Actions of three structurally distinct sea anemone toxins on crustacean and insect sodium channels.

The membrane actions of three recently isolated polypeptide neurotoxins from the sea anemones Stichodactyla helianthus (toxin ShI), Condylactis gigantea (toxin CgII) and Calliactis parasitica (toxin CpI) were investigated on action potentials and voltage-clamp membrane currents of the giant axon of the crayfish Procambarus clarkii. The first two toxins were also tested on the cockroach (Periplaneta americana) giant axon. All three toxins were particularly lethal to crustaceans, moderately toxic to an insect (cockroach), and essentially non-toxic to a mammal (mouse). ShI and CgII were 50- to 100-fold more potent on crayfish than on cockroach axons; this difference in activity was correlated with the relative reversibility of their effects on these arthropod axons. The crustacean selectivity of these toxins is therefore due largely to their greater affinity for crustacean sodium channels. All three toxins prolonged crayfish giant axon action potentials by selectively slowing Na channel inactivation without greatly affecting activation. Before toxin treatment, inactivation was nearly exponential, with a time constant less than 1 msec. After treatment, the inactivation time course could be described as the sum of two exponentially decaying components, plus a large steady-state component. The major component possessed the slower (10-20 msec) time constant. The steady-state component increased with depolarization, causing the sodium channel steady-state inactivation curve to reach a minimum between -60 and -20 mV and then increase at more positive potentials. All three toxins shifted the peak sodium current-voltage relation to the left. This voltage shift was greater at 20 degrees C than at 10 degrees C. Maintained membrane depolarization during toxin wash-in delayed the appearance of modified Na channels. Also, prolonged depolarization of toxin-treated axons converted modified sodium channels back to normal ones. The toxins did not affect potassium and leakage currents. Our results indicate that the three crustacean-active sea anemone toxins share a common electrophysiological action on arthropod sodium channels, at least at the macroscopic level.     

Slowing Na+ channel inactivation prolongs QT interval and aggravates adrenaline-induced arrhythmias.

We investigated the effects of prolonged repolarization induced by slowed inactivation of Na+ channel on adrenaline-induced arrhythmias in halothane anesthetized, closed-chest dogs. We used sea anemone toxins (ATX-II and Anthopleurin-A) to prolong ventricular repolarization and examined their effects on adrenaline arrhythmias. Sea anemone toxins prolonged the QTc- and JTc-intervals (P<0.01), but did not affect the PQ interval, QRS duration, heart rate and mean blood pressure. Although sea anemone toxins did not induce any arrhythmias by themselves, under the treatment with these toxins, arrhythmias were induced by non-arrhythmia-inducing doses of adrenaline in four dogs out of seven and the control arrhythmias induced by adrenaline were aggravated. These results indicate that, similar to the inhibition of K+ channels by class III drugs, which we have already reported, slowing Na+ channel inactivation with QTc prolongation also aggravates adrenaline-induced arrhythmias.     

Sea anemone venom as a source of insecticidal peptides acting on voltage-gated Na+ channels.

Sea anemones produce a myriad of toxic peptides and proteins of which a large group acts on voltage-gated Na+ channels. However, in comparison to other organisms, their venoms and toxins are poorly studied. Most of the known voltage-gated Na+ channel toxins isolated from sea anemone venoms act on neurotoxin receptor site 3 and inhibit the inactivation of these channels. Furthermore, it seems that most of these toxins have a distinct preference for crustaceans. Given the close evolutionary relationship between crustaceans and insects, it is not surprising that sea anemone toxins also profoundly affect insect voltage-gated Na+ channels, which constitutes the scope of this review. For this reason, these peptides can be considered as insecticidal lead compounds in the development of insecticides.     

Purification and characterization of Hainantoxin-V, a tetrodotoxin-sensitive sodium channel inhibitor from the venom of the spider Selenocosmia hainana.

