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Expression of neurotrophin trk and p75 receptors in quail embryos undergoing gastrulation and neurulation

We have previously demonstrated the presence of mRNA for the full‐length neurotrophin receptors trkA, trkB and trkC in quail embryos from stages 1 through 6 using reverse transcription followed by the polymerase chain reaction (RT‐PCR; Yao et al. [1994] Dev. Biol. 165: 727–730). Furthermore, we showed that mRNA for the neurotrophins brain‐derived neurotrophic factor and neurotrophin‐3 was present from stage 1 onward, while nerve growth factor mRNA began to be expressed at stage 5. In the present study, wholemount in situ hybridization was used to localize full‐length trk mRNA in embryos from stages 3 through 10. Structures expressing trkC mRNA included the primitive streak and Hensen's node, the neural plate or notochord, somites and the rostral neural tube. trkA and trkB mRNA were expressed at much lower levels than trkC mRNA; however, staining was detected on the primitive streak and Hensen's node. In addition to trk mRNA, we have also demonstrated the presence of full‐length Trk protein in embryos from stages 3 through 11, suggesting that the trk mRNA detected at these early stages is translated into functional cell surface receptors. To support this hypothesis, we have shown that neurotrophins can induce phosphorylation of Trk on tyrosine residues, at least at stage 11. We also detected mRNA and protein for the nontyrosine kinase neurotrophin receptor, p75, at similar stages. The presence of neurotrophin receptors, as well as neurotrophin mRNA, in embryos undergoing gastrulation and neurulation leads to speculation that neurotrophins may be playing a role in these processes. © 1996 Wiley‐Liss, Inc.