Senior mice treated with THC improved on learning and memory tests
Picture the stereotypical pot smoker: young, dazed and confused. Marijuana has long been known for its psychoactive effects, which can include cognitive impairment. But new research published this week in Nature Medicine suggests the drug might affect older users very differently than young ones—at least in mice. Instead of impairing learning and memory as it does in young people, the drug appears to reverse age-related declines in the cognitive performance of elderly mice.
Researchers led by Andreas Zimmer of the University of Bonn in Germany gave low doses of delta 9-tetrahydrocannabinol, or THC, marijuana’s main active ingredient, to young, mature and aged mice. As expected, young mice treated with THC performed slightly worse on behavioral tests of memory and learning. For example, after THC young mice took longer to learn where a safe platform was hidden in a water maze, and they had a harder time recognizing another mouse to which they had previously been exposed. Without the drug, mature and aged mice performed worse on the tests than young ones did. But after receiving THC the elderly animals’ performances improved to the point that they resembled those of young, untreated mice. “The effects were very robust, very profound,” Zimmer says.
Other experts praised the study but cautioned against extrapolating the findings to humans. “This well-designed set of experiments shows that chronic THC pretreatment appears to restore a significant level of diminished cognitive performance in older mice, while corroborating the opposite effect among young mice,” Susan Weiss, director of the Division of Extramural Research at the National Institute on Drug Abuse who was not involved in the study, wrote in an e-mail. Nevertheless, she added, “While it would be tempting to presume the relevance of these findings [extends] to aging humans…further research will be critically needed.”
When the researchers examined the brains of the treated, elderly mice for an explanation, they noticed neurons in the hippocampus—a brain area critical for learning and memory—had sprouted more synaptic spines, the points of contact for communication between neurons. Even more striking, the gene expression pattern in the hippocampi of THC-treated aged mice was radically different from that of untreated elderly mice. “That is something we absolutely did not expect: the old animals [that received] THC looked most similar to the young, untreated control mice,” Zimmer says.