The major function of primary sensory neurons, such as those in dorsal root ganglia or trigeminal ganglia, is to convey information to the central nervous system about the location, nature and intensity of peripheral stimuli. Thus, during embryonic development sensory neurons must connect precisely with the appropriate central and peripheral targets. The experiments outlined in this proposal are designed to ascertain how sensory neurons accomplish this task. The overall goals are (1) to determine whether sensory neurons are specified prior to axon outgrowth and target innervation, and (2) to elucidate the nature and specificity of the guidance cues that govern the development of sensory innervation patterns as axons grow to and distribute within peripheral targets. To investigate these problems, surgical manipulations will be made in chick embryos and the resulting innervation patterns of sensory neurons in dorsal root ganglia will he mapped with physiological and anatomical techniques. In addition, the behavior of cutaneous afferents (from trigeminal ganglia) and muscle afferents (from the trigeminal mesencephalic nucleus) dissected from embryos of different ages will be studied in various in vitro assays, including (a) time lapse video microscopy of growth cones interacting with potential targets and axons and (b) different types of adhesion assays. Differences in the response of the two populations of neurons in these assays at stages prior to target innervation would indicate that the neurons are specified prior to axon outgrowth. These in vitro studies will also provide information about the specificity of different guidance cues. Growth cones of trigeminal cutaneous and muscle afferents from embryos of different ages will be stained with a battery of antibodies to adhesion molecules to ascertain whether there are quantitative or qualitative differences in the surfaces of these two populations of neurons. To determine which molecules in or on targets play a role in the development of sensory innervation patterns, sections of skin will be stained with antibodies to both extracellular matrix (ECM) molecules and axons to delineate the spatial relationship between ingrowing axons and these ECM components. Finally, antibody perturbation experiments will be performed both in vivo and in vitro, to determine the importance of particular molecules on growth cones and targets in mediating interactions of cutaneous and muscle afferents with the targets and with other axons, and in regulating the distribution of sensory axons within the target tissue. Together, these experiments are designed to provide an understanding of the mechanisms responsible for the initial development of sensory innervation and the extent to which these mechanisms persist at later stages. This knowledge should aid in designing clinical strategies to promote or enhance the extent or specificity of regeneration of injured or diseased sensory axons.
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