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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.
Genes and social forces may conspire to turn people into addicts but do not doom them to remain so. Consider the case of Rafael Rios, who grew up in a housing project in New York City's drug-infested South Bronx. For 18 years, until he turned 31, Rios, whose father died of alcoholism, led a double life. He graduated from Harvard Law School and joined a prestigious Chicago law firm. Yet all the while he was secretly visiting a shooting gallery once a day. His favored concoction: heroin spiked with a jolt of cocaine. Ten years ago, Rios succeeded in kicking his habit for good, he hopes. He is now executive director of A Safe Haven, a Chicago-based chain of residential facilities for recovering addicts.
How central is dopamine's role in this familiar morality play? Scientists are still trying to sort that out. It is no accident, they say, that people are attracted to drugs. The major drugs of abuse, whether depressants like heroin or stimulants like cocaine, mimic the structure of neurotransmitters, the most mind-bending chemicals nature has ever concocted. Neurotransmitters underlie every thought and emotion, memory and learning; they carry the signals between all the nerve cells, or neurons, in the brain. Among some 50 neurotransmitters discovered to date, a good half a dozen, including dopamine, are known to play a role in addiction.