by Michelle Frank
figures by Jovana Andrejevic
You’ve undoubtedly experienced the feeling of thirst: it’s a slight itch in the back of your throat, a distracting urge to turn away from whatever you’re doing and find something to drink. It drives you to guzzle water on hot days and to drink something along with your meals. Our need for water is as omnipresent and critical as our need for food or oxygen–it’s an essential cog that keeps our bodies working normally. The craving to pick up a beverage when we’re feeling parched might feel intuitive, but our bodies rely on an intricate set of biological processes to make sure we stay properly hydrated, as consuming both too much or too little water can lead to problems.
What makes us thirsty?
When your body starts to run low on water, a number of changes take place: for one, the volume of your blood decreases, causing a change in blood pressure. Because the amount of salt and other minerals in your body is staying constant as the volume of liquids decreases, their relative concentration increases (the same number of particles in a smaller volume means that the particles are more concentrated). This concentration of particles in bodily fluids relative to the total amount of liquid is known as osmolality, and it needs to be kept in a narrow range to keep the cells in your body functioning properly. Your body also needs a steady supply of fluids to transport nutrients, eliminate waste, and lubricate and cushion joints. To some extent, the body can compensate for water depletion by altering heart rate and blood pressure and by tweaking kidney function to retain more water. For you, though, the most noticeable indication that your body is running low on fluids is likely the feeling of thirst, as you increasingly feel like you need to drink some water.
So how does your body know that these responses are necessary, and how are they coordinated across so many different organ systems? Scientists are still trying to uncover how this process works, but research over the past several decades indicates that a highly specialized part of the brain called the lamina terminalis is responsible for guiding many of these thirst responses (Figure 1). Brain cells within the lamina terminalis can sense when the body is running low on water and whether you’ve had anything to drink recently. When researchers manipulate this brain region, they can also drive animals to seek out or avoid water, regardless of how hydrated that animal might be.
The lamina terminalis is located towards the front of the brain and occupies a prime location just below a fluid reservoir called the third ventricle. Unlike much of the rest of the brain, many cells in the lamina terminalis aren’t guarded by a blood-brain barrier. This barrier prevents many circulating factors in the blood and other fluids from interacting with cells in the brain, offering the brain protection against potentially dangerous invaders like certain bacteria, viruses, and toxins. However, the blood-brain barrier also cuts the brain off from many circulating signals that might hold useful information about the body’s overall status. Because certain cells in the lamina terminalis lie outside the blood-brain barrier, these cells can also interact with the fluid in the third ventricle to keep tabs on factors that indicate whether the body needs more or less water. In particular, these cells can monitor the fluid in the ventricle to determine its osmolality and the amount of sodium present.
When other parts of the brain detect information that’s relevant to understanding the body’s water needs, they frequently pass it along to the lamina terminalis, as well (Figure 2). In this way, the lamina terminalis also collects information about things like blood pressure, blood volume, and whether you’ve eaten recently (even before food can cause any change in circulating salt or water levels, your body tries to maintain a balance between these factors by encouraging you to drink water every time you eat). Information from the part of the brain that controls the circadian clock also gets forwarded to the lamina terminalis, encouraging animals to drink more water before sleeping to avoid becoming dehydrated during long periods of sleep. Collectively, this information gives the lamina terminalis the resources needed to make a call about whether the body needs more or less water. In turn, cells in the lamina terminalis project to many other areas of the brain, sending out their verdict about current water needs. Although scientists are still trying to figure out exactly how information from the lamina terminalis affects other brain regions, it’s clear that this output can influence an animal’s motivation to seek out water, as well as physiological factors like kidney function and heart rate (Figure 2).
What makes water so refreshing?
After a while standing outside in the hot sun, a cold drink of water tends to feel instantly refreshing. You might also find that drinking a very sugary beverage feels equally refreshing but leaves you feeling thirsty again later. In both cases, it takes tens of minutes for that drink to have any effect on attributes like osmolality or blood pressure, the body’s main indicators of hydration status. Instead, the brain must rely on some other cue to tell you to stop drinking and give you that instant feeling of refreshment.
One clue came from the discovery that neurons in the lamina terminalis actually respond to the physical act of swallowing liquids, even before there are any changes in the amount of water in the blood. Researchers in Zack Knight’s lab at UCSF identified a group of neurons in the lamina terminalis whose activity is required for drinking behaviors: when you artificially turn off the activity in these cells, mice no longer drink water, even when they are water deprived. When the researchers recorded the activity of these cells as animals drank water, they found that the cell’s activity decreased in lockstep with each sip of water, far before any physiological changes in blood pressure or osmolality could have an effect. They also found that this change in activity only happened when the mice drank water, not when they drank a salt solution. This study suggests that our brains have a built-in mechanism to compare how much water we need with the amount of water we’re currently drinking, telling us when we’ve had enough and leaving us feeling instantly hydrated. Still, scientists don’t know exactly how the brain can tell water apart from other liquids, or why drinking some non-water beverages can leave you feeling instantly hydrated, as well.
Another group of researchers led by Yuki Oka at Caltech set out to tackle the problem of why we find drinking water so rewarding when we’re thirsty. Neuroscientists have long known that most reward signals are carried by a molecule called dopamine. In order to look at the role that this molecule has drinking behaviors, Oka’s team used a new kind of sensor that glows in the presence of dopamine. By putting this sensor into a mouse’s brain, they were able to record dopamine levels in real time as the mouse went about its tasks (Figure 3).
