Colocalization of TrkB and brain-derived neurotrophic factor proteins in green-red-sensitive cone outer segments

PURPOSE: To examine the distribution of neurotrophins (NTs) and their catalytic receptors in adult rat photoreceptors. METHODS: Immunocytochemistry and Western blot analyses were performed using primary antibodies raised against NTs (nerve growth factor NGF], brain-derived neurotrophic factor BDNF], NT-3, and NT-4/5) and NT receptors (TrkA, TrkB, TrkC, and p75NTR). Double-labeling of retinal sections with opsin-specific antibodies was performed to identify each photoreceptor type. Competitive experiments using excess recombinant NT or Trk receptors confirmed the binding specificity of each antibody. RESULTS: TrkB and BDNF immunoreactivity was colocalized in cone outer segments. TrkB and BDNF were detected in all green-red-sensitive cones, but not in blue-UV cones or rods, and other NTs and NT receptors were not detected in any of the photoreceptor types. CONCLUSIONS: The findings suggest a specific role for BDNF through its signaling receptor TrkB in the function and maintenance of green-red cones, the predominant cone type in the rat retina.