Winter is here. We’ve already had a preview with a cold front that swept South Africa last week, and we know the challenge of waking up on those cold, dark days. But why is it so hard?
Temperature affects the behaviour of nearly all living creatures, but there is still much to be learned about the link between sensory neurons and neurons controlling the sleep-wake cycle.
“This helps explains why — for both flies and humans — it is so hard to wake up in the morning in winter,” says Marco Gallio, associate professor of neurobiology in the Weinberg College of Arts and Sciences. “By studying behaviors in a fruit fly, we can better understand how and why temperature is so critical to regulating sleep.”
The study, led by Gallio and conducted in Drosophila melanogaster, was published on May 21 in the journal Current Biology. The paper is titled “A circuit encoding absolute cold temperature in Drosophila.”
The paper describes for the first time “absolute cold” receptors
residing in the fly antenna, which respond to temperature only below the
fly’s “comfort zone” of approximately 77 degrees Fahrenheit. Having
identified those neurons, the researchers followed them all the way to
their targets within the brain. They found the main recipients of this
information are a small group of brain neurons that are part of a larger
network that controls rhythms of activity and sleep. When the cold
circuit they discovered is active, the target cells, which normally are
activated by morning light, are shut down.
Drosophila is a classic
model system for circadian biology, the area in which researchers study
the mechanisms controlling our 24-hour cycle of rest and activity. The
focus of much current work is on how changes in external cues such as
light and temperature impact rhythms of activity and sleep and how the
cues reach the specific brain circuits that control these responses.
While
detection of environmental temperature is critical for small
“cold-blooded” fruit flies, humans are still creatures of comfort and
are continually seeking ideal temperatures. Part of the reason humans
seek optimal temperatures is that core and brain temperatures are
intimately tied to the induction and maintenance of sleep. Seasonal
changes in daylight and temperature are also tied to changes in sleep.
“Temperature sensing is one of the most fundamental sensory modalities,” says Gallio, whose group is one of only a few in the world that is systematically studying temperature sensing in fruit flies. “The principles we are finding in the fly brain — the logic and organization — may be the same all the way to humans. Whether fly or human, the sensory systems have to solve the same problems, so they often do it in the same ways.”
Gallio is the corresponding author of the paper.
Michael H. Alpert, a postdoctoral fellow in Gallio’s lab, and Dominic D.
Frank, a former Ph.D. student in Gallio’s lab, are the paper’s co-first
authors.
“The ramifications of impaired sleep are numerous — fatigue, reduced concentration, poor learning and alteration of a myriad of health parameters — yet we still do not fully understand how sleep is produced and regulated within the brain and how changes in external conditions may impact sleep drive and quality,” Alpert says.
The
study, a collaborative effort many years in the making, was performed in
the Gallio lab by a range of scientists at different stages of their
careers, ranging from undergraduate students to the principal
investigator.
“It is crucial to study the brain in action,” Frank says. “Our findings demonstrate the importance of functional studies for understanding how the brain governs behavior.”
Overall, the study
heavily relied on the ability to study both the activity of neurons and
the role of these neurons on behavior. To do this, the researchers
developed new tools and used a combination of functional and anatomical
studies, neurogenetic and behavioral monitoring approaches to conduct
these experiments in both wild type and transgenic flies.
