Many people assume that an addict’s substance abuse is responsible for the damage done to his or her brain. New research shows that some of that “damage” may have been there to begin with.

Chronic drug users have fewer dopamine D2 receptors than nonusers in the reward pathways of their brain, which often makes them less sensitive to natural pleasures such as food and attractive mates. Scientists believe this receptor deficit may reinforce addiction by causing users to seek from drugs what they are unable to get naturally—the “high” caused by a surge of dopamine.

Now Jeffrey Dalley and his colleagues at the University of Cambridge have shown that some people may be born with an abnormally low D2 receptor count, predisposing them to impulsive behavior and drug addiction.

The team compared the brains of six impulsive rats and six normal rats and then allowed the animals to self-administer cocaine. The impulsive rats became addicted more quickly than their nonimpulsive lab mates, and they showed a significantly lower number of D2 receptors in the ventral striatum, a brain region associated with reward anticipation and craving. The researchers found no differences in the dorsolateral striatum, an area involved in compulsive drug-seeking behavior. A decrease in D2 receptors within this brain area, according to past findings, is seen most commonly after habitual drug use.

“This last point is crucial because it suggests that progressive drug use produces progressive changes in the brain,” Dalley says.

The scientists proposed a hypothesis: some drug addicts are born with a localized reduction of D2 receptors in the ventral striatum. This anomaly predisposes them to high levels of impulsivity, which may lead to their initial experimentation with drugs. Long-term drug abuse, in turn, may cause damage in the dorsolateral striatum and other parts of the brain’s reward pathway, causing addicts to compulsively seek out drugs.

If the researchers are correct, D2 receptors may one day be used to identify people at high risk for drug abuse.