Abstract: The present invention provides for assay systems that may be used to detect and/or measure neurotrophin activity or to identify agents that exhibit neurotrophin-like activity, and for methods of using such assay systems. It is based, at least in part, on the discovery that the trkB protooncogene encodes a tyrosine kinase receptor that may serve as a functional binding protein for BDNF and NT-3. Such assay systems may be of particular value in identifying new neurotrophins or agents with neurotrophin-like activity. In various embodiments, the assay systems and methods of the invention may be used to detect and/or measure the binding of neurotrophin to trkB, either using direct binding studies or the detection of the secondary affects of trkB/neurotrophin binding. The present invention also has diagnostic and therapeutic utilities. In particular embodiments of the invention, methods of detecting aberrancies in trkB function or expression may be used in the diagnosis of neurological disorders.