Old experiments have led to the notion that frogs boiled gradually will not try to escape, while a sudden dunk in hot water will result in the frog jumping out. While the truth of that can be debated, it is known that many animals, including humans, are not only sensitive to temperature but also the rate at which temperature changes. It’s not well understood exactly why, however.
Researchers at the University of California Santa Barbara (UCSB), led by Professor Craig Montell wanted to investigate this further. The team used fruit fly larvae to uncover mechanisms underling behavioral changes that are seen when the larvae are subjected to fast and slow increases in temperature.
Publishing in Nature Neuroscience, the investigators determined that a fast 25 degree Fahrenheit rise in temperature resulted in a writhing behavioral response in fruit fly larvae. Conversely, when the same change in temperature happened slowly, fewer animals displayed the writhing response and among those that did, the temperature at which the behavior started was higher.
"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?"
Montell and his team were able to answer that question. They found the themosensory neurons of the brain that sense the rate at which temperature changes. That led them to discover the mechanism that behind the process.
"When there is a really rapid change in temperature, you want to protect the brain, particularly in fly larvae because they're cold-blooded and their body temperature equilibrates to the outside," Montell explained. "If their brains feel a rapid increase in temperature, that stimulates the writhing response."
The researchers learned that a particular receptor is responsible for sensing fast temperature changes. The cell has a sensor of cellular temperature, a molecule called TRPA1. The activation of TRP1A is dependent, however, on the rate of changes in temperature. A rapid increase in temperature resulted in a fast activation of TRPA1, exciting the thermosensory neurons. TRP1A was less activated by a slow increase in temperature.
"There's a feedback mechanism that turns off this protein as quickly as it's turned on," Montell said. "When it's turned on quickly, it stimulates the pathway to cause the writhing response. But when the process occurs slowly, the on and off mechanisms cancel each other out."
"We think similar mechanisms occur in other animals—for example, the frog," Montell continued. The investigators suggest that the sensation of rapid changes in temperature could help an organism respond quickly to a dangerous change in its environment.
"It could be that related mechanisms affect temperature-sensitive TRP channels in humans as well. Ultimately, if a common mechanism is conserved across species, our findings may provide insight into how different animals adapt to the rate of temperature change," concluded Montell.
Sources: Phys.org via UCSB, Nature Neuroscience