The findings may be an argument for developing synthetic compounds rather than saying "smoke pot for pain", said one researcher. But they also suggest that people smoking marijuana for medical reasons are "experiencing pain relief", not just enjoying a high, he said. Marijuana's role in easing pain was a key issue in California's hotly contested vote last year to allow doctors to recommend it for patients such as cancer or Aids sufferers.
Supporters claim it relieves the symptoms of pain and nausea without the addictive side-effects of opiates. The new studies will only add fuel to the controversy, at a time when some opponents of medical marijuana in the US have grudgingly conceded that there should at least be studies of its possible effects. Some results suggested that cannabinoids may relieve the symptoms both of arthritis and hyperalgesia, extreme sensitivity to pain linked to nerve diseases and spinal cord injuries. Researchers from the University of California at San Francisco, from Brown University and the University of Michigan, reported their findings at a meeting of the Society for Neuroscience in New Orleans. They were based on animal studies using THC, the active ingredient in marijuana, and a synthetic cannabinoid, WIN 55212.
The studies showed that "substances similar to or derived from marijuana, known as cannabinoids, could benefit the more than 97 million Americans who experience some form of pain every year," the Society of Neuroscience said in a press release. They have a "direct effect on pain signals in the central nervous system and peripheral tissues," and "could be manipulated to form a new type of pain reliever".
At San Francisco, the research included measuring pain suppression in rats by seeing how long it took them to remove their tails from an "uncomfortable heat source". Injections of less than a tenth of a millionth of an ounce caused a profound loss of pain sensitivity. At the University of Michigan, similar experiments were conducted on three rhesus monkeys, confirming that after a dose of cannabinoids they were slower to remove their tails from a tub of water kept at 50 degrees Celsius. But along with these simple observations, researchers also monitored the biochemical reactions involved. The research suggests cannabinoids can, like morphine, reduce pain in two different ways, said Dr Barton Manning of UC San Francisco.
Firstly, they can directly suppress neurons in the peripheral and central nervous system that convey pain information from the body to the brain. But they can also "talk" to pain-transmission neurons in the spinal cord and influence the number of pain signals that are allowed to reach the brain.