The brain’s biological clock likely explains why we want to down a glass of water before going to sleep. New findings based on research conducted in mice offer the first insight into how the clock regulates a physiological function.

Scientists knew that rodents show a surge in water intake during the last two hours before sleep.

McGill University researchers found that restricting access to water during the surge period resulted in significant dehydration towards the end of the sleep cycle. So the increase in water intake before sleep is a preemptive strike that guards against dehydration and serves to keep the animal healthy and properly hydrated.

Then the researchers looked for the mechanism that sets this thirst response in motion. It’s well established that the brain harbors a hydration sensor with thirst neurons in that sensor organ. So they wondered if the suprachiasmatic nuclei (SCN), the brain region that regulates circadian cycles, could be communicating with the thirst neurons.

Vasopressin Sniffer Cells

[caption id=“attachment_81252” align=“alignleft” width=“327”]Thirst neurons Thirst neurons. Credit: Bourque lab[/caption]

The team suspected that vasopressin, a neuropeptide produced by the SCN, a tiny region of the brain in the hypothalamus, might play a critical role.

To confirm, they used so-called “sniffer cells” designed to fluoresce in the presence of vasopressin.

When they applied these cells to rodent brain tissue and then electrically stimulated the SCN.

“We saw a big increase in the output of the sniffer cells, indicating that vasopressin is being released in that area as a result of stimulating the clock,”

says Charles Bourque, a neurology professor at McGill University and senior author of the study

To explore if vasopressin was stimulating thirst neurons, the researchers employed optogenetics, a cutting-edge technique that uses laser light to turn neurons on or off. Using genetically engineered mice whose vasopressin neurons contain a light activated molecule, the researchers were able to show that vasopressin does, indeed, turn on thirst neurons.

“Although this study was performed in rodents, it points toward an explanation as to why we often experience thirst and ingest liquids such as water or milk before bedtime,” Bourque says. “More importantly, this advance in our understanding of how the clock executes a circadian rhythm has applications in situations such as jet lag and shift work.

All our organs follow a circadian rhythm, which helps optimize how they function. Shift work forces people out of their natural rhythms, which can have repercussions on health. Knowing how the clock works gives us more potential to actually do something about it.”

C. Gizowski, C. Zaelzer & C. W. Bourque Clock-driven vasopressin neurotransmission mediates anticipatory thirst prior to sleep Nature 537, 685–688 (29 September 2016) doi:10.1038/nature19756

Image: Myrtle Beach TheDigitel/Flickr

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