Ontogeny of high- and low-affinity nerve growth factor receptors in the lumbar spinal cord of the developing chick embryo

The binding of ¹²⁵I-labeled nerve growth factor-β (NGF) to soluble extracts of intact or dissociated embryonic chick lumbar cords was used to investigate the kinetic properties and to quantify the levels of NGF receptors (NGFRs) in the developing chick between Embryonic Day 6 (E6) and E10. Both high-affinity (type I; K_d = 7.4 × 10^-11 M) and low-affinity (type II; K_d = 2.4 × 10^-9 M) NGFRs were detected by Scatchard analysis of ¹²⁵I-NGF binding to E6 spinal cord extracts. A total of 4 × 10⁹ type I and 5 × 10¹⁰ type II receptors/cord were found in extracts of E6·cords. As development progressed, there was a decline of both types of NGFRs; however, the decline of type I receptors occurred more rapidly than that of type II. Between E6 and E8 >90% of the type I but only 25% of the type II receptors were lost. These relative rates of loss were maintained over the next week of development, with type I receptors no longer detectable by E12, and type II receptors reduced to 0.025% of their E6 numbers by E15. Analyses of NGFR levels in subpopulations of E6 and E8 lumbar cord cells, prepared by metrizamide density gradient centrifugation, showed that during this period there is an enrichment of both types of NGFRs in the motoneuron-containing subpopulation, relative to other cell populations. The loss of NGFRs does not appear to be influenced by those peripheral-trophic interactions which control other aspects of motoneuron development: curarization of the embryos between E6 and E9 increased motoneuron number in E10 embryos by 30%, but did not significantly affect the loss of NGFRs. These results provide the first quantitative evidence that type I and type II NGFRs are differentially regulated in the spinal cord during embryonic development and raise the possibility that distinct cellular mechanisms may govern their expression.