Scorpion toxin could help solve chronic pain mystery, say scientists
Scorpion toxin and wasabi are typically associated with causing pain, not reducing it.
But scientists say the former induces sensations associated with a hit of the green stuff, and may actually help solve the mystery of chronic pain.
Researchers at the University of California, San Francisco, and the University of Queensland, have discovered a scorpion toxin that targets the "wasabi receptor".
This is the chemical-sensing protein found in nerve cells that is responsible for the sinus-jolting sting of wasabi and the flood of tears associated with chopping onions.
Scientists say that because the toxin triggers a pain response through a previously unknown mechanism, it can be used to analyse chronic pain and inflammation.
The study, published in the Cell journal, suggests it may even lead to the development of new kinds of non-opioid pain relievers.
Researchers isolated the toxin, a short protein they dubbed the "wasabi receptor toxin" (WaTx), from the venom of the Australian Black Rock scorpion.
They were looking for compounds in animal venom that could activate the wasabi receptor – a protein called TRPA1 that is embedded in sensory nerve endings throughout the body – and be used to study it.
When activated, TRPA1 opens to reveal a channel that allows sodium and calcium ions to flow into the cell, which can induce pain and inflammation.
Lead author John Lin King said: "Think of TRPA1 as the body's 'fire alarm' for chemical irritants in the environment.
"When this receptor encounters a potentially harmful compound – specifically, a class of chemicals known as 'reactive electrophiles,' which can cause significant damage to cells – it is activated to let you know you're being exposed to something dangerous that you need to remove yourself from."
The receptor can be activated by chemicals in pungent foods like wasabi, onions, mustard, ginger and garlic.
According to researchers, it may have evolved to discourage animals from eating these plants.
WaTx appears to have evolved for the same reason, researchers said.
While many animals use venom to paralyse or kill their prey, the scorpion toxin seems to serve a purely defensive purpose as it only activates the version of TRPA1 found in mammals, which are not on the menu for Black Rock Scorpions.
Scientists found the toxin was able to work its way through a cell's membrane, a feat that only a few other proteins are able to manage.
Mr Lin King added: "It was surprising to find a toxin that can pass directly through membranes. This is unusual for peptide toxin.
"But it's also exciting, because if you understand how these peptides get across the membrane, you might be able to use them to carry things – drugs, for example – into the cell that can't normally get across membranes."
The researchers also observed that like plant and environmental irritants that trigger the wasabi receptor, the scorpion toxin triggered a pain response.
But unlike them, it did not cause inflammation.
Researchers said their findings indicated it was possible to decouple the protective acute pain response from the inflammation that establishes chronic pain.
They hope their findings will lead to a better understanding of acute pain, as well as the link between chronic pain and inflammation, which were previously thought to be experimentally indistinguishable.