Fruit fly research reveals a thermometer brain circuit that promotes midday siesta on hot days
On the hottest days of summer, you can fall asleep in the middle of the day. In some parts of the world, it is a cultural norm to schedule ‘siesta’s and close businesses during the hottest hours of the day. It turns out that biology, not just culture, is behind this.
Temperature influences the duration of human behavior, from eating and activity levels to sleep-wake cycles. We have a harder time sleeping in the summer and getting out of bed slowly on colder mornings. But the link between sensory neurons and neurons that control this cycle is not fully understood.
Neurobiologists at Northwestern University have found a few clues as to what’s going on. In a new study, published today (Aug. 17) in the journal Current Biology, researchers found that fruit flies are preprogrammed to take a nap in the middle of the day. A follow-up to their paper that identified a brain thermometer that was only active in cold weather, the new paper explores a similar “thermometer” circuit for high temperatures.
“Changes in temperature have a strong effect on the behavior of both humans and animals, signaling animals that it is time to adapt to the changing seasons,” said Marco Gallio, an associate professor of neurobiology at the Weinberg College of Arts. and Sciences. “The effect of temperature on sleep can be quite extreme, with some animals deciding to sleep in for an entire season — think of a hibernating bear — but the specific brain circuits that mediate the interaction between temperature and sleep centers remain largely unknown.”
Gallio led the research, saying fruit flies are a particularly good model for studying big questions, such as “why do we sleep?” and “what does sleep do for the brain?” because they don’t try to disturb the instinct in the same way that humans do when, for example, we travel through the night. They also enable researchers to study the influence of external signals such as light and temperature on cellular pathways.
Cells that stay on longer
The paper is the first to identify “absolute heat” receptors in a fly’s head, which respond to temperatures above about 77 degrees Fahrenheit — the fly’s preferred temperature. It turns out that the common lab fruit fly (Drosophila) has colonized almost the entire planet by forming close bonds with humans. Not surprisingly, his preferred temperature also matches that of many people.
Just as they expected based on the results of their previous paper on cold temperatures, researchers found that brain neurons that receive information about heat are part of the broader system that regulates sleep. When the warm circuit, which runs parallel to the cold circuit, is active, the target cells that promote midday sleep stay on longer. This results in an increase in midday sleep that keeps flies away from the hottest part of the day.
The study was made possible by a 10-year initiative that produced the first completed map of neural connections in an animal (a fly), the connectome. With the connectome, researchers have access to a computer system that tells them all the possible brain connections for each of the fly’s ~100,000 brain cells. But even with this extremely detailed roadmap, researchers still have to figure out how information in the brain moves from point A to B. This article will help fill that gap.
The different warm versus cold temperature circuits make sense to Gallio, because “hot and cold temperatures can have very different effects on physiology and behavior,” he said. This separation may also reflect evolutionary processes based on Earth’s heat and cold cycles. For example, based on this work, the possibility can now be explored that brain centers for sleep are targeted directly at humans by a specific sensory circuit.
Next, Gallio’s team hopes to figure out the common goals of the cold and warm circuits, to discover how each might affect sleep.
“We identified one neuron that could be a site of integration for the effects of hot and cold temperatures on sleep and activity in Drosophila,” said Michael Alpert, the paper’s lead author and a postdoctoral researcher in the Gallio lab. “This would be the start of interesting follow-up studies.”
Gallio added that the team is interested in looking at the long-term effects of temperature on behavior and physiology to understand the impact of global warming, and how adaptable species can change.
“People can choose to take an afternoon nap on a hot day, and in some parts of the world this is a cultural norm, but what do you choose and what is programmed into you?” said Gallio. “Of course it’s not cultured in flies, so there could be a very strong underlying biological mechanism that is being overlooked in humans.”
Michael H. Alpert et al, A high temperature thermometer circuit adapts Drosophila behavior to sustained heat, Current Biology (2022). DOI: 10.116/j.cub.2022.07.060
Dominic D. Frank et al, Early integration of temperature and humidity stimuli in the Drosophila brain, Current Biology (2017). DOI: 10.116/j.cub.2017.06.077
Quote: Fruit fly research reveals a thermometer brain circuit that promotes midday siesta on hot days (2022, August 17), retrieved August 17, 2022 from https://phys.org/news/2022-08-fruit-reveals -thermometer-brain-circuit. html
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