Plasticity of Primary Gustatory Afferents

The first synaptic relay of the gustatory system is located in the brainstem in the nucleus of the solitary tract (NTS). This relay integrates input from three distinct gustatory nerves: the greater superficial petrosal nerve, the chorda tympani nerve, and the glossopharyngeal nerve. However, key information is lacking regarding the anatomical relationships among the terminations of these three nerves. Furthermore, the normal organization of these terminations in the gustatory brainstem is profoundly influenced by dietary sodium restriction during early development. The experiments in this dissertation sought to obtain an accurate characterization of the normal interactions of the three gustatory afferents in the NTS in order to further explore the neural plasticity resulting from early dietary sodium restriction. Chapter II describes a novel, triple-fluorescent confocal microscopy technique implemented to simultaneously visualize the three gustatory terminal fields in the NTS of control and sodium-restricted rats. Under normal developmental conditions, individual gustatory afferents terminated in an ordered yet overlapping array. However, dietary sodium restriction resulted in dramatic and specific alterations of these termination patterns, which increased the extent of convergence of the gustatory terminal fields. Chapter III describes the distinct ultrastructural morphology and synaptic relationships of the axon terminals of the three gustatory afferents in control rats. Chapter IV reveals evidence for plasticity in the synaptic arrangements of all three gustatory afferents as a result of dietary sodium restriction. The results of these three experiments provide new insight on how the circuitry of the gustatory brainstem is wired during distinct phases of development and how they are modified by early dietary III manipulations. These findings provide fundamental information about how taste stimulus information may be functionally processed centrally and how it may be manifested behaviorally. IV DEDICATION To the mentors that have shaped and informed my aspirations, and to the family and friends who have supported my every endeavor, I am most grateful.

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