The heavy, long-term use of cannabis is associated with negative changes in parts of the brain not previously implicated, and is linked to deficits in learning and memory, new research suggests.
“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.
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
Researchers have long racked their brains whether marijuana damages IQ. According to a recent study, addictive cannabis use causes the gray matter to become smaller. However, the white matter compensates for the damage by increased neuron connectivity.
A team of scientists from the US claim their research - the latest in a long line of marijuana effects studies - is the first to describe comprehensively the puzzling changes in human brain’s structure and function, associated with cannabis use.
Overall the IQ suffers along with the orbitofrontal cortex (OFC), a part of the brain whose volume decreases - and medics notice that its damage often leads to disinhibited behavior. That is, poor social interaction, excessive swearing and hypersexuality.
To obtain such abnormalities on a detectable level, as participants demonstrated on average, one has to consume the drug about three times per day. But the term ‘chronic user’ is applied to usage of at least four times per week.
“Cannabis shares a negative impact on dopaminergic transmission with other drugs, only with a different regional profile,” explained Dr Abi-Dargham.
The study, published in the Journal of Neuroscience, is the first to link casual marijuana use to major changes in the brain. And according to the researchers, the degree of abnormalities is based on the number of joints you smoke in a week.
Using different types of neuroimaging, researchers examined the brains of 40 young adults between the ages of 18 and 25 who were enrolled in Boston-area colleges. Twenty of them smoked marijuana at least once a week. The other 20 did not use pot at all.
The marijuana smokers were asked to track their cannabis use for 90 days. All were given high-resolution MRIs, and users and non-users’ results were compared.
Researchers examined regions of the brain involved in emotional processing, motivation and reward, called the nucleus accumbens and the amygdala. They analyzed volume, shape and density of grey matter - where most cells in brain tissue are located.
“I think the findings that there are observable differences in brain structure with marijuana even in these young adult recreational users indicate that there are significant effects of marijuana on the brain,” says Dr. Jodi Gilman, lead author and a researcher in the Massachusetts General Center for Addiction Medicine. “Those differences were exposure-dependent, meaning those who used more marijuana had greater abnormalities.”
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.