Axonal neurofilament-H immunolabeling in the rabbit retina

A monoclonal antibody directed at the multiphosphorylated epitope of axonal neurofilament-H (NF-H) was used to label axon-like fibers in the rabbit retina. NF-H-immunopositive fibers were found in the outer plexiform layer (OPL), inner plexiform layer (IPL), and optic fiber layer (OFL). The morphological characteristics of the labeled processes identified those in the OPL as horizontal cell axons and axon terminals and fibers in the OFL as axons of ganglion cells. The NF-H-positive profiles in the OPL formed a subset of horizontal cell processes labeled for calbindin. In the IPL, NF-H-immunoreactive profiles lay at all levels but were detected most often in the middle strata, 2-4. Occasionally, we observed NF-H-immuoreactive processes emerging from the IPL and entering either the GCL or the inner nuclear layer (INL). The labeled fibers in the IPL were typically very thin, less than 1 microm in diameter, and could often be followed for over 1 mm as they ran laterally across the retina. Cell bodies were never labeled by the immunoserum. To identify the NF-H-immunopositive fibers in the IPL, standard immunocytochemical double-labeling techniques were applied, using antibodies directed against several neurotransmitters or modulators thought to be expressed by axon-bearing amacrine cells. The NF-H-positive processes in the IPL were found to correspond to those labeled for tyrosine hydroxylase, somatostatin, substance P, and NADPH diaphorase activity. However, the NF-H labels did not colocalize with those against the vasoactive intestinal peptide-associated protein PHM27. Our results indicate that putative axons in the retina possess the multiphosphorylated NF-H protein found within classic axons in the central nervous system. These results thus support the idea that certain subtypes of amacrine and horizontal cells maintain true axons in the mammalian retina.