An international team of scientists has linked the sharp stabbing tooth pain that some of us might experience when eating or drinking something very cold, to the cold-sensing role of an ion channel, TRPC5, which is found in a layer of cells known as odontoblasts, in tooth pulp. The results indicate that when someone with a dentin-exposed tooth drinks or eats something very cold, the TRPC5-packed cells pick up on the cold sensation and an “ow!” signal then speeds to the brain.
The results of the scientists’ research in mice, and their closer look at human teeth, suggest that developing drugs which specifically target this ion channel sensor could feasibly eliminate tooth sensitivity to cold. “Once you have a molecule to target, there is a possibility of treatment.” suggested electrophysiologist Katharina Zimmermann, PhD, who led the work at Friedrich-Alexander University Erlangen-Nürnberg in Germany.
“We found that odontoblasts, which support the shape of the tooth, are also responsible for sensing cold,” added pathologist Jochen Lennerz, MD, PhD, one of the paper’s senior authors and medical director of the Center for Integrated Diagnostics at Massachusetts General Hospital (MGH). “This research contributes a new function to this cell, which is exciting from a basic-science standpoint. But we now also know how to interfere with this cold-sensing function to inhibit dental pain.”
The investigators’ findings, reported in Science Advances, may also offer up an explanation for how one age-old home remedy for toothache—clove oil—might help to dull the pain of cold sensitivity, as the main ingredient in clove oil contains a chemical that blocks the cold sensor protein. Zimmermann and colleagues report on their studies in a paper titled, “Odontoblast TRPC5 channels signal cold pain in teeth.”
Teeth decay when films of bacteria and acid eat away at the enamel, the hard, whitish covering of teeth. As enamel erodes, pits called cavities form. Worldwide, approximately 2.4 billion people—about a third of the world’s population—have untreated cavities in permanent teeth, which can cause intense pain, including extreme cold sensitivity, the research team wrote. “Inflamed teeth are extremely cold sensitive, perceived as a short, sharp intense neuralgic pain.” So, for people with tooth decay, drinking a cold beverage can be agony. “It’s a unique kind of pain,” commented co-author David Clapham, PhD, vice president and chief scientific officer of the Howard Hughes Medical Institute (HHMI). “It’s just excruciating.”
Tooth pain that occurs on exposure to cold can occur for other reasons. Teeth can also become very sensitive to cold from gum erosion due to aging. Some cancer patients treated with platinum-based chemotherapies have extreme cold sensitivity all over their bodies. “A breeze on the face registers as extreme pain in the teeth, which may even cause some patients to stop therapy,” said Lennerz.
To date it’s not really been understood how teeth sense the cold, though scientists had proposed one main theory. This suggests that tiny canals inside the teeth contain fluid that moves when the temperature changes, and that somehow, nerves can the sense the direction of this movement, which signals whether a tooth is hot or cold. “We can’t rule this theory out,” said Clapham, a neurobiologist at HHMI’s Janelia Research Campus. However, as the team noted, “Functional experimental evidence for this theory is lacking.”
Zimmermann, Clapham, and their colleagues didn’t originally set out to study teeth. Their work focused primarily on ion channels. These pores in cells’ membranes act like molecular gates. After detecting a signal—a chemical message or temperature change, for example— the channels either clamp shut or open wide and let ions flood into the cell. This creates an electrical pulse that transmits from cell to cell. It’s a rapid way to send information, and is critical for signaling in the brain, heart, and other tissues.
About fifteen years ago, when Zimmermann was a postdoc in Clapham’s lab, the team discovered that an ion channel called TRPC5 was highly sensitive to the cold. But the team didn’t know where in the body TRPC5’s cold-sensing role came into play. It wasn’t the skin, as their research found that mice lacking the ion channel could still sense the cold.
After that, “we hit a dead end,” Zimmermann acknowledged. The team was sitting at lunch one day discussing the problem when the idea finally hit. “David said, ‘Well, what other tissues in the body sense the cold?’” Zimmermann recalls. The answer was teeth. And by examining specimens from human adults, study coauthor Jochen Lennerz, PhD, a pathologist from Massachusetts General Hospital, discovered that TRPC5 does reside in teeth—and more so in teeth with cavities.
