Ciguatoxins activate specific cold pain pathways to elicit burning pain from cooling

Marine-derived bioactive compounds for neuropathic pain: pharmacology and therapeutic potential

Neuropathic pain, a challenging condition often associated with diabetes, trauma, or chemotherapy, impairs patients' quality of life. Current treatments often provide inconsistent relief and notable adverse effects, highlighting the urgent need for safer and more effective alternatives. This review investigates marine-derived bioactive compounds as potential novel therapies for neuropathic pain management. Marine organisms, including fungi, algae, cone snails, sponges, soft corals, tunicates, and fish, produce a diverse range of secondary metabolites with significant pharmacological properties. These include peptides (e.g., conopeptides, piscidin 1), non-peptides (e.g., guanidinium toxins, astaxanthin, docosahexaenoic acid, fucoidan, apigenin, fumagillin, aaptamine, flexibilide, excavatolide B, capnellenes, austrasulfones, lemnalol), and crude extracts (e.g., Spirulina platensis, Dunaliella salina, Cliothosa aurivilli). These compounds exhibit diverse mechanisms of action, such as modulating ion channels (e.g., transient receptor potential channels, voltage-gated sodium, calcium, and potassium channels, and G protein-coupled inwardly rectifying potassium channels), interacting with cell-surface receptors (e.g., nicotinic acetylcholine, NMDA, kainate, GABAB​, and neurotensin receptors), inhibiting norepinephrine transporters, reducing oxidative stress, and attenuating neuroinflammation. These effects collectively contribute to alleviating nerve degeneration and symptoms of neuropathic pain, including hyperalgesia, allodynia, and associated psychomotor disturbances. Marine-derived bioactive compounds represent promising alternatives to conventional neuropathic pain treatments, to advance their development and assess their integration into neuropathic pain management strategies.

Marine toxins in environment: Recent updates on depuration techniques

Marine toxins pose a significant safety risk, leading to human intoxications and causing substantial economic losses in seafood-producing regions. The development of rapid, cost-effective, efficient, and reliable approaches for the containment of these substances is therefore crucial in order to mitigate the adverse impact of marine toxins. This research conducted a comprehensive review on the toxicity and influencing factors of marine toxins production. Additionally, depuration technologies, including adsorption, advanced oxidation processes, biodegradation, heating treatment, temporary maintenance and purification, and drug inhibition, were systematically summarized. The study also provided a comparative analysis of the advantages and disadvantages of various depuration technologies and proposed strategies for future development.

Topical Oxaliplatin Produces Gain- and Loss-of-Function in Multiple Classes of Sensory Afferents

The platinum chemotherapeutic oxaliplatin produces dose-limiting pain, dysesthesia, and cold hypersensitivity in most patients immediately after infusion. An improved understanding of the mechanisms underlying these symptoms is urgently required to facilitate the development of symptomatic or preventative therapies. In this study, we have used skin-saphenous nerve recordings in vitro and behavioral experiments in mice to characterize the direct effects of oxaliplatin on different types of sensory afferent fibers. Our results confirmed that mice injected with oxaliplatin rapidly develop mechanical and cold hypersensitivities. We further noted profound changes to A fiber activity after the application of oxaliplatin to the receptive fields in the skin. Most oxaliplatin-treated Aδ- and rapidly adapting Aβ-units lost mechanical sensitivity, but units that retained responsiveness additionally displayed a novel, aberrant cold sensitivity. Slowly adapting Aβ-units did not display mechanical tachyphylaxis, and a subset of these fibers was sensitized to mechanical and cold stimulation after oxaliplatin treatment. C fiber afferents were less affected by acute applications of oxaliplatin, but a subset gained cold sensitivity. Taken together, our findings suggest that direct effects on peripheral A fibers play a dominant role in the development of acute oxaliplatin-induced cold hypersensitivity, numbness, and dysesthesia. PERSPECTIVE: The chemotherapeutic drug oxaliplatin rapidly gives rise to dose-limiting cold pain and dysesthesia. Here, we have used behavioral and electrophysiological studies of mice to characterize the responsible neurons. We show that oxaliplatin directly confers aberrant cold responsiveness to subsets of A-fibers while silencing other fibers of the same type.

hnRNPA1 SUMOylation promotes cold hypersensitivity in chronic inflammatory pain by stabilizing TRPA1 mRNA

TRPA1 is pivotal in cold hypersensitivity, but its regulatory mechanisms in inflammatory cold hyperalgesia remain poorly understood. We show here that the upregulation of SUMO1-conjugated protein levels in a complete Freund's adjuvant (CFA)-induced inflammatory pain model enhances TRPA1 mRNA stability, ultimately leading to increased expression levels. We further demonstrate that hnRNPA1 binds to TRPA1 mRNA, and its SUMOylation, upregulated in CFA-induced inflammatory pain, contributes to stabilizing TRPA1 mRNA by accumulating hnRNPA1 in the cytoplasm. Moreover, we find that wild-type hnRNPA1 viral infection in dorsal root ganglia neurons, and not infection with the SUMOylation-deficient hnRNPA1 mutant, can rescue the reduced ability of hnRNPA1-knockdown mice to develop inflammatory cold pain hypersensitivity. These results suggest that hnRNPA1 is a regulator of TRPA1 mRNA stability, the capability of which is enhanced upon SUMO1 conjugation at lysine 3 in response to peripheral inflammation, and the increased expression of TRPA1 in turn underlies the development of chronic inflammatory cold pain hypersensitivity.

Ciguatera Fish Poisoning in the Caribbean Sea and Atlantic Ocean: Reconciling the Multiplicity of Ciguatoxins and Analytical Chemistry Approach for Public Health Safety

Ciguatera is a major circumtropical poisoning caused by the consumption of marine fish and invertebrates contaminated with ciguatoxins (CTXs): neurotoxins produced by endemic and benthic dinoflagellates which are biotransformed in the fish food-web. We provide a history of ciguatera research conducted over the past 70 years on ciguatoxins from the Pacific Ocean (P-CTXs) and Caribbean Sea (C-CTXs) and describe their main chemical, biochemical, and toxicological properties. Currently, there is no official method for the extraction and quantification of ciguatoxins, regardless their origin, mainly due to limited CTX-certified reference materials. In this review, the extraction and purification procedures of C-CTXs are investigated, considering specific objectives such as isolating reference materials, analysing fish toxin profiles, or ensuring food safety control. Certain in vitro assays may provide sufficient sensitivity to detect C-CTXs at sub-ppb levels in fish, but they do not allow for individual identification of CTXs. Recent advances in analysis using liquid chromatography coupled with low- or high-resolution mass spectrometry provide new opportunities to identify known C-CTXs, to gain structural insights into new analogues, and to quantify C-CTXs. Together, these methods reveal that ciguatera arises from a multiplicity of CTXs, although one major form (C-CTX-1) seems to dominate. However, questions arise regarding the abundance and instability of certain C-CTXs, which are further complicated by the wide array of CTX-producing dinoflagellates and fish vectors. Further research is needed to assess the toxic potential of the new C-CTX and their role in ciguatera fish poisoning. With the identification of C-CTXs in the coastal USA and Eastern Atlantic Ocean, the investigation of ciguatera fish poisoning is now a truly global effort.

