Pain, pain, go away

 
 

The discovery of a mouse resistant to a scorpion sting could have positive implications for treating chronic pain, writes Sara Holbrook

A desert mouse has been discovered in the south-western US deserts that cannot feel pain from the sting of a scorpion. These grasshopper mice have evolved to the point that they are now immune to the sting of the Arizona bark scorpion, a species only found in the same area as the mice themselves.

This scorpion is generally able to protect itself from predators with its extremely painful venom, but now there is a strain of mouse that is essentially higher up on the food chain than this once lethal predator.

According to research published in Science on October 24th of this year, there have been small changes in these mice that allow for this pain immunity. These changes in question have been to the amino acids in a pain-transmitting sodium channel. While the mice do consume the venom from the sting, the pain is not transmitted and so, while they are experiencing the pain, they technically cannot feel it.

It was evolutionary neurobiologist Ashlee Rowe who first noticed that this strain of mouse surprisingly did not react to the venom of a scorpion that was known to have a painful sting, both from human accounts and from the reactions of other mammals.

Rowe continued her work at the University of Texas, Austin where she explored the role of Nav1.7 and Nav1.8, two sodium channels that are found on pain-transmitting neurons called nociceptors. The reactions of the neurons in the grasshopper mouse were compared to those of the house mouse.

It was discovered that in both mice, the venom bound and activated Nav1.7, but that in the grasshopper mouse it constrained the activity of the Nav1.8 channel. This means that while both mice were affected by the venom, it was only the regular house mice that felt the pain while the pain transmission was blocked in the grasshopper mice.

Pain is a warning signal to both humans and animals. It is a simplified feeling induced to remind animals in the wild that their actions could result in death. These mice cannot feel the pain that other mammals experience when stung by the scorpion, and so they need no longer fear the bark scorpion.

Neuroscientist Thomas Park, of the University of Illinois in Chicago, who was not involved in the research, has highlighted that pain is very important and it is not that the mice can feel no pain; they are only resistant to the pain caused by the sting from this specific scorpion.

He said, “The grasshopper mouse has found a way, very cleverly, to disconnect the pain pathway.” The mice can still feel other sources of pain, which is very important as complete loss of pain means that the mice would have no way of knowing that they were hurt and no reason to avoid danger or predators. This would decrease their survival rate substantially.

Could this pain resistance have any impact for humans? The blocking of all pain transmissions would be just as dangerous in humans as it would be in mice. Pain is a signal to a person that something is wrong within the body, if humans could not feel pain, then serious damage could be done to the body without the person noticing.

When you pick up a hot plate and you feel pain in your fingertips, it is that pain that makes your hand release the plate before the burning gets too severe. If you could not experience pain, you would continue to hold the hot plate until your fingers were damaged beyond repair.

Also, a sudden appearance of pain is the body’s way of notifying people that they could have a serious disease. Modifying a person so that they were unable to feel any pain would in fact be more dangerous for a person than being able to fully feel pain.

This method of pain blocking could be useful in cases of chronic pain. Chronic pain is when pain signals are still active in the nervous system long after the time of healing and while acute pain is necessary as a warning signal; chronic pain is of no use to anyone.

This new discovery could be used to treat chronic pain, such as phantom limb painn that persists after a limb has been amputated. In these types of cases, it could be very useful for a person to have the same immunity that the desert mouse has developed.

The Nav1.8 channel has already been looked at in terms of solving cases of inexplicable pain and this discovery could give scientists new ideas on how to deal with pain. Instead of having to stop the pain at its source, they can block the transmission of the pain along the nervous system.

If it could be used in humans in a way that just blocked one type of pain, in the same way the mice are only immune to the scorpion sting, it could be very effective. It could increase the quality of life from those suffering from conditions such as arthritis. If the pain from just this disease could be prevented from being transmitted, it could exponentially benefit sufferers of the disease.

This novel pain-blocking mechanism could be useful in certain human cases and it is fascinating that such a small amount of variation in the make up of these mice can have such a large effect on their entire being. It really is the little things that make all the difference.

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