Colocalization of TrkB and Brain-Derived Neurotrophic Factor Proteins in Green-Red-Sensitive Cone Outer Segments

To examine the distribution of neurotrophins (NTs) and their catalytic receptors in adult rat photoreceptors. 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. 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. 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.