During development of the taste system in mammals, there is a striking increase in neural responsiveness to NaCl compared to other salts. Developmental differences in salt taste responses are accompanied by neural rearrangements that alter number of fungiform papillae and taste buds in receptive fields of first and second order taste neurons on the anterior tongue. Continuing neurophysiological and neuroanatomical studies of the development of salt taste sensation in fetal and postnatal sheep are proposed, to test ideas related to formation of receptive fields and related characteristics of taste neurons. To learn whether there is a developmental period of hyperinnervation and subsequent fiber elimination in fungiform papillae, with associated alterations in taste bud size and number, the fungiform papillae and taste buds from sheep in five age groups will be examined, after ipsilateral transection of the lingual branch of the trigeminal nerve. Numbers of taste buds and nerve fibers within papillae will be determined. Taste bud morphology also will be studied. In the nucleus of the solitary tract (NST) in the medulla, there is convergence of chorda tympani afferents onto second order cells in fetal and postnatal sheep, so receptive fields can be very large, and the degree of convergence increases developmentally. However, even with extensive convergence, the relation between receptive field size and salt taste responses is maintained centrally. Neurons with different response and receptive field properties might be clustered in the NST in a functional and/or morphological microorganization. To test these ideas, neurophysiological responses will be recorded from clusters of cells in NST and the morphology of neurons will be determined from reconstructions of cells after HRP microinjections. Receptive field organization on the posterior tongue is probably very different from that on anterior tongue because large numbers of taste buds are located in relatively few circumvallate papillae. To begin to characterize receptive fields on posterior tongue, the number, location and salt taste responses of circumvallate papillae and taste buds in receptive fields of single glossopharyngeal nerve fibers and NST cells will be determined. These experiments on taste receptive field and neuron characteristics will contribute to understanding salt taste sensation, and how salt taste develops and alters as a function of age. Knowledge of developmental differences in salt taste sensation contributes to understanding changing patterns of salt preference and intake throughout the life cycle.
