Scorpion alpha and alpha-like toxins differentially interact with sodium channels in mammalian CNS and periphery

Methionine 58 is a key residue in the modulation of BmK scorpion toxin AGP-SYPU2 activity through in silico and in vivo studies

Protein dynamic networks play an important role in the regulation of many protein systems. Some residues that are far away from the interface between proteins and their targets have a critical role in modulating the activity of some scorpion toxins. Here, conservation analysis combined with an in vivo experiment has reveals that Met58 is a key residue of BmK scorpion toxin AGP-SYPU2 in the modulation of analgesic activity. Molecular dynamics simulations clearly reveal the conformational changes that allow the loop between the β2 and β3 sheets to be exposed on the toxin surface to interact with its targets. Our results emphasize specific roles for the residue Met58 in the NC domain and our work gives valuable information for further modification of scorpion toxins to obtain new analgesic peptides with enhanced activity. Communicated by Ramaswamy H. Sarma.

Amazonian scorpions and scorpionism: integrating toxinological, clinical, and phylogenetic data to combat a human health crisis in the world's most diverse rainfores

Venom from Amazonian scorpions of the genus Tityus contains components capable of eliciting a distinct clinical, mostly neurological, syndrome. This contrasts with the mainly autonomic manifestations produced after envenomation by congeneric southern and northern South American species. Herein, we summarize Pan-Amazonian scorpionism by synthesizing available toxinological, clinical, and molecular data gathered from all affected areas in Amazonia, including Brazil, Ecuador, Colombia, Peru, Venezuela, and French Guiana. We searched multiple databases, as well as our own records, for reports of scorpion envenomations in Amazonia by confirmed Tityus spp., and compared the clinical manifestations. To help uncover clinical and venom relationships among problematic species, we explored phylogenetic relationships with a rate-calibrated analysis of mitochondrial COI data from available species. The possible existence of diversity gradients for venom toxic and immunogenic components despite the predicted strong phylogenetic association among species is underscored by discussed clinical and toxinological findings. A multicentric effort, involving all nations affected by this neglected disease, is urgently needed to offer alternatives for treating and understanding this pathology, including the preparation of neutralizing antibodies with a broad range of efficacy.

Scorpion toxin peptide action at the ion channel subunit level

This review categorizes functionally validated actions of defined scorpion toxin (SCTX) neuropeptides across ion channel subclasses, highlighting key trends in this rapidly evolving field. Scorpion envenomation is a common event in many tropical and subtropical countries, with neuropharmacological actions, particularly autonomic nervous system modulation, causing significant mortality. The primary active agents within scorpion venoms are a diverse group of small neuropeptides that elicit specific potent actions across a wide range of ion channel classes. The identification and functional characterisation of these SCTX peptides has tremendous potential for development of novel pharmaceuticals that advance knowledge of ion channels and establish lead compounds for treatment of excitable tissue disorders. This review delineates the unique specificities of 320 individual SCTX peptides that collectively act on 41 ion channel subclasses. Thus the SCTX research field has significant translational implications for pathophysiology spanning neurotransmission, neurohumoral signalling, sensori-motor systems and excitation-contraction coupling. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Two recombinant α-like scorpion toxins from Mesobuthus eupeus with differential affinity toward insect and mammalian Na(+) channels

α-Scorpion toxins are modulators of voltage-gated Na(+) channels (Navs), which bind to the receptor site 3 to inhibit the fast inactivation of the channels. MeuNaTxα-12 and MeuNaTxα-13 are two new α-scorpion toxin-like peptides identified by cDNA cloning from the scorpion Mesobuthus eupeus with unknown functions. Here, we report their recombinant production, oxidative refolding, structural and functional features. By in vitro renaturation from bacterial inclusion bodies and further purification through reverse phase high-performance liquid chromatography, we obtained high purity recombinant products with a native-like conformation identified by circular dichroism analysis. Two-electrode voltage clamp recordings on five cloned mammalian Nav subtypes (rNav1.1, rNav1.2, rNav1.4, rNav1.5, and mNav1.6) and the insect counterpart DmNav1, all expressed in Xenopus laevis oocytes, showed that these two peptides inhibited rapid inactivation of the sensitive Na(+) channels with significant preference for DmNav1. The half maximal effective concentrations (EC50) of MeuNaTxα-12 and MeuNaTxα-13 for this channel are 19.95 ± 2.99 nM and 65.50 ± 7.28 nM, respectively, showing 45 and 38 folds higher affinities than for rNav1.1, the most sensitive mammalian channel among the five isoforms. Our functional data confirms that these two peptides belong to the α-like scorpion toxin group. A combined analysis of the site 3 sequences and the pharmacological data illuminates the importance of the loop LD4:S5-S6 of the channel in interacting with the toxins whereas affinity variations between MeuNaTxα-12 and MeuNaTxα-13 highlight a key functional role of a cationic side chain at position 28 of MeuNaTxα-12. Successful expression together with structural and functional characterization of these two new α-like scorpion toxins lays basis for further studies of their structure-function relationship.

Molecular determination of selectivity of the site 3 modulator (BmK I) to sodium channels in the CNS: a clue to the importance of Nav1.6 in BmK I-induced neuronal hyperexcitability

BmK I, a site-3-specific modulator of VGSCs (voltage-gated sodium channels) from the Chinese scorpion Buthus martensi Karsch, can induce spontaneous nociception and hyperalgesia and generate epileptiform responses in rats, which is attributed to the modulation of VGSCs in the neural system. However, which VGSC subtype is targeted by BmK I remains to be identified. Using two-electrode voltage-clamp recording, we studied the efficacy and selectivity of BmK I to three neuronal VGSCs co-expressed with the auxiliary β1 subunit in Xenopus oocytes. Results revealed that BmK I induced a large increase in both transient and persistent currents in mNav1.6α/β1 (where m indicates mouse), which correlated with a prominent reduction in the fast component of inactivating current. In comparison, BmK I-increased currents of rNav1.2α/β1 (where r indicates rat) and rNav1.3α/β1 were much smaller. The EC50 values of BmK I for rNav1.2α/β1 (252±60 nM) and mNav1.6α/β1 (214±30 nM) were similar and roughly half of that for rNav1.3α/β1 (565±16 nM). Moreover, BmK I only accelerated the slow inactivation development and delay recovery of mNav1.6α/β1 through binding to the channel in the open state. Residue-swap analysis verified that an acidic residue (e.g. Asp1602 in mNav1.6) within the domain IV S3-S4 extracellular loop of VGSCs was crucial for the selectivity and modulation pattern of BmK I. Our findings thus provide the molecular determinant explaining the divergent and intriguing behaviour of neuronal VGSCs in response to site-3-specific modulators, indicating that these subtypes play different roles in BmK I-induced hyperexcitablity in rat models.

