Hyperosmolar D-mannitol reverses the increased membrane excitability and the nodal swelling caused by Caribbean ciguatoxin-1 in single frog myelinated axons

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.

An Updated Review of Ciguatera Fish Poisoning: Clinical, Epidemiological, Environmental, and Public Health Management

Ciguatera Fish Poisoning (CFP) is the most frequently reported seafood-toxin illness in the world. It causes substantial human health, social, and economic impacts. The illness produces a complex array of gastrointestinal, neurological and neuropsychological, and cardiovascular symptoms, which may last days, weeks, or months. This paper is a general review of CFP including the human health effects of exposure to ciguatoxins (CTXs), diagnosis, human pathophysiology of CFP, treatment, detection of CTXs in fish, epidemiology of the illness, global dimensions, prevention, future directions, and recommendations for clinicians and patients. It updates and expands upon the previous review of CFP published by Friedman et al. (2008) and addresses new insights and relevant emerging global themes such as climate and environmental change, international market issues, and socioeconomic impacts of CFP. It also provides a proposed universal case definition for CFP designed to account for the variability in symptom presentation across different geographic regions. Information that is important but unchanged since the previous review has been reiterated. This article is intended for a broad audience, including resource and fishery managers, commercial and recreational fishers, public health officials, medical professionals, and other interested parties.

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.

Gene expression profiling in brain of mice exposed to the marine neurotoxin ciguatoxin reveals an acute anti-inflammatory, neuroprotective response

BACKGROUND

Ciguatoxins (CTXs) are polyether marine neurotoxins and potent activators of voltage-gated sodium channels. This toxin is carried by multiple reef-fish species and human consumption of ciguatoxins can result in an explosive gastrointestinal/neurologic illness. This study characterizes the global transcriptional response in mouse brain to a symptomatic dose of the highly toxic Pacific ciguatoxin P-CTX-1 and additionally compares this data to transcriptional profiles from liver and whole blood examined previously. Adult male C57/BL6 mice were injected with 0.26 ng/g P-CTX-1 while controls received only vehicle. Animals were sacrificed at 1, 4 and 24 hrs and transcriptional profiling was performed on brain RNA with Agilent whole genome microarrays. RT-PCR was used to independently validate gene expression and the web tool DAVID was used to analyze gene ontology (GO) and molecular pathway enrichment of the gene expression data.

RESULTS

A pronounced 4°C hypothermic response was recorded in these mice, reaching a minimum at 1 hr and lasting for 8 hrs post toxin exposure. Ratio expression data were filtered by intensity, fold change and p-value, with the resulting data used for time course analysis, K-means clustering, ontology classification and KEGG pathway enrichment. Top GO hits for this gene set included acute phase response and mono-oxygenase activity. Molecular pathway analysis showed enrichment for complement/coagulation cascades and metabolism of xenobiotics. Many immediate early genes such as Fos, Jun and Early Growth Response isoforms were down-regulated although others associated with stress such as glucocorticoid responsive genes were up-regulated. Real time PCR confirmation was performed on 22 differentially expressed genes with a correlation of 0.9 (Spearman's Rho, p < 0.0001) with microarray results.

CONCLUSIONS

Many of the genes differentially expressed in this study, in parallel with the hypothermia, figure prominently in protection against neuroinflammation. Pathologic activity of the complement/coagulation cascade has been shown in patients suffering from a chronic form of ciguatera poisoning and is of particular interest in this model. Anti-inflammatory processes were at work not only in the brain but were also seen in whole blood and liver of these animals, creating a systemic anti-inflammatory environment to protect against the initial cellular damage caused by the toxin.

Analysis of Caribbean ciguatoxin-1 effects on frog myelinated axons and the neuromuscular junction

Caribbean ciguatoxin-1 (C-CTX-1) induced, after about 1h exposure, muscle membrane depolarisation and repetitive post-synaptic action potentials (APs) in frog neuromuscular preparations. This depolarising effect was also observed in a Ca(2+)-free medium with a strong enhancement of spontaneous quantal transmitter release, compared with control conditions. The ciguatoxin-induced increase in release could be accelerated when Ca(2+) was present in the extracellular medium. C-CTX-1 also enhanced nerve-evoked quantal acetylcholine (ACh) release. At normal neuromuscular junctions loaded with the fluorescent dye FM1-43, C-CTX-1 induced swelling of nerve terminals, an effect that was reversed by hyperosmotic d-mannitol. In myelinated axons, C-CTX-1 increased nodal membrane excitability, inducing spontaneous and repetitive APs. Also, the toxin enlarged the repolarising phase of APs in control and tetraethylammonium-treated axons. Overall, our data suggest that C-CTX-1 affects nerve excitability and neurotransmitter release at nerve terminals. We conclude that C-CTX-1-induced up-regulation of Na(+) channels and the inhibition of K(+) channels, at low nanomolar concentrations, produce a variety of functional dysfunctions that are in part responsible for the human muscle skeletal symptoms observed in ciguatera. All these dysfunctions seem to result from the subtle balance between ionic currents, intracellular Na(+) and Ca(2+) concentrations, and engaged second messengers.

