Mighty mouse uses scorpion venom as painkiller

Here we go: two weeks in a row. I think that technically counts as a habit, right?

Photo by Ashlee Rowe via /msutoday.msu.edu

Anyway, this is a pretty cool study out of the Department of Zoology at Michigan State University (actually it looks the lead author did most of the research at the University of Texas but I am choosing to ignore that; cf. sucks, Texas). The adorably vicious grasshopper mouse in this video is obviously unphased by the stings it receives from its lunch: a common bark scorpion. Apparently, the venom of this scorpion is strong enough to kill other mammals of the same size and is extremely painful to larger animals including humans. The venom normally works by specifically binding to and opening one of the sodium channels (Na_v1.7) that is expressed in pain receptor neurons. Opening of one of these channels causes depolarization of the membrane and thus an action potential which travels to the central nervous system and creates the sensation of pain. What this group found out is that, in grasshopper mice, the venom also binds to a different sodium channel (Na_v1.8). Both channels are found on the same neurons and serve the same function. When the venom binds to the 1.8 channel, however, it has an inhibitory effect which means that it closes the channel and keeps it from opening which prevents action potentials from propagating. In other words, not only is the mouse immune to the venom, but it actually has an analgesic effect. But why doesn't the same thing happen when the scorpion stings other animals? Apparently the venom does not bind to the Na_v1.8 channel of other mammals. The reason is a substitution mutation of a single amino acid in the protein structure of the Na_v1.8 channel that has only been found in this particular variety of mouse. The authors note that they do not have the full story and there is likely more to the mechanism of the venom that may contribute to the mouse's immunity, but it is a nice illustration of the neverending evolutionary arms race that goes on between predators and prey. The bark scorpion has few predators precisely because of its potent venom, but a single mutation carried by the grasshopper mouse has rendered the scorpion completely defenseless. For the mouse it means that it is not only safe from the venom, but it has access to a food source that no other competing predator can use.