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Does Drug Addiction Rewire the Brain?

It can become 'sensitized' to respond to addiction cues, researchers say

MONDAY, Dec. 24, 2001 (HealthDayNews) -- Getting past withdrawal symptoms may be just the beginning of the end of a drug habit.

Addicts may spend years fighting relapses because the deepest corners of the brain have learned to associate sights or sounds with the drug of choice, new research suggests.

The brain quickly becomes "sensitized" in the early stages of addiction to respond to "cues" -- the sight of a needle or even the sound of clinking ice cubes, for example. These cues then trigger a desire to have whatever drug the brain has come to associate with that cue, University of Michigan researchers say.

"Drug use is known to sensitize certain neural systems within the brain, causing changes that are relatively permanent," says psychologist Kent C. Berridge, a co-author of the study that focused on laboratory rats.

"This study shows that the brain is vulnerable to cues that trigger irrational wanting, even after a long period of remaining drug-free, once sensitized by prior drug use or exposure," he adds.

In the study, the rats were trained to associate a 30-second tone with receiving sugar pellets.

One group of rats received a series of amphetamine injections, while a control group received saline injections.

After a 10- to 14-day period to allow the drugs to clear the rats' systems, the rodents who'd been given the drugs showed a much greater desire to obtain the sugar pellets after hearing the tone.

"In the sensitized rats, the cue triggered excessive wanting," says Berridge. "Whenever a sugar cue [the tone] occurred, rats pressed in a frenzy on a lever that had previously earned them a sugar reward."

In fact, the sensitized rats pressed the lever 200 percent more than rats in the control group, the researchers found.

Berridge suspects that the effect of sensitization in humans probably translates to a "wanting" for the drug itself.

"For the addict, the drug cues aren't only paired with the drug reward, but they're also paired with the sensitizing [of the brain caused by the drug]. And we suspect the combination of that is what really gives the super focus of addictive wanting on the specific drugs that the addict is used to taking," he says.

Perhaps the study's most disturbing finding, Berridge says, was the fact that the symptoms in the rats lasted for at least a year after the drugs had been stopped -- about half an average rat's life span.

"It might even last longer than a year, but that's the longest anyone has looked at this," he says.

Although heavy drugs, such as heroin, are believed to have the strongest sensitizing effect, Berridge says alcohol and nicotine also are known to have similar influences.

Also, some people are more sensitive to drug cues than other people. That offers a possible explanation as to why some people are more susceptible to addictions, he says.

"You might have one person who takes a certain amount of cocaine and quickly becomes a sensitized addict and is vulnerable to relapse for a long time," Berridge says. "And then you might have someone else who can just give it up with just some withdrawal and then they're free."

Norman Hoffmann, a clinical associate professor of community health at Brown University, says Berridge's research adds to a growing understanding of who is susceptible to drug addiction, why, and how they might be helped.

"This could provide some assistance either for assessment of an addiction or for looking at better ways of helping people deal with that kind of stimuli," he says.

Traditional methods of fighting off relapse temptations have included thinking about the unpleasant aspects of an addiction whenevera craving starts.

"You want to pair the negative consequence into the equation to make it a less potent stimulus," says Hoffmann.

Berridge cautions that because his study involved animals, much more research is needed to fully understand how the addiction cue process works in humans.

But, he adds, the study pinpoints the specific neurological process that may be at the core of addiction.

What to Do: For more information on addiction, visit the American Academy of Addiction Psychiatry. Or try the National Institute on Drug Abuse for a primer on cocaine addiction.

SOURCES: Interviews with Kent C. Berridge, Ph.D., professor of psychology at the University of Michigan, Ann Arbor; Norman Hoffmann, Ph.D., clinical associate professor of community health, Brown University, Providence, R.I.; October 2001 issue of the Journal of Neuroscience
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