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How a mouse evolved to ignore a scorpion's sting

The tiny grasshopper mouse doesn't seem to be bothered in the slightest by the sting of the Arizona bark scorpion, thanks to specially evolved nerve cells, finds new research. 

By Stephanie PappasLivescience.com Senior Writer / October 24, 2013

A southern grasshopper mouse eats the Arizona bark scorpions that it has just killed.

Ashlee and Matthew Rowe

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The sting of the Arizona bark scorpion is so fierce that humans say the pain is like being hit by a hammer. But the tiny grasshopper mouse shakes off the sting like it's nothing.

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Now, researchers have found for the mouse, the sting really is nothing. Instead of causing pain, the scorpion venom blocks it, a fact that could lead to the development of new pain-blocking drugs for people.

"The venom actually blocks the pain signal that the venom is trying to send" to the mouse, said study researcher Ashlee Rowe of Michigan State University. "We don't want to try to sound too cute or anything, but it is sort of like an evolutionary martial art, where the grasshopper mice are turning the tables. They're using their opponents' strength against them." 

Fierce mouse

Southern grasshopper mice (Onychomys torridus) are carnivorous desert-dwellers. Among their favorite meals are the Arizona bark scorpions (Centruroides sculpturatus). The scorpions' sting would kill any other rodent the size of the grasshopper mouse, but the little rodent can absorb many stings in the course of attacking a scorpion. In studying this phenomenon, Rowe noticed not only did the mice survive, but they also seemed unconcerned. [See Video of a Mouse Attacking a Bark Scorpion]

"I was really intrigued by the fact that the mice, if they get stung, they just groom a little bit and then it's over," Rowe told LiveScience.

Clearly, the mice had evolved to handle the pain. To find out how, Rowe and her colleagues analyzed how the toxin acts on the nerve cells called nociceptors that pick up and relay pain to the mouse's brain.

Nerve cells communicate pain to the brain by translating stimuli into electric pulses. To do so, tiny channels in the cell membrane, called ion channels, open and close. One ubiquitous type of ion channel, the sodium/potassium channel, is present in cells throughout the body. This channel makes critical bodily functions, from breathing to muscle contractions, possible.

Typically, scorpion venom acts directly on sodium/potassium channels in nociceptors to create the sensation of pain. A specialized channel known as channel 1.7 is responsible for picking up the pain signal, whereas a channel called channel 1.8 carries it to the brain.

"They just turn [the nerve] on and send that signal to the brain," Rowe said.

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