Smokers enjoy their habit because it stimulates the flow of "feel good" chemicals in the brain, according to a new study involving just a handful of test subjects.
The system of the brain affected is the same one that is stimulated by heroin and morphine.
The study is the first to show smoking affects the brain's natural system of chemicals called endogenous opioids, which also help quell painful sensations and heightening positive emotions, the researchers said in a statement today. The system includes the release of endorphins that produce the oft-sought "runner's high."
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Participants did not smoke for 12 hours before the test. Then they smoked two cigarettes which had the nicotine removed from them, followed later by two cigarettes with nicotine.
Their brains were monitored the whole time, and they were also asked how they felt at each step.
"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," said David Scott, a graduate student in the University of Michigan Neuroscience Program. "This change in flow as seen on a brain scan correlated with changes in how the smokers themselves reported feeling before and after smoking."
The orange dots on these brain scans show the areas where the biggest changes in opioid activity took place after smokers began smoking a regular cigarette. On the left is the cingulate, where activity increased 20 percent. On the right is the amygdala, where activity decreased by more than 20 percent. Credit: University of Michigan
The study involved just six smokers, however, all males in their 20s who said they normally puffed 14 cigarettes a day. Scott and his colleagues say that despite the small number of participants, they were surprised at the large effect on opioid levels. The research will be expanded to include more participants.
Further study, the scientists suggest, might reveal why the habit is so hard to kick.
"The interaction of tobacco, and especially nicotine, with brain chemistry is a fascinating area that we're just beginning to understand, especially when it comes to correlating neurochemistry with behavior," said study leader Jon-Kar Zubieta, a psychiatrist and neuroscientist at the university. "Just as with the 'hard' drugs of abuse, such as heroin and cocaine, the phenomena of pleasure, addiction, increased tolerance and craving from tobacco are firmly rooted in neurochemistry."
The research will be presented Tuesday in San Diego at the annual meeting of the Society for Neuroscience.
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