Effect of 8-bromo-cAMP on the tetrodotoxin-resistant sodium (Nav 1.8) current in small-diameter nodose ganglion neurons

We examined whether 8-bromo-cAMP (8-Br-cAMP)-induced modification of tetrodotoxin-resistant (TTX-R) sodium current in neonatal rat nodose ganglion neurons is mediated by the activation of protein kinase A (PKA) and/or protein kinase C (PKC). In 8-Br-cAMP applications ranging from 0.001 to 1.0 mM, 8-Br-cAMP at 0.1 mM showed a maximal increase in the peak TTX-R Na+ (Nav1.8) current and produced a hyperpolarizing shift in the conductance–voltage (G–V) curve. The PKC inhibitor bisindolylmaleimide Ro-31-8425 (Ro-31-8425, 0.5 μM) decreased the peak Nav 1.8 current. The Ro-31-8425-induced modulation of the GV1/2 baseline (a percent change in G at baseline V1/2) was not affected by additional 8-Br-cAMP application (0.1 mM). The maximal increase in Nav 1.8 currents was seen at 0.1 μM after the application of a PKC activator, phorbol 12-myristate 13-acetate (PMA) and forskolin. The PMA-induced increase in Nav 1.8 currents was not significantly affected by additional 0.1 mM 8-Br-cAMP application. Intracellular application of a PKA inhibitor, protein kinase inhibitor (PKI, 0.01 mM), inhibited the baseline Nav 1.8 current, significantly attenuated the 8-Br-cAMP-and PMA-induced increase in the peak Nav 1.8 current, and caused a significant increase in the slope factor of the inactivation curve. The PKI application at a higher concentration (0.5 mM) greatly inhibited the PMA (0.1 μM)-induced increase in the peak Nav 1.8 current amplitude and further enhanced the Ro-31-8425-induced decrease in the current. These results suggest that the 8-Br-cAMP-induced increase in Nav 1.8 currents may be mediated by activation of both PKA and PKC.