IQGAP1 promotes chronic pain by regulating the trafficking and sensitization of TRPA1 channels

TRPA1 channels have been implicated in mechanical and cold hypersensitivity in chronic pain. But how TRPA1 mediates this process is unclear. Here we show that IQ-motif containing GTPase activating protein 1 (IQGAP1) is responsible using a combination of biochemical, molecular, Ca2+ imaging and behavioural approaches. TRPA1 and IQGAP1 bind to each other and are highly colocalised in sensory DRG neurons in mice. The expression of IQGAP1 but not TRPA1 is increased in chronic inflammatory and neuropathic pain. However, TRPA1 undergoes increased trafficking to the membrane of DRG neurons catalysed by the small GTPase Cdc42 associated with IQGAP1, leading to functional sensitization of the channel. Activation of PKA is also sufficient to evoke TRPA1 trafficking and sensitization. All these responses are, however, completely prevented in the absence of IQGAP1. Concordantly, deletion of IQGAP1 markedly reduces mechanical and cold hypersensitivity in chronic inflammatory and neuropathic pain in mice. IQGAP1 thus promotes chronic pain by coupling the trafficking and signalling machineries to TRPA1 channels.

Cold avoidance and heat pain hypersensitivity in neuronal nucleoredoxin knockout mice

Nucleoredoxin is a thioredoxin-like oxidoreductase that mainly acts as oxidase and thereby regulates calcium calmodulin kinase Camk2a, an effector of nitric oxide mediated synaptic potentiation and nociceptive sensitization. We asked here if and how NXN affects thermal sensation and nociception in mice using pan-neuronal NXN deletion driven by Nestin-Cre, and sensory neuron specific deletion driven by Advillin-Cre. In a thermal gradient ring, where mice can freely choose the temperature of well-being, Nestin-NXN^-/- mice avoided unpleasant cold temperatures. In neuropathic and inflammatory nociceptive models, Nestin-NXN^-/- and Advillin-NXN^-/- mice displayed subtle phenotypes of heightened heat nociception. Abnormal thermal in vivo responses were associated with heightened calcium influx upon stimulation of transient receptor channels, with heightened oxygen consumption upon disruption of the mitochondrial membrane potential and with higher density of neurite trees of primary sensory neurons of the dorsal root ganglia in cultures. The data suggest that loss of NXN's balancing redox functions leads to maladaptive changes in sensory neurons that manifest in vivo as polyneuropathy-like abnormal cold sensitivity and heat "pain".

Gambierdiscus and Its Associated Toxins: A Minireview

Gambierdiscus is a dinoflagellate genus widely distributed throughout tropical and subtropical regions. Some members of this genus can produce a group of potent polycyclic polyether neurotoxins responsible for ciguatera fish poisoning (CFP), one of the most significant food-borne illnesses associated with fish consumption. Ciguatoxins and maitotoxins, the two major toxins produced by Gambierdiscus, act on voltage-gated channels and TRPA1 receptors, consequently leading to poisoning and even death in both humans and animals. Over the past few decades, the occurrence and geographic distribution of CFP have undergone a significant expansion due to intensive anthropogenic activities and global climate change, which results in more human illness, a greater public health impact, and larger economic losses. The global spread of CFP has led to Gambierdiscus and its toxins being considered an environmental and human health concern worldwide. In this review, we seek to provide an overview of recent advances in the field of Gambierdiscus and its associated toxins based on the existing literature combined with re-analyses of current data. The taxonomy, phylogenetics, geographic distribution, environmental regulation, toxin detection method, toxin biosynthesis, and pharmacology and toxicology of Gambierdiscus are summarized and discussed. We also highlight future perspectives on Gambierdiscus and its associated toxins.

Current Trends and New Challenges in Marine Phycotoxins

Marine phycotoxins are a multiplicity of bioactive compounds which are produced by microalgae and bioaccumulate in the marine food web. Phycotoxins affect the ecosystem, pose a threat to human health, and have important economic effects on aquaculture and tourism worldwide. However, human health and food safety have been the primary concerns when considering the impacts of phycotoxins. Phycotoxins toxicity information, often used to set regulatory limits for these toxins in shellfish, lacks traceability of toxicity values highlighting the need for predefined toxicological criteria. Toxicity data together with adequate detection methods for monitoring procedures are crucial to protect human health. However, despite technological advances, there are still methodological uncertainties and high demand for universal phycotoxin detectors. This review focuses on these topics, including uncertainties of climate change, providing an overview of the current information as well as future perspectives.

Modality-Specific Modulation of Temperature Representations in the Spinal Cord after Injury

Different types of tissue injury, such as inflammatory and neuropathic conditions, cause modality-specific alternations on temperature perception. There are profound changes in peripheral sensory neurons after injury, but how patterned neuronal activities in the CNS encode injury-induced sensitization to temperature stimuli is largely unknown. Using in vivo calcium imaging and mouse genetics, we show that formalin- and prostaglandin E2-induced inflammation dramatically increase spinal responses to heating and decrease responses to cooling in male and female mice. The reduction of cold response is largely eliminated on ablation of TRPV1-expressing primary sensory neurons, indicating a crossover inhibition of cold response from the hyperactive heat inputs in the spinal cord. Interestingly, chemotherapy medication oxaliplatin can rapidly increase spinal responses to cooling and suppress responses to heating. Together, our results suggest a push-pull mechanism in processing cold and heat inputs and reveal a synergic mechanism to shift thermosensation after injury.SIGNIFICANCE STATEMENT In this paper, we combine our novel in vivo spinal cord two-photon calcium imaging, mouse genetics, and persistent pain models to study how tissue injury alters the sensation of temperature. We discover modality-specific changes of spinal temperature responses in different models of injury. Chemotherapy medication oxaliplatin leads to cold hypersensitivity and heat hyposensitivity. By contrast, inflammation increases heat sensitivity and decreases cold sensitivity. This decrease in cold sensitivity results from the stronger crossover inhibition from the hyperactive heat inputs. Our work reveals the bidirectional change of thermosensitivity by injury and suggests that the crossover inhibitory circuit underlies the shifted thermosensation, providing a mechanism to the biased perception toward a unique thermal modality that was observed clinically in chronic pain patients.