Molecular requirements for recognition of brain voltage-gated sodium channels by scorpion alpha-toxins

The scorpion alpha-toxin Lqh2 (from Leiurus quinquestriatus hebraeus) is active at various mammalian voltage-gated sodium channels (Na(v)s) and is inactive at insect Na(v)s. To resolve the molecular basis of this preference we used the following strategy: 1) Lqh2 was expressed in recombinant form and key residues important for activity at the rat brain channel rNa(v)1.2a were identified by mutagenesis. These residues form a bipartite functional surface made of a conserved "core domain" (residues of the loops connecting the secondary structure elements of the molecule core), and a variable "NC domain" (five-residue turn and the C-tail) as was reported for other scorpion alpha-toxins. 2) The functional role of the two domains was validated by their stepwise construction on the similar scaffold of the anti-insect toxin LqhalphaIT. Analysis of the activity of the intermediate constructs highlighted the critical role of Phe(15) of the core domain in toxin potency at rNa(v)1.2a, and has suggested that the shape of the NC-domain is important for toxin efficacy. 3) Based on these findings and by comparison with other scorpion alpha-toxins we were able to eliminate the activity of Lqh2 at rNa(v)1.4 (skeletal muscle), hNa(v)1.5 (cardiac), and rNa(v)1.6 channels, with no hindrance of its activity at Na(v)1.1-1.3. These results suggest that by employing a similar approach the design of further target-selective sodium channel modifiers is imminent.

Effects of BmKNJX11, a bioactive polypeptide purified from Buthus martensi Karsch, on sodium channels in rat dorsal root ganglion neurons

A long-chain polypeptide BmKNJX11 was purified from the venom of Asian scorpion Buthus martensi Karsch (BmK) by a combination of gel filtration, ion-exchange chromatography, and reverse-phase high-performance liquid chromatography. The molecular mass was found to be 7036.85 Da by electrospray ionization mass spectrometry. The first 15 N-terminal amino acid sequence of BmKNJX11 was determined to be GRDAY IADSE NCTYT by Edman degradation. With whole cell recording, BmKNJX11 inhibited tetrodotoxin-sensitive voltage-gated sodium channels (TTX-S VGSC) in freshly isolated rat dorsal root ganglion (DRG) neurons in a concentration- and voltage-dependent manner. At a concentration of 40 mug/ml BmKNJX11 lowered the activation threshold and produced negative shifting of TTX-S sodium current (I(Na)) activation curve. In addition, BmKNJX11 induced shifting of the steady-state inactivation curve to the left, delayed the recovery of TTX-S I(Na) from inactivation, and also reduced the fraction of available sodium channels. These results suggested that BmKNJX11 might exert effects on VGSC by binding to a specific site. Considering that TTX-S VGSC expressed in DRG neurons play a critical role in nociceptive transmission, the interaction of BmKNJX11 with TTX-S VGSC might lead to a change in excitability of nociceptive afferent fibers, which may be involved in the observed peripheral pain expression.

Mammalian Skeletal Muscle Voltage-Gated Sodium Channels Are Affected by Scorpion Depressant “Insect-Selective” Toxins when Preconditioned

Among scorpion beta- and alpha-toxins that modify the activation and inactivation of voltage-gated sodium channels (Na(v)s), depressant beta-toxins have traditionally been classified as anti-insect selective on the basis of toxicity assays and lack of binding and effect on mammalian Na(v)s. Here we show that the depressant beta-toxins LqhIT2 and Lqh-dprIT3 from Leiurus quinquestriatus hebraeus (Lqh) bind with nanomolar affinity to receptor site 4 on rat skeletal muscle Na(v)s, but their effect on the gating properties can be viewed only after channel preconditioning, such as that rendered by a long depolarizing prepulse. This observation explains the lack of toxicity when depressant toxins are injected in mice. However, when the muscle channel rNa(v)1.4, expressed in Xenopus laevis oocytes, was modulated by the site 3 alpha-toxin LqhalphaIT, LqhIT2 was capable of inducing a negative shift in the voltage-dependence of activation after a short prepulse, as was shown for other beta-toxins. These unprecedented results suggest that depressant toxins may have a toxic impact on mammals in the context of the complete scorpion venom. To assess whether LqhIT2 and Lqh-dprIT3 interact with the insect and rat muscle channels in a similar manner, we examined the role of Glu24, a conserved "hot spot" at the bioactive surface of beta-toxins. Whereas substitutions E24A/N abolished the activity of both LqhIT2 and Lqh-dprIT3 at insect Na(v)s, they increased the affinity of the toxins for rat skeletal muscle channels. This result implies that depressant toxins interact differently with the two channel types and that substitution of Glu24 is essential for converting toxin selectivity.

The unique pharmacology of the scorpion α-like toxin Lqh3 is associated with its flexible C-tail

The affinity of scorpion alpha-toxins for various voltage-gated sodium channels (Na(v)s) differs considerably despite similar structures and activities. It has been proposed that key bioactive residues of the five-residue-turn (residues 8-12) and the C-tail form the NC domain, whose topology is dictated by a cis or trans peptide-bond conformation between residues 9 and 10, which correlates with the potency on insect or mammalian Na(v)s. We examined this hypothesis using Lqh3, an alpha-like toxin from Leiurus quinquestriatus hebraeus that is highly active in insects and mammalian brain. Lqh3 exhibits slower association kinetics to Na(v)s compared with other alpha-toxins and its binding to insect Na(v)s is pH-dependent. Mutagenesis of Lqh3 revealed a bi-partite bioactive surface, composed of the Core and NC domains, as found in other alpha-toxins. Yet, substitutions at the five-residue turn and stabilization of the 9-10 bond in the cis conformation did not affect the activity. However, substitution of hydrogen-bond donors/acceptors at the NC domain reduced the pH-dependency of toxin binding, while retaining its high potency at Drosophila Na(v)s expressed in Xenopus oocytes. Based on these results and the conformational flexibility and rearrangement of intramolecular hydrogen-bonds at the NC domain, evident from the known solution structure, we suggest that acidic pH or specific mutations at the NC domain favor toxin conformations with high affinity for the receptor by stabilizing the bound toxin-receptor complex. Moreover, the C-tail flexibility may account for the slower association rates and suggests a novel mechanism of dynamic conformer selection during toxin binding, enabling alpha-like toxins to affect a broad range of Na(v)s.

