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Rats induced into hibernation-like state

Injection of compound causes animals to slow heartbeat, lower body temperature

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Rats spent hours in a state of chilly suspended animation after researchers injected a compound into the animals in a cold room.  The animals’ heart rates slowed, brain activity became sluggish and body temperature plummeted.

The research joins a small number of studies that attempt to induce the metabolically lethargic state known as torpor in animals that can’t normally slow their metabolism. “It’s a breakthrough” in understanding aspects of torpor, says neuroscientist Kelly Drew of the University of Alaska Fairbanks.

Lowering the body temperature of a nonhibernating mammal is really hard, says Domenico Tupone of Oregon Health & Science University in Portland. As temperatures inside the body fall, several failsafe systems spring into action. Blood vessels near the skin squeeze tight to hold warmth in, the body starts to shiver and brown fat, a tissue that’s especially plentiful in newborns, starts to produce heat.

But Tupone and colleagues bypassed the rats’ defenses against the cold with a compound that’s similar to adenosine, a molecule in the body that signals sleepiness. After about an hour in a room chilled to 15° Celsius, the rats grew lethargic. Their brain waves slowed, their heart rates dropped and their heart occasionally skipped beats.

The rats’ core temperature dropped from about 38°  to about 28° C, or 82° Fahrenheit, the authors report  in the Sept. 4 Journal of Neuroscience. Tupone and his colleagues measured even lower temperatures in further experiments — rats’ core body temperature reached 15° C or about 59° F. “That is a pretty amazing temperature. No one has done this before,” he says.

The rats weren’t in a coma, nor were they asleep or truly hibernating. Hibernating animals’ metabolisms plummet and their temperatures sink much lower; an Arctic ground squirrel, for instance, cools to about —3° C when it hibernates.

“It’s a new state,” Tupone says. “We don’t really know what it is.”

In the experiment, loud noises and tail pinches failed to arouse the rats. They didn’t eat or drink. Occasionally, one would slither into a corner, but for the most part, the animals stayed still for up to 6 hours. In unpublished experiments, Tupone has kept the animals in the unresponsive state for 24 hours, he says.

Warming the room coaxed the rats out of their torpor. The recovery process takes about 12 hours, during which the animals ate and drank voraciously. After recovering, the animals were alert, moved around their cages normally and slept when tired.

The rat experiment could one day have implications for another non-hibernating mammal, humans. A safe and reversible way to allow people to lower their temperatures would be an important tool for doctors, says neurologist Midori Yenari of the University of California, San Francisco. 

When people have heart attacks or strokes, clinicians can use ice packs or frigid water to chill people and prevent further tissue damage. But those methods of cooling take time and can have dangerous side effects. The compound “looks like another lead,” Yenari says.

A safe way to induce torpor in humans is also the fanciful dream of people with interstellar intentions: Humans would be able to travel much farther in space in a suspended animation state.

Editor's Note: This story was updated on September 10, 2013 to correct the rats' core body temperatures and to remove the assertion that the rats' blood pressure dropped.

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