People who regularly use marijuana show significant abnormalities in areas of the brain associated with motivation and emotion, a team of Swiss researchers has found. The scientists discovered changes in the volume of grey matter — the tissue containing brain cells — in regular marijuana users compared with occasional ones.
Marijuana, the most commonly used illicit drug in the United States, is associated with various cognitive impairments. Animal studies have uncovered structural changes in brain regions rich in cannabinoid CB1 receptors, but little is known about how the drug affects the structure of the human brain.
In their study of 47 marijuana users, which was published in
Neuropsychopharmacology, the researchers used Magnetic Resonance Imaging (MRI) technology to compare the brains of regular and occasional users. Regular users smoked a joint at least 10 times a month, while occasional users smoked at least one joint per month but not more than one joint per week.
“Brain structure changes were investigated in a group of regular cannabis smokers and compared with a group of occasional smokers enrolled in our previous functional study,” the researchers wrote. “The subjects in the two groups did not use any drug other than cannabis and were free from psychiatric disorders. We then stratified the two groups according to the age of first cannabis use in order to assess the effect of cannabis on the developing brain.”
In some brain regions, the regular users showed decreased gray matter compared to occasional users, while in other regions the regular users showed increased gray matter.
The researchers found lower gray matter volume in regular marijuana users in the medial temporal cortex, temporal pole, parahippocampal gyrus, left insula, and orbitofrontal cortex. These brain regions are associated with decision making, emotion, and motivation.
Changes to the insula “have also been confirmed in alcohol addiction where the decrease in insular activation seems to reflect an inability to switch from interoceptive cravings to cognitive control for suppressing internal needs,” the researchers explained.
The researchers found higher gray matter volume in regular marijuana users in the cerebellum — the brain’s motor control center. The finding suggests that marijuana use could impair the normal “pruning back” of nerve cells in the cerebellum during adolescence and early adulthood.
“One possible reason for abnormal pruning could be the toxic effect of THC at a critical period of brain maturation,” the researchers explained. “Exogenous cannabinoids might disturb this system by competing for the receptors, thus inhibiting the pruning particularly in receptor-rich areas like the cerebellum or the prefrontal cortex.”
A similar brain imaging study, published in The Journal of Neuroscience, found the nucleus accumbens of marijuana users was abnormally large compared to non-users.
Some studies have found the brain abnormalities associated with marijuana use return to normal after periods of abstinence. The Swiss scientists called for longitudinal studies to better understand these brain changes.
“The design of our study cannot address whether the structural alterations observed are permanent or reversible,” they wrote.