Dual action of a dinoflagellate-derived precursor of Pacific ciguatoxins (P-CTX-4B) on voltage-dependent K+ and Na+ channels of single myelinated axons

Evaluation of relative potency of calibrated ciguatoxin congeners by near-infrared fluorescent receptor binding and neuroblastoma cell-based assays

Ciguatera fish poisoning (CFP) is a foodborne illness affecting > 50,000 people worldwide annually. It is caused by eating marine invertebrates and fish that have accumulated ciguatoxins (CTXs). Recently, the risk of CFP to human health, the local economy, and fishery resources have increased; therefore, detection methods are urgently needed. Functional assays for detecting ciguatoxins in fish include receptor binding (RBA) and neuroblastoma cell-based assay (N2a assay), which can detect all CTX congeners. In this study, we made these assays easier to use. For RBA, an assay was developed using a novel near-infrared fluorescent ligand, PREX710-BTX, to save valuable CTXs. In the N2a assay, a 1-day assay was developed with the same detection performance as the conventional 2-day assay. Additionally, in these assays, we used calibrated CTX standards from the Pacific determined by quantitative NMR for the first time to compare the relative potency of congeners, which differed significantly among previous studies. In the RBA, there was almost no difference in the binding affinity among congeners, showing that the differences in side chains, stereochemistry, and backbone structure of CTXs did not affect the binding affinity. However, this result did not correlate with the toxic equivalency factors (TEFs) based on acute toxicity in mice. In contrast, the N2a assay showed a good correlation with TEFs based on acute toxicity in mice, except for CTX3C. These findings, obtained with calibrated toxin standards, provide important insights into evaluating the total toxicity of CTXs using functional assays.

Gambierol Blocks a K+ Current Fraction without Affecting Catecholamine Release in Rat Fetal Adrenomedullary Cultured Chromaffin Cells

Gambierol inhibits voltage-gated K⁺ (K_V) channels in various excitable and non-excitable cells. The purpose of this work was to study the effects of gambierol on single rat fetal (F19–F20) adrenomedullary cultured chromaffin cells. These excitable cells have different types of K_V channels and release catecholamines. Perforated whole-cell voltage-clamp recordings revealed that gambierol (100 nM) blocked only a fraction of the total outward K⁺ current and slowed the kinetics of K⁺ current activation. The use of selective channel blockers disclosed that gambierol did not affect calcium-activated K⁺ (K_Ca) and ATP-sensitive K⁺ (K_ATP) channels. The gambierol concentration necessary to inhibit 50% of the K⁺ current-component sensitive to the polyether (IC₅₀) was 5.8 nM. Simultaneous whole-cell current-clamp and single-cell amperometry recordings revealed that gambierol did not modify the membrane potential following 11s depolarizing current-steps, in both quiescent and active cells displaying repetitive firing of action potentials, and it did not increase the number of exocytotic catecholamine release events, with respect to controls. The subsequent addition of apamin and iberiotoxin, which selectively block the K_Ca channels, both depolarized the membrane and enhanced by 2.7 and 3.5-fold the exocytotic event frequency in quiescent and active cells, respectively. These results highlight the important modulatory role played by K_Ca channels in the control of exocytosis from fetal (F19–F20) adrenomedullary chromaffin cells.

Determination of the toxicity equivalency factors for ciguatoxins using human sodium channels

Ciguatoxins (CTXs) which are produced by dinoflagellates of the genus Gambierdiscus and Fukuyoa and share a ladder-shaped polyether structure, are causative compounds of one of the most frequent foodborne illness disease known as ciguatera fish poisoning (CFP). CFP was initially found in tropical and subtropical areas but nowadays the dinoflagellates producers of ciguatoxins had spread to European coasts. Therefore, this raises the need of establishing toxicity equivalency factors for the different compounds that can contribute to ciguatera fish poisoning, since biological methods have been replaced by analytical techniques. Thus, in this work, the effects of six compounds causative of ciguatera, on their main target, the human voltage-gated sodium channels have been analyzed for the first time. The results presented here led to the conclusion that the order of potency was CTX1B, CTX3B, CTX4A, gambierol, gambierone and MTX3. Furthermore, the data indicate that the activation voltage of sodium channels is more sensitive to detect ciguatoxins than their effect on the peak sodium current amplitude.

