November 14, 2013
ADHD Drug Ritalin Shown To Control Impulsiveness In Rhesus Monkeys
redOrbit Staff & Wire Reports - Your Universe Online
Impulsive monkeys were more likely to accept delayed gratification after being given a dose of methylphenidate, the active ingredient of the drug Ritalin, according to new research presented during Neuroscience 2013, the annual meeting of the Society for Neuroscience.
According to Luis Populin, associate professor of neuroscience at University of Wisconsin-Madison, 11 percent of all American children between the ages of four and 17 have been diagnosed with attention deficit hyperactivity disorder (ADHD), and one of the primary symptoms each of those youngsters face is impulsiveness – the predisposition to act without forethought.
In scientific terms, impulsiveness can manifest itself when someone opts for a smaller, immediate reward over a larger one requiring patience. Populin said that choosing between immediate and long-term rewards is an essential requirement in schooling, adding that “do your homework so you will get a good grade at the end of the quarter” is less appealing to an impulsive child than “let’s play baseball this afternoon instead of studying chemistry.”
To study this phenomenon, Populin and his colleagues analyzed a pair of rhesus macaque monkeys, one of which had a calm personality and the other which was described as nervous, fidgety and impulsive. Both creatures were trained to stare at a dot on a screen, and to select one of two pictures after it disappeared.
One of the pictures gave the monkeys a small but immediate sip of water, while the other allowed them to have a larger drink, but only after a delay of up to 16 seconds. Naturally, the calm monkey soon figured out that the better reward was worth the wait, but the impulsive one continually opted for immediate gratification.
“This willingness to take a smaller reward right away rather than a larger, delayed reward, called ‘temporal discounting,’ is a common feature of ‘combined type’ ADHD, which specifically lists impulsivity among its diagnostic criteria,” the university explained in a statement.
However, when the monkeys were given methylphenidate, both of them selected the delayed reward more often. In fact, the impulsive monkey demonstrated the same preference for the delayed reward as the unmedicated calm one. Even so, there were identifiable differences in the monkeys’ performance illustrating that while the Ritalin ingredient improved the condition in the fidgety monkey, it did not completely eliminate it.
“There is no perfect animal model of ADHD, but many studies are performed on rodents,” said study coauthor Abigail Zdrale Rajala. “This one was done in a non-human primate, which is much closer to humans.”
She added that methylphenidate changes the elimination of the reward-neurotransmitter dopamine. As a result, more of the chemical remains in the brain, and is most likely the reason for the medicated monkeys’ altered reward-related processing.
“Some scientists have thought that temporal discounting in ADHD may result from cognitive processing, which relies on the highly evolved frontal cortex in the brain,” the university added. “The new results support an alternative, but less common, hypothesis: that temporal discounting is linked to the reward-processing mechanism, which is governed by more primitive parts of the brain.”