Do you drink a glass of water before going to bed? Then, hate to break it to you, but you have something in common with a mouse.

Overview

A new paper suggests that a mouse’s biological clock increases its thirst shortly before it sleeps.¹ Why? The study’s authors suggest this increased drive to drink, allows a mouse to drink a little bit of extra water, which will help maintain the mouse’s balance of water and salt throughout the night as it loses fluid through for example through sweat.

The study shows the mouse’s Brain is hard-wired to anticipate fluid will be lost throughout the night. The part of the brain that keeps track of the biological clock, the SCN, sends a Connection that signals to the part of the brain that regulates thirst, the OVLT,² and this connection releases Vasopressin as a neurotransmitter.

Brain Slices: Scientists anesthetize the mouse and remove it’s brain keeping it in an oxygenated fluid with nutrients, so the cells stay alive. In this case, they cut the brain at a 34 degree angle to obtain a slice that contains the OVLT, SNC, and connection between them.

Then experimenters looked at a ‘slice’ of the mouse’s brain, a thin strip of brain tissue that maintained the connection between the SCN and OVLT. They showed that the OVLT Neurons that respond to vasopressin, increased in the time shortly before the animal would sleep.

When scientists stimulated the SCN they could increase activity in the vasopressin-detecting neurons in the OLVT. Crucially, however, when the scientists stimulated the SCN, while simultaneously blocking the receptors that sense Vasopressin, the OVLT showed no increase in activity.

These experiments suggest that the SCN’s releases of vasopressin increases the activity of OVLT neurons, and the activity of OVLT neurons has been shown to correlate with drinking behavior in other experiments.

To prove that this effect was real in the context of the whole brain, the experimenters bred mutant mice that express optogenetic proteins only in Vasopressin releasing neurons. These optogenetic proteins allowed the researchers to shine a light into the mouses brain and inhibit or excite the Vasopressin-releasing neurons that project from the SCN to the OVLT.³ When they inhibited this connection during the pre-sleep period, they could decrease animals pre-sleep drinking behavior. When they activated this connection outside of the pre-sleep period the animals increased the amount they drunk.

Importantly, this study was performed in mice. Although circadian rhythms are similar and many hypothalamic processes are conserved throughout evolution, they are not exactly the same–mice are for example nocturnal. Therefore, it is still unclear whether this exact mechanism occurs in other animals such as humans. (So maybe I was premature in comparing you to a mouse, my apologies.)

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I’m curious about your own experience. I’ve found I always wake up from naps extremely thirsty (and I don’t think it’s just from the drool). I wonder if that too might relate to an interplay between circadian rhythm, sleep, and thirst.

What’s your experience–do you feel you get thirsty right before bed? Do you sleep with a glass of water on your nightstand?

Some nerdy comments for those who care and References:

Additionally, while this experiment showed statistically significant effects and makes an extremely strong case for the existence of this pathway and its role in modulating pre-sleep thirst, other pathways may also play roles. For example vasopressin levels into the blood also follow circadian rhythms and may have effects on OVLT neurons.⁴

1. Gizowski, C., Zaelzer, C. & Bourque, C. W. Nature 537, 685–688 (2016).

2. Both of these regions are within the hypothalamus, an area which regulates maintains homeostatic balance of things like body temperature, food intake, etc.

3. Though technically back-propagating action potentials could lead to other brain areas being affected as well, especially if the SCN neurons have branching connections.

4. Trudle, E. & Bourque, C. W. Nature Neurosci. doi:10.1038/nn.2503 (2010).