Development and Validation of an LC-MS/MS Method for the Ultra-Trace Analysis of Pacific Ciguatoxins in Fish

BACKGROUND

Ciguatera fish poisoning (CFP) poses a serious threat to both public health and the use of aquatic resources from the various warm-water regions of the world. Hence, a process for the efficient determination of the relevant toxins is required.

OBJECTIVE

We sought to develop and validate the first LC-MS/MS method to quantify the major toxins prevalent in fish from the Pacific Ocean.

METHOD

Toxins were extracted from fish flesh (2 g) using a methanol-water mixture (9:1, v/v). The extract was heated at 80°C, and low-polarity lipids were eliminated using hexane, initially from the basic solution and later from the acidic solution. The cleanup was performed using solid-phase extraction, Florisil, silica, reversed-phase C18, and primary secondary amine columns. A validation study was conducted by spiking fish flesh with two representative toxins having different skeletal structures and polarities and was calibrated by NMR (qNMR) spectroscopy.

RESULTS

The validation parameters for the ciguatera toxins CTX1B and CTX3C at spiked levels of 0.1 µg/kg were as follows: repeatabilities of 2.3-3.5% and 3.2-5.3%; intermediate precisions of 6.3-9.8% and 6.0-7.4%; recoveries of 80-107% and 95-120%, respectively. The lowest detection levels were 0.004 µg/kg for CTX1B, 0.005 µg/kg for 51-hydroxyCTX3C, and 0.009 µg/kg for CTX3C.

CONCLUSIONS

The described method practically clears the international action level of 0.01 µg/kg CTX1B equivalents set by the U.S. Food and Drug Administration and the European Food Safety Authority and satisfies the global standards set by Codex and AOAC INTERNATIONAL.

HIGHLIGHTS

A validation study for an LC-MS/MS method for ciguatoxin detection was completed for the first time using calibrated toxin standards.

Screening for Predictors of Chronic Ciguatera Poisoning: An Exploratory Analysis among Hospitalized Cases from French Polynesia

Ciguatera poisoning is a globally occurring seafood disease caused by the ingestion of marine products contaminated with dinoflagellate produced neurotoxins. Persistent forms of ciguatera, which prove to be highly debilitating, are poorly studied and represent a significant medical issue. The present study aims to better understand chronic ciguatera manifestations and identify potential predictive factors for their duration. Medical files of 49 patients were analyzed, and the post-hospitalization evolution of the disease assessed through a follow-up questionnaire. A rigorous logistic lasso regression model was applied to select significant predictors from a list of 37 patient characteristics potentially predictive of having chronic symptoms. Missing data were handled by complete case analysis, and a survival analysis was implemented. All models used standardized variables, and multiple comparisons in the survival analyses were handled by Bonferroni correction. Among all studied variables, five significant predictors of having symptoms lasting ≥3 months were identified: age, tobacco consumption, acute bradycardia, laboratory measures of urea, and neutrophils. This exploratory, hypothesis-generating study contributes to the development of ciguatera epidemiology by narrowing the list from 37 possible predictors to a list of five predictors that seem worth further investigation as candidate risk factors in more targeted studies of ciguatera symptom duration.

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.

Cyanobacteria, Cyanotoxins, and Neurodegenerative Diseases: Dangerous Liaisons

The prevalence of neurodegenerative disease (ND) is increasing, partly owing to extensions in lifespan, with a larger percentage of members living to an older age, but the ND aetiology and pathogenesis are not fully understood, and effective treatments are still lacking. Neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis are generally thought to progress as a consequence of genetic susceptibility and environmental influences. Up to now, several environmental triggers have been associated with NDs, and recent studies suggest that some cyanotoxins, produced by cyanobacteria and acting through a variety of molecular mechanisms, are highly neurotoxic, although their roles in neuropathy and particularly in NDs are still controversial. In this review, we summarize the most relevant and recent evidence that points at cyanotoxins as environmental triggers in NDs development.

Silent cold-sensing neurons contribute to cold allodynia in neuropathic pain

Patients with neuropathic pain often experience innocuous cooling as excruciating pain. The cell and molecular basis of this cold allodynia is little understood. We used in vivo calcium imaging of sensory ganglia to investigate how the activity of peripheral cold-sensing neurons was altered in three mouse models of neuropathic pain: oxaliplatin-induced neuropathy, partial sciatic nerve ligation, and ciguatera poisoning. In control mice, cold-sensing neurons were few in number and small in size. In neuropathic animals with cold allodynia, a set of normally silent large diameter neurons became sensitive to cooling. Many of these silent cold-sensing neurons responded to noxious mechanical stimuli and expressed the nociceptor markers Nav1.8 and CGRPα. Ablating neurons expressing Nav1.8 resulted in diminished cold allodynia. The silent cold-sensing neurons could also be activated by cooling in control mice through blockade of Kv1 voltage-gated potassium channels. Thus, silent cold-sensing neurons are unmasked in diverse neuropathic pain states and cold allodynia results from peripheral sensitization caused by altered nociceptor excitability.

Critical Review and Conceptual and Quantitative Models for the Transfer and Depuration of Ciguatoxins in Fishes

We review and develop conceptual models for the bio-transfer of ciguatoxins in food chains for Platypus Bay and the Great Barrier Reef on the east coast of Australia. Platypus Bay is unique in repeatedly producing ciguateric fishes in Australia, with ciguatoxins produced by benthic dinoflagellates (Gambierdiscus spp.) growing epiphytically on free-living, benthic macroalgae. The Gambierdiscus are consumed by invertebrates living within the macroalgae, which are preyed upon by small carnivorous fishes, which are then preyed upon by Spanish mackerel (Scomberomorus commerson). We hypothesise that Gambierdiscus and/or Fukuyoa species growing on turf algae are the main source of ciguatoxins entering marine food chains to cause ciguatera on the Great Barrier Reef. The abundance of surgeonfish that feed on turf algae may act as a feedback mechanism controlling the flow of ciguatoxins through this marine food chain. If this hypothesis is broadly applicable, then a reduction in herbivory from overharvesting of herbivores could lead to increases in ciguatera by concentrating ciguatoxins through the remaining, smaller population of herbivores. Modelling the dilution of ciguatoxins by somatic growth in Spanish mackerel and coral trout (Plectropomus leopardus) revealed that growth could not significantly reduce the toxicity of fish flesh, except in young fast-growing fishes or legal-sized fishes contaminated with low levels of ciguatoxins. If Spanish mackerel along the east coast of Australia can depurate ciguatoxins, it is most likely with a half-life of ≤1-year. Our review and conceptual models can aid management and research of ciguatera in Australia, and globally.

