Chronic Marijuana Users' Brains Produce Less Dopamine, Study Finds

Levels of dopamine - a chemical in the brain linked to reward-motivated behavior - tend to be lower in chronic marijuana users, according to a new study out of the United Kingdom.

Researchers at Imperial College London, University College London and King's College London report a trend of dopamine levels in a part of the brain called the striatum being lower in those who started using marijuana earlier and also in those who smoke more of the plant.

The 19 marijuana users in the study were chosen to participate because they all reported the presence of psychotic-like symptoms -- such as experiencing strange sensations or having bizarre thoughts like being threatened by an unknown force -- while smoking the drug. Researchers used PET brain imaging to look at dopamine production in the striatum of the marijuana users' brains. The data was compared against PET scans from 19 non-users of the drug of matching age and gender.

Researchers suspected that dopamine production would be higher in such a particular group of marijuana smokers because elevated dopamine production has been linked with psychosis. Counter to their expectations, the researchers found the 19 marijuana users had lower levels of dopamine.

The researchers suggest that cannabis use may be the cause of the difference in dopamine levels, while also suggesting that low dopamine levels may be a marker of addiction severity, as the lowest dopamine levels were seen in users that met the diagnostic criteria for cannabis use or dependence.

"The results weren't what we expected, but they tie in with previous research on addiction, which has found that substance abusers - people who are dependent on cocaine or amphetamine, for example - have altered dopamine systems," Imperial College London's Michael Bloomfield, lead researcher in the study, said in a news release.

"Although we only looked at cannabis users who have had psychotic-like experiences while using the drug, we think the findings would apply to cannabis users in general, since we didn't see a stronger effect in the subjects who have more psychotic-like symptoms. This needs to be tested though."

Bloomfield added that the lower dopamine levels reported could also explain the controversial "antimotivational syndrome" which has been described in marijuana users.

Other studies have found that dopamine levels in former marijuana users are no different than non-users, suggesting the effects seen in the latest study are likely to be reversible.

The research is published in the journal Biological Psychiatry.