Fruit fly study reveals a thermometer brain circuit promoting a midday siesta on hot days
From Northwestern Now
By Win Reynolds
August 17, 2022 -- On
the hottest summer days, you may find yourself dozing off in the middle of the
day. In some parts of the world, it’s a cultural norm to schedule “siestas” and
shutter businesses during the warmest hours of the day. As it turns out, biology, not just culture,
may be behind this.
Temperature affects the
span of human behavior, 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 Aug. 17 in 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 centers 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.
77 degrees is a
favorite temperature
The paper is the first
to identify “absolute heat” receptors in fly head, which respond to
temperatures above about 77 degrees Fahrenheit — the fly’s favorite
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 favorite 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.
Figuring common targets
of hot, cold circuit
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.
“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.”
https://news.northwestern.edu/stories/2022/08/why-heat-makes-us-sleepy
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