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Eat, sleep, stay warm
How our bodies find the right balance
Wednesday, Aug. 12, 2009
James Krueger, WSU sleep research
PULLMAN –A new study led by scientists at WSU shows that alternate products of a single gene help control whether an animal sleeps or stays awake, craves food or doesn’t, and maintains its body temperature or plunges deep into hypothermia.
Mice in which the preproghrelin (pre-pro-GRAY’-lin) gene had been “knocked out” behaved like normal mice as long as their surroundings were warm and they had plenty of food to eat. But when the temperature was dialed down to 17o C (62.6o F) for two days, and then their food was removed, they became inactive and their body temperature dropped so steeply they appeared to be dead.
“The EEG goes flat,” said James Krueger, leader of the research team. “In all my years in science, I’ve never had a result this dramatic.”
The results provided new understanding about the genetic control of sleep, hunger and metabolism, and thermoregulation in a challenging environment.
Scientists from Fordham University and Baylor College of Medicine contributed to the study. Their paper, to be published in the Proceedings of the National Academy of Sciences, is available online at www.pnas.org/cgi/doi/10.1073/pnas.0903090106.
Krueger said that in mammals, there’s a trade-off between activity and food-seeking on the one hand, and sleep on the other. The balance between them is regulated by several peptide hormones, including products of the preproghrelin gene. Normal mice that have a functional preproghrelin gene cope with lower ambient temperature and lack of food by mobilizing energy stored in their fat. The knockout mice weren’t able to do that. Their lack of a preproghrelin gene (and its products) caused no problem as long as the mice had access to food, even if their surroundings were chilled slightly; they were able to eat and use the incoming calories to keep their body temperature up. However, if their food was removed so their only source of energy was their own stores of fat, they could not stay warm. A few hours after they lost access to food, their body temperature dropped by about 4o C. Between 18 and 24 hours after food was removed, their temperature plunged by another 10o C and their brain activity flat-lined.
Since the preproghrelin gene codes for multiple products, the scientists then tried to determine which of those products was the key hormone whose absence led to the extreme temperature drop. They started with the best-known product of the preproghrelin gene, small peptide hormone called ghrelin (GRAY’-lin) that makes us feel hungry and inhibits sleep.
To find out whether the inactivity and temperature drop of the knockout mice was due to a lack of ghrelin, the researchers produced mice lacking the gene for the ghrelin receptor (which is needed for ghrelin to function). Those mice lacked functional ghrelin but were able to maintain their body temperature just fine, even in the cold and with no food available.
“If it wasn’t ghrelin [that was responsible], what was it?” said Krueger. The group’s attention turned to obestatin (oh-be-STAT’-in), another small hormone coded for by the preproghrelin gene. The role of obestatin has been more elusive and controversial than that of ghrelin, but an earlier experiment by Krueger and his colleagues provided a clue. When they injected obestatin into rats, the rats slept more. In the current work, the researchers rigged osmotic minipumps to deliver tiny amounts of obestatin to the knockout mice. Adding obestatin to their systems partially corrected for the lack of the preproghrelin gene. The mice were able to enter a fairly normal torpor state without letting their body temperature plummet.
Krueger said that result has convinced him that obestatin inhibits feeding behavior and promotes sleep, just the opposite of what ghrelin does. That one gene codes for products with opposing actions leads to questions for further research, such as what determines which product gets made in what amounts? And how is that related to the animal’s environmental conditions?
Understanding the links between sleep, hunger and body temperature could lead to major advances in weight control and induction of hypothermia in surgical patients, said Krueger.
Although talk of sleep, cold and suspension of feeding could also lead to speculation about hibernation, Krueger said the inactivity of knockout mice in this study differs from hibernation in key respects. For example, hibernating polar bears lower their body temperature very little, by about 1o C. And they’re not driven into hibernation by cold ambient temperatures, but by lack of food. Even if the temperature stays balmy, if they have little to eat, they will hibernate. Finally, hibernation is tightly regulated by brain hormones and circuits. The current study did not show whether the temperature drop in knockout mice was actively regulated, or the passive result of the mice simply being unable to rev up their metabolism enough to maintain their temperature in the cold.