For some time, people have known that using cannabis during adolescence
increases the risk of developing cognitive impairment and mental illness (e.g.
depression, anxiety or schizophrenia) later in life. Importantly however, the
mechanisms responsible for this vulnerability are not well understood.
A new study, published in Brain,
shows that long-term cannabis use that starts during adolescence damages the
neural pathways connecting brain regions, and that this may cause the later
development of cognitive and emotional problems.
The authors used diffusion tensor
imaging (DTI), a MRI technique that measures water diffusion, to examine the
microstructure of white matter in 59 heavy cannabis users, who used cannabis at
least twice a month for three years or longer, as well as 33 non-users. In the
human brain, white matter pathways are formed by bundles of axons, which carry
the neural signals, and myelin, which coat the axons and speeds up signal
transfer. These white matter pathways are crucial for normal brain function as
they enable disparate regions of the brain to communicate, and act together.
When the authors investigated
white matter microstructure in the cannabis users, they found damage in the
white matter pathways of the hippocampus, crucial for memory, and the corpus
callosum, which connects the brain’s two hemispheres. Both pathways are
critical for normal brain function. The authors suggest that impaired
connectivity due to damage in these pathways may be the cause of the cognitive
impairment and vulnerability to schizophrenia, depression and anxiety seen in
long-term users.
The authors also show an inverse
relationship between the amount of white matter damage and the age of first
use. That is, participants who started using cannabis younger had more white
matter damage and showed poorer brain connectivity. Adolescence is a critical
period in the development of white matter in the brain, when the neural
connections we rely on in adulthood are being finally formed. The authors point
out that white matter cells have cannabinoid receptors (those susceptible to
cannabis) during adolescence, which disappear as the brain matures.
This new study demonstrates a
mechanism that may help explain how cannabis use in adolescence causes
long-term changes in brain function. The cannabis users in the study had
significantly higher levels of depression and anxiety compared to the
non-users.
This important new study suggests
that young people’s brains are at risk of white matter injury due to cannabis,
and that cannabis exposure during adolescence may permanently damage white
matter development. Future research must address the question; can white matter
pathways and connectivity recover when a person quits using cannabis?
References
Zalesky A, Solowij N, Yücel M,
Lubman DI, Takagi M, Harding IH, Lorenzetti V, Wang R, Searle K, Pantelis C,
& Seal M (2012). Effect of long-term cannabis use on axonal fibre
connectivity. Brain : a journal of neurology, 135 (Pt 7), 2245-55 PMID: 22669080
Image via Amihays / Shutterstock.
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