The differential preference of scorpion α-toxins for insect or mammalian sodium channels: Implications for improved insect control

Receptor site-3 on voltage-gated sodium channels is targeted by a variety of structurally distinct toxins from scorpions, sea anemones, and spiders whose typical action is the inhibition of sodium current inactivation. This site interacts allosterically with other topologically distinct receptors that bind alkaloids, lipophilic polyether toxins, pyrethroids, and site-4 scorpion toxins. These features suggest that design of insecticides with specificity for site-3 might be rewarding due to the positive cooperativity with other toxins or insecticidal agents. Yet, despite the central role of scorpion alpha-toxins in envenomation and their vast use in the study of channel functions, molecular details on site-3 are scarce. Scorpion alpha-toxins vary greatly in preference for sodium channels of insects and mammals, and some of them are highly active on insects. This implies that despite its commonality, receptor site-3 varies on insect vs. mammalian channels, and that elucidation of these differences could potentially be exploited for manipulation of toxin preference. This review provides current perspectives on (i) the classification of scorpion alpha-toxins, (ii) their mode of interaction with sodium channels and pharmacological divergence, (iii) molecular details on their bioactive surfaces and differences associated with preference for channel subtypes, as well as (iv) a summary of the present knowledge about elements involved in constituting receptor site-3. These details, combined with the variations in allosteric interactions between site-3 and the other receptor sites on insect and mammalian sodium channels, may be useful in new strategies of insect control and future design of anti-insect selective ligands.

Allosteric interactions between scorpion toxin receptor sites on voltage‐gated Na channels imply a novel role for weakly active components in arthropod venom

Scorpion beta and alpha-toxins modify the activation and inactivation of voltage-gated sodium channels. Although the two types of toxin bind at two distinct receptor sites on the same sodium channel, they exhibit synergic effects when coinjected into insects. To clarify the basis of this synergism we examined the mutual effects of alpha and beta toxin representatives in radio-ligand binding assays. We found positive allosteric interactions between receptor site-4 of the excitatory Bj-xtrIT and the depressant LqhIT2 beta toxins and receptor site-3 of the alpha toxin LqhalphaIT, on locust neuronal membranes. Unexpectedly, a nontoxic mutant Bj-xtrIT-E15R, which binds with high affinity to receptor site-4, was able to enhance LqhalphaIT binding and toxicity similarly to the unmodified Bj-xtrIT. This result indicates that mere binding of a nontoxic ligand to receptor site-4 ("silent binding") induces a conformational change that does not alter channel gating, but influences toxin binding at receptor site-3 leading to enhanced toxicity. This finding suggests a new functional role for weakly toxic polypeptides in that they enhance the effect of other active neurotoxins in the arthropod venom. Such silent binding may have also valuable implications in attempts to improve drug efficacy by combining potent drugs with nonactive allosteric enhancers.

Oxaliplatin induces hyperexcitability at motor and autonomic neuromuscular junctions through effects on voltage-gated sodium channels

Oxaliplatin, an effective cytotoxic treatment in combination with 5-fluorouracil for colorectal cancer, is associated with sensory, motor and autonomic neurotoxicity. Motor symptoms include hyperexcitability while autonomic effects include urinary retention, but the cause of these side-effects is unknown. We examined the effects on motor nerve function in the mouse hemidiaphragm and on the autonomic system in the vas deferens. In the mouse diaphragm, oxaliplatin (0.5 mM) induced multiple endplate potentials (EPPs) following a single stimulus, and was associated with an increase in spontaneous miniature EPP frequency. In the vas deferens, spontaneous excitatory junction potential frequency was increased after 30 min exposure to oxaliplatin; no changes in resting Ca(2+) concentration in nerve terminal varicosities were observed, and recovery after stimuli trains was unaffected. In both tissues, an oxaliplatin-induced increase in spontaneous activity was prevented by the voltage-gated Na(+) channel blocker tetrodotoxin (TTX). Carbamazepine (0.3 mM) also prevented multiple EPPs and the increase in spontaneous activity in both tissues. In diaphragm, beta-pompilidotoxin (100 microM), which slows Na(+) channel inactivation, induced multiple EPPs similar to oxaliplatin's effect. By contrast, blockers of K(+) channels (4-aminopyridine and apamin) did not replicate oxaliplatin-induced hyperexcitability in the diaphragm. The prevention of hyperexcitability by TTX blockade implies that oxaliplatin acts on nerve conduction rather than by effecting repolarisation. The similarity between beta-pompilidotoxin and oxaliplatin suggests that alteration of voltage-gated Na(+) channel kinetics is likely to underlie the acute neurotoxic actions of oxaliplatin.

Efficacy of antiepileptic isomers of valproic acid and valpromide in a rat model of neuropathic pain

Antiepileptic drugs (AEDs) are often utilized in the treatment of neuropathic pain. The major AED valproic acid (VPA) is of particular interest as it is thought to engage a variety of different neural mechanisms simultaneously. However, the clinical use of VPA is limited by two rare but life-threatening side effects: teratogenicity and hepatotoxicity. We synthesized VPA's corresponding amide: valpromide (VPD), two of VPAs isomers and their corresponding amides; valnoctic acid (VCA), valnoctamide (VCD), diisopropyl acetic acid (DIA), diisopropylacetamide (DID), and VPD's congener: N-methyl-VPD (MVPD). VCD, DID and VPD are nonteratogenic, potentially nonhepatotoxic, and exhibit better anticonvuslant potency than VPA. In this study, we assessed the antiallodynic activity of these compounds in comparison to VPA and gabapentin (GBP) using the rat spinal nerve ligation model of neuropathic pain (SNL, Chung model). VCA and MVPD were inactive. However, VPD (20-100 mg kg(- 1)), VCD (20-100 mg kg(- 1)) and DID (20-90 mg kg(- 1)) produced dose-related reversal of tactile allodynia with ED50 values of 61, 52 and 58 mgkg(- 1), respectively. All the amides were more potent than VPA (ED50=269 mgkg(- 1)). The antiallodynic effect of VPA, VPD, VCD and DID was obtained at plasma concentrations of 125, 24, 18 and 7 mg l(- 1), respectively, with a good pharmacokinetic-pharmacodynamic correlation and a minimal lag response. VCD and DID were found to have minimal motor and sedative side effects at analgesic doses, and were equipotent to GBP, currently the leading drug in neuropathic pain treatment. Consequently, VCD and DID have potential to become new drugs for the treatment of neuropathic pain.

