Adults with *ADHD do not respond as well as children do to Ritalin (methylphenidate). Ritalin boosts brain dopamine levels. According to an article in Archives of General Psychiatry (JAMA/Archives), it is suggested that dopamine dysfunction may be linked to ADHD symptoms, and may be a factor in substance abuse that often takes place simultaneously.

* (ADHD = attention-deficit/hyperactivity disorder)

ADHD is the most common psychiatric disorder among kids, say the authors. “Despite decades of research, the specific neurobiological mechanisms underlying this disorder still remain unclear,” the authors write. “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.”

Nora D. Volkow, M.D., National Institute on Drug Abuse, Bethesda, Md., and team studied 19 adults, average age 32 – they all suffered from ADHD. None of them had received medication. They were compared to 24 healthy controls, with an average age of 30.

All the participants had their brains scanned using PET (positron emission tomography) and raclopride labeled with carbon 11 – this binds with dopamine receptors. They were all scanned twice, after a placebo injection and after receiving methylphenidate injection. It was a blind trial – the participants did not know when they were receiving the placebo or the methylphenidate. The volunteers were asked to describe the severity of their ADHD symptoms, and also whether they could sense the drug. They were also asked whether the drug was pleasant, unpleasant, whether it made them anxious, restless, alert, tired, and if it made them feel high.

The methylphenidate caused less of a decrease in the amount of Carbon 11 that was bound to dopamine receptors in areas of the brain linked to attention among those with ADHD than those without ADHD. Given that methylphenidate blood levels were identical in both groups, it seems that the people with ADHD released less dopamine in response to the drug compared to the controls. This poorer response was associated with symptoms of inattention. Exploratory analyses discovered evidence the Carbon 11 levels were binding in reduced numbers in the hippocampus and amygdala among those with ADHD. The hippocampus and amygdala are part of the limbic system which deals with emotional responses and consolidating and retrieving memories.

The authors wrote “The findings of reduced dopamine release in subjects with ADHD are consistent with the notion that the ability of stimulant medications to enhance extracellular dopamine underlies their therapeutic effects in ADHD.”

Those with ADHD said they liked methylphenidate more than those without ADHD, the researchers said. They added “The reinforcing responses to methylphenidate were negatively correlated with the dopamine increases, suggesting that decreased dopaminergic activity may also be involved in modulating the magnitude of the reinforcing effects of methylphenidate. This suggests that dopamine dysfunction is involved with symptoms of inattention but may also contribute to substance abuse comorbidity in ADHD.”

Arch Gen Psychiatry. 2007;64(8):932-940.

Written by: Christian Nordqvist