One way most drugs cause addiction when they hit the brain is by stimulating certain cells to make dopamine, the biochemical most crucial to the experience of pleasure. But according to the researchers, smoking does a good deal more than that. Using rats, they discovered that the first time a brain is subjected to nicotine, the substance not only stimulates brain cells to make more dopamine, but also interferes with the brain's ability to regulate the chemical. Consequently, a smoker's high lingers as long as 45 minutes after the nicotine has disappeared from the bloodstream. The smoker experiences a runaway feel-good sensation that the brain records in its reward center and craves again and again.