Effects of δ-Conotoxins PVIA and SVIE on Sodium Channels in the Amphibian Sympathetic Nervous System

δ-Conotoxins are a family of small, disulfide-rich peptides found in the venoms of predatory cone snails ( Conus). We examined in detail the effects of δ-conotoxin PVIA from the fish hunting cone snail Conus purpurascens on sodium currents in dissociated sympathetic neurons from the leopard frog Rana pipiens. We also compared this toxin’s effects with those of δ-conotoxin SVIE from Conus striatus, another piscivorous cone snail. d-PVIA slowed the time-course of inactivation of δ sodium currents and shifted the voltage-dependence of activation and steady-state inactivation to more hyperpolarized potentials. Similar, albeit more pronounced, effects were seen with d-SVIE. While the effects of d-PVIA were reversed by washing, those of d-SVIE were largely irreversible over the time-course of these experiments. The effects of d-PVIA could be suppressed by conditioning depolarizations in a voltage- and time-dependent manner, whereas the effects of d-SVIE were largely resistant to conditioning depolarizations. Last, in intact sympathetic nervous system preparations, d-PVIA inhibited evoked trains of compound action potentials. Many of these effects of d-PVIA and d-SVIE are remarkably similar to those of toxins that bind to site 3 on voltage-gated sodium channels.

Structure-activity relationship of an alpha-toxin Bs-Tx28 from scorpion (Buthus sindicus) venom suggests a new alpha-toxin subfamily

Scorpion venoms are among the most widely known source of peptidyl neurotoxins used for callipering different ion channels, e.g., for Na(+), K(+), Ca(+) or Cl(-). An alpha-toxin (Bs-Tx28) has been purified from the venom of scorpion Buthus sindicus, a common yellow scorpion of Sindh, Pakistan. The primary structure of Bs-Tx28 was established using a combination of MALDI-TOF-MS, LC-ESI-MS, and automated Edman degradation analysis. Bs-Tx28 consists of 65 amino acid residues (7274.3+/-2Da), including eight cysteine residues, and shows very high sequence identity (82-94%) with other long-chain alpha-neurotoxins, active against receptor site-3 of mammalian (e.g., Lqq-IV and Lqh-IV from scorpions Leiurus sp.) and insect (e.g., BJalpha-IT and Od-1 from Buthotus judaicus and Odonthobuthus doriae, respectively) voltage-gated Na(+) channels. Multiple sequence alignment and phylogenetic analysis of Bs-Tx28 with other known alpha- and alpha-like toxins suggests the presence of a new and separate subfamily of scorpion alpha-toxins. Bs-Tx28 which is weakly active in both, mammals and insects (LD(50) 0.088 and 14.3microg/g, respectively), shows strong induction of the rat afferent nerve discharge in a dose-dependent fashion (EC(50)=0.01microg/mL) which was completely abolished in the presence of tetrodotoxin suggesting the binding of Bs-Tx28 to the TTX-sensitive Na(+)-channel. Three-dimensional structural features of Bs-Tx28, established by homology modeling, were compared with other known classical alpha-mammal (AaH-II), alpha-insect (Lqh-alphaIT), and alpha-like (BmK-M4) toxins and revealed subtle variations in the Nt-, Core-, and RT-CT-domains (functional domains) which constitute a "necklace-like" structure differing significantly in all alpha-toxin subfamilies. On the other hand, a high level of conservation has been observed in the conserved hydrophobic surface with the only substitution of W43 (Y43/42) and an additional hydrophobic character at position F40 (L40/A/V/G39), as compared to the other mentioned alpha-toxins. Despite major differences within the primary structure and activities of Bs-Tx28, it shares a common structural and functional motif (e.g., transRT-farCT) within the RT-CT domain which is characteristic of scorpion alpha-mammal toxins.

Overview of scorpion toxins specific for Na+ channels and related peptides: biodiversity, structure-function relationships and evolution

Scorpion venoms contain a large number of bioactive components. Several of the long-chain peptides were shown to be responsible for neurotoxic effects, due to their ability to recognize Na(+) channels and to cause impairment of channel functions. Here, we revisited the basic paradigms in the study of these peptides in the light of recent data concerning their structure-function relationships, their functional divergence and extant biodiversity. The reviewed topics include: the criteria for classification of long-chain peptides according to their function, and a revision of the state-of-the-art knowledge concerning the surface areas of contact of these peptides with known Na(+) channels. Additionally, we compiled a comprehensive list encompassing 191 different amino acid sequences from long-chain peptides purified from scorpion venoms. With this dataset, a phylogenetic tree was constructed and discussed taking into consideration their documented functional divergence. A critical view on problems associated with the study of these scorpion peptides is presented, drawing special attention to the points that need revision and to the subjects under intensive research at this moment, regarding scorpion toxins specific for Na(+) channels and the other related long-chain peptides recently described.

Sodium Channel Inactivation: Molecular Determinants and Modulation

Voltage-gated sodium channels open (activate) when the membrane is depolarized and close on repolarization (deactivate) but also on continuing depolarization by a process termed inactivation, which leaves the channel refractory, i.e., unable to open again for a period of time. In the “classical” fast inactivation, this time is of the millisecond range, but it can last much longer (up to seconds) in a different slow type of inactivation. These two types of inactivation have different mechanisms located in different parts of the channel molecule: the fast inactivation at the cytoplasmic pore opening which can be closed by a hinged lid, the slow inactivation in other parts involving conformational changes of the pore. Fast inactivation is highly vulnerable and affected by many chemical agents, toxins, and proteolytic enzymes but also by the presence of β-subunits of the channel molecule. Systematic studies of these modulating factors and of the effects of point mutations (experimental and in hereditary diseases) in the channel molecule have yielded a fairly consistent picture of the molecular background of fast inactivation, which for the slow inactivation is still lacking.

