Our long-term goal is to reveal processes responsible for somatotopic pattern formation. Lesion, deprivation and descriptive paradigms have implicated ordered peripheral connections in the development of somatotopy; yet, impulse blockade has no effect on pattern formation. Thus, it is unclear what signalling mechanism is used by the periphery in establishing CNS patterns. We will employ the rodent whisker/barrel model to address the following general hypothesis: Somatotopic patterns result as a consequence of neurotrophically regulated, naturally occurring cell death in the trigeminal (V) ganglion. Preliminary data suggest novel causal relationships between nerve growth factor (NGF) level, V ganglion cell number, and the development of barrels, suggesting that regressive processes produce somatotopic patterns. To assess the validity and implications of these observations, and to begin to explore how neurotrophic factors impact on ganglion cell survival and CNS pattern formation, the following specific hypotheses will be tested in rats: 1: Whisker-related patterns emerge in fetal V brainstem nuclei during a period when large numbers of V ganglion cells die of natural causes. We will use an array of histochemical markers and image analysis to assay the development of whisker-related patterns, as well as stereological methods to determine if ganglion cell number predicts the emergence and relative maturity of somatotopic patterns. 2: V ganglion cells express mRNA message for different neurotrophic receptors during different phases of fetal development. In situ hybridization experiments will be carried out on fetal V ganglia to determine patterns of expression of the protein product of the trk proto-oncogene which has similar properties to the NGF high-affinity receptor, the trk-B gene product that binds brain-derived neurotrophic factor (BDNF), and the trk-C gene product that binds neurotrophin-3 (NT-3). A retrograde tracer will also be used to correlate projection status with receptor gene expression over the period of naturally occurring ganglion cell death and CNS pattern formation. 3: systemic delivery of NGF, BDNF and/or NT-3 will rescue V ganglion cells from naturally occurring cell death. We will determine the ages at which each or all of these neurotrophic factors is most effective in sustaining supernumerary V ganglion cells. A slow-release, chronic delivery procedure will be used for convenient, long-term delivery of NGF, BDNF, and/or NT-3 at varying fetal ages. 4: Central V patterns do not develop when supernumerary V ganglion cells are sustained by neurotrophic augmentation. Histochemical markers, image analysis, and ganglion cell stereology will be used in each animal to reveal relationships between ganglion cell number and whisker-related patterns in the brainstem, thalamus, and barrel cortex. Postnatal cases will also be studied to determine if ganglion cells are permanently rescued and if pattern formation is permanently interrupted. These studied may uncover general principles controlling somatosensory development in humans in light of recent indications that humans have somatotopically parcellated, barrel-like aggregations in V and dorsal column nuclei. Biological actions of neurotrophic factors will also be revealed that may be of clinical value.
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