Brain differences may herald drug addiction.

Differences in the behavior and the brain receptors of rats seem to predict which of the rodents will become cocaine addicted, scientists report. The finding supports the idea that some people are predisposed to drug addiction.

Scientists have long suspected that certain personality traits, including thrill seeking, impulsivity, and a tendency to be antisocial, go hand in hand with drug addiction. Studies have also shown that the brains of monkeys and people addicted to stimulants such as cocaine or amphetamine have significantly fewer receptors for dopamine, a brain chemical that regulates emotion, motivation, and feelings of pleasure.

However, the connection between these mental and physical characteristics and drug addiction has posed a chicken-and-egg problem for researchers. It's unclear whether drug addicts have these qualities before they begin using drugs or whether taking drugs over the long term changes a person's personality and brain properties.

Jeffrey Dalley of the University of Cambridge in England and his colleagues report results in the March 2 Science that shed light on this problem. The researchers taught a group of lab rats to poke their noses in a hole to retrieve a treat after seeing a light flash. The team found that about 7 percent of the animals consistently acted impulsively. Rather than wait to collect the treat that appeared after the light blinked, those animals frequently poked their noses into the hole before the treat arrived.

Dalley's team next scanned the rats' brains. In a region known as the nucleus accumbens, the impulsive animals had significantly fewer of the D2/3 type of dopamine receptor than the more patient rats did. Previous studies in people had connected the nucleus accumbens to reward feelings, such as those gained from eating, winning at gambling, and taking drugs.

Finally, the researchers hooked the impulsive and normal animals to a machine that delivered cocaine intravenously when the rats pressed a lever. The impulsive animals learned to self-administer the drug more quickly and took more of it than the other rats did. Within days, the impulsive rats were using cocaine at nearly twice the rate of the patient ones.

Because the impulsive rats had fewer D2/3 receptors before using drugs, Dalley says, the traits of impulsivity and a low number of these D2/3 receptors seem to be characteristics that make an animal vulnerable to drug addiction.

David Jentsch of the University of California, Los Angeles, who studies the long-term effects of drugs on the brain, calls the study "very exciting." He adds, "This will help us understand what leads people on a pathway to drug abuse."

Jentsch cautions that it's premature to say that all people who become addicts had pre-existing anomalies in personality and brain traits. Rather, long-term drug use may cause these anomalies in some people who didn't have them before.

"It's unlikely that it's an either-or phenomenon," Jentsch says.