French scientists say they have identified a chemical compound which explains how nicotine becomes addictive and which could help point the way to new drugs for people who want to stop smoking.

A scientist at the Pasteur Institute in Paris, Marina Picciotto, said: "For the first time, one particular molecule has been shown to be critical for the events leading up to nicotine addiction."

The discovery was made when scientists found the first of 11 sub-units, or molecules, of the nicotine receptor in the brain of mice.

Advances in treating drug abuse may follow

Humans have the same so-called 'b2' sub-unit.

"It's the first step in identifying the other components of that receptor and that pathway [that triggers addiction]," she said.

Because the habit-forming properties of nicotine are similar to those of other drugs, such as cocaine and heroin, the finding could also have important implications for treating drug abuse.

Like many drugs, the addictive elements of nicotine are connected with the release of the neurotransmitter, dopamine, in the brain.

People get addicted because of the rapid activation that leads to the dopamine release.

The discovery could lead to the development of a drug that would block the receptor responsible for addiction or activate it in a slower way.

"The idea is that the more you know about exactly which molecules are important to nicotine addiction, the more you can intelligently design a drug that would interrupt specifically that molecule and not other nicotine receptors," she said.

Nicotine addiction occurs because of a rapid dopamine release in the brain.

In a report in the scientific journal Nature, the scientists described how they found that nicotine stimulated the release of dopamine in the brains of normal mice but not the brains of mutant mice lacking the b2 sub-unit.

In a series of experiments they discovered that the mutant mice did not react to nicotine.

The study also showed that nicotine improved the performance of mice on avoidance-learning tests, which could explain why some smokers claim it enhances their concentration and memory.

"There are some good things that nicotine does," said Mrs Picciotto, adding that clinical trials have shown that it enhances memory in Alzheimer's patients.

The 11 receptor sub-units are similar in structure but have different functions.

Scientists have some ideas of what the sub-units do but are only at the very beginning of identifying which molecule does what.

"We think the endogenous role of this receptor is not to make you addictive to nicotine but to regulate dopamine release in the brain under normal circumstances," Mrs Picciotto explained.

The scientists are already conducting experiments to see how their finding could influence the addictive properties of other drugs.
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