Efficacy of antiepileptic tetramethylcyclopropyl analogues of valproic acid amides in a rat model of neuropathic pain

Antiepileptic drugs (AEDs) are widely utilized in the management of neuropathic pain. The AED valproic acid (VPA) holds out particular promise as it engages a variety of different anticonvulsant mechanisms simultaneously. However, the clinical use of VPA is limited by two rare but potentially life-threatening side effects: teratogenicity and hepatotoxicity. We have synthesized several tetramethylcyclopropyl analogues of VPA amides that are non-teratogenic, and are likely to be non-hepatotoxic, and that exhibit good antiepileptic efficacy. In the present study we have assessed the antiallodynic activity of these compounds in comparison to VPA and gabapentin (GBP) using the rat spinal nerve ligation (SNL) model of neuropathic pain. TMCA (2,2,3,3-tetramethylcyclopropanecarboxylic acid, 100-250 mg/kg), TMCD (2,2,3,3-tetramethylcyclopropanecarboxamide, 40-150 mg/kg), MTMCD (N-methyl-TMCD, 20-100 mg/kg), and TMCU (2,2,3,3-tetramethylcyclopropanecarbonylurea, 40-240 mg/kg) all showed dose-related reversal of tactile allodynia, with ED(50) values of 181, 85, 41, and 171 mg/kg i.p., respectively. All were more potent than VPA (ED(50)=269 mg/kg). An antiallodynic effect was obtained for TMCD, MTMCD and TMCU at plasma concentrations as low as 23, 6 and 22 mg/L, respectively. MTMCD was found to be non-toxic, non-sedative and equipotent to gabapentin, currently the leading AED in neuropathic pain treatment. Tetramethylcyclopropyl analogues of VPA amides have potential to become a new series of drugs for neuropathic pain treatment.

BjalphaIT: a novel scorpion alpha-toxin selective for insects--unique pharmacological tool

Long-chain neurotoxins derived from the venom of the Buthidae scorpions, which affect voltage-gated sodium channels (VGSCs) can be subdivided according to their toxicity to insects into insect-selective excitatory and depressant toxins (beta-toxins) and the alpha-like toxins which affect both mammals and insects. In the present study by the aid of reverse-phase HPLC column chromatography, RT-PCR, cloning and various toxicity assays, a new insect selective toxin designated as BjalphaIT was isolated from the venom of the Judean Black Scorpion (Buthotus judaicus), and its full primary sequence was determined: MNYLVVICFALLLMTVVESGRDAYIADNLNCAYTCGSNSYCNTECTKNGAVSGYCQWLGKYGNACWCINLPDKVPIRIPGACR (leader sequence is underlined). Despite its lack of toxicity to mammals and potent toxicity to insects, BjalphaIT reveals an amino acid sequence and an inferred spatial arrangement that is characteristic of the well-known scorpion alpha-toxins highly toxic to mammals. BjalphaITs sharp distinction between insects and mammals was also revealed by its effect on sodium conductance of two cloned neuronal VGSCs heterloguously expressed in Xenopus laevis oocytes and assayed with the two-electrode voltage-clamp technique. BjalphaIT completely inhibits the inactivation process of the insect para/tipE VGSC at a concentration of 100 nM, in contrast to the rat brain Na(v)1.2/beta1 which is resistant to the toxin. The above categorical distinction between mammal and insect VGSCs exhibited by BjalphaIT enables its employment in the clarification of the molecular basis of the animal group specificity of scorpion venom derived neurotoxic polypeptides and voltage-gated sodium channels.

Molecular basis of the high insecticidal potency of scorpion alpha-toxins

Scorpion alpha-toxins are similar in their mode of action and three-dimensional structure but differ considerably in affinity for various voltage-gated sodium channels (NaChs). To clarify the molecular basis of the high potency of the alpha-toxin LqhalphaIT (from Leiurus quinquestriatus hebraeus) for insect NaChs, we identified by mutagenesis the key residues important for activity. We have found that the functional surface is composed of two distinct domains: a conserved "Core-domain" formed by residues of the loops connecting the secondary structure elements of the molecule core and a variable "NC-domain" formed by a five-residue turn (residues 8-12) and a C-terminal segment (residues 56-64). We further analyzed the role of these domains in toxin activity on insects by their stepwise construction onto the scaffold of the anti-mammalian alpha-toxin, Aah2 (from Androctonus australis hector). The chimera harboring both domains, Aah2(LqhalphaIT(face)), was as active to insects as LqhalphaIT. Structure determination of Aah2(LqhalphaIT(face)) by x-ray crystallography revealed that the NC-domain deviates from that of Aah2 and forms an extended protrusion off the molecule core as appears in LqhalphaIT. Notably, such a protrusion is observed in all alpha-toxins active on insects. Altogether, the division of the functional surface into two domains and the unique configuration of the NC-domain illuminate the molecular basis of alpha-toxin specificity for insects and suggest a putative binding mechanism to insect NaChs.

Combinatorial interaction of scorpion toxins Lqh-2, Lqh-3, and LqhalphaIT with sodium channel receptor sites-3

Scorpion alpha-toxins LqhalphaIT, Lqh-2, and Lqh-3 are representatives of three groups of alpha-toxins that differ in their preference for insects and mammals. These alpha-insect, antimammalian, and alpha-like toxins bind to voltage-gated sodium channels and slow down channel inactivation. Sodium channel mutagenesis studies using various alpha-toxins have shown that they interact with receptor site 3, which is composed mainly of a short stretch of amino-acid residues between S3 and S4 of domain 4. Variation in this region results in marked differences between various subtypes of sodium channels with respect to their sensitivity to the three Lqh toxins. We incorporated the S3-S4 linker of domain 4 from hNaV1.2/hNaV1.1, hNaV1.3, hNaV1.6, and hNaV1.7 channels as well as individual point mutations into the rNaV1.4 skeletal muscle sodium channel. Our data show that the affinity of Lqh-3 and LqhalphaIT to sodium channels is markedly determined by an aspartate residue (Asp1428 in rNaV1.4); when mutated to glutamate, as is present in NaV1.1-1.3 channels, Lqh-3-channel interactions are abolished. The interaction of Lqh-2 and LqhalphaIT, however, is strongly reduced when a lysine residue (Lys1432 in rNaV1.4) is replaced by threonine (as in hNaV1.7), whereas this substitution is without effect for Lqh-3. The influence of Lys1432 on Lqh-2 and LqhalphaIT strongly depends on the context of the Asp/Glu site at position 1428, giving rise to a wide variety of toxicological phenotypes by means of a combinatorial mixing and matching of only a few residues in receptor site 3.

