Feb. 21 (HealthDay News) -- Smoking produces long-lasting changes in the brains of smokers and former smokers alike, a new study suggests.

For the study, which is published in the Feb. 21 issue of The Journal of Neuroscience, researchers from the National Institute on Drug Abuse (NIDA) examined eight samples of human brain tissue from each of three groups: long-term smokers who smoked until death, previous long-term smokers, and nonsmokers. The samples were taken from the nucleus accumbens and the ventral midbrain, two brain regions that play a part in controlling addiction-related behaviors.

All of the participants had died of causes not related to smoking.

The researchers analyzed levels of two specific enzymes found inside brain cells that have been associated with addictive-related behaviors in animals exposed to cocaine, heroin and other illicit drugs.

Levels of these enzymes were elevated in smokers and, more interestingly, former smokers, compared with nonsmokers.

According to lead author Bruce Hope, of NIDA, these findings suggest that the brain changes persist long after smoking has ceased and could contribute to future drug relapse.

"The parallel between the new study and the animal studies is important, because a causal role has been shown in animal studies between increased levels of these neuronal signaling enzymes in these brain regions and addiction-related behaviors. This strongly suggests that the similar changes observed in smokers and former smokers contributed to their addiction," Hope said in a prepared statement.

Hope pointed out that although his findings support previous research, it is not yet clear that these biochemical changes actually cause addiction-related behaviors.