According to new research reported in the journal Science, potentially deadly stings of scorpions are nothing more than a slight nuisance to grasshopper mice.
Grasshopper mice are desert-dwelling rodents in the genus Onychomys, which includes three species – O. arenicola, O. leucogaster and O. torridus.
These species are well-known for their resistance to the venom of bark scorpions (Centruroides) on which they often prey.
“This venom kills other mammals of similar size. The grasshopper mouse has developed the evolutionary equivalent of martial arts to use the scorpions’ greatest strength against them,” said lead author Dr Ashlee Rowe from Michigan State University.
To test whether the grasshopper mice felt pain from the scorpion venom, Dr Rowe with colleagues injected small amounts of venom or nontoxic saline solution in the mice’s paws.
Surprisingly, the mice licked their paws much less when injected with the scorpion toxin than when injected with a nontoxic saline solution.
“This seemed completely ridiculous. One would think that the venom would at least cause a little more pain than the saline solution. This would mean that perhaps the toxin plays a role as an analgesic. This seemed very far out, but we wanted to test it anyway,” explained co-author Prof Harold Zakon from the University of Texas at Austin.
The scientists discovered that the bark scorpion toxin acts as an analgesic by binding to sodium channels in the mouse pain neurons, and this blocks the neuron from firing a pain signal to the brain. Pain neurons have a couple of different sodium channels, called 1.7 and 1.8, and the study has shown that when toxins bind to 1.7 channels, the channels open, sodium flows in and the pain neuron fires.
By sequencing the genes for both the 1.7 and 1.8 sodium channels, the researchers discovered that channel 1.8 in the grasshopper mice has amino acids different from mammals that are sensitive to bark scorpion stings, such as house mice, rats and humans. They then found that the scorpion toxin binds to one of these amino acids to block the activation of channel 1.8 and thus inhibit the pain response.
“Incredibly, there is one amino acid substitution that can totally alter the behavior of the toxin and block the channel,” Prof Zakon said.
Some resistance to prey toxins in mammals has been found in other species. The mongoose, for example, is resistant to the cobra. And naked mole rats’ eyes do not burn in pain when carbon dioxide builds up in their underground tunnels. This study, however, is the first to find that an amino acid substitution in sodium channel 1.8 can have an analgesic effect.
Bibliographic information: Rowe AH et al. 2013. Voltage-Gated Sodium Channel in Grasshopper Mice Defends Against Bark Scorpion Toxin. Science, vol. 342, no. 6157, pp. 441-446; doi: 10.1126/science.1236451