Every single day, half a liter or so of water flows from the blood to the brain to reinforce the cerebrospinal fluid (CSF) – this is a bit like the brain's armor in as far as it protects the organ from concussion. Scientists know that the water does so by passing through an extremely thin tissue called the plexus choroideusm. How exactly it manages to permeate the barrier, however, has been a bit of a mystery. Until now.
It was assumed to be through a process of osmosis. That is when molecules seep through a semipermeable membrane from a higher concentration liquid to a lower concentration liquid until it reaches a point when the liquid on both sides of the barrier is equally concentrated. Now, researchers from the University of Copenhagen have shown that most water that ends up in the brain gets there with the help of something they refer to as a co-transporter. The study has been published in the journal Nature Communications.
Suspecting that osmosis would not be enough to sustain the required rates of fluid production, the researchers used a mouse model where the conditions necessary for osmosis were missing and likely water transporters could be "turned off". By inhibiting various transporters and measuring fluid production, they discovered that a previously unknown ion transporter, the NKCC1 co-transporter, was responsible for roughly half of all fluid production, which would make it the brain's principal water transporter.
"It is brand new knowledge on a very important physiological process involving the by far most complex organ in the human body, namely the brain," co-author Nanna MacAulay, an associate professor at the Department of Neuroscience at the University of Copenhagen, said in a statement.
Of course, the experiment was conducted on mice, which does not always translate perfectly to humans. The researchers do, however, point out that the cell membrane in the plexus chorideus of mice has the same structure as it does in humans and so the same mechanisms should apply.
"[I]t would be ground-breaking if we were able to use this mechanism as a target for medical treatment and turn down the inflow of water to the brain to reduce intracranial pressure," MacAulay explained.
"There are no effective medical treatments for a lot of disorders involving increased intracranial pressure. And at worst, the patient may suffer permanent damage and even die as a result of increased pressure. Therefore, this basic mechanism is an important find to us."
Next, the researchers will try to find out how to exploit this now-identified process of the flow of water to the brain, and how it may be controlled. If they are successful, it could pave the way for treatments for disorders that involve intracranial pressure, like stroke and hydrocephalus (aka "water in the head").