The Journal of the American Medical Association (JAMA. 2009;302(10):1084-1091) recently published work by Dr. Nora D. Volkow, MD, et al regarding evaluation of the biological bases that may reveal a reward/motivational deficit present in the brains of persons with ADHD.
Volkow and her colleagues theorized that ADHD may be connected to reward-motivation deficits. Volkow investigated whether lack of motivation and its relationship to reward could be traced to depression of dopamine in various areas of the brain.
To determine whether dopamine was depressed in ADHD persons, the researchers used positron emission tomography (PET scans) to measure dopamine levels in 53 nonmedicated ADHD adults and 44 healthy non ADHD adults between 2001-2009.
Since the biological mechanisms of ADHD are unknown, studies of this type have become the holy grails of research. While Volkow’s credentials are quite impressive (NIH, NIDA, etc.) this research is not new or conclusive. The theory that dopamine dysfunction/depression may be involved with ADHD symptoms has been researched for many years.
Furthermore, Volkow’s small sample size consisted only of adults and therefore should not be extrapolated to include the child population. The small sample size alone should prevent it from being generalized to the entire adult ADHD population. One has a problem of antecedence here; is ADHD caused by dopamine depression in the brain? Or is the dopamine depression the result of ADHD that was acquired by other biological means? This research cannot answer that question.
What does the research tell us? It tells us that for some adults, dopamine may play a role in ADHD. For those adults, taking a stimulant medication may increase dopaminergic activity thus increasing reward/motivation responses and thus increasing attention to task. That might be a stretch.
On the downside, persons with depressed dopamine levels would probably greatly enjoy using stimulants. Study participants reported this. This may contribute to the frequent incidences of substance abuse among ADHD persons.
The authors write,"Despite decades of research, the specific neurobiological mechanisms underlying this disorder still remain unclear. Genetic, clinical and imaging studies point to a disruption of the brain dopamine system, which is corroborated by the clinical effectiveness of stimulant drugs (methylphenidate hydrochloride and amphetamine), which increase extracellular dopamine in the brain."
Unfortunately, the study leaves us with more questions than answers. Does it tell us what happens long term? Does it tell us of side effects? Does it tell us if this actually applies to children? Can we conclusively determine a causal relationship between reward/motivation and ADHD? Does it solve the problem of antecedence? Do we know anything conclusively about all ADHD adults. No. There’s still a long road ahead.