ADDICTED: WHY DO PEOPLE GET HOOKED?

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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.

The neurons that produce this molecular messenger are surprisingly rare. Clustered in loose knots buried deep in the brain, they number a few tens of thousands of nerve cells out of an estimated total of 100 billion. But through long, wire-like projections known as axons, these cells influence neurological activity in many regions, including the nucleus accumbens, the primitive structure that is one of the brain's key pleasure centers. At a purely chemical level, every experience humans find enjoyable--whether listening to music, embracing a lover or savoring chocolate--amounts to little more than an explosion of dopamine in the nucleus accumbens, as exhilarating and ephemeral as a firecracker.

Dopamine, like most biologically important molecules, must be kept within strict bounds. Too little dopamine in certain areas of the brain triggers the tremors and paralysis of Parkinson's disease. Too much causes the hallucinations and bizarre thoughts of schizophrenia. A breakthrough in addiction research came in 1975, when psychologists Roy Wise and Robert Yokel at Concordia University in Montreal reported on the remarkable behavior of some drug-addicted rats. One day the animals were placidly dispensing cocaine and amphetamines to themselves by pressing a lever attached to their cages. The next they were angrily banging at the lever like someone trying to summon a stalled elevator. The reason? The scientists had injected the rats with a drug that blocked the action of dopamine.