- Researchers followed 799 adolescents for 5 years and studied their brain MRIs at a mean age of 14 and agsin at a mean age of 19.
- After 5 years of using cannabis, adolescents' brain structure was altered; the more cannabis they used, the more change there was.
- Researchers also found that adolescents had more attentional impulsiveness after 5 years of cannabis use.
Does the use of cannabis early in life influence adolescents’ brain maturation, and if it does, is cannabis beneficial or detrimental for adolescents’ brain development?
Matthew Albaugh of the University of Vermont College of Medicine and colleagues from 8 different European sites studied the brains of 799 adolescents (450 females and 349 males) over five years, from a mean age of 14 to a mean age of 19.
The researchers performed 1,598 MRIs (Magnetic Resonance Imaging) studies, half before any of the subjects used cannabis and the other half five years later, after the subjects had used cannabis.
Albaugh and colleagues found that cannabis use was associated with decreased thickness of the cerebral cortex, especially in the prefrontal regions (located at the extreme front of the brain just above the eyes, they are particularly rich in cannabinoid receptors type 1). And, heavier cannabis use over five years was associated with faster and more pronounced thinning of the prefrontal cortex after five years. There didn’t seem to be any statistical difference between males and females.
Having established that the brains of adolescents changed with cannabis use, the researchers next wanted to know if these changes correlated with any cognitive or attention problems.
The researchers found that cortical thinning associated with cannabis use, when it was in the right prefrontal cortex, was associated with attentional impulsiveness, which means that the adolescents who used most cannabis had less ability to focus their attention.
Indeed, in previous studies, the prefrontal cortex has been described as responsible for high executive functions, including receiving inputs from other brain regions, paying attention to those inputs, synthesizing data then planning and executing tasks while preventing irrelevant distractions from interfering.
So, it would make sense that thinning of the prefrontal cortex could lead to less focused attention.
But what could be happening with cannabis at the cellular level?
Albaugh and colleagues hypothesize that cannabis use in adolescents could induce a premature shortening of neuronal dendrites (branch-like structures growing out of neuronal cell bodies that receive most of the inputs a typical neuron receives from other neurons). Such premature dendritic shortening, also called pruning, has been described by Miller and colleagues from Columbia University when they exposed male adolescent rats to THC (tetrahydrocannabinol), the main active ingredient of cannabis.
If cannabis does indeed prune extensions of neurons in adolescents, what will happen as more and more states legalize recreational cannabis?
The researchers cite that, already in 2013, 78% of first-time cannabis users were between 12 and 20 years of age (results from the 2013 National Survey on Drug Use and Health) and that in 2017, more than one-third of 12th graders in the US reported using cannabis (World Drug Report 2018). Given those numbers, the study's authors worry that, with increased cannabis legalization, more young adolescents will use cannabis in the future while their brain isn’t fully mature.
Albaugh and colleagues conclude that it is essential to do more longitudinal studies to follow changes in adolescents’ brain MRIs exposed to cannabis over several years to assess the potential damage of the drug in young brains.