Drug abuse produces long-term changes in the reward circuitry of the brain. Knowledge of the cellular and molecular details of these adaptations could lead to new treatments for the compulsive behaviors that underlie addiction.
White lines on a mirror. A needle and spoon. For many users, the sight of a drug or its associated paraphernalia can elicit shudders of anticipatory pleasure. Then, with the fix, comes the real rush: the warmth, the clarity, the vision, the relief, the sensation of being at the center of the universe. For a brief period, everything feels right. But something happens after repeated exposure to drugs of abuse—whether heroin or cocaine, whiskey or speed.
The amount that once produced euphoria doesn’t work as well, and users come to need a shot or a snort just to feel normal; without it, they become depressed and, often, physically ill. Then they begin to use the drug compulsively. At this point, they are addicted, losing control over their use and suffering powerful cravings even after the thrill is gone and their habit begins to harm their health, finances and personal relationships.
Neurobiologists have long known that the euphoria induced by drugs of abuse arises because all these chemicals ultimately boost the activity of the brain’s reward system: a complex circuit of nerve cells, or neurons, that evolved to make us feel flush after eating or sex—things we need to
do to survive and pass along our genes. At least initially, goosing this system makes us feel good and encourages us to repeat whatever activity brought us such pleasure. But new research indicates that chronic drug use induces changes in the structure and function of
the system’s neurons that last for weeks, months or years after the last fix. These adaptations, perversely, dampen the pleasurable effects of a chronically abused substance yet also increase the cravings that trap the addict in a destructive spiral of escalating use and increased fallout at work and at home. Improved understanding of these neural alterations should help provide better interventions for addiction, so that people who have fallen prey to habit-forming drugs can reclaim their brains and their lives.