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This essay on the polymerase chain reaction is one of a series developed as part of FASEB's efforts to educate the general public, and the legislators whom it elects, about the benefits of fundamental biomedical research-particularly how investment in such research leads to scientific progress, improved health, and economic well-being.

In daily use for centuries by hundreds of millions of people, nicotine has only lately been investigated for its therapeutic potential in a long list of common ills

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Features Addiction and the brainThe dopamine pathway is helping researchers find their way through the addiction maze ur flourishing knowledge of the brain is in large part the product of research on addiction. Identifying what happens in the brain when a drug is inhaled, injected, or eaten, why it leads to compulsive drug seeking, and learning how to disrupt that process has seemed like the last best hope for a permanent fix for addiction. Which is why, according to Alan Leshner, director of the National Institute on Drug Abuse (NIDA), researchers know more about drugs in the brain than they know about anything else in the brain.Among the revelations: addiction is now seen to be a brain disease triggered by frequent use of drugs that change the biochemistry and anatomy of neurons and alter the way they work. Scientists have developed a basic model of addiction that presents these changes as the desperate attempt of the brain to carry on business-as-usual-to make neurons less responsive to the drugs and so restore homeostasis-while under extreme chemical siege.But the adaptations the drugs force on the brain can be long term or even permanent. With sustained drug use, the brain adapts to this saturation bombardment, and giving up drugs leaves it bereft and demanding a return to the new homeostasis. Thus, even the brains of people who have quit using drugs and urgently wish to stay clean remain vulnerable to relapse. Deprived addicts are no longer seeking to get high, they just want to feel normal.Genetic factors, environmental factors, and-most important-the intricate and still mysterious inter-action of the two are assumed to be fundamental to the addiction process. But a great many critical details are emerging from studies of events in the brain. The common pathwayThe most compelling revelation about addiction and the brain may even deserve that tattered encomium "breakthrough." The discovery that startled the scientists? Although each drug employs it in a somewhat different way, addictions center around alterations in a single pathway in the brain: the "reward" circuit whose chief centers of action lie in the ancient part of the brain known as the limbic system. This pathway is involved in drug addictions of all kinds-not just addiction to illegal drugs such as heroin and cocaine, but also addiction to alcohol, tobacco, and even caffeine. Marijuana appears to employ this pathway too. And perhaps-a big perhaps because addiction experts are divided on this point-the pathway also figures in "addictions" that do not involve drugs, for example, the compulsive and destructive pursuit of eating, exercise, gambling, or sex.The addiction pathway is the brain system that governs motivated behavior. When the pathway was first discovered, almost a half-century ago, people called it the pleasure center. Scientists now call it the brain reward region and have confirmed its role as the addiction pathway in countless animal studies (mostly with rats and mice) and many brain-imaging studies of human addicts.The pathway is hid...