Neurobiologists have found that fruit flies are pre-programmed to take a nap in the middle of the day. The latest article delves into a “thermometer” circuit for hot temperatures.
Temperature influences everything from eating habits and activity levels to sleep-wake cycles. We may have trouble sleeping in the summer and wake up slowly on colder mornings. However, the connection between sensory neurons and neurons that control this cycle is not fully understood.
Fruit flies are pre-programmed to nap
Northwestern University neurobiologists have discovered a few hints as to what is going on. Researchers discovered that fruit flies are pre-programmed to nap in the middle of the day in a new study published August 17 in the journal Current Biology.
The new paper, which is a follow-up to their 2020 Biology paper that discovered a brain thermometer that is only active in cold weather, investigates a similar “thermometer” circuit for hot temperatures.
“Changes in temperature have a strong effect on behavior in both humans and animals, and offer animals a cue that is time to adapt to the changing seasons,” said Marco Gallio, associate professor of neurobiology in the Weinberg College of Arts and Sciences. “The effect of temperature on sleep can be quite extreme, with some animals deciding to sleep off an entire season — think of a hibernating bear — but the specific brain circuits that mediate the interaction between temperature and sleep centers remain largely unmapped.”
Gallio, the study’s lead author, says fruit flies are a particularly good model for studying big questions like “why do we sleep?” and “what does sleep do for the brain” because they don’t try to disrupt instinct in the same way humans do when we pull all-nighters, for example. They also enable researchers to investigate the effects of external cues such as light and temperature on cellular pathways.
The study is the first to identify “absolute heat” receptors in the fly head, which respond to temperatures above 77 degrees Fahrenheit – the fly’s preferred temperature. The common laboratory fruit fly (Drosophila) has colonised nearly the entire planet by forming close associations with humans, as it turns out. Not surprisingly, its preferred temperature is similar to that of many humans.
Researchers discovered that brain neurons receiving information about heat are part of the larger system that regulates sleep, just as they predicted based on the findings of their previous paper on cold temperature. When the hot circuit, which runs parallel to the cold circuit, is activated, the target cells that promote midday sleep remain active for a longer period of time. This increases midday sleep, which keeps flies away from the hottest part of the day.
The study was made possible by the connectome, a 10-year initiative that produced the first complete map of neural connections in an animal (a fly). Researchers can use the connectome to access a computer system that tells them all possible brain connections for each of the fly’s 100,000 brain cells. Even with this extremely detailed road map, researchers must still figure out how information in the brain moves from point A to point B. This paper contributes to filling that void.
Next steps
Next, Gallio’s team hopes to figure out the common targets of the cold and hot circuit, to discover how each can influence 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 first author and a post-doctoral 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, looking at how adaptable species are to change.
Gallio believes that the different circuits for hot and cold temperatures make sense because “hot and cold temperatures can have quite different effects on physiology and behaviour.” This separation may also reflect evolutionary processes based on the Earth’s heat and cold cycles. Based on this research, it is now possible to investigate whether brain centres for sleep can be directly targeted in humans by a specific sensory circuit.
