Pre-adolescent rats given the popular ADHD drug Ritalin are more likely to show signs of depression in adulthood, according to a Harvard study.
The study suggests that stimulants, at least in the normally developing brain, can have unsuspected effects in adulthood. The findings also underscore the importance of an accurate diagnosis for ADHD, attention deficit hyperactivity disorder.
William Carlezon, director of McLean Hospital's Behavioral Genetics Laboratory and associate professor of psychiatry at Harvard Medical School, presented the findings at the annual meeting of the American College of Neuropsychopharmacology in Puerto Rico.
Carlezon said that because there are no animal models for ADHD, the young rats in the Harvard study were normal. Experts say many people with ADHD go unrecognized and untreated, and others are misdiagnosed.
The study hints at what might be going on in the ADHD brain.
"These are important issues," said Dr. Peter Jensen, director of the Center for the Advancement of Children's Mental Health at Columbia University Medical Center.
"We know that depression occurs more often in adults with ADHD. What we don't know is whether it's because the disorder wasn't treated when it should have been, or was treated and the depression is a consequence of treatment, or it's a result of ADHD itself."
The animals were exposed to Ritalin during the same developmental stage as a human between ages 4 and 12. The animals spent 15 days on the stimulant. Otherwise, they grew up with untreated littermates.
In adulthood, the animals were given tests to tap the brain circuits thought to trigger symptoms of ADHD - hyperactivity, impulsiveness, difficulty focusing.
The stimulants work on regulating the brain chemical dopamine, involved in learning, attention, motivation and reward.
The tests showed "the animal's brain reward system is altered" by drug treatment in pre-adolescence, Carlezon said.
In one experiment, the animals were placed in water and forced to swim to safety. The treated animals gave up much more quickly, scientists said - an indication of depression.
Other studies also showed the brain's reward system was not working properly. The animals seemed uninterested in working for rewards that would normally be pleasurable.
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