GUSTATORY NST--PATHWAYS, SYNAPSES AND NEUROCHEMICALS

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Studies of the cell types of the rostral nucleus of the solitary tract (NST), their connections and synaptic relationships, are essential for understanding the basic organization of the mammalian gustatory system. A major goal of such studies is to develop a writing diagram for the gustatory NST in which connections between different components are identified at the synaptic level on the basis of their morphologies, their cells of origin and their neurochemical or neurotransmitter characteristics. Present information about the organization of synaptic circuits in the rostral NST is limited. The proposed research will provide detailed information about the morphological and neurochemical organization of the NST in light of recent findings on its neuronal architecture and synaptic organization in the hamster. The synaptic interactions between afferent axons, projection neurons and Gamma aminobutyric acid-containing (GABAergic) interneurons will be characterized by electron microscopy combined with axonal and cellular labelling methods and immunocytochemistry. The approach involves a series of labelling studies focused on the rostral central subdivision of the NST. This region receives the densest input from gustatory primary afferent axons and contains most of parabrachial (PBN)-projection cells an many GABAergic cells. Axon terminals will be labelled with horseradish peroxidase (HRP) or cholera toxin (Ct), for neurochemical content (e.g. GABA), or identified using anterograde degeneration. In some material a second, contrasting label will be used to demonstrate cell types on the basis of connections (HRP or CT) or chemical characteristics (e.g. GABA). These studies will determine whether taste afferent axons synapse with PBN-projection cells, with intramedullary projection cells, or with GABAergic interneurons. Convergence of inputs from different afferent axons (e.g. the chorda tympani and the glossopharyngeal nerve) on identified cell types will be evaluated. The fine structure and sites of synapsis of axons descending from the forebrain and axons characterized immunocytochemically (e.g. for GABA) will also be determined. Data from these studies should allow the formulation of hypotheses about how the circuitry of the solitary nucleus processes and disseminates gustatory information. Knowledge of the microcircuitry of the gustatory system is required for more complete understanding of taste disorders and their progress.