“The grasshopper mouse has developed the evolutionary equivalent of martial arts to use scorpions’ greatest strength against them.” That is how Ashlee Rowe describes her recent research, published in Science. Bark scorpions are known for their intensely painful and often lethal stings. But to grasshopper mice, which feed on the scorpions, the stings are merely a little annoying, distracting them briefly from their feast.
“This venom kills other mammals of similar size,” said Rowe, now an assistant professor at Michigan State University. Her work shows how the mice have evolved numbness to the venom, but it is still unclear why the toxins are not lethal as in comparable species.
Mice will typically lick their paws in response to injection with a toxin. In response to the venom of the bark scorpion, house mice dramatically increase their rate of paw licking compared to when a nontoxic saline control is used. But grasshopper mice lick their paws less in response to the venom than when saline is given.
“This seemed completely ridiculous,” said Harold Zakon, a professor at the University of Texas at Austin, where the study took place. It was suggested that the toxin may have a role as an analgesic (painkiller) in the mice. “This seemed very far out, but we wanted to test it anyway,” Zakon said.
In the nervous systems of mammals, acute pain is transmitted by sodium channels called Nav1.7 and Nav1.8. In most mammals, scorpion toxins bind Nav1.7, but have no effect on Nav1.8.
However, what Rowe and Zakon showed is that grasshopper mice have small changes in their Nav1.8 protein. As a result of one amino acid substitution, Nav1.8 in these mice can bind the scorpion toxin. This inhibits painful sodium currents and has an analgesic effect.
While it is now known which parts of the channel are involved in this, Rowe believes there is more to uncover. She said: “There’s something else that’s playing a role, and that’s what I’m focusing on next.” She is also hopeful that this study and others like it will contribute to the future development of new painkillers that are both highly selective and nonaddictive.