June 8th 2015
"In light of a current trend toward legalizing marijuana, with potentially increased exposure of adolescents, we believe our findings are important to consider," said investigator Jodi Weinstein, MD, from the Columbia University Medical Center and the New York State Psychiatric Institute in New York City.
If used daily, cannabis "can be as bad as other drugs in terms of consequences," said senior investigator Anissa Abi-Dargham, MD, also from the Columbia University Medical Center and the New York State Psychiatric Institute.
"People often think of cannabis as a lighter, harmless drug. This study shows that it is not and that it has negative consequences," she told Medscape Medical News.
The study results were presented here at the Society of Nuclear Medicine and Molecular Imaging 2015 Annual Meeting.
The team used the highly specific carbon 11-labeled (+)-4-propyl-9-hydroxynaphthoxazine radiotracer — known as [11C]PHNO — to image the impact of cannabis on the brain. The radiotracer binds preferentially to dopamine D₃ receptors. Because it is a different class of compound than most other D₂ and D₃ radiotracers — an agonist rather than an antagonist — it is sensitive to dopamine release.
People often think of cannabis as a lighter, harmless drug. This study shows that it is not and that it has negative consequences.
With [11C]PHNO, the investigators showed that heavy chronic cannabis use is associated with lower dopamine release in the associative striatum and the sensory motor striatum, regions involved in cognition.
In contrast, previous reports have suggested that other drugs affect the limbic striatum, which processes reward information, Dr Weinstein explained
"Cannabis shares a negative impact on dopaminergic transmission with other drugs, only with a different regional profile," explained Dr Abi-Dargham.
An exploratory analysis showed a significant association between lower dopamine release capacity in the associative striatum and decreased cognitive measures in probabilistic category learning and working memory tasks, Dr Weinstein reported.