A new painkilling substance, produced naturally by the human body, has been discovered to be up to six times more potent than morphine when tested on rats, researchers said.
Natural painkillers are very rare and researchers hope that this recent find might be harnessed for clinical treatment, Nature magazine reported on Wednesday.
The compound Opiorphin seems to work by prolonging the body's own defences against pain, explained Catherine Rougeot of the Pasteur Institute in Paris, France, and her colleagues.
It does so by preventing the breakdown of chemicals called enkephalins, which in turn activates opiate receptors that block pain signals from reaching the brain.
Naturally produced painkillers might help to avoid some of the side effects experienced by patients treated with synthetic compounds such as morphine, including addiction and tolerance with prolonged use, the report said but cautioned that the new substance will first have to be tested to confirm whether it will be an effective drug.
This is the first natural substance to be found in humans that exploits this mechanism to relieve pain, the report said.
But it's unlikely that Opiorphin ordinarily has a painkilling role in the body, said Alistair Corbett, a specialist on opioids at Glasgow Caledonian University, UK.
The substance could play a general role in protecting chemicals in the body from being broken down, he said.
Rougeot's team, Nature said, tracked down the new compound after previously finding a similar natural painkiller in rats, called sialorphin.
They wondered whether humans might produce something similar and by analysing saliva samples, hit upon Opiorphin.
Opiorphin, the report said, seems to work on pain induced by both chemical and physical means. When it was given to rats that had been given a painful injection, they exhibited diminished pain responses, such as licking the injected paw.
It also reduced acute pain when the rats walked across a pin-covered surface.
Researchers have previously tried to make synthetic compounds to perform the same function of protecting enkephalins, with little success.
"Inhibiting this enzyme would be a good target for drugs. But nobody has looked for a natural inhibitor," Rougeot said.
The researchers reported their discovery in 'Proceedings of the National Academy of Sciences'.