Students who smoked marijuana as rarely as once a week still showed significant brain abnormalities in terms of the organ’s size, density, and volume, a new study finds. The two areas most affected by the marijuana use were the amygdala and the nucleus accumbens, which regulate, among other things, emotion and motivation.

The study, published in the Journal of Neuroscience, is the first to examine marijuana’s effects on casual smokers. It found that the number of joints a person smokes directly relates to the degree of brain impairment, challenging the notion that recreational use is somehow exempt from causing neurological damage.

“People think a little recreational use shouldn't cause a problem, if someone is doing OK with work or school,” said co-senior study author Hans Breiter in a statement. “Our data directly says this is not the case.”

Subjects for the study included 40 students, aged 18 to 25, split up evenly according to whether they currently smoked marijuana. Before conducting a range of neuroimaging tests, the researchers ensured none of the 20 current users exhibited any signs of addiction. Later, they tested for the density of subjects’ gray matter, the size of their brains, and the overall volume. They also compared the two groups’ brains to see how marijuana may have altered users’ responses to motivation and emotion.

"These are core, fundamental structures of the brain," said co-senior study author Anne Blood, director of the Mood and Motor Control Laboratory at Massachusetts General Hospital and assistant professor of psychiatry at Harvard Medical School. "They form the basis for how you assess positive and negative features about things in the environment and make decisions about them."

Overall, subjects who smoked had a larger-than-average nucleus accumbens and prominently displayed changes in size and shape. Relying on former studies in rat models, the researchers concluded that marijuana’s key psychoactive ingredient, tetrahydrocannabinol (THC), was conferring the wiring changes in the subjects’ brains.

"It may be that we're seeing a type of drug learning in the brain," lead author and psychology instructor at HMS, Dr. Jodi Gilman, said. "We think when people are in the process of becoming addicted, their brains form these new connections."

Marijuana is the most commonly used illicit drug in the U.S. A 2008 survey found 15.2 million people had used marijuana in the past month. And despite ongoing legal actions around the country, most of which shines a positive light on the plant, and growing public support for the drug’s medicinal effects, medical science knows little about marijuana’s actual effects on the brain. Such studies require long-term investigation to untangle the external factors from what’s actually going on.

What scientists do know is that drugs like marijuana can cause the brain to release more dopamine than it would for other pleasure-seeking activities, such as food, socializing, and sex. Even if they don’t physically addict people like the nicotine in cigarettes, marijuana addictions are problematic because they gradually numb a person’s response to dopamine. Activities that were once pleasurable turn unappealing.

Weed is also stronger today than it was in the 1960s and 70s. Forty years ago, the THC content hovered between one and three percent. Now, it typically ranges in potency from five to nine percent. This concerns people like Gilman and Breiter because it means that pot’s growing acceptance could mean greater damage on still-developing brains, despite many advocates’ well-meaning intentions.

"I've developed a severe worry,” Breiter said, “about whether we should be allowing anybody under age 30 to use pot unless they have a terminal illness and need it for pain."

 

Source: Gilman J, Kuster J, Lee S, et al.  Cannabis Use is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users. Journal of Neuroscience. 2014.