Nicotine builds up gradually in smokers' brains rather than spiking after each puff, according to a study that might help point to new ways to help people quit smoking.

Dr. Jed E. Rose of Duke University reports in Monday's online edition of Proceedings of the National Academy of Sciences that nicotine buildup in the brain was gradual over several minutes.

Scientists have theorized that there is a spike of nicotine in the brain about seven seconds after each puff, but almost no measurements had been taken until now, Rose said in a telephone interview.

"We were surprised to find that the rate of uptake was much different from what one commonly hears," said Rose, who directs the Duke Center for Nicotine and Smoking Cessation Research, a part of the university's School of Medicine.

Rose used brain scans to measure the nicotine levels in 13 regular smokers and 10 people who smoke only occasionally, an indication they were not addicted to nicotine.

Maximum brain levels of nicotine were reached in 3 to 5 minutes, and built up slower in addicted smokers than in casual ones, the researchers found.

"This slower rate resulted from nicotine staying longer in the lungs of dependent smokers, which may be a result of the chronic effects of smoke on the lungs," Rose suggested.

"Now that we know there are not these spikes" that had been expected, Rose said, researchers may be better able to develop new approaches to help smokers get what they need from cigarettes, but in a way that's not addictive.

His laboratory, for example, is working on a mist inhaler to deliver nicotine without any combustion.

Still in question: Why do some people become addicted to cigarettes and others don't? The difference in the rate of nicotine buildup in the brain doesn't explain this, the researchers said.

The research was funded by the giant tobacco companies Philip Morris USA and Philip Morris International. The researchers said the companies had no role in designing or carrying out the research or analyzing the results.

Rose's findings confirm his earlier work on blood levels of nicotine, and "the brain is what really matters," commented Dr. Kenneth A. Perkins, a professor at the University of Pittsburgh who also studies smoking addiction.

The assumption was that a critical effect of smoking was a shot of nicotine with each puff, then another shot with the next puff, and so on, Perkins said.

"He is showing that, at least when you look at the blood and brain concentration levels, that's not really what's going on, it's much more gradual," Perkins, who was not part of Rose's research team, said in a telephone interview.

"Clinically, what you do with that, I'm not quite sure," he added.