Marijuana use dampens brain's response
to reward over time, study finds
By
Dr.Fourkan Ali
Changes may increase
risk of continued drug use and addiction
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 activity in
the part of the brain that responds to rewards. But for marijuana users, that
rush just isn't as big -- and gets smaller over time, a new study finds. And
that may open them up to more risk of addiction.
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 marijuana, 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 brain
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 reward 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 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."
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 nucleus accumbens. 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 brain scans. 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.
Story
Source:
The above post is
reprinted from materials provided by University of Michigan Health System. Note: Content may be edited for style
and length.
0 comments:
Post a Comment