Avoiding Brain Freeze:
Hibernating Ground Squirrel Brains Weather the Cold

October 9, 2000

The brisk air and changing color of leaves tell you that winter is on its way. Although we humans go about our business as usual during the winter, some animals engage in a special kind of sleep called hibernation. Hibernation helps animals survive the cold winter when it is difficult for them to find food. The body temperature of some animals drops to near freezing as they prepare to hibernate when heart rate and other body functions slow. Although body temperature gets quite low, these hibernating animals wake up and warm up in the spring without any damage. New research provides some clues about how their brains survive such cold temperatures.

Scientists at the National Institutes of Health (NIH) examined the brains of ground squirrels at different stages of hibernation. Using light and electron microscopes, the researchers saw "slits" inside neurons and glial cells of hibernating ground squirrels. These slits were never seen in tissue from non-hibernating animals. The slits appeared to be hardened patches on the endoplasmic reticulum without any protein. The endoplasmic reticulum is a structure in the cell that is responsible for producing proteins, lipids and other materials for cell metabolism.

The NIH scientists believe that the lipids and proteins that make up the membrane of the endoplasmic reticulum were rearranged during hibernation to form areas with and without proteins. Because the proteins moved to areas that were not hardened, it is possible that they continued to function and keep the cells alive.

Exactly how the proteins and lipids became rearranged is not known. Further studies may provide a better understanding of the changes that occur during hibernation and may uncover ways to preserve and protect tissue from cold temperatures.


Azzam, N.A., Hallenbeck, J.M. and Kachar, B. "Membrane changes during hibernation," Nature, Vol. 407, September 21, 2000, pp. 317-318.

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