Ultrasound brain pulse turns on the “hibernation switch”

Arctic ground squirrels are champions of hibernation, staying in this state for 7 to 8 months a year while keeping their body temperature below zero. CREDIT: INGO ARNDT/MINDEN PICTURES

Scientists have been working on how to put humans to sleep to lessen the damage caused by diseases such as heart disease and stroke, and to reduce the stress and cost of long-distance space travel in the future. In a study published today in Nature Metabolism, scientists report that they induced mice to initiate hibernation-like states — slumber — by directing ultrasound pulses on the head. Some experts say this is a major technological advance in the future to achieve this feat in humans.

“It’s an amazing paper.” Frank van Breukelen, a biologist who studies hibernation at the University of Nevada, Las Vegas, commented. The work builds on a series of recent studies that pinpointed specific populations of neurons in the preoptic area (POA) of the hypothalamus. These cells act like switches that activate hibernation, and when animals are hungry or cold, they go into a slumber state. In previous studies, scientists genetically engineered these neurons to respond to light or certain chemicals, and found that even mice went into sleep when warm and well-nourished. However, Breukelen notes that this invasive technique is difficult to apply to humans.

The new ultrasound study, led by the team of Hong Chen, a bioengineer at Washington University in St. Louis, did not require genetic engineering. Chen learned from previous research that some neurons have special pores, called TRPM2 ion channels, that change shape with ultrasound, including controlling mouse POA cell subsets.

Chen’s team glued a device like a tiny speaker to the mouse’s head to focus the ultrasound on the POA. Under a series of 3.2 MHz pulses, the core body temperature of these rodents dropped by about 3 degrees Celsius. Bruekelen noted that the mice’s body temperature dropped, heart rate and metabolism slowed, which are classic signs of stinging. When the animal’s body temperature began to rise, the researchers automatically fired additional ultrasonic pulses that kept the mice dormant for up to 24 hours. When they muted the micro speaker, the mice returned to normal and did not have any adverse reactions.

Breukelen said that when the researchers directed ultrasound to other areas of the brain, the mice did not appear to go into sleep. This suggests that the slowdown in metabolism in these animals is indeed caused by specifically stimulating neurons in POA. Breukelen said that while humans don’t hibernate naturally, almost all mammals, from fat-tailed lemurs in Madagascar to arctic ground squirrels, have the ability to hibernate. Perhaps humans, like mice, also possess a hidden ability to go into sleep.

However, others are not convinced by the findings. Shaun Morrison of Oregon Health & Science University said that ultrasound stimulation warms the brain, so researchers may actually activate temperature-sensitive neurons in the region, causing the animals to lower their body temperatures in response. Even if the effect were real, he doubted that ultrasound could be used to put astronauts in suspended animation. Morrison noted that human brains are much larger than mouse brains, and POAs are buried deeper. “This ultrasound technique is unlikely to work in humans as well as it does in mice.” (Source: Li Huiyu, China Science News)

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