Characterization of Amm VIII from Androctonus mauretanicus mauretanicus: a new scorpion toxin that discriminates between neuronal and skeletal sodium channels

The venom of the scorpion Androctonus mauretanicus mauretanicus was screened by use of a specific serum directed against AaH II, the scorpion alpha-toxin of reference, with the aim of identifying new analogues. This led to the isolation of Amm VIII (7382.57 Da), which gave a highly positive response in ELISA, but was totally devoid of toxicity when injected subcutaneously into mice. In voltage-clamp experiments with rat brain type II Na+ channel rNa(v)1.2 or rat skeletal muscle Na+ channel rNa(v)1.4, expressed in Xenopus oocytes, the EC50 values of the toxin-induced slowing of inactivation were: 29+/-5 and 416+/-14 nM respectively for AmmVIII and 2.6+/-0.3 nM and 2.2+/-0.2 nM, respectively, for AaH II interactions. Accordingly, Amm VIII clearly discriminates neuronal versus muscular Na+ channel. The Amm VIII cDNA was amplified from a venom gland cDNA library and its oligonucleotide sequence determined. It shows 87% sequence homology with AaH II, but carries an unusual extension at its C-terminal end, consisting of an additional Asp due to a point mutation in the cDNA penultimate codon. We hypothesized that this extra amino acid residue could induce steric hindrance and dramatically reduce recognition of the target by Amm VIII. We constructed a model of Amm VIII based on the X-ray structure of AaH II to clarify this point. Molecular modelling showed that this C-terminal extension does not lead to an overall conformational change in Amm VIII, but drastically modifies the charge repartition and, consequently, the electrostatic dipole moment of the molecule. At last, liquid-phase radioimmunassays with poly- and monoclonal anti-(AaH II) antibodies showed the loss of conformational epitopes between AaH II and Amm VIII.

Distinct primary structures of the major peptide toxins from the venom of the spider Macrothele gigas that bind to sites 3 and 4 in the sodium channel

Six peptide toxins (Magi 1-6) were isolated from the Hexathelidae spider Macrothele gigas. The amino acid sequences of Magi 1, 2, 5 and 6 have low similarities to the amino acid sequences of known spider toxins. The primary structure of Magi 3 is similar to the structure of the palmitoylated peptide named PlTx-II from the North American spider Plectreurys tristis (Plectreuridae). Moreover, the amino acid sequence of Magi 4, which was revealed by cloning of its cDNA, displays similarities to the Na+ channel modifier delta-atracotoxin from the Australian spider Atrax robustus (Hexathelidae). Competitive binding assays using several 125I-labelled peptide toxins clearly demonstrated the specific binding affinity of Magi 1-5 to site 3 of the insect sodium channel and also that of Magi 5 to site 4 of the rat sodium channel. Only Magi 6 did not compete with the scorpion toxin LqhalphaIT in binding to site 3 despite high toxicity on lepidoptera larvae of 3.1 nmol/g. The K(i)s of other toxins were between 50 pM for Magi 4 and 1747 nM for Magi 1. In addition, only Magi 5 binds to both site 3 in insects (K(i)=267 nM) and site 4 in rat brain synaptosomes (K(i)=1.2 nM), whereas it showed no affinities for either mammal binding site 3 or insect binding site 4. Magi 5 is the first spider toxin with binding affinity to site 4 of a mammalian sodium channel.

An 'Old World' scorpion beta-toxin that recognizes both insect and mammalian sodium channels

Scorpion toxins that affect sodium channel (NaCh) gating in excitable cells are divided into alpha- and beta-classes. Whereas alpha-toxins have been found in scorpions throughout the world, anti-mammalian beta-toxins have been assigned, thus far, to 'New World' scorpions while anti-insect selective beta-toxins (depressant and excitatory) have been described only in the 'Old World'. This distribution suggested that diversification of beta-toxins into distinct pharmacological groups occurred after the separation of the continents, 150 million years ago. We have characterized a unique toxin, Lqhbeta1, from the 'Old World' scorpion, Leiurus quinquestriatus hebraeus, that resembles in sequence and activity both 'New World'beta-toxins as well as 'Old World' depressant toxins. Lqhbeta1 competes, with apparent high affinity, with anti-insect and anti-mammalian beta-toxins for binding to cockroach and rat brain synaptosomes, respectively. Surprisingly, Lqhbeta1 also competes with an anti-mammalian alpha-toxin on binding to rat brain NaChs. Analysis of Lqhbeta1 effects on rat brain and Drosophila Para NaChs expressed in Xenopus oocytes revealed a shift in the voltage-dependence of activation to more negative membrane potentials and a reduction in sodium peak currents in a manner typifying beta-toxin activity. Moreover, Lqhbeta1 resembles beta-toxins by having a weak effect on cardiac NaChs and a marked effect on rat brain and skeletal muscle NaChs. These multifaceted features suggest that Lqhbeta1 may represent an ancestral beta-toxin group in 'Old World' scorpions that gave rise, after the separation of the continents, to depressant toxins in 'Old World' scorpions and to various beta-toxin subgroups in 'New World' scorpions.

Exploration of the functional site of a scorpion alpha-like toxin by site-directed mutagenesis

Scorpion alpha-neurotoxins can be classified into distinct subgroups according to their sequence and pharmacological properties. Using toxicity tests, binding studies, and electrophysiological recordings, BmK M1, a toxin from the Asian scorpion Buthus martensi Karsch, was experimentally identified as an alpha-like toxin. Being the first alpha-like toxin available in a recombinant form, BmK M1 was then modified by site-directed mutagenesis for investigation of the molecular basis of its activity. The results suggested a functional site which protrudes from the molecular scaffold as a unique tertiary arrangement, constituted by the five-residue reverse turn 8-12 and the C-terminal segment. The C-terminal basic residues Lys62 and His64 together with Lys8 in the turn, which are critical for the bioactivities, may directly interact with the receptor site on the sodium channel. Residues Asn11 and Arg58, indispensable for the activities, are mainly responsible for stabilizing the distinct conformation of the putative bioactive site. Among others, His10 and His64 seem to be involved in the preference of BmK M1 for phylogenetically distinct target sites. The comparison of BmK M1 with Aah2 (classical alpha-toxin) and Lqh(alpha)IT (alpha-insect toxin) showed that the specific orientation of the C-terminus mediated by the reverse turn might be relevant to the preference of alpha-toxin subgroups for phylogenetically distinct yet closely related receptor sites. The Y5G mutation indicated the "conserved hydrophobic surface" might be structurally important for stabilizing the beta-sheet in the alpha/beta-scaffold. The observations in this work shed light on the nature and roles of the residues possibly involved in the biological activity of a scorpion alpha-like toxin.

