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July 01, 1998

Nicotine may not be the only psychoactive component in tobacco smoke, according to a study funded in part by NIDA. Using positron emission tomography, an advanced neuroimaging technology, Dr. Joanna S. Fowler and her colleagues at Brookhaven National Laboratory in Upton, New York, have produced images showing that smoking decreases the brain levels of an important enzyme that breaks down the neurotransmitter dopamine. The amount of the enzyme, called monoamine oxidase (MAO), is reduced by 30 to 40 percent in the brains of smokers, compared to nonsmokers or former smokers, the brain scans show. The reduction in brain MAO levels may result in an increase in levels of dopamine, which scientists associate with the reinforcing effects of drugs of abuse.

Although nicotine causes increases in brain dopamine, it does not affect MAO levels, research has shown. Thus it appears that another component of tobacco smoke is inhibiting MAO. "Whatever is inhibiting MAO could be acting in concert with nicotine to enhance dopamine's activity by preventing its breakdown," says Dr. Fowler.

see captionThe lighted areas in these brain images show levels of an enzyme called monoamine oxidase (MAO). A marked decrease in MAO is apparent in the brains of smokers compared to those of nonsmokers, but nicotine is not responsible.

The concept that the smoking-related reduction of MAO activity may synergize with nicotine's stimulation of dopamine levels to produce the diverse behavioral effects of smoking suggests that MAO inhibitor drugs may be useful as an additional therapy in smoking cessation efforts, she adds. MAO inhibitor drugs are used to treat depression and Parkinson's disease. One such drug, moclobemide, is already being used experimentally to assist persons trying to quit smoking.

Dr. Fowler's research was funded by NIDA, the National Institute of Neurological Diseases and Stroke, and the Department of Energy's Office of Health and Environmental Research.


  • Fowler, J.S.; Volkow, N.D.; et al. Inhibition of monoamine oxidase B in the brains of smokers. Nature 379:733-736, 1996.
  • Fowler, J.S.; Volkow, N.D.; et al. Brain monoamine oxidase inhibition in cigarette smokers. Proceedings of the National Academy of Sciences 93:14065-14069, 1996.