MRI Used To Investigate The Pain-Relieving Effect Of Cannabis
December 22, 2012

Understanding The Neurology Of Marijuana’s Pain-Relieving Effects

redOrbit Staff & Wire Reports - Your Universe Online

Rather than reducing the intensity of pain, taking cannabis actually makes the discomfort more bearable, a team of UK researchers discovered during a recent, small-scale, brain-imaging study.

Dr. Michael Lee of the Oxford University Centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB) and colleagues recruited a dozen healthy male subjects and carried out a series of MRI scans with each participant.

Before the scan, however, each was given either a 15mg tablet of THC (the psychoactive ingredient found in cannabis) or placebo, and each had one of two creams -- one that contained 1% capsaicin, an ingredient found in chilli's that can cause a painful burning sensation, or a "dummy" cream that lacked the substance, the university explained in a statement Thursday.

Each subject was given a series of four separate MRI examinations in order to account for all possible combinations of pills and creams, and were asked to describe how intense and unpleasant the pain was -- that is, how hot the cream felt, and how much that burning sensation bothered them, Dr. Lee explained.

"We found that with THC, on average people didn't report any change in the burn, but the pain bothered them less," he said. While the university said that the effect was "statistically significant," they also reported different subjects reported different levels of changes in the unpleasantness of the pain thanks to THC's effects. In fact, only half of the 12 subjects said there was "a clear change" in how much they were bothered by the discomfort.

"The brain imaging results substantiate the reports of the participants," they added. "The change in unpleasantness of pain was matched with a suppression of activity in the part of the brain called the anterior mid-cingulate cortex. This structure sits in a deep part of the brain and is involved in many functions, and has previously been implicated in the emotional aspects of pain."

Furthermore, Dr. Lee's team also observed changes in the activity of the right side of the amygdala -- changes they said correlated to the lessening of the pain thanks to the marijuana ingredient -- as well as a connection between that part of the brain and the primary sensorimotor area of the cortex.

They observed a link between that connection's strength with the pain-relieving effects of THC experienced by the different subjects. That discovery could help them learn which people would most benefit from the pain relieving effects of the drug.

"We may in future be able to predict who will respond to cannabis, but we would need to do studies in patients with chronic pain over longer time periods," Dr. Lee said. "Cannabis does not seem to act like a conventional pain medicine. Some people respond really well, others not at all, or even poorly."

"Brain imaging shows little reduction in the brain regions that code for the sensation of pain, which is what we tend to see with drugs like opiates," he added. "Instead cannabis appears to mainly affect the emotional reaction to pain in a highly variable way."

The study, which was funded by the UK Medical Research Council (MRC) and the National Institute for Health Research (NIHR) Oxford Biomedical Research Centre, is detailed in the latest edition of the peer-reviewed journal Pain.