A new study has found that male teens who use cannabis (marijuana), particularly those who have genetic risk factors for schizophrenia, have decreased cortical thickness of their brain. The cortex is the outer area of the brain and its thickness is associated with cognitive ability (intelligence). The findings were published online on August 26 in the journal JAMA Psychiatry by an international team of researchers.
The study authors note that cannabis use during adolescence is known to increase the risk for schizophrenia in men. Sex differences in the process of brain maturation during the teens may be of particular importance in regard to vulnerability of the male brain to cannabis exposure. Therefore, they conducted a study to assess whether the association between cannabis use and cortical maturation in adolescents is affected by a polygenic risk score for schizophrenia. (A polygenic risk score is a measurement of genetic risk factors for schizophrenia.
For the study, the researchers reviewed data from three studies of teens of both genders: 1,024 adolescents of both genders from the Canadian Saguenay Youth Study (SYS); follow-up in 426 adolescents of both genders from the IMAGEN Study from eight European cities; and 504 male teens from the Avon Longitudinal Study of Parents and Children (ALSPAC) based in England. The study group comprised a total of 1,577 adolescents (aged 12-21 years (899; 57.0% male).Information was available on: (1) cannabis use; (2) imaging studies of the brain; and (3) a polygenic risk score for schizophrenia across 108 genetic locations identified by the Psychiatric Genomics Consortium. Data analysis was conducted from March 1 through December 31, 2014.
The main outcome measurement of the study was cortical thickness derived from magnetic resonance images (MRIs). Statistical analysis was used to determine the relationships between cannabis use, cortical thickness, and risk score.
The investigators found that among the three groups, a negative association was observed between cannabis use in early teens and cortical thickness in male participants with a high polygenic risk score, meaning that cannabis use was associated with a thinner brain cortex in these teens. This association was not found for low-risk male adolescents or for the low- or high-risk female participants. In SYS male teens, cannabis use interacted with risk score in regard to cortical thickness; higher scores were associated with lower thickness only in males who used cannabis. Similarly, in the IMAGEN male teens, cannabis use interacted with increased risk score in regard to a decrease in cortical thickness from 14.5 to 18.5 years of age. In the ALSPAC high-risk group of male adolescents, those who used cannabis most frequently (61 or more occasions) had lower cortical thickness than those who never used cannabis (difference in cortical thickness and those with light use (less than 5 occasions).
The authors concluded that cannabis use in the early tens influenced the association between the genetic risk for schizophrenia and cortical maturation among males. Therefore, this finding associates processes underlying cortical maturation in affecting the link between cannabis use and liability to schizophrenia.