Investigating how our brains determine when our bodies should have a drink
Your skin may seem dry, your mouth may be parched, but the experience of thirst is all in your brain. To be more precise, it’s in your ‘thirst center,’ a bundle of brain cells found in your hypothalamus. These neurons are tasked with the job of telling an animal when to get a drink, and just as importantly, when to stop sipping. Weirdly, that second command takes place well before the rest of the body can benefit from any new fluids, which has made untangling the exact functionality of this system a surprising tricky problem.
When the body wants water
The thirst center isn’t making us go for a drink at random. The hypothalamus takes in information about a lot of our physiology, including blood pressure, sodium concentration and more. When these systems are out of balance and more water is needed, one response is to create more of the hormone vasopressin in the pituitary gland, which then makes the kidneys try to reclaim water from any available urine. The second, but probably preferable response is to have the thirst center have us look for a drink.
As obvious as that response may seem, it’s taken years for researchers to isolate which cells were actually in charge of feeling thirsty. To identify exact which parts of the brain were tied to thirst, researchers turned to optogenetics, a technique that uses engineered neurons to be activated by light via implanted fiber optics. In this case, mice had their brain cells monitored and stimulated until they either became thirsty, or artificially had their thirst quenched without needing a drink.
In this process, it became clear that the amount of water an animal drinks is entirely up to the brain. Water won’t do a body much good until 10 to 15 minutes after it’s been ingested, whereas mice’s thirst center stopped them from drinking after only a minute each time. Somehow the brain felt it had had enough liquid, and that it was time to stop drinking.
This may sound pointlessly academic, but that’s only because it works so well for most of us. Whatever mechanism the thirst center may be using to measure our water intake, it’s generally helping us obtain a safe amount of water. If we ignore the thirst center, as some marathon runners have done, or the mechanism is damaged, as with people with psychogenic polydipsia, the results can be dangerous or even fatal. Cells swell and burst like balloons as they try to absorb the incoming water, a fate particularly damaging when it occurs in the brain. On the flip side, as people age they often have a weaker thirst response, and may forget to hydrate on a regular basis.
At this point, the investigation into how we feel thirst is actually moving out of the brain. Researchers are interested in muscle cells in the throat that help with swallowing, as there’s a chance that our sip-stopping thirst center may be gauging our water intake based on how quickly we swallow.
Source: Still Thirsty? It's Up To Your Brain, Not Your Body by Jon Hamilton, The Salt