Finding the cells that put our brain to sleep
Finding the cells that put our brain to sleep
寻找让大脑进入睡眠状态的细胞
For a long time, sleep research has treated the cerebral cortex as a passive follower reacting to signals from the deep brain. “Usually, sleep is associated with being controlled by subcortical regions,” said Geoffrey Terral, a neuroscientist at the Albert Einstein College of Medicine in New York. The cortex is where the slow rhythms of deep sleep can be seen, but researchers assumed the signals that triggered them originated elsewhere. In a recent Nature study, Terral and Renata Batista-Brito, who runs the lab, report a population of cortical cells that challenges that assumption. These cortical cells make up only around one percent of the cortex’s inhibitory neurons, and switching them on in a mouse puts the animal to sleep. “What our work shows is that the cortex can not only see this rhythm but also initiate it by itself, and this is sufficient to promote sleep,” Terral said.
长期以来，睡眠研究一直将大脑皮层视为被动跟随者，认为它只是对来自深层大脑的信号做出反应。纽约阿尔伯特·爱因斯坦医学院的神经科学家杰弗里·特拉尔（Geoffrey Terral）表示：“通常，睡眠被认为是由皮层下区域控制的。”虽然深层睡眠的慢节律是在皮层中观察到的，但研究人员此前认为触发这些节律的信号源于其他地方。在最近发表于《自然》（Nature）的一项研究中，特拉尔和该实验室负责人雷娜塔·巴蒂斯塔-布里托（Renata Batista-Brito）报告了一群皮层细胞，挑战了这一假设。这些皮层细胞仅占皮层抑制性神经元的百分之一左右，而在小鼠体内激活它们会使动物进入睡眠状态。特拉尔说：“我们的研究表明，皮层不仅能感知这种节律，还能自行启动它，这足以促进睡眠。”
The 1 percent
那 1% 的细胞
The cells are called Sst-Chodl neurons, after two genes that are active in them. Long-range inhibitory neurons marked by the activity of these genes have been known for some time, based on studies in monkeys, but researchers couldn’t figure out how to manipulate them using these genes. Aiming at either one of these genes captures a huge, heterogeneous family of different neurons. “A single gene is not able to target these cells,” Batista-Brito said. Inhibitory neurons account for about a fifth of all cortical neurons, so Sst-Chodl cells amount to roughly one neuron in a thousand in the cortex. “If you’re not really specific, the contaminants are going to be much more dominant than the specific cells,” Batista-Brito said.
这些细胞被称为 Sst-Chodl 神经元，得名于在其中活跃的两个基因。基于对猴子的研究，人们早已知道以这些基因活动为标志的长程抑制性神经元，但研究人员一直无法弄清楚如何利用这些基因来操纵它们。针对这两个基因中的任何一个，都会捕获一个庞大且异质的神经元家族。巴蒂斯塔-布里托说：“单一基因无法精准定位这些细胞。”抑制性神经元约占所有皮层神经元的五分之一，因此 Sst-Chodl 细胞在大脑皮层中大约只占千分之一。巴蒂斯塔-布里托补充道：“如果你不够精确，杂质细胞的影响力将远超目标细胞。”
Building a strategy that labels a cell only when both genes are on took her team years. “Going after these cells was really a high-risk project because the likelihood of seeing anything with 0.1 percent of neurons in the cortex is really low,” Batista-Brito said. “I wrote a bunch of grants on these projects that were always rejected because it was too high-risk.” Her first look at the anatomy of Sst-Chodl neurons proved the risk worth taking. “At first, I saw two or three cell bodies in the whole brain,” Batista-Brito told Ars. “Despite that, there was massive, massive arborization all over the visual cortex, like I never saw with any other neuron.”
她的团队花费了数年时间才建立起一种仅在两个基因同时开启时才标记细胞的策略。巴蒂斯塔-布里托说：“研究这些细胞是一个高风险项目，因为在皮层中仅占 0.1% 的神经元，观察到任何现象的可能性都非常低。我为这些项目写过很多申请书，但总是被拒绝，因为风险太高了。”她第一次观察 Sst-Chodl 神经元的解剖结构时，证明了这种冒险是值得的。巴蒂斯塔-布里托告诉《Ars Technica》：“起初，我在整个大脑中只看到了两三个细胞体。尽管如此，视觉皮层中却布满了极其庞大的树突分支，这是我在其他任何神经元中从未见过的。”
That “massive arborization” is a tree-like branching that connects neurons to other parts of the brain, making them part of a wider communication network. It was also very unusual, as inhibitory neurons are almost always local. They receive inputs from different, sometimes distant regions in the brain and exert control over their immediate surroundings—their own small patch. “Sst-Chodl neurons are kind of the opposite,” Batista-Brito said. “They are receiving inputs that are quite precise, but then they are broadcasting that information everywhere.” A single Sst-Chodl cell branches out to the entire visual cortex, then sends axons to the areas of the brain responsible for touch, hearing, spatial memory, navigation, and voluntary movements.