Sulfo-Gambierones, Two New Analogs of Gambierone Produced by Gambierdiscus excentricus

Ciguatera poisoning is caused by the ingestion of fish or shellfish contaminated with ciguatoxins produced by dinoflagellate species belonging to the genera Gambierdiscus and Fukuyoa. Unlike in the Pacific region, the species producing ciguatoxins in the Atlantic Ocean have yet to be definitely identified, though some ciguatoxins responsible for ciguatera have been reported from fish. Previous studies investigating the ciguatoxin-like toxicity of Atlantic Gambierdiscus species using Neuro2a cell-based assay identified G. excentricus as a potential toxin producer. To more rigorously characterize the toxin profile produced by this species, a purified extract from 124 million cells was prepared and partial characterization by high-resolution mass spectrometry was performed. The analysis revealed two new analogs of the polyether gambierone: sulfo-gambierone and dihydro-sulfo-gambierone. Algal ciguatoxins were not identified. The very low ciguatoxin-like toxicity of the two new analogs obtained by the Neuro2a cell-based assay suggests they are not responsible for the relatively high toxicity previously observed when using fractionated G. excentricus extracts, and are unlikely the cause of ciguatera in the region. These compounds, however, can be useful as biomarkers of the presence of G. excentricus due to their sensitive detection by mass spectrometry.

Deep-Water Fish Are Potential Vectors of Ciguatera Poisoning in the Gambier Islands, French Polynesia

Ciguatera poisoning (CP) cases linked to the consumption of deep-water fish occurred in 2003 in the Gambier Islands (French Polynesia). In 2004, on the request of two local fishermen, the presence of ciguatoxins (CTXs) was examined in part of their fish catches, i.e., 22 specimens representing five deep-water fish species. Using the radioactive receptor binding assay (rRBA) and mouse bioassay (MBA), significant CTX levels were detected in seven deep-water specimens in Lutjanidae, Serranidae, and Bramidae families. Following additional purification steps on the remaining liposoluble fractions for 13 of these samples (kept at -20 °C), these latter were reanalyzed in 2018 with improved protocols of the neuroblastoma cell-based assay (CBA-N2a) and liquid chromatography tandem mass spectrometry (LC-MS/MS). Using the CBA-N2a, the highest CTX-like content found in a specimen of Eumegistus illustris (Bramidae) was 2.94 ± 0.27 µg CTX1B eq. kg^-1. Its toxin profile consisted of 52-epi-54-deoxyCTX1B, CTX1B, and 54-deoxyCTX1B, as assessed by LC-MS/MS. This is the first study demonstrating that deep-water fish are potential ciguatera vectors and highlighting the importance of a systematic monitoring of CTXs in all exploited fish species, especially in ciguatera hotspots, including deep-water fish, which constitute a significant portion of the commercial deep-sea fisheries in many Asian-Pacific countries.

An Update on Ciguatoxins and CTX-like Toxicity in Fish from Different Trophic Levels of the Selvagens Islands (NE Atlantic, Madeira, Portugal)

The Selvagens Islands, which are a marine protected area located at the southernmost point of the Portuguese maritime zone, have been associated with fish harboring ciguatoxins (CTX) and linked to ciguatera fish poisonings. This study reports the results of a field sampling campaign carried out in September 2018 in these remote and rarely surveyed islands. Fifty-six fish specimens from different trophic levels were caught for CTX-like toxicity determination by cell-based assay (CBA) and toxin content analysis by liquid chromatography with tandem mass spectrometry (LC-MS/MS). Notably, high toxicity levels were found in fish with an intermediate position in the food web, such as zebra seabream (Diplodus cervinus) and barred hogfish (Bodianus scrofa), reaching levels up to 0.75 µg CTX1B equivalent kg⁻¹. The LC-MS/MS analysis confirmed that C-CTX1 was the main toxin, but discrepancies between CBA and LC-MS/MS in D. cervinus and top predator species, such as the yellowmouth barracuda (Sphyraena viridis) and amberjacks (Seriola spp.), suggest the presence of fish metabolic products, which need to be further elucidated. This study confirms that fish from coastal food webs of the Selvagens Islands represent a high risk of ciguatera, raising important issues for fisheries and environmental management of the Selvagens Islands.

