NT-3 is found in neurons of the central nervous system. NT-3 is expressed also in muscles and its expression is down-regulated in denervated muscles. Many human gliomas express and secrete NT-3 into the conditioned medium. Some protein domains of NT-3 are identical with those of NGF and BDNF. NT-3 selectively supports the survival of neuronal cell populations. NT-3 has been shown recently to prevent death of cultured embryonic rat spinal motor neurons at picomolar concentrations. NT-3 has been shown to enhance sprouting of corticospinal tract during development and after adult spinal cord lesion. The activities of NT-3 and BDNF are additive in some systems. The biological activities of NT-3 are mediated by a receptor belonging to the trk family of receptors with intrinsic tyrosine-specific protein kinase activity. NT-3 only binds weakly to the trk receptor which is a high-affinity receptor for NGF.