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The calcium-sensing receptor (CaR) is expressed in neurons of the adult central and peripheral nervous systems (PNS). There are currently no data describing the expression or function of the CaR during neuronal development. CaR mRNA expression was detected in sympathetic and sensory mouse ganglia at a number of embryonic (E) and postnatal (P) ages where, in superior cervical ganglia (SCG), CaR mRNA expression peaked at El8. At this age CaR protein was localised to the cell soma of dissociated neurons and throughout the neurite field. When compared to neurons grown in low (0.7mM) extracellular Ca2+ concentration (Ca2+ 0), high (2.3mM) Ca 2+ 0 increased SCG neurite outgrowth within a specific developmental window, E18-P0, correlating with a time when these neurons are exposed to a systemic Ca 0 of 1.7mM in vivo. SCG neurite outgrowth within this window was susceptible to pharmacological modulation of the CaR by calcimimetic and calcilytic compounds, increasing and decreasing neurite growth, respectively. Expression of a dominant negative CaR (R185Q) reduced outgrowth of El8 neurite arbors, whereas overexpression of a wild-type CaR re-introduced Ca 0 sensitivity of neurons outside the developmental window. Consistent with these data, Car' SCG neurons displayed impaired neurite arborisation in vitro and sympathetic target innervation in vivo when compared to Car+/ and Car+/+ neurons. This role for the CaR is not limited to the PNS. Overexpression of the dominant negative CaR in P4 hippocampus reduced neurite outgrowth of CA2 and CA3 pyramidal neurons. Preliminary data also suggest a role for the CaR in neuronal survival, where increasing Ca2+ 0 from 0.7 to 2.3mM increased El8 SCG survival in a dose-dependent manner. Further, high Ca 0 enhances NGF-dependent survival, an effect that can be reduced using the calcilytic compound. These data are the first to show that Ca 0 modulates neurite growth and neuronal survival during development and that these effects of Ca2+ 0 are mediated though the CaR.

Aspects of the development and distribution of the primary retinal afferent projection to the contralateral optic tectum were studied by light microscopy, electron microscopy, and microelectrode recording from adult and tadpole optic nerves and optic tecta of the toad Xenopus laevis (Daudin). The adult tectum was found to be innervated by four populations of optic nerve afferents with different conduction velocities and retinal receptive field properties. Current source density analysis of the postsynaptic potentials recorded extra cellularly from tectal cell populations revealed the spatial distribution of optic nerve afferent synapses. The dendrites of cells that constitute tectal cell layer 8 were found to receive the majority of retinal afferent fibre input. The earliest time at which ganglion cell terminal activity was recorded in the tectum was tadpole Stage 46, at which time the optic nerve was entirely composed of unmyelinated fibres. The excitatory receptive fields of these units were large, and these units were easily habituated. Stage 46 was also the earliest time at which postsynaptic field potentials ('u' waves) could be recorded in the tectum and synapses observed morphologically. Evoked potentials ('m' waves), characteristic of a myelinated fibre innervation of the tectum, were first seen at tadpole Stage 59, as was the appearance and development of optic nerve myelination. No obvious correlations, other than the division of optic nerve axons into my elimated and unmyelinated fibre populations, could be made between optic nerve fibre conduction velocity groups and fibre diameter histograms. However, an obvious correlation between optic nerve conduction velocity groups and postsynaptic field potentials recorded in the tectum was observed. In addition, - a clear correlation was discovered between the distributions of the current sinks of postsynaptic activity and the location of recording sites of ganglion cell axon terminals having known receptive field properties, A gradual emergence of the adult pattern of tectal innervation by optic nerve afferents, during the time at which the tadpole undergoes metamorphosis, was observed.

Neural crest cells are a transient population of pluripotent cells unique to vertebrate embryos. Their importance in vertebrate development is highlighted by the diversity of their final fates, which include cells forming the cranialfacial cartilage, the peripheral nervous system, and melanocytes, amongst others. Neural crest cells maintain pluripotency until after they delaminate from the surrounding epithelium and migrate to their sites of final differentiation. This multi-step process allowing neural crest delamination and migration is known as epithelial-mesenchymal transition (EMT). Key molecular events of EMT include the downregulation of E-cadherin and upregulation of many transcription factors, including Snail and Slug. Snail and Slug are key regulators of EMT, and they are also markers for neural crest identity. This intimate link between neural crest fate and EMT provides a system whereby cell fate specification and EMT can be studied together. Many signalling pathways are known to affect neural crest induction and EMT. However, the molecular details of how these signals mediate this developmental program are poorly understood. I have carried out an overexpression screen in Xenopus laevis embryos to look for factors affecting neural crest induction and identified several novel candidates of interest. Included in the hits identified in the screen are known genes that have already been implicated in neural crest induction, which validates the screen. After some preliminary experiments, I focused my studies on a regulator of gene expression, SNW domain-containing 1 (SNW1). I discovered that SNW1 regulates the BMP gradient in Xenopus embryos responsible for dorsal-ventral patterning. Manipulation of SNW1 protein level in whole embryos alters the BMP gradient so that the graded response responsible for patterning the ectoderm is disrupted, with severe consequences for neural crest induction. Two downstream targets of SNW1 were identified using microarray analysis and these may mediate the effect of SNW1 on the BMP gradient, and they may also have direct effects on neural crest induction. Neural crest cells undergo EMT to migrate to their final sites of differentiation, and require transcription factors of the Snail family to drive this process. This is reminiscent of cases of EMT observed in some metastatic cancer cells. Loss of SNW1 prevents neural crest induction and expression of Snail, hence SNW1 may play an important role in cancer progression as well.

In this thesis I present studies on the structure, function and development of the avian neuronal nucleus for sound localisation. The ability to locate sound sources is an important faculty for both predator and prey alike and the precision of sound localisation ability is known as acuity. Using theoretical methods I have determined that there is a limitation on acuity for sound localisation using interaural time difference (ITD) detection. Acuity limitation is due to the geometry of a sound source and a listener and acuity can not be improved by varying the ITD detection mechanism. This has implications for ITD detection structures. In birds, the nucleus Laminaris (nL) is a population of ITD detecting neurons with an unusual laminar structure. I have investigated nL structural development from a rounded cluster of cells to a one cell thick sheet. The structure and function of cell populations are interlinked and their relationship can be investigated through understanding their development. These studies show that cell death is not a significant contributor to nL structural development and relative cell movement is the dominant mechanism. There is also spatiotemporal patterning of nL structure development which is similar to the patterning found for neuron birth times. Intercellular interactions, such as adhesion, can affect relative cell movements during development. By considering the maximum adhesion configuration for a theoretical population of cells, I have demonstrated that population structure can be affected by individual cell morphology. This may provide a mechanism for nL laminar structure development. The role of adhesion in nL development and cell migration has also been explored using in vivo perturbation and in vitro assays.