Electrophysiological Analysis of the Neurotoxic Action of a Funnel-Web Spider Toxin, d-atracotoxin-Hv1a on Insect Voltage-Gated Sodium Channels

Abstract

The effects of d-ACTX-Hv1a, purified from the venom of the funnel-web spider Hadronyche versuta, were studied on the isolated giant axon and dorsal unpaired median (DUM) neurones of the cockroach Periplaneta americana under current- and voltage-clamp conditions using the double oil-gap technique for single axons and the patchclamp technique for neurones. In parallel, the effects of the toxin were investigated on the excitability of rat dorsal root ganglion (DRG) neurones. In both DRG and DUM neurones, d-ACTX-Hv1a induced spontaneous repetitive firing accompanied by plateau potentials. However, in the case of DUM neurones, plateau action potentials were facilitated when the membrane was artificially hyperpolarized. In cockroach giant axons, d-ACTX-Hv1a also produced plateau action potentials, but only when the membrane was pre-treated with 3-4 diaminopyridine. Under voltage-clamp conditions, d-ACTX-Hv1a specifically affected voltage-gated Na+ channels in both axons and DUM neurones. Both the current/voltage and conductance/ voltage curves of the d-ACTX-Hv1a-modified inward current were shifted 10 mV to the left of control curves. In the presence of d-ACTX-Hv1a, steady-state Na+ channel inactivation became incomplete, causing the appearance of a non-inactivating component at potentials more positive than -40 mV. The amplitude of this non-inactivating component was dependent on the holding potential. From this study, it is concluded that, in insect neurones, d-ACTXHv1a mainly affects Na+ channel inactivation by a mechanism that differs slightly from that of scorpion a- toxins.