Allosteric interactions among pyrethroid, brevetoxin, and scorpion toxin receptors on insect sodium channels raise an alternative approach for insect control

Intensive pyrethroid use in insect control has led to resistance buildup among various pests. One alternative to battle this problem envisions the combined use of synergistically acting insecticidal compounds. Pyrethroids, scorpion alpha- and beta-toxins, and brevetoxins bind to distinct receptor sites on voltage-gated sodium channels (NaChs) and modify their function. The binding affinity of scorpion alpha-toxins to locust, but not rat-brain NaChs, is allosterically increased by pyrethroids and by brevetoxin-1. Brevetoxin-1 also increases the binding of an excitatory beta-toxin to insect NaChs. These results reveal differences between insect and mammalian NaChs and may be exploited in new strategies of insect control.

Isoform-specific effects of a novel BmK 11(2) peptide toxin on Na channels

BmK 11(2) is a 7216Da polypeptide toxin purified from the venom of the scorpion Buthus martensii Karsch. Nanomolar concentrations of the toxin prolong amphibian nerve action potentials without attenuation of the amplitude. The pharmacological action of the toxin and its sequence similarity to other alpha-scorpion toxins suggest that BmK 11(2) selectively alters voltage-gated Na channels. In order to test whether BmK 11(2) preferentially modulates the gating or kinetics of certain channel isoforms, we applied BmK 11(2) to muscle, heart and neuronal Na channels. 100nM BmK 11(2) increased the peak current amplitude of skeletal muscle (micro1) and neuronal (N1E-115) Na currents by 40 and 20%, respectively, and reduced the cardiac Na (hH1) current by 15%. The toxin slowed current decay of all isoforms, most prominently in N1E-115 (tau(BmK)/tau(Control)=12), micro1 (11), and less so for hH1 (1.3). BmK 11(2) shifted the voltage dependence of activation of micro1 and N1E-115 currents. BmK 11(2) had no effect on steady-state inactivation, use-dependent availability, and the kinetics of entry into slowly recovering inactivated states.

Characterization of Na(+) currents in isolated dorsal unpaired median neurons of Locusta migratoria and effect of the alpha-like scorpion toxin BmK M1

A primary cell culture was developed for efferent dorsal unpaired median (DUM) neurons of the locust. The isolated somata were able to generate Tetrodotoxin (TTX)-sensitive action potentials in vitro. The alpha-like scorpion toxin BmK M1, from the Asian scorpion Buthus martensi Karsch, prolonged the duration of the action potential up to 50 times. To investigate the mechanism of action of BmK M1, the TTX-sensitive voltage gated Na(+) currents were studied in detail using the whole cell patch clamp technique. BmK M1 slowed down and partially inhibited the inactivation of the TTX-sensitive Na(+) current in a dose dependent manner (EC50=326.8+/-34.5 nM). Voltage and time dependence of the Na(+) current were described in terms of the Hodgkin-Huxley model and compared in control conditions and in the presence of 500 nM BmK M1. The BmK M1 shifted steady state inactivation by 10.8 mV to less negative potentials. The steady state activation was shifted by 5.5 mV to more negative potentials, making the activation window larger. Moreover, BmK M1 increased the fast time constant of inactivation, leaving the activation time constant unchanged. In summary, BmK M1 primarily affected the inactivation parameters of the voltage gated Na(+) current in isolated locust DUM neurons.

Differential sensitivity of sodium channels from the central and peripheral nervous system to the scorpion toxins Lqh-2 and Lqh-3

The scorpion alpha-toxins Lqh-2 and Lqh-3, isolated from the venom of the Israeli yellow scorpion Leiurus quinquestriatus hebraeus, were previously shown to be very potent in removing fast inactivation of rat skeletal muscle sodium channels (Chen et al., 2000). Here, we show that tetrodotoxin-sensitive neuronal channels NaV1.2 and NaV1.7, which are mainly expressed in mammalian central and peripheral nervous systems, respectively, are differentially sensitive to these two toxins. rNaV1.2 and hNaV1.7 channels were studied with patch-clamp methods upon expression in mammalian cells. While Lqh-3 was about 100-times more potent in removing inactivation in hNaV1.7 channels compared with rNaV1.2, Lqh-2 was about 20-times more active in the other direction. Site-directed mutagenesis showed that the differences in the putative binding sites for these toxins, the S3-4 linkers of domain 4, are of major importance for Lqh-3, but not for Lqh-2.

Molecular determinant of Na(v)1.8 sodium channel resistance to the venom from the scorpion Leiurus quinquestriatus hebraeus

The scorpion venom from Leiurus quinquestriatus (LQTX) alters the kinetics of tetrodotoxin (TTX)-sensitive channels such as the skeletal muscle sodium channel Na(v)1.4. In this study, we tested the effects of LQTX on the TTX-resistant sodium current generated by Na(v)1.8 channels in sensory neurons. Na(v)1.8 current was found to be resistant to LQTX, whereas LQTX slowed inactivation of the current generated by Na(v)1.4 and induced a persistent current. LQTX has been shown to bind the S3-S4 linker of domain four (D4S3-S4) of rat brain Na(v)1.2 sodium channels. Sequence analysis shows that the D4S3-S4 linker is longer in Na(v)1.8 than in Na(v)1.4 by four amino acids: Serine; Leucine; Glutamic acid; and Aspargine (SLEN). Na(v)1.4-SLEN, a chimera construct carrying SLEN at the analogous position in the D4S3-S4 linker, was also found to be resistant to LQTX. Therefore, we conclude that the tetrapeptide SLEN at the D4S3-S4 linker region is sufficient to make Na(v)1.8 resistant to LQTX.