A neurotoxic peptide, named Hainantoxin-V (HNTX-V), was isolated from the venom of the Chinese bird spider Selenocosmia hainana. The complete amino acid sequence of HNTX-V has been determined by Edman degradation and found to contain 35 amino acid residues with three disulfide bonds. Under whole-cell patch-clamp mode, HNTX-V was proved to inhibit the tetrodotoxin-sensitive (TTX-S) sodium currents while it had no any effects on tetrodotoxin-resistant (TTX-R) sodium currents on adult rat dorsal root ganglion neurons. The inhibition of TTX-S sodium currents by HNTX-V was tested to be concentrate-dependent with the IC(50) value of 42.3nM. It did not affect the activation and inactivation kinetics of currents and did not have the effect on the active threshold of sodium channels and the voltage of peak inward currents. However, 100nM HNTX-V caused a 7.7mV hyperpolarizing shift in the voltage midpoint of steady-state sodium channel inactivation. The results indicated that HNTX-V inhibited mammalian voltage-gated sodium channels through a novel mechanism distinct from other spider toxins such as delta-ACTXs, micro -agatoxins I-VI which bind to receptor site three to slow the inactivation kinetics of sodium currents.     

Site-3 toxins and cardiac sodium channels.

Site-3 toxins are small polypeptide venoms from scorpions, sea anemones, and spiders that bind with a high specificity to the extracellular surface of voltage-gated Na channels. After binding to a site near the S4 segment in domain IV the toxin causes disruption of the normal fast inactivation transition resulting in a marked prolongation of the action potentials of excitable tissues including those of cardiac and skeletal muscle and nerve. In this review we discuss the specific binding interactions between residues of the toxin and those of the Na channel, and the specific modification of Na channel kinetic behavior leading to a change in fast inactivation focusing on interactions deduced primarily from the study of sea anemone toxins and the cardiac Na channel (Na(V)1.5). We also illustrate the usefulness of site-3 toxins in the study of altered Na channel behavior by drug-modification.     

The interaction of sea anemone and scorpion neurotoxins with tetrodotoxin-resistant Na+ channels in rat myoblasts. A comparison with Na+ channels in other excitable and non-excitable cells

The properties of interactions of several polypeptide neurotoxins isolated from sea anemone and scorpion venom with Na+ channels of rat myoblasts, chick myotubes, neuroblastoma cells, and fibroblasts have been compared. Tetrodotoxin (TTX)-resistant Na+ channels appear to be much more sensitive to the action of sea anemone toxins than TTX-sensitive Na+ channels but have the same affinity for scorpion neurotoxins. This conclusion holds both for Na+ channels that can be activated electrically and for silent forms of Na+ channels. The sensitivity to sea anemone toxins of the different types of Na+ channels that have been studied suggests the existence of multiple forms of Na+ channels.     

APETx1, a new toxin from the sea anemone Anthopleura elegantissima,blocks voltage-gated human ether-a-go-go-related gene potassium channels

A new peptide, APETx1, which specifically inhibits human ether-a-go-go-related gene (HERG) channels, was purified from venom of the sea anemone Anthopleura elegantissima. APETx1 is a 42-amino acid peptide cross-linked by three disulfide bridges and shares 54% homology with BDS-I, another sea anemone K+ channel inhibitor. Although they differ in their specific targets, circular dichroism spectra and molecular modeling indicate that APETx1 and BDS-I have a common molecular scaffold and belong to the same structural family of K+ channel blocking peptides. APETx1 inhibits HERG currents in a heterologous system with an IC50 value of 34 nM by modifying the voltage dependence of the channel gating. Central injections in mice failed to induce any neurotoxic symptoms. APETx1, which has no sequence homologies with scorpion toxins acting on HERG, defines a new structural group of HERG gating modifiers isolated from a sea anemone.     

Two novel sodium channel inhibitors from Heriaeus melloteei spider venom differentially interacting with mammalian channel's isoforms

Two new polypeptide toxins named Hm-1 and Hm-2 were isolated from the venom of the crab spider Heriaeus melloteei. These toxins consist of 37 and 40 amino acid residues, respectively, contain three intramolecular disulfide bonds, and presumably adopt the inhibitor cystine knot motif. Hm-1 is C-terminally amidated and shows a low degree of homology to spider toxins agelenin and mu-agatoxin-II, whereas Hm-2 has no relevantly related peptide sequences. Hm-1 and Hm-2 were found to act on mammalian voltage-gated Na(+) channels. Both toxins caused a strong decrease of Na(+) current peak amplitude, with IC(50) values of 336.4 and 154.8nM, respectively, on Na(V)1.4. Hm-1 and Hm-2 did not shift the voltage-dependence of activation, nor did they change the kinetics of fast inactivation of the Na(+) currents. Interestingly, both toxins negatively shifted the steady-state inactivation process, which might have important functional consequences in vivo. However, this hyperpolarizing shift cannot by itself explain the observed inhibition of the Na(+) current, indicating that the two presented toxins could provide important structural information about the interaction of polypeptide inhibitors with voltage-gated Na(+) channels.     

