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Marijuana Abuse Hampers the Brain’s Response to Dopamine

give me weed

Image credit: Ian Sane via Flickr CC BY 2.0.

According to a latest research published in Proceedings of the National Academy of Sciences recently, abuse of marijuana could blunts the brain’s ability in its reaction to dopamine, the neurotransmitter responsible for human feelings, such as pleasure, motivation as well as reward. The impact of that “high” could eventually cause depression and anxiety,

Although marijuana abuse is quite prevailing, scientists have to find more about its effect on the brain. Many abused substances could help stimulate brain dopamine signaling, which greatly underlies the rewarding effects caused by drugs, food, and sex.

The previous studies demonstrated that cocaine and alcohol definitely enhanced dopamine release in the brain’s reward region, but as for marijuana, this relationship had not been consistently shown.

In order to identify marijuana’s impact on the human brain, the research team led by Nora Volkow from the U.S. National Institute on Drug Abuse started with their tests by recruiting twenty-four participants, who were regular marijuana users.  In their daily life, on average they smoked five joints a day, five days a week for a decade.

By applying personality questionnaires and two types of brain imaging, Nora Volkow’s team had examined how the marijuana users’ brains responded to the drug methylphenidate, kind of a stimulant promoting dopamine, which is usually used for treatment of ADHD and narcolepsy. In their experiment they also recruited 24 control participants for comparison.

As the result, after taking the drug, both groups generated much extra dopamine than usual. However, the behavioral, cardiovascular, and brain dopamine of the marijuana abusers were obviously blunted in their reaction to the stimulant. In addition, in comparison with the control participants, their heart rate and blood pressure became lower together with felt restlessness and anxiety.

With same amount of dopamine but weaker physical responses, it is suggested that the reward circuitry in marijuana abusers’ brains are damaged. Being different from alcohol and cocaine abusers, marijuana abusers seem to generate the same amount of dopamine with methylphenidate as those who have never used the drug, but their brains are unable to know what to deal with it.

Their lower scores on positive and higher scores on negative tests of emotional activity are closely linked with limited sensitivity to reward and motivation in one way, and with increased irritability and stress on the other.

From their study, the scientists think that it is the effect of dampened dopamine responses on the brain’s reward region that could be attributed to addictive behaviors and a tendency toward anxiety and depression. However, it is hoped that Volkow’s team would go further in their research to identify  exact cause and effect in this respect.

Source: Volkow, Nora D., et al. “Decreased dopamine brain reactivity in marijuana abusers is associated with negative emotionality and addiction severity.”Proceedings of the National Academy of Sciences (2014): 201411228.