Researchers from the University of Maryland in US found that exposure to marijuana during the critical period of adolescence, but not adulthood, impaired cognition.
"Over the past 20 years, there has been a major controversy about the long-term effects of marijuana, with some evidence that use in adolescence could be damaging," said the study's senior author Asaf Keller, from the University of Maryland School of Medicine.
"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.
"There likely is a genetic susceptibility, and then you add marijuana during adolescence and it becomes the trigger," Keller said.
The scientists examined 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.
They exposed young mice to very low doses of the active ingredient in marijuana for 20 days, and then allowed them to return to their siblings and develop normally.
"Adolescence is the critical period during which marijuana use can be damaging. We wanted to identify the biological underpinnings and determine whether there is a real, permanent health risk to marijuana use," said the study's lead author, Sylvina Mullins Raver.
"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 Raver.
"We also found impaired cognitive behavioural 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," Raver said.
Researchers repeated the experiment, this time administering marijuana ingredients to adult mice that had never been exposed to the drug before.
Their cortical oscillations and ability to perform cognitive behavioural tasks remained normal, indicating that it was only drug exposure during the critical period of adolescence that impaired cognition through this mechanism.
Researchers looked at different regions of the brain and 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.
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