Naked mole-rats know no pain
This comical creature has a rather unenviable claim to fame. Along with the blobfish, the naked mole-rat is often described as "one of the ugliest creatures in the world."
It all sounds quite mean, but it can't be denied. Perhaps the naked mole-rat is simply missing a little fur. Otherwise, he would be as cute and cuddly as a hamster.
But, in reality, we could - and perhaps should - learn a lesson or two from Heterocephalus glaber, also known as the sand puppy or desert mole rat. Not only is the naked mole-rat cancer-resistant and incredibly long-living. It is also virtually immune to pain.
Scientists from Berlin's Max Delbrück Center (MDC) for Molecular Medicine published their findings about the animal on Tuesday, in the scientific journal Cell Reports.
The researchers studied sensitivity to a particular type of pain, so-called thermal hyperalgesia, or hypersensitivity to heat where there is inflammation. A good example would be sunburn. Even the mildest rays of sun on an already-burnt skin can cause a disproportionate amount of pain.
That's how it is for most animals, but not for the naked mole-rat. It barely registers the pain.
The reason? A receptor molecule is slightly altered in the unusual rodent, which lives in desert regions around the Horn of Africa. In an existing inflammation, nerve growth factor (NGF) molecules normally bind to a receptor known as TrkA in the sensory neurons.
In this way, a series of signals - a cascade effect - is set in motion that triggers a pain alarm in the brain. The cascade is not completely switched off, but it takes ten times the amount of NGF for a naked mole-rat to react like other creatures.
"Although the naked mole-rat version of the TrkA receptor is almost identical to that of a mouse or a rat, there is a significant effect on the ability of the animals to feel pain," said Gary Lewin from MDC.
While thermal hyperalgesia has a protective effect for us, in preventing further damage to injured or inflamed tissues, it doesn't for the naked mole-rat.
This is a really sensible step in the animal's evolution," said Lewin, adding that the animals already live in a state of permanent metabolic deprivation. The naked mole-rat saves energy by switching off every bodily system that is not necessary for the body to function.
Lewin is very enthusiastic about naked mole-rats. Six colonies, comprising some 120 animals in total, are housed at the MDC, and they are a source of constant astonishment for the researcher.
The animals also seem to have an inner fountain of youth. They don't get cancer and their cells hardly age. Their lifespan of some 30 years far exceeds that of other rodents. A mouse, by comparison, lives for just two years on average.
The researchers have also found that the animals barely respond to chemical stimuli such as acid or chili extract, something that's probably due to the extreme living environment of the animal. They live in narrow, dark caves - densely packed in colonies with up to 300 companions. As a result, oxygen content of the air is very low, with the carbon dioxide levels so high that a human could barely survive.
Such high carbon dioxide would usually lead to the permanent activation of pain receptors. In the case of naked mole-rats, that mechanism has been shut down over the course of evolution.
What might we learn from all of this? The researchers hope to gain insight into the normal pain sensations of mammals and, more specifically, humans.
A new study is underway into the inhibition of the NGF function with antibodies. If this is successful, a new means of pain relief might become available. "It would be using exactly the same mechanism that the naked mole-rat already uses," said Lewin.