Have you ever noticed a tendency to drink some water or other liquid sustenance right before going to bed? It’s a common occurrence although the reason behind this action has not been well understood. This unfortunately has led to a rather large-scale debate regarding the potential health benefits and risks of having a swig before sleep.
Over the years, some researchers have suggested the action is based on a physiological need, such as elevated body temperature or low water concentration in blood. Others have suggested this action is psychological rather than biological in nature as it increases the chances for REM sleep and dreaming. Then there are those who feel this action has no health value at all. After all, drinking immediately before sleep means you will no doubt have to disturb your regular period of rest for a quick bathroom break.
For a group of researchers at McGill University led by Dr. Charles Bourque, none of these arguments appeared to be correct. They felt there was a nascent, neurological reason for this swig before slumber. They decided to explore this trait in mice in the hopes of finding the mechanism behind the nightly thirst. Their discovery, which now published in the journal Nature, revealed the reason for this process may more important than once believed.
The first step was to act as mythbusters and show thirst had nothing to do with physiological needs. They did this by restricting various components of diet and monitoring various vital signs such as temperature, blood salinity, and water concentration in blood. None of these changes amounted to any difference in water intake. The animals still went for water right before bed.
To remove the second theory – psychological motivation – the team explored which areas of the brain were responsible for this need for hydration. This was not all that difficult as they already had one area in mind for testing. It’s called the organum vasculosum lamina terminalis, or OVLT. It’s a small, bead-like structure near the hypothalamus and is responsible for thirst . In addition, this area has no involvement in the production of dreams.
The team examined the activity of the OVLT during normal wakefulness, which they called the basal period; and right before bed, known as the anticipatory period. As expected, the OVLT was more active right before bedtime. This meant the urge for hydration had little to nothing to do with dreaming and instead was regulated by some other factor.
For the authors, the most likely molecule causing this sudden need for thirst was vasopressin. It’s long been known to be involved in water retention and acts as a neurotransmitter in the brain. They also had figured on the area of the brain responsible for this action. It’s known as the suprachiasmatic nucleus, though most call it the SCN. This area is known to be involved in circadian rhythm and contain vasopressin neurons.
When the team examined the SCN for the electrical activity of vasopressin neurons, they saw, as expected, a rise in their activity during the anticipatory period in comparison to the basal period. This observation suggested there was a link between the SCN and the OVLT, known as a projection and that vasopressin was the signal between the two regions.
To determine if this was indeed the route of vasopressin-based electrical activity, the team used optogenetics to modify the release of vasopressin. As expected, when vasopressin was released, the mice drank more in the anticipatory period. But when vasopressin was shut off, there was no difference in intake compared to the basal level.
With the mechanism all but determined, there was only one last experiment to perform. They had to knock out the actual receptor for vasopressin, appropriately known as vasopressin 1a receptor, or V1aR. When they did, the drinking rate stayed the same for both basal and anticipatory periods.
From the collection of results, the study reveals exactly how, at least in mice, the urge to have that beverage before bedtime occurs. As the body begins to anticipate the sleep period, the SCN begins to send vasopressin to the OVLT. Once here, the hormone acts as a neurotransmitter with V1aR to activate the neurons in that area. When they get excited, they send signals to initiate the thirst response.
The results of this study reveal the need to drink before bed is most likely a natural process. As to why this happens, the data shows that in the absence of water intake the mice become dehydrated overnight. Whether this is true for humans remains to be determined.
This study also offers a fascinating glimpse into the impressive spectrum of brain activities. In this case, the brain – using the clock – can predict physiological trouble and take preventative action to avoid imbalance. This concept of foreshadowing unveiled by the authors reveals how much we still haven’t uncovered amid the spectrum of brain functions. With more research such as theirs, we are certain to continue to gain insight – and no doubt captivation – about the yet undiscovered wonders of the brain.
Read the original research article here:
Clock-driven vasopressin neurotransmission mediates anticipatory thirst prior to sleep’, by C. Gizowski, C. Zaelzer, and C.W. Bourque is published in Nature, doi:10.1038/nature19756
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