PEOPLE who smoke cannabis believe that it eases pain, but its analgesic
powers have been little studied. Now researchers in the US have found that the
active ingredient in marijuana, THC, targets the same pain centres in the brain
as morphine.
The ability of marijuana to soothe has been hard to fathom from animal
studies. Scientists often test the power of painkillers by timing how long it
takes a drugged rat to flick its tail away from a hot lamp, but since cannabis
slows down motor neurons, rats given cannabis may just be too high to react
quickly.
To tease apart these two neural pathways, Ian Meng and his colleagues at the
University of California Medical Center in San Francisco analysed THC鈥檚 effect
on a specific pain centre in the brain, the rostral ventromedial medulla or RVM.
The RVM can amplify or block pain signals travelling from the spinal cord to the
brain, and opioids such as morphine activate pain-blocking cells in the RVM.
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The researchers took rats and inserted a tube through their skulls and into
the RVM. After recovering from the surgery, the rats received an intravenous
injection of a THC-like substance. As expected, they were much slower to flick
their tails away from a hot lamp. Meng then showed that he could restore the
rats鈥� heat sensitivity by shutting down the RVM with a neural inhibitor injected
through the skull tube (Nature, vol 395, p 381).
To prove that inactivating the RVM did not just restore motor coordination,
Meng placed drugged rats on a 鈥渞otor rod鈥� which rotates under their feet like a
log in a river. 鈥淕ive them a cannabis drug and they just fall right off,鈥� he
says. They continued to do so even after the RVM was inactivated.
鈥淭his is very important work,鈥� says Daniele Piomelli, a neurologist at the
University of California鈥檚 Irvine campus. 鈥淚f medical scientists start to look
with greater interest at cannabis as a result, that鈥檚 a major achievement.鈥