Tooth biology is, however, difficult to study. Scientists have to cut through the enamel—the hardest substance in the human body—and through the dentin layer, without pulverizing the tooth’s soft pulp and the blood vessels and nerves within it. Sometimes, the whole tooth “will just fall to pieces,” Zimmermann commented.
So for their research, the team first conducted experiments on mice whose molars were drilled under anesthesia. Mice with dental injuries manifest pain with their behavior; they drink up to 300% more sugar water than their littermates without dental injuries, for example. The team set out to identify the roles of TRPC5 and two other ion channels, TRPA1, and TRPM8, in dental cold sensitivity. TRPA1 and TRPM8 are found in the skin, where they play a role in sensing cold. “In the skin, TRPM8 and TRPA1 act synergistically and represent the key sensors of environmental cooling as well as painful cold,” the scientists explained.
The team first deactivated the gene for one of the three ion channels in different lineages of mice and compared their responses to dental pulp injuries. The results showed that only mice with deactivated TRPC5 channels responded to their injuries in the same way as uninjured control group, suggesting that TRPC5 is necessary for mice to perceive inflammatory tooth pain.
Next, to study the entire tooth sensory system, the researchers developed a mouse jaw-nerve model, which involved extracting a nerve preparation from the lower jaw, placing it in an organ bath, and exposing it to chemical compounds and cold temperatures. Using this novel jaw-nerve preparation the researchers could record electrical activity from intact teeth. Experiments with the system showed that administering TRPC5 blockers significantly reduced the nerve preparation’s cold responses.
“This experimental model allowed us to close an important gap in our understanding of tooth pain as it is the first model that allows the assessment of the tooth sensory system in its entire anatomical and, necessarily, physiological context in transgenic mice,” the scientists wrote.”We now have definitive proof that the temperature sensor TRCP5 transmits cold via the odontoblast and triggers nerves to fire, creating pain and cold hypersensitivity,” says Lennerz. “This cold sensitivity may be the body’s way to protect a damaged tooth from additional injury.”
The team traced TRPC5’s location to a specific cell type, the odontoblast, which resides on teeth between the pulp and the dentin.
Specifically, in response to cold, the TRCP5 protein opens channels in the membrane of odontoblasts, enabling other molecules, such as calcium, to enter and interact with the cell. If the tooth’s pulp is inflamed from a deep cavity, for example, TRCP5 is overabundant, causing increased electrical signaling via the nerves emerging from the root of the tooth and running to the brain, where pain is perceived. When gums recede from aging, teeth can become hypersensitive because the odontoblasts are sensing cold in a newly exposed region of the tooth. “Most cells and tissues slow their metabolism in the presence of cold, which is why donor organs are put on ice,” says Lennerz. “But TRPC5 makes cells more active in cold, and the odontoblasts’ ability to sense cold via TRPC5 makes this discovery so exciting.”
Lennerz confirmed the presence of the TRPCS protein in extracted human teeth, which was a technical tour de force. “Our teeth aren’t meant to be cut into ultra-thin layers so they can be studied under the microscope,” says Lennerz, who first had to decalcify the teeth and put them in epoxy resin before slicing them and identifying the TRPC5 channels in the odontoblasts.
From their combined studies in mice, and also of adult human teeth, the team concluded, “Our data provide concrete functional evidence that equipping odontoblasts with the cold-sensor TRPC5 expands traditional odontoblast functions and renders it a previously unknown integral cellular component of the dental cold sensing system … The higher percentage of TRPC5 in pulpitic teeth and the presence of dentinal fibers within the normal and degenerating dentinal tubules suggest that TRPC5 also acts as a cold sensor in human teeth.”
The results point to the potential to target TRPC5 for treating dentin hypersensitivity and inflammatory tooth pain, the team noted. Interestingly “Oil of cloves’ (Syzygium aromaticum) main ingredient, eugenol, has been used for centuries as an analgesic in dentistry and inhibits TRPC5 currents, they pointed out.
Toothpastes containing eugenol are already on the market, but the newly reported findings may lead to more potent applications to treat teeth that are hypersensitive to cold. And there may be novel applications for eugenol, such as treating patients systemically for extreme cold sensitivity from chemotherapy. “I’m excited to see how other researchers will apply our findings,” said Lennerz.