The underlying causes of Attention Deficit Hyperactivity Disorder (ADHD) have yet to be well characterized. But a new study utilizing brain imaging has found an abnormality in the pathway responsible for the motivation/reward system in patients with ADHD. The finding may lead to more effective treatments for the condition as well as a greater understanding of ADHD behavior.
A hallmark of ADHD is lack of attention. Especially seen in the classroom, both children and adults with the disorder lack the ability to focus for extended periods of time. Scientists suspected the symptom was due to a deficit in motivation and reward system--a process which can hone focus with the understanding that a reward (or at least not a punishment) will be given if successful.
Studying that pathway is a difficult task. It relies on the chemical dopamine, which can be easily affected by ADHD treatment or drug abuse which is common in adult ADHD sufferers. Tests to this point have been relatively small, but a push by lead author Nora Volkow, Director of the National Institute on Drug Abuse, finally saw a sizable cohort of participants investigated.
53 adults with ADHD who had never received treatment were subjected to a PET scan along with 44 healthy controls. Researchers looked at both ends of the dopamine pathway--"dopamine receptors, to which the chemical messenger binds to propagate the "reward" signal, and dopamine transporters, which take up and recycle excess dopamine after the signal is sent."
The study showed those with ADHD had lower levels of both receptors and transporters. This was especially clear in the acumbens and midbrain, both of which are regions important to the motivation/reward process.
Understanding the deficit in dopamine can help change the way ADHD patients interact with the world. Volkow stated, "[The pathway's] involvement in ADHD supports the use of interventions to enhance the appeal and relevance of school and work tasks to improve performance."
Though the dopamine problems have not been a solid fact until this moment, the medication that has been used for decades were on the right track. "Our results also support the continued use of stimulant medications — the most common pharmacological treatment for ADHD — which have been shown to increase attention to cognitive tasks by elevating brain dopamine," Volkow said.
The team also hopes that this study will help adults with ADHD who tend towards drug abuse and obesity. The lack of dopamine makes the rewards system difficult to trigger, so overeating and over use of stimulant drugs may be seen as a dangerous form of compensation, an unconscious move to help bolster the feeling of reward. Developing therapies that help attenuate the need for drugs and binge eating will greatly improve quality of life. Thanks to the examiner