Brevenal inhibits pacific ciguatoxin-1B-induced neurosecretion from bovine chromaffin cells

Ciguatoxins and brevetoxins are neurotoxic cyclic polyether compounds produced by dinoflagellates, which are responsible for ciguatera and neurotoxic shellfish poisoning (NSP) respectively. Recently, brevenal, a natural compound was found to specifically inhibit brevetoxin action and to have a beneficial effect in NSP. Considering that brevetoxin and ciguatoxin specifically activate voltage-sensitive Na⁺ channels through the same binding site, brevenal has therefore a good potential for the treatment of ciguatera. Pacific ciguatoxin-1B (P-CTX-1B) activates voltage-sensitive Na⁺ channels and promotes an increase in neurotransmitter release believed to underpin the symptoms associated with ciguatera. However, the mechanism through which slow Na⁺ influx promotes neurosecretion is not fully understood. In the present study, we used chromaffin cells as a model to reconstitute the sequence of events culminating in ciguatoxin-evoked neurosecretion. We show that P-CTX-1B induces a tetrodotoxin-sensitive rise in intracellular Na⁺, closely followed by an increase in cytosolic Ca²⁺ responsible for promoting SNARE-dependent catecholamine secretion. Our results reveal that brevenal and β-naphtoyl-brevetoxin prevent P-CTX-1B secretagogue activity without affecting nicotine or barium-induced catecholamine secretion. Brevenal is therefore a potent inhibitor of ciguatoxin-induced neurotoxic effect and a potential treatment for ciguatera.

Food Poisoning

Food poisoning is encountered throughout the world. Many of the toxins responsible for specific food poisoning syndromes are no longer limited to isolated geographic locations. With increased travel and the ease of transporting food products, it is likely that a patient may present to any emergency department with the clinical effects of food poisoning. Recognizing specific food poisoning syndromes allows emergency health care providers not only to initiate appropriate treatment rapidly but also to notify health departments early and thereby prevent further poisoning cases. This article reviews several potential food-borne poisons and describes each agent's mechanism of toxicity, expected clinical presentation, and currently accepted treatment.

Toxins: Bacterial and Marine Toxins

The term toxin refers in a specific way to a toxic substance of biologic origin; that is, a true toxin is a poison produced by a living organism. The purpose of this article is to review some of the most potentially dangerous toxins of concern today. Mechanisms of action, routes of exposure, diagnostic tools, and treatment recommendations are addressed. In addition, current therapeutic uses for certain toxins are discussed.

Mechanisms involved in the swelling of erythrocytes caused by Pacific and Caribbean ciguatoxins

The mechanisms underlying the swelling of frog red blood cells (RBC), induced by Pacific (P-CTX-1) and Caribbean (C-CTX-1) ciguatoxins (CTXs), were investigated by measuring the length, width and surface of their elliptic shape. P-CTX-1 (0.5 to 5 nM) and C-CTX-1 (1 nM) induced RBC swelling within 60 min. The CTXs-induced RBC swelling was blocked by apamin (1 microM) and by Sr(2+) (1 mM). P-CTX-1-induced RBC swelling was prevented and inhibited by H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (27 microM), an inhibitor of soluble guanylate cyclase (sGC), and NOS blockade by NG methyl-l-arginine (l-NMA; 10 microM). Cytochalasin D (cytD, 10 microM) increased RBC surface and mimicked CTX effect but did not prevent the P-CTX-1-induced l-NMA-sensitive extra increase. Calculations revealed that P-CTX-1 and cytD increase RBC total surface envelop and volume. These data strongly suggest that the molecular mechanisms underlying CTXs-induced RBC swelling involve the NO pathway by an activation of the inducible NOS, leading to sGC activation which modulates intracellular cGMP and regulates L-type Ca(2+) channels. The resulting increase in intracellular Ca(2+) content, in turn, disrupts the actin cytoskeleton, which causes a water influx and triggers a Ca(2+)-activated K(+) current through SK2 isoform channels.

