Insensitivity to the cold helps hibernators sleep

21 December 2017
Posted by Chris Smith.

Not feeling the cold could be key to the ability of hibernators, like squirrels and hamsters, to snooze their way through winter, a new study has shown...

Animals that hibernate can withstand prolonged periods with a profoundly low body temperature. We don't know how their bodies do this, but at least part of the answer must include deactivating normal responses to the cold.

Low temperatures see us reaching for a hat and gloves, throwing extra logs on the fire and shivering to warm up, but hibernators suppress these reflexes and now we have a new clue as to how.

Writing in Cell Reports this week, Yale scientist Vanessa Matos-Cruz and her colleagues offered hamsters and squirrels, which habitually hibernate, as well as mice, which don't, a choice between lying on a warm plate at 30 degrees, or one at 10 to 20 degrees.

The mice almost exclusively plumped for the warm plate, while the squirrels and hamsters showed no such preference. In fact, the temperature of the cold plate had to be dropped below 5 degrees before the squirrels vacated it in favour of the warmer surface.

The nervous system uses specific "cold-sensing" nerves to detected a drop in temperature, but each of the three animal groups had roughly the same numbers of cold-sensing nerve fibres in their skin. But, Matos-Cruz found, the cold-detectors in the hibernators are much less sensitive to low temperatures than those in mice and rats.

Skin nerves contain cold-sensing proteins called TRPM8. As the temperature falls, the protein subtly changes shape, altering the activity of the nerve cell. By comparing the structure of the hibernators TRPM8 with the rat equivalent the Yale team have found that 6 of the amino acid building blocks at TRPM8 protein's core have been changed in the hibernators compared with the rats.

These changes render the hibernator's TRPM8 much less sensitive to the cold and potentially allows the body temperature to drop for long periods of time without causing the animal to feel stressed, thus triggering their seasonal slumber.

"If these animals were to feel cold, they would not be able to hibernate because their sensory system would tell the rest of the body that they need to warm up first," says senior author Elena Gracheva. "They would not be able to survive as a species."

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