Although methylphenidate (Ritalin) is the most frequently prescribed drug for treating attention deficit hyperactivity disorder (ADHD), its mechanism of action and its effects on the human brain have been poorly understood.
In an article in the January 12 online issue of the Journal of Neuroscience, researchers have for the first time assessed the effects of therapeutic doses of oral Ritalin on the levels of dopamine in the human brain. Dopamine imbalances appear to be closely related to ADHD symptoms.
"This research begins to explain how Ritalin works to treat ADHD," says NIDA Director Alan I. Leshner, Ph.D. "And in doing so it helps us to better understand the biology of ADHD. Understanding this process can help doctors appropriately diagnose and treat children with the disorder and could eventually provide practitioners with a clearer framework for prescribing the medication."
ADHD is the most commonly diagnosed behavioral disorder in childhood. An estimated 3 to 5 percent of the general population suffers from the disorder, which is characterized by agitated behavior and an inability to focus on tasks.
Although previous research demonstrated that methylphenidate administered intravenously increases dopamine levels in the brain, this study is the first to assess the effects on dopamine levels when Ritalin is administered orally, the way most patients receive the drug.
A team of investigators from the Brookhaven National Laboratory in Upton, New York, and the University of New York at Stony Brook used positron emission tomography to confirm that administration of Ritalin resulted in increased dopamine levels in healthy, adult male subjects.
According to Dr. Nora D. Volkow, the lead investigator, Ritalin is a dopamine transmitter blocker "and hence it amplifies [dopamine] release....One could therefore speculate that the amplification of weak dopamine signals in subjects with ADHD by [Ritalin] would enhance task-specific signaling," and improve attention and decrease distractibility, the most common symptoms of the disorder. The research team also measured the effects of different doses, finding that the drug's plasma half-life at commonly prescribed dosage (10 to 20 mg administered 2 to 4 times a day) is approximately 4 hours.
The researchers note, however, that the 11 subjects who participated in the study were healthy, male adults, and were tested in stress-free conditions. Additional research on subjects who have been given a task would be the logical next step in analyzing the effects of Ritalin on the brain, according to the investigators.
Note to reporters: The full text of this article is available on the Journal of Neuroscience's website (www.jneurosci.org).
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