The gustatory system is situated at the entrance to the alimentary canal and serves a variety of reflex functions including oropharyngeal control, e.g. swallowing and gagging, as well as regulation of the gastrointestinal (GI) tract. The proposed studies are designed to test the hypothesis that two or more gustatory submodalities exist, each being involved in different reflex systems, and therefore each maintaining different connections within the brainstem. The hypoethesis will be tested by comparing, by anatomical and physiological means, the reflex connections of the primary gustatory nuclei devoted to facial nerve taste, glossopharyngeal nerve taste, or vagal nerve taste. Such studies on the interrelationship of the gustatory and general visceral systems are not feasible in mammals since the primary gustatory and primary general visceral nuclei are not clearly segregated within the medulla. In highly gustatory fish such as catfish and goldfish, however, these studies are practical. In the first phase of this study, anatomical tracing methods will be used to delineate the medullary general visceral sensory and motor nuclei involved in innervation of the GI tract. The three different primary gustatory nuclei, facial lobe, glossopharyngeal lobe, and vagal lobe, then will be injected with anterograde tracers to determine whether direct connections exist from any of the primary gustatory nuclei onto any of the GI tract sensory or motor nuclei found in the first experiments. Extracellular electrophysiological methods will be used to characterize the nature of any gustatory information reaching these general viscerl control nuclei. In the second phase of the project, the previously demonstrated reflex system connecting vagal taste centers to the nuc. ambiguus will be investigated by anatomical and electrophysiological means, including both in vivo and in vitro methods. The final two phases of this study will investigate whether hypothalamic visceromotor control centers are involved in gustatory regulation of general visceral activity. Two specific questions will be addressed: first, whether the gustatory submodalities remain separate at hypothalamic levels, and second, whether gustatory afference reaches areas of the hypothalamus that project onto the visceromotor nuclei defined in the first phase of th experiment. At their completion, these studies should delineate the pathways involved in gustatory modulation of general visceromotor activity.
