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Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine involved in the development and maintenance of inflammatory and neuropathic pain conditions. The mechanisms by which TNF-alpha elicits pain behavior are still incompletely understood. Numerous studies suggest that TNF-alpha sensitizes primary afferent neurons. Most recently, it was shown that TNF-alpha induced an enhancement of TTX-R Na(+) current in dorsal root ganglion (DRG) cells. In the present study, we have tested the effect of acute application of TNF-alpha on voltage-gated potassium, calcium and sodium channel currents as well as its influence on membrane conductance in isolated rat DRG neurons. We report that voltage-gated potassium channel currents of nociceptive DRG neurons are not influenced by TNF-alpha (100 ng/ml), while voltage-gated calcium channel currents were decreased voltage-dependently by -7.73+/-6.01% (S.D.), and voltage-activated sodium channels currents were increased by +5.62+/-4.27%, by TNF-alpha. In addition, TNF-alpha induced a significant increase in IV ramps at a potential of +20 mV, which did not exist when the experiments were conducted in a potassium-free solution, indicating that this effect is mainly the result of a change in potassium conductance. These different actions of TNF-alpha might help to explain how it sensitizes primary afferent neurons after nerve injury and thus facilitates pain.

Modulation of intracellular calcium ([Ca2+](i)) has a major impact on processing of nociceptive signals. While activation of the transient receptor potential vanilloid-1 (TRPV-1) receptor/channel complex increases [Ca2+](i) by Ca2+ entry from the extracellular space, as well as by Ca2+ release from intracellular stores, the Ca2+ entry through voltage-activated calcium channels (VACCs) is modulated simultaneously. To clarify the relations between [Ca2+](i) and the activation of TRPV-1 receptor and VACC currents [I(TRPV-1) and I(Ca(V))], we performed voltage clamp experiments using Ba2+ as well as Ca2+ as a charge carrier. The TRPV-1 receptor was activated by the application of 0.5 microM capsaicin, and the currents through TRPV-1 and VACC [I(TRPV-1) and I(Ca(V))] were measured either when Ca2+ release from intracellular stores was pharmacologically promoted or prevented. With Ba2+ as the divalent charge carrier, capsaicin (0.5 microM) reduced I(Ca(V)) (elicited by a depolarization to 0 mV) to 52.7 +/- 4.5% of baseline, and the elicited current through the TRPV-1 receptor/channel complex was 6.6 +/- 0.9% [relative to peak I(Ca(V))]. These currents were significantly different when Ca2+ was used as charge carrier: the I(Ca(V)) reductions were decreased to 17.8 +/- 5.9% of baseline, while the I(TRPV-1) was as high as 57.1 +/- 9.1% of I(Ca(V)). Increases of [Ca2+](i) by releasing Ca2+ from intracellular stores (using caffeine, 10 mM) before the application of capsaicin increased the I(TRPV-1) (14.1 +/- 7%), while the I(Ca(V)) was decreased to 51.6 +/- 4.9% compared with control. A preexperimental partial reduction of the Ca2+ release from the stores by dantrolene (5 microM) resulted in less pronounced effects [24.5 +/- 8.8%, relative to peak I(Ca(V))] for I(TRPV-1), and a reduction to 35.4 +/- 3% of baseline for I(Ca(V)) after capsaicin application.