Neurological Disturbances of Ciguatera Poisoning: Clinical Features and Pathophysiological Basis

Ciguatera fish poisoning (CFP), the most prevalent seafood poisoning worldwide, is caused by the consumption of tropical and subtropical fish contaminated with potent neurotoxins called ciguatoxins (CTXs). Ciguatera is a complex clinical syndrome in which peripheral neurological signs predominate in the acute phase of the intoxication but also persist or reoccur long afterward. Their recognition is of particular importance in establishing the diagnosis, which is clinically-based and can be a challenge for physicians unfamiliar with CFP. To date, no specific treatment exists. Physiopathologically, the primary targets of CTXs are well identified, as are the secondary events that may contribute to CFP symptomatology. This review describes the clinical features, focusing on the sensory disturbances, and then reports on the neuronal targets and effects of CTXs, as well as the neurophysiological and histological studies that have contributed to existing knowledge of CFP neuropathophysiology at the molecular, neurocellular and nerve levels.

Revisiting the Neuroblastoma Cell-Based Assay (CBA-N2a) for the Improved Detection of Marine Toxins Active on Voltage Gated Sodium Channels (VGSCs)

The neuroblastoma cell-based assay (CBA-N2a) is widely used for the detection of marine biotoxins in seafood products, yet a consensus protocol is still lacking. In this study, six key parameters of CBA-N2a were revisited: cell seeding densities, cell layer viability after 26 h growth, MTT incubation time, Ouabain and Veratridine treatment and solvent and matrix effects. A step-by-step protocol was defined identifying five viability controls for the validation of CBA-N2a results. Specific detection of two voltage gated sodium channel activators, pacific ciguatoxin (P-CTX3C) and brevetoxin (PbTx3) and two inhibitors, saxitoxin (STX) and decarbamoylsaxitoxin (dc-STX) was achieved, with EC50 values of 1.7 ± 0.35 pg/mL, 5.8 ± 0.9 ng/mL, 3 ± 0.5 ng/mL and 15.8 ± 3 ng/mL, respectively. When applied to the detection of ciguatoxin (CTX)-like toxicity in fish samples, limit of detection (LOD) and limit of quantification (LOQ) values were 0.031 ± 0.008 and 0.064 ± 0.016 ng P-CTX3C eq/g of flesh, respectively. Intra and inter-assays comparisons of viability controls, LOD, LOQ and toxicity in fish samples gave coefficients of variation (CVs) ranging from 3% to 29%. This improved test adaptable to either high throughput screening or composite toxicity estimation is a useful starting point for a standardization of the CBA-N2a in the field of marine toxin detection.

Global impact of ciguatoxins and ciguatera fish poisoning on fish, fisheries and consumers

Ciguatera fish poisoning (CFP) is one of the most devastating food-borne illnesses caused by fish consumption. Ciguatoxins (CTXs) are potent neurotoxins synthesized by the benthic microalgae Gambierdiscus spp. and Fukuyoa spp. that are transmitted to fish by grazing and predation. Despite the high incidence of CFP, affecting an estimated number of 50,000 persons per year in tropical and subtropical latitudes, the factors underlying CTXs occurrence are still not well understood. Toxin transfer and dynamics in fish and food-webs are complex. Feeding habits and metabolic pathways determine the toxin profile and toxicity of fish, and migratory species may transport and spread the hazard. Furthermore, CTX effect on fish may be a limiting factor for fish recruitment and toxin prevalence. Recently, new occurrences of Gambierdiscus spp. in temperate areas have been concomitant with the detection of toxic fish and CFP incidents in non-endemic areas. CFP cases in Europe have led to implementation of monitoring programs and fisheries restrictions with considerable impact on local economies. More than 400 species of fish can be vectors of CTXs, and most of them are high-valued commercial species. Thus, the risk uncertainty and the spread of Gambierdiscus have serious consequences for fisheries and food safety. Here, we present a critical review of CTXs impacts on fish, fisheries, and humans, based on the current knowledge on CFP incidence and CTXs prevalence in microalgae and fish.

Small Molecule Natural Products and Alzheimer's Disease

Alzheimer's disease (AD) is a progressive and deadly neurodegenerative disease that is characterized by memory loss, cognitive impairment and dementia. Several hypotheses have been proposed for the pathogenesis based on the pathological changes in the brain of AD patients during the last few decades. Unfortunately, there is no effective agents/therapies to prevent or control AD at present. Currently, only a few drugs, which function as acetylcholinesterase (AChE) inhibitors or N-methyl-Daspartate (NMDA) receptor antagonists, are available to alleviate symptoms. Since many small molecule natural products have shown their functions as agonists or antagonists of receptors, as well as inhibitors of enzymes and proteins in the brain during the development of central nervous system (CNS) drugs, it is likely that natural products will play an important role in anti-AD drug development. We review recent papers on using small molecule natural products as drug candidates for the treatment of AD. These natural products possess antioxidant, anti-inflammatory, anticholinesterase, anti-amyloidogenic and neuroprotective activities. Moreover, bioactive natural products intended to be used for preventing AD, reducing the symptoms of AD and the new targets for treatment of AD are summarized.

