Alcohol Effects on Brain : Adolescents Are Less Sensitive

by National Institute of Health

(Page 4 of 6)

Researchers also have investigated the mechanisms underlying age-related differences in sensitivity to alcohol's effects on memory. These analyses have demonstrated that alcohol-induced inhibition of LTP and of NMDA receptor-mediated activity were greater in brain slices from adolescent rats than in brain slices from adult rats. For example, in studies using hippocampal slices taken from adolescent and adult rats, repeated stimulation in the absence of alcohol induced LTP in samples taken from both age groups. In fact, in the absence of alcohol, the LTP was more pronounced in adolescent than in adult brain tissue. When alcohol was added, however, LTP induction was reduced substantially or almost completely blocked in the adolescent tissue, whereas it took much higher alcohol concentrations to inhibit the LTP process in tissue from adults.

Similar experiments compared the activity of the glutamate/NMDA system in response to stimulation in the presence or absence of alcohol in hippocampal brain slices from adolescent and adult rats. It took significantly higher concentrations of alcohol to reduce NMDA receptor activity in the adult brain slices, compared with those taken from adolescent animals.

All of these studies confirm the heightened susceptibility of the adolescent rodent brain to alcohol-induced inhibition of hippocampal function and memory formation.

Adolescents Are Less Sensitive Than
Adults to Other Alcohol Effects

As the preceding section has shown, adolescent animals are uniquely sensitive to some of alcohol's effects on memory. However, adolescents seem less sensitive than adults to other effects of drinking, such as impairment of motor coordination, sedation, and susceptibility to seizures during withdrawal.

Motor Coordination

One of the most obvious effects of alcohol consumption in humans as well as laboratory animals is the impairment of motor activity and coordination. Alcohol interferes with a person's ability to perform tasks that require balance and motor coordination, such as standing still, walking in a straight line, or driving an automobile. In animals, alcohol's effects on motor coordination can be demonstrated using the tilting plane test, in which an animal is placed on a horizontal platform that is gradually tilted, so that the animal must adjust its position to maintain its balance.

Motor coordination is one of the primary functions of the cerebellum, an area at the base of the brain. Because the cerebellum continues to develop during adolescence, it is reasonable to assume that alcohol might affect motor coordination in adolescents differently than in adults. To investigate this possibility, White and colleagues analyzed the motor coordination of adolescent and adult rats using the tilting plane test before, and at various time points after, administering alcohol at three different doses (1.0, 2.0, and 3.0 g/kg body weight). These researchers found that the lowest alcohol dose did not affect the animals' performance in either age group. At almost all time points after the administration of the two higher doses, however, the adult animals were more impaired than the adolescent animals. These findings clearly demonstrate that, in contrast to alcohol's effects on memory, adolescent rats appear to be less sensitive to alcohol's effects on motor coordination than adult rats. It is not clear precisely why the adolescent animals were less sensitive to alcohol-induced motor impairment. It is clear, however, that the cerebellum, which plays a critical role in motor coordination, still is developing quite rapidly during adolescence. If the cerebellum is less sensitive to alcohol during this period, this could account for the developmental difference in sensitivity to alcohol. Currently, it is not known if this difference in sensitivity also applies to human adolescents.

Sedation

Another common effect of alcohol consumption that can be observed both in humans and in animals is sedation. With increasing alcohol consumption, drinkers tend to become sleepy and eventually may even pass out. In laboratory animals, sedation can be assessed by observing the righting reflex that normally helps the animals get back on all four feet if they fall over. When treated with sedative agents, animals temporarily lose this righting reflex, and the duration of this loss is a measure of the sedative potency of an agent.

To better characterize alcohol's effects on the developing organism, researchers also have evaluated alcohol's sedative effects in rats of different ages. Little and colleagues injected animals from three age groups - juvenile animals, adolescent animals, and adult animals - with three different alcohol doses, and found the following:

When treated with the lowest alcohol dose, none of the adolescent animals lost their righting reflex, whereas one-half of the juvenile rats and two-thirds of the adult rats did.

Adolescent animals lost the righting reflex for a significantly shorter period of time than adult animals. When they regained the reflex, adolescent animals also had significantly higher blood alcohol concentrations than the adult animals had when they regained the righting reflex.

The juvenile animals also lost the righting reflex for a significantly shorter time than the adult animals, although not as short as the adolescent animals.

These observations demonstrate that, as with alcohol's motor-impairing effects, adolescent animals are substantially less sensitive to alcohol's sedative effects. Mechanisms that may contribute to this lower sensitivity are discussed in the following section.

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.