Activity-dependent mechanisms of visual system development

The brain consists of billions of neurons that communicate with each other at specialized sites of contact called synapses. What developmental mechanisms guide the precise formation of such vast numbers of synaptic connections? This question is at the heart of understanding brain development, but it also has implications for understanding the processes of learning and memory, and the underlying basis of neurological diseases such as epilepsy, schizophrenia, and autism. Unlike a computer, which must be fully assembled before it can function, the brain begins to function as it develops. This early activity, which is modulated by experience and the surrounding environment, is utilised by the developing brain to shape and refine synaptic connections. From a developmental point of view, this provides a mechanism to adapt the developing brain to a diverse and changing environment. The purpose of this project is to investigate how activity influences the behaviour of developing neurons to bring about changes in synaptic connectivity. We will use precise manipulations of activity in neurons within the intact developing brain of zebrafish larvae. Because zebrafish larvae are translucent we are able to use time-lapse imaging to see how such manipulations of activity influence the behaviour of neurons in real-time. By using these approaches we will gain detailed and novel insights into the precise roles of activity in shaping the developing nervous system. Such insights may prove to be invaluable for understanding the causes and progression of neurological diseases.

Neuronal activity can modulate patterns of synaptic connectivity in the developing brain. The aims of this proposal are to examine the precise mechanisms by which such activity refines neural connections using the retinotectal system of zebrafish as a model system. For this purpose, targeted expression of neurotoxins will be used to suppress synaptic activity in single retinal ganglion cells (RGCs) in vivo. Time-lapse imaging of silenced RGC axons expressing a fluorescently tagged presynaptic marker protein will be used to examine the consequences of suppressing synaptic transmission on dynamic aspects of axonal arbour growth and synaptogenesis. To test whether differences in synaptic activity between axons modulates connectivity we will compare time-lapse data obtained from suppressing activity in a single RGC to data obtained from suppressing activity in all RGCs. As a complementary approach to examining the role of relative activity levels in shaping retinotectal connectivity we will generate a relative increase in synaptic activity of a single axon relative to its neighbours. This will be achieved by expressing a toxin-insensitive mutant protein in single RGCs in a background of RGCs silenced by neurotoxin expression. By silencing synaptic activity specifically in RGCs we will also address the role of sensory input in regulating the growth and branching of tectal cell dendrites- the targets of RGC axons. Time-lapse imaging of single tectal cells will be used to provide a detailed description of how synaptic input modulates the dynamics of dendrite growth and synaptogenesis. Lastly, we will examine specifically the role of spike activity in regulating the structural plasticity of developing tectal cell dendrites. This will be achieved through expression of the light-activated channel (channelrhodopsin-2) in single tectal neurons that are deprived of synaptic input from RGCs. It is hoped that by combining time-lapse microscopy with precise and specific manipulations of neural activity we will gain novel insight into activity-dependent mechanism of neural development.