Selective alteration of sodium channel gating by Australian funnel-web spider toxins

The actions of potent mammalian neurotoxins isolated from the venom of two Australian funnel-web spiders were investigated using both electrophysiological and neurochemical techniques. Whole-cell patch clamp recording of sodium currents in rat dorsal root ganglion neurons revealed that versutoxin (VTX), isolated from the venom of Hadronyche versuta, produced a concentration-dependent slowing or removal of tetrodotoxin-sensitive (TTX-S) sodium current inactivation and a reduction in peak TTX-S sodium current. In contrast, VTX had no effect on tetrodotoxin-resistant (TTX-R) sodium currents or potassium currents. VTX also shifted the voltage dependence of sodium channel activation in the hyperpolarizing direction and increased the rate of recovery from inactivation. Ion flux studies performed in rat brain synaptosomes also revealed that robustoxin (RTX), from the venom of Atrax robustus, and VTX both produced a partial activation of ²²Na ⁺ flux and an inhibition of batrachotoxin-activated ²²Na⁺ flux. This inhibition of flux through batrachotoxin-activated channels was not due to an interaction with neurotoxin receptor site 1 since [³H]saxitoxin binding was unaffected. In addition, the partial activation of ²²Na⁺ flux was not enhanced in the presence of α-scorpion toxin and further experiments suggest that VTX also enhances [³H]batrachotoxin binding. These selective actions of funnel-web spider toxins on sodium channel function are comparable to those of α-scorpion and sea anemone toxins which bind to neurotoxin receptor site 3 on the channel to slow channel inactivation profoundly. Also, these modifications of sodium channel gating and kinetics are consistent with actions of the spider toxins to produce repetitive firing of action potentials.     

The interaction of polypeptide neurotoxins with tetrodotoxin-resistant Na+ channels in mammalian cardiac cells. Correlation with inotropic and arrhythmic effects

This paper describes the interaction of several polypeptide neurotoxins isolated from sea anemone toxins and scorpion venom with the tetrodotoxin-resistant Na+ channel of rat cardiac cells. The 22Na+ flux and tension development were measured to examine in parallel the cardiotonic and cardiotoxic effects of these polypeptides. Inotropic effects and arrhythmias were seen in the concentration range in which an action of the toxins on the Na+ channel was observed. The maximal inotropic effect was systematically observed at toxin concentrations below the concentration value observed for half-maximal stimulation of 22Na+ flux through the Na+ channel. Arrhythmias began at concentrations near the value for half-maximal stimulation of 22Na+ flux by the toxins. Toxins extracted from the sea anemones Anemonia sulcata and Anthopleura xanthogrammica were more active than scorpion toxins and sea anemone Radianthus paumotensis toxins. The most interesting among all the toxins tested for potential use in cardiotherapy was toxin II from Anthopleura xanthogrammica.     

Effects of hydrogen peroxide on sodium current in acutely isolated rat hippocampal CA1 neurons

The effects of hydrogen peroxide (H2O2) on sodium currents (Na+ currents) in freshly dissociated rat hippocampal neurons were studied using the whole-cell patch-clamp techniques. H2O2 caused a reversible increase of the voltage-activated Na+ currents in a concentration- and voltage-dependent manner. The half-increasing concentration (EC50) of H2O2 on Na+ currents was 10.79 microM. In addition, 10 microM H2O2 shifted the steady-state inactivation curve of Na+ currents toward positive potential (control Vh = -64.58 +/- 1.22 mV, H2O2 Vh = -53.55 +/- 0.94 mV, n = 10, P < 0.01 without changing the slope factor). However, the steady-state activation curve was not affected. These results indicated that H2O2 could increase the amplitudes of Na+ currents and change the inactivation properties of Na+ channels even in very low concentration.     

Effects of a Lasiodora spider venom on Ca2+ and Na+ channels.

