Physiology: Ultrasound induces a hibernation-like state in mice and rats

A reversible hibernation-like state can be induced in mice and rats using ultrasound pulses directed at the head, reports a study published in Nature Metabolism. The non-invasive approach temporarily activates nerve cells in the brain that lead to a reduction in body temperature and slow down metabolism, and could have applications in the future in medicine or potentially long-distance space flight.

Torpor is a physiological state, like hibernation, wherein mammals suppress metabolism, reduce body temperature and slow down other processes to conserve energy. This state is believed to be controlled by the central nervous system. Ultrasound waves can noninvasively penetrate the skull to focus on the brain and, if focused on neurons, have been shown to activate nerve cells in animals.

Hong Chen and colleagues developed an ultrasound emitter that can be mounted on the heads of freely moving mice. They targeted 10-second ultrasonic pulses on the hypothalamus preoptic area, a brain region known to regulate hibernation. This triggered an immediate drop in body temperature by several degrees (on average 3–3.5 °C), coupled with a reduced heart rate and reduced oxygen consumption in both male and female mice; within two hours, the animals had fully recovered. The authors then combined their ultrasound emitter with an automated system that administers a repeated ultrasonic pulse once body temperature starts to rise, and were able to maintain animals in this torpor-like state for up to 24 hours, without signs of damage or discomfort. The authors also demonstrate that the technique works in 12 rats, an animal that does not naturally hibernate, although their body temperature only dropped by an average of 1–2°C. They suggest this indicates that the physiological processes regulating the metabolic response might be present in non-hibernating mammals.

Although more research is needed to investigate whether the approach would work safely in humans, the authors argue that a non-invasive and reversible technique to slow down metabolism and reduce body temperature could have applications in medicine, potentially following acute emergencies or acute severe disease, or have implications for future long-distance space travel.