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circadian rhythms

Even tiny temp changes affect these ‘clock neurons’

Circadian clock neurons use thermoreceptors to constantly monitor the temperature of their environment, a new study using fruit flies suggests.

In the study, researchers found even mild changes in temperature have physiological effects on clock neurons that control sleep timing.

“The circadian system produces a daily rhythm in temperature which is an important cue for when it’s time to go to sleep.”

This discovery will help researchers understand how neurons are using environmental temperature in addition to light to regulate sleep timing in mammals, including humans.

“Decades of work from recent Nobel Prize winners and many other labs have actually worked out the details of how light is able to adjust the clock, but the details of how temperature was able to adjust the circadian clock were not well understood,” says Swathi Yadlapalli, first author of the study and a postdoctoral researcher in the University of Michigan’s molecular, cellular, and developmental biology department.

“Going forward, we can ask questions of how these two stimuli are processed and integrated into the clock system, and how this has effects on our sleep behavior and other physiological processes,” Yadlapalli says.

Circadian clocks are biochemical mechanisms that allow living things to organize their sleep and waking across the 24-hour cycle of a day. Researchers know that circadian clocks in mammals control the internal body temperature to drive sleep patterns, says Orie Shafer, principal investigator of the study. For example, we think of the human body temperature as a steady 98.6 degrees, Shafer says, but actually, our body temperature changes throughout the day.

“In fact, it’s fluctuating,” Shafer says. “The circadian system produces a daily rhythm in temperature which is an important cue for when it’s time to go to sleep.”

As we’re coasting toward bedtime, these circadian clocks cool our internal body temperature. As we’re gliding toward wakefulness, these clocks turn up the heat. This is regardless of the temperature of the room we’re sleeping in.

Showing that circadian clock neurons in fruit flies use external temperature to trigger sleep, however, suggests that some clock neurons in humans could be similarly sensitive.

To study how the fruit fly neurons responded to external temperature, Yadlapalli worked with Chang Jiang, a postdoctoral researcher in the labs of Pramod Reddy and Edgar Meyhofer of the mechanical engineering department. Together, they developed an optical imaging and temperature control system that enabled them to take a snapshot of neural activity in the circadian clock network of fruit flies when the flies are exposed to heat or cold stimulus.

Light sensor in fly eye offers clues to circadian rhythm

“It looks like clock neurons are able to get the temperature information from external thermoreceptors, and that information is being used to time sleep in the fly in a way that’s fundamentally the same as it is in humans,” Shafer says. “As temperature drops, these neurons that promote sleep become excited, and that really entrains the sleep activity cycle to external temperature cycles. It’s precisely what happens to sleep in mammals when internal temperature drops.”

Funding for the work came from the Damon Runyon Cancer Research Foundation, the Howard Hughes Medical Institute, a M-Cubed grant from the University of Michigan, the National Science Foundation, and the National Institutes of Health.

The researchers report their findings in the journal Nature.

Source: University of Michigan

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