Brain change: two very different ways.

Teens who experience real thrills often feel satisfied afterwards.

But in the search for a kick, some teens make the dangerous turn to drugs. People who use drugs may feel depressed and crave more drugs. To understand why, you have to look inside the brain.

During an actual thrilling experience, brain cells, or neurons, create feelings by sending and receiving chemicals called neurotransmitters.

Teens who use drugs might experience a rush of feelings, too, but in their brains something very different is happening. Once in the brain, drugs of abuse are similar in size shape to brain chemicals or neurotransmitters. Because of that, drugs can create fake messages. They may also cause floods of neurotransmitters. Most drugs of abuse act on the brain's emotional center, so these fake messages and floods create intense feelings. That's how drugs, in a sense, trick the brain.

Neurons respond to the floods of neurotransmitters by altering their chemical message system. Now, the brain doesn't have enough, or can't use, its owe natural chemicals to create normal feelings. The drug user feels flat and even depressed. "Kids who are using drugs may not find as much pleasure in hanging out with their friends, or swimming on a nine day," says Dr. Spear. "So much of their behavior is directed toward finding more drugs." When drug users search out drugs, even when they know it's wrong, they are addicted. Addiction is a brain disease; the brain is changed and does not function normally. The way out of addiction is through the hard work of treatment and therapy.

In these brain scans, receptors that use the natural brain chemical dopamine show up as red. The cocaine-addicted brain has fewer dopamine receptors than the healthy brain.