Pacific-Ciguatoxin-2 and Brevetoxin-1 Induce the Sensitization of Sensory Receptors Mediating Pain and Pruritus in Sensory Neurons

Ciguatera fish poisoning (CFP) and neurotoxic shellfish poisoning syndromes are induced by the consumption of seafood contaminated by ciguatoxins and brevetoxins. Both toxins cause sensory symptoms such as paresthesia, cold dysesthesia and painful disorders. An intense pruritus, which may become chronic, occurs also in CFP. No curative treatment is available and the pathophysiology is not fully elucidated. Here we conducted single-cell calcium video-imaging experiments in sensory neurons from newborn rats to study in vitro the ability of Pacific-ciguatoxin-2 (P-CTX-2) and brevetoxin-1 (PbTx-1) to sensitize receptors and ion channels, (i.e., to increase the percentage of responding cells and/or the response amplitude to their pharmacological agonists). In addition, we studied the neurotrophin release in sensory neurons co-cultured with keratinocytes after exposure to P-CTX-2. Our results show that P-CTX-2 induced the sensitization of TRPA1, TRPV4, PAR2, MrgprC, MrgprA and TTX-r NaV channels in sensory neurons. P-CTX-2 increased the release of nerve growth factor and brain-derived neurotrophic factor in the co-culture supernatant, suggesting that those neurotrophins could contribute to the sensitization of the aforementioned receptors and channels. Our results suggest the potential role of sensitization of sensory receptors/ion channels in the induction or persistence of sensory disturbances in CFP syndrome.

Odontoblast TRPC5 channels signal cold pain in teeth

Teeth are composed of many tissues, covered by an inflexible and obdurate enamel. Unlike most other tissues, teeth become extremely cold sensitive when inflamed. The mechanisms of this cold sensation are not understood. Here, we clarify the molecular and cellular components of the dental cold sensing system and show that sensory transduction of cold stimuli in teeth requires odontoblasts. TRPC5 is a cold sensor in healthy teeth and, with TRPA1, is sufficient for cold sensing. The odontoblast appears as the direct site of TRPC5 cold transduction and provides a mechanism for prolonged cold sensing via TRPC5's relative sensitivity to intracellular calcium and lack of desensitization. Our data provide concrete functional evidence that equipping odontoblasts with the cold-sensor TRPC5 expands traditional odontoblast functions and renders it a previously unknown integral cellular component of the dental cold sensing system.

Calcium Increase and Substance P Release Induced by the Neurotoxin Brevetoxin-1 in Sensory Neurons: Involvement of PAR2 Activation through Both Cathepsin S and Canonical Signaling

Red tides involving Karenia brevis expose humans to brevetoxins (PbTxs). Oral exposition triggers neurotoxic shellfish poisoning, whereas inhalation induces a respiratory syndrome and sensory disturbances. No curative treatment is available and the pathophysiology is not fully elucidated. Protease-activated receptor 2 (PAR2), cathepsin S (Cat-S) and substance P (SP) release are crucial mediators of the sensory effects of ciguatoxins (CTXs) which are PbTx analogs. This work explored the role of PAR2 and Cat-S in PbTx-1-induced sensory effects and deciphered the signaling pathway involved. We performed calcium imaging, PAR2 immunolocalization and SP release experiments in monocultured sensory neurons or co-cultured with keratinocytes treated with PbTx-1 or P-CTX-2. We demonstrated that PbTx-1-induced calcium increase and SP release involved Cat-S, PAR2 and transient receptor potential vanilloid 4 (TRPV4). The PbTx-1-induced signaling pathway included protein kinase A (PKA) and TRPV4, which are compatible with the PAR2 biased signaling induced by Cat-S. Internalization of PAR2 and protein kinase C (PKC), inositol triphosphate receptor and TRPV4 activation evoked by PbTx-1 are compatible with the PAR2 canonical signaling. Our results suggest that PbTx-1-induced sensory disturbances involve the PAR2-TRPV4 pathway. We identified PAR2, Cat-S, PKA, and PKC that are involved in TRPV4 sensitization induced by PbTx-1 in sensory neurons.

Evidence for the Range Expansion of Ciguatera in French Polynesia: A Revisit of the 2009 Mass-Poisoning Outbreak in Rapa Island (Australes Archipelago)

Ciguatera poisoning (CP) results from the consumption of seafood contaminated with ciguatoxins (CTXs). This disease is highly prevalent in French Polynesia with several well-identified hotspots. Rapa Island, the southernmost inhabited island in the country, was reportedly free of CP until 2007. This study describes the integrated approach used to investigate the etiology of a fatal mass-poisoning outbreak that occurred in Rapa in 2009. Symptoms reported in patients were evocative of ciguatera. Several Gambierdiscus field samples collected from benthic assemblages tested positive by the receptor binding assay (RBA). Additionally, the toxicity screening of ≈250 fish by RBA indicated ≈78% of fish could contain CTXs. The presence of CTXs in fish was confirmed by liquid chromatography tandem mass spectrometry (LC-MS/MS). The potential link between climate change and this range expansion of ciguatera to a subtropical locale of French Polynesia was also examined based on the analysis of temperature time-series data. Results are indicative of a global warming trend in Rapa area. A five-fold reduction in incidence rates was observed between 2009 and 2012, which was due in part to self-regulating behavior among individuals (avoidance of particular fish species and areas). Such observations underscore the prominent role played by community outreach in ciguatera risk management.

