A new study has found evidence in heavy users of marijuana of a compromised dopamine system.
According to researchers at Columbia University Medical Center, lower dopamine release was found in the striatum, a region of the brain involved in working memory, impulsive behavior, and attention.
Previous studies have shown that addiction to other drugs, such as cocaine and heroin, have similar effects on dopamine release. But such evidence for cannabis was missing until now, according to the researchers.
“In light of the more widespread acceptance and use of marijuana, especially by young people, we believe it is important to look more closely at the potentially addictive effects of cannabis on key regions of the brain,” said Anissa Abi-Dargham, M.D., a professor of psychiatry at Columbia University Medical Center and a lead author of the paper.
The study included 11 adults between the ages of 21 and 40 who were severely dependent on cannabis and a control group of 12 matched healthy adults.
On average, the cannabis group started using at age 16, became dependent by age 20, and have been dependent for the past seven years. In the month prior to the study, nearly all users in this study smoked marijuana daily, the researchers reported.
Using positron emission tomography (PET) to track a radio-labelled molecule that binds to dopamine receptors in the brain, the scientists measured dopamine release in the striatum and its subregions, as well as in several brain regions outside the striatum, including the thalamus, midbrain and globus pallidus.
The researchers note that the cannabis users stayed in the hospital for a week of abstinence to ensure that the PET scans were not measuring the acute effects of the drug.
Participants were scanned before and after being given an oral amphetamine to elicit dopamine release. The percent change in the binding of the radiotracer was taken as an indicator of capacity for dopamine release, the scientists explain.
Compared with the healthy group, the cannabis users had significantly lower dopamine release in the striatum, including subregions involved in associative and sensorimotor learning, and in the globus pallidus, the study found.
The researchers also explored the relationship between dopamine release in a key area of the striatum and cognitive performance on learning and working memory tasks.
Although there was no difference between groups in task performance, in all participants lower dopamine release was associated with worse performance on both tasks, the researchers report.
“We don’t know whether decreased dopamine was a preexisting condition or the result of heavy cannabis use,” said Abi-Dargham. “But the bottom line is that long-term, heavy cannabis use may impair the dopaminergic system, which could have a variety of negative effects on learning and behavior.”