BioEd Online

The body's clock may lose track of time during winter hibernation, scientists have found in a species of hamster.

The genes responsible for regulating circadian rhythms in the brain normally follow a 24-hour cycle, with their activity waxing and waning in step with day and night. But what happens during hibernation?

Brain activity resembles that of deep slumber, and the body slows its metabolism to a crawl. The internal temperature of arctic ground squirrels, for instance, can plummet below freezing. It's thought that hibernation evolved from sleep as a way to save energy during lean winter months.

Some studies have hinted that factors such as body temperature continue to oscillate up and down in a daily cycle during this winter period, although not nearly so much as during normal conditions. But no one had tapped directly into the brain or looked at the genes that control the body clock to see what was happening there.

Stopped clock

Biologists Florent Revel and Paul Pévet of Louis Pasteur University in Strasbourg, France, investigated hibernation's effect on the brain in European hamsters, which normally stay burrowed in their nests between December and March, rarely venturing above ground. During these months they follow a regular schedule: three to four days of hibernation followed by two to three days of activity.

The team tricked some animals into entering hibernation by turning off the lab lights and turning down the thermostat to 8 degrees Celsuis. They then probed the activity of several clock genes in the brain's suprachiasmatic nucleus, a grape-seed-sized structure nestled in the hypothalamus. Activity of genes called Per1, Per2, Bmal1 and arginine vasopressin remained constant throughout hibernation, they report in the Proceedings of the National Academy of Science¹. Normally, the activity of these genes fluctuate throughout the day.

"This is the first time we have seen a physiological condition in which the clock is not working," says Pévet.

Body heat

Craig Heller, a biologist at Stanford University in California, had previously detected circadian rhythms in hibernating golden mantled ground squirrels by measuring small fluctuations in their body heat. He says the new work is interesting, but it may not tell us about animals other than hamsters. And, he adds, monitoring gene activation in the brain might not be a sensitive enough way to watch for daily body cycles.

Because animals are nearly dormant during hibernation, their circadian rhythms hush to a murmur, he says. Perhaps it is easier to spot tiny changes in temperature than it is to spot tiny changes in molecular activity.

"Maybe you can't measure the molecular mechanism, but that doesn't mean it's not there."

Pévet agrees that their findings apply only to European hamsters. But he says understanding hibernation and its link to circadian rhythms could help doctors induce a similar state in people undergoing surgery.