In classic experiments on frogs, scientists found that the amphibians’ urge to escape from dangerously hot water decreased significantly when the water temperature rose very gradually. In fact, sensitivity of many animals to temperature — including humans — is similarly affected by the rate of increase. Exactly why, however, has not been understood. “We know a lot about how animals sense large and sudden increases in temperature,” said Montell, the Patricia and Robert Duggan Professor of Neuroscience in UCSB’s Department of Molecular, Cellular, and Developmental Biology. “They respond to noxious heat by initiating an escape response. But how is it that animals are so much less sensitive to the same hot temperature when the change is really slow?” The answer to that question turned out to have two parts. First, Montell and his team identified the thermosensory neurons in the brain responsible for sensing the rate of temperature change, which helped to define the underlying molecular mechanism. Second, the researchers discovered that the fast response to rapidly increasing heat depended on a transient receptor potential (TRP) channel. The activation of the cellular temperature sensor, a protein called TRPA1, was not simply a function of the absolute temperature but rather depended on the rate of temperature change. If the temperature increase was rapid, TRPA1 turned on quickly and excited those thermosensory brain neurons. When the temperature increased slowly, TRPA1 was less active.
The scientists posit that the ability to sense the rate of temperature change is a critical survival mechanism that allows an animal to quickly respond to and escape from a noxious thermal landscape before it is too late.