Regulation of aberrant neurofilament phosphorylation in neuronal perikarya. III. Alterations following single and continuous /gB,ß′-iminodipropionitrile administrations

Abstract

β,ß′-Iminodipropionitrile (IDPN) administration produces giant neurofilament-filled axonal swellings in the first proximal internodes of large myelinated sensory and motor fibers without any accompanying axonal degeneration. In the present study, we asked whether proximal giant axonal swellings are sufficient to elicit aberrant neurofilament (NF) phosphorylation in neuronal perikarya. Rats were given a single intraperitoneal (i.p.) injection of IDPN (2 g/kg) followed by IDPN (0.1%) in the drinking water (continuous IDPN exposure) or tap water (single IDPN exposure) for two days to 7 weeks. Immunoreactivity to phosphorylated NF (pNF) epitopes (using monoclonal antibodies 6–17 and 7–05) was observed in L 4 and L 5 dorsal root ganglia (DRG) neurons beginning between one and 5 days, corresponding to the development of proximal giant axonal swellings. Quantitation of DRG neurons demonstrated maximal numbers of immunoreactive cell bodies to pNF epitopes (46–51%) by one week. The number of immunostained DRG cells was maintained in animals given continuous IDPN exposure, but declined significantly ( P < 0.001) in rats given a single injection of IDPN to 26 ± 0.80% and 6 ± 0.04% at 3 and 5 weeks, respectively. Ventral and dorsal root fibers, which undergo axonal atrophy distal to axonal swellings, showed intense immunoreactivity to pNF epitopes and a marked reduction or a complete lack of immunostaining to antibody 2–135 (directed against non-phosphorylated NF epitopes); pretreatment with alkaline phosphatase reversed this staining pattern. In a separate study, a similar alkaline phosphatase-sensitive lack of staining to antibody 2–135 was also observed in atrophic motor fibers in the DRG 4 weeks following nerve crush. It is suggested that aberrant NF phosphorylation in DRG neuronal cell bodies from IDPN-treated rats arises secondarily to an alteration in a retrogradely transported ‘trophic’ signal(s) to the neuron due to the presence of giant axonal swellings. Furthermore, pNFs in atrophic axons may correspond to stationary or slowly moving NFs in the axoplasm.