Scientists discover the brain circuit that times a state of low metabolism known as torpor, with implications for medicine and space travel 

You have gone without food for days, and the temperature drops to near freezing. What do you do? For some animals, the answer is influenced by the brain’s circadian clock. Hummingbirds, bats, and mice are among the animals that can enter torpor, which reduces body temperature and metabolism. Scientists suspected that the brain’s circadian clock controls the timing of torpor, but until now the exact mechanism was not known.  
 
Researchers at Nagoya University in Japan have identified the specific neural circuit responsible for this survival strategy. They have shown that the brain’s circadian clock, a small cluster of neurons located in the hypothalamus at the base of the brain, sends silencing signals through this circuit to a nearby temperature-regulating region, suppressing torpor during the day. The findings were published in Nature Communications. 
 

Torpor from midnight to dawn 
 
"The brain's preoptic area (POA) controls body temperature and has an important role in initiating torpor,” said senior author and lecturer Daisuke Ono from the Research Institute of Environmental Medicine at Nagoya University. “During the day, the brain’s circadian clock suppresses torpor, which occurs between midnight and dawn in mice.” 
 
Using light-based tools (optogenetics) to switch specific neurons on or off, the researchers showed that activating the circadian clock-POA pathway suppressed torpor. When the circadian clock was disrupted, mice either entered torpor at irregular, unpredictable times or showed a marked reduction in torpor. 
  

Additionally, the specific clock cell type responsible for sending these signals was identified. Neurons that produce a protein called arginine vasopressin (AVP neurons) in the circadian clock inhibit neurons in the POA. Mice with impaired inhibitory signaling from AVP neurons to the POA showed abnormal torpor timing, demonstrating that this pathway plays a key role in determining when torpor occurs. 
 

The research team also discovered that the POA becomes more active at night. “The clock does not actively trigger torpor. Instead, it reduces its inhibitory influence at night, allowing neural circuits involved in thermoregulation and energy balance to promote torpor when environmental conditions are favorable. The three systems work in tandem to create the right conditions,” Ono explained. 

Implications for medicine and space travel

A clearer understanding of how the brain times metabolic shutdown may inform a technique that uses controlled cooling to limit tissue damage after injury or surgery (induced hypothermia). The findings may also be relevant to extended spaceflight, where controlled reduction of metabolism could protect the body. 
 

Although humans do not naturally enter torpor, understanding the neural mechanisms that regulate metabolic suppression in mammals could provide clues for developing controlled hypometabolic states in the future. Rare accounts of people surviving extreme cold exposure with dangerously low body temperatures hint at this possibility. Understanding the brain circuits that control these states in mammals may one day bring researchers closer to inducing suspended animation in humans, a state long imagined for deep space travel.