为什么人夜晚睡觉不会口渴

 　　The body's internal clock helps to regulate a water-storing hormone¹ so that nightly dehydration² 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 mechanism³ by which the body's circadian system, or internal clock, controls water regulation1. By allowing cells that sense water levels to activate⁴ 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 physiological⁵ 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 intake⁶ through thirst with water loss through urine production. People don't drink during sleep, so the body has to minimize water loss to remain sufficiently⁷ 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 retention⁹ and less urine production during sleep.

　　To do this, they isolated¹⁰ thin slices of rat brain containing intact sensory⁸, vasopressin-releasing and clock neurons. Even when removed from the brain, clock neurons continue to mark time.

　　The duo then stimulated¹¹ 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 activated¹² 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 secretory¹³ 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 remains¹⁴ 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 physiology¹⁵ related to circadian rhythms.

　　《自然》杂志新闻（NatureNews）安德鲁·贝内特·赫尔曼（AndrewBennett Hellman）

　　生物钟是控制身体水分流失的荷尔蒙调节开关。

　　大脑细胞使动物在睡觉期间保持水分，而不觉口渴。

　　研究显示，身体内部的生物钟帮助调节一种水分储存荷尔蒙，因此人晚上一般不会感到口渴或者频繁上厕所。

　　2月28日发表在《自然神经科学》杂志（Nature Neuroscience）上的一篇论文中，加拿大蒙特利尔麦吉尔大学健康中心研究所（Research Institute of the McGill University Health Centre）的神经生理学家埃里克·特鲁德尔（Eric Trudel）和查尔斯·布尔克（Charles Bourque）提出了一种机制，通过这种机制身体的昼夜系统（circadiansystem），或者说内部生物钟能够控制水分的调节。通过允许感知水含量的细胞去激活能够释放抗利尿激素（vasopressin）的细胞，抗利尿激素是一种指示身体进行储水的荷尔蒙，昼夜系统就能使身体在睡觉时储存水分（而不觉口渴）。

　　“多年来我们已经知道，当你在睡觉时抗利尿激素的水平会升高。但是没有人知道这是如何发生的。而这个研究团队提出了一种关于其如何发生的非常具体的生理机制，”神经科学家克里斯托弗·科尔韦尔（Christopher Colwell）说，他在加州大学洛杉矶分校（UCLA）大卫·格芬医学院（DavidGeffen School of Medicine）从事睡眠与昼夜节律的研究。

　　身体主要通过平衡因口渴而饮水与因产生尿液而排水来调节水含量。人们在睡觉时不会喝水，所以身体必须使水分流失最小化来保持充足的水分。科学家知道低含水量会激活一组被称为渗透感觉（osmosensory）神经元的细胞，它们能指示另一组神经元以释放抗利尿激素到血液中。在睡眠期间抗利尿激素水平增加；同时，生物钟神经元变得更加安静。

　　特鲁德尔和布尔克检验了这个想法，即更低的生物钟神经元活动可能允许渗透感觉神经元更容易激活抗利尿激素释放神经元，这将意味着在睡眠期间人体会保存更多的水并产生更少的尿液。

　　为了检验这个想法，他们将小白鼠的大脑分离成薄片，这些薄片仍然包含未受损伤的完整的感觉神经元、抗利尿激素释放神经元和生物钟神经元。即使当这些薄片从大脑里取出后，生物钟神经元仍然能够继续记录时间。

　　然后这两名研究者对感觉神经元施加刺激，并记录抗利尿激素释放神经元中的任何电活动来检测这两组细胞间的沟通。此后，研究者转而考察使用这种方法后生物钟神经元的效应。当他们未对处于昼夜周期中“睡眠”时期的生物钟细胞进行激活时，感觉细胞更容易与抗利尿激素释放细胞沟通。相反，当他们激活（睡眠时期的）生物钟细胞时，这种沟通明显下降。

　　这些结果指出了生物钟细胞作为身体水分控制开关的功能。当它们的活动很强时，它们阻止感觉细胞指示分泌细胞（secretory cell）去释放抗利尿激素。相反，当生物钟细胞激活水平比较弱时，感觉细胞能够更容易地指示分泌细胞释放抗利尿激素，以保证身体在睡眠时保持足够的蓄水量。

