Individuals would have often wondered why a bite of warm cherry pie infuses their mouth with sweetness, but the same slice taken directly from the refrigerator is not as tempting as before?
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While researchers are well aware of this phenomenon, the mechanism behind it has not been understood well.
Craig Montell, a Distinguished Professor from the University of California, Santa Barbara (UC Santa Barbara) has identified a process that is responsible for this phenomenon, and he made this discovery by using fruit flies as his subjects.
Montell’s research team, which comprises Qiaoran Li, Nicolas DeBeaubien, and Takaaki Sokabe, discovered that the appeal of sweetness is suppressed by cool temperatures.
But such conditions did not influence the sugar neurons themselves and instead acted through other sensory cells through a protein that was initially found to sense light in the eye.
In spite of this, the perceived coolness in sugary food is not changed by light. The study results were reported in the Current Biology journal.
The appeal of food is influenced by more than just chemical composition. We already know that cool temperatures reduce the delectability of sweetness in humans.”
Craig Montell, Duggan Professor, Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara
Montell and his collaborators speculated if this was also true in the case of fruit flies and if so, what were the fundamental mechanisms?
The researchers identified a major difference in the fruit flies’ interest in feeding between the temperature of 23 °C (73.4 °F) and 19 °C (66.2 °F). But in spite of the behavioral change, the researchers did not find any difference in the activity of the sweet-sensing taste neurons of the flies.
Since the temperature is not directly affecting the sugar neurons, it must be affecting some other types of cells, which then indirectly affect the propensity to consume sugar.”
Craig Montell, Duggan Professor, Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara
Fruit flies use one type of taste neuron to detect sugar. While another type of neuron senses bitter, mechanosensory neurons sense the texture of food, like hardness. But the temperature sensation is not as simple as it sounds.
Both mechanosensory and bitter neurons are also involved in perceiving coolness. The brain will interpret that as a cool signal only when both neurons are stimulated.
All these stimuli appear to decrease the animal’s preference to feed, Montell explained. Bitter neurons are activated by bitter compounds, which inform the fly to cease feeding. The mechanosensory neurons, triggered by hard foods, also inform the fly to cease feeding. And cool temperatures activate both neurons, to the same effect.
Rhodopsin 6 is a protein that is crucial to this response. Rhodopsins are most commonly linked with vision, but in the past few years, the Montell research team has linked rhodopsins to a wide range of other senses.
In fact, only two weeks before, Montell’s laboratory published the first study linking different members of this group of protein to chemical taste.
Montell added, “The bitter neurons express this rhodopsin called Rh6, and if you get rid of it, then cool temperatures no longer suppress the appeal of sugar.”
But without rhodopsin 6, the bitter-and-cool-detecting neurons were not triggered any more by low temperatures. Considering that cool sensation involves triggering numerous and different types of neurons, the loss of rhodopsin 6 inhibits the fly from detecting the lower temperature and thus eliminates the diminished attraction to sugary food.
The surprise was finding that it was really the other neurons, not the sugar neurons, whose activity went up, and that the cool activation of other neurons was indirectly suppressing the sugar neurons.”
Craig Montell, Duggan Professor, Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara
At low temperatures, the sweet-sensing neurons are still stimulated by sugars but the stimulation of these other neurons by reduced temperature inhibits the interaction between the animal’s brain and the sweet-detecting neurons. This is probably acquired by an inhibitory neurotransmitter discharged by the bitter/cool-activated neurons.
With regards to why fruit flies avoid food during cool conditions, Montell believes that it is because of their metabolism. The metabolism and thus the food requirements of fruit flies are influenced by temperature.
Lower temperatures imply slower metabolisms and minimal requirements for food. On the whole, cold food means a cold fly.
The fly generation time—that is, the time tak
en by an egg to transform into an adult fly—increases by twofold between 10 and 20 days when the temperature is reduced from 25 °C to 18 °C.
Montell added, “Everything is just slowed down, and that’s why feeding is reduced. You don’t want to eat the same amount when your metabolism is slowed down.”
For warm-blooded animals like humans, this explanation does not hold true even if they display an analogous behavior.
In the days to come, Montell and Qiaoran Li, the first author of the study, are planning to additionally study the mechanosensory side of food appeal by observing how the size of particles impacts the feeding behavior.
As a case in point, Montell provides the evident difference between refrozen and fresh ice cream. In spite of having the same temperature and chemical composition, a majority of the people will favor ice cream that has not melted and refrozen into a block.
Talking about the unexpected discovery, Montell concluded, “It’s great for your expectations to be wrong, as long as you can then figure out what’s right.”
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