Nitric oxide (NO) is a highly diffusible signalling molecule that serves a wide range of physiological functions. During development, biosynthetic enzymes for NO are often expressed in nascent neurons, although little is known of how this molecule regulates in vivo nervous system development. This thesis aims to address this problem by examining the functional roles of NO signalling in the zebrafish embryo, focusing specifically on nitrergic regulation of spinal locomotor network assembly.\ud The first aim of this study is to characterise the spatiotemporal distribution of NO synthase 1 (NOS1), the enzyme responsible for NO biosynthesis in the zebrafish nervous system. NOS1 transcript and protein was observed in discrete regions of the brain as well as a distinct class of spinal interneuron from early stages of embryonic development.\ud The second aim was to examine functional roles of NO signalling during in vivo spinal cord development. Using molecular antisense and pharmacological approaches, NO levels were disrupted during early life and the consequences to spinal circuit maturation assessed. NO was found to specifically regulate the growth of spinal motoneurons that innervate axial trunk muscles. Abrogation of NO signalling dramatically increased the number of motor axon branches formed within the muscle across the first three days of life whilst exogenous elevation of NO levels had the opposite effect.\ud The third aim was to determine downstream signalling pathways underpinning NOs effects. Pharmacological studies revealed that NO regulates motoneuron branching through the cyclic guanosine monophosphate pathway and subsequent analysis revealed that this pathway can modify neuromuscular synapse density, with high NO levels suppressing synaptogenesis and retarding locomotor maturation and low NO levels having the converse effect.\ud In summary, the work presented in this thesis identifies a novel and important role for NO signalling, demonstrating that it functions to sculpt neuromuscular synapse assembly and modify locomotor maturation
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