Animal toxins - Nature's evolutionary-refined toolkit for basic research and drug discovery

Venomous animals have evolved toxins that interfere with specific components of their victim's core physiological systems, thereby causing biological dysfunction that aids in prey capture, defense against predators, or other roles such as intraspecific competition. Many animal lineages evolved venom systems independently, highlighting the success of this strategy. Over the course of evolution, toxins with exceptional specificity and high potency for their intended molecular targets have prevailed, making venoms an invaluable and almost inexhaustible source of bioactive molecules, some of which have found use as pharmacological tools, human therapeutics, and bioinsecticides. Current biomedically-focused research on venoms is directed towards their use in delineating the physiological role of toxin molecular targets such as ion channels and receptors, studying or treating human diseases, targeting vectors of human diseases, and treating microbial and parasitic infections. We provide examples of each of these areas of venom research, highlighting the potential that venom molecules hold for basic research and drug development.

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.

PAR2, Keratinocytes, and Cathepsin S Mediate the Sensory Effects of Ciguatoxins Responsible for Ciguatera Poisoning

Ciguatera fish poisoning is caused by the consumption of fish contaminated with ciguatoxins (CTXs). The most distressing symptoms are cutaneous sensory disturbances, including cold dysesthesia and itch. CTXs are neurotoxins known to activate voltage-gated sodium channels, but no specific treatment exists. Peptidergic neurons have been critically involved in ciguatera fish poisoning sensory disturbances. Protease-activated receptor-2 (PAR2) is an itch- and pain-related G protein‒coupled receptor whose activation leads to a calcium-dependent neuropeptide release. In this study, we studied the role of voltage-gated sodium channels, PAR2, and the PAR2 agonist cathepsin S in the cytosolic calcium increase and subsequent release of the neuropeptide substance P elicited by Pacific CTX-2 (P-CTX-2) in rat sensory neurons and human epidermal keratinocytes. In sensory neurons, the P-CTX-2‒evoked calcium response was driven by voltage-gated sodium channels and PAR2-dependent mechanisms. In keratinocytes, P-CTX-2 also induced voltage-gated sodium channels and PAR2-dependent marked calcium response. In the cocultured cells, P-CTX-2 significantly increased cathepsin S activity, and cathepsin S and PAR2 antagonists almost abolished P-CTX-2‒elicited substance P release. Keratinocytes synergistically favored the induced substance P release. Our results demonstrate that the sensory effects of CTXs involve the cathepsin S-PAR2 pathway and are potentiated by their direct action on nonexcitable keratinocytes through the same pathway.

The (Un)Conscious Mouse as a Model for Human Brain Functions: Key Principles of Anesthesia and Their Impact on Translational Neuroimaging

In recent years, technical and procedural advances have brought functional magnetic resonance imaging (fMRI) to the field of murine neuroscience. Due to its unique capacity to measure functional activity non-invasively, across the entire brain, fMRI allows for the direct comparison of large-scale murine and human brain functions. This opens an avenue for bidirectional translational strategies to address fundamental questions ranging from neurological disorders to the nature of consciousness. The key challenges of murine fMRI are: (1) to generate and maintain functional brain states that approximate those of calm and relaxed human volunteers, while (2) preserving neurovascular coupling and physiological baseline conditions. Low-dose anesthetic protocols are commonly applied in murine functional brain studies to prevent stress and facilitate a calm and relaxed condition among animals. Yet, current mono-anesthesia has been shown to impair neural transmission and hemodynamic integrity. By linking the current state of murine electrophysiology, Ca2+ imaging and fMRI of anesthetic effects to findings from human studies, this systematic review proposes general principles to design, apply and monitor anesthetic protocols in a more sophisticated way. The further development of balanced multimodal anesthesia, combining two or more drugs with complementary modes of action helps to shape and maintain specific brain states and relevant aspects of murine physiology. Functional connectivity and its dynamic repertoire as assessed by fMRI can be used to make inferences about cortical states and provide additional information about whole-brain functional dynamics. Based on this, a simple and comprehensive functional neurosignature pattern can be determined for use in defining brain states and anesthetic depth in rest and in response to stimuli. Such a signature can be evaluated and shared between labs to indicate the brain state of a mouse during experiments, an important step toward translating findings across species.

The pathology of Chironex fleckeri venom and known biological mechanisms

The large box jellyfish Chironex fleckeri is found in northern Australian waters. A sting from this cubozoan species can kill within minutes. From clinical and animal studies, symptoms comprise severe pain, welts, scarring, hypotension, vasospasms, cardiac irregularities and cardiac arrest. At present, there is no cure and opioids are used to manage pain. Antivenom is available but controversy exists over its effectiveness. Experimental and combination therapies performed in vitro and in vivo have shown varied efficacy. These inconsistent results are likely a consequence of the different methods used to extract venom. Recent omics analysis has shed light on the systems of C. fleckeri venom action, including new toxin classes that use pore formation, cell membrane collapse and ion channel modulation. This review covers what is known on C. fleckeri pathomechanisms and highlights current gaps in knowledge. A more complete understanding of the mechanisms of C. fleckeri venom-induced pathology may lead to novel treatments and possibly, the discovery of novel cell pathways, novel drug scaffolds and novel drug targets for human disease.

Toxicity Characterisation of Gambierdiscus Species from the Canary Islands

In the last decade, several outbreaks of ciguatera fish poisoning (CFP) have been reported in the Canary Islands (central northeast Atlantic Ocean), confirming ciguatera as an emerging alimentary risk in this region. Five Gambierdiscus species, G. australes, G. excentricus, G. silvae, G. carolinianus and G. caribaeus, have been detected in macrophytes from this area and are known to produce the ciguatoxins (CTXs) that cause CFP. A characterization of the toxicity of these species is the first step in identifying locations in the Canary Islands at risk of CFP. Therefore, in this study the toxicity of 63 strains of these five Gambierdiscus species were analysed using the erythrocyte lysis assay to evaluate their maitotoxin (MTX) content. In addition, 20 of the strains were also analysed in a neuroblastoma Neuro-2a (N2a) cytotoxicity assay to determine their CTX-like toxicity. The results allowed the different species to be grouped according to their ratios of CTX-like and MTX-like toxicity. MTX-like toxicity was especially high in G. excentricus and G. australes but much lower in the other species and lowest in G. silvae. CTX-like toxicity was highest in G. excentricus, which produced the toxin in amounts ranging between 128.2 ± 25.68 and 510.6 ± 134.2 fg CTX1B equivalents (eq) cell⁻¹ (mean ± SD). In the other species, CTX concentrations were as follows: G. carolinianus (100.84 ± 18.05 fg CTX1B eq cell⁻¹), G. australes (31.1 ± 0.56 to 107.16 ± 21.88 fg CTX1B eq cell⁻¹), G. silvae (12.19 ± 0.62 to 76.79 ± 4.97 fg CTX1B eq cell⁻¹) and G. caribaeus (<LOD to 90.37 ± 15.89 fg CTX1B eq cell⁻¹). Unlike the similar CTX-like toxicity of G. australes and G. silvae strains from different locations, G. excentricus and G. caribaeus differed considerably according to the origin of the strain. These differences emphasise the importance of species identification to assess the regional risk of CFP.