The venom of a Brazilian spider, Lasiodora sp (Mygalomorphae, Theraphosidae), was screened for activity against ion channels using Ca2+ imaging and whole-cell patch clamp in GH3 cells. When tetrodotoxin (TTX) was present to block Na+ channels, the venom abolished the Ca2+ oscillations that are normally present in these cells and reduced the basal level of intracellular Ca2+. Under patch clamp, the venom reduced the L-type Ca2+ channel conductance and caused a positive shift in its voltage dependence of activation. In addition to these effects, when applied without TTX, the venom also caused a slow and noisy increase in intracellular Ca2+. The sensitivity of this second effect to TTX suggested an effect on Na+ channels, which was tested using patch clamp. Control Na+ currents inactivated completely as a single exponential. Treatment with the venom did not affect the amplitude of I(Na), but caused it to divide in two slower exponential components plus a sustained component, all of which were suppressed by TTX. The venom also caused a negative shift in the voltage dependence of activation and steady-state inactivation of I(Na). The observed effects of this venom on whole-cell currents explain the changes it causes in intracellular Ca2+ in GH3 cells and demonstrate that the venom of this spider is a source of toxins active against ion channels.     

Modulation of voltage-dependent sodium channels by the delta-agonist SNC80 in acutely isolated rat hippocampal neurons

Following activation, voltage-gated Na+ currents (I(Na)) inactivate on two different time scales: fast inactivation takes place on a time scale of milliseconds, while slow inactivation takes place on a time scale of seconds to minutes. Both fast and slow inactivation processes govern availability of Na+ channels. In this study, the effects of the delta-opioid receptor agonist SNC80 on slow and fast inactivation of I(Na) in rat hippocampal granule cells were analyzed in detail. Following application of SNC80, a block of the peak Na+ current amplitude (EC50: 50.6 microM, Hill coefficient: 0.518) was observed. Intriguingly, SNC80 (50 microM) also caused a selective effect on slow but not fast inactivation processes, with a notable increase in the fraction of Na+ channels undergoing slow inactivation during prolonged depolarization. In addition, recovery from slow inactivation was considerably slowed. At the same time, fast recovery processes were unaffected. The effects of SNC80 were not mimicked by the peptide delta-receptor agonist DPDPE (10 microM), and were not inhibited by the opioid receptor antagonists naloxone (50-300 microM) or naltrindole (10 and 100 microM), indicating an opioid receptor independent modulation of Na+ channels. These data suggest that SNC80 not only affects delta-opioid receptors, but also voltage-gated Na+ channels. SNC80 is to our knowledge hitherto the only substance that selectively influences slow but not fast inactivation processes and could provide an important tool in unraveling the mechanism underlying these distinct biophysical processes.     

The purification and N-terminal sequencing of a polypeptide that prolongs action potentials by altering Na channel inactivation from the venom of Buthus martensii Karsch

A polypeptide that extensively prolongs action potentials (APs) in frog nerve has been isolated and purified from the venom of the scorpion Buthus martensii Karsch (BMK). The polypeptide was purified using gel filtration, ion exchange, FPLC, and HPLC chromatography. APs recorded in the presence of nanomolar concentrations of the polypeptide were extensively prolonged without much attenuation in their heights. The N-terminal sequence of BMK 11(2) was found to be: VRDGYIADDKD-AYF-GRDAYYDDDEKKKD. Sequence similarity comparisons to other alpha-scorpion toxins suggest that the two blanks in the sequences are cysteines. The molecular weight (M.W.) of BMK 11(2) was determined by LC/MS/MS to be 7216 Da. Voltage-clamp experiments conducted on plasmid-transfected human kidney cells expressing the alpha and beta subunits of the rat sodium channel showed that BMK 11(2) acted to prolong Na channel inactivation. Also, in the presence of 100-200 nM BMK 11(2), a persistent non-activating Na current was induced when the membrane was depolarized from a -120 mV holding potential. BMK 11(2) caused Na channel fast inactivation to be further slowed when the holding potential was increased, suggesting that BMK 11(2) effects are voltage dependent. Na channel slow inactivation and return from slow inactivation were unaffected by the presence of BMK 11(2). Since the polypeptide prolongs APs when both K+ and Ca+ channels were blocked and shows sequence similarity to other alpha-neurotoxins, it appears likely that BMK 11(2) acts to selectively alter Na channel inactivation to produce its effect.     

