Dopamine is a phenethylamine naturally produced by the human body, which acts as a neurotransmitter in the brain.
The study was conducted by a team of researchers led by Nora D. Volkow at the National Institute on Drug Abuse, Bethesda, Maryland.
As part of the study, researchers examined 19 adults with with ADHD (average age 32) who had never received medication and 24 healthy controls (average age 30) and performed brain scans using positron emission tomography (PET) and a drug known as raclopride labelled with carbon 11 ([11C]raclopride), which binds with dopamine receptors.
Researchers scanned the patients twice after injecting them with placebo and methylphenidate. The participants were asked to report the severity of their ADHD symptoms, whether they could detect the drug, if they liked or disliked it, and if it made them feel 'high', tired, alert, anxious or restless.
The study found that methylphenidate caused less of a decrease in the amount of [11C]raclopride that bound to dopamine receptors in areas of the brain associated with attention in individuals with ADHD.
Researchers suggested that since levels of methylphenidate in the blood were same in both groups, those with ADHD released less dopamine in response to the drug than controls. This blunted response was associated with symptoms of inattention.
"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," the authors wrote.
The findings of the study were published in the August issue of Archives of General Psychiatry, one of the JAMA/Archives journals.