DEVELOPMENT OF TROPHIC INTERACTIONS IN THE VERTEBRATE PERIPHERAL NERVOUS-SYSTEM

During embryogenesis, the neurons of vertebrate sympathetic and sensor y ganglia become dependent on neurotrophic factors, derived from their targets, for survival and maintenance of differentiated functions. Ma ny of these interactions are mediated by the neurotrophins NGF, BDNF, and NT3 and the receptor tyrosine kinases encoded by genes of the trk family. Both sympathetic and sensory neurons undergo developmental cha nges in their responsiveness to NGF, the first neurotrophin to be iden tified and characterized. Subpopulations of sensory neurons do not req uire NGF for survival, but respond instead to BDNF or NT3 with enhance d survival. In addition to their classic effects on neuron survival, n eurotrophins influence the differentiation and proliferation of neural crest-derived neuronal precursors. In both sympathetic and sensory sy stems, production of neurotrophins by target cells and expression of n eurotrophin receptors by neurons are correlated temporally and spatial ly with innervation patterns. In vitro, embryonic sympathetic neurons require exposure to environmental cues, such as basic FGF and retinoic acid to acquire neurotrophin-responsiveness; in contrast, embryonic s ensory neurons acquire neurotrophin-responsiveness on schedule in the absence of these molecules.