The finding helps researchers understand why smokers have such a tough time quitting despite all the health dangers. They presented their report at the annual meeting of the Society for Neuroscience, held this week in San Diego.
It's the first human study to show that smoking cigarettes stimulates the brain's production of chemicals called opioids. The opioids are known to play a role in soothing pain, increasing positive emotions, and creating a sense of reward. Both morphine and heroin trigger this same chemical flow.
The new study also confirms previous findings -- from animal studies -- that smoking cigarettes affects the flow of another feel-good brain chemical called dopamine. Researchers are now investigating the interaction between the two chemicals in the brains of smokers and nonsmokers.
"It appears that smokers have an altered opioid flow all the time, when compared with non-smokers, and that smoking a cigarette further alters that flow by 20 to 30 percent in regions of the brain important to emotions and craving," says lead researcher David J. Scott, a graduate student in neuroscience at the University of Michigan in Ann Arbor, in a news release.
Smoking Cigarettes During Brain Scans
Scott's research team spent several years testing a way of using PET imaging (positive emission tomography) to study the opioid system in the brain. The scans don't show the flow of opioids directly, but they do show opioid receptor activity in the brain.
Morphine and heroin are among the drugs that bind to these receptors.
The University of Michigan researchers have also created a system that allows someone to smoke cigarettes while lying in the PET scanner having his or her brain scanned.
That's how they conducted this new study, which involved six healthy men who smoked one pack per day. All refrained from smoking cigarettes for at least 10 hours before the study began. During their brain scans, each first smoked a cigarette almost devoid of nicotine, and then smoked a regular cigarette.
Researchers also asked the men to rate their feelings at various times during the study, including before and after smoking cigarettes.
Significant differences in the flow of opioids were seen when the smoker smoked low-nicotine versus high-nicotine cigarettes.
While the men smoked low-nicotine cigarettes, their brains started to show changes in opioid flow. But as soon as they smoked regular cigarettes, opioid levels increased significantly in the brain area involved in emotion and emotion-memory processing, Scott reports.
However, other brain regions were 20% to 30% less active -- the areas involved in memory, emotion, and pleasure. This matched the men's feelings at the time: They reported feeling more relaxed, less alert, less nervous, and having fewer cravings than before smoking cigarettes.
When the men smoked low-nicotine cigarettes, their brains started to show changes in opioid flow. But as soon as they smoked regular cigarettes, opioid receptor activity increased significantly in the brain areas involved in emotion and emotion-memory processing, Scott reports. This meant that far more "feel good" chemicals were being released.
However, other regions in the brain involved in memory, emotion, and pleasure were 20% to 30% less active. The researchers aren't sure why these other areas of the brain respond differently. But this decreased activity allows other chemicals -- like nicotine -- to affect these areas. The men reported feeling more relaxed fewer alerts, less nervous, and having fewer cravings than before smoking cigarettes. This suggests that nicotine binds to the opioid receptors, causing these brain areas to activate "feel good" hormones.