July 1, 2013
Motivation Lacking For Many Marijuana Users
Lee Rannals for redOrbit.com - Your Universe Online
Long-time marijuana smokers have a lack of motivation to work or pursue normal interests, according to a new study.
Scientists at Imperial College London, University College London (UCL) and King's College London found long-term marijuana users tend to have lower levels of dopamine in a part of the brain called the striatum. They said this finding could explain why some users appear to have a lack of motivation.
The team used PET brain imaging to look at dopamine levels in the striatum of 19 regular cannabis users and 19 non-users of matching age and sex. The users in the study had all experienced psychotic-like symptoms while smoking the drug, such as strange sensations or bizarre thoughts like feeling as though they are being threatened.
The researchers expected the dopamine levels might be higher in this group because increased levels of this chemical has been linked with psychosis. However, they found the opposite effect.
Cannabis users in the study had their first experience with the drug between the ages of 12 and 18 years of age. The team saw a trend for lower dopamine levels in those who started earlier, and also in those who smoke more cannabis. They said marijuana use may be the cause of the difference in dopamine levels.
The lowest dopamine levels were seen in users who meet diagnostic criteria for cannabis abuse or dependence, which could be a marker of addiction severity.
Past studies found marijuana users have a higher risk of mental illnesses that involve repeated episodes of psychosis, such as schizophrenia.
"It has been assumed that cannabis increases the risk of schizophrenia by inducing the same effects on the dopamine system that we see in schizophrenia, but this hasn't been studied in active cannabis users until now," said Dr Michael Bloomfield, from the Institute of Clinical Sciences at Imperial, who led the study. "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."
He said they only looked at cannabis users who have had psychotic-like experiences with the drug. However, he added they believe the findings would apply to marijuana users in general because they didn't see a stronger effect in subjects who have more psychotic-like symptoms.
"It could also explain the 'amotivational syndrome' which has been described in cannabis users, but whether such a syndrome exists is controversial," Bloomfield said.
In 2011, researchers published in the British Medical Journal that they found a link between marijuana use and psychosis. They found cannabis use almost doubled the risk of incident psychotic symptoms later in life, even after accounting for other factors like age, sex, socioeconomic status, use of other drugs and other psychiatric diagnoses.