Alcohol's Effects on the Adolescent Brain

by National Institute of Health

Because of legal and ethical constraints on alcohol research in human adolescents, many studies of alcohol's effects on the developing brain have been conducted in animal models, primarily rats and mice. The adolescent brain may be uniquely sensitive to alcohol's effects because major changes in brain structure and function occur during this developmental period. For example, adolescent animals are more sensitive than adults to the effects on memory and learning that result from alcohol's actions on the hippocampus. Conversely, adolescent animals appear to be less sensitive than adults to alcohol-related motor impairment, alcohol-induced sedation, and the development of seizures during withdrawal. Alcohol exposure during adolescence can have long-lasting effects and may interfere with normal brain functioning during adulthood.

Adolescence and young adulthood are developmental stages of transition during which humans, as well as members of many other species, mature physically and behaviorally into their adult state. Adolescents and young adults need to acquire the physical and behavioral skills that will allow them to live independently of their parents, sustain themselves, and reproduce. This period is marked by more frequent and sophisticated social interactions with peers, exploration of new situations and behaviors, and an increased willingness to take risks. In humans, this often involves the initiation of alcohol and other drug use.

At the same time, the brain undergoes considerable structural and functional changes, at least in part in response to the individual's many new experiences. Connections among nerve cells (neurons) in the brain can change based on which neurons or groups of neurons are regularly stimulated, a characteristic known as plasticity. This natural process serves to eliminate unnecessary or unused nerve cell connections, allowing the survival of only those neurons that make meaningful contacts with other neurons. (Human infants are born with far more neurons than are found in the adult brain. Based on a child's interactions with the environment, the neurons and connections that are most meaningful can be selected.) This winnowing of neurons is influenced by, among other factors, the adolescent's interactions with and experiences in the outside world.

Adolescence is such a critical phase in brain development that the actions of alcohol and other drugs on the brain can be assumed to have a particularly profound impact during this developmental period. Indeed, research has shown that compared with the adult brain, the adolescent brain is particularly sensitive to some effects of alcohol, yet more resistant to other effects. Much of this research, especially investigations of specific effects of acute alcohol administration, has been conducted in animals because studies involving administration of alcohol to human adolescents are subject to very stringent regulations, and certain studies of alcohol's effects on the adolescent brain can be conducted only using animal models. This article reviews some of the differences in alcohol's effects on the adolescent and adult brain that were identified using these animal models. The accompanying article by Tapert and colleagues summarizes information that has been obtained in studies of human adolescents and young adults.

Major Changes in Brain Structure and
Function during Adolescence

Adolescence in humans is broadly defined as the second decade of life, although some researchers consider ages up to 25 years as "late adolescence." The corresponding period in laboratory animals that are frequently used as study subjects is just as loosely defined. In rats it typically spans postnatal days 30-50. In both humans and animal models, adolescence is a period when the brain undergoes extensive remodeling. New connections among neurons are being formed; at the same time, a substantial number of existing connections are lost. It is hypothesized that this plasticity allows the individual's brain to be sculpted based on his or her personal experiences and interactions with the outside world.

One brain region where particularly extensive remodeling occurs is the frontal region of the outer layer of the brain - the prefrontal cortex - which is thought to be involved in working memory, voluntary motor behavior, impulse control, rule learning, spatial learning, planning, and decisionmaking. These changes are especially extensive in humans. Although the number of neurons and neuronal connections in the prefrontal cortex appear to decline during adolescence, the relative importance of the frontal lobes increases.

Developmental changes in the behavioral relevance of certain brain areas are accompanied by increases or decreases in the activities of chemicals called neurotransmitters, which help transmit nerve signals from one neuron to another. This signaling takes place when neurotransmitters released by one neuron bind to protein molecules called receptors on the surface of the receiving neuron. The interaction between the neurotransmitter and its receptor initiates chemical and electrical changes in the signal-receiving neuron that influence the generation of a new nerve signal in that cell. In this way, nerve cells and circuits communicate and drive behavior. Excitatory neurotransmitters promote the generation of new nerve signals, whereas inhibitory neurotransmitters make it more difficult to generate a nerve signal in a signal-receiving neuron. Numerous neurotransmitters and their receptors have been identified that act on specific cells or groups of cells and have specific effects on those cells.

Two important neurotransmitter systems that undergo substantial changes during adolescence and are affected by alcohol consumption are dopamine and gamma-aminobutyric acid. Dopamine can have both excitatory and inhibitory effects, depending on the cells it acts on. Dopamine-releasing and dopamine-receiving cells are found in numerous brain areas. One prominent region, which lies deep within the brain, is called the striatum. It consists of several components that are involved in behaviors such as learning to automatically execute complex movements triggered by a voluntary command. Another dopamine-using area is the nucleus accumbens, which plays a role in learning and performing certain behaviors in response to incentive stimuli. Activity in the nucleus accumbens in part accounts for the fact that people perceive the effects of drinking alcohol or taking other drugs as pleasurable.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.