Of all the rodents used for pain research, the naked mole-rat (Heterocephalus glaber) certainly stands out. This African animal, which can live for over 30 years and is resistant to cancer, also shows insensitivity to different kinds of noxious stimuli. To bring attention to this unusual creature and what it has taught pain researchers, Ewan St. John Smith, University of Cambridge, UK, presented a virtual seminar hosted by the IASP Pain Research Forum on April 13, 2021. A Q&A session moderated by Thomas Park, University of Illinois at Chicago, US, followed the presentation.
During his talk, “Lessons in Nociception from the Naked Mole-Rat,” Smith showed that naked mole-rats do not show any behavioral response to capsaicin or acid, and that nerve growth factor fails to evoke thermal hyperalgesia in these animals. Mechanisms underlying these unique characteristics of the nociceptive system include a lack of key nociceptive neuropeptides in cutaneous sensory fibers, increased developmental pruning of C-fibers, and amino acid changes in the voltage-gated sodium channel Nav1.7, among others. Smith stressed that the study of the extreme biology of naked mole-rats can help advance understanding of nociception from an evolutionary perspective and deepen the pain field's overall grasp of how other animals, including humans, detect noxious stimuli and feel pain.
A recording of the seminar is freely available to IASP members here.
Oddities in the pain system
Smith began by orienting the audience to basic facts about naked mole-rats. These animals are subterranean rodents found in Ethiopia, Kenya, and Somalia. They are eusocial, meaning they live in large colonies with a queen that is the only breeding female, and poikilothermic (cold-blooded), with their body temperature largely regulated by ambient temperature. They can also live for more than three decades.
Naked mole-rats also stand out when it comes to nociception. A 2008 study from Smith, Park, and colleagues showed that naked mole-rats showed hardly any behavioral response to hind paw administration of capsaicin or acid (pH 3.5), in contrast to mice (Park et al., 2008). Additionally, naked mole-rats did not develop thermal hyperalgesia after capsaicin or nerve growth factor (NGF) injection. This first set of data raised the question: Do naked mole-rats have “normal” nociceptors?
To address that question, the researchers also investigated the electrophysiological features of naked mole-rat sensory fibers in the above study. Skin-nerve recordings revealed normal heat and mechanical responses, with the animals looking very similar to mice in that regard. However, when looking at the composition of naked mole-rat saphenous nerves with electron microscopy, the investigators found that the animals had about a 1:1 ratio of C- to A-fibers, in contrast to mice and most mammals, which usually show an approximately 4:1 ratio.
Was this lower ratio in naked mole-rats the result of fewer C-fibers, or more A-fibers, and was it species specific? To address that question, Smith and colleagues compared naked mole-rats to other members of the African mole-rat family (Smith et al., 2012). Naked mole-rats had a deficit of C-fibers only in cutaneous nerves, with muscle-innervating nerves containing a C- to A-fiber ratio similar to other rodent species tested. Compared to body surface area, a C-fiber deficit was shown to underpin the low cutaneous C- to A-fiber ratio. Knowing that the naked mole-rat is the only naturally hairless rodent, the investigators asked if a paucity of hair follicles could account for this observation. Using a conditional knockout mouse that loses its hair, the researchers found that the ratio of C- to A-fibers in the knockouts was only slightly lower than its wild-type control, leading to the conclusion that the naked mole-rat's lack of hair did not account for the altered C- to A-fiber ratio.
Still seeking an answer to the riddle, Smith and colleagues would find a more extravagant developmental pruning of C-fibers in naked mole-rats compared to mice (Omerbašić et al., 2016). In particular, the naked mole-rats exhibited a greater decrease in C-fibers from a young age into adulthood in the saphenous and peroneal nerves.
Next, Smith reviewed data on naked mole-rat neuronal function to understand where the pain insensitivity in the animals comes from. Park and colleagues had demonstrated that cutaneous nerves from naked mole-rats lack CGRP and substance P, two important neuropeptides involved in nociceptive transmission (Park et al., 2003). And, as noted earlier, naked mole-rats are behaviorally insensitive to capsaicin, yet their C-fibers did respond to this chemical in skin-nerve recordings. However, additional results revealed altered central connectivity of capsaicin-sensitive afferents with second-order neurons in the superficial laminae of the dorsal horn of the spinal cord, compared to mice. Intrathecal injection of substance P rescued behavioral capsaicin sensitivity in naked mole-rats by boosting superficial dorsal horn signaling. A similar mechanism underpins the absence of histamine-induced itch in the naked mole-rat, which can also be rescued by intrathecal substance P (Smith et al., 2010).
Acid insensitivity
Smith next turned to the acid insensitivity of naked mole-rats, asking whether it could result from a lack of peripheral input to the central nervous system.