Toxicological Investigations on the Sea Urchin Tripneustes gratilla (Toxopneustidae, Echinoid) from Anaho Bay (Nuku Hiva, French Polynesia): Evidence for the Presence of Pacific Ciguatoxins

The sea urchin Tripneustes gratilla (Toxopneustidae, Echinoids) is a source of protein for many islanders in the Indo-West Pacific. It was previously reported to occasionally cause ciguatera-like poisoning; however, the exact nature of the causative agent was not confirmed. In April and July 2015, ciguatera poisonings were reported following the consumption of T.gratilla in Anaho Bay (Nuku Hiva Island, Marquesas archipelago, French Polynesia). Patient symptomatology was recorded and sea urchin samples were collected from Anaho Bay in July 2015 and November 2016. Toxicity analysis using the neuroblastoma cell-based assay (CBA-N2a) detected the presence of ciguatoxins (CTXs) in T.gratilla samples. Gambierdiscus species were predominant in the benthic assemblages of Anaho Bay, and G.polynesiensis was highly prevalent in in vitro cultures according to qPCR results. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses revealed that P-CTX-3B was the major ciguatoxin congener in toxic sea urchin samples, followed by 51-OH-P-CTX-3C, P-CTX-3C, P-CTX-4A, and P-CTX-4B. Between July 2015 and November 2016, the toxin content in T.gratilla decreased, but was consistently above the safety limit allowed for human consumption. This study provides evidence of CTX bioaccumulation in T.gratilla as a cause of ciguatera-like poisoning associated with a documented symptomatology.

Immune effects of the neurotoxins ciguatoxins and brevetoxins

Ciguatoxins (CTXs) and brevetoxins (PbTxs) are phycotoxins that can accumulate along the marine food chain and thus cause seafood poisoning in humans, namely "ciguatera fish poisoning" (CFP) and "neurotoxic shellfish poisoning" (NSP), respectively. CFP is characterized by early gastrointestinal symptoms and typical sensory disorders (paraesthesia, pain, pruritus and cold dysaesthesia), which can persist several weeks and, in some cases, several months or years. NSP is considered a mild form of CFP with similar but less severe symptoms. After inhaled exposure, PbTxs can also cause respiratory tract irritation in healthy subjects and asthma exacerbations in predisposed subjects, whose respiratory functions may be disrupted for several days following PbTx inhalation. Mechanistically, it is well established that CTX- or PbTx-induced disturbances are primarily mainly due to voltage-gated sodium channel activation in sensory and motor peripheral nervous system. However, little is known about the pathophysiology or a potential individual susceptibility to long lasting effects of CFP/NSP. In addition to their action on the nervous system, PbTxs and CTXs were also shown to exert effects on the immune system. However, their role in the pathophysiology of syndromes induced by CTX or PbTx exposure is poorly documented. The aim of this review is to inventory the literature thus far on the inflammatory and immune effects of PbTxs and CTXs.

Tetracyclic Truncated Analogue of the Marine Toxin Gambierol Modifies NMDA, Tau, and Amyloid β Expression in Mice Brains: Implications in AD Pathology

Gambierol and its two, tetra- and heptacyclic, analogues have been previously proved as promising molecules for the modulation of Alzheimer's disease (AD) hallmarks in primary cortical neurons of 3xTg-AD fetuses. In this work, the effect of the tetracyclic analogue of gambierol was tested in vivo in 3xTg-AD mice (10 months old) after 1 month of weekly treatment with 50 μg/kg. Adverse effects were not reported throughout the whole treatment period and no pathological signs were observed for the analyzed organs. The compound was found in brain samples after intraperitoneal injection. The tetracyclic analogue of gambierol elicited a decrease of amyloid β_1-42 levels and a dose-dependent inhibition of β-secretase enzyme-1 activity. Moreover, this compound also reduced the phosphorylation of tau at the 181 and 159/163 residues with an increase of the inactive isoform of the glycogen synthase kinase-3β. In accordance with our in vitro neuronal model, this compound produced a reduction in the N2A subunit of the N-methyl-d-aspartate (NMDA) receptor. The combined effect of this compound on amyloid β_1-42 and tau phosphorylation represents a multitarget therapeutic approach for AD which might be more effective for this multifactorial and complex neurodegenerative disease than the current treatments.

