In contrast to the abundance of information available regarding the development and organization of the visual, somatosensory and auditory systems, we know relatively little about the role of the central nervous system in gustation. The experiments described in this proposal are designed to provide data that will significantly improve our understanding of gustatory function. The proposed studies are based on the following three hypotheses: 1) Morphologically defined cell types of the rat nucleus of the solitary tract (NST) differ in their response properties; 2) The dendritic synaptic architecture of NST neurons reflects the response properties of these cells; 3) Changes in gustatory sensibility during development are correlated with specific morphological changes in the NST (this hypothesis will be addressed upon the completion of studies in adult animals). We will address the first hypothesis by labeling individual physiologically-characterized NST gustatory neurons in the adult rat with the intracellular injection of horseradish peroxidase (HRP). Using computerized three dimensional reconstruction techniques, we will correlate each neuron's morphological traits with its physiological response properties. The second hypothesis will be addressed by determining the density of presynaptic terminals on the dendrites of adult gustatory neurons. Individual NST neurons will be labeled with HRP and processed for electron microscopy. Segments of the first-, second-, and third-order dendrites will be reconstructed in three dimensions and the type, location and number of synapses will be ascertained. We will begin to test the third hypothesis by employing the intracellular labeling technique in postnatal animals aged 10 days (permitting optimal contrast between the immature and adult NST). At the conclusion of the proposed funding period, we expect to have described the morphological characteristics of physiologically-defined cell types in the NST. In addition, we will have characterized the dendritic synaptology of these cells. Finally, we will have begun to determine the relationship between changes in the morphology and physiology of NST neurons in developing animals.
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