Pain, unpleasant though it may be, is essential to most mammals’ survival, a warning to back off before we lose a limb or worsen a wound. So it was curious when, in a 2008 study, molecular physiologist Gary Lewin and his colleagues found that, unlike most mammals, naked mole rats (Heterocephalus glaber) didn’t lick or flick a limb that had been injected with a small amount of capsaicin—the hot in hot chili pepper. The mole rats turned out to be similarly nonchalant when exposed to dilute hydrochloric acid. “We wondered, first of all, how they became insensitive to these things,” says Lewin, who heads up a lab at Berlin’s Max Delbrück Center for Molecular Medicine.
The team took an evolutionary approach to finding the answer. Several group members traveled to the naked mole rat’s native territory of East Africa to try out three common pain-causing substances on seven other mole rat species, plus the more distantly related East African root rat. They found that, in addition to the naked mole rat, the Natal mole rat was insensitive to capsaicin, while the Cape mole rat and the root rat didn’t seem to feel a burn from the hydrochloric acid. Most startlingly, one species, the highveld mole rat (Cryptomys hottentotus pretoriae), didn’t flinch when injected with a few milliliters of a highly diluted solution of an irritant present in mustard and wasabi known as AITC—an agent that even the naked mole rat reacted to.
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When team member Karlien Debus donned a gas mask to inject a similar amount of 100 percent AITC under the skin of a highveld mole rat, there was still no response. “Probably the AITC was the most interesting because AITC is a substance that actually every [other] animal in the entire animal kingdom avoids,” Lewin says. An electrophilic compound, AITC can crosslink an animal’s proteins and damage its cells.
Intrigued, the researchers took a look at the highveld mole rats’ TRPA1 gene, which codes for the receptor that detects AITC, and found some differences in sequence compared with the same gene in other rodent species. But they suspected these differences couldn’t completely explain the level of insensitivity they saw. So the researchers compared the sequences and expression levels of more than 6,000 of the highveld mole rat’s genes with those of the other, AITC-sensitive species the team had studied. One hit was a gene that codes for a sodium channel protein called NALCN: while the sequence was nearly identical among the species, the highveld mole rat was churning out far more RNA transcripts from the gene in a pain-sensing region of the spine known as the dorsal root ganglia. Unlike some ion channels, NALCN channels are always open, Lewin says, allowing sodium ions into the cell. This means that in neurons that overproduce the channel, “the membrane potential is slightly depolarized, and it’s also harder for electrical stimuli to excite action potentials.”
To test whether NALCN was really key to the species’ insensitivity to AITC, the researchers injected the agent into highveld mole rats that had been given a drug to block the channel. These animals displayed normal pain behaviors, confirming the channel’s role (Science, 364:852–59, 2019).
We wondered, first of all, how they became insensitive to these things.—Gary Lewin, Max Delbrück Center for Molecular Medicine
Lewin and colleagues were left with the question of why highveld mole rats had evolved not to mind such a noxious substance. Daniel Hart, a postdoc at the University of Pretoria who trapped some of the animals used in the study, suggested an answer: highveld mole rats frequently live in burrows alongside Natal droptail ants (Myrmicaria natalensis), an aggressive species that chomps animals with its mandibles and then injects them with a venom made in its abdomen. Could the highveld mole rats’ surfeit of NALCN ion channels in its dorsal root ganglia be a way of enabling them to live comfortably alongside these unpleasant roommates?
To find out, the researchers ground up the abdomens of Natal droptail ants to make what Lewin terms “ant juice” for use in a new pain test, and injected it into highveld mole rats. They also tried formic acid, which works similarly to AITC and which many ant species use for defense. Sure enough, the rodents remained stoic, while other mole rat species reacted in the same way they had to AITC.
The researchers suspect that insensitivity to the ants’ sting “would probably give this particular species an advantage because it was able, then, to populate areas of Africa which are infested with the Natal droptail ant, which other mole rat species would basically avoid,” Lewin says.
Lindsey Macpherson, a taste researcher at the University of Texas at San Antonio who has studied the TRPA1 protein but was not involved in the study, agrees with that interpretation. “If they have to live there and they have no choice but to interact with these biting ants, I guess their best option is to eventually become insensitive to their stings,” she says. Were they to live in different environments with higher risk of predation or other harm, “pain would actually be more of a benefit to them and they might not have that advantage anymore.”
The study’s combination of fieldwork, molecular biology, and physiology makes for “a fascinating story,” says Paul Heppenstall, a neuroscientist at the European Molecular Biology Laboratory in Rome who did a postdoc with Lewin but was not involved in the new study. The identification of the NALCN channel as being involved in pain sensing, in particular, is novel, he says, and blocking the channel with a drug to determine its involvement “was just such a clear, beautiful experiment that showed that this was the main hinge which these mole rats were using to regulate the pain sensitivity.”
Indeed, the study’s authors suggest that the channel, whose sequence is highly conserved among mammals, could be a target for the development of new pain therapies. “It’s now actually quite conceivable that you could design a drug that would bring NALCN channels more to the membrane in a mouse or human” to shut down pain responses, Lewin says. Heppenstall agrees this is an exciting angle to explore, though he concedes it’s not yet clear whether NALCN mediates types of pain beyond those caused by the compounds tested in the study. “A big question is: What else is [overproduction of this channel] doing? . . . I don’t think that they really answered that.”
Jianguo Cheng, an anesthesiologist and pain researcher at the Cleveland Clinic who was not involved in the study, also sees potential in the NALCN finding for new, safer avenues toward treating pain. “It’s exciting because it demonstrates [that] evolution has figured out a way to render [a] certain pain insensitivity that still enables the animals to survive and have a normal function.”
Shawna Williams is an associate editor at The Scientist. Email her at firstname.lastname@example.org or follow her on Twitter @coloradan.