Voltage-sensor conformation shapes the intra-membrane drug binding site that determines gambierol affinity in Kv channels

Marine ladder-shaped polyether toxins are implicated in neurological symptoms of fish-borne food poisonings. The toxin gambierol, produced by the marine dinoflagellate Gambierdiscus toxicus, belongs to the group of ladder-shaped polyether toxins and inhibits Kv3.1 channels with nanomolar affinity through a mechanism of gating modification. Binding determinants for gambierol localize at the lipid-exposed interface of the pore forming S5 and S6 segments, suggesting that gambierol binds outside of the permeation pathway. To explore a possible involvement of the voltage-sensing domain (VSD), we made different chimeric channels between Kv3.1 and Kv2.1, exchanging distinct parts of the gating machinery. Our results showed that neither the electro-mechanical coupling nor the S1-S3a region of the VSD affect gambierol sensitivity. In contrast, the S3b-S4 part of the VSD (paddle motif) decreased gambierol sensitivity in Kv3.1 more than 100-fold. Structure determination by homology modeling indicated that the position of the S3b-S4 paddle and its primary structure defines the shape and∖or the accessibility of the binding site for gambierol, explaining the observed differences in gambierol affinity between the channel chimeras. Furthermore, these findings explain the observed difference in gambierol affinity for the closed and open channel configurations of Kv3.1, opening new possibilities for exploring the VSDs as selectivity determinants in drug design.

Synthetic ciguatoxin CTX 3C induces a rapid imbalance in neuronal excitability

Ciguatera is a human global disease caused by the consumption of contaminated fish that have accumulated ciguatoxins (CTXs), sodium channel activator toxins. Symptoms of ciguatera include neurological alterations such as paraesthesiae, dysaesthesiae, depression, and heightened nociperception, among others. An important issue to understand these long-term neurological alterations is to establish the role that changes in activity produced by CTX 3C represent to neurons. Here, the effects of synthetic ciguatoxin CTX 3C on membrane potential, spontaneous spiking, and properties of synaptic transmission in cultured cortical neurons of 11-18 days in vitro (DIV) were evaluated using electrophysiological approaches. CTX 3C induced a large depolarization that decreased neuronal firing and caused a rapid inward tonic current that was primarily GABAergic. Moreover, the toxin enhanced the amplitude of miniature postsynaptic inhibitory currents (mIPSCs), whereas it decreased the amplitude of miniature postsynaptic excitatory currents (mEPSCs). The frequency of mIPSCs increased, whereas the frequency of mEPSCs remained unaltered. We describe, for the first time, that a rapid membrane depolarization caused by CTX 3C in cortical neurons activates mechanisms that tend to suppress electrical activity by shifting the balance between excitatory and inhibitory synaptic transmission toward inhibition. Indeed, these results suggest that the acute effects of CTX on synaptic transmission could underlie some of the neurological symptoms caused by ciguatera in humans.

The voltage-gated sodium channel: a major target of marine neurotoxins

Voltage-gated sodium channels (Nav) are key components for nerve excitability. They initiate and propagate the action potential in excitable cells, throughout the central and peripheral nervous system, thus enabling a variety of physiological functions to be achieved. The rising phase of the action potential is driven by the opening of Nav channels which activate rapidly and carry Na(+) ions in the intracellular medium, and ends with the Na(+) current inactivation. The biophysical properties of these channels have been elucidated, through the use of pharmacological agents that disrupt the molecular mechanism of the channel functioning. Among them, marine toxins produced by venomous animals or microorganisms have been crucial to map the different allosteric binding sites of the channels, understand their mode of action and represent an emerging source of therapeutic agents to alleviate or cure Na(+) channels-linked human diseases. In this article, we review recent discoveries on the molecular and biophysical properties of the Na(+) channel as a target for marine neurotoxins, and present the ongoing developments of pharmacological agents as therapeutic tools.

