Smokers have more nicotine receptors in their brains than nonsmokers, making it more difficult for them to quit, according to researchers at Yale University.

Researchers used brain-scanning technology to compare the nicotine receptors of 16 smokers who had abstained for four days with scans from a group of 16 nonsmokers. They found that the density of common nicotine receptors was higher among smokers during early abstinence, contributing to withdrawal symptoms.

"Nicotine craving is an important factor associated with relapse," said lead author Julie Staley. "This study paves the way for determining whether medications normalize the number of receptors and why some smokers, such as women and those with neuropsychiatric disorders, have more difficulty quitting smoking."

The study appears in the Aug. 21, 2006 issue of the Journal of Neuroscience.