These efforts led later to the characterization
of the NGF-related proteins brain-derived neurotrophic
factor (BDNF), neurotrophin 3 (NT-3), and NT-4/5, which
together comprise the family of neurotrophins (reviewed in Ip and Yancopoulos, 1996; Lewin and Barde, 1996).
With the characterization of the biological effects of the neurotrophins, the neurotrophic hypothesis was modified (Davies, 1988). In addition to acting as target-derived factors, the neurotrophins were found to promote neuronal survival locally in both an autocrine and paracrine manner
(Davies, 1996).