Every time we go to sleep, our bodies turn off our senses of sight, sound and smell. However, there is another sense that always remains active, that is affected by almost no diseases and is more crucial to our survival than any other: pain. Without pain, we would never know when we were injured, which could be a potentially fatal oversight.
Past research at UC San Francisco was focused on identifying the pain transmitters in humans. They experimented by injecting capsaicin, the compound in peppers that makes them spicy, into animals. What they found was that the pain receptors are also responsible for transmitting sensations of heat. UC Davis researchers are now further investigating that link.
“These proteins work like the thermostat in your room,” said Jie Zheng, leader of the current research and an associate professor of physiology and membrane biology at the UC Davis School of Medicine. “Once temperature deviates slightly from where it should be, it sends a signal to the hypothalamus [the part of the brain that controls many automatic functions] to either produce more heat through metabolism or trigger heat dissipation through sweating.”
What the researchers found is that there are six proteins responsible for transmitting temperature sensations. Some of those proteins sense increases in temperature and others sense drops in temperature.
“Other proteins that sense cold also respond to menthol, giving us the cooling sensation,” Zheng said. “Vanilla and cinnamon give us a warm feeling because they activate other sensors. All of these [sensors] contribute to our body being able to maintain at 98.6 degrees.”
Even though there are only six proteins responsible for temperature sensation, they can activate in many possible combinations. At a certain temperature, the proteins activate in a certain combination, sending a specific temperature sensation to the brain.
“Each [combination] is activated at a different temperature,” said Fan Yang, a graduate student in physiology and membrane biology working with Zheng.
Now that researchers understand how these sensations are transmitted, they are looking for ways to selectively interfere with the transmission of these signals to the brain. Unfortunately, there are severe side effects to inhibiting our heat and pain sensitivity.
“These are the sensors that regulate body temperature,” Zheng said. “When we cut those out, we see extreme cases of hypothermia [extreme chilling] or hyperthermia [extreme overheating].”
There are multiple genetic diseases that deteriorate our sight and hearing over time, but there is only one known genetic disorder that removes sensitivity to pain and heat. According to Zheng and Fang, that fact indicates that the heat and pain senses are fundamental necessities for human survival, and any disease affecting them has been removed over time through natural selection.
Upon closer inspection, the researchers found that although heat and pain senses function the same at the cellular level, they function differently at the molecular level. This means that there is the potential to turn off either pain or heat. Medications could potentially alleviate pain in specific areas while maintaining other sensory functions or cut off the heat sensations in burn victims.
“Understanding temperature is crucial to human life,” said Peter Cala, chair of the department of physiology at UC Davis. “[We are] looking at the sense in enough detail that will allow us to fully understand how it all works.”
HUDSON LOFCHIE can be reached at science@theaggie.org.
