ATLANTA, Georgia (CNN) -- Scientists say cigarettes' most addictive component -- nicotine -- may also lead to degeneration in a region of the brain that affects emotional control, sexual arousal, REM sleep and seizures.
The findings, reported in a new study, could help doctors pinpoint the part of the brain that gives some people increased susceptibility to chronic smoking or drug addiction, researchers said.
"Nicotine causes the most selective degeneration in the brain that I have ever seen," said the study's lead author, neuroscientist Gaylord Ellison. He and other colleagues at the University of California, Los Angeles, published their findings in the latest issue of Neuropharmacology, a Bristol, England-based medical journal.
In previous research, Ellison and others had shown that amphetamines, cocaine, Ecstasy and other addictive drugs damage one half of the fasciculus retroflexus. The bundle of nerve fibers emerges from a region of the brain just above the thalamus.
In the latest study, the researchers found that nicotine causes degeneration in the other half of the fasciculus retroflexus, or what Ellison describes as a region "affected more by chronic drug use than other tracts in the brain."
To make matters worse, the study found drugs or chemicals that damage one half of the nerve bundle do not damage the nicotine-degenerated side. That selectivity, Ellison said, makes nicotine damage even more insidious and harder to chart.
"Only one track of the brain is affected," he said.
Calls by CNN.com to the National Tobacco Council and Friends of Tobacco, both pro-tobacco industry groups, for comment on the study were not immediately returned. The researchers administered nicotine to a group of rats for five days through a mini-pump inserted under their skin. Human brains were not examined for the study, and it's authors were careful to note that more research is needed on smokers' neurological makeup.
But the study does say the human fasciculus retroflexus "shows a special vulnerability to nicotine-induced neurotoxicity."
At high doses cigarette smoke contains more than 4,000 chemicals, according to the American Cancer Society, which lists lung cancer as the largest single cause of cancer death in the United States. The group said nicotine occurs naturally in tobacco and does not cause cancer -- but does keep people addicted to smoking.
This month, the cancer society celebrates its 24th annual Great American Smokeout pro-cessation day on November 16.
Ellison's study was funded in part by the Tobacco Related Disease Research Program, which is funded by California's tobacco tax.