Experiments detailed in this proposal examine the extent to which central gustatory pathways exhibit plasticity during postnatal development, and continue our long-term focus on the contribution of peripheral gustatory receptors and orochemical transduction to the development of primary gustatory axons in the nucleus of the solitary tract (NST) and parabrachial nucleus (PBN). Information regarding developmental plasticity in central taste pathways is needed to further understand factors that contribute to individual differences in preferences for dietary salts, sugars, and fats. Such information is important because environmental influences on the development of taste-guided behaviors must be taken into consideration when evaluating the etiology of health problems such as feeding disorders, obesity, dental disease, cardiovascular disease and cancer. Series 1 of proposed experiments evaluates the potential for various orochemical stimulants contained in mothers milk to induce normal development of primary gustatory axons in the NST. Normal taste stimulation is prevented during critical periods of development by artificially-rearing (AR) rats with a milk replacement diet delivered via intragastric cannulae. During AR subgroups of animals receive experimental taste stimulation with either NaCl, lactose, or rat milk to determine whether such stimulation is effective in promoting normal anatomical development of the gustatory NST. Anatomical experiments are performed at maturity with highly sensitive fluorescent tract-tracing dyes to study the organization of NST axons in AR animals and normal animals. In Series 2 thermal cautery is induced to circumvallate and/or foliate papilla(e) of the posterior tongue at various postnatal ages to determine whether sensory transduction via posterior tongue receptors is necessary for normal development of primary gustatory axons in intermediate and caudal divisions of the gustatory NST. Although it is known that taste transduction via fungiform papillae of the anterior tongue is required for normal development of the rostral gustatory NST, information regarding similar transduction relations in development of the intermediate NST is lacking. In these experiments fluorescent tracing procedures are used to assess the organization of NST terminal fields following receptor damage. In Series 3, thermal cautery of fungiform receptors of the anterior tongue is induced during critical periods of development, and corresponding effects of damage on growth of efferent projections from the NST to the PBN is evaluated by the use of fluorescent tracing methods. Results of these experiments will compliment existing data to extend our understanding of developmental plasticity of central gustatory pathways.
