Embryonic sensory development: local expression of neurotrophin-3 and target expression of nerve growth factor

Development and maintenance of peripheral sensory and sympathetic neurons are regulated by target-derived neurotrophins, including nerve growth factor (NGF). To determine whether trophins are potentially critical prior to and during target innervation, for neuronal survival or axon 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, respectively. Trks A and C were detected in dorsal root sensory ganglia (DRG) on 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. Somitic expression 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 actions on sensory neurons. NT-3 was also expressed in the ventral spinal cord at low levels on E13.5. We examined the trigeminal ganglion to determine whether cranial sensory neurons are similarly regulated. Trk A was detected in the trigeminal ganglion, while NGF was expressed in the central myelencephalon target, paralleling observations in the DRG and spinal cord. However, NT-3 and trk C were undetectable, in contrast to DRG, suggesting that the environment or different neural crest lineages govern expression of different trophins and trks. Apparently, multiple trophins regulate sensory neuron development through local as well as transient target mechanisms prior to innervation of definitive targets.