Study leader Dr. Michael Bloomfield of the Institute of Clinical Sciences at Imperial College London and colleagues at University College London and King's College London used PET brain imaging to look at dopamine production in the brains of 19 regular marijuana users and 19 non-users of matching age and sex.
The marijuana users all experienced psychotic-like symptoms while smoking the drug, such as experiencing strange sensations or having bizarre thoughts like feeling as though they are being threatened by an unknown force.
The researchers expected dopamine production might be higher in this group, since increased dopamine production has been linked with psychosis, but they found the opposite effect, the study said.
The marijuana users in the study had their first experience with pot between the ages of 12-18. There was a trend for lower dopamine levels in those who started earlier, and also in those who smoked higher levels of marijuana.
The researchers said these findings suggest marijuana use might be the cause of the difference in dopamine levels.
"The results weren't what we expected, but they tie in with previous research on addiction, which found substance abusers -- people who are dependent on cocaine or amphetamine, for example -- have altered dopamine systems," Bloomfield said. "It could also explain the 'amotivational syndrome' which has been described in marijuana users, but whether such a syndrome exists is controversial."
The study was published in the journal Biological Psychiatry.