivation and reward experiences of the individual. The study was headed by Nora Volkow, a long time neuroimaging research collaborator at the Brookhaven National Laboratory and Director at the National Institute on Drug Abuse.

Participants of the study consisted of 53 ADHD adult patients who never had any form of treatment and 44 healthy adults who became the control group. All 97 participants had been screened according to study criteria in order to eliminate confounding variables. A PET or positron emission tomography was utilized to determine the levels of two dopamine system markers, which are the dopamine receptors and dopamine transporters, in each participant.

The propagation of the reward signal is highly reliant on dopamine receptors while the job of uptake and recycling of the excess falls on the dopamine transporters.

While lying in a PET scanner, participants were injected with radiotracer, a radioactive compound designed to bind to a specific target. Multiple radiotracers were employed to account for the range of targets that were measured. The researchers used the data to determine the location and concentration of the dopamine receptors and transporters.

The findings indicated that patients with ADHD have smaller amounts of dopamine transporters and receptors in two regions of the brain – the midbrain and accumbens – two areas that are responsible for motivation and reward processes. Scientists also found that the markers showed high correlation with symptoms of adult ADHD, such as inability to focus and inattention.

According to Volkow, the data suggested that deficiencies in the dopamine pathway result in a patient’s lowered attention and focus level and that it could underlie ADHD patients’
reactions to reward and motivation. She further noted the study showed that the use of stimulants to enhance levels of dopamine in the brain could enhance the patient’s attention level during mental tasks.

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