Nicotine is addictive, but the new research shows that there are other components in cigarette smoke which have a similar effect on the brain to drugs such as heroin and cocaine.
Joanna Fowler, a chemist at Brookhaven National Laboratory in New York, and her colleagues have not identified the new chemical, but have monitored its effects by studying scans of the brains of both smokers and non-smokers.
They found that it acts indirectly on dopamine, a chemical messenger that carries signals between nerves and affects mood. Dopamine has been implicated in the "rewarding properties" of drugs of abuse.
The study, published in today's issue of the journal Nature, could explain why cigarettes are so addictive and expose tobacco as a so-called "gateway" drug, making people more susceptible to addiction. It may also shed some light on the finding that smokers seem to suffer less than the norm from Parkinson's Disease.
Dr Fowler and her colleagues discovered that smokers have lower concentrations of the chemical monoamine oxidase (MAO) B in their brains than ex- or non-smokers.
This chemical breaks down dopamine, so if the concentrations of MAO are reduced by cigarette smoke, levels of dopamine are increased - leading to the rewarding, addictive effect of tobacco smoke.
Dr Fowler said this was the first such study on living volunteers. "It's amazing how little is known about the effects of smoke on the brain," she said.
French researchers reported last year that they were using an antidepressant which inhibits MAO to help smokers quit. MAO inhibitors are also used to treat Parkinson's, a disease that causes uncontrollable shaking and which is associated with low levels of dopamine.
Dr Fowler has noted that depressed and schizophrenic people are likely to smoke. "There is some speculation that people who smoke and who are depressed are self-medicating," she said.Reuse content