Cannabis use linked to brain changes in emotion and motivation

Smoking cannabis alters the size and shape of two key brain regions involved in emotion and motivation, research has shown.

Even light use of the drug once or twice a week warped the brains of young adults, US scientists found.

Hans Breiter, one of the researchers from Northwestern University in Chicago, said: “This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences.”

The scientists analysed magnetic resonance imaging brain scans of 20 cannabis users aged 18 to 25 and 20 non-users.

They found major differences in two areas, the nucleus accumbens and amygdala. Both are linked to emotions and motivation, and also associated with addiction.

The nucleus accumbens of cannabis users was unusually large, while the amygdala was deformed.

“Some of these people only used marijuana to get high once or twice a week,” said Prof Breiter. “People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case.”

Co-author Anne Blood, from Harvard Medical School, said: “These are core, fundamental structures of the brain. They form the basis for how you assess positive and negative features about things in the environment and make decisions about them.”

Drug users in the study smoked cannabis at least once a week but were not psychologically dependent.

The scientists, whose findings are reported in the Journal of Neuroscience, believe the effects are due to users’ brains adapting to low-level cannabis exposure. Previous research shows rats given the psychoactive compound in cannabis, THC, “rewire” their brains.

“It may be that we’re seeing a type of drug learning in the brain,” said study leader Jodie Gilman, an instructor in psychology at Harvard Medical School. “We think when people are in the process of becoming addicted, their brains form these new connections.”

Peter Jones, professor of psychiatry at Cambridge University, said: “This is an interesting piece of research. However, the research is limited as it is only a small study. It is not known whether the reported changes in the brain are necessarily bad [the main example being an enlarged nucleus accumbens] and nor is it known how significant the amount of the active ingredient THC is [it was not measured in the study].

“Furthermore, there were only 40 people in the study and as they didn’t measure the brains before and after, it’s possible that people with a larger accumbens are more likely to take cannabis.”