US researchers have managed to induce a semi-lethargic state in mice by directing an ultrasound beam at a precise area of the brain, the supraoptic area of the hypothalamus (which is also present in humans).
Some mammals and birds have a very efficient system for conserving energy and warmth in potentially fatal environmental conditions, such as extreme cold or lack of food: this is torpor, during which body temperature and metabolic rate decrease for periods of various lengths to ensure survival. In its most extreme form, torpor becomes hibernation.
Reproducing animal torpor in humans is something that has been studied for many decades, as lowering the metabolism to a point where it almost stops without any consequences, and being able to reactivate it at the right time, could be very useful for humans, both during long space missions and in various clinical situations such as waiting for an organ transplant or performing particularly delicate operations like open-heart surgery.
Now this aim, which seems like science fiction, might be a bit closer, thanks to interesting research carried out by bioengineers at Washington University in Saint Louis (USA), published in the scientific journal Nature Metabolism. Researchers have developed a non-invasive tool, to place on the skull, that can direct an ultrasound beam to a specific area of the brain (the hypothalamus), called the supraoptic area, where the nerve cells that regulate the torpor process in animals are located.
The researchers used the ultrasound on mice (animals subject to torpor), inducing a drop in body temperature of 3 degrees in one hour, and changing their metabolism from a mixed consumption of sugars and lipids to one almost exclusively of lipids (fats are complex molecules to metabolise and take time: this is why they function as a reserve of energy), a typical feature of hibernation. Ultrasound also lowered their heart rate by 47%: another extremely specific feature of the state of torpor.
Programmable 'hibernation’
The depth and duration of the semi-lethargic state increases as the duration and intensity of the ultrasound applied increases. The instrument developed by the US researchers can even programme the duration and intensity of the state of torpor, as well as parameters like temperature. In the future, if this technique is also tested (with extreme caution) on humans, these options could become very useful in space travel or in surgery.
How can torpor be induced?
As for the mechanism by which ultrasound induces torpor, researchers have shown that it all happens because on the surface of the nerve cells of the hypothalamus there are special proteins that open up and form real channels through which ions that can modify the electrical currents of the cell itself, called TRPM2, and which are sensitive to ultrasound, pass.
Further evidence lies in the fact that if TRPM2 ion channels are eliminated, suppressing the gene that 'produces' them, the animals no longer hibernate, despite being stimulated with ultrasound. But can the system developed at Washington University cause problems in the brain? “Immunohistological tests after ultrasound treatment revealed no visible brain damage or inflammation” the researchers wrote in Nature Metabolism.
Bioengineers have also tried to use their instrument on animals like rats (a different type of rodent from mice), which are not subject to natural periods of torpor, and experimentation produced less dramatic effects. However, ultrasound still induced a result and this, the researchers write, is an important starting point.
