BDNF and NT-4 differentially modulate neurite outgrowth in developing retinal ganglion cells

We show here that neurite outgrowth of ganglion cells (RGCs) was selectively enhanced following treatment with BDNF or NT-4 in short-term cultures of dissociated cells derived from the neuroretina of postnatal rats. NT-4 was more effective than BDNF. The effect of NT-3 was variable, whereas NGF and CNTF had no effects upon neurite elongation. The neuritogenic responses of RGCs to both BDNF and NT-4 were prevented by competition with soluble TrkB receptor, and abolished by K252a, a selective inhibitor of the tyrosine kinase activity of Trks. These results indicate that the differentiating effects of BDNF and NT-4 are mediated by TrkB receptors, naturally expressed by RGCs. Developing RGCs treated with these TrkB ligands displayed distinct, albeit partially overlapping, patterns of neurite morphology. BDNF supported predominantly polarized outgrowth, whereas NT-4 induced the appearance of intensely branched symmetrical arbors. The lack of RGCs showing combined morphologies (e.g., highly arborized unipolar cells) suggests distinct mechanisms underlying either elongation or branching, and implicates distinct responses of RGC subsets. We conclude that neurite growth in vitro is extensively promoted by neurotrophins in developing RGCs. Moreover, highly homologous neurotrophins such as BDNF and NT-4, presumably activating via TrkB receptors, selectively control the differentiation of distinct ganglion cell neuritic morphologies.