Formation of habitual use drives cannabis addiction

Individual differences in brain systems for habitual behavior distinguish heavy cannabis users who develop an addiction

June 13, 2019

Science Daily/Elsevier

A shift from brain systems controlling reward-driven use to habit-driven use differentiates heavy cannabis users who are addicted to the drug from users who aren't, according to a study in Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, published by Elsevier. The findings help explain how the brain becomes dependent on cannabis, and why not all cannabis users develop an addiction, even with long-term regular use.

In the study, researchers at the University of Electronic Science and Technology of China and University of Bonn, Germany, used brain imaging to monitor neural activity when cannabis users viewed images associated with cannabis use, referred to as drug cues. Although all of the cannabis users in the study reported heavy use, only some were dependent on the drug. Both dependent and non-dependent cannabis users had exaggerated responses in a brain region that processes reward -- the ventral striatum -- compared with people who didn't use cannabis. Interestingly, the dependent users also had larger responses in a brain region that forms habits -- the dorsal striatum.

"The present findings reflect that heavy cannabis use is promoted by changes in the brain's reward system -- however, these changes alone may not fully explain addictive use. Addictive use may rather be driven by changes in brain systems that promote habitual -- automatic -- use, which also may explain the fact that addicts continue use despite a lack of experiencing rewarding effects of the drug. As such, their behavior has become under the control of the drug cues, rather than the actual reward expectation," said lead author Benjamin Becker, PhD.

Dependent users also had increased responses in other regions throughout the brain, including regions that attribute importance to things, for example, drug cues. This suggests that the development of cannabis addiction incorporates additional brain regions that may strengthen a person's desire to seek the drug.

"Cannabis is now legal for medical and recreational use in many parts of the United States and the health impacts of this development are still being understood," said Cameron Carter, MD, Editor of Biological Psychiatry: Cognitive Neuroscience and Neuroimaging. "These findings are important insights that can help us better understand why some individuals might be more likely to become addicted to cannabis," he added.

Differentiating the unique brain circuits behind dependent cannabis use could also be useful for understanding how to combat the problem of cannabis addiction. "The identification of the dorsal striatum and habitual behavior as a driver of addiction may allow the development of more specific treatment approaches to increase treatment success," said first author Xinqi Zhou.

https://www.sciencedaily.com/releases/2019/06/190613095213.htm

Heavy cannabis use associated with reduced dopamine release in brain

Effect similar to other addictions

April 14, 2016

Science Daily/Columbia University Medical Center

In a recent study, researchers found evidence of a compromised dopamine system in heavy users of marijuana. Lower dopamine release was found in the striatum -- a region of the brain that is involved in working memory, impulsive behavior, and attention. Previous studies have shown that addiction to other drugs of abuse, such as cocaine and heroin, have similar effects on dopamine release, but such evidence for cannabis was missing until now.

"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, MD, professor of psychiatry (in radiology) at Columbia University Medical Center (CUMC) 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 12 matched healthy controls. On average, the cannabis group started using at age 16, became dependent on cannabis by age 20, and have been dependent for the past 7 years. In the month prior to the study, nearly all users in this study smoked marijuana daily.

Using positron emission tomography (PET) to track a radiolabelled 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 cannabis users in this study 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 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.

Compared with the controls, 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 investigators 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.

"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."

Jeffrey Lieberman, MD, Chair of Psychiatry at CUMC and past president of the American Psychiatric Association, noted that "these findings add to the growing body of research demonstrating the potentially adverse effects of cannabis, particularly in youth, at the same time that government policies and laws are increasing access and use."

https://www.sciencedaily.com/releases/2016/04/160414214826.htm