Cannabis dampens peoples’ response to rewards, according to a new study that sheds light on why some users become addicted to the drug and other substances.
Scientists at the 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.
Participants, who were in their early 20s, played games that required them to click a button when they saw a target on screen. Beforehand, they were told they might win 20 cents, or $5, or that they might lose that amount, have no reward or loss.
At the same time, participants’ brains were scanned by an MRI scanner which enabled researchers to assess activity in the nucleus accumbens, the brain’s reward centre.
The cells of the nucleus accumbens pump out the pleasure chemical dopamine when someone is anticipating reward, which makes people more likely to repeat the behaviour. But researchers found that the more cannabis use volunteers reported, the smaller the response over time to rewards, monetary or not.
“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.”
Researchers didn’t assess volunteers’ responses to cannabis-related cues, but research has found that people who use a high-inducing drug repeatedly often have a greater response to cues related to that 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.”
The pool of 108 participants, three quarters of whom were male and nearly all of whom were white, were taking part in a larger study of substance use and had brain scans at three points over four years.
Heitzeg added that the earlier in life someone tries marijuana, the faster their transition to becoming dependent on the drug, or other substances.
The study was published in the journal JAMA Psychiatry.
High profile research last month revealed that cannabinoids, a component in cannabis, can fight proteins which cause Alzheimer’s disease.