Inhibition of Voltage-Gated Na+ Currents in Sensory Neurons by the Sea Anemone Toxin APETx2

Abstract

ASICs are neuronal H+-gated channel that are transiently opened by extracellular acidification. Functional ASIC channels are made of homotrimeric or heterotrimeric complexes of different ASIC subunits (ASIC1a, −1b, −2a, −2b and 3). APETx2, a toxin from the sea anemone Anthropleura elegantissima inhibits homotrimeric ASIC3 with IC50<100 nM. In rat sensory neurons (DRG), where heterotrimeric channels are expressed, higher concentration are necessary to block ASIC3 containing channels (0.1-2μM). Several animal studies relied on APETx2 as a selective pharmacological tool to study ASIC3 physiological role.A large number of toxins from sea anemone have been previously shown to modulate voltage-gated Na+ channel (Nav) function. The aim of this study was to test whether APETx2, at concentrations used to block heterotrimeric ASIC3 containing channels, affects Nav function in sensory neurons. The effect of APETx2 on Nav function was studied using the whole-cell patch-clamp technique on acutely dissociated small-diameter rat dorsal root ganglion (DRG) neurons.In our study, APETx2 inhibited the tetrodotoxin (TTX)-resistant Nav1.8 currents of DRG neurons in a concentration-dependent manner with an IC50 of ∼3 μM. TTX-sensitive currents were inhibited to a smaller extent. The observed inhibition of Nav1.8 currents is due to a rightward shift in the voltage dependence of activation, and a reduction of the maximal macroscopic conductance. In current-clamp experiments in DRG neurons the number of action potentials induced by injection of a current ramp was reduced by APETx2.APETx2 inhibits, in addition to ASIC3, Nav1.8 channels at concentrations used in in vivo studies. The limited specificity of this toxin should be taken into account when using APETx2 as a pharmacological tool. Its dual action will be an advantage for the use of APETx2 or its derivatives as analgesic drugs.