Differential modulation of Nav1.7 and Nav1.8 peripheral nerve sodium channels by the local anesthetic lidocaine

1 Voltage-gated Na+ channels are transmembrane proteins that are essential for the propagation of action potentials in excitable cells. Nav1.7 and Nav1.8 dorsal root ganglion Na+ channels exhibit different kinetics and sensitivities to tetrodotoxin (TTX). We investigated the properties of both channels in the presence of lidocaine, a local anesthetic (LA) and class I anti-arrhythmic drug. 2 Nav1.7 and Nav1.8 Na+ channels were coexpressed with the beta1-subunit in Xenopus oocytes. Na+ currents were recorded using the two-microelectrode voltage-clamp technique. 3 Dose-response curves for both channels had different EC50 (dose producing 50% maximum current inhibition) (450 microm for Nav1.7 and 104 microm for Nav1.8). Lidocaine enhanced current decrease in a frequency-dependent manner. Steady-state inactivation of both channels was also affected by lidocaine, Nav1.7 being the most sensitive. Only the steady-state activation of Nav1.8 was affected while the entry of both channels into slow inactivation was affected by lidocaine, Nav1.8 being affected to a larger degree. 4 Although the channels share homology at DIV S6, the LA binding site, they differ in their sensitivity to lidocaine. Recent studies suggest that other residues on DI and DII known to influence lidocaine binding may explain the differences in affinities between Nav1.7 and Nav1.8 Na+ channels. 5 Understanding the properties of these channels and their pharmacology is of critical importance to developing drugs and finding effective therapies to treat chronic pain.     

Isoflurane and propofol inhibit voltage-gated sodium channels in isolated rat neurohypophysial nerve terminals

Mounting electrophysiological evidence indicates that certain general anesthetics, volatile anesthetics in particular, depress excitatory synaptic transmission by presynaptic mechanisms. We studied the effects of representative general anesthetics on voltage-gated Na+ currents (INa) in nerve terminals isolated from rat neurohypophysis using patch-clamp electrophysiological analysis. Both isoflurane and propofol inhibited INa in a dose-dependent and reversible manner. At holding potentials of -70 or -90 mV, isoflurane inhibited peak INa with IC50 values of 0.45 and 0.56 mM, and propofol inhibited peak INa with IC50 values of 4.1 and 6.0 microM, respectively. Isoflurane (0.8 mM) did not significantly alter the V1/2 of activation; propofol caused a small positive shift. Isoflurane (0.8 mM) or propofol (5 microM) produced a negative shift in the voltage dependence of inactivation. Recovery of INa from inactivation was slower from a holding potential of -70 mV than from -90 mV; isoflurane and propofol further delayed recovery from inactivation. In conclusion, the volatile anesthetic isoflurane and the intravenous anesthetic propofol inhibit voltage-gated Na+ currents in isolated neurohypophysial nerve terminals in a concentration- and voltage-dependent manner. Marked effects on the voltage dependence and kinetics of inactivation and minimal effects on activation support preferential anesthetic interactions with the fast inactivated state of the Na+ channel. These results are consistent with direct inhibition of oxytocin and vasopressin release from the neurohypophysis by isoflurane and propofol. Inhibition of voltage-gated Na+ channels may contribute to the presynaptic effects of general anesthetics on nerve terminal excitability and neurotransmitter release.     

Electrophysiological effect of l-cis-diltiazem, the stereoisomer of d-cis-diltiazem, on isolated guinea-pig left ventricular myocytes

l-cis-Diltiazem, the stereoisomer of the L-type Ca(2+) channel blocker d-cis-diltiazem, protects cardiac myocytes from ischemia and reperfusion injury in the perfused heart and from veratridine-induced Ca(2+) overload. We determined the effect of l-cis-diltiazem on the voltage-dependent Na(+) current (I(Na)) and lysophosphatidylcholine-induced currents in isolated guinea-pig left ventricular myocytes by a whole-cell patch-clamp technique. l-cis-Diltiazem inhibited I(Na) in a dose-dependent manner without altering the current-voltage relationship for I(Na) (K(d) values : 729 and 9 microM at holding potentials of -140 and -80 mV, respectively). A use-dependent block of I(Na), the leftward shift of the steady-state inactivation curve and the delay of recovery from inactivation suggest that l-cis-diltiazem has a higher affinity for the inactivated state of Na(+) channels. In addition to I(Na), the lysophosphatidylcholine-induced currents were inhibited by l-cis-diltiazem in a similar concentration range. It is suggested that inhibition of both Na(+) channels and lysophosphatidylcholine-activated non-selective cation channels contributes to the cardioprotective effect of l-cis-diltiazem.     

