Neurobiologists discovered that fruit flies are pre-programmed to take a nap in the middle of the day. The new research examines a ‘thermometer’ circuit for hot temperatures.
Temperature affects the span of human behaviour, from eating and activity
levels to sleep-wake cycles. We may have a harder time sleeping in the summer
and be slow to get out of bed on colder mornings.
But the link between sensory neurons and neurons that control this cycle are not understood completely.
Northwestern University neurobiologists have found a few clues about what’s
happening. In a new study, published recently the journal Current
Biology, researchers found that fruit flies are pre-programmed to take a nap in
the middle of the day.
A follow-up to their 2020 Biology paper that identified a brain thermometer only active in cold weather, the new paper explores 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 centres remain largely unmapped.”
Gallio led the study and said fruit flies are a particularly good model to study
big questions like “why do we sleep,” and “what does sleep do for the brain”
because they don’t attempt to disrupt instinct in the same way humans do when we pull all-nighters, for example. They also allow researchers to study the
influence of external cues like light and temperature on cellular pathways.
Cells that stay on longer.
The paper is the first to identify “absolute heat” receptors in the fly’s head, which
respond to temperatures above about 77 degrees Fahrenheit – the fly’s favourite
temperature. As it turns out, the common laboratory fruit fly (Drosophila) has
colonized nearly the entire planet by forming a close association with humans.
Not surprisingly, its favourite temperature also matches that of many humans.
Just as they expected based on the results of their previous paper on cold
temperature, researchers found that brain neurons receiving information about
heat are part of the broader system that regulates sleep.
When the hot 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 enabled by a 10-year initiative that produced the first completed
map of neural connections in an animal (a fly), called the connectome. With the
connectome, researchers have access to a computer system that tells them all
possible brain connections for each of the fly’s ~100,000 brain cells.
However. even with this extremely detailed road map, researchers still need to figure out how information in the brain goes from point A to B. This paper helps fill that gap.
The different circuits for hot versus cold temperatures make sense to Gallio
because “hot and cold temperatures can have quite different effects on
physiology and behavior,” he said.
This separation may also reflect evolutionary processes based on heat and cold cycles of the Earth. For example, the possibility that brain centers for sleep may be directly targeted in humans by a specific sensory circuit is now open to be investigated based on this work.
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 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.
“People may 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?” Gallio said.
“Of course, it’s not culture in flies, so there actually might be a very strong underlying biological mechanism that is overlooked in humans.”
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