Smith and colleagues had shown that naked mole-rats were insensitive to acid at pH 3.5, spending no time at all licking the injected paw compared to the vigorous licking exhibited by mice. Moreover, naked mole-rat saphenous nerve fibers did not respond to low pH in electrophysiological recordings – that is, a lack of peripheral input explained the absence of behavior (Park et al., 2008). Pointing to findings from a 2011 study in Science(Smith et al., 2011see related PRF news story), Smith said that naked mole-rat dorsal root ganglion (DRG) neurons rarely fired action potentials in response to acid. In addition, when cloned TRPV1 and acid-sensing ion channels 1a/b (ASIC 1a/1b) from mice or naked mole-rats were expressed heterologously, there was normal sensitivity of these proton-gated ion channels to low pH, a finding that suggested that the ability of acid to activate naked mole-rat neurons is similar to what is observed in mice.
Still seeking an explanation to the acid insensitivity in naked mole-rats, Smith next turned to voltage-gated sodium channels. Smith explained that low pH inhibits these channels, pointing to data from mouse and naked mole-rat DRG neurons, with the latter showing a greater inhibition of voltage-gated inward currents in response to protons.
To understand why, Smith’s group asked whether there were molecular differences in the composition of the sodium channels. They found that naked mole-rats exhibited two amino acid changes in the Nav1.7 sodium channel, which render it more sensitive to proton block than other vertebrate sodium channels.
Smith then focused on ASIC3, since this highly proton-sensitive ASIC channel is known to play a role in pain and central carbon dioxide chemosensing. Smith and colleagues discovered that naked mole-rat ASIC3, when in homomeric form, was unresponsive to protons, yet could form functional heteromers (Schuhmacher et al., 2018). From an evolutionary point of view, the formation of nonfunctional homomers most likely further blunts nociceptor sensitivity to acid, perhaps serving as an adaptation to the hypercapnic [increased carbon dioxide] subterranean environment in which naked mole-rats live.
Nerve growth factor and thermal hyperalgesia
Smith then discussed why nerve growth factor (NGF) fails to induce thermal hyperalgesia in naked mole-rats, a phenomenon that depends on sensitization of TRPV1. One of Smith and colleagues' studies reported an absence of NGF-induced TRPV1 sensitization in isolectin B4 (IB4)-negative neurons (which show immunoreactivity to TrkA, the high-affinity receptor of NGF) from naked mole-rat sensory neurons (Omerbašić et al., 2016). But transfection of naked mole-rat TRPV1 into sensory neurons from TRPV1 knockout mice restored NGF sensitization, suggesting that the absence of thermal hyperalgesia in naked mole-rats is not because naked mole-rat TRPV1 cannot be sensitized.
Consequently, Smith and colleagues investigated the role of TrkA. They showed that amino acid variations in the naked mole-rat TrkA kinase domain rendered it hypofunctional, which explained the absence of NGF-induced thermal hyperalgesia in the animals.
Finally, Smith highlighted some more recent research from other groups, including the first nerve injury study, published last year, using naked mole-rats (Poulson et al., 2020). This work showed that naked mole-rats with a spared nerve injury, a model of neuropathic pain, develop mechanical hypersensitivity but not cold hypersensitivity. Another 2020 study investigated the descending modulation of pain in naked mole-rats, demonstrating that the noradrenergic receptor system is present and involved in formalin test-related antinociceptive mechanisms, similarly to other mammals (Mwobobia et al., 2020). Lastly, Smith discussed a 2019 study that identified pain insensitivity in other mole-rat species (Eigenbrod et al., 2019related PRF news story).
Food for thought
After the seminar, Smith provided PRF his perspective by email on some important questions relevant to his talk. First, how can findings on the pain insensitivity of naked mole-rats be translated to novel therapeutics for pain treatment in people?
“With regard to drug development, I would argue that although we have learned a lot from mice and rats, I think there is great potential to learn even more about pain neurobiology by studying a species that has highly different nociception compared to humans, i.e., the naked mole-rat,” Smith wrote. “From our work on acid insensitivity in naked mole-rats, we identified the voltage-gated sodium channel Nav1.7 as being a key component, such that amino acid variations result in greater acid inhibition of naked mole-rat Nav1.7 than human Nav1.7. Considering the role of tissue acidosis in inflammatory pain, this work builds on that of others in mice and humans, which has shown the importance of Nav1.7 in nociception, and indeed there are clinical trials taking place targeting Nav1.7. So overall thus far, although there is no drug designed purely off the back of naked mole-rat research, it is still early days, and I believe that there is still a lot to learn, especially with regard to mechanisms underpinning their diminished inflammatory pain.”
Another question is whether male and female naked mole-rats look similar in their lack of pain responses.
“It is rather complicated in naked mole-rats,” Smith wrote. “This is because there is usually only one breeding male and female in each colony, with the other animals being kept in a prepubescent state, such that they are not fully sexually mature animals. Breeding males and females are rather precious, and so to my knowledge no one has done a full comparison of nocifensive behavior in subordinate males/females versus breeding males/females, but it would certainly be interesting to look at, considering the significant differences in nociception and neuroimmune interactions between males and females of other species.”
PRF Correspondent Francisco Isaac Fernandes Gomes, DDS, is a PhD student at the University of São Paulo, Brazil.