'Opiorphin' is a natural painkiller more powerful than morphine that can be found where?

A new painkilling substance has been discovered that is up to six times more potent than morphine when tested in rats and it is produced naturally in saliva by the human body. Natural painkillers are very rare, and researchers hope that this recent find might be harnessed as a clinical treatment.

The compound, dubbed opiorphin, seems to work by prolonging the body's own defences against pain, explains Catherine Rougeot of the Pasteur Institute in Paris, France, and her colleagues, who report the discovery in Proceedings of the National Academy of Sciences. It does so by preventing the breakdown of chemicals called enkephalins, which in turn activate opiate receptors that block pain signals from reaching the brain.