Localization of p75 neurotrophin receptor in the retina of the adult SD rat: an immunocytochemical study at light and electron microscopic levels.

The low-affinity neurotrophin receptor p75(NTR), or p75, is a 75-kDa cell surface glycoprotein that binds all neurotrophins with similar affinity and is thought to help to ensure the specificity of each neurotrophin. In order to better understand the role of p75 and how it is involved in the neurotrophic effects in the retina, we have examined its cellular localization in the adult rat retina by immunocytochemistry at both light and electron microscopic levels. The similarity between the staining pattern of p75 and that of the distribution of Müller cell processes, as marked by antibodies against S-100 and vimentin, suggests that p75 may be on the Müller cell processes and not on the retinal ganglion cells (RGCs) as previously reported. The failure to detect p75 immunoreactivity on Fluoro-Gold retrogradely labeled RGCs in the radially sectioned retinae also indicates that it is not expressed on RGCs. The results from the light microscopic immunohistochemical studies are supported at the ultrastructural level, showing that p75-immunopositive staining is localized on Muller cell processes and not on RGC bodies. Muller cell processes not only form the inner limiting membrane but also partially wrap around the RGC bodies. Our results lead us to conclude that the previously reported immunopositive staining of p75 on RGCs might belong to the surrounding Miller cell processes. Thus, the pathway of neurotrophic effects on RGCs might be, at least partially, through a glial-neuronal pathway rather than on RGCs directly.