This research is directed toward understanding the morphological and synapt c organization of the primary gustatory relay nuclues and how it functions in taste. The gustatory system plays a vital role in the health and survival of the organism. It is responsible for evaluating potential food substance for acceptance or rejection, and is also an important afferent component in both complex learned behaviors influencing diet selection and coordinated orofacial reflexes essential for food ingestion. The rostral one-third of the nucleus of the solitary tract is the recipient zone for gustatory afferent information. It is also the point from which the gustatory system diverges into an ascending component, underlying the perceptual and hedonic aspects of taste, and a brainstem component contributing to coordination of ingestive reflexes. Although they underlie different behavioral functions, these two subsystems are not well understood as to their relative sensory functions. The properties of NST organization that generate this subdivisi n are also unknown. The prolonged studies will use morphological and electrophysiological analysis of output neurons in rostrol NST to begin an in-depth investigatio of NST organization. These cells project either rostrally, into an ascendi g subsystem, or caudally, into a brainstem pathway. Morphological experiment are planned, in which dendritic morphology and distribution of output neurons, identified by retrograde transport of fluorescent tracers, is revealed by intracellular injection of Lucifer Yellow. Electrical stimulation of afferent peripheral nerves will be used to compare neuronal responses in terms of the sources of afferent input, relative contributions of sources, and response characteristics. Temporally complex pulse train stimulation will also be used to investigate second order effects, either resulting from the neurons' intrinsic properties or produced via local synaptic circuitry, that modulate postspike responsiveness in these cells. Similar recording experiments will be carried out in an in vitro preparatio of the rostral NST, to establish the correspondence between output neuron function in vivo and in vitro. The brain slice will establish a preparatio in which interrelationships between neuronal cell types in rostral NST can be investigated using intracellular morphological and electrophysiological techniques. These studies involve the first application of several new and power techniques to gustatory physiology. These experiments will define classes of output neurons based on 1) intrins c NST processing, 2) access to afferent input, and 3) morphological features. They will provide information essential for future experiments comparing taste coding in identified subsets of NST neurons, and for defining hypotheses about synaptic interactions among NST neurons for future in vitr work. This research will contribute to understanding of the organization o brainstem and higher functions of the gustatory system, and basic sensory system processes throughout the nervous system.
