Some mammals such as bats, squirrels, skunks, and bears hibernate. That is, they actively lower their body temperature for energy conservation to survive food scarcity in the winter. Mice are not on that list, but they do have a daily state of hypometabolism known as torpor. Although both of these states are thought to be controlled by the nervous system, the mechanism remains undiscovered.
Researchers from the University of Tsukuba in Japan opened the door to answering questions such as “why do some animals hibernate while others do not?” and “do all animals have the potential to hibernate even if they never do so in nature?” To do this, they found specific cells in the mouse brain that can trigger a hibernation-like state when activated.
The study, “A discrete neuronal circuit induces a hibernation-like state in rodents,” was published in Nature.
Even though mice do not hibernate, researchers led by Takeshi Sakurai, PhD, professor at the International Institute for Integrative Sleep Medicine at the University of Tsukuba and Genshiro Sunagawa, MD, PhD, a postdoctoral researcher at the RIKEN Center for Biosystems Dynamics Research, showed that activating a specific type of cell in the mouse brain—quiescence-inducing neurons (Q neurons)—caused them to enter a hibernation-like state for several days.
The hypothalamic neuronal circuit in rodents induced “a long-lasting hypothermic and hypometabolic state similar to hibernation,” the authors wrote. During this state of Q-neuron-induced hypothermia and hypometabolism (QIH), they noted, “although body temperature and levels of oxygen consumption are kept very low, the ability to regulate metabolism still remains functional, as in hibernation.” They added that there “was no obvious damage to tissues and organs or abnormalities in behavior after recovery from this state.”
“The mice exhibited distinctive qualities that met the criteria for hibernation,” noted Sakurai. “In particular, the body temperature set-point lowered from about 96.8°F [36°C] to about 81°F [27°C], and the body functioned normally to maintain a lower body temperature around 22°C, even when the surrounding ambient temperature was dramatically reduced.” The mice also showed all the signs of a reduced metabolism that are common during hibernation, including reduced heart rate, oxygen consumption, and respiration.
Being able to send mice into a hibernation-like state for days simply by artificially exciting Q neurons was somewhat unexpected. “Even more surprising,” said first author Tohru Takahashi, faculty of medicine, University of Tsukuba, “is that we were able to induce a similar hypometabolic state in rats, a species that neither hibernates nor has daily torpor.” Although we do not know the answer yet, the possibility that humans also have Q neurons that can be used to induce a similar state is tantalizing.
“People might not want to hibernate for the same reasons as animals,” explained Sunagawa. “But there are medical reasons for wanting to place people in suspended animation, such as during emergency transport or critically ill conditions as in severe pneumonia, when the demand for oxygen cannot meet the supply.”
Sparing oxygen is not only for medicine. “In the future,” Sakurai added, “we may put human in a hibernation-like state for missions to Mars and beyond.”