这种“庞大的树突分支”是一种树状结构，将神经元与其他大脑区域连接起来，使它们成为更广泛通信网络的一部分。这也是非常罕见的，因为抑制性神经元几乎总是局部的。它们接收来自大脑不同（有时是遥远）区域的输入，并对其直接周围环境——即它们所在的小区域——施加控制。巴蒂斯塔-布里托说：“Sst-Chodl 神经元恰恰相反。它们接收非常精确的输入，然后将这些信息广播到各处。”单个 Sst-Chodl 细胞的分支覆盖整个视觉皮层，然后将轴突发送到负责触觉、听觉、空间记忆、导航和自主运动的大脑区域。
The quiet hours
静谧时刻
To see when the cells fire, the team imaged them while tracking the mice’s pupil size, muscle tone, facial movement, running, and cortical electrical activity at the same time. Ninety-five out of 111 imaged cells lit up during slow-wave sleep and quiet, motionless wakefulness, and fell silent during running and REM sleep. During slow-wave sleep, the cortex alternates between UP states of vigorous firing and DOWN states of near-silence; the Sst-Chodl neurons broke this pattern. “Their activity is even higher at the termination of the UP state,” Terral said. “Once all the other neurons start to decrease their activity, those neurons start to increase it even more.”
为了观察这些细胞何时放电，研究团队在对它们进行成像的同时，追踪了小鼠的瞳孔大小、肌肉张力、面部运动、奔跑状态以及皮层电活动。在 111 个被成像的细胞中，有 95 个在慢波睡眠和安静、静止的清醒状态下被激活，而在奔跑和快速眼动（REM）睡眠期间则保持沉默。在慢波睡眠期间，皮层在剧烈放电的“UP 状态”和近乎静默的“DOWN 状态”之间交替；而 Sst-Chodl 神经元打破了这种模式。特拉尔说：“在 UP 状态结束时，它们的活动甚至更高。当所有其他神经元开始降低活动时，这些神经元反而进一步增加了活动。”
They also skip the rebound that other cells show coming out of a DOWN state. “Those cells behave totally differently to any other cells we can measure around that transition,” Terral said. To really understand what they do, the team made the SSt-Chodl neurons fire on command using optogenetics, which involves inserting the gene for a light-activated ion channel that can stimulate the nerve cells to fire.
它们还跳过了其他细胞在脱离 DOWN 状态时表现出的反弹现象。特拉尔说：“在那种转换过程中，这些细胞的表现与我们能测量的任何其他细胞完全不同。”为了真正了解它们的功能，研究团队利用光遗传学技术，通过植入一种光激活离子通道基因，实现了对 Sst-Chodl 神经元的按需放电刺激。
The long reach
远距离影响
Optogenetic stimulation of the Sst-Chodl cells in the visual cortex drove delta power—the slowest, highest-amplitude brain waves measured during deep, restorative sleep—up across every layer. It also tightened spike timing and made DOWN states more frequent and longer. But overall firing barely changed. “Those neurons are not changing so much the firing rate,” Terral said. “They change just the coordination—whether the neurons fire together or not.” Normally, driving a population at a given frequency entrains the network at that frequency. Here, it didn’t matter. “If we did a flat stimulus, or delta, or 20 hertz, or 60 hertz, you still induced the same kind of oscillation,” Batista-Brito said. “There’s something about the intrinsic properties of these cells that, once they trigger, they go on this one frequency.” She emphasized that we don’t currently know how they do this.
对视觉皮层中 Sst-Chodl 细胞的光遗传学刺激，驱动了每一层皮层的 delta 波（深层修复性睡眠期间测得的最慢、振幅最高的脑波）增强。它还收紧了尖峰放电的时间，并使 DOWN 状态变得更频繁、更持久。但整体放电率几乎没有变化。特拉尔说：“这些神经元改变的不是放电频率，而是协调性——即神经元是否同步放电。”通常，以特定频率驱动一个神经元群体会使网络同步到该频率。但在本研究中，频率并不重要。巴蒂斯塔-布里托说：“无论是平稳刺激、delta 波、20 赫兹还是 60 赫兹，诱导出的振荡都是一样的。这些细胞的内在特性决定了它们一旦触发，就会以这种特定的频率运行。”她强调，目前我们尚不清楚它们是如何做到这一点的。
The reach of these neurons also proved unusually long. “Their cell bodies were in the visual cortex, but they could regulate the activity of most of the neurons we were recording,” Terral said. Direct inhibitory currents appeared in a third of cells two millimeters out, and stimulating the visual cortex had measurable effects as far as frontal motor areas. In the final step, the team focused on behavioral experiments. They checked if stimulating the Sst-Chodl neurons would put the mice to sleep. It did.
这些神经元的影响范围也被证明异常广泛。特拉尔说：“它们的细胞体位于视觉皮层，但它们能够调节我们记录的大多数神经元的活动。”在两毫米外的三分之一细胞中出现了直接抑制电流，刺激视觉皮层甚至对额叶运动区产生了可测量的影响。最后，研究团队进行了行为实验，验证刺激 Sst-Chodl 神经元是否能让小鼠入睡。结果证实确实如此。
A sleep switch
睡眠开关
Activating the cells across the cortex of freely moving mice increased slow-wave and REM sleep, cut the time taken to fall asleep, and sent the animals into their nests during the day. Then the team tried it during the dark phase, when nocturnal mice are mostly awake.
在自由活动的小鼠皮层中激活这些细胞，增加了慢波睡眠和 REM 睡眠的时间，缩短了入睡时间，并使小鼠在白天回到了巢穴。随后，研究团队在黑暗阶段（夜行性小鼠通常处于清醒状态的时间）进行了同样的实验。