Purpose: Application of ciliary neurotrophic factor (CNTF) can rescue mature photoreceptors from lesion-induced and hereditary degeneration. In the chick retina, expression of the CNTF receptor is present in a subpopulation of photoreceptor cells. The present study was undertaken to identify the CNTF receptor-expressing photoreceptors and to describe the subcellular localization of the receptor protein.
Methods: The localization of the CNTF receptor was analyzed by light and electron microscopic immunocytochemistry in chick retinal wholemount preparations, with an antibody for CNTF receptor alpha (CNTFRalpha). Immunoreactive cells were identified by double labeling with immunocytochemical markers for photoreceptor subpopulations.
Results: The CNTFRalpha antibody labeled evenly distributed outer segments (OS) of a photoreceptor subpopulation. CNTFRalpha-positive OS were associated with oil droplets of uniform size. Receptor immunoreactivity did not colocalize with markers for rods and red-green cones. Complete overlap was found after double labeling with the antibody CERN 933, which recognizes violet-sensitive cones in the chick retina. Ultrastructurally, the CNTFRalpha-immunoreactive OS showed rodlike properties: an elongated shape and stacks of membrane discs separated from the plasma membrane. Immunoreactivity was completely restricted to the plasma membrane of the OS and the inner membrane sheet of the photoreceptor calices present in avian retinas.
Conclusions: CNTFRalpha expression identifies a unique type of photoreceptors in the avian retina which does not fit into the classic morphologic definition of rods and cones. The specific expression in violet-sensitive photoreceptors suggests that CNTF may have a neuroprotective role related to the specific function of these cells.