Molecular mechanisms of cold pain

The sensation of cooling is essential for survival. Extreme cold is a noxious stimulus that drives protective behaviour and that we thus perceive as pain. However, chronic pain patients suffering from cold allodynia paradoxically experience innocuous cooling as excruciating pain. Peripheral sensory neurons that detect decreasing temperature express numerous cold-sensitive and voltage-gated ion channels that govern their response to cooling in health and disease. In this review, we discuss how these ion channels control the sense of cooling and cold pain under physiological conditions, before focusing on the molecular mechanisms by which ion channels can trigger pathological cold pain. With the ever-rising number of patients burdened by chronic pain, we end by highlighting the pressing need to define the cells and molecules involved in cold allodynia and so identify new, rational drug targets for the analgesic treatment of cold pain.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

NaV 1.6 regulates excitability of mechanosensitive sensory neurons

KEY POINTS

Voltage-gated sodium channels are critical for peripheral sensory neuron transduction and have been implicated in a number of painful and painless disorders. The β-scorpion toxin, Cn2, is selective for Na_V 1.6 in dorsal root ganglion neurons. Na_V 1.6 plays an essential role in peripheral sensory neurons, specifically at the distal terminals of mechanosensing fibres innervating the skin and colon. Na_V 1.6 activation also leads to enhanced response to mechanical stimulus in vivo. This works highlights the use of toxins in elucidating pain pathways moreover the importance of non-peripherally restricted Na_V isoforms in pain generation.

ABSTRACT

Peripheral sensory neurons express multiple voltage-gated sodium channels (Na_V ) critical for the initiation and propagation of action potentials and transmission of sensory input. Three pore-forming sodium channel isoforms are primarily expressed in the peripheral nervous system (PNS): Na_V 1.7, Na_V 1.8 and Na_V 1.9. These sodium channels have been implicated in painful and painless channelopathies and there has been intense interest in them as potential therapeutic targets in human pain. Emerging evidence suggests Na_V 1.6 channels are an important isoform in pain sensing. This study aimed to assess, using pharmacological approaches, the function of Na_V 1.6 channels in peripheral sensory neurons. The potent and Na_V 1.6 selective β-scorpion toxin Cn2 was used to assess the effect of Na_V 1.6 channel activation in the PNS. The multidisciplinary approach included Ca²⁺ imaging, whole-cell patch-clamp recordings, skin-nerve and gut-nerve preparations and in vivo behavioural assessment of pain. Cn2 facilitates Na_V 1.6 early channel opening, and increased persistent and resurgent currents in large-diameter dorsal root ganglion (DRG) neurons. This promotes enhanced excitatory drive and tonic action potential firing in these neurons. In addition, Na_V 1.6 channel activation in the skin and gut leads to increased response to mechanical stimuli. Finally, intra-plantar injection of Cn2 causes mechanical but not thermal allodynia. This study confirms selectivity of Cn2 on Na_V 1.6 channels in sensory neurons. Activation of Na_V 1.6 channels, in terminals of the skin and viscera, leads to profound changes in neuronal responses to mechanical stimuli. In conclusion, sensory neurons expressing Na_V 1.6 are important for the transduction of mechanical information in sensory afferents innervating the skin and viscera.

Sodium channels and pain: from toxins to therapies

Voltage-gated sodium channels (NaV channels) are essential for the initiation and propagation of action potentials that critically influence our ability to respond to a diverse range of stimuli. Physiological and pharmacological studies have linked abnormal function of NaV channels to many human disorders, including chronic neuropathic pain. These findings, along with the description of the functional properties and expression pattern of NaV channel subtypes, are helping to uncover subtype specific roles in acute and chronic pain and revealing potential opportunities to target these with selective inhibitors. High-throughput screens and automated electrophysiology platforms have identified natural toxins as a promising group of molecules for the development of target-specific analgesics. In this review, the role of toxins in defining the contribution of NaV channels in acute and chronic pain states and their potential to be used as analgesic therapies are discussed.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

An SAR study of hydroxy-trifluoromethylpyrazolines as inhibitors of Orai1-mediated store operated Ca2+ entry in MDA-MB-231 breast cancer cells using a convenient Fluorescence Imaging Plate Reader assay

The proteins Orai1 and STIM1 control store-operated Ca²⁺ entry (SOCE) into cells. SOCE is important for migration, invasion and metastasis of MDA-MB-231 human triple negative breast cancer (TNBC) cells and has been proposed as a target for cancer drug discovery. Two hit compounds from a medium throughput screen, displayed encouraging inhibition of SOCE in MDA-MB-231 cells, as measured by a Fluorescence Imaging Plate Reader (FLIPR) Ca²⁺ assay. Following NMR spectroscopic analysis of these hits and reassignment of their structures as 5-hydroxy-5-trifluoromethylpyrazolines, a series of analogues was prepared via thermal condensation reactions between substituted acylhydrazones and trifluoromethyl 1,3-dicarbonyl arenes. Structure-activity relationship (SAR) studies showed that small lipophilic substituents at the 2- and 3-positions of the RHS and 2-, 3- and 4-postions of the LHS terminal benzene rings improved activity, resulting in a novel class of potent and selective inhibitors of SOCE.

