The neurotrophins are members of a family of four related proteins that allow the survival and differentiation of specific sub-sets of embryonic vertebrate neurons. On neurons, two types of neurotrophin receptors can be distinguished on the basis of their dissociation constants: low affinity receptors (Kd 10(-9) M) and high affinity receptors (Kd 10(-11) M). Several genes coding for neurotrophin receptors have been cloned and the expression in fibroblasts of the recombinant membrane proteins allows comparisons to be made between the binding properties of the neurotrophins on such cell lines and neurons. As a result, it appears that much of the low affinity binding sites detected on neurons for all neurotrophins can be attributed to a single molecular entity, the low affinity neurotrophin (or NGF) receptor. This receptor binds all known neurotrophins with similar affinity but different binding kinetics. Its role in neurotrophic signal transduction remains to be established. In addition to this receptor, three members of the trk-subfamily of tyrosine kinase receptors have recently been identified as receptors for the neurotrophins. These receptors (whose intrinsic tyrosine kinase activity can be stimulated by the various neurotrophins) bind the neurotrophins with higher affinity and higher ligand specificity when compared with the low affinity receptor. However, the observation has been made that some of the recombinant trk-receptors on cell lines bind more than one neurotrophin (though typically with lower affinity than their own ligands).(ABSTRACT TRUNCATED AT 250 WORDS)