“The bark scorpion is, according to Wikipedia, the most venomous scorpion in North America, wielding an intensely painful – and potentially lethal – sting that stuns and deters snakes, birds and other predators. People unfortunate enough to have experienced the sting say that it produces an immediate burning sensation, followed by prolonged throbbing pain that can last for hours.
But the grasshopper mouse is completely resistant to the bark scorpion’s venom. In fact, it actively preys upon scorpions and other poisonous creatures. As the film clip below shows, it responds to the bark scorpion’s sting by licking its paw for a second or two, before resuming its attack, then killing and eating the scorpion, starting with the stinger and the bulb containing the venom. Researchers have now established exactly why this is – paradoxically, the venom has an analgesic, or pain-killing, effect on the grasshopper mouse.
The animal’s secret lies in two proteins, the sodium channels Nav1.7 and Nav1.8, which are found in a subset of sensory nerve fibres called nociceptors. These cells express numerous other proteins that are sensitive to damaging chemicals, excessive mechanical pressure, and extremes in temperature, and have fibres that extend from just beneath the skin surface into the spinal cord.
The sensor proteins relay these signals to Nav1.7 and Nav1.8, which then change their structure in response, so that their pores, which span the nerve cell membrane, open up, allowing sodium ions to flood into the cell. This causes the nociceptors to generate nervous impulses, which are transmitted along the fibre into the spinal cord. From there, the signals are relayed to second-order sensory neurons, which then carry the signals up into the brain…
Ashlee Rowe of the University of Texas in Austin and her colleagues started off by injecting scorpion venom, formaldehyde and salt water into the hind paws of southern grasshopper mice and common house mice, and compared their behavioural responses.
The house mice licked their paws furiously for several minutes after being injected with venom or formaldehyde, but not when they were injected with salt water. By contrast, the grasshopper mice seemed completely oblivious to the venom, and barely licked their paws at all after being injected with it. They found the formaldehyde to be far more irritating, and the venom actually reduced the amount of time they spent licking their paws when the two were injected together.
Further investigation revealed that the scorpion venom directly binds to, and potently inhibits, Nav1.8 sodium channels from the grasshopper mice, but not the house mice.
As a result of these tiny structural changes, scorpion venom binds to Nav1.8 and switches it off, perhaps by plugging the pore or making it impermeable to sodium ions in some other way, thus blocking the transmission of… signals into the spinal cord.”
WARNING – video of mouse eating scorpion following
If you watch carefully at the beginning, you can see the mouse react to what I presume is the ‘mechanical insult’ of the sting, but then nothing, no effect from the venom at all, just a hungry mouse and nom, nom, nom.
I guess humans are not the only ones that are, in Pat Wall’s words, “Evolved animals….. evolved, challenged by certain historical problems in how to remain alive”
– Tim Cocks
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