A long-term study performed at the University of Michigan Medical School found that prolonged use of marijuana inhibits the brain’s reward system.
The reward system is formed by neural structures and neurotransmitters causing incentive, pleasure and positive reinforcement, in order to motivate and reinforce behaviors. For example, most people feel a degree of excitement when they are about to win some money. This is their brains’ reward system working, which in that moment would show a lot of activity, if seen through a MRI.
The group of Michigan researchers found that such excitement decreases in marijuana users, decreasing further over the time. The problem is that an inhibited reward system increases the risk of becoming an addict to marijuana itself, and to other drugs.
These results, published in JAMA Psychiatry, are yield from the first long-term study that tracked brain responses to rewards of young marijuana users over time. The changes in the brain activity of marijuana consumers remain even after taking into account other factors such as the use of cigarette smoking and alcohol.
“What we saw was that over time, marijuana use was associated with a lower response to a monetary reward,” says senior author and U-M neuroscientist Mary Heitzeg, Ph.D. “This means that something that would be rewarding to most people was no longer rewarding to them, suggesting but not proving that their reward system has been ‘hijacked’ by the drug, and that they need the drug to feel reward — or that their emotional response has been dampened.”
The study involved 108 volunteers in their early 20s, who had brain scans at three points over four years. The individuals were scanned while playing a game that may allow winning or losing 20 cents or 5 dollars.
The researchers focused in the moment of anticipation, reflected in the nucleus accumbens (brain reward center), when the participants knew they might win some money, anticipating the performance of a simple task they needed to do to earn the money.
According with the authors, this result may indicate that long-term marijuana use reduces a person’s emotional response, which is aligned with a previous study by the research group that showed that marijuana use impacts emotional functioning.
“We are all born with an innate drive to engage in behaviors that feel rewarding and give us pleasure,” says co-author Elisa Trucco, Ph.D., psychologist at the Center for Children and Families at Florida International University. “We now have convincing evidence that regular marijuana use impacts the brain’s natural response to these rewards. In the long run, this is likely to put these individuals at risk for addiction.”
Source: University of Michigan
