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
