Is the taste system composed of discrete information channels each served by independent subsets of cells, or do all taste neurons contribute to every taste experience? This question is fundamental to an understanding of taste coding and consequently an enormous research effort has been expended in the attempt to answer it- even so, a definitive answer has proven elusive since most data can be accommodated by either theoretical perspective. Recently, however, results from experiments that induced changes in the animals' physiological state or produced profound and selective receptor blockade by application of specific inhibitors have tipped the balance in favor of the discrete channel hypothesis. These data demonstrate that sodium and lithium salts are coded within a specific gustatory channel, established at the periphery and maintained throughout the central nervous system. The goal of the proposed experiments is to extend this hypothesis, which is most secure for sodium-lithium salts, to embrace a sugar channel as well. I will use a sweet(1) taste inhibitor, ziziphin, to ascertain the existence of a specific gustatory channel that is responsible for coding sweet stimuli. A behavioral study (Experiment 1) will be conducted to confirm the effectiveness and determine the selectivity of ziziphin in blocking the perception of sweet stimuli by Wistar rats. In Experiment 2 electrophysiological recordings will be taken from single neurons in the nucleus tractus solitarius (NTS) prior to and following oral application of ziziphin. These data will demonstrate whether the NTS contains a subset of sugar-sensitive cells that are differentially susceptible to ziziphin blockade. This will be followed by intracellular recording at the receptor level (Experiment 3) so that inferences may be drawn regarding the nature of the projections NTS cells receive from the periphery. Taken together, data obtained from experiments 2 and 3 will define the sweet channel through second order taste neurons. The functional organization of gustation--whether an integrated system or a collection of autonomous units subserving each of the basic tastes- remains in question. The demonstration of an independent coding channel for sweet stimuli, when added to the already convincing evidence for the existence a salt channel, would nearly seal the case for the latter view. 1The term """"""""sweet, when used in this proposal, refers to stimuli with a variety of chemical structures that human subjects report as having a predominantly sweet taste and for which rats express a preference.
