The thirteen-striped squirrels native to North America hibernate for up to eight months. During this time they do not consume any liquid and do not feel thirsty, even during periods of activity. A study now shows which neural mechanisms are behind it. Accordingly, the chipmunks have a significant lack of fluids, but their thirst neurons are so downregulated that they do not pass on the signals.
If we haven’t drunk enough, sooner or later we will feel thirsty – a vital signal from our body that prompts us to drink fluids again. The feeling of thirst is triggered by an interaction between the kidneys and the brain. If the blood volume decreases, the kidney releases an enzyme that triggers the production of the hormone angiotensin II. This hormone ensures that less water is excreted in the kidneys. In the brain it binds to receptors that trigger the need to drink. The same mechanism is found in most other mammals. For some, however, it seems to be suspended at least temporarily: during hibernation.
Eight months without water
“Despite more than a century of research, the question of the neurophysiological mechanism that allows hibernating animals to survive for months without drinking remains unanswered,” explains a team led by Madeleine Junkins from Yale University School of Medicine. To answer this question, the researchers examined the thirteen-striped squirrels (Ictidomys tridecemlineatus), which are native to North America.
“In winter, chipmunks hibernate for six to eight months without drinking,” reports the research team. During this time, the animals remain in their underground burrow and switch between two states: In the so-called torpor phases, the body temperature drops to two to four degrees Celsius and the animals are completely inactive. Torpor is interrupted every two to three weeks by short periods of activity in which the body temperature rises to 37 degrees Celsius for one to two days. “During these phases, the animals move around in their burrow, but do not leave it in search of water,” say the researchers. “Even when given the opportunity, they refuse to drink.”
Thirst neurons inhibited
But what mechanisms are behind it? Junkins and her team found clear signs of dehydration in the blood of chipmunks during a winter activity phase: the blood volume was reduced and the angiotensin II level doubled. Actually, such a high level of this hormone should trigger intense thirst. But that’s obviously not the case, as the croissant’s refusal to drink shows. The researchers therefore suspected that the reason must lie in the brain: Could the receptors in the brain not detect angiotensin II during hibernation?
As Jungkins and her team discovered, this obvious explanation doesn’t apply. “Surprisingly, the neuronal mechanism for thirst suppression appears to be independent of the angiotensin II signaling pathway,” they report. “The thirst neurons clearly have the ability to recognize and respond to angiotensin II throughout the entire period of time.” Instead, the researchers found that the basic activity of the thirst neurons is reduced to such an extent by inhibitory neurons that the stimuli are not passed on. In addition, the thirst neurons in the experiment reacted more strongly to the inhibitory messenger GABA.
“We suspect that chipmunks use a two-pronged strategy to optimize water consumption while minimizing thirst,” the researchers write. Angiotensin II ensures that the body loses as little water as possible. The downregulation of thirst neurons in the brain also prevents the animals from developing the need to leave their burrow in winter to look for water. Because that would cost them too much energy and endanger their survival. “Our work demonstrates a remarkable ability to control fluid balance to enable long-term survival without water,” the team said.
Source: Madeleine Junkins (Yale University School of Medicine, CT, USA) et al., Science, doi: 10.1126/science.adp8358