Voltage-Independent Inhibition of the Tetrodotoxin-Sensitive Sodium Currents by Oxotremorine and Angiotensin II in Rat Sympathetic Neurons

Tetrodotoxin-sensitive Na+ currents have been extensively studied because they play a major role in neuronal firing and bursting. In this study, we showed that voltage-dependent Na+ currents are regulated in a slow manner by oxotremorine (oxo-M) and angiotensin II in rat sympathetic neurons. We found that these currents can be readily inhibited through a signaling pathway mediated by G proteins and phospholipase C (PLC) β1. This inhibition is slowly established, pertussis toxin–insensitive, partially reversed within tens of seconds after oxo-M washout, and not relieved by a strong depolarization, suggesting a voltage-insensitive mechanism of inhibition. Specificity of the M1 receptor was tested by the MT-7 toxin. Activation and inactivation curves showed no shift in the voltage dependency under the inhibition by oxo-M. This inhibition is blocked by a PLC inhibitor (U73122, 1-(6-{[(17β)-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino}hexyl)-1H-pyrrole-2,5-dione), and recovery from inhibition is prevented by wortmannin, a PI3/4 kinase inhibitor. Hence, the pathway involves Gq/11 and is mediated by a diffusible second messenger. Oxo-M inhibition is occluded by screening phosphatidylinositol 4,5-bisphosphate (PIP2)–negative charges with poly-l-lysine and prevented by intracellular dialysis with a PIP2 analog. In addition, bisindolylmaleimide I, a specific ATP-competitive protein kinase C (PKC) inhibitor, rules out that this inhibition may be mediated by this protein kinase. Furthermore, oxo-M–induced suppression of Na+ currents remains unchanged when neurons are treated with calphostin C, a PKC inhibitor that targets the diacylglycerol-binding site of the kinase. These results support a general mechanism of Na+ current inhibition that is widely present in excitable cells through modulation of ion channels by specific G protein–coupled receptors.