Characterization of scorpion alpha-like toxin group using two new toxins from the scorpion Leiurus quinquestriatus hebraeus

Two novel toxins, Lqh6 and Lqh7, isolated from the venom of the scorpion Leiurus quinquestriatus hebraeus, have in their sequence a molecular signature (8Q/KPE10) associated with a recently defined group of alpha-toxins that target Na channels, namely the alpha-like toxins [reviewed in Gordon, D., Savarin, P., Gurevitz, M. & Zinn-Justin, S. (1998) J. Toxicol. Toxin Rev. 17, 131-159]. Lqh6 and Lqh7 are highly toxic to insects and mice, and inhibit the binding of alpha-toxins to cockroach neuronal membranes. Although they kill rodents by intracerebroventricular injection, they do not inhibit the binding of antimammal alpha-toxins (e.g. Lqh2) to rat brain synaptosomes, not even at high concentrations. Furthermore, in voltage-clamp experiments, rat brain Na channels IIA (rNav1.2A) expressed in Xenopus oocytes are not affected by Lqh6 nor by Lqh7 below 3 micro m. In contrast, muscular Na channels (rNav1.4 and hNav1.5) expressed in the same cells respond to nanomolar concentrations of Lqh6 and Lqh7 by slowing of Na current inactivation and a leftward shift of the peak conductance-voltage curve. The structural and pharmacological properties of the new toxins are compared to those of other scorpion alpha-toxins in order to re-examine the hallmarks previously set for the alpha-like toxin group.

Domain 2 of Drosophila para voltage-gated sodium channel confers insect properties to a rat brain channel

The ability of the excitatory anti-insect-selective scorpion toxin AahIT (Androctonus australis hector) to exclusively bind to and modify the insect voltage-gated sodium channel (NaCh) makes it a unique tool to unravel the structural differences between mammalian and insect channels, a prerequisite in the design of selective pesticides. To localize the insect NaCh domain that binds AahIT, we constructed a chimeric channel composed of rat brain NaCh alpha-subunit (rBIIA) in which domain-2 (D2) was replaced by that of Drosophila Para (paralytic temperature-sensitive). The choice of D2 was dictated by the similarity between AahIT and scorpion beta-toxins pertaining to both their binding and action and the essential role of D2 in the beta-toxins binding site on mammalian channels. Expression of the chimera rBIIA-ParaD2 in Xenopus oocytes gave rise to voltage-gated and TTX-sensitive NaChs that, like rBIIA, were sensitive to scorpion alpha-toxins and regulated by the auxiliary subunit beta(1) but not by the insect TipE. Notably, like Drosophila Para/TipE, but unlike rBIIA/beta(1), the chimera gained sensitivity to AahIT, indicating that the phyletic selectivity of AahIT is conferred by the insect NaCh D2. Furthermore, the chimera acquired additional insect channel properties; its activation was shifted to more positive potentials, and the effect of alpha-toxins was potentiated. Our results highlight the key role of D2 in the selective recognition of anti-insect excitatory toxins and in the modulation of NaCh gating. We also provide a methodological approach to the study of ion channels that are difficult to express in model expression systems.

Variations in receptor site-3 on rat brain and insect sodium channels highlighted by binding of a funnel-web spider delta-atracotoxin

Delta-atracotoxins (delta-ACTXs) from Australian funnel-web spiders differ structurally from scorpion alpha-toxins (Sc(alpha)Tx) but similarly slow sodium current inactivation and compete for their binding to sodium channels at receptor site-3. Characterization of the binding of 125I-labelled delta-ACTX-Hv1a to various sodium channels reveals a decrease in affinity for depolarized (0 mV; Kd=6.5 +/- 1.4 nm) vs.polarized (-55 mV; Kd=0.6 +/- 0.2 nm) rat brain synaptosomes. The increased Kd under depolarized conditions correlates with a 4.3-fold reduction in the association rate and a 1.8-increase in the dissociation rate. In comparison, Sc(alpha)Tx binding affinity decreased 33-fold under depolarized conditions due to a 48-fold reduction in the association rate. The binding of 125I-labelled delta-ACTX-Hv1a to rat brain synaptosomes is inhibited competitively by classical Sc(alpha)Txs and allosterically by brevetoxin-1, similar to Sc(alpha)Tx binding. However, in contrast with classical Sc(alpha)Txs, 125I-labelled delta-ACTX-Hv1a binds with high affinity to cockroach Na+ channels (Kd=0.42 +/- 0.1 nm) and is displaced by the Sc(alpha)Tx, Lqh(alpha)IT, a well-defined ligand of insect sodium channel receptor site-3. However, delta-ACTX-Hv1a exhibits a surprisingly low binding affinity to locust sodium channels. Thus, unlike Sc(alpha)Txs, which are capable of differentiating between mammalian and insect sodium channels, delta-ACTXs differentiate between various insect sodium channels but bind with similar high affinity to rat brain and cockroach channels. Structural comparison of delta-ACTX-Hv1a to Sc(alpha)Txs suggests a similar putative bioactive surface but a 'slimmer' overall shape of the spider toxin. A slimmer shape may ease the interaction with the cockroach and mammalian receptor site-3 and facilitate its association with different conformations of the rat brain receptor, correlated with closed/open and slow-inactivated channel states.

Effect of depolarization on binding kinetics of scorpion alpha-toxin highlights conformational changes of rat brain sodium channels

Binding of scorpion alpha-toxins to receptor site 3 on voltage-gated sodium channels inhibits sodium current inactivation and is voltage-dependent. To reveal the direct effect of depolarization, we analyzed binding kinetics of the alpha-toxin Lqh-II (from Leiurus quinquestriatus hebraeus) to rat brain synaptosomes and effects on rat brain II (rBII) channels expressed in mammalian cells. Our results indicated that the 33-fold decrease in toxin affinity for depolarized (0 mV, 90 mM [K(+)](out), K(d) = 5.85 +/- 0.5 nM) versus polarized (-55 mV, 5 mM [K(+)](out), K(d) = 0.18 +/- 0.04 nM) synaptosomes at steady state results from a 48-fold reduction in the association rate (k(on) at 5 mM [K(+)] = (12.0 +/- 4) x 10(6) M(-1) s(-1) and (0.25 +/- 0.03) x 10(6) M(-1) s(-1) at 90 mM [K(+)](out)) with nearly no change in the dissociation rate. Electrophysiological analyses of rBII channels expressed in mammalian cells revealed that approximately 75% and 40% of rBII occupied fast- and slow-inactivated states, respectively, at resting membrane potential of synaptosomes (-55 mV), and Lqh-II markedly increased the steady-state fast and slow inactivation. To mimic electrophysiological conditions we induced fast depolarization of toxin-bound synaptosomes, which generated a biphasic unbinding of Lqh-II from toxin-receptor complexes. The first fast off rate closely resembled values determined electrophysiologically for rBII in mammalian cells. The second off rate was similar to the voltage-independent steady-state value, attributed to binding to the slow-inactivated channel states. Thus, the Lqh-II voltage-dependent affinity highlights two independent mechanisms representing conformational changes of sodium channels associated with transitions among electrically visible and invisible inactivated states.