How Do We Know We are Thirsty?
We normally do not ponder what the mechanisms that let us know we need fluid are. We just want to drink. Period. How the body understands there is not enough water has been a mystery for a long time, and it actually remains to be one, but now researchers have found that our brain can detect when we are drinking.
Have you ever seen an animal drinking? When cats (or any other creature) start drinking, they do it because their bodies are dehydrated – there is not enough fluid to maintain body functions, so this need triggers thirst. However, it takes time for the body to become hydrated. Why do animals stop drinking after a minute or so? If they had to wait until the liquid consumed is absorbed, they would drink a lot more than they do. How do they (and we) do it?
How is thirst regulated?
More than a decade ago, it was discovered that thirst is regulated with the help of a special part of the brain, which can be called thirst center and is located deep in the brain, in the hypothalamus. In this center, there are different types of cells; one of them can sense dehydration and trigger processes that make a person want to drink. The brain area involved in the processes of thirst regulation is a special structure that has the name of lamina terminalis. Two of the three areas of which this brain part is comprised are not separated from the bloodstream, so they can monitor the sodium concentration in the blood and determine when it is time for hydration. Three years ago, another study followed that found the excitatory neurons located in the LT elicit drinking behavior when stimulated. Now the same team of researchers led by Yuki Oka has carried out a research in which they demonstrated that the body can differentiate between the state of “drinking” and the state of “eating”, and this “water meter” helps stop drinking when the water is consumed but not yet absorbed, thus helping avoid excessive fluid intake.
It order to hydrate the body using the liquid you have just consumed, your body needs 10-15 minutes. No “drinking session” lasts that long, except for the disorder we will mention below. To find out how the brain gives the “stop” command, the investigators used lab mice and an advanced technique called optogenetics. This approach allowed the researchers to switch on and off the circuits in the mice’s brains to identify which cells react to drinking.
The findings turned out to be surprising: there are special cells in the LT that seem to react to drinking, which they detect by sensing how muscles in the throat move. If the movements are characteristic of drinking (they are fast = gulping), thirst is quenched; if the movements are slow, it means the person is eating, and it has nothing to do with thirst, so no actions are taken.
The investigators also found that by means of stimulating these special cells they could affect the circuit regulating thirst and actually “turn off” drinking behavior.
They suppose it is the disruption of the circuit processes that is behind psychogenic polydipsia, a disorder that makes a person drink large amounts of fluid. The researchers hope that they will manage to develop some kind of treatment for those suffering from it using the information they have obtained.
A lot of work to do
However, the findings do not explain how the body maintains the feeling of satiety, which is followed by thirst when the sodium concentration changes. The neurons in question are not the only part of this complex system, and there is a lot of work to do to find out what other processes involved in thirst regulation there are.