ADDICTED

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The idea that a single chemical could be associated with everything from snorting cocaine and smoking tobacco to getting good grades and enjoying sex has electrified scientists and changed the way they look at a wide range of dependencies, chemical and otherwise. Dopamine, they now believe, is not just a chemical that transmits pleasure signals but may, in fact, be the master molecule of addiction.

This is not to say dopamine is the only chemical involved or that the deranged thought processes that mark chronic drug abuse are due to dopamine alone. The brain is subtler than that. Drugs modulate the activity of a variety of brain chemicals, each of which intersects with many others. "Drugs are like sledgehammers," observes Dr. Eric Nestler of the Yale University School of Medicine. "They profoundly alter many pathways."

Nevertheless, the realization that dopamine may be a common end point of all those pathways represents a signal advance. Provocative, controversial, unquestionably incomplete, the dopamine hypothesis provides a basic framework for understanding how a genetically encoded trait--such as a tendency to produce too little dopamine--might intersect with environmental influences to create a serious behavioral disorder. Therapists have long known of patients who, in addition to having psychological problems, abuse drugs as well. Could their drug problems be linked to some inborn quirk? Might an inability to absorb enough dopamine, with its pleasure-giving properties, cause them to seek gratification in drugs?

Such speculation is controversial, for it suggests that broad swaths of the population may be genetically predisposed to drug abuse. What is not controversial is that the social cost of drug abuse, whatever its cause, is enormous. Cigarettes contribute to the death toll from cancer and heart disease. Alcohol is the leading cause of domestic violence and highway deaths. The needles used to inject heroin and cocaine are spreading aids. Directly or indirectly, addiction to drugs, cigarettes and alcohol is thought to account for a third of all hospital admissions, a quarter of all deaths and a majority of serious crimes. In the U.S. alone the combined medical and social costs of drug abuse are believed to exceed $240 billion.

For nearly a quarter-century the U.S. has been waging a war on drugs, with little apparent success. As scientists learn more about how dopamine works (and how drugs work on it), the evidence suggests that we may be fighting the wrong battle. Americans tend to think of drug addiction as a failure of character. But this stereotype is beginning to give way to the recognition that drug dependence has a clear biological basis. "Addiction," declares Brookhaven's Volkow, "is a disorder of the brain no different from other forms of mental illness."

That new insight may be the dopamine hypothesis' most important contribution in the fight against drugs. It completes the loop between the mechanism of addiction and programs for treatment. And it raises hope for more effective therapies. Abstinence, if maintained, not only halts the physical and psychological damage wrought by drugs but in large measure also reverses it.