Neurotrophin-3 receptors in the developing chicken retina

Neurotrophin 3 (NT-3) had specific high-affinity receptors (HNT-3R) in the developing chick retina at all ages between embryonic day (E) 4 and E14. The affinity of HNT-3R for 125I-NT-3 did not change with the developmental state. A dissociation constant (kd) of 13 pM was obtained. However, the amount of HNT-3R appeared to be developmentally regulated; the number of receptors/cell increased from E4 up to E6-7 (coinciding with the main onset of neuronal differentiation), then decreased until E9 and increased again by E12, when all retinal cells were differentiated. Kinetic and cross-linking experiments showed that HNT-3R from two prototypical developmental ages, E7 and E14, were different. E7 and E14 HNT-3R could be distinguished from each other on the basis of different inhibition patterns of 125I-NT-3 binding in the presence of nerve growth factor or brain-derived neurotrophic factor. Chemical cross-linking of increasing concentrations of 125I-NT-3 to its receptors showed (a) one 100-kDa band corresponding to neurotrophin low-affinity receptors in both E7 and E14 cells; (b) one 130-kDa band also present in both E7 and E14 cells. Densitometric measurements showed that this 130-kDa band behaved as HNT-3R in E14 cells (kd approximately 10 pM) but not in E7 cells (kd > or = 0.2 nM). Furthermore, the 130-kDa band in both E7 and E14 retinal cells displayed a trk-like immunoreactivity. Our data show that, in neurons, one particular neurotrophin may induce different actions mediated through distinct and specific receptors.