With more than 25% of high school seniors reporting recent use and 6.5% of 12th graders being daily users, marijuana (MJ) is the most frequently used illicit substance among adolescents. Across all age groups over 70% of new drug initiates start with using MJ at an average age of 18 years.
MJ use in early adolescence is associated with increased risk of greater substance use, legal problems, disrupting education, developing mental illness and cognitive changes. However, despite its high prevalence, the impact of MJ use on adolescent brain development is not fully known. Now, a study from researchers at The University of Texas at Dallas shows that the age at which an adolescent begins using marijuana may affect typical brain development. The team describe how marijuana use, and the age at which use is initiated, may adversely alter brain structures that underlie higher order thinking. The opensource study is published in the journal Developmental Cognitive Neuroscience.
Previous structural neuroimaging studies have indicated that volumes of several brain areas are smaller in heavy adult MJ users especially in areas enriched with cannabinoid 1 receptors, such as medial temporal lobe, and prefrontal cortex. Among different characteristics of MJ involvement, the age of initial MJ use is a factor that has been associated with smaller brain volumes in users. Studies comparing early adolescent MJ use to users initiating MJ use in later adolescence provide further evidence for the potential of MJ to cause enduring change. Functional and behavioural differences were reported between early (before age 16) and late (after age 16) MJ users, with decreased white matter integrity neuroimaged in early onset vs. late onset MJ users. These studies highlight the importance of clarifying the neural effects of early- and late-adolescent onset use.
The current study analyzed MRI scans of 42 heavy marijuana users; twenty participants were categorized as early onset users with a mean age of 13.18 and 22 were labeled as late onset users with a mean age of 16.9. According to self-reports, all participants, ages 21-50, began using marijuana during adolescence and continued throughout adulthood, using cannabis at least one time per week. Results show that participants who began using marijuana at the age of 16 or younger demonstrated brain variations that indicate arrested brain development in the prefrontal cortex, the part of the brain responsible for judgment, reasoning and complex thinking. Data findings show that individuals who started using marijuana after age 16 showed the opposite effect and demonstrated signs of accelerated brain aging.
The team explain that in typical adolescent brain development, the brain prunes neurons, which results in reduced cortical thickness and greater gray and white matter contrast. They go on to add that typical pruning also leads to increased gyrification, which is the addition of wrinkles or folds on the brain’s surface. However, their MRI results reveal that the more marijuana early onset users consumed, the greater their cortical thickness, the less gray and white matter contrast, and the less intriate the gyrification, as compared to late onset users.
The lab state that these three indexes indicate that when participants began using marijuana before age 16, the extent of brain alteration was directly proportionate to the number of weekly marijuana use in years and grams consumed. The findings show that those who began using marijuana after age 16 showed brain change that would normally manifest later in life, namely, thinner cortical thickness, stronger gray and white matter contrast. The group stress that a longitudinal study would be necessary to establish a causal relationship between brain alterations and marijuana use.
The team surmise their findings verify that the amount of marijuana used and the amount of times MJ is used strongly relates to the degree to which brain development does not follow the normal pruning pattern. They go on to add that the effects observed are in-line with the current literature that suggest that cannabis use during adolescence can have long-term consequences. For the future, the researchers state their studies will explore cognitive and behavioural changes associated with structural brain change and consider the different patterns of development within the adolescent period and how these patterns could lead to non-linear effects.
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