A research team comprised of investigators from Columbia University Medical Center and the New York State Psychiatric Institute in New York City has found that ” heavy, long-term use of cannabis is associated with negative changes in parts of the brain not previously implicated”, according to an article posted on Medscape on June 8, 2015.
The research team used a radiotracer that is highly sensitive to dopamine D3 release to image the impact of cannabis on the brain. They found that heavy use was associated with lower dopamine release in areas of the brain involved in cognition. (Dopamine is, as the Medscape article points out, “a key chemical messenger for motivation and attention”. Previous studies have demonstrated an adverse impact on dopamine transmission in reward regions of the brain.)
This was a small study in which 11 heavy cannabis users were compared with 12 healthy control subjects. However, the results were dramatic. The subjects underwent an initial PET scan after an inpatient abstinence period of 5-7 days. They were scanned again 3 hours after an oral administration of amphetamine. Cannabis users had significantly less overall dopamine release after amphetamine administration and also significantly lower dopamine release in the associative striatum, the sensory motor striatal subregions and the pallidum. The investigators noted that “exploratory analysis suggests that the deficits we are seeing in dopamine release in the striatum have a functional significance — that lower dopamine release is associated with lower working memory and learning performance”. They also observed that the study points to “a potential mechanism by which cannabis may be contributing to the risk for psychosis or the severity of psychosis, specifically by interfering with dopamine transmission in one key brain region involved in psychosis — the head of the caudate,” added Dr Abi-Dargham, of Columbia University Medical Center. “If this is repeatedly occurring at a young age, it could result in abnormal salience and could have offsite effects on the circuitry”.