How the gut overrides the throat in telling the brain whether it's thirsty
Feeling thirsty is a natural bodily function to get you to drink some water – but why does the feeling go away so quickly after downing a glass? Shouldn't it take a bit of time for the water to be absorbed? Strangely enough, the mechanisms behind thirst and satiation are somewhat of a mystery to science. So now researchers from Caltech have investigated and found two connections between body and brain that control the process.
The new research paints a more complete picture of the complex process that plays out when we take a drink. It starts, obviously, in the mouth and throat. When nerves in these parts of the body detect a liquid, they send a signal to a part of the brain called the subfornical organ (SFO).
The SFO is responsible for regulating thirst signals, to make sure an animal stays properly hydrated. Neurons in this region are activated to generate a thirsty sensation, and when it receives a "drinking" signal from the mouth and throat, those neurons are switched off.
But switching off the thirst response is a two-stage process, and it's this second step that the team has newly discovered. It turns out that a second signal is sent from the gut to the SFO when the liquid finally reaches the stomach, and this can effectively override the signal from the throat.
Nerves in the gut sense the concentration of water in that liquid, and if there's enough to hydrate the body, the thirst neurons stay off. But if there isn't much hydration happening – say, if the drink was soda or alcohol – then those neurons are switched back on, and the sensation of being thirsty will come back quickly.
This "failsafe" system seems to serve an evolutionary purpose. It should make animals seek out liquids when they're thirsty, while the refreshing feeling from the mouth signals stops them from drinking too much before the body can decide whether that liquid is good for you.
"The gulping signal is important to prevent the ingestion of too much of any liquid that will not hydrate you," says Vineet Augustine, first author of the study. "If you drank salty water, for example, this gulping signal is like a brake that stops you from continuing to drink until the second signal can make sure that what you drank was water."
The team discovered these two separate pathways by separating them in animal tests in the lab. They had some mice drink water normally through the mouth, while others received their liquids directly into the stomach through a process called intragastric infusion.
They found that when thirsty mice drank water, they got a hit of dopamine in the brain. That indicates that the animals got pleasure from it, which makes sense. But when the water went directly to the gut, no dopamine was released, even though the thirst neurons were still shut off.
The researchers say that identifying these two pathways could lead to new treatments for eating disorders, which may result from too much or too little pleasure from eating.
"As is the case with drinking, we eat because it is pleasurable," says Augustine. "Sometimes, even if you are not hungry, you eat because of the pleasure you feel. So, if we can separate these pathways completely, then we can maybe manipulate them individually. For example, if we could satiate appetite without stimulating the pleasure pathway, we could regulate overeating. We could possibly also reduce anorexia, which may occur because a person gets too little pleasure from eating."
The research was published in the journal Neuron.