爲什麼人夜晚睡覺不會口渴
The body's internal clock helps to regulate a water-storing hormone so that nightly dehydration or trips to the toilet are not the norm, research suggests.
In an article published in Nature Neuroscience today, neurophysiologists Eric Trudel and Charles Bourque at the Research Institute of the McGill University Health Centre in Montreal, Canada, propose a mechanism by which the body's circadian system, or internal clock, controls water regulation1. By allowing cells that sense water levels to activate cells that release vasopressin, a hormone that instructs the body to store water, the circadian system keeps the body hydrated during sleep.
"We've known for years that there's a rhythm of vasopressin that gets high when you're sleeping. But no one knew how that occurred. And this group identified a very concrete physiological mechanism of how it occurs," says Christopher Colwell, a neuroscientist who studies sleep and circadian rhythms at the David Geffen School of Medicine at the University of California, Los Angeles.
The body regulates its water content mainly by balancing water intake through thirst with water loss through urine production. People don't drink during sleep, so the body has to minimize water loss to remain sufficiently hydrated. Scientists knew that low water levels excite a group of cells called osmosensory neurons, which direct another set of neurons to release vasopressin into the bloodstream. Vasopressin levels increase during sleep; clock neurons, meanwhile, get quieter.
Trudel and Bourque tested the idea that lower clock-neuron activity might allow osmosensory neurons to more easily activate vasopressin-releasing neurons, which would mean more water retention and less urine production during sleep.
To do this, they isolated thin slices of rat brain containing intact sensory, vasopressin-releasing and clock neurons. Even when removed from the brain, clock neurons continue to mark time.
The duo then stimulated the sensory neurons and recorded any electrical activity in the vasopressin-releasing neurons to monitor communication between the two cell groups. The researchers then moved on to look at the effect of the clock cells on this pathway. When they did not activate the clock cells during the 'sleep' part of their cycle, it was easier for the sensory cells to communicate with vasopressin-releasing cells. Conversely, when they activated the clock cells, this communication decreased markedly.
The results suggest that clock cells function as a dimmer switch for water control. When their activity is high, they prevent sensory cells from instructing secretory cells to release vasopressin. Then, when clock cells are less active, sensory cells can easily instruct secretory cells to release vasopressin, ensuring that the body holds on to its water reserves.
Colwell points out that the study was done in rats, which are nocturnal. Although the vasopressin cycle and clock-neuron activity are similar in rats and humans, the question of whether the same mechanism occurs in animals that sleep at night remains to be answered.
"We show this for this one circuit, but it's possible that clock neurons regulate other circuits in a similar manner and this remains to be studied," says Bourque. He speculates that future studies might reveal whether the same mechanism regulates hunger, sleepiness and other aspects of physiology related to circadian rhythms.
《自然》雜誌新聞（NatureNews）安德魯·貝內特·赫爾曼（AndrewBennett Hellman）
生物鐘是控制身體水分流失的荷爾蒙調節開關。
大腦細胞使動物在睡覺期間保持水分，而不覺口渴。
研究顯示，身體內部的生物鐘幫助調節一種水分儲存荷爾蒙，因此人晚上一般不會感到口渴或者頻繁上廁所。
2月28日發表在《自然神經科學》雜誌（Nature Neuroscience）上的一篇論文中，加拿大蒙特利爾麥吉爾大學健康中心研究所（Research Institute of the McGill University Health Centre）的神經生理學家埃裏克·特魯德爾（Eric Trudel）和查爾斯·布爾克（Charles Bourque）提出了一種機制，通過這種機制身體的晝夜系統（circadiansystem），或者說內部生物鐘能夠控制水分的調節。通過允許感知水含量的細胞去激活能夠釋放抗利尿激素（vasopressin）的細胞，抗利尿激素是一種指示身體進行儲水的荷爾蒙，晝夜系統就能使身體在睡覺時儲存水分（而不覺口渴）。
“多年來我們已經知道，當你在睡覺時抗利尿激素的水平會升高。但是沒有人知道這是如何發生的。而這個研究團隊提出了一種關於其如何發生的非常具體的生理機制，”神經科學家克里斯托弗·科爾韋爾（Christopher Colwell）說，他在加州大學洛杉磯分校（UCLA）大衛·格芬醫學院（DavidGeffen School of Medicine）從事睡眠與晝夜節律的研究。
身體主要通過平衡因口渴而飲水與因產生尿液而排水來調節水含量。人們在睡覺時不會喝水，所以身體必須使水分流失最小化來保持充足的水分。科學家知道低含水量會激活一組被稱爲滲透感覺（osmosensory）神經元的細胞，它們能指示另一組神經元以釋放抗利尿激素到血液中。在睡眠期間抗利尿激素水平增加；同時，生物鐘神經元變得更加安靜。
特魯德爾和布爾克檢驗了這個想法，即更低的生物鐘神經元活動可能允許滲透感覺神經元更容易激活抗利尿激素釋放神經元，這將意味着在睡眠期間人體會保存更多的水併產生更少的尿液。
爲了檢驗這個想法，他們將小白鼠的大腦分離成薄片，這些薄片仍然包含未受損傷的完整的感覺神經元、抗利尿激素釋放神經元和生物鐘神經元。即使當這些薄片從大腦裏取出後，生物鐘神經元仍然能夠繼續記錄時間。
然後這兩名研究者對感覺神經元施加刺激，並記錄抗利尿激素釋放神經元中的任何電活動來檢測這兩組細胞間的溝通。此後，研究者轉而考察使用這種方法後生物鍾神經元的效應。當他們未對處於晝夜週期中“睡眠”時期的生物鐘細胞進行激活時，感覺細胞更容易與抗利尿激素釋放細胞溝通。相反，當他們激活（睡眠時期的）生物鐘細胞時，這種溝通明顯下降。
這些結果指出了生物鐘細胞作爲身體水分控制開關的功能。當它們的活動很強時，它們阻止感覺細胞指示分泌細胞（secretory cell）去釋放抗利尿激素。相反，當生物鐘細胞激活水平比較弱時，感覺細胞能夠更容易地指示分泌細胞釋放抗利尿激素，以保證身體在睡眠時保持足夠的蓄水量。
科爾韋爾指出這項研究是在小白鼠上進行的，而老鼠恰恰是夜間活動動物。雖然老鼠和人類體內的抗利尿激素釋放週期和生物鐘神經元活動都很相似，但是動物夜間睡眠期間是否都發生了相同的機制這個問題仍需回答。
“我們只是顯示了這一個生物反饋迴路的結論，但是可能生物鐘神經元也以同樣的方式調節其他的反饋迴路，這仍需繼續研究，”布爾克說。他預測未來的研究將可能揭示是否相同的機制也調節了飢餓、嗜睡以及與晝夜節律相關的其他生理問題。
