Drinking too much water can be harmful. So how do we know when to stop?
Like most substances known to man, water when consumed in excessive amounts can be detrimental to your health. Polydipsia is a condition characterized by abnormal and excessive thirst that can sometimes be life-threatening as it upsets the balance of salts in the body. But how do we know when we are thirsty and when to stop drinking in order to avoid consuming too much water?
When our body gets dehydrated, our brain makes the decision to consume fluids. But how does the brain sense the need for water?
The “thirst center” is located in a tiny region of the brain called the subfornical organ. This brain region lacks the protective blood-brain barrier that acts as a highly selective filtering system that prevents most blood-borne substances from entering the brain. The absence of this barrier puts the thirst center directly in contact with the body’s blood circulation. When we get dehydrated, the composition, volume, and tonicity of blood changes. These changes are detected by the thirst center which then alerts the body to increase its water consumption. These signals are perceived by the body as thirst.
There is, however, a clear disconnect between the time it takes the body to return to its normal blood composition and the rapidity with which the feeling of thirst is quenched. While thirst satiation occurs within a minute, restoration of normal blood composition after fluid intake takes much longer (10 to 15 minutes). If blood composition was the sole metric by which water intake was regulated, we should in theory continue to consume water for a much longer duration which would then result in detrimental effects due to over-dilution of blood. This suggests that the brain predicts changes in blood composition much before they actually occur to alter thirst perception and drinking behaviors accordingly.
In order to identify the brain circuits that control this behavior, scientists studied the firing patterns of neurons in the thirst center of live, freely moving mice. They identified both excitatory and inhibitory neurons in this brain region. Excitatory neurons stimulate the firing of electrical impulses, whereas inhibitory neurons prevent it. The excitatory neurons in the thirst center were activated when mice were deprived of water but this activity began to decline almost as soon as mice started drinking water. When neuron activity returned to baseline levels (approximately 1 min), mice stopped drinking. On the other hand, when these neurons were inhibited using laser light (a technique called optogenetics) they did not drink water even when dehydrated. These thirst center neurons were also activated during eating explaining why many animals, including humans, feel the need to drink water during meals. The reduction in activity of these neurons was found to be directly related to the temperature of the water being ingested. Neuron activity declined rapidly when cold water was consumed, but much gradually when warm water was consumed. This explains why cold water is found to be much more satisfying in quenching thirst!
So is the mere sight of water or indication of access to water enough to stop the thirst neurons from firing? Turns out that allowing mice to see or lick an empty water bottle did not stop the activity of these neurons. The activity of these neurons was tightly linked to the act of water ingestion itself. Interestingly, neuron firing did not decline in response to food intake. This suggests that although food and fluid ingestion is controlled by the same throat muscles, the thirst center is able to distinguish between the rapid gulping motion associated with drinking as opposed to the slower act of chewing and swallowing.
If the sensation of fluid in the mouth was the only criteria that determined when to stop drinking, what would happen if we consumed sea water when thirsty? The neuron activity in mice that were given salt water was initially indistinguishable from mice that received purified water, in that the firing decreased over time. However, while the neuron activity returned to baseline levels in mice provided purified water, it remained significantly elevated in the mice provided salty water, therefore not completely satisfying the feeling of thirst in the latter. This indicates that the thirst center relies on two types of feedback mechanisms that dictate when we should stop drinking: the first is an immediate mechanism that tracks fluid ingestion while the second slightly delayed mechanism tracks the tonicity/composition of the consumed fluid.
Scientists believe that unraveling the brain circuits that control thirst could provide an understanding of psychogenic polydipsia, a rare disorder that causes people to drink copious amounts of water. Psychogenic polydipsia may be observed in people with severe mental illness, including anxiety and depression. In the larger scheme of things, being able to manipulate and record neurons in living laboratory animals is providing newer, hereto unexplored insights into the mysterious workings of the brain.
