Alcohol : Effects on Memory

by National Institute of Health

(Page 2 of 6)

During adolescence, the dopamine system in the striatum appears to undergo substantial changes. For example, studies in rats have found that dopamine receptor levels in the striatum increase during early adolescence but then decrease during late adolescence and young adulthood. At the same time, dopamine receptor levels in the nucleus accumbens increase dramatically.

GABA is the primary inhibitory neurotransmitter in the brain - that is, it represses the activity of other brain cells. Alcohol generally enhances the effects of GABA on its receptors. This enhanced GABA activation may play a role in mediating the sedative effects of alcohol and other sedating agents. In addition, alcohol's effects on GABA and its receptors are thought to contribute to the development of tolerance to and dependence on alcohol. Like dopamine, the GABA system changes substantially during adolescence. Studies in rats have found that the number of GABA receptors, and thus the activity of the GABA system, increases markedly in several brain structures during early adolescence.

In addition to these two neurotransmitter systems, a system using the neurotransmitter glutamate also appears to undergo changes during adolescence. Glutamate interacts with several receptors, including one called the NMDA receptor. Evidence from animal studies indicates that the NMDA receptor complex changes during postnatal development, and these changes may continue into adolescence.

Although it is beyond the scope of this article to review the changes occurring in various brain structures and neurotransmitter systems in more detail, this brief description demonstrates that adolescence is a period of profound alterations in brain function. Therefore, it is reasonable to expect that alcohol's effects on the brain and behavior may differ for adolescents and adults. The following sections will review some of the differences in sensitivity to alcohol that have been identified using animal models.

Adolescents Are More Sensitive than Adults
to Alcohol's Memory-Impairing Effects

Alcohol's Effects on Memory

Among its many effects on the brain and brain function - such as impairing balance, motor coordination, and decisionmaking - alcohol interferes with the drinker's ability to form memories (i.e., it is an amnestic agent). However, alcohol does not impair all types of memory equally. Alcohol disrupts a person's ability to form new, lasting memories to a far greater extent than it interferes with the ability to recall previously established memories or to hold new information in memory for just a few seconds. One study conducted with young adults ages 21 to 29 found that intoxicated study participants could recall items on word lists immediately after the lists were presented, but they had greater difficulty recalling the information 20 minutes later. Interestingly, this effect was much more powerful in the younger subjects in this age group - that is, people in their early twenties. In addition, alcohol particularly affects the ability to form explicit memories - that is, memories of facts (e.g., names and phone numbers) or events (e.g., what the drinker did the previous night). Because different brain areas play a role in the formation of different types of memories, this pattern of alcohol-related memory impairment allows researchers to make assumptions about the brain regions that are most affected by alcohol. Thus, the pattern of memory impairment observed after intoxication is similar to that found in patients with damage to a brain area called the hippocampus.

Alcohol and the Hippocampus

The hippocampus is located deep under the brain's surface but is extensively connected with the outer layer of the brain. It consists of only a few layers of cells arranged in a characteristic shape with several bends and folds. The primary cells in the hippocampus are called pyramidal cells because of their shape. The hippocampus can be divided into several areas, and studies in humans have found that in some patients with an inability to form new explicit memories, brain damage was limited to a single region of hippocampal neurons called the CA1 region. In rodents, the activity of CA1 cells correlates strikingly with behavior: Each CA1 neuron tends to emit signals primarily when the animal is in a specific area of its environment. For example, cell A may be active predominantly when the animal is in the northeast corner of its cage, whereas cell B may become active when the animal enters the southwest corner of the cage. As a result, these cells can play a very strong and specific role in spatial learning (e.g., the ability to learn the path through a maze or the location of a certain item, such as a food reward).

Researchers have used this characteristic of the CA1 cells to assess the effects of alcohol exposure and other interventions on hippocampal cell activity in intact, living rodents. In one study, electrodes were implanted in the hippocampus of rats that then were able to move freely around their cages. After the animals were administered alcohol, the activities of their CA1 cells were measured. This study found that the activity of the CA1 cells was reduced when alcohol levels reached at least 0.5 grams per kilogram of body weight and ceased almost completely at higher alcohol doses. This finding is consistent with the hypothesis that alcohol can interfere with the formation of new explicit memories by disrupting hippocampal function.

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.