The researchers, who publish their work in the journal Molecular Psychiatry, say that heavy cocaine and heroin use have been shown to decrease dopamine release, but until now, such data regarding cannabis was notably absent.
Lead author Dr. Anissa Abi-Dargham, of the Columbia University Medical Center (CUMC) in New York, says the recent “widespread acceptance and use of marijuana” makes looking into the effects of cannabis on the brain extremely important.
She and her team explain that most drug addictions blunt dopamine release during the chronic phase of drug dependence, which results in poor outcomes.
To further investigate whether marijuana dependence is linked with similar effects, the researchers conducted their study in 11 adults aged 21-40 years who were heavily dependent on marijuana, and they matched them with 12 healthy controls.
The adults in the marijuana group started using it at around age 16 on average, became dependent by age 20 and had been dependent for around 7 years.
Nearly all users in the study smoked marijuana daily in the month leading up to the study, the researcher say.
‘Heavy use may negatively affect learning and behavior’
The researchers used positron emission tomography (PET) scans to track a radiotracing molecule that binds to the brain’s dopamine receptors. From this, they were able to measure the release of dopamine in the striatum, which is a brain region involved in memory, impulsive behavior and attention.
Additionally, the team was able to track dopamine release in other brain regions, including the thalamus, midbrain and globus pallidus.
During the study, the marijuana users stayed in the hospital for a week, during which they abstained from using it. This was to ensure that the scans were not measuring the drug’s effects.
Both before and after being given oral amphetamine to draw out dopamine release, the participants’ brains were scanned. The researchers used the percent change in the binding of the radiotracer as a sign of dopamine release capacity.
Results showed that, compared with the control group, the marijuana users’ striatum had lower dopamine release. There was also lower release in subregions that play a role in associative and sensorimotor learning, as well as in the globus pallidus.
Upon investigating the link between dopamine release in the striatum and cognitive performance on learning and working memory tasks, the researchers did not observe a difference in performance between the two groups.
However, they do note that among all participants, those who had lower dopamine release performed worse on both tasks.
Commenting on the findings, Dr. Abi-Dargham says:
”We don’t know whether decreased dopamine was a preexisting condition or the result of heavy cannabis use. 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.”
The researchers conclude their study by noting that the lower dopamine release is linked with inattention and negative symptoms in marijuana users, and with “poorer working memory and probabilistic category learning performance” in both groups.