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ADHD May Be Linked To Low Dopamine Brain Activity |
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Tuesday, 14 August 2007 |
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A new study (Aug 2007) published in the Archives of
General Psychiatry shows a direct correlation between ADHD and
low dopamine activity in the brain. This could shed light on what
seems to be large numbers of ADHD suffers among ex-methamphetamine
users. Article follows:
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." |
Neuroscience 
