Details of a study released today suggest that the brains of long-term cannabis users produce less dopamine, a neurotransmitter linked to motivation.
The study, funded by the Medical Research Council and published in Biological Psychiatry, analyzed PET brain scans to determine the level of dopamine production in the striatum of 19 regular cannabis users and 19 non-users of matching age and sex.
The cannabis users began using the drug between the ages of 12 and 18, and researchers — scientists at the Imperial College London, UCL and King's College London — discovered that the earlier they began and the more they smoked, the lower their levels of dopamine.
Those who met the diagnostic criteria for cannabis abuse or dependence had the lowest dopamine levels.
"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," said Dr. Michael Bloomfield, the study's leader from the Institute of Clinical Sciences at Imperial. "It could also explain the 'amotivational syndrome' which has been described in cannabis users, but whether such a syndrome exists is controversial."
The press release noted that the effects seen in the study are likely reversible since other studies have analyzed the dopamine levels in former cannabis users and found that they didn't differ from those of people who haven't used cannabis.