Tumor Necrosis Factor-α Suppresses Activation of Sustained Potassium Currents in Rat Small Diameter Sensory Neurons

Abstract

Tumor necrosis factor-α (TNF-α), a pro-inflammatory cytokine, produces pain and hyperalgesia by activating and/or sensitizing nociceptive sensory neurons. In the present study, using whole-cell patch clamp techniques, the regulation of potassium currents by TNF-α was examined in acutely dissociated small dorsal root ganglion neurons. We found that acute application of TNF-α inhibited, in a dose-dependent manner, the non-inactivating sustained potassium current without changing the rapidly inactivating transient current or the kinetics of steady-state inactivation. The effects of TNF-α on potassium currents were similar to that of prostaglandin E2 as reported previously and also demonstrated in the current study. Furthermore, indomethacin, a potent inhibitor for both cyclo-oxygenase (COX) -1 and COX-2, completely blocked the effect of TNF-α on potassium currents. These results suggest that TNF-α may sensitize or activate sensory neurons by suppressing the sustained potassium current in nociceptive DRG neurons, possibly via stimulating the synthesis/release of endogenous prostaglandins.

Figure 1

Pharmacological isolation of the K current reveals 2 distinct classes of small DRG neurons. A: A typical neuron with a large amount of non-inactivating sustained component and a smaller component of rapidly inactivating transient component. B: A typical small neuron with both non-inactivating sustained and fast inactivating components. Note that the rapidly inactivating component is sensitive to 4-AP; and the non-inactivating sustained component is blocked by TEA.

Figure 2

Effects of TNF-α on potassium currents. TNF-α (1 ng/ml) suppressed the activation of total (A) and sustained K currents (B), but did not change the rapidly inactivating transient current (C). The K current was normalized by the maximum conductance, and fitted by the Boltzmann equation, and plotted as a function of membrane potential. The activation protocol consisted of 500-ms depolarizing voltage steps to potentials between −60 and 50 mV after a 1-s voltage step to either −120 (A) or −30 mV (B). Currents were normalized by the G_max obtained from control at +50 mV.

Figure 3

The potassium current suppression by TNF-α is not associated with a shift in the voltage dependence for activation. Currents from control and TNF-α treatment were independently normalized by its own G_max obtained at +50 mV and plotted as a function of membrane potential.

Figure 4

Acute bath application of TNF-α did not change the steady-state inactivation of the total K current. Current was normalized by the maximum conductance and fitted by the Boltzmann equation, and plotted as a function of membrane potential. The inactivation protocol consisted of a 1-s voltage step to potentials from -100 to -10 mV followed by depolarizing voltage step to +50 mV.

Figure 5

The dose-dependence of the inhibitory effect of TNF-α on non-inactivating sustained K current. TNF-α-induced K current suppression was tested at different concentrations (0.001-5 ng/ml). Each data point represents the percent decrease of the normalized current (G/G_max) over control after TNF-α application.

Figure 6

Acute application of PGE2 for 5 minute inhibited the non-inactivating K current without affecting steady-state inactivation.

Figure 7

Pretreatment of the DRG cells with indomethacin completely blocked TNF-α-induced K current inhibition. Indomethacin alone had no effect on the potassium currents.