A large number of neuronal phenotypes are generated during the course of development. An important phenotype of a neuron is the set of connections it has with other neurons and peripheral targets. The formation of these specific connections entails a number of steps that include target selection, stabilization of appropriate connections, and elimination of inappropriate connections. The cellular processes underlying these steps are still poorly understood. In this proposal, the rules governing the formation of specific connections will be examined in a simple in -vitro system consisting of identified neurons isolated from the abdominal ganglion of the marine mollusk Aplysis californica. Isolated Aplysia neurons maintained in cell culture can form chemical synapses. Some cells appear to form connections only with their appropriate target cells while others are more permissive and form connections indiscriminately. Combining electrophysiological, fluorescent and differential interference contrast light microscopic, and electron microscopic techniques, the studies outlined in this proposal will examine the nature of the cell-cell interactions, and the types of intercellular signals and intracellular events underlying the formation of specific chemical connections. These studies will also attempt to support the hypotheses that a) competitive interactions between presynaptic cells for postsynaptic space involve the same mechanisms as target selection and avoidance, and b) the signal for stopping the growth of a presynaptic neuron following contact with a target cell is the boundary between the target cell and the surrounding substrate.
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