Marijuana Use in Adolescents May Lead to Long-term Cognitive Impairment

Adolescents, but not adults, who regularly use marijuana may permanently impair their brain function and cognition and may also be at an increased risk of developing schizophrenia, according to new preclinical research by the University of Maryland School of Medicine.

Asaf Keller, a professor of anatomy and neurobiology at UM and lead study author, says that the topic of whether marijuana use in adolescents can cause long-term heath effects is controversial, but that the latest study provides further evidence that regular marijuana use in adolescents can be harmful.

"Previous research has shown that children who started using marijuana before the age of 16 are at greater risk of permanent cognitive deficits, and have a significantly higher incidence of psychiatric disorders such as schizophrenia," Keller said. "There likely is a genetic susceptibility, and then you add marijuana during adolescence and it becomes the trigger."

The study began by examining cortical oscillations in mice. Cortical oscillations are patterns of the activity of neurons in the brain and are believed to underlie the brain's various functions. These oscillations are very abnormal in schizophrenia and in other psychiatric disorders.

Humans were not involved in the study. Instead, Keller and his colleagues used adolescent mice that had never been exposed to tetrahydrocannabinol, the main psychoactive compound in marijuana, subjecting the mice to low doses of the active compound for 20 days before reintroducing them to their sibling mice and allowing them to develop normally.

"In the adult mice exposed to marijuana ingredients in adolescence, we found that cortical oscillations were grossly altered, and they exhibited impaired cognitive abilities," said Sylvina Mullins Raver, a graduate researcher involved in the study. "We also found impaired cognitive behavioral performance in those mice. The striking finding is that, even though the mice were exposed to very low drug doses, and only for a brief period during adolescence, their brain abnormalities persisted into adulthood."

The researchers repeated the experiment with adult mice that had never been exposed to marijuana before and found that their cortical oscillations and ability to perform cognitive behavioral tasks remained normal, which indicated that marijuana only impairs cognition when exposed to during adolescence.

"We looked at the different regions of the brain," Keller said. "The back of the brain develops first, and the frontal parts of the brain develop during adolescence. We found that the frontal cortex is much more affected by the drugs during adolescence. This is the area of the brain controls executive functions such as planning and impulse control. It is also the area most affected in schizophrenia."

Keller said the purpose of their study of cortical oscillations was to see whether it's possible to reverse the effects marijuana has on adolescent brains.

"We are hoping we will learn more about schizophrenia and other psychiatric disorders, which are complicated conditions. These cognitive symptoms are not affected by medication, but they might be affected by controlling these cortical oscillations."

The research is published in the journal Neuropsychopharmacology.