The addicted brain

Drug addiction has been a stubborn problem for thousands of years, but only in the last generation have scientists come to understand clearly one of the reasons: It causes lasting changes in brain function that are difficult to reverse. That means many altered brains — nearly 2 million heroin and cocaine addicts, perhaps 15 million alcoholics, and tens of millions of cigarette smokers in the United States alone. No simple solution is in sight, but we know much more than we did 20 or even 5 years ago about how the brain responds to addictive drugs, and that knowledge is beginning to affect treatment and prevention.

Why does the brain prefer opium to broccoli?

The question of addiction has been put that way by Steven Hyman, a former director of the National Institute of Mental Health. The answer involves the nucleus accumbens, a cluster of nerve cells that lies beneath the cerebral hemispheres. When a human being or other animal performs an action that satisfies a need or fulfills a desire, the neurotransmitter dopamine is released into the nucleus accumbens and produces pleasure. It serves as a signal that the action promotes survival or reproduction, directly or indirectly. The system is called the reward pathway. When we do something that provides this reward, the brain records the experience and we are likely to do it again. Damage to the nucleus accumbens and drugs that block dopamine release in the region make everything less rewarding.
In nature, rewards usually come only with effort and after a delay. Addictive drugs provide a shortcut. Each in its own way sets in motion a biological process that results in flooding the nucleus accumbens with dopamine. The pleasure is not serving survival or reproduction, and evolution has not provided our brains with an easy way to withstand the onslaught. In a person who becomes addicted through repeated use of a drug, overwhelmed receptor cells call for a shutdown. The natural capacity to produce dopamine in the reward system is reduced, while the need persists and the drug seems to be the only way to fulfill it. The brain is losing its access to other, less immediate and powerful sources of reward. Addicts may require constantly higher doses and a quicker passage into the brain. It's as though the normal machinery of motivation is no longer functioning; they want the drug even when it no longer gives pleasure.

The brain's reward system

Addictive drugs provide a shortcut to the brain's reward system by flooding the nucleus accumbens with dopamine. The hippocampus lays down memories of this rapid sense of satisfaction, and the amygdala creates a conditioned response to certain stimuli. Stressors or something associated with substance use can trip the mental machinery of relapse.