Immunohistochemical studies on neurofilamentous hypertrophy in degenerating retinal terminals of the olivary pretectal nucleus in the rat

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Abstract
Following section of the optic nerve, degenerating retinal terminals reveal an accumulation of neurofilaments (neurofilamentous hypertrophy) as demonstrated by silver impregnation techniques or electron microscopy. The present study examined degenerating retinal terminals by means of immunohistochemistry and antibodies specific for the triplet of neurofilament proteins of low (NF‐L), medium (NF‐M), and high (NF‐H) molecular weight class. Following unilateral optic nerve section in the rat and survival 1,2,4,8, and 21 days, brains were perfused with aldehyde fixative, sliced on a vibratome and stained for neurofilaments by using the peroxidase‐antiperoxidase technique. Other brains were frozen, cut in the native state, and slid‐mounted sections were fixed by acetone. Side comparisons in visual pathways were made in frontal sections, taking advantage of the near complete crossing of retinal fibers in the rat. Anterograde degeneration of axons occurred in the optic tract and brachium colliculi. Changes of terminals were investigated in the olivary pretectal nucleus, which contains a dense aggregation of retinal terminals in the core region. The optic tract and brachium coliculi showed a reduction in imunostaining for neurofilament proteins following axotomy. Within the core region of the olivary pretectal nucleus, strong increases of immunoreactivity of NF‐L and NF‐M were detected begining at 2 days postlesion and persisting at 8 days. No changes in NF‐H proteins were found in the terminal regions with three different antibody probes. The increase in immunostaining reflects the accumulation of neurofilament proteins in the degenerating retinal terminals, i.e., neurofilamentous hypertrophy. A combination of increased influx of neurofilaments into the terminals and decreased local degradation by calciumactivated neutral protease might explain the accumulations. The selective occurrence of NF‐L and NF‐M suggests molecular specificity of the degenerative process, which may be related to differences in axonal transport, integration into the stationary cytoskeleton, and phosphorylation state of different neurofilament proteins.

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