Neurophysiological and morphological studies are planned to learn how developing peripheral and central nervous system taste neurons change functionally and structurally during normal development and how these processes are influenced by early environmental factors. Neurophysiological taste responses from single chorda tympani nerve fibers will be recorded in normal rats and in rats that have an attenuated taste response selectively to sodium salts as a result of being fed a sodium-deficient diet throughout development. In vivo voltage clamp experiments in combination with taste stimulation will be used to examine the response properties and the voltage sensitivity of these taste fibers and to determine the site of response alterations in taste receptor cells due to dietary sodium restriction (Specific Aim 1). Experiments are also planned to further our understanding of the relationship between taste bud size and number of neurons that innervate each taste bud during normal development and in sodium-restricted rats (Specific Aim 2). In addition to the studies that focus on the peripheral gustatory system, two specific aims are devoted to examine the structural and functional plasticity of the developing central gustatory system. Namely, the arborization pattern of single chorda tympani fibers in normal and sodium-restricted rats will be examined morphologically in the nucleus of the solitary tract (NST), the first central gustatory relay (Specific Aim 3), and the functional critical period for dietary effects on taste responses will be identified for neurons in the NST (Specific Aim 4). The proposed studies will provide new information on: 1) the chemical specificities of single chorda tympani fibers in control and sodium restricted rats, and localization and quantification of functional transduction elements for sodium taste, 2) how taste bud size and number of innervating taste fibers match each other during postnatal development and how early NaCl restriction affects such a match, 3) how the arborization and projection patterns of single neurons are affected by early NCI restriction, and 4) what the developmental endpoints are for the critical period in which early NaCl restriction affects central gustatory system function. The findings will provide new information on the development of peripheral and central nervous system taste responses and the neuroanatomical organization of the developing gustatory system. Such information will ultimately be used to better understand the biological basis for the development of taste preferences and aversions, and the role that prenatal diets have on shaping taste system structure and function.
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