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

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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.