Sleeper Finding: Hormone key to hibernation?

A recently discovered hormone may play a major role in triggering and maintaining hibernation. The finding could shed light on this annual period of slumber, which largely remains a mystery even after decades of research.

Each year, species ranging from amphibians to rodents settle in for a long winter’s nap, which helps them conserve energy and other resources during harsh weather. Their body temperatures plummet to near freezing, and metabolisms slow.

Researchers are eager to understand hibernation because it seems to protect slumbering animals from a variety of ills, including hypothermia, strokes, muscle atrophy, infections, and cancer—a defense that might someday be emulated in people.

But because hibernating animals’ metabolisms drop so dramatically, it’s difficult to detect what molecules might control the phenomenon, notes biologist Noriaki Kondo of Kanagawa Academy of Science and Technology in Tokyo.

Several years ago, Kondo and his colleagues identified a hormone that’s abundant in Siberian chipmunks’ blood during summer but fades away during the winter. Suspecting that the hormone’s seasonal fluxes might be related to hibernation, the researchers decided to give it a closer look. The hormone is made of four proteins and is called hibernation-protein complex (HPc).

First, to make sure that HPc doesn’t fluctuate simply because of body temperature or light, the researchers kept a group of chipmunks in cold and darkness year-round. Other chipmunks were kept warm throughout the year, with regular 24-hour cycles of daylight and darkness. Regardless of these conditions, HPc continued to fluctuate on a seasonal schedule.

Next, Kondo’s team examined whether HPc makes its way to the brain, the presumed control site for hibernation. Though blood concentrations of the hormone were highest during the summer, the researchers couldn’t find HPc in the fluid bathing the chipmunks’ brains during that season. However, concentrations of the hormone rose in the brain fluid as hibernation began and remained steady during hibernation.

When the scientists injected hibernating chipmunks’ brains with antibodies that blocked the hormone’s action, some of the animals cut short their hibernations. This result suggests that HPc keeps hibernation going, the researchers report in the April 7 Cell.

The study is “a nice first step” toward understanding hibernation’s molecular mechanism, says Sandy Martin of the University of Colorado School of Medicine in Aurora. However, she warns, “more work needs to be done before anyone writes this into the textbooks.” Martin notes that no study has found HPc in other hibernating animals. She adds that even in Siberian chipmunks, it’s unclear what HPc might do in the body to trigger and maintain hibernation.