"Earlier studies of older participants showed that the smokers had structural differences in various brain regions," said senior author Edythe D. London.
And in studies of adolescent animals, nicotine damaged and killed brain cells, added London, from the Semel Institute for Neuroscience and Human Behavior at UCLA and the David Geffen School of Medicine in Los Angeles.
"While the results do not prove causation, they suggest that there are effects of cigarette exposure on brain structure in young smokers, with a relatively short smoking history," London said.
She and her team at UCLA mapped the brains of 42 people ages 16 to 21 using magnetic resonance imaging (MRI) and asked them about their smoking history and cravings.
Eighteen of the participants were smokers. They had typically started smoking around age 15 and smoked six to seven cigarettes per day.
There were no clear differences in the brains of smokers versus non-smokers. However, among smokers, those who reported smoking more cigarettes tended to have a thinner insula, a region of the cerebral cortex involved in decision making, according to results published in the journal Neuropsychopharmacology. The effects seemed confined to the right insula.
Previous studies have suggested the insula plays a central role in tobacco dependence, with the highest density of nicotine receptors in the brain.
The researchers also found a thinner insula in the brains of people who had more cravings and felt more dependent on cigarettes. Their study was funded by Philip Morris USA, makers of Marlboro and Virginia Slims.
Young people ages 18 to 25 have the highest smoking rates in the U.S. at 30 percent, London said.
"Because the brain is still undergoing development, smoking during this critical period may produce neurobiological changes that promote tobacco dependence later in life," she said. Changing the structure of the insula may affect future smoking dependence and other substance abuse.
"It is possible that changes in the brain from prolonged exposure help maintain dependence," she said.
People who start smoking early in life seem to have more trouble quitting and have more serious health consequences than those who start later, London said.
But since the study only assessed smokers at one point in time, it doesn't prove that cigarettes changed their brains.
"It is possible that such changes pre-dated the smoking, i.e. they were not caused by smoking," Dr. Nasir H. Naqvi told Reuters Health in an email. "The only way to know this is to take a group of adolescents who have never smoked, follow them over time, and then see who starts smoking, and then compare them to the adolescents who never started smoking."
Naqvi, a substance abuse researcher at Columbia University in New York City, was not involved in the study.
He studies the insula and said that area drives drug addiction like a "gas pedal" and also controls decision making like a "brake pedal."
Since the insula was thinner in heavier smokers, it could be they have reduced power over the "brake pedal" and less control over cravings, he said.
"The key question is whether these changes are reversible with smoking cessation, or whether they persist," Naqvi said. But few studies have measured changes in the brains of people who stop taking a drug.
"What we do know is that once you are addicted to smoking, you will always have a high likelihood of relapse, even if you are abstinent for many years," Naqvi said.
The study is interesting but quite small, especially for a study on this age group, Simone Kuhn said.
Kuhn studies brain plasticity across the lifespan at the Max Planck Institute for Human Development in Berlin and was not involved in the new research.
"Between 16 and 21 the prefrontal cortex changes considerably; this might make research such as this slightly difficult," she told Reuters Health. "Though I clearly agree that brain structural effects of smoking in adolescence is an extremely interesting topic that could be used to spice up anti-smoking campaigns, addressing this age group with scientific facts."
"It would be useful to do this kind of study in a larger number of people, starting before the initiation of smoking and continuing with repeated scans," London said. "This is practical. It just requires funding."