Even casual use of marijuana appears to cause significant structural changes in key brain structures of young adults, a new study finds. In a report in the April 16 Journal of Neuroscience, researchers from Massachusetts General Hospital (MGH) and Northwestern University describe finding differences between casual users of marijuana and non-users in the size, shape, and structure of brain regions involved with motivation, emotion and reward.
“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,” says Jodi Gilman, PhD, of the MGH Department of Psychiatry, lead author of the paper. “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, of Northwestern University Feinberg School of Medicine.
In these cross-sectional MR images, taken at different points within the brain, the red and yellow coloring indicates that grey matter density in the left nucleus accumbens – a region involved with reward and motivated behavior – is significantly greater in recreational marijuana users than in non-users. (Reprinted with permission: GILMAN, et al. The Journal of Neuroscience 2014.)
Previous studies in animals have found that THC – the principal psychoactive compound in marijuana – causes abnormal changes in cell structure within the nucleus accumbens, a brain region known to be involved in reward and addiction. Other studies 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 investigate 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 studies of participants’ brains, focusing on the amygdala and nucleus accumbens, were conducted at the Martinos Center for Biomedical Imaging at MGH. Results indicated that all three measures – volume, density and shape – of both structures were abnormal in marijuana users compared with non-using control participants. In addition, how the three structural measures related to each other was altered in marijuana users. All of these changes were even more pronounced in users who reported using marijuana more frequently during an average week and smoking more joints on those occasions.
In these MR brain images, the amygdala (top) and nucleus accumbens (bottom) are circled in red, and the yellow coloring indicates where the shapes of the structures are altered in recreational marijuana users. (Reprinted with permission: GILMAN, et al. The Journal of Neuroscience 2014.)
“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,” says Blood. “It also is possible that the brain is adapting to marijuana exposure and that these new connections may encourage further marijuana use.”
Breiter, who is also corresponding author of the Journal of Neuroscience report, adds, “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.”
Breiter is a professor of Psychiatry and Behavioral Science at Northwestern University and maintains leadership of the Laboratory of Neuroimaging and Genetics in MGH Psychiatry. Gilman is an MGH neuroscientist and an instructor in Psychiatry at Harvard Medical School (HMS). Blood is director of the Mood and Motor Control Laboratory in MGH Psychiatry and an assistant professor of Psychiatry at HMS. Additional co-authors are John Kuster and Nikos Makris, MD, MGH Psychiatry; Andre van der Kouwe, PhD, MGH Radiology; and Sang Lee, Myung Joo Lee, and Byoung Woo Kim, Northwestern University.
Support for the study includes National Institute on Drug Abuse grants 14118, 026002, 026104, 027804 and 034093; Office of National Drug Control Policy grants DABK39-03-0098 and DABK39-03-C-0098; and National Institute of Neurological Disorders and Stroke grant 052368.
Massachusetts General Hospital, founded in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH conducts the largest hospital-based research program in the United States, with an annual research budget of more than $775 million and major research centers in AIDS, cardiovascular research, cancer, computational and integrative biology, cutaneous biology, human genetics, medical imaging, neurodegenerative disorders, regenerative medicine, reproductive biology, systems biology, transplantation biology and photomedicine.