Crossing the Line to Addiction: How and When Does It Happen?

Compared to a normal brain, the brain of a cocaine abuser shows reduced metabolic activity, shown in these PET scans in the warmer colors.
"No one becomes addicted the first time they try a drug," says George Koob, M.D., a professor in the neuropharmacology department at The Scripps Research Institute in La Jolla, California. Although there are some cases where a person's reaction to first use is so positive that they immediately begin to abuse a drug, Koob says most addiction has a subtler start. It usually doesn't take place until the person has been using chronically. The person has become an addict when his or her brain has literally been changed by this chronic use of the drug.
Many substances and activities, from food to sex, exert control over human behavior by motivating us to indulge in them. But addictive drugs, such as alcohol, nicotine, cocaine, and heroin, can affect the structure and function of the brain -- and hence our motivations -- in long-lasting ways. They can actually alter and "usurp," in one scientist's term, the "circuits" in the brain that are involved in the control of emotions and motivation, impairing an addicted person's will. "What addiction really is, is a result of brain changes that over time get translated into behavior changes," says National Institute on Drug Abuse (NIDA) director Alan Leshner, Ph.D.
If a person uses drugs, at a high enough dose, frequently enough and for a long period of time, these drugs change the way the brain works. "You change the way nerve cells communicate in such a way that you develop this compulsive, out-of-control use despite knowing that all kinds of terrible things can happen to you, and despite even experiencing many of those things," says National Institute of Mental Health director Steven Hyman, M.D.
Studies using new technologies show the precise effects of drugs on the brain. "In many cases, we can actually see changes in the structure of synapses and in the shapes of [brain] cells," says Hyman. A NIDA study released in 1996 provided the first direct evidence that chronic use of opiates (such as morphine and heroin) is linked with structural changes in the size and shape of specific neurons. Researchers at the Yale University School of Medicine found that rats chronically given morphine experienced marked structural changes in critical brain "circuits." Other NIDA studies have shown that altered brain circuits could be responsible for the major differences in brain functioning between an occasional cocaine user and a cocaine addict.
-- Janet Firshein