Teen smokers might crave nicotine in part because their brains respond differently than adults to seeing people light up, a small study suggests.

To see if young minds might be hard-wired to desire cigarettes, researchers did magnetic resonance imaging (MRI) scans of teens and adults - including smokers and nonsmokers - who watched videos of adolescents and young adults smoking.

With teen smokers in particular, researchers saw heightened responses in brain regions rich in the neurotransmitter dopamine, a chemical that modulates pleasure and reward centers and helps regulate emotions.

"We interpret these data to mean that the teen brain is more responsive to the rewarding and thrilling aspects of smoking, thus making craving more psychologically salient to them," said study co-author Adriana Galvan of the University of California Los Angeles.

"The dopamine system undergoes significant maturation during the teenage years, rendering the teen brain more reactive to rewards and perhaps more vulnerable to addictive substances," Galvan added by email.

Galvan and co-author Kathy Do of the University of Illinois Urbana-Champaign analyzed MRI results for 39 teens and 39 adults who watched a series of brief video clips of actors smoking cigarettes.

Roughly half of the participants said they were smokers, and those who admitted to this habit reported more cravings when they watched the videos.

Adolescent smokers reported the same level of craving in response to the videos, even though they had a much briefer history of smoking than the adults in the study.

When researchers asked participants after the MRIs whether they wanted to smoke, and how badly, only teen smokers appeared to have a connection between activation of reward and pleasure centers in the brain and a subsequent desire for cigarettes.

Besides the small study size, another limitation is that researchers didn't ask participants to refrain from smoking for a set period of time before the start of the experiment, which might be an independent influence on cravings, the authors note in the Journal of Adolescent Health.

More on this...

It's also possible that the study found different responses to the videos in teen and adult smokers because the older participants didn't identify with the images featuring young people, noted Adam Leventhal, director of the Health, Emotion and Addiction Laboratory at the University of Southern California.

"For the adults in the study, the video of young people smoking might not have been realistic enough to produce the natural response that they might have had if encountering smoking in the real world," Leventhal, who wasn't involved in the study, said by email.

Teens might have also responded more to the videos because during adolescence, parts of the brain that react to pleasure-seeking cues develop much faster than those involved in impulse control, Leventhal added.

"Consequently, teens are generally more interested in seeking out new, exciting, and pleasurable experiences than adults, which could generalize to addictive behaviors like smoking," he said.

To the extent that teen brains may be more sensitive to nicotine than adult brains, it makes sense to set the age for buying cigarettes at 21 to help keep adolescents from using tobacco when they may be more vulnerable to cravings, said Dr. Nancy Rigotti, a researcher at Harvard University and director of the Tobacco Research and Treatment Center at Massachusetts General Hospital in Boston.

The study doesn't definitively prove this is the case, "but it is very nice supporting data," Rigotti, who wasn't involved in the study, said by email.