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Grasshopper mice resist pain, thanks to evolution

A southern grasshopper mouse (Onychomys torridus) approaches and sniffs the Arizona bark scorpion (Centruroides sculpturatus) that it has just killed.
[Image courtesy of Matthew and Ashlee Rowe]

Bark scorpions have one of the most painful stings in the animal kingdom—but the grasshopper mouse doesn't know that. In fact, these rodents can get stung multiple times while they're eating bark scorpions, and they hardly even seem to notice! That's because grasshopper mice have evolved a unique kind of resistance to the scorpion's venom over the years, researchers say in Science.

This particular type of evolution is rare because pain signals—like the ones that would let you know if you've been stung by a scorpion—are generally a good thing. They alert the body to tissue damage or other health problems, so that you don't touch a hot surface for too long or hurt yourself without even knowing it.

However, Ashlee Rowe and colleagues compared the effects of bark scorpion venom on grasshopper mice and regular, pain-sensitive mice. They discovered that the toxins in the scorpion venom prevented certain neurons from firing in the grasshopper mice but activated them in the regular mice. With a series of experiments, they found that the scorpion's venom activated a particular pain receptor, known as Na¬V1.7, in the regular mice—as it does in most mammals—but not in the grasshopper mice.

After performing more experiments, the researchers realized that a different pain receptor in the grasshopper mice—NaV1.8—has special amino acids that bind to the bark scorpion's toxins and silence other nearby pain receptors. In fact, the bark scorpion venom seems to temporarily numb grasshopper mice from all kinds of pain—not just the pain brought on by the venom—for a little while, they say.