Researchers examined 20 heavy smokers who were given two drugs that either increased or decreased the brain's level of dopamine, a neurotransmitter that affects motor function. Dopamine, a chemical in the central nervous system, is believed to affect emotion, and research on animals has shown that nicotine causes a release of dopamine in brain areas that are associated with feelings of pleasure.
Test subjects smoked less when given a drug that mimics the effects of dopamine in the brain than they did when given a second drug that impedes dopamine's effects. When the subjects were given the dopamine-mimicking drug, their desire to smoke dropped, but when the same smokers were given the dopamine-impeding drug, they took longer and more frequent puffs of smoke.
"Overall, these results imply that smoking behavior can be manipulated within the same subjects in opposite directions by alternately stimulating and blocking dopamine, which strongly suggests the importance of dopamine in reinforcement from cigarette smoking," notes lead researcher Nicholas H. Caskey, Ph.D., of the Veterans Affairs Greater Los Angeles Healthcare System and the Neuropsychiatric Institute at UCLA's David Geffen School of Medicines
The dopamine-mimicking drug used by the researchers, bromocriptine, is used to treat menstrual problems, some tumors and Parkinson's disease. Administration of the drug over a five-hour period caused the subjects to smoke fewer cigarettes and take fewer puffs and have a shorter total puffing time compared with their smoking behavior when given the dopamine-impeding drug.
Prior to this study, very few studies examined the effects of nicotine on dopamine levels in humans. Dopamine is also believed to play a role in regulating the addictive effects of other stimulant drugs such as cocaine and amphetamines.
Researchers monitored the level of nausea in each subject to ensure that smoking levels did not change because of side effects of the drugs. "The … results suggest that nausea played some role in the … response in smoking behavior to the two drugs, but also clearly indicate that there is a substantial drug effect independent of nausea," they write.
The study, which is published in the September issue of Nicotine & Tobacco Research, was funded by a grant from the National Institutes of Health.
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Nicotine & Tobacco Research