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DURHAM, N.C. -- The main active compound in marijuana leads to greater short-term impairment of learning in adolescent rats than in adults, according to researchers at Duke University Medical Center. The study directly compared the effects of delta9-tetrahydrocannabinol (THC) on the memory ability of adolescent and adult rats in order to learn if rats – and possibly humans – are more sensitive to THC at certain life stages.

Their results also suggest that chronic exposure to THC during adolescence may not lead to long-term damage that persists into adulthood, the researchers said. But this finding should be interpreted cautiously, as other studies have suggested that THC's damage may be longer-lasting.

"Just because we did not see lasting effects of chronic exposure does not mean effects on learning aren't present," said the study's lead author, Young May Cha, an undergraduate who completed her bachelor's degree at Amherst College in 2004 and now works as a researcher in the department of psychiatry and behavioral sciences at Duke University Medical Center and the Durham Veterans Affairs Medical Center. "There is the possibility that users under the influence could be suffering at least temporary memory damage."

The team's findings appear in the March 2006 issue of the journal Pharmacology, Biochemistry and Behavior, which is being published on June 8. The research was funded by the National Institutes of Health and through two Veterans Affairs Senior Research Career Scientist Awards.

The researchers assessed the effects of both acute and chronic THC exposure on memory and learning by dosing rats with varying amounts of the compound, training them to navigate their way through a "water maze" and then measuring how well the rats remember how to navigate themselves to a stationary platform in the water.

In all, they tested 192 male rats, half of which were adolescent and half adults, in a series of experiments that measured the distance the animals swam before finding the platform. When given acute doses of THC, adolescent rats were not able to remember where to swim as well as the adult rats. For example, adolescent rats that received a 2.5 milligram per kilogram (mg/kg) dose of THC in solution swam 64 percent more than adult rats given the same amount of THC. Adolescent rats that were given 10 mg/kg of THC in solution swam 47 percent more than adults provided with the same amount of THC. Adolescent and adult rats that were given a "control" solution that did not contain THC performed equally as well on the series of tasks.

In the series of tests designed to examine the effects of chronic THC exposure, the researchers gave 20 adult rats and 20 adolescent rats either a 5 mg/kg dose of THC or a control solution once a day for three weeks. Roughly a month later, after the THC had cleared the rats' systems and the adolescents had reached full maturity, the researchers tested how well they performed in the water maze.

The chronic exposure appeared to have no effect on the rats' later learning abilities, the researchers said.

"Even though this study was done in rats, we can relate the effects of THC in rats to what would be true for humans," Cha said. "This is the case particularly with the spatial learning tasks that rely heavily on the hippocampus. In humans, the hippocampus is very important in memory and the brains of humans and rats are a lot more similar, evolutionarily speaking, than one might think. To an extent, learning can transcend species."

Indeed, the researchers noted that much remains to be learned about many aspects of how THC affects both the adolescent and the adult brain. For example, acute exposure to THC is known to affect the hippocampus, a primitive structure located within the brain's temporal lobe, which is closely linked to cognition in humans. But cannabinoid receptors -- specific sites to which THC binds -- are located throughout the brain, which means that other regions are likely to be affected by THC but have yet to be identified, they added.

Still, even though much remains to be learned, current evidence already makes one thing clear, according to the scientists.

"There are plenty of good reasons for adolescents to not smoke pot," said H. Scott Swartzwelder, Ph.D., professor of psychiatry at Duke and the Durham Veterans Affairs Medical Center and senior author on the study. "The teen years require a lot of learning and preparation for adulthood.

"Kids need to be aware that they risk having their learning impaired, at least for the short term, which could pose problems with their education and socialization," he said. "Our findings give parents, counselors and educators sound reasons -- based on science, not scare tactics -- they can use when talking with kids about the importance of not using marijuana."

Because adults and adolescents have markedly distinct brain differences, it is important to make age-related comparisons rather than simply to look at the overall effect of a drug on one or the other group, the researchers noted. Comparing the two groups also provides a better sense of whether the brain is more sensitive to certain drugs at certain developmental stages than others, they said.

Other researchers involved in the study include Aaron White, Cynthia Kuhn and Wilkie A. Wilson of Duke University Medical Center and the Durham Veterans Affairs Medical Center.