Marijuana use dampens brain's response to reward over time, study finds

marijuana
A dried flower bud of the Cannabis plant. Credit: Public Domain

Most people would get a little 'rush' out of the idea that they're about to win some money. In fact, if you could look into their brain at that very moment, you'd see lots of activity in the part of the brain that responds to rewards.

But for people who've been using , that rush just isn't as big - and gets smaller over time, a new study finds.

And that dampened, blunted response may actually open marijuana users up to more risk of becoming addicted to that drug or others.

The new results come from the first long-term study of young marijuana users that tracked responses to rewards over time. It was performed at the University of Michigan Medical School.

Published in JAMA Psychiatry, it shows measurable changes in the brain's system with marijuana use - even when other factors like alcohol use and cigarette smoking were taken into account.

"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 has been 'hijacked' by the drug, and that they need the drug to feel reward—or that their emotional response has been dampened."

Watching the reward centers

The study involved 108 people in their early 20s - the prime age for marijuana use. All were taking part in a larger study of substance use, and all had brain scans at three points over four years. Three-quarters were men, and nearly all were white.

While their brain was being scanned in a functional 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. Before each round, they were told they might win 20 cents, or $5 - or that they might lose that amount, have no reward or loss.

The researchers were most interested at what happened in the reward centers of the volunteers' brains - the area called the . And the moment they cared most about was that moment of anticipation, when the volunteers knew they might win some money, and were anticipating performing the simple task that it would take to win.

In that moment of anticipating a reward, the cells of the nucleus accumbens usually swing into action, pumping out a 'pleasure chemical' called dopamine. The bigger the response, the more pleasure or thrill a person feels - and the more likely they'll be to repeat the behavior later.

But the more marijuana use a volunteer reported, the smaller the response in their nucleus accumbens over time, the researchers found.

While the researchers didn't also look at the volunteers' responses to marijuana-related cues, other 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. And that can make it harder to stop seeking out the drug and using it.

If this is true with marijuana users, says first author Meghan Martz, doctoral student in developmental psychology at U-M, "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."

Change over time

Regardless, the new findings show that there is change in the reward system over time with marijuana use. Heitzeg and her colleagues also showed recently in a paper in Developmental Cognitive Neuroscience that marijuana use impacts emotional functioning.

The new data on response to potentially winning money may also be further evidence that long-term marijuana use dampens a person's emotional response - something scientists call anhedonia.

"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."

Marijuana's reputation as a "safe" drug, and one that an increasing number of states are legalizing for small-scale recreational use, means that many young people are trying it - as many as a third of college-age people report using it in the past year.

But Heitzeg says that her team's findings, and work by other addiction researchers, has shown that it can cause effects including problems with emotional functioning, academic problems, and even structural brain changes. And, the earlier in life someone tries marijuana, the faster their transition to becoming dependent on the drug, or other substances.

"Some people may believe that marijuana is not addictive or that it's 'better' than other drugs that can cause dependence," says Heitzeg, who is an assistant professor of psychiatry at the U-M Medical School and member of the U-M Addiction Research Center. "But this study provides evidence that it's affecting the brain in a way that may make it more difficult to stop using it. It changes your brain in a way that may change your behavior, and where you get your sense of reward from."

She is among the neuroscientists and psychologists leading a nationwide study called ABCD, for Adolescent Brain Cognitive Development. That study will track thousands of today's pre-teens nationwide over 10 years, looking at many aspects of their health and functioning, including brain development via . Since some of the teens in the study are likely to use marijuana, the study will provide a better chance of seeing what happens over time.


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Study: Long-term marijuana use changes brain's reward circuit

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Jul 06, 2016
Yes, of course! Clearly, less excitement over the stimulus of receiving money means that the brain is now blunted. In other words, if money doesn't get your juices flowing, nothing will!

Did the sages who engineered this study happen to test other stimuli? What a bunch of junk science.

If they were really serious about examining this phenomenon instead of fueling the anti-marijuana camp (by drawing such heroic conclusions over such rudimentary evidence), they might consider examining other "rewards" or even investigating if the "marijuana brain" is turned on by things that the control group brains are not.

