Cannabis Harms Brain, Imaging Shows

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.