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Think about how you feel when something good happens—maybe your team wins a game or you're praised for something you've done well—that's your limbic system at work. Because natural pleasures in our lives are necessary for survival, the limbic system creates an appetite that drives you to seek out those things.
The first time someone uses a drug of abuse, he or she experiences unnaturally intense feelings of pleasure. The reward circuitry is activated—with dopamine carrying the message. Of course, drugs have other effects, too; a first-time smoker also may cough and feel nauseated from toxic chemicals in a tobacco or marijuana cigarette. But the brain starts changing as a result of the unnatural flood of neurotransmitters. Because they sense more than enough dopamine, neurons may begin to reduce the number of dopamine receptors or simply make less dopamine.
The result is less dopamine signaling in the brain, what the scientists call “down regulation.” Because some drugs are toxic, some neurons also may die. As a result, dopamine’s ability to activate circuits to cause pleasure is severely weakened. The person feels flat, lifeless, and depressed. In fact, without drugs, life may seem joyless. Now the person needs drugs just to bring dopamine levels up to normal. Larger amounts of the drug are needed to create a dopamine flood, or “high”—an effect known as “tolerance.”
These brain changes drive a person to seek out and use drugs compulsively, despite negative consequences such as stealing, losing friends, family problems, or other physical or mental problems brought on by drug abuse—this is addiction. Although we know what happens to the brain when someone becomes addicted, we can’t predict how many times a person must use a drug before becoming addicted. A person's genetic makeup, the genes that make each of us who we are, and the environment each play a role. What we do know is that a person who uses drugs risks becoming addicted, craving the drug despite its potentially devastating consequences.