Modification of sodium channel gating in frog myelinated nerve fibres by Centruroides sculpturatus scorpion venom.

Abstract

SUAMARY1. The effect of Centruroides sculpturatus scorpion venom on single frog myelinated nerve fibres was studied. Sodium currents through the nodal membrane were measured under voltage-clamp conditions before and after exposure to venom in Ringer solution 1-5 ,ug/ml. for 1-3 min.2. Centruroides venom brings about repetitive firing and increased membrane potential noise. Spontaneous firing was also observed. Eventually the nodal membrane becomes inexcitable following venom treatment.3. Under voltage clamp with a step depolarization of the membrane potential, activation and inactivation of sodium currents appear normal. However, upon repolarization a novel sodium current turns on, reaches a peak within about 25 msec, and then declines over several hundred milliseconds. As the amplitude and duration of the depolarizing pulse are increased, the size of the venom-induced current that follows also increases.4. The venom-induced current turns on exponentially with a time constant near the value of the time constant for recovery from inactivation, Th' at the resting membrane potential. A depolarizing pulse inactivates this new current component, while a hyperpolarizing pulse leads to a larger venom-induced current immediately after the hyperpolarization. Its time course and membrane potential dependence indicate that the venominduced current is modulated by the sodium inactivation process.5. The membrane potential dependence of sodium activation in some channels is shifted by 40-S50 mV in the hyperpolarizing direction. Depolarization increases the proportion of channels with shifted activation gating by first-order kinetics. Following a depolarizing pulse the activation * Present address: Deparment of Physiology, University of Rochester, Rochester, New York. M. D. CAHALAN parameter, i3, remains elevated for hundreds of milliseconds, allowing channels to reopen as recovery from inactivation occurs.6. A kinetic model with normal inactivation gating and shifted activation gating in some channels accounts for the observed voltage-clamp currents and for the repetitive firing evoked by Centruroides venom. In the model normal channels are converted to channels with shifted activation gating by a voltage dependent reaction.7. The results suggest limits to possible coupling between sodium channel activation and inactivation. Transitions of the inactivation parameter, h, can occur normally in channels with a shifted membrane potential dependence for activation.

Resurgent Current and Voltage Sensor Trapping Enhanced Activation by a β-Scorpion Toxin Solely in Nav1.6 Channel

“…4). To some extent, these studies explained previous observations of scorpion venom actions (7) and the effects of ␤-scorpion toxins on frog and toad Ranvier nodes (8,9). In contrast, sea anemone toxins and ␣-scorpion toxins disrupt the inactivation process by interacting with the extracellular IVS3-S4 loop (3).…”

“…These values are all about three times smaller than the rec (V m ) at the same membrane potentials, suggesting a mechanism much faster than that conventionally operating in control and, thus, different from that previously hypothesized for the scorpion venom, which is known to be a mixture of many peptides (7).…”

“…For the other concentrations (35,16, and 8 nM), we maintained fixed all parameters except the amplitude of the Cn2-shifted activation curve, and the amplitudes that resulted were 0. Assuming that the channels exist in two forms (6,7), with and without toxin, the data were fitted by the sum of two Boltzmann relationships with concentration-dependent amplitudes (see legend of Fig. 1D).…”

Structure and Function of the Voltage Sensor of Sodium Channels Probed by a β-Scorpion Toxin

“…Binding of toxins to IVS3-S4 is thought to slow inactivation by preventing the normal outward movement of the IVS4 transmembrane segment during channel gating (23,26). In contrast to these toxins that inhibit inactivation gating, ␤-scorpion toxins bind to neurotoxin receptor site 4 on sodium channels and enhance activation by shifting its voltage dependence to more negative potentials (27)(28)(29)(30)(31)(32)(33)(34). Our previous results (34) implicate the extracellular loops S1-S2 and S3-S4 in domain II in formation of neurotoxin receptor site 4.…”

Purification and characterization of a β‐toxin from the venom of the African scorpion Leiurus quinquestriatus

“…INTRODUCTION The venom of the African (mideast) scorpion Leiurus quinquestriatus markedly inhibits inactivation of voltage-sensitive sodium channels in excitable membranes [l]. The venom of the North American species Centruroides sculpturatus acts quite differently [2]. Among the toxins isolated from this venom [3], only toxin V causes a drastic inhibition of inactivation [4], while toxins I, III, IV, VI and VII of C. sculpturatus produce a transient shift of Na activation to more negative potentials [5].…”