Axonal transport of TNF-alpha in painful neuropathy: distribution of ligand tracer and TNF receptors.

Tumor necrosis factor-alpha (TNF-alpha) is a key player in peripheral nerve injury. In the inflammatory chronic constriction injury (CCI) model of sciatic neuropathy, upregulation of TNF-alpha mRNA and protein at the site of nerve injury has been associated with pain. We now report the distribution of endogenous TNF-alpha protein and its receptors along normal and CCI-injured sciatic nerves, and within the corresponding lumbar dorsal root ganglia (DRG). Using Western blotting, TNF-alpha was found to be distinctly increased at the injury site, as well as in the axons just distal to the corresponding DRG. The TNF-alpha signal between the injury site and DRG (midaxonal) was induced between 2 and 5 days post-CCI, suggesting activation of TNF-alpha axonal transport. Endogenous TNF-alpha was localized in small-diameter, presumably nociceptive, and large-diameter, presumably mechanoceptive, DRG sensory neurons in both normal and CCI animals. Intraneural microinjection of biotin-labeled TNF-alpha showed specific axonal uptake at the injection site, as detected by avidin-biotin-peroxidase histochemistry, and confirmed by co-localization with neurobiotin tracer. In control animals, fast retrograde transport of biotinylated TNF-alpha to both L4 and L5 DRG neurons was apparent 6 h following injection. TNF receptors TNFRI and TNFRII co-localized with biotinylated TNF-alpha tracer along the nerve trunk, suggesting that TNF-alpha transport may be receptor-mediated. In animals with CCI neuropathy, uptake of biotinylated TNF-alpha by neuronal soma was inhibited. Instead, there was signal accumulation in the axons immediately distal to the DRG, and TNFRI and RII were increased at this same anatomic location. These findings highlight a dynamic process of TNF-alpha protein and receptor regulation throughout the peripheral neural axis that bears on both the normal function of DRG neurons and the pathogenesis of painful neuropathies.