Scientists have known for years
that smokers are less likely than nonsmokers to develop Parkinson's disease, but
they could not explain why.
Now researchers at Virginia Tech in Blacksburg, Virginia have identified a
chemical in tobacco smoke that inhibits an enzyme in the brain, called monoamine
oxidase (MAO), which normally destroys excess dopamine. The findings were
presented here at the American Chemical Society meeting.
Without sufficient dopamine, people develop the tremors and jerky movements
characteristic of Parkinson's disease. The new research suggests that cigarette
smoking probably keeps dopamine levels higher than they would otherwise be.
What's more, separate studies at Brookhaven National Laboratory in Upton,
New York, confirm that smokers have lower levels of the enzyme than nonsmokers.
The smokers' brain chemistry essentially mimics those of nonsmokers taking MAO
inhibitors, one of the first treatments ever discovered for depression.
Together, these findings suggest that smokers light up to make themselves
feel better, just as drug abusers take street drugs to make themselves feel
better, according to Dr. Joanna Fowler of Brookhaven.
"Dopamine is a central chemical in the reward system. All drugs of abuse
cause elevations in dopamine," Fowler said.
The studies are designed to learn more about Parkinson's and do not justify
smoking or suggest any health benefits of the habit.
"Do not start smoking, and if you are smoking, stop," said study author Dr.
Kay Castagnoli, an expert in Parkinson's disease at Virginia Tech. "The
incidence of Parkinson's disease (among nonsmokers) does not come close to
outweighing the risks of smoking."
The chemical identified by Castagnoli and colleagues is a derivative of
naphthoquinone, already known to be one of the thousands of chemicals in tobacco
smoke.
To test whether the chemical could protect against Parkinson's, Castagnoli
injected it into mice, and then injected them with MPTP, a drug that can cause
Parkinson's-like symptoms.
MPTP first was recognized as Parkinson's-inducing after some drug abusers in
the 1980s developed irreversible damage when the drug was consumed as a
contaminant in heroin.
Castagnoli showed that mice given naphthoquinone were much less likely to
develop Parkinson's symptoms when given MPTP. But she noted that other chemicals
in tobacco smoke, as yet unidentified, might also contribute to the effect in
humans.
