fMRI shows marijuana users’ brains light up less over nondrug rewards

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A new fMRI study has shown how marijuana use dulls neural activity in the nucleus accumbens (NAcc), a key part of the brain’s “reward pathway.” The imaging also captured the effect associating with sluggish anticipatory excitement over the promise of nondrug rewards such as money.

Over time, the study authors postulate, the NAcc dulling may increase the risk for continued drug use and later addiction.

The study was posted online July 6 in JAMA Psychiatry.

Meghan Martz, MS, University of Michigan, and colleagues recruited 108 young adults from the Michigan Longitudinal Study, an ongoing study of youth at high risk for substance abuse. They also recruited a contrast sample of control families.

The team sent all participants for three consecutive fMRI scans—the first at around age 20, the second at around age 22 and the third at around age 24.

Martz and colleagues also collected participants’ self-reported data on their use of marijuana and other drugs going back to age 11.

During the scans, the researchers told the participants they could win 20 cents or $5, or that they might lose that amount or have no reward or loss at all.

When the monetary rewards were offered, the team found greater marijuana use correlated with increasingly blunted activation in the NAcc over time (scan 1 to scan 2: β = −0.26; scan 2 to scan 3: β = −0.25).

In their discussion, Martz et al. write that their findings indicate that “continued marijuana use may result in a blunted NAcc response to nondrug rewards, even when controlling for previous and concurrent substance use.”

As their model also controlled for potential risk factors, including familial risk for substance-use disorders and baseline differences in NAcc activation, this work “provides robust evidence that marijuana use has long-term associations with anticipatory reward processing.”

The authors express hope that the work will help “dispel the notion that marijuana use produces no lasting neural influences and may elucidate the potential harms associated with this substance.”

In a feature article on the study produced by the University of Michigan Health System’s news office, co-author Elisa Trucco, PhD, of Florida International University points out that everyone is born with an innate drive to seek out situations or outcomes that feel rewarding.   

“We now have convincing evidence that regular marijuana use impacts the brain’s natural response to these rewards,” Trucco says. “In the long run, this is likely to put these individuals at risk for addiction.”