Cellular Mechanisms Underlying Retinal Wave Generation

The intricate patterning and complex circuitry of the nervous system beg the question of how such precise and ordered connections are formed during development. Genetically encoded information certainly instructs coarse projections of neuronal processes, and it has long been appreciated that sensory experience can fine tune connections. However, many functional neuronal circuits are present prior to birth and therefore must be organized in the absence of sensory experience. What instructs the formation of these circuits?Spontaneous activity is prevalent in the developing nervous system and may play a role in organizing functional circuits. Spontaneous activity has been most extensively studied in the developing retina, where bursts of action potentials propagate among neighboring ganglion cells in a wave like fashion. These `retinal waves' exist for an extended period of time prior to vision during which connections between retinal projection neurons and target regions of the brain are being refined. It is thought that information relevant for the refinement of these connections is provided in the spatial and temporal patterns of retinal waves.In my thesis work, I set out to determine how the developing retina generates the distinct spatial and temporal patterns of retinal waves. A population of interneurons called starburst amacrine cells generates retinal waves during the first week after birth in mice. These starburst cells release acetylcholine onto neighboring starburst cells and retinal ganglion cells. How does this excitatory network generate precise patterns of activity?Using a combination of calcium imaging, electrophysiology, and a cell-based sensor of acetylcholine, I characterized the features of the developing starburst cell network that give rise to the initiation, propagation, and termination of retinal waves. I incorporated these features of the starburst network into a computational model to explain how the spatial and temporal features of waves rely upon the properties of the underlying network. Finally, I described the molecular mechanisms of a key feature in starburst cells, a minute long slow afterhyperpolarization, that sets the frequency and spatial coverage of waves. These findings provide a detailed understanding of how spatial and temporal patterns of spontaneous activity are generated in the retina, and provide a basis for further experiments to determine the relevant information within these patterns that instruct the wiring of visual circuits.Last, I provide evidence that the slow afterhyperpolarizartions in starburst amacrine cells are mediated by two-pore potassium channels, which are traditionally thought to be leak channels. Our findings suggest that slow variations of second messengers, including cAMP and IP3, are read out by two-pore channels to alter neuronal excitability. These findings would have relevance to similar conductances found throughout the nervous system