The sense of taste plays a pivotal role in the selection of potential food items or rejection of potential toxins. Thus appetitive and aversive stimuli must be processed in different ways or in separate neural networks in order to produce different responses. The proposed experiments are designed to delineate the neural networks and transmitters used in dealing with appetitive and aversive stimuli. The primary sensory nucleus for taste in mammals, the nucleus of the solitary tract, is difficult to study because of its compact, complex nature and relatively undifferentiated organization. Accordingly, the proposed work centers on the vagal lobe, a distinctly laminated and highly organized primary gustatory nucleus in a non-mammalian vertebrate. The vagal lobe is organized incortex-like fashion complete with discrete layers,and with columns of incoming afferents. The vagal lobe not only carries out the initial sensory processing of gustatory inputs, but also contains motoneurons equivalent to the nucleus ambiguus that drive the gustatory-related feeding behaviors. Initial experiments will test the animal's behavioral reaction to various amino acids or quinine in order to establish thresholds and classes of stimuli. Then, detailed electrophysiological studies will be carried out in order to determine whether stimulus quality is related to either layer or functional column within the lobe. Output systems of the lobe will be studied by use of intracellular recording and dye-filling of different motoneuron systems. The neurotransmitters and receptors utilized by the primary gustatory afferents and interneuron systems in the lobe will be studied by means of in vitro electrophysiology, immunocytochemistry, in vitro physiology and pharmacology, and ligand binding methods.In particular, the role of acetylcholine, excitatory amino acids and ATP will be tested. Finally, the possibility that cholecystokinin (CCK), a peptide implicated in regulation of feeding behavior and present in the primary gustatory nucleus, plays a role in modulation of gustatory responses will be studied by application of the peptide in the in vitro recording paradigm. Collectively, these studies will reveal the basic circuitry and neurotransmitters involved in gustatory reflex systems controlling essential physiological functions such as swallowing and choking.
