We are all familiar with the drawbacks of dehydration, but we rarely hear about the harmful effects of overhydration. It is known that excess fluid accumulation can lead to dangerously low sodium levels in the blood or hyponatremia - a life-threatening condition that can result in brain swelling. Similarly, more is known about the mechanisms in the body that detect and drive thirst while little is known about how the brain detects a state of overhydration.

"[Hyponatremia] occurs in common pathological conditions, including brain injury, sepsis, cardiac failure and in the use of drugs, such as MDMA (ecstasy)," says the senior author of the study featured in Cell Reports.

"Our specific data will be important for people studying hydromineral and fluid electrolyte homeostasis, and clinicians who treat patients faced with hyponatremia," reports another author. This condition is more common in elderly patients and can cause cognitive problems and seizures in this vulnerable group. While it remains uncertain how hyponatremia develops, a defect in the hydration sensing mechanism of the brain could be the culprit.

The new research shows that overhydration activates Trpv4, which is a cellular gatekeeper implicated in maintaining the balance of water in the body. Trpv4 is a calcium channel that can be found in glial cells, which are cells that act to surround hydration sensing neurons.

"Our study shows that it is in fact glial cells that first detect the overhydrated state and then transfer this information to turn off the electrical activity of the [hydration sensing] neurons," explains the author. The researchers also found that it is the release of the amino acid taurine that acts to inhibit hydration sensing neurons. Essentially, when overhydration is detected by glial cells, the Trpv4 channel triggers the release of taurine, which acts as a trip wire to inhibit hydration sensing neurons.

The brain's ability to detect excess hydration is essential to maintaining fluid balance in the body and preventing conditions like hyponatremia. "Preclinical models of hyponatremia will be used to examine if the mechanism we report is affected in this condition with the long-term objective of designing new treatments or diagnostic tools," says the author.

"Hyponatremia is common with as many as one-quarter of hospitalized patients suffering from a traumatic brain injury developing the condition," says neurosurgeon author.

https://muhc.ca/newsroom/news/harmful-dehydration

https://www.cell.com/cell-reports/abstract/S2211-1247(18)30668-5