Regular use of cannabis over many years lowers levels of dopamine, a key chemical in the brain’s reward center, says an international team of researchers led by Imperial College London’s Professor Oliver Howes.
Cannabis use in humans is associated with reduced dopamine in the striatum. PET studies have shown lower striatal dopamine synthesis and release capacity in cannabis users. Lower dopamine synthesis capacity in the dorsal striatum is directly associated with reduced motivation levels, whereas reduced dopamine release in the ventral striatum is directly associated with negative emotion levels and severity of addiction. Image credit: Michael A. P. Bloomfield et al, doi: 10.1038/nature20153.
Cannabis is a widely used recreational drug. Over half of young Americans have used the drug, whereas in Europe, cannabis has now overtaken heroin as the most widely reported illegal drug used amongst people entering specialist addiction services.
This provides the background to political debates worldwide concerning changes to the legal status of the drug.
Although causality has not been conclusively demonstrated, heavy cannabis use is associated with increased risk of mental disorders including psychosis, addiction, depression, suicidality, cognitive impairment and amotivation.
According to scientists, there are no effective treatments and this is a pressing concern for global mental health.
“There is now conclusive evidence that cannabis blunts dopamine levels, and that it’s critical to understand exactly how this happens and what it means for the health of users,” Prof. Howes and co-authors said.
In a review of the state of current research, they examined the action of dopamine through detailed analysis of multiple studies involving brain scans of long-term cannabis users.
“We urgently need to better understand how cannabis affects the brain, to help policy makers and individuals make informed decisions,” Prof. Howes said.
“If it turns out that it’s bad news to take cannabis then we need to know now, before people take the gamble.”
“The dopamine system is central to learning and motivation. We conclude that long-term cannabis use in people is linked to blunting of the dopamine system.”
“In this respect cannabis is similar to cocaine or amphetamine, although probably not to quite the same degree.”
“The changing patterns of cannabis use, including ‘cannavaping’ and edible products, mean it’s vital that we understand the long-term effects of cannabis on the brain,” added co-author Dr. Michael Bloomfield, from University College London and Imperial College London.
This research, according to Dr. Bloomfield, helps to explain how some people get addicted to cannabis, by showing that tetrahydrocannabinol, the main psychoactive component of cannabis, alters a delicate balance of brain chemicals.
Tetrahydrocannabinol is known to alter dopamine levels, and has been linked to the harmful effects of cannabis, including addiction and psychosis. The concentration of tetrahydrocannabinol in cannabis is increasing as growers select plants that induce the strongest ‘high’, and some new synthetic drugs mimic its action.
If researchers can work out how tetrahydrocannabinol changes dopamine levels, they may be able to develop ways to treat or even prevent cannabis-induced illness.
Such treatments may prove particularly important in those whose brains are still developing, such as adolescents, and are therefore most vulnerable. The brains of infants in the womb may also be at risk.
The findings were published in the Nov. 17 issue of the journal Nature.
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Michael A. P. Bloomfield et al. 2016. The effects of Δ9-tetrahydrocannabinol on the dopamine system. Nature 539: 369-377; doi: 10.1038/nature20153
