Drugs

Long-Term Marijuana Use Alters Brain's Reward System, Decreasing Motivation

Longtime Marijuana Use Lowers Motivation
Regardless of other aspects of the cost-benefit analysis of marijuana, long-term use of the drug compound tends to lover motivation, a new study claims. Creative Commons

As support for legalizing marijuana grows in the United States, new research bolsters claims that long-term marijuana use lowers motivation and reward-seeking behavior — as the brain produces less dopamine over time.

In a small sampling, researchers in the United Kingdom scanned the brains of 19 long-term cannabis smokers along with 19 nonusers of the same age and sex, measuring the distribution of chemicals throughout the brain via positron emission tomography.

Those who smoked marijuana were found to produce less dopamine than others, in a part of the brain called the striatum.

However, "whether such a syndrome exists is controversial," Michael Bloomfield, a researcher at the Institute of Clinical Sciences at Imperial College London, told reporters of the study he led. Although dopamine is thought to be associated with reward-seeking behavior, empirical evidence of so-called "amotivational syndrome" is lacking.

A similar study using the same brain imaging technology reported a lowered dopamine production among chronic users of marijuana, with Columbia University researchers last April calling for additional work in that area.

In this sampling, the marijuana smokers reported heavy use of the drug compound and a history of use beginning between ages 12 and 18. Moreover, they all reported experiencing symptoms of psychosis while under the influence of marijuana, including strange sensations or bizarre thoughts, such as paranoia — a threat from an unknown force, for example.

Given the association between heightened dopamine production and psychosis, researchers were surprised to find lower levels of the chemical. While previous neuroscience study has shown that long-term marijuana use may trigger inflammation in the brain, affecting coordination and learning, the new findings about dopamine suggest a deepening mystery with regard to marijuana and mental illness — particularly an increased risk of schizophrenia.

"It has been assumed that cannabis increases the risk of schizophrenia by inducing the same effects on the dopamine system that we see in schizophrenia, but this hasn't been studied in active cannabis users until now," Bloomfield said.

The study results conform to previous findings with regard to altered dopamine systems in users of other drugs, and may explain behaviors observed in marijuana users, including those suffering psychosis or dependence.

However, researchers offered a silver lining in this cloud of marijuana smoke: the brain changes to the dopamine were likely reversible, given that previous studies showed normal dopamine production in former marijuana smokers.

 

Sources: Bloomfield M. Biological Psychiatry. 2013.

Urban NB, Slifstein M, Thompson JL, et al. Dopamine Release In Chronic Cannabis Users. Biological Psychology. 2012.

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