WASHINGTON -- 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.

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.