MICHIGAN, United States, Tuesday July 12, 2016 – Marijuana dampens users’ response to rewards, according to a new American study that sheds light on why some users become addicted to the drug and other substances.
Scientists at the University of Michigan Medical School found a negative correlation between how much cannabis a user reported taking and their sense of reward over time.
“What we saw was that over time, marijuana use was associated with a lower response to a monetary reward,” said 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.”
Dr Heitzeg added that the earlier in life someone tries marijuana, the faster their transition is to becoming dependent on the drug, or other substances.
The study, which was published in the journal JAMA Psychiatry, involved 108 people in their early twenties, which is considered the prime age for marijuana use.
These volunteers were taking part in a larger study of substance use, and all had brain scans at three points over four years.
While their brain was being scanned in an MRI scanner, they played a game that asked them to click a button when they saw a target on a screen in front of them.
At the start of each round, they were told they might win 20 cents, or US$5 – or that they might lose that amount, have no reward or loss.
The researchers were most interested at what happened in the reward centres of the volunteers’ brains – the area called the nucleus accumbens – at the moment when the volunteers knew they might win some money, and were anticipating performing the simple task that it would take to win.
At the moment of anticipating a reward, the cells of the nucleus accumbens usually swing into action, pumping out the “pleasure chemical” dopamine.
The greater the response, the more pleasure or thrill a person feels, and the more likely they’ll be to repeat the behaviour later.
However, the more marijuana use a volunteer reported, the smaller the response in their nucleus accumbens over time, the researchers found.
While the researchers did not look at the volunteers’ responses to marijuana-related cues, previous research has shown that the brains of people who use a high-inducing drug repeatedly often respond more strongly when they’re shown cues related to that drug.
The increased response means the drug has become associated in their brains with positive, rewarding feelings, which makes it harder to stop using the drug.
First author Meghan Martz, doctoral student in developmental psychology at U-M, said: “It may be that the brain can drive marijuana use, and that the use of marijuana can also affect the brain. We’re still unable to disentangle the cause and effect in the brain’s reward system, but studies like this can help that understanding.”