EMBRYONIC SENSORY DEVELOPMENT - LOCAL EXPRESSION OF NEUROTROPHIN-3 AND TARGET EXPRESSION OF NERVE GROWTH-FACTOR

Development and maintenance of peripheral sensory and sympathetic neur ons are regulated by target-derived neurotrophins, including nerve gro wth factor (NGF). To determine whether trophins are potentially critic al prior to and during target innervation, for neuronal survival or ax on guidance, in situ hybridization was performed in the rat embryo. We examined the expression of genes encoding NGF, neurotrophin-3 (NT-3), and their putative high-affinity receptors, trk A and trk C, respecti vely. Trks A and C were detected m dorsal root sensory ganglia (DRG) o n embryonic day 12.5 (E12.5), implying early responsiveness to NGF and NT-3. NGF mRNA was expressed in the central spinal cord target and by the peripheral somite, at this early time, which thereby may function as a transient ''guidepost'' target for sensory fibers. Semitic expre ssion was transient and was undetectable by E17.5. NT-3 was expressed in the DRG itself from E13.5 to 17.5, suggesting local transient actio ns on sensory neurons. NT-3 was also expressed in the ventral spinal c ord at low levels on E13.5. We examined the trigeminal ganglion to det ermine whether cranial sensory neurons are similarly regulated. Trk A was detected in the trigeminal ganglion, while NGF was expressed in th e central myelencephalon target, paralleling observations in the DRG a nd spinal cord. However, NT-3 and trk C were undetectable, in contrast to DRG, suggesting that the environment or different neural crest lin eages govern expression of different trophins and trks. Apparently, mu ltiple trophins regulate sensory neuron development through local as w ell as transient target mechanisms prior to innervation of definitive targets. (C) 1994 Wiley-Liss, Inc.