Involvement of both sodium influx and potassium efflux in ciguatoxin-induced nodal swelling of frog myelinated axons

Ciguatoxins, mainly produced by benthic dinoflagellate Gambierdiscus species, are responsible for a complex human poisoning known as ciguatera. Previous pharmacological studies revealed that these toxins activate voltage-gated Na+ channels. In frog nodes of Ranvier, ciguatoxins induce spontaneous and repetitive action potentials (APs) and increase axonal volume that may explain alterations of nerve functioning in intoxicated humans. The present study aimed determining the ionic mechanisms involved in Pacific ciguatoxin-1B (P-CTX-1B)-induced membrane hyperexcitability and subsequent volume increase in frog nodes of Ranvier, using electrophysiology and confocal microscopy. The results reveal that P-CTX-1B action is not dependent on external Cl- ions since it was not affected by substituting Cl- by methylsulfate ions. In contrast, substitution of external Na+ by Li+ ions suppressed spontaneous APs and prevented nodal swelling. This suggests that P-CTX-1B-modified Na+ channels are not selective to Li+ ions and/or are blocked by these ions, and that Na+ influx through Na+ channels opened during spontaneous APs is required for axonal swelling. The fact that the K+ channel blocker tetraethylammonium modified, but did not suppress, spontaneous APs and greatly reduced nodal swelling induced by P-CTX-1B indicates that K+ efflux might also be involved. This is supported by the fact that P-CTX-1B, when tested in the presence of both tetraethylammonium and the K+ ionophore valinomycin, produced the characteristic nodal swelling. It is concluded that, during the action of P-CTX-1B, water movements responsible for axonal swelling depend on both Na+ influx and K+ efflux. These results pave the way for further studies regarding ciguatera treatment.

The ladder-shaped polyether toxin gambierol anchors the gating machinery of Kv3.1 channels in the resting state

Voltage-gated potassium (Kv) and sodium (Nav) channels are key determinants of cellular excitability and serve as targets of neurotoxins. Most marine ciguatoxins potentiate Nav channels and cause ciguatera seafood poisoning. Several ciguatoxins have also been shown to affect Kv channels, and we showed previously that the ladder-shaped polyether toxin gambierol is a potent Kv channel inhibitor. Most likely, gambierol acts via a lipid-exposed binding site, located outside the K⁺ permeation pathway. However, the mechanism by which gambierol inhibits Kv channels remained unknown. Using gating and ionic current analysis to investigate how gambierol affected S6 gate opening and voltage-sensing domain (VSD) movements, we show that the resting (closed) channel conformation forms the high-affinity state for gambierol. The voltage dependence of activation was shifted by >120 mV in the depolarizing direction, precluding channel opening in the physiological voltage range. The (early) transitions between the resting and the open state were monitored with gating currents, and provided evidence that strong depolarizations allowed VSD movement up to the activated-not-open state. However, for transition to the fully open (ion-conducting) state, the toxin first needed to dissociate. These dissociation kinetics were markedly accelerated in the activated-not-open state, presumably because this state displayed a much lower affinity for gambierol. A tetrameric concatemer with only one high-affinity binding site still displayed high toxin sensitivity, suggesting that interaction with a single binding site prevented the concerted step required for channel opening. We propose a mechanism whereby gambierol anchors the channel’s gating machinery in the resting state, requiring more work from the VSD to open the channel. This mechanism is quite different from the action of classical gating modifier peptides (e.g., hanatoxin). Therefore, polyether toxins open new opportunities in structure–function relationship studies in Kv channels and in drug design to modulate channel function.

A comparative study of the effect of ciguatoxins on voltage-dependent Na+ and K+ channels in cerebellar neurons

Ciguatera is a global disease caused by the consumption of certain warm-water fish (ciguateric fish) that have accumulated orally effective levels of sodium channel activator toxins (ciguatoxins) through the marine food chain. The effect of ciguatoxin standards and contaminated ciguatoxin samples was evaluated by electrophysiological recordings in cultured cerebellar neurons. The toxins affected both voltage-gated sodium (Nav) and potassium channels (Kv) although with different potencies. CTX 3C was the most active toxin blocking the peak inward sodium currents, followed by P-CTX 1B and 51-OH CTX 3C. In contrast, P-CTX 1B was more effective in blocking potassium currents. The analysis of six different samples of contaminated fish, in which a ciguatoxin analogue of mass 1040.6, not identical with the standard 51-OH CTX 3C, was the most prevalent compound, indicated an additive effect of the different ciguatoxins present in the samples. The results presented here constitute the first comparison of the potencies of three different purified ciguatoxins on sodium and potassium channels in the same neuronal preparation and indicate that electrophysiological recordings from cultured cerebellar neurons may provide a valuable tool to detect and quantify ciguatoxins in the very low nanomolar range.