The results of a recent study have found that heavy marijuana use may decrease the level of dopamine released in the brain. Dopamine is a chemical transmitter that sends signals to the brain and is involved in movement, as well as learning, memory, cognition and pleasure.

Dr. Anissa Abi-Dargham, a professor of psychiatry at Columbia University Medical Center and a lead author of the research paper, said that it was important to look at the “addictive effects of cannabis on key portions of the brain.” The researchers found evidence of lower dopamine release in the striatum, which is the portion of the brain involved in attention, working memory and impulsive behavior.

The results of previous research studies have found a link between addiction to “harder drugs,” like heroin and cocaine, result in similar effects on dopamine release in the brain. Evidence of similar effects on dopamine release had not been found up to this point.

Study participants were 11 adults aged 21-40 who were described as being “severely dependent” on marijuana and 12 healthy control subjects who did not use cannabis. The group who were marijuana users started using the drug at an average age of 16, were dependent on it at about age 20, and were dependent on it for seven years. In the 30 days before the start of the study, almost all users in this group had smoked marijuana every day.

Positron emission tomography (PET) was used to track a radio-labelled molecule which binds to dopamine receptors in the brain. Researchers noted the level of dopamine released in the striatum, its subregions, and other regions outside of the striatum (thalamus, midbrain and globes pallidus).

To ensure that the marijuana group did not continue using the drug during the study, they stayed in the hospital for a week to ensure that they practiced abstinence. The participants were scanned before and after taking an oral amphetamine, which would cause the brain to release dopamine. The percent change in the binding of the radio tracer was used to indicate the participants’ capacity for dopamine release in their brain.

When compared with the control group, the cannabis users had much lower dopamine release in their striatum. Researchers also looked at the connection between dopamine release in a specific area of the striatum and cognitive performance on working memory and learning tasks. Lower dopamine release resulted in worse performance in both types of tasks, and this result was seen in all members of the marijuana group.

Dr. Abi-Dargham said that researchers were unable to determine whether the decreased dopamine was present before they started using marijuana or the result of their heavy drug use. She did say that long-term heavy marijuana use may impair a person’s dopaminergic system, resulting in a number of negative effects on both learning and behavior.