A study by Gilman, et al. in the April 2014 edition of The Journal of Neuroscience found that marijuana exposure, even in young recreational users is associated with exposure-dependent alterations of core reward structures in the human brain.
The investigators noted that while marijuana use is associated with the impairment of cognitive functions such as learning, memory, attention and decision-making, it is still the most widely used illicit drug on college campuses and, partly due to changing social and legal perspectives about the drug, use is also increasing among adolescents. Animal studies show changes in the brain structures that underlie impaired cognitive functions and this study aimed to determine whether similar alterations occur in the human brain, even after casual use of marijuana.
Twenty young adults (age 18–25 years) who were current marijuana users and 20 control subjects participated in this research. Participants were matched with respect to age, sex (9 males and 11 females in each group), handedness, race, and years of education. Marijuana participants used marijuana at least once a week, but were not dependent, according to results obtained from administration of the Structured Clinical Interview for the DSM-IV (SCID).
Marijuana users completed were asked about their use of the drug over the past 90 days, including the days they used, how many separate times per day they used and how many joints they consumed per each smoking occasion. All participants in the study supplied similar information concerning their alcohol use.
The researchers performed high-resolution MRI scans on young adult recreational marijuana users and nonusing controls and analyses of the scans “revealed greater gray matter density in marijuana users than in control participants in the left nucleus accumbens extending to subcallosal cortex, hypothalamus, sublenticular extended amygdala, and left amygdala, even after controlling for age, sex, alcohol use, and cigarette smoking”. As the authors point out, these discoveries are consistent with animal research that shows structural changes to key reward structures after exposure to Δ9-tetrahydrocannabinol (THC), the main psychoactive component of cannabis.
Read more about addiction and the family in Dr. Wood’s books: Children of Alcoholism: The Struggle for Self and Intimacy in Adult Life and Raising Healthy Children in an Alcoholic Home