EASING THE AGONY

Marijuana Does More Than Merely Make You Stoned

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 irLgredient 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's
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
painblocking cells in the RVM.

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 THClike 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' 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 "rotor rod" which rotates under their feet
like a log in a river. "Give them a cannabis drug and they just fall right
off," he says. They continued to do so even after the RVM was inactivated.

"This is very important work," says Daniele Piomelli, a neurologist at the
University of California's Irvine campus. "If medical scientists start to
look with greater interest at cannabis as a result, that's a major
achievement."

Checked-by: Richard Lake