Tectus niloticus (Tegulidae, Gastropod) as a Novel Vector of Ciguatera Poisoning: Clinical Characterization and Follow-Up of a Mass Poisoning Event in Nuku Hiva Island (French Polynesia)

Ciguatera fish poisoning (CFP) is the most prevalent non-bacterial food-borne form of poisoning in French Polynesia, which results from the consumption of coral reef fish naturally contaminated with ciguatoxins produced by dinoflagellates in the genus Gambierdiscus. Since the early 2000s, this French territory has also witnessed the emergence of atypical forms of ciguatera, known as ciguatera shellfish poisoning (CSP), associated with the consumption of marine invertebrates. In June 2014, nine tourists simultaneously developed a major and persistent poisoning syndrome following the consumption of the gastropod Tectus niloticus collected in Anaho, a secluded bay of Nuku Hiva Island (Marquesas Archipelago, French Polynesia). The unusual nature and severity of this event prompted a multidisciplinary investigation in order to characterize the etiology and document the short/long-term health consequences of this mass-poisoning event. This paper presents the results of clinical investigations based on hospital medical records, medical follow-up conducted six and 20 months post-poisoning, including a case description. This study is the first to describe the medical signature of T. niloticus poisoning in French Polynesia and contributed to alerting local authorities about the potential health hazards associated with the consumption of this gastropod, which is highly prized by local communities in Pacific island countries and territories.

Comparative studies of saxitoxin (STX) -induced cytotoxicity in Neuro-2a and RTG-2 cell lines: An explanation with respect to changes in ROS

Saxitoxin (STX), a paralytic shellfish toxin (PST) produced from toxic bloom-forming dinoflagellates, was selected to comparatively investigate the induction of cytotoxicity and apoptosis and a possible mechanism based on changes in the antioxidant defence system of two cellular strains: the mouse neuroblastoma cell line Neuro-2a and the rainbow trout fish cell line RTG-2. Increasing concentrations of STX (0-256 nM) presented little cytotoxic or apoptotic effects on the two cell lines. Measurements of cellular viability, lethal ratio and LDH leakage showed slight changes in Neuro-2a and RTG-2 cells (p > 0.05), and similar results were observed for cellular morphology and apoptotic rates. The contents of the main reactive oxygen species (ROS) components, superoxide anion (O₂^-) and hydrogen peroxide (H₂O₂), were markedly increased in Neuro-2a cell with STX exposure at middle (15 nM) and high (150 nM) concentrations (p < 0.05), and the simultaneous increase of the ratio of reduced/oxidized glutathione (GSH/GSSG) (p < 0.05) inferred the occurrence of oxidative stress. However, little difference was observed in all treated groups of RTG-2 cells. The activities of three antioxidant enzymes, superoxide dismutase (SOD), catalase (CAT) and glutathione reductase (GR), were significantly enhanced in Neuro-2a cells in the middle and high concentration groups (p < 0.05), while glutathione peroxidase (GPX) obviously decreased (p < 0.05) in all treated groups. Little change was found in RTG-2 cells with the same exposures. These results provided evidence that STX exposure altered the redox status of Neuro-2a cells and resulted in oxidative stress, but the same exposure exerted little effect on RTG-2 cells. Therefore, Neuro-2a cells are more sensitive than reproductive cells to STX exposure, and the antioxidant systems appears to be partly responsible for this differentiation response.

Regulation of Pain and Itch by TRP Channels

Nociception is an important physiological process that detects harmful signals and results in pain perception. In this review, we discuss important experimental evidence involving some TRP ion channels as molecular sensors of chemical, thermal, and mechanical noxious stimuli to evoke the pain and itch sensations. Among them are the TRPA1 channel, members of the vanilloid subfamily (TRPV1, TRPV3, and TRPV4), and finally members of the melastatin group (TRPM2, TRPM3, and TRPM8). Given that pain and itch are pro-survival, evolutionarily-honed protective mechanisms, care has to be exercised when developing inhibitory/modulatory compounds targeting specific pain/itch-TRPs so that physiological protective mechanisms are not disabled to a degree that stimulus-mediated injury can occur. Such events have impeded the development of safe and effective TRPV1-modulating compounds and have diverted substantial resources. A beneficial outcome can be readily accomplished via simple dosing strategies, and also by incorporating medicinal chemistry design features during compound design and synthesis. Beyond clinical use, where compounds that target more than one channel might have a place and possibly have advantageous features, highly specific and high-potency compounds will be helpful in mechanistic discovery at the structure-function level.

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.

Nociceptors: thermal allodynia and thermal pain

The sensation of pain plays a vital protecting role, alerting organisms about potentially damaging stimuli. Tissue injury is detected by nerve endings of specialized peripheral sensory neurons called nociceptors that are equipped with different ion channels activated by thermal, mechanic, and chemical stimuli. Several transient receptor potential channels have been identified as molecular transducers of thermal stimuli in pain-sensing neurons. Skin injury or inflammation leads to increased sensitivity to thermal and mechanic stimuli, clinically defined as allodynia or hyperalgesia. This hypersensitivity is also characteristic of systemic inflammatory disorders and neuropathic pain conditions. Mechanisms of thermal hyperalgesia include peripheral sensitization of nociceptor afferents and maladaptive changes in pain-encoding neurons within the central nervous system. An important aspect of pain management involves attempts to minimize the development of nociceptor hypersensitivity. However, knowledge about the cellular and molecular mechanisms causing thermal hyperalgesia and allodynia in human subjects is still limited, and such knowledge would be an essential step for the development of more effective therapies.

Pain-Causing Venom Peptides: Insights into Sensory Neuron Pharmacology

Venoms are produced by a wide variety of species including spiders, scorpions, reptiles, cnidarians, and fish for the purpose of harming or incapacitating predators or prey. While some venoms are of relatively simple composition, many contain hundreds to thousands of individual components with distinct pharmacological activity. Pain-inducing or "algesic" venom compounds have proven invaluable to our understanding of how physiological nociceptive neural networks operate. In this review, we present an overview of some of the diverse nociceptive pathways that can be modulated by specific venom components to evoke pain.

Biotechnological and Pharmacological Applications of Biotoxins and Other Bioactive Molecules from Dinoflagellates

The long-lasting interest in bioactive molecules (namely toxins) produced by (microalga) dinoflagellates has risen in recent years. Exhibiting wide diversity and complexity, said compounds are well-recognized for their biological features, with great potential for use as pharmaceutical therapies and biological research probes. Unfortunately, provision of those compounds is still far from sufficient, especially in view of an increasing demand for preclinical testing. Despite the difficulties to establish dinoflagellate cultures and obtain reasonable productivities of such compounds, intensive research has permitted a number of advances in the field. This paper accordingly reviews the characteristics of some of the most important biotoxins (and other bioactive substances) produced by dinoflagellates. It also presents and discusses (to some length) the main advances pertaining to dinoflagellate production, from bench to large scale-with an emphasis on material published since the latest review available on the subject. Such advances encompass improvements in nutrient formulation and light supply as major operational conditions; they have permitted adaptation of classical designs, and aided the development of novel configurations for dinoflagellate growth-even though shearing-related issues remain a major challenge.

