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A genetic study found that grizzly bears' internal clocks continue to tick during hibernation. This tenacity highlights the significance of circadian cycles in the metabolism of many organisms, including humans.
The genetic study, led by Washington State University, verified observational data that bears' energy production still swells and wanes on a daily basis, even when they sleep for several months without eating. The researchers also discovered that the amplitude of energy output was blunted during hibernation, implying that the range of highs and lows was reduced. The peak occurred later in the day during hibernation than it did during the active season, although the daily variability remained. "This underscores the importance of the circadian rhythms themselves--that they give organisms the flexibility to still function in a state as extreme as a hibernating bear," said Heiko Jansen, a professor in WSU's integrative physiology and neuroscience department and senior author on the study in the Journal of Comparative Physiology B. Other research has shown that circadian rhythms, the 24-hour physical cycles common to most living animals on Earth, have ties to metabolic health. In humans, major disruptions to these patterns, such as those occurring in night shift work, have been linked to metabolic problems like weight gain and a higher prevalence of diabetes. In some sense, bears are extreme shift workers, taking as much as six months off when they hibernate. Researchers like Jansen's team are attempting to figure out how they engage in seemingly unhealthy habits of gaining excessive weight then going without food and not moving much for several months - all without detrimental effects like loss of bone mass or diseases like diabetes. Unlike hibernating rodents, who are almost comatose, bears do move around occasionally during this dormant period. Through observation studies of grizzly bears at the WSU Bear Center, researchers found these movements tend to follow a circadian rhythm with more activity during the day than at night. In the current study, the researchers looked to see if that circadian rhythm was expressed on the cellular level. They took cell samples from six bears during active and hibernating seasons, then cultured those cells to conduct an array of genetic analyses. To mimic hibernation, the researchers examined the cells at the bears' typical lowered body temperature during hibernation at about 34 degrees Celsius (93.2 degrees Fahrenheit) and compared that to 37 C (98.6 F) during the active season. They found thousands of genes were expressed rhythmically in hibernating bear cells. This translated into rhythms of energy through the rise and fall of the production of adenosine triphosphate or ATP, the body's cellular source of energy. ATP was still produced in a daily pattern under hibernation but the production had a blunted amplitude with lower peaks and valleys. The highest production point also shifted to later in the day under hibernation than under active-season conditions. Maintaining a circadian rhythm requires some energy itself. The researchers believe that altering this rhythm during hibernation may allow bears to still get some energetic benefit from the daily cycle without incurring as much cost, which likely helps them survive going without food for months. "It's like setting a thermostat. If you want to conserve some energy, you turn down the thermostat, and this is essentially what the bears are doing," Jansen said. "They're using the ability to suppress the circadian rhythm, but they don't stop the clock from running. It's a really novel way of fine-tuning a metabolic process and energy expenditure in an animal." (ANI)
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