Neuroprotectant effects of iso-osmolar D-mannitol to prevent Pacific ciguatoxin-1 induced alterations in neuronal excitability: a comparison with other osmotic agents and free radical scavengers

The basis for the neuroprotectant effect of D-mannitol in reducing the sensory neurological disturbances seen in ciguatera poisoning, is unclear. Pacific ciguatoxin-1 (P-CTX-1), at a concentration 10 nM, caused a statistically significant swelling of rat sensory dorsal root ganglia (DRG) neurons that was reversed by hyperosmolar 50 mM D-mannitol. However, using electron paramagnetic resonance (EPR) spectroscopy, it was found that P-CTX-1 failed to generate hydroxyl free radicals at concentrations of toxin that caused profound effects on neuronal excitability. Whole-cell patch-clamp recordings from DRG neurons revealed that both hyper- and iso-osmolar 50 mM D-mannitol prevented the membrane depolarisation and repetitive firing of action potentials induced by P-CTX-1. In addition, both hyper- and iso-osmolar 50 mM D-mannitol prevented the hyperpolarising shift in steady-state inactivation and the rise in leakage current through tetrodotoxin (TTX)-sensitive Na(v) channels, as well as the increased rate of recovery from inactivation of TTX-resistant Na(v) channels induced by P-CTX-1. D-Mannitol also reduced, but did not prevent, the inhibition of peak TTX-sensitive and TTX-resistant I(Na) amplitude by P-CTX-1. Additional experiments using hyper- and iso-osmolar D-sorbitol, hyperosmolar sucrose and the free radical scavenging agents Trolox and L-ascorbic acid showed that these agents, unlike D-mannitol, failed to prevent the effects of P-CTX-1 on spike electrogenesis and Na(v) channel gating. These selective actions of D-mannitol indicate that it does not act purely as an osmotic agent to reduce swelling of nerves, but involves a more complex action dependent on the Na(v) channel subtype, possibly to alter or reduce toxin association.

Neurotoxic marine poisoning

Marine poisoning results from the ingestion of marine animals that contain toxic substances and causes substantial illness in coastal regions. Three main clinical syndromes of marine poisoning have important neurological symptoms-ciguatera, tetrodotoxin poisoning, and paralytic shellfish poisoning. Ciguatera is the commonest syndrome of marine poisoning and is characterised by moderate to severe gastrointestinal effects (vomiting, diarrhoea, and abdominal cramps) and neurological effects (myalgia, paraesthesia, cold allodynia, and ataxia), but is rarely lethal. Tetrodotoxin poisoning and paralytic shellfish poisoning are less common but have a higher fatality rate than ciguatera. Mild gastrointestinal effects and a descending paralysis are characteristic of these types of poisoning. In severe poisoning, paralysis rapidly progresses to respiratory failure. Diagnosis of all types of marine poisoning is made from the circumstances of ingestion (type of fish and location) and the clinical effects. Because there are no antidotes, supportive care, including mechanical ventilation in patients with severe paralysis, is the mainstay of treatment.

Muscarinic effects of the Caribbean ciguatoxin C-CTX-1 on frog atrial heart muscle

The effects of Caribbean ciguatoxin (C-CTX-1) isolated from horse-eye jack (Caranx latus) on the electrical and mechanical activities of frog auricle were studied. C-CTX-1 (1 pM-50 nM) dose-dependently shortened the duration of the plateau and the repolarizing phase of the action potential (AP). The AP shortening induced by C-CTX-1 (50 nM) was suppressed or prevented either by tetrodotoxin (TTX; 0.6 nM) or by atropine (0.1mM). C-CTX-1 (50 nM) prolonged the TTX (0.6 nM)-sensitive electrical response of the vagus nerve branches, which innervate the auricle. The C-CTX-1 (50 nM)-induced shortening of the plateau and of the repolarization phase were prevented or reversed by gallamine (20 microM) and pirenzepine (0.5 microM), respectively. C-CTX-1 (50 nM) decreased the amplitude of the peak contraction and shortened its duration. In the presence of gallamine (20 microM), C-CTX-1 decreased the amplitude of the peak contraction and shortened its duration in the presence of pirenzepine (0.5 microM). C-CTX-1 (50 nM) decreased the time constant of the relaxation phase of the peak contraction suggesting that it increased the Na(+)/Ca(2+) exchange activity. Acetylcholine (ACh; 1 pM) shortened APD, decreased the peak contraction and mimics the effects of C-CTX-1. In conclusion, the presented data show that C-CTX-1 released ACh from atrial cholinergic nerve terminals which activated M(1) and M(2) muscarinic receptors subtype (mAChR). Our findings suggest that M(1) and M(2) mAChR are present in frog atrial tissue and play a previously unrecognized role in the modulation of the AP duration and of the mechanical activity of cardiac tissue.