　　科尔韦尔指出这项研究是在小白鼠上进行的，而老鼠恰恰是夜间活动动物。虽然老鼠和人类体内的抗利尿激素释放周期和生物钟神经元活动都很相似，但是动物夜间睡眠期间是否都发生了相同的机制这个问题仍需回答。

　　“我们只是显示了这一个生物反馈回路的结论，但是可能生物钟神经元也以同样的方式调节其他的反馈回路，这仍需继续研究，”布尔克说。他预测未来的研究将可能揭示是否相同的机制也调节了饥饿、嗜睡以及与昼夜节律相关的其他生理问题。

1 hormone   

n.荷尔蒙，激素，内分泌

参考例句：

• Hormone implants are used as growth boosters.激素植入物被用作生长辅助剂。
• This hormone interacts closely with other hormones in the body.这种荷尔蒙与体內其他荷尔蒙紧密地相互作用。

2 dehydration   

n.脱水，干燥

参考例句：

• He died from severe dehydration.他死于严重脱水。
• The eyes are often retracted from dehydration.眼睛常因脱水而凹陷。

3 mechanism   

n.机械装置；机构，结构

参考例句：

• The bones and muscles are parts of the mechanism of the body.骨骼和肌肉是人体的组成部件。
• The mechanism of the machine is very complicated.这台机器的结构是非常复杂的。

4 activate   

vt.使活动起来，使开始起作用

参考例句：

• We must activate the youth to study.我们要激励青年去学习。
• These push buttons can activate the elevator.这些按钮能启动电梯。

5 physiological   

adj.生理学的，生理学上的

参考例句：

• He bought a physiological book.他买了一本生理学方面的书。
• Every individual has a physiological requirement for each nutrient.每个人对每种营养成分都有一种生理上的需要。

6 intake   

n.吸入，纳入；进气口，入口

参考例句：

• Reduce your salt intake.减少盐的摄入量。
• There was a horrified intake of breath from every child.所有的孩子都害怕地倒抽了一口凉气。

7 sufficiently   

adv.足够地，充分地

参考例句：

• It turned out he had not insured the house sufficiently.原来他没有给房屋投足保险。
• The new policy was sufficiently elastic to accommodate both views.新政策充分灵活地适用两种观点。

8 sensory   

adj.知觉的，感觉的，知觉器官的

参考例句：

• Human powers of sensory discrimination are limited.人类感官分辨能力有限。
• The sensory system may undergo long-term adaptation in alien environments.感觉系统对陌生的环境可能经过长时期才能适应。

9 retention   

n.保留，保持，保持力，记忆力

参考例句：

• They advocate the retention of our nuclear power plants.他们主张保留我们的核电厂。
• His retention of energy at this hour is really surprising.人们惊叹他在这个时候还能保持如此旺盛的精力。

10 isolated   

adj.与世隔绝的

参考例句：

• His bad behaviour was just an isolated incident. 他的不良行为只是个别事件。
• Patients with the disease should be isolated. 这种病的患者应予以隔离。

11 stimulated   

a.刺激的

参考例句：

• The exhibition has stimulated interest in her work. 展览增进了人们对她作品的兴趣。
• The award has stimulated her into working still harder. 奖金促使她更加努力地工作。

12 activated   

adj. 激活的 动词activate的过去式和过去分词

参考例句：

• The canister is filled with activated charcoal.蒸气回收罐中充满了活性炭。

13 secretory   

adj.分泌的，能分泌的，促分泌的n.分泌腺，分泌器官

参考例句：

• Secretory granules are apparently built up from fusion of smaller vesicles. 分泌颗粒显然是从较小的小泡融合而成的。 来自辞典例句
• The secretory system of the respiratory tract is probably the most appreciated. 呼吸道的分泌系统可能是最先被意识到的。 来自辞典例句

14 remains   

n.剩余物，残留物；遗体，遗迹

参考例句：

• He ate the remains of food hungrily.他狼吞虎咽地吃剩余的食物。
• The remains of the meal were fed to the dog.残羹剩饭喂狗了。

15 physiology   

n.生理学，生理机能

参考例句：

• He bought a book about physiology.他买了一本生理学方面的书。
• He was awarded the Nobel Prize for achievements in physiology.他因生理学方面的建树而被授予诺贝尔奖。