Marijuana craving appears to activate the same reward circuits in the brain as other addictive drugs or behaviors, including alcohol and gambling, scientists have found, offering new fodder for the controversial argument over just how potent the drug is.
“There has been a great debate as to whether marijuana is an addictive substance,” says lead researcher Francesca Filbey, a psychology researcher at the University of New Mexico in Albuquerque. “We do show that it at least has the same underlying mechanisms as other drugs of abuse or other addictive behaviors.”
Much of the uncertainty arises because marijuana is much more difficult to study that these other drugs. Animals “can’t self-administer marijuana,” for instance, Filbey says, unlike the case with alcohol or opiates mixed into drinking water. Getting objective data about drug use from people, meanwhile, is tricky, especially when the substance in question is mostly outlawed and has become highly politicized.
For the new study, which appeared online July 27 in Proceedings of the National Academy of Sciences, Filbey and her colleagues decided to skirt those problems by specifically studying craving behavior in 38 habitual marijuana users who had quit using the drug for three days. Functional magnetic resonance imaging (fMRI) was used to scan the participants’ brains after they were given either a marijuana-associated cue—a pipe—or a neutral cue, a pencil.
The researchers found that only the pipe caused increased brain activity in reward areas of the brain, notably the ventral tegmental, thalamus, anterior cingulate, insula and amgydala areas. According to the scientists, these regions specifically deal with “motivated behavior”—activity geared toward receiving a known reward—and also light up in people craving alcohol, nicotine and cocaine. “These regions all separately have been related to reward processes in a cue,” Filbey says. The reward areas also show increased activation during gambling.
In addition, the researchers tested the participants using a “marijuana problem scale,” a well-established questionnaire assessing whether and how greatly their drug use impaired their daily lives. The higher the score, the scientists found, the greater the activity observed in the orbitofrontal cortex and nucleus accumbens, two other areas associated with both reward activity and with relapse after drug treatment. In other words, the brain scans tended to mirror the level of each person’s marijuana use.
“I find these results interesting because they describe the brain’s reaction to marijuana craving, and shows that the brain areas involved in this craving are similar to those involved in craving for other drugs of abuse,” says Andres Ozaita, a neuropharmacologist at the Universitat Pompeu in Barcelona. Ozaita was principal investigator on a recent Nature Neuroscience paper that found that marijuana seems to impair memory by shutting down essential protein synthesis pathways in the hippocampus.
Filbey says that one of the reasons the findings are noteworthy is because they suggest new ways of understanding and treating drug users. These might include specifically targeting reward brain pathways with new pharmaceuticals or developing interventions that minimize the intensity of cravings.
But the results aren’t completely conclusive, she acknowledges. For cost reasons the researchers didn’t include a control group of marijuana non-users, and they didn’t verify marijuana abstinence using urine tests. And not all the participants preferred a pipe when using marijuana, though the general association between the two is pretty well established, Filbey says.
Nor is this work alone likely to settle the long-running debates over marijuana. “Many people would argue that it is definitely addictive,” she says. “People do go to rehab. They do have to get treatment. The question is whether it is as addictive as other drugs. This wouldn’t settle that debate.”