Con: Marijuana Alters Brain Structure, Could Change Behavior
In February 2013, Dr. Sharon Levy, MD, MPH, who is currently the Assistant Professor of Pediatrics at Harvard Medical School, addressed an open letter to the Massachusetts Department of Public Health (DPH) in which she expressed a concern that “marijuana does cause changes in the same areas of the brain as other addictive drugs (in particular the nucleus accumbens).”
The nucleus accumbens, sometimes abbreviated to NAc or NAcc, is a group of neurons (nerve cells) clustered in the basal forebrain, which is located near the front and bottom of the brain. The basal forebrain is responsible for producing a chemical called acetylcholine, which impacts neuroplasticity (the ability to form new neural connections) and therefore learning abilities. The nucleus accumbens is what people are talking about when they describe the brain’s “reward center” (i.e. increased dopamine levels). Contrary to popular belief, dopamine levels rise in response to both positive and negative experiences, as indicated by this 2013 study published in the Journal of Neuroscience.
Dr. Levy’s statement appears to have been confirmed by a 2014 medical study – also published in the Journal of Neuroscience – whose title neatly sums up its message: “Cannabis Use Is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users.” (It should be noted that the study examined teens and young adults in the 18- to 25-year-old age range – not children.) The study used a sample size of 20 participants, all of whom reported using Cannabis at least once a week, “but were not dependent.” The study compared MRI scans from the participants against a control group.
The study found that “gray matter density analyses revealed greater gray matter density in marijuana users than in control participants in the left nucleus accumbens… even after controlling for age, sex, alcohol use, and cigarette smoking.” Additionally, the study noted that “significant shape differences were detected in the left nucleus accumbens and right amygdala,” which deals with emotional responses.
The study summarized that “the results… indicate that in young, recreational marijuana users, structural abnormalities in gray matter density, volume, and shape of the nucleus accumbens and amygdala can be observed.”
In a response to the Neuroscience study, Dr. Anne Blood, PhD, from the Massachusetts General Hospital’s Department of Psychiatry, stated that “these abnormal structural changes in the amygdala and nucleus accumbens could indicate that the experience with marijuana alters brain organization and may produce changes in function and behavior. It also is possible that the brain is adapting to marijuana exposure and that these new connections may encourage further marijuana use.”
13