Ciguatoxin-induced oscillations in membrane potential and action potential firing in rat parasympathetic neurons

The actions of ciguatoxins from the Pacific (P-CTX-1) and Caribbean (C-CTX-1) regions were investigated in isolated parasympathetic neurons from rat intracardiac ganglia using patch-clamp recording techniques. Under current-clamp conditions, bath application of P-CTX-1 (1-10 nm) or C-CTX-1 (10-30 nm) caused a gradual depolarization that was accompanied by oscillation of the membrane potential leading to tonic action potential firing. Membrane potential oscillations were observed between -45 and -60 mV and had an amplitude of 10-20 mV and a mean frequency of 10 Hz. Oscillation frequency was temperature-dependent with a Q10 of 2.0. Membrane oscillations were temporarily inhibited by hyperpolarizing current pulses and potentiated by weak depolarizing current pulses. The amplitude of oscillations was reduced upon lowering the external Na+ concentration and inhibited by tetrodotoxin (TTX), tetracaine or Zn2+. Tetraethylammonium, 4-aminopyridine, Cs+, Cd2+, Ba2+, 1,4,4'-diothiocyanato-2,2'-stilbenedisulphonic acid (DIDS) and ouabain had no effect on the CTX-1-induced membrane depolarization and oscillations. Brevetoxin (PbTx-3, 100 nm), in contrast to CTX-1, caused a membrane depolarization that was not associated with oscillation of the membrane potential. Under voltage-clamp conditions, P-CTX-1 inhibited the peak amplitude of the voltage-dependent Na+ current and shifted the activation curve to more negative potentials, but membrane oscillations were not seen in this configuration. These results suggest that ciguatoxins cause oscillation of the membrane potential in mammalian autonomic neurons by modifying the activation and inactivation properties of a population of TTX-sensitive Na+ channels.

Characterisation of multiple Caribbean ciguatoxins and congeners in individual specimens of horse-eye jack (Caranx latus) by high-performance liquid chromatography/mass spectrometry

We studied the variation in toxin profiles of purified extracts of 10 individual specimens and two pools of ciguateric Caranx latus. High-performance liquid chromatography/mass spectrometry (HPLC/MS) identified in all individual samples at least seven Caribbean ciguatoxins (C-CTXs) comprising C-CTX-1 and its epimer C-CTX-2 ([M+H](+) m/z 1141.58), and five new C-CTX congeners with pseudo-molecular ions at m/z 1141.58, 1143.60, 1157.57, 1159.58, and 1127.57. In some samples, additional C-CTX isomers were detected with [M+H](+) ions at m/z 1141.58 (two), 1143.60 (one) and 1157.57 (two). The two low-toxic pools contained only four to six ciguatoxins. The comparison in relative proportions of four different mass classes ([M+H](+) at m/z 1141, 1143, 1157 and 1127) showed that the group at m/z 1157 increased (2-20%) with flesh toxicity. More than 80% of group m/z 1141 comprised C-CTX-1, C-CTX-2 and their isomer C-CTX-1a whose level in this group correlated with fish toxicity. Contrary to low-toxic fishes, high-risk specimens had C-CTX-1 levels <50% and were subjected to large losses of activity on purification indicating that unstable ciguatoxins were present. A possible conversion of C-CTX-1 into C-CTX-1a was identified when flesh was cooked, without changes in toxicity. In conclusion, HPLC/MS characterised 12 C-CTXs accumulated by C. latus at variable levels.

Ciguatera fish poisoning in the Caribbean islands and Western Atlantic

Ciguatera fish poisoning (ciguatera), a common poisoning caused by fish ingestion, is reviewed in the Western Atlantic and the Caribbean waters. It is endemic from Florida coasts (northern limit) to Martinique Island (southern limit), with outbreaks occurring from time to time. In the Caribbean, ciguatera causes a polymorphic syndrome with gastrointestinal, cardiovascular, and neurological signs and symptoms. Neurological and muscular dysfunctions can be treated by intravenous injection of D-mannitol. The lipid-soluble toxins involved are ciguatoxins that are likely produced by the dinoflagellate Gambierdiscus toxicus. G. toxicus strains are endemic in the Caribbean Sea and in theWestern Atlantic. Although it is likely that blooms of G. toxicus are ingested by herbivorous fishes, they are not implicated in ciguatera in the Caribbean. Rather, large carnivores (barracudas, jacks, snappers, groupers), consumers of smaller benthic fish, are often involved in ciguatera. Fish toxicity depends on fishing area and depth, fish size and tissues, and climatic disturbances. Ciguatoxins have been isolated and purified from Caribbean fish species. The structure of two epimers, C-CTX-1 and C-CTX-2 from horse-eye jack, comprise 14 trans-fused ether-linked rings and a hemiketal in terminal ring. Caribbean ciguatoxins are mainly detected in the laboratory by chicken, mouse, mosquito, or cell bioassays, and by analytical HPLC/tandem mass spectrometry down to parts per billion (ppb). A ciguatera management plan that integrates epidemiology, treatment, and a simple method of detection is required to ensure the protection of consumers.