Marijuana quashes brain's response to reward over time

New York: Smoking weed may dampen the brain's response to reward over time and put people more at risk of becoming addicted to the drug or other substances, finds a new study.

The findings showed that the reward system of the brain has been 'hijacked' by the drug and that the users need the drug to feel reward -- or that their emotional response has been dampened.

Humans are born with an innate drive to engage in behaviours that feel rewarding and give pleasure, but over time marijuana use was associated with a lower response to a monetary reward.

"This means that something that would be rewarding to most people was no longer rewarding to them," said Mary Heitzeg, Neuroscientist and Assistant Professor at the University of Michigan in the US. 

Further, marijuana use was also found to impact the emotional functioning of the brain.

Marijuana can cause effects, including problems with emotional functioning, academic problems and even structural brain changes, said the paper published in JAMA Psychiatry. 

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," Heitzeg said adding, "but marijuana changes your brain in a way that may change your behaviour, and where you get your sense of reward from. It affects the brain in a way that may make it more difficult to stop using it." 

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 that 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, said the researchers.

"If this is true with marijuana users, it may be that the brain can drive the use of the drug, and that this use can also affect the brain," said lead author Meghan Martz, doctoral student at the University of Michigan. 

For the study, the team involved 108 people in their early 20s -- the prime age for marijuana use. 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 focussed on the nucleus accumbens -- the reward centres of the volunteers' brains. 

When a reward is being anticipated, the cells of the nucleus accumbens usually swing into action, pumping out dopamine -- a 'pleasure chemical.' 

The bigger 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 the marijuana use, the smaller was the response found in nucleus accumbens over time.