These researchers looked at dopamine levels after thirsty mice drank water and other liquids. They also recorded dopamine levels after they injected water directly into the gastrointestinal system; this procedure hydrated thirsty animals, but meant that the mice didn’t actually drink any water. Oka’s group found that thirsty mice had a large surge in dopamine levels after drinking either water or saline and that these dopamine changes happened even before drinking would have any effect on blood fluid levels. In contrast, the animals didn’t release any dopamine after water was pumped into their gastric systems, suggesting that it’s the act of drinking itself that’s rewarding—not the feeling of being hydrated. This effect also starts to explain why drinking beverages other than water can be so satisfying, even when they leave you feeling thirsty later: the dopamine spike that comes from drinking liquids when animals are thirsty doesn’t depend on what kind of liquid they’re drinking, even though not all liquids are equally hydrating.
These two studies highlight the varying strategies the brain uses to monitor essential nutrients like water; because no single sensor can tally current water levels and predict future water needs, the brain relies on myriad complementary sensations and cues. As researchers get closer to unraveling the mystery of thirst, they’re sure to identify even more ways that the nervous system accounts for our innate need for water.
Michelle Frank is a PhD Candidate in Neurobiology at Harvard Medical School.
Jovana Andrejevic is a fourth-year Applied Physics Ph.D. student in the School of Engineering and Applied Sciences at Harvard University.
For More Information:
- See this scientific review article summarizing recent progress in our understanding of the neurobiology of thirst
- Here is a summary of recent research about why thirst is rewarding
- An overview of why water is important for biology in another Special Edition article
This article is part of our special edition on water. To read more, check out our special edition homepage!
19 thoughts on “The Neuroscience of Thirst: How your brain tells you to look for water”
Can you please tell me if the lamina terminalis is responsible for both, letting know the body needs hydration and also for telling the body it is also hydrated? Is it true that the brain sends more hydrated messages rather than dehydrated in older people, leaving seniors to drink less and less as they age? eg, grandma has a cup of tea and ten minutes later heads to the bathroom to pee?
1. One of the things I noticed with COVID patients is the drop in thirst drive.
2. And I also noticed it a patient that had a tiny brain bleed which didn’t show until the next day when she was more symptomatic of damage.
3. When former president Obama brought the Ebola patients to our country for treatment ;
One of the winning treatments was hydration.
So would it be possible to review labs to see if that had been a contributing factor.
I know that checking dopamine levels was probably not done.
Are there X-rays or ct tests that would show those tiny organs?
Don’t know about the lamina, so hopefully someone will answer that But:
1. Caffeine is a diuretic
2. You could ask her if she has talked to her doctor about frequency. It might be just a need for pelvic floor exercises
3. Ask what meds she is on
4. She might be afraid of an ‘accident’, several elderly people don’t want to ‘have to go’ and will limit their fluid intake
Can a head injury injure the neurons in the lamina terminalis? What affects how you tell if you’re hungry?
I have not been drinking water for very, very long time and am having serious headache, painnimn my neck. The headache is in front of my head, left side of my head to by my ear, and my neck. Can lack of drinking water for very very long time cause that?
Yes It affects hypothalamus and activates the hormones that regulates your body’s blood pressure and your vitals causing you major headaches, weakness, fatigue etc . Water is very essential
Not just the insane, but 100% of jazz musicians are drinking water or other, apparently identically dopaminergic, liquids. One particularly disturbing aspect to all this is that enjoyment is involved. Government agencies need to set up Task Forces to round up these addicts, counteract the dangerous lure of rewarding experiences, and in the rare cases where water may be required to treat medical conditions, some kind of non-dopaminergic form should be synthesised to prevent water barons from exploiting the illicit market, so that ordinary citizens can sleep safe in the knowledge that those with water dependence can get the help they need, while ensuring the populace remains safe, and as dry as people who work in government offices, the police and justice systems.
Lol. Please do not give them any ideas
Now replace “water” with “dihydrogen monoxide”
Hey, I hope you are doing great!
I Came across your blog, and I am Impressed with the regular and frequent updates on it .
A 81yo friend only ever drinks very very strong black coffee. NO water.
She is now diagnosed with Parkinsonism and several falls.
Can lack of water play a part?
I like this article about The Neuroscience of Thirst: How your brain tells you to look for water, it has great info, and the drinking water safe you from a lot of diseases, thanks for sharing
Thank you for this invaulable insight into mechnisms behind thirst. Would be very interested to know your thoughts of the delicate balancing act in combat sports where athletes drop water weight and regain it fast – would can lead to a lack of “normal functioning” in the way the cerebro-spinal fluid is able to act as a protective casing for the brain which then is primed for maximum damage from blunt force trauma to the head. Is there any benefit here in legalising IV based rehydration in this special case? Or are there other ways to make the rehydration process faster/better/more effective?
Just commenting so I can receive follow up comments that are posted as I am very interested in what people may say. I, myself, never feel thirsty and it has caused me problems in my life, at times it has become quite serious and I can’t find a doctor that has even a guess at an answer as to why my brain never tells me to drink.
Hello Eileen, I’m the same here. I’m never thirsty either. I try to drink 64 oz a day because I’ve read this is the amount a human needs, but I end up drinking closer to 32 oz or less. Other signals tell me I need to drink water, e.g. the color of my urine, headache, tiredness, dry mouth, or headache. I’m hungry all the time however and sometimes have an aversion to drinking liquids. I would like to find out why and correct it. Good luck to you.
Caan someone make a mind map or short essay about thirst sensation. My teacher won’t explain it to me. I need it for my exams.
I have a Friend who faints many times, when taken to hospital, nothing wrong is found. what could be the possible cause?
IDK, maybe lack of nutrien, another disclaimer: IDK, but I would sugest looking into better diets and increase of water intake, but don’t take my word Idk enough to be giving you an answer, just that I have family who have similar problems
Great blog and really impressive.