Jul 06, 2016
Chill. Its just a study.

Jul 06, 2016
Would be interesting to include "non-users" in the study... i.e. people who are inclined to be long-term marijuana users might just generally not be as excited about monetary rewards... or more "chilled" to use a scientific term :)

They should have got random non-smokers: make them smoke every day for a few months then test and compare to control

Jul 06, 2016
Would chronic coffee use not have a similar reward dampening effect?

JRi
Jul 07, 2016
I think it would be good if one was able to dampen his reward center so he could get easier rid of more serious drugs such as heroin.

Jul 07, 2016
http://archpsyc.j...=2532234

"One hundred eight young adults were recruited from the Michigan Longitudinal Study, an ongoing study of youth at high risk for substance use disorder..."

Well, if you start with people already at "high risk" for what you're looking for, guess what you're going to find? What you're looking for.

From the conclusions: " Over time, marijuana use may alter anticipatory reward processing in the NAcc, which may increase the risk for continued drug use and later addiction."

Well, at least in this generation they're allowing that it *may* be a gateway drug, instead of insisting that it's just the first step on a short road to IV heroin use.

Jul 07, 2016
Well, at least in this generation they're allowing that it *may* be a gateway drug


That sentence didn't imply it was a gateway drug, more than just saying it may turn into an addiction.


I think it would be good if one was able to dampen his reward center so he could get easier rid of more serious drugs such as heroin.


You don't want to do that, because dampening the reward mechanism actually means people will take more of the drug to get the same high. In case of Heroin, that eventually means an overdose.

There are better drugs that kill the reward mechanism almost entirely for substances like alcohol or opiates, such as https://en.wikipe...ltrexone

You need to go all the way or the effect works against you. Imagine if an alcoholic needed two beers instead of one to get a good buzz going - well they would simply drink twice as much. If that never happens, they become bored of alcohol very quickly.

Jul 07, 2016
The flaw in this study is fundamental: addiction, so called, is a matter of personality, not chemistry. That is why almost all people who are exposed to supposedly addicting opiods --- hospital patients, wounded soldiers, etc. --- do not become junkies.

As a concept, the term "addiction" is nearly meaningless. Essentially, it is a value judgment. Air, after all, is highly "addicting" --- once you've tasted air, you simply must have it at any cost. So can be sugar, or pornography. But, until recently, people wrung their hands over "addiction to porn," while no one worried about sugar "addiction." Values are changing, though, and who knows, maybe sugar "addiction" will become the target of a repurposed DEA someday?

I think the point with cannabis is that this most Buddhist of drugs causes people to be more reflective and less prone to chase pointless or destructive enthusiasms, like the Vietnam war.


Jul 07, 2016
The flaw in this study is fundamental: addiction, so called, is a matter of personality, not chemistry.


Then why does the Sinclair method work?

Some people have different brain chemistry responses to different substances. People who are alcoholics describe the sensation of drinking alcohol as "hair raising" in pleasure, and when put on naltrexone they report a lack of such sensations, and go "Huh, is this how other people feel like drinking? This is nothing."

I think the point with cannabis is that this most Buddhist of drugs causes people to be more reflective and less prone to chase pointless or destructive enthusiasms, like the Vietnam war.


That's funny, because one precept of buddhism is to abstain from drugs in order to think clearly.

Jul 07, 2016
Too few participants too litle time no peer review listed, BS.

Jul 09, 2016
"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."
That's some seriously backwards logic. The actual result is an acquired pseudo-immunity from addictions – a mind less distracted by the usual carrot and stick motivations, and a mind less prone to irrational anger. The capitalists and warmongers hate weed because a free mind is harder to influence and control. Of course declaring its use a crime was their solution and it's been working out extremely well for them since the Nixon administration.
cont'd >

Jul 09, 2016
> cont'd

Quoting from the above link:
"The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people. You understand what I'm saying. We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities. We could arrest their leaders, raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did." – John Ehrlichman (served as Tricky Dick's domestic policy chief)

Speaking of heroin, how's that [America's longest] war in Afghanistan coming along? Unbelievable....

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