HOW SMOKERS GET HOOKED

TAKE A PUFF OF TOBACCO, AND A BLAST OF 4,000 CHEMICALS fills your lungs, blood and brain. The smoke delivers a strong hit–about 2 mg in each cigarette–of nicotine, a compound the U.S. Surgeon General in 1988 deemed an addictive drug. But is nicotine alone what hooks people on tobacco? Apparently not. According to a new report, smoking may exert yet another powerfully addictive influence, one that enhances the effect of nicotine in what a leading researcher calls a “diabolical synergism.”

In the new research, published in the journal Nature, scientists ran brain scans on smokers and abstainers and found that smokers had 40% less of a brain enzyme known as monoamine oxidase B, or MAO B. The enzyme breaks down dopamine, a chemical messenger in the brain associated with feelings of pleasure. Because of its exquisitely satisfying effects, says Joanna Fowler, a chemist at Brookhaven National Laboratory and one of the study’s authors, “dopamine is crucially important in reinforcing and motivating behavior.”

Thus smoking appears to create a self-perpetuating cycle: less MAO B leads to more dopamine, leads to more pleasure, leads to more smoking, leads to less MAO B and so on.

Scientists have not yet identified what factor in smoke lowers levels of MAO B, but Fowler speculates that it may be working synergistically with nicotine to boost dopamine levels. Earlier research showed that nicotine also increases dopamine levels by gripping like Velcro to receptors clustered in the forebrain.

There may be an even more dangerous synergy at work, according to Alan Leshner, director of the National Institute on Drug Abuse. “Every substance that is addicting leads to an increase in dopamine levels in the brain,” he says. That may help explain why people who abuse one substance so often abuse another. Fowler likens the effect to creating a biochemical pathway or channel: “A drug may leave an imprint in the brain, so that the next drug becomes more pleasurable than it would otherwise.” In short, the brain gets into a rut that just grows deeper and deeper.

–By Anastasia Toufexis. Reported by Alice Park/New York