The same effect is seen with drugs like heroin and cocaine.
Researchers in the Department of Psychiatry at Columbia University have revealed some new findings about the effects of heavy cannabis use — chronic marijuana users have compromised dopamine systems.
Previous research has shown that addiction to other drugs like cocaine and heroin have similar effects on dopamine release, but until now, the evidence for cannabis was missing.
"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 lead author Anissa Abi-Dargham, professor of psychiatry at Columbia University Medical Center (CUMC), in a press statement.
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The results, which have been published in the journal Molecular Psychiatry, show that heavy cannabis users showed a reduced level of dopamine release in the striatum — a brain region that regulates memory, attention, and impulsive behavior.
The study sample, which was small in size, involved 11 adults between the ages of 21 and 40 who were severely dependent on cannabis, and 12 healthy controls. On average, the cannabis smokers started using at age 16 and became dependent on the drug by age 20, and all had been dependent on weed for the past 7 years. In the month leading up to the study, nearly all of them smoked marijuana daily.
The researchers used positron emission tomography (PET) to track a specially labeled molecule that binds to dopamine receptors in the brain. The scientists measured dopamine release in the striatum and its subregions, as well as a number of brain regions outside of the striatum, including the thalamus, midbrain, and globus pallidus.
To ensure that the PET scans were not measuring the acute effects of cannabis use, the cannabis users stayed in the hospital for a week of abstinence. All of the participants were scanned before and after they were given an oral amphetamine to elicit dopamine release.
In comparison to the non-smoking controls, the cannabis smokers had significantly lower dopamine release in the striatum and some of its subregions involving associative and sensorimotor learning. Dopamine release was also reduced in the globus pallidus, which is a brain structure that’s part of the basal ganglia and regulates voluntary movement.
Additionally, the researchers investigated the relationship between dopamine release in the striatum and cognitive performance on learning and memory tasks. They report that lower dopamine release was associated with worse performance on both tasks.
"We don't know whether decreased dopamine was a preexisting condition or the result of heavy cannabis use," said Dr. 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."
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