A study published this week in Science suggests that the active ingredient in marijuana, cannabinoids—whichinclude THC and other chemicals—may limit the body's ability to tampdown pain responses, and, as a result, turn short term acute pain intomore long term discomfort. While there has been a growing amount ofresearch analyzing the pain reduction effects of cannabis, aninternational group of researchers led by Hanns Ulrich Zeilhofer fromthe University of Zurich's Institute of Pharmacology and Toxology and collaborators from the University of Texas Medical Branch at Galveston think they may have uncovered evidence that marijuana can achieve just the opposite effect.

By applying a mimic of endocannibanoids, the body's homegrownversion of cannibanoids, to the spinal columns of mice, researchers sawthat instead of amplifying the capacity of pain inhibitor neurons, thechemical managed to shut them down. That is, the electrical signalsthat indicate pain inhibition is underway effectively went dark. In asecond trial, they attempted the same technique on mice that had beengenetically engineered to lack the specific receptors necessary forendocannibanoids. In this group, pain inhibition neurons functionednormally.

Yet, does the apparent neuronal on/off switch flicked by cannabischemicals actually signify an increased sensation of pain? To test thatout, researchers put rats who had endocannabinoids in their systemsunder anesthesia, and then injected small amounts of capsaicin—thechemical that makes chillies spicy—into their hindpaws. The result wasa quick uptick in pain response. And for these rats, even stimuli thatwouldn't have caused discomfort earlier now produced a pain response, aphenomenon known as allodynia. When the researchers administered anendocannabinoid blocker, however, "this increase was reversed," theywrite.

Finally, to see if this trend persisted in humans, they subjectedvolunteers to mild electrical shocks on the forearm to inducehyperalgesia—heightened pain sensitivity—in that specific region. Theythen gave half of the volunteers a placebo, and half theendocannabinoid receptor blocker rimonabant to take for amonth, after which time they came back to get tested again. There waslittle change in the placebo group, but, consistent with their findingsin mice and rats, the researchers say, there was a marked decline inhyperalgesia and allodynia in the previously tested regions of thosewho had taken the blocker. There was not, however, any reduction inacute pain—the discomfort caused at the site of the new round of shocks.

Their conclusion? While the researchers began their project with theidea that cannabis should help dull pain, in fact they found thatendocannabinoids play an "unexpected role" in governing pain-inhibitorneurons in the spine, and may even possibly increase the risk forturning short term, sharp pain, into more long term pain. And, sincethe active ingredient in pot is so chemically similar toendocannabinoids, marijuana could potentially cause the same problems.Yet, while these results certainly open the door for furtherresearch—and might provide a word of caution for recreational potsmokers—many more studies still have to be done before these findingscan upend the existing evidence for the predominately pain-reducingqualities of cannabis.

Source: http://wellness.blogs.time.com/2009/08/14/can-smoking-pot-lower-your-threshold-for-pain/