Cannabis and the Adolescent Brain
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?
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