In a new study, a group of international researchers investigated whether adolescent cannabis use has any effect on cortical thickness in the brains of teenagers.

Very few studies have looked at the link between cannabis use in adolescents and its impact on neurodevelopment, and although studies in animal models have pointed towards altered neurodevelopment with lasting behavioral effects with early cannabis use, studies in humans have provided conflicting results.

Now, reporting in the journal JAMA Psychiatry, researchers show that cannabis use during youth might be associated with changes in neurodevelopment – specifically, the thinning of cortical regions rich in cannabinoid 1 receptors (CB1 receptors).

In the study, the researchers used longitudinal neuroimaging and behavioral data (self-reported) from the IMAGEN study data set and investigated 2,223 school children from 8 different European regions who were all approximately 14 years of age.

Out of that cohort, the authors identified 799 participants (450 Female and 349 Male) that had not used cannabis at a baseline of roughly 14 years. They were followed up after a five-year period to determine changes in their cannabis use habits during adolescence while having Magnetic Resonance Imaging (MRI) data of their brains available. 

Accounting for various factors such as age, sex, alcohol use, and lifetime cannabis use, the brain imaging data showed that cannabis had a dose-dependent effect on age-related cortical thinning of certain brain regions in teenagers that regularly used it over the five year period. Those that used more cannabis during this period had more cortical thinning in these regions, according to the MRI data. 

The researchers argue that the baseline cortical thickness was not associated with lifetime cannabis use, which suggests the thinning they observed was not associated with any pre-existing differences in anatomical brain structure, but rather was a result of their cannabis use during this development stage during adolescence.

Furthermore, the authors pointed out that regions that had cortical thinning were also regions that are normally rich in CB1 receptors, one of the receptors that form part of the endocannabinoid system, and to which both THC and CBD – the two main compounds in cannabis – can bind. 

It is worth mentioning that the study does come with some caveats. Although it might be one of the largest longitudinal brain imaging studies of adolescent cannabis use to date, the findings are still reliant on self-reporting to determine things like the amount of substance used. The researchers also had no information on the type and quality of cannabis that was used, making it difficult to generalize the findings. 

It is also worth pointing out that because of ethical limitations that involve positron emission tomography (PET) scanning in minors due to certain risks that are involved, the authors could not perform PET scans on the participants and had to use a different data set to quantify the levels of CB1 receptors in the cortical regions of interest and correspond that to the thinning regions in the 799 adolescent participants. So, therefore, they can't say for certain that those areas that showed cortical thinning in the study participants were rich in CB1 receptors, but only that those areas, in general, are rich in CB1 receptors.

Nevertheless, more brain imaging studies will have to be done to further ascertain what impact early cannabis use might have on the developing human brain, and how thinning of cortical regions might result in behavioral changes later in life.

"We report evidence of an association between adolescent cannabis use and altered cortical thickness development in a longitudinal sample of youths." the authors wrote in their write-up. "The findings underscore the importance of further longitudinal studies of adolescent cannabis use, particularly given increasing trends in the legalization of recreational cannabis use."