Sunday, April 17, 2016

Cannabis Addicts Have Reduced Dopamine Release in Brain

Summary: Various reported studies have shown that addiction to drugs of abuse such as cocaine and heroin, have serious effects on dopamine release in human brain, but such evidence for cannabis was missing until now. A latest publication in the journal Molecular Psychiatry provides 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.
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Study: The study was carried involving 11 adults between the ages of 21 and 40 who were severely dependent on cannabis (CD) and 12 matched healthy controls (HC). 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. Moreover, during the study period the heavy marijuana users stayed in the hospital for a week, during which they abstained from using it. This was to ensure that the experiments (scans) were not measuring the drug's effects.
Experimental Details: The researchers used positron emission tomography (PET) scans to track a radio-tracing molecule that binds to the brain's dopamine receptors. In scientific terminology, 11 CD and 12 HC completed two positron emission tomography scans with [11C]-(+)-PHNO, before and after oral administration of d-amphetamine. Magnetic resonance spectroscopy (MRS) measures of glutamate in the striatum and hippocampus were obtained in the same subjects. Percent change in [11C]-(+)-PHNO-binding potential (delta-BPND) was compared between groups and correlations with MRS glutamate, subclinical psychopathological and neurocognitive parameters were examined.
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.
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.
Results: 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.
On further 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.
Conclusions: 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. 
Article citation: Abi-Dargham, A.; et. al. Deficits in striatal dopamine release in cannabis dependence. Molecular Psychiatry 2016 | doi:10.1038/mp.2016.21