Retinal ganglion cells: when and how do they contribute to the design and function of the developing visual system?

理解神经功能是如何产生的无疑是当前神经科学中最具挑战性的问题之一。破译神经功能的一个有效方法是研究生命早期神经连接是如何形成的。大脑连接最初是粗糙的，由遗传密码和分子信号决定。后来，在电活动的影响下，连接变得精细，电活动在发育中的中枢神经系统（CNS）中无处不在。它在怀孕期间就开始了，远在感官体验成为可能之前。它是自发驱动的，在发育中的中枢神经系统的不同部位有不同的形式。人们普遍认为，在早期自发活动中编码的精确的空间和时间模式对于指导神经连接的完善是极其重要的。我们感兴趣的是了解早期的神经活动如何指导视觉系统的成熟。在有限的围产期，自发活动的波扫过视网膜神经节细胞（RGCs）层，视网膜的输出通道。许多研究表明，视网膜波指导视网膜功能的发展和视网膜中央投射的组织，主要是丘脑的背外侧膝状核（dLGN）和上丘（SC）。在未成熟的大脑中，来自双眼的RGC投射是混杂的和弥漫性的。随着发育，它们分离成特定于眼睛的区域，并被组织成精确的地形图，相邻的rgc投射到dLGN和SC的相邻细胞上，从而保存了我们视觉世界的连贯地图。视网膜波在眼特异性分离和地图细化过程中起着至关重要的作用。然而，以往的研究没有考虑到波浪的动力特性随着发展而发生深刻的变化。我们最近在小鼠（我们的实验模型）中表明，波最初并不频繁（因此不可能同时发生在两只眼睛中），它们非常大，并且它们只在其轨迹上招募一小部分RGCs（许多RGCs保持沉默）。后来，它们变得更频繁，空间更小，速度更慢，密度更大，招募大多数邻近的rgc。最后，在视觉体验开始前不久，波变得几乎静止，变成覆盖视网膜的小而频繁的热点。它们在睁开眼睛后立即完全消失。我们假设早期的波对于眼睛特定的隔离（在眼睛之间产生竞争）是重要的，后期的波对于地形图的改进是有用的（当波变得更密集时）。在最近的一段时间里，波对于视网膜网络的成熟是很重要的，视网膜网络是在rgc中产生光反应的基础上形成的，在这个时候，投射大多是成熟的。我们将使用药物遗传学来验证这一假设，这是一种强大的方法，可以让我们有选择地、可逆地沉默rgc。为此，我们将生成一个小鼠系，其中大多数RGCs表达由设计药物特异性激活的设计受体（DREADDs）。当暴露于设计药物氯氮平n-氧化物时，表达dreadd的RGCs被沉默，因此它们不参与波。使用这种方法，我们将在不同的围产期长期沉默rgc，当波传递不同的空间和时间信息时。利用电生理学、神经解剖学和行为学，我们将研究产后不同时期的沉默波如何影响成年人的视觉功能，重点是：1。视网膜连接性和rgc对光的反应。SC3中的映射细化。dLGN和SC4的眼特异性分离。图像细节辨别（视觉敏锐度）和对比敏感度的行为测量。该项目将阐明视网膜波如何引导视觉系统发展的不同阶段，以及该过程的可塑性。

Understanding how neural function is generated is undoubtedly one of the most challenging current questions in neuroscience. One powerful approach to decipher neural function is to study how neural connections are formed in early life. Brain connectivity is initially coarse, determined by the genetic code and molecular signals. Later on, connections become refined under the influence of electrical activity which is ubiquitous in the developing central nervous system (CNS). It starts during gestation, long before sensory experience is even possible. It is spontaneously driven and takes distinct forms in different parts of the developing CNS. It is widely recognised that the precise spatial and temporal patterns encoded in early spontaneous activity are extremely important for guiding the refinement of neural connections. We are interested in understanding how early neural activity guides the maturation of the visual system. Waves of spontaneous activity sweep across the layer of retinal ganglion cells (RGCs), the output channels of the retina during a limited perinatal period. Many studies have demonstrated that retinal waves guide the development of retinal function and the organisation of retinal central projections, principally the dorsal lateral geniculate nucleus (dLGN) of the thalamus and the superior colliculus (SC). In the immature brain, RGC projections from both eyes are intermingled and diffuse. With development, they segregate into eye-specific areas and become organised into precise topographic maps, with neighbouring RGCs projecting to adjacent cells in the dLGN and SC, resulting in the preservation of a coherent map of our visual world. Retinal waves play a crucial role during the process of eye-specific segregation and map refinement. However, previous studies have not considered that the dynamic properties of the waves profoundly change with development. We have recently shown in mouse (our experimental model) that waves are initially infrequent (hence unlikely to occur simultaneously in both eyes), very large and they recruit only a fraction of RGCs on their trajectory (many RGCs remain silent). Later on, they become more frequent, spatially smaller, slower and much denser, recruiting most neighbouring RGCs. Finally, shortly before the onset of visual experience, waves become almost stationary, turning into small and frequent hotspots that tile the retina. They completely disappear immediately after eye opening. We hypothesise that early waves are important for eye-specific segregation (generating competition between the eyes), later waves are instrumental for the refinement of topographic maps (when waves become denser). In the latest period, waves are important for the maturation of retinal networks underlying the generation of light responses in RGCs, at a time when projections are mostly mature.We will test this hypothesis using pharmacogenetics, a powerful approach allowing us to selectively and reversibly silence RGCs. For this purpose, we will generate a mouse line where most RGCs express Designer Receptors Exclusively Activated by Designer Drugs (DREADDs). When exposed to the designer drug clozapine n-oxide, DREADD-expressing RGCs are silenced, hence they do not participate in waves. Using this approach, we will chronically silence RGCs during distinct perinatal periods, when waves convey different spatial and temporal information. Using electrophysiology, neuroanatomy and behaviour, we will investigate how silencing waves at different postnatal periods affects visual function in adults, focusing on:1. Retinal connectivity and how RGCs respond to light.2. Map refinement in the SC3. Eye-specific segregation in the dLGN and SC4. Behavioural measures of image details discrimination (visual acuity) and sensitivity to contrastThis project will elucidate how retinal waves guide different phases of the development of the visual system and how plastic the process is.