Neuroimaging Shows Brain Structures Altered with Casual Marijuana Use

Researchers found differences between casual users of marijuana and non-users in the size, shape, and structure of brain areas involved with motivation, emotion, and reward.

The researchers, from Massachusetts General Hospital (MGH; Boston, MA, USA) and Northwestern University (Evanston, IL, USA), published their findings April 16, 2014, in the Journal of Neuroscience. “Most studies associating brain abnormalities with marijuana have been conducted in heavy, habitual users, so this is the first to find abnormalities in recreational users,” stated Jodi Gilman, PhD, of the MGH department of psychiatry, lead author of the study. “We also found these abnormalities to be dose dependent—in other words, they were more pronounced in those who used greater amounts of marijuana.” Co-senior authors of the report are Anne Blood, PhD, of the MGH department of psychiatry, and Hans Breiter, MD, from Northwestern University Feinberg School of Medicine.

Earlier research in animals have found that THC (Tetrahydrocannabinol) causes abnormal changes in cell structure within the nucleus accumbens, a brain region known to be involved in reward and addiction. Other research in heavy marijuana users have found alterations in the volume or size of the amygdala, an area of the brain with a role in emotional processing, and in the density of cells within that structure. The current study was designed to explore whether similar abnormalities occurred in young adult recreational users and whether the amount of marijuana used made any difference.

The study enrolled 40 young adults ages 18 to 25, recruited from Boston-area colleges: 20 who used marijuana at least once a week and 20 who did not use the drug. All users participated in psychiatric interviews to ensure they did not meet criteria for dependence, which include factors such as whether marijuana use interfered with their studies, work or other activities and whether they had needed to increase their usage to get the same effects.

Structural MRI scans of participants’ brains, focusing on the amygdala and nucleus accumbens, were conducted at the Martinos Center for Biomedical Imaging at MGH. Findings indicated that all three measures—volume, density, and shape—of both structures were abnormal in marijuana users compared with non-using control participants. Furthermore, how the three structural measures related to each other was changed in marijuana users. All of these alterations were even more pronounced in users who reported using marijuana more frequently during an average week and smoking more marijuana cigarettes on those instances.

“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,” said Dr. Blood. “It also is possible that the brain is adapting to marijuana exposure and that these new connections may encourage further marijuana use.”

Dr. Breiter, who is also corresponding author of the report, added, “These two brain regions have been broadly implicated in processes underlying addiction, so it’s a real problem that people claiming their marijuana use does not negatively impact their lives show significant changes in these structures. Our findings, which need to be followed up with longer-terms studies, raise serious concerns about efforts to legalize recreational marijuana use, particularly for young adults.”

Related Links:
Massachusetts General Hospital 
Northwestern University