Early expression of the nerve growth factor receptor ctrkA in chick sympathetic and sensory ganglia

Avian sympathetic and sensory ganglia are useful models to study the biological effects of nerve growth factor (NGF) in vitro as well as in vivo. In order to examine the expression pattern of the NGF tyrosine kinase receptor during embryogenesis, we cloned a full-length cDNA encoding chick trkA (ctrkA). Compared with human trkA, the sequence identity is 46% in the extracellular domain and is unevenly distributed between the subdomains. Between embryonic Days 6.5 and 16 (E6.5 and E16), a single 3-kb ctrkA transcript is detected in sympathetic and sensory ganglia, and in situ hybridization experiments reveal the presence of ctrkA mRNA in both ganglia from E4.5 onward. The detection of ctrkA in the primary sympathetic chain is unexpected in view of previous experiments with cultured sympathetic neurons indicating a lack of a survival response to NGF at early developmental stages. However, it fits with the observation that, in vivo, the administration of NGF markedly affects cell numbers substantially before the period of target control of neuronal survival in the sympathetic chain.