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DURHAM, N.C. -- A growing body of evidence on alcohol's effects shows that just one drink can impair learning and memory in both young animals and young humans, but has no memory effect on adults, according to researchers from Duke University Medical Center and the Durham VA Medical Center.

The investigators said their research offers the first scientific evidence that alcohol has a markedly different effect depending on the age of the drinker. In addition, they said their studies provide the first hard evidence to support the ban on under-age drinking, which up until now has been based on moral, political or religious reasons.

"Historically, there has been no compelling reason to deter the youth of America from drinking, other than a moral or authoritarian message," said neuropsychologist Scott Swartzwelder, lead investigator of two new studies being published in April. "At least now we can back our message with scientific evidence showing that even occasional and moderate drinking could impair a young person's memory systems much more than an adult's."

Swartzwelder said the memory loss persisted as long as the subject was under the influence of alcohol, and that none of the information presented during that time was memorized. The long-term effects of chronic drinking are not known. According to the new research, young animals respond differently to alcohol in three ways:

They suffer memory and learning impairments from as little as one drink, yet adults do not.

They develop more rapid tolerance to the drug than adults -- an incentive to drink more to get the same high.

They experience less sedation from the drug, meaning they can drink far more than adults before falling asleep. This puts adolescents at greater risk for a variety of dangerous outcomes, from memory and learning impairments to drunk-driving and impulsive sexual behavior, the researchers said.

The research is funded by the Alcoholic Beverage Medical Research Foundation, the National Institute on Alcoholism and Alcohol Abuse, and the U.S. Dept. of Veterans Affairs.

In one new study, to be published in the April issue of Alcoholism: Clinical and Experimental Research, Swartzwelder showed that just a single dose of alcohol prevented adolescent rats from learning how to swim to a platform in a water-filled maze, yet adult rats given the same dose easily learned and remembered the task. The amount of alcohol was not enough to sedate the rats or even affect their swimming abilities -- in the range of .08 percent blood alcohol level -- but it strongly impaired learning and memory in the adolescent rats.

This finding, which supports President Clinton's recent initiatives aimed at lowering the legal blood alcohol level in all states to .08 percent for drunk driving, was confirmed in preliminary human studies reported by Swartzwelder's team last year at the Research Society on Alcoholism meeting.

In that study, younger people given alcohol had a harder time recognizing words from a list read to them 20 minutes earlier, compared with older subjects who received an equivalent dose. While alcohol decreased the performance of all subjects, who ranged in age from 21 to 30, there was a strong correlation between their ages and their ability to learn and recognize the words after a dose of alcohol. Those under 25 performed markedly worse than those over the age of 30, he said.

"Quite simply, the younger the age, the worse they performed on the memory tests when given the equivalent of two drinks," he said. "If alcohol's effects varied that much within such a narrow age range, then there's a compelling reason to believe its effects are even stronger in adolescents and children." Swartzwelder said obvious ethical and legal constraints have prevented him from studying alcohol's effects in people younger than 21, although he plans to continue his research in animals and, whenever ethically appropriate, in humans.

In his second new study, to be published in the April issue of the journal Alcohol, Swartzwelder showed that young rats developed tolerance to alcohol's temperature-lowering effects much more rapidly than adult animals. At first, all the animals given alcohol experienced a significant drop in body temperature -- an effect that occurs consistently among people and animals. But after several doses at two-day intervals, the adolescent animals had lost far less body temperature than the adults, indicating they had developed tolerance to one of alcohol's most basic effects.

The Alcohol study also showed that adolescents develop more rapid tolerance to the sedative effects of alcohol, indicating that the differences between adolescents and adults extend to multiple brain regions that control a variety of different functions. In two 1995 rat studies, Swartzwelder showed precisely how alcohol impairs memory: it blocks the action of a specific nerve receptor in the hippocampus, the brain's center for learning and memory. When hippocampal "NMDA" receptors are inhibited, they cannot receive electrical signals from other nerve cells, thereby preventing the acquisition and storage of new information. Those findings were published in the May and December 1995 issues of Alcoholism: Clinical and Experimental Research.

"Young brains are built to learn," Swartzwelder said. "They have more NMDA receptors than adult brains, and their receptors are formulated with a different balance of proteins. This could account for why young brains experience such a dramatic decrease in memory-related activity when they're exposed to low doses of alcohol."