Addiction and the Brain

How do drugs change the brain and lead to addiction?

Synapses make the brain work by transferring messages from cell to cell, doing the conscious work of thinking and the less conscious work of producing feelings and emotions. Can you guess what would happen if a chemical changed the way those synapses worked? If you guessed that such chemicals might change behavior, you'd be right.

FIGURE 31–9 Drugs and Society The damage to the brain is only the start of the damage that drugs cause. For example, alcohol abuse costs the United States about $185 billion a year in health care costs, treatment services, property damage, and lost productivity.

Nearly every addictive substance, including illegal drugs such as heroin, methamphetamine, and cocaine, and legal drugs, such as tobacco and alcohol, affect brain synapses. Although the chemistry of each drug is different, they all produce changes in one particular group of synapses. These synapses use the neurotransmitter dopamine and are associated with the brain's pleasure and reward centers.

When we engage in an activity that brings us pleasure, whether it's eating a tasty snack or being praised by a friend, neurons in the hypothalamus and the limbic system release dopamine. Dopamine molecules stimulate other neurons across these synapses, producing the sensation of pleasure and a feeling of wellbeing.

Addictive drugs act on dopamine synapses in a number of ways. Methamphetamine releases a flood of dopamine, producing an instant “high.” Cocaine keeps dopamine in the synaptic region longer, intensifying pleasure and suppressing pain. Drugs made from opium poppies, like heroin, stimulate receptors elsewhere in the brain that lead to dopamine release. Nicotine, the addictive substance in tobacco, and alcohol, the most widely abused drug in the United States, also cause increased release of dopamine.

The brain reacts to excessive dopamine levels by reducing the number of receptors for the neurotransmitter. As a result, normal activities no longer produce the sensations of pleasure they once did. Addicts feel depressed and sick without their drugs. Because there are fewer receptors, larger amounts of tobacco, alcohol, and illegal drugs are required to produce the same high. The result is a deeper and deeper spiral of addiction that is difficult to break.

31.2 Assessment

Review Key Concepts

1. 1. Review What are the three major regions of the brain?

   2. Describe Explain the role of the spinal cord.

   3. Infer How do reflexes protect the body from injury?

2. 1. Review Describe three ways that drugs affect synapses that use the neurotransmitter dopamine.

   2. Apply Concepts Why do many drug users begin to take more and more of the drug they abuse?

Apply the Big idea

Homeostasis

3. Explain the brain's role in homeostasis in regard to the body as a whole. How does homeostasis within the brain differ from the rest of the body? How must it be similar?

Table of Contents