Back in the late 1980s, researchers at the Max Planck Institute of Psychiatry injected the right hind paw of a rat with Freund’s complete adjuvant, a compound that triggers inflammation, as part of a test of chronic pain treatments. One of the first painkillers they administered was morphine.
When they probed the paw for tenderness, they found the morphine had numbed the flesh as expected.
But the scientists also noticed something strange. The other hind paw, which they did not inflame beforehand, remained sensitive to touch. With all the morphine coursing through the rodent’s veins, that foot should have been numb as well. According to the textbooks, morphine does its analgesic magic in the brain and central nervous system. Therefore, the injection should have desensitized the rat’s whole body equally.
The Planck Institute experiment suggested that the drug was acting locally. Somehow the nerve cells, or neurons, in the inflamed paw were responding to morphine, and something about the inflammation gave the painkiller its zing.
At first, “we didn’t have an explanation,” says Christoph Stein, who led the research.
