Neurofilament immunoreactivity in populations of rat primary afferent neurons: a quantitative study of phosphorylated and non-phosphorylated subunits.

Neurofilament subunits in rat dorsal root ganglion (DRG) neurons were examined using five antibodies: NFH, RT97 and NFHP- recognise the 200 kDa subunit (NF200); NFH recognises both phosphorylated and non-phosphorylated forms of NF200 whereas RT97 and NFHP- are specific for the phosphorylated and non-phosphorylated forms respectively; 155 and anti-68 kD recognise the 155 kDa and 68 kDa subunits respectively. All the antibodies apart from NFHP- distinguished between the two populations of neurons corresponding to the light (L) and small dark (SD) cell types as previously shown for RT97. This demonstrates that L and SD neurons contain different levels of neurofilament and that the ability to discriminate between them is not unique to the antibody RT97. It is also evident that DRG neurons contain neurofilament composed of all three subunits. Since NFH and RT97, but not NFHP-, distinguished between the two populations, it appears that it is the presence of the phosphorylated form of NF200 that provides the basis for discrimination between the two cell types. After dephosphorylation of the neurofilament, NFHP- also discriminated between the two populations, indicating that there is more NF200 regardless of phosphorylation state in the L neurons. Observations made from unfixed DRGs indicate that all neurons contain some neurofilament and the neurofilament rich and neurofilament poor populations were also apparent. The use of colchicine apparently caused a small increase in neurofilament levels in at least some perikarya, presumably due to its blocking effect on axoplasmic transport. This caused some SD neurons to become neurofilament rich. We conclude that L neurons contain more neurofilament than SD neurons since both cell types contain non-phosphorylated NF200, but the L neurons also contain a much greater amount of the phosphorylated form.