The brain makes less dopamine, a chemical involved in both pleasure and addiction, when people smoke but this temporary deficit may be reversed when smokers kick the habit, a small experiment suggests.

"It is assumed that the brain adapts to the repeated nicotine-induced release of dopamine by producing less dopamine," said lead study author Dr. Lena Rademacher of Lubeck University in Germany.

It's still not clear if dopamine production reduced by long-term smoking bounces back in ex-smokers, so the researchers did brain scans of 15 never-smokers and 30 smokers.

Then, they offered cessation treatment to the smokers and did another set of brain scans three months later on the subset of 15 people in this group who had quit.

More on this...

On the first set of scans, smokers had a 15 percent to 20 percent lower capacity for dopamine production than the nonsmokers, researchers report in the journal Biological Psychiatry.

But in the second set of scans, there was no longer a difference between nonsmokers and the smokers who successfully quit during the study.

This is important because some researchers think certain people could possess naturally low dopamine production that predisposes them to addiction.

Nicotine addiction is known to be associated with abnormalities in the dopamine system. But scientists are uncertain if smoking induces those abnormalities or if they already exist in some people and make them more vulnerable to getting hooked on nicotine.

Because the study found that most nicotine abnormalities went away after smokers quit, it suggests they are a byproduct of smoking, Rademacher said.

"In case of a predisposing trait, abnormalities are expected to persist with abstinence," Rademacher said. "Conversely, if dopamine function normalizes with abstinence this rather indicates that alterations were induced by substance consumption."

One limitation of the study is its small size, which makes it difficult to draw statistically meaningful conclusions, the authors note. The study also only included men, making it hard to say whether the findings would apply to women.

Even so, the results are encouraging because they suggest that brain function is plastic, or modifiable, and that an ex-smoker's brain can return to more normal functioning over time, said Joseph McClernon, a psychiatry researcher at Duke University School of Medicine in Durham, North Carolina, who wasn't involved in the study.

The findings also may have implications beyond just addiction to cigarettes because the dopamine system is involved in a broad range of functions including learning, motivation and behavior control, McClernon added by email.

"To the extent that smoking or other drug use alters how this system functions normally can have impacts on behavior that increase the likelihood that one continues to use drugs or has difficulty in quitting drug use," McClernon said.

"Dopamine regulation of motivation for instance, is likely involved in the tendency of drug users to be overly preoccupied with drug use" to the exclusion of other forces in their lives like work and relationships, McClernon added.