Smoking Lowers Enzyme, Which May Push Addiction

Smoking lowers the brain’s supply of a certain enzyme, a process that might heighten the addictive power of nicotine, a study suggests.

If so, drugs now used to treat Parkinson’s disease might help people quit smoking, said researcher Joanna Fowler.

Fowler is director of PET imaging at the Brookhaven National Laboratory in Upton, N.Y. She and colleagues at Brookhaven and elsewhere presented the federally funded work in today’s issue of the journal Nature.

They used PET, which stands for positron emission tomography, to scan the brains of eight smokers, eight non-smokers and four ex-smokers. They measured the levels of an enzyme called monamine oxidase B, or MAO B.

This enzyme is produced by brain cells, and it is involved in the natural destruction of dopamine. Dopamine is a chemical signal some brain cells use to communicate.

Nicotine is addictive because it stimulates the release of dopamine as well as other chemical messengers. Some researchers believe the heightened dopamine release is the key effect that produces nicotine addiction.

The brain scans found that smokers average about 40 percent less MAO B than non-smokers had. The former smokers had about the same amount as people who had never smoked.

Fowler speculated that the reduced level of the enzyme in smokers might increase the addictive power of nicotine. That’s because with less of the enzyme around, the extra dopamine produced in response to nicotine is less subject to destruction, she said.

Scientists have not yet identified what compound in cigarette smoke is reducing the levels of the MAO B enzyme, she said. Previous studies show it isn’t nicotine, she said.

Drugs now used to treat Parkinson’s disease inhibit the effect of the enzyme, she said. They might be useful in weaning smokers off cigarettes in the same way that nicotine gum or nicotine skin patches are, by easing the drop in dopamine that comes with quitting, she said.

George F. Koob, an expert in brain chemistry at the Scripps Research Institute in La Jolla, Calif., said he believes it’s worth studying whether reductions in the enzyme boost nicotine’s ability to hook smokers when they’re just starting the habit.

Dr. Neal Benowitz, a professor of medicine at the University of California at San Francisco who studies the effect of nicotine on the brain and body, said the study could explain why most people who abuse alcohol or use cocaine also smoke cigarettes.

The lowering of the enzyme level by cigarette smoke could make alcohol and cocaine more pleasurable, because like nicotine, those two substances also make the brain release more dopamine to cause pleasure, Benowitz said.

Fowler said the new finding might also help explain why other studies have found that smoking reduces the risk of Parkinson’s disease.