Interactions among neurons and glial cells help to shape functional assemblies of neurons in the developing nervous system. We are studying neuron-neuron and neuron-glia interactions that lead to the formation of neuropil compartments, the olfactory glomeruli, in a tractable model system, the antennal lobe of the brain of the moth Manduca sexta. In previous years, we have shown that afferent axons induce the formation of glomerular branching patterns in neurons of the antennal lobe, and that this inductive influence requires the presence of glial cells. We hypothesized that olfactory receptor axons exert at least part of their influence by using the glial cells as intermediaries - in other words, that the receptor axons lay down a template, around which the glial cells build a scaffold, and subsequent neurite growth by antennal-lobe neurons is constrained to the template by the glial scaffold. We recently have discovered that the protoglomerular template laid down by the axons dissolves almost immediately in glia-deficient antennal lobes. Thus, we now plan to ask: (Specific Aim I) How do olfactory receptor axons establish the pattern of protoglomeruli? when and in what ways does this pattern deviate in the absence of sufficient numbers of glial cells? We also have found recently that members of one class of neuron, the uniglomerular projection neurons of the antennal lobe (similar in some ways to the mitral/tufted cells of the vertebrate olfactory bulb), interact much earlier with the incoming receptor axons than do other classes of neurons that we have studied in the past. Moreover, these projection neurons form a single glomerulus-like structure at the point where glomerulus formation ordinarily is initiated, even if sensory axons are prevented from growing in and inducing glomeruli. To elucidate the level of autonomy of the sensory axons and/or antennal-lobe glial cells in glomerulus development, we will ask: (Specific Aim 2) what roles does this class of CNS neuron play in glomerulus development? In addition, we will use heterotopic grafting tests to ask: (Specific Aim 3) How specific are the interactions between receptor owns and CNS neuronal and glial cells in developing glomeruli? Using the experimentally favorable insect nervous system, we will be able to do experiments not as readily performed in vertebrates. Answers to the questions we have posed should provide important insights into the cellular and molecular bases for the roles of afferent axons and glial cells in shaping subunits or compartments within the developing brain.
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