Chronic Cannabis Use May Cause Brain Inflammation

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Chronic marijuana use may cause inflammation in the brain that leads to problems with coordination and learning, a new study in animals suggests.

The study also teased out why this brain inflammation leads to motor and learning problems, and found a surprising answer — cannabis activates immune cells that appear to play a critical role in how a brain region called the cerebellum works.

In the study, mice were given delta9-tetrahydrocannabinol, or THC, the active ingredient of marijuana, for six days. Then the mice were required to perform several tasks to test their coordination, as well as their ability to learn to associate a sound with a puff of air to the eye. During the latter task, called eyeblink conditioning, mice should learn to anticipate the air puff, and blink when they hear the sound.

Mice given THC showed impairments in both tasks. (A previous study in people also found that cannabis users had trouble in an eyeblink conditioning task.)

The researchers also found that THC activated microglial cells, which are immune cells in the cerebellum. These microglial cells then induced inflammation in the cerebellum, which resulted in the learning and coordination problems.

These problems went away when the researchers used a drug to prevent activation of microglial cells. This result could offer "interesting new therapeutic approaches for treating these cannabis-induced side effects, the researchers write in the July 1 issue of the Journal of Clinical Investigation..

Study researcher Andrés Ozaita, of the Universitat Pompeu Fabra in Barcelona, noted that these side effects typically go away when the mice are no longer exposed to THC.

Chronic use of marijuana is known to lower the number of cannabinoid receptors (receptors that bind to THC) in the brain. The reduced number of receptors is what appears to eventually lead to the activating of the microglial cells, Ozaita said.

More research is needed to determine what kinds of problems these "cerebellar deficits" cause people to have in everyday life, Ozaita said. It's possible that microglial cell activation in the cerebellum causes hard-to-detect problems — such as a slightly delayed reaction time while driving — that researchers wouldn't know about unless they tested for them, he said.

In addition to brain inflammation, smoking cannabis has also been linked to lower IQ and an increased risk of schizophrenia.

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Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.