Differential blockade of neuronal voltage-gated Na(+) and K(+) channels by antidepressant drugs

The effects of a range of antidepressants were investigated on neuronal voltage-gated Na(+) and K(+) channels. With the exception of phenelzine, all antidepressants inhibited batrachotoxin-stimulated 22Na(+) uptake, most likely via negative allosteric inhibition of batrachotoxin binding to neurotoxin receptor site-2 on the Na(+) channel. Imipramine also produced a differential action on macroscopic Na(+) and K(+) channel currents in acutely dissociated rat dorsal root ganglion neurons. Imipramine produced a use-dependent block of Na(+) channels. In addition, there was a hyperpolarizing shift in the voltage-dependence of steady-state Na(+) channel inactivation and slowed repriming kinetics consistent with imipramine having a higher affinity for the inactivated state of the Na(+) channel. At higher concentrations, imipramine also blocked delayed-rectifier and transient outward K(+) currents in the absence of alterations to the voltage-dependence of activation or the kinetics of inactivation. These actions on voltage-gated ion channels may underlie the therapeutic and toxic effects of these drugs.     

Structure and function of delta-atracotoxins: lethal neurotoxins targeting the voltage-gated sodium channel.

Delta-atracotoxins (delta-ACTX), isolated from the venom of Australian funnel-web spiders, are responsible for the potentially lethal envenomation syndrome seen following funnel-web spider envenomation. They are 42-residue polypeptides with four disulfides and an "inhibitor cystine-knot" motif with structural but not sequence homology to a variety of other spider and marine snail toxins. Delta-atracotoxins induce spontaneous repetitive firing and prolongation of action potentials resulting in neurotransmitter release from somatic and autonomic nerve endings. This results from a slowing of voltage-gated sodium channel inactivation and a hyperpolarizing shift of the voltage-dependence of activation. This action is due to voltage-dependent binding to neurotoxin receptor site-3 in a similar, but not identical, fashion to scorpion alpha-toxins and sea anemone toxins. Unlike other site-3 neurotoxins, however, delta-ACTX bind with high affinity to both cockroach and mammalian sodium channels but low affinity to locust sodium channels. At present the pharmacophore of delta-ACTX is unknown but is believed to involve a number of basic residues distributed in a topologically similar manner to scorpion alpha-toxins and sea anemone toxins despite distinctly different protein scaffolds. As such, delta-ACTX provide us with specific tools with which to study sodium channel structure and function and determinants for phyla- and tissue-specific actions of neurotoxins interacting with site-3.     

Potent modulation of the voltage-gated sodium channel Nav1.7 by OD1, a toxin from the scorpion Odonthobuthus doriae

Voltage-gated sodium channels are essential for the propagation of action potentials in nociceptive neurons. Nav1.7 is found in peripheral sensory and sympathetic neurons and involved in short-term and inflammatory pain. Nav1.8 and Nav1.3 are major players in nociception and neuropathic pain, respectively. In our effort to identify isoform-specific and high-affinity ligands for these channels, we investigated the effects of OD1, a scorpion toxin isolated from the venom of the scorpion Odonthobuthus doriae. Nav1.3, Nav1.7, and Nav1.8 channels were coexpressed with beta1-subunits in Xenopus laevis oocytes. Na+ currents were recorded with the two-electrode voltage-clamp technique. OD1 modulates Nav1.7 at low nanomolar concentrations: 1) fast inactivation is dramatically impaired, with an EC50 value of 4.5 nM; 2) OD1 substantially increases the peak current at all voltages; and 3) OD1 induces a substantial persistent current. Nav1.8 was not affected by concentrations up to 2 microM, whereas Nav1.3 was sensitive only to concentrations higher than 100 nM. OD1 impairs the inactivation process of Nav1.3 with an EC50 value of 1127 nM. Finally, the effects of OD1 were compared with a classic alpha-toxin, AahII from Androctonus australis Hector and a classic alpha-like toxin, BmK M1 from Buthus martensii Karsch. At a concentration of 50 nM, both toxins affected Nav1.7. Nav1.3 was sensitive to AahII but not to BmK M1, whereas Nav1.8 was affected by neither toxin. In conclusion, the present study shows that the scorpion toxin OD1 is a potent modulator of Nav1.7, with a unique selectivity pattern.