The reduction of pain in the course of antiinflammatory therapy can result from an attenuation of the inflammatory process and/or from the neutralization of endogenous mediators of inflammation that act directly on nociceptive neurons. The purpose of this study was to investigate whether analgesic effects of the neutralization of tumor necrosis factor α (TNFα) are due to an attenuation of inflammation or whether direct neuronal effects significantly contribute to pain relief in the course of therapy.
Locomotor and pain-related behavior and histology were assessed in rats with chronic antigen-induced arthritis (AIA) in the knee joint, and the rats were treated with systemic saline, etanercept, or infliximab. The expression of TNF receptors (TNFRs) in dorsal root ganglia was measured using immunohistochemical analysis and polymerase chain reaction. Action potentials were recorded from afferent Aδ fibers and C fibers of the medial knee joint nerve, and etanercept and infliximab were injected intraarticularly into normal or inflamed knee joints (AIA or kaolin/carrageenan-induced inflammation).
In rats with AIA, both etanercept and infliximab significantly decreased inflammation-induced locomotor and pain-related behavior, while joint swelling was only weakly attenuated and histomorphology still revealed pronounced inflammation. A large proportion of dorsal root ganglion neurons showed TNFRI- and TNFRII-like immunoreactivity. Intraarticular injection of etanercept reduced the responses of joint afferents to mechanical stimulation of the inflamed joint starting 30 minutes after injection, but had no effect on responses to mechanical stimulation of the uninflamed joint.
Overall, these data show the pronounced antinociceptive effects of TNFα neutralization, thus suggesting that reduction of the effects of TNFα on pain fibers themselves significantly contributes to pain relief.