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.
