The tumor necrosis factor alpha (TNFa) plays an important role as inflammatory and algesic mediator. Now we tested, i) whether injection of TNFa into a non-inflamed rat knee joint would enhance responses of nociceptive nerve endings in vivo, and ii), whether such an influence can be blocked by different drugs that were either applied at the nociceptive nerve ending or systemically and interacted with the TNFa signal transduction pathway.

Healthy adult WISTAR rats were anesthetized with sodium thiopentone (100 mg/kg, i.p.). The knee joint was mechanically stimulated by innocuous (20 mNm) or noxious (40 mNm) rotations of the lower leg against the fastened femoral bone for 15 sec each. Recording of action potentials (APs) was performed at least for 1 hour prior to and for 3 hours after injection of test substances. Stimulation of the knee was performed at intervals of 15 minutes. Nerve fibers were classified as C- or as Ad-fibers by their conduction velocity (<1.4 m/s or <10 m/s, respectively). Compounds were injected into the joint cleft at a volume of 0.1 ml each.

A single intraarticular injection of 5 ng TNFa increased the net response rate of C-fibers to innocuous stimulation within three hours by 53.6 APs/15 sec and to noxious stimulation by 202.8 APs/15 sec. The net response rate of Ad-fibers increased in the same time interval by 14.3 and 68.5 APs/15 sec, respectively. These effects were dose-dependent. A single injection of 0.5 ng TNFa increased the net response rate only in C-fibers but not in Ad-fibers. A dose of 0.05 ng TNFa finally did not affect the net response rate. Simultaneous peripheral administration of the biological etanercept that blocks TNFa (100 mg) together with 5 ng TNFa prevented an increase in APs both to noxious or innocuous stimulation of the knee. When the non-selective COX-inhibitor Diclofenac was injected intraperitoneally 2 hours prior to intraarticular application of 5 ng TNFa, a dose-dependent inhibition of the increase in AP responses to noxious and innocuous stimulation was seen: 2 mg/kg body weight Diclofenac slowed and diminished the previously seen effect of TNFa, 4 mg/kg Diclofenac completely abolished the increase of the action potentials. A simultaneous injection of the p38 kinase inhibitor SB 203580 (10 mg) together with 5 ng TNFa completely inhibited the increase of AP responses both to noxious and to innocuous stimuli.

Our data show that TNFa is able to sensitize nociceptive afferents of the joint in vivo. The injection of TNFa, however, did not induce an increase in resting activity of nociceptive nerve fibers. The cyclooxygenase-pathway at least in part mediates the effects of TNFa. The p38 kinase pathway at the peripheral nerve ending contributes to the sensitizing effect.