Crotalphine desensitizes TRPA1 ion channels to alleviate inflammatory hyperalgesia

Crotalphine is a structural analogue to a novel analgesic peptide that was first identified in the crude venom from the South American rattlesnake Crotalus durissus terrificus. Although crotalphine's analgesic effect is well established, its direct mechanism of action remains unresolved. The aim of the present study was to investigate the effect of crotalphine on ion channels in peripheral pain pathways. We found that picomolar concentrations of crotalphine selectively activate heterologously expressed and native TRPA1 ion channels. TRPA1 activation by crotalphine required intact N-terminal cysteine residues and was followed by strong and long-lasting desensitization of the channel. Homologous desensitization of recombinant TRPA1 and heterologous desensitization in cultured dorsal root ganglia neurons was observed. Likewise, crotalphine acted on peptidergic TRPA1-expressing nerve endings ex vivo as demonstrated by suppression of calcitonin gene-related peptide release from the trachea and in vivo by inhibition of chemically induced and inflammatory hypersensitivity in mice. The crotalphine-mediated desensitizing effect was abolished by the TRPA1 blocker HC030031 and absent in TRPA1-deficient mice. Taken together, these results suggest that crotalphine is the first peptide to mediate antinociception selectively and at subnanomolar concentrations by targeting TRPA1 ion channels.

Ciguatoxins Evoke Potent CGRP Release by Activation of Voltage-Gated Sodium Channel Subtypes NaV1.9, NaV1.7 and NaV1.1

Ciguatoxins (CTXs) are marine toxins that cause ciguatera fish poisoning, a debilitating disease dominated by sensory and neurological disturbances that include cold allodynia and various painful symptoms as well as long-lasting pruritus. Although CTXs are known as the most potent mammalian sodium channel activator toxins, the etiology of many of its neurosensory symptoms remains unresolved. We recently described that local application of 1 nM Pacific Ciguatoxin-1 (P-CTX-1) into the skin of human subjects induces a long-lasting, painful axon reflex flare and that CTXs are particularly effective in releasing calcitonin-gene related peptide (CGRP) from nerve terminals. In this study, we used mouse and rat skin preparations and enzyme-linked immunosorbent assays (ELISA) to study the molecular mechanism by which P-CTX-1 induces CGRP release. We show that P-CTX-1 induces CGRP release more effectively in mouse as compared to rat skin, exhibiting EC₅₀ concentrations in the low nanomolar range. P-CTX-1-induced CGRP release from skin is dependent on extracellular calcium and sodium, but independent from the activation of various thermosensory transient receptor potential (TRP) ion channels. In contrast, lidocaine and tetrodotoxin (TTX) reduce CGRP release by 53–75%, with the remaining fraction involving L-type and T-type voltage-gated calcium channels (VGCC). Using transgenic mice, we revealed that the TTX-resistant voltage-gated sodium channel (VGSC) Na_V1.9, but not Na_V1.8 or Na_V1.7 alone and the combined activation of the TTX-sensitive VGSC subtypes Na_V1.7 and Na_V1.1 carry the largest part of the P-CTX-1-caused CGRP release of 42% and 34%, respectively. Given the contribution of CGRP to nociceptive and itch sensing pathways, our findings contribute to a better understanding of sensory symptoms of acute and chronic ciguatera that may help in the identification of potential therapeutics.

TRPA1 channels: molecular sentinels of cellular stress and tissue damage

TRPA1 is a non-selective cation channel expressed in mammalian peripheral pain receptors, with a major role in chemonociception. TRPA1 has also been implicated in noxious cold and mechanical pain sensation. TRPA1 has an ancient origin and plays important functions in lower organisms, including thermotaxis, mechanotransduction and modulation of lifespan. Here we highlight the role of TRPA1 as a multipurpose sensor of harmful signals, including toxic bacterial products and UV light, and as a sensor of stress and tissue damage. Sensing roles span beyond the peripheral nervous system to include major barrier tissues: gut, skin and lung. Tissue injury, environmental irritants and microbial pathogens are danger signals that can threaten the health of organisms. These signals lead to the coordinated activation of the nociceptive and the innate immune system to provide a homeostatic response trying to re-establish physiological conditions including tissue repair. Activation of TRPA1 participates in protective neuroimmune interactions at multiple levels, sensing ROS and bacterial products and triggering the release of neuropeptides. However, an exaggerated response to danger signals is maladaptive and can lead to the development of chronic inflammatory conditions.

Ladder-Shaped Ion Channel Ligands: Current State of Knowledge

Ciguatoxins (CTX) and brevetoxins (BTX) are polycyclic ethereal compounds biosynthesized by the worldwide distributed planktonic and epibenthic dinoflagellates of Gambierdiscus and Karenia genera, correspondingly. Ciguatera, evoked by CTXs, is a type of ichthyosarcotoxism, which involves a variety of gastrointestinal and neurological symptoms, while BTXs cause so-called neurotoxic shellfish poisoning. Both types of toxins are reviewed together because of similar mechanisms of their action. These are the only molecules known to activate voltage-sensitive Na⁺-channels in mammals through a specific interaction with site 5 of its α-subunit and may compete for it, which results in an increase in neuronal excitability, neurotransmitter release and impairment of synaptic vesicle recycling. Most marine ciguatoxins potentiate Nav channels, but a considerable number of them, such as gambierol and maitotoxin, have been shown to affect another ion channel. Although the extrinsic function of these toxins is probably associated with the function of a feeding deterrent, it was suggested that their intrinsic function is coupled with the regulation of photosynthesis via light-harvesting complex II and thioredoxin. Antagonistic effects of BTXs and brevenal may provide evidence of their participation as positive and negative regulators of this mechanism.

Cold Temperature Encoding by Cutaneous TRPA1 and TRPM8-Carrying Fibers in the Mouse

Previous research identified TRPM8 and TRPA1 cold transducers with separate functions, one being functional in the non-noxious range and the second one being a nociceptive transducer. TRPM8-deficient mice present overt deficits in the detection of environmental cool, but not a lack of cold avoidance and TRPA1-deficient mice show clear deficits in some cold nocifensive assays. The extent of TRPA1's contribution to cold sensing in vivo is still unclear, because mice lacking both TRPM8 and TRPA1 (DKO) were described with unchanged cold avoidance from TRPM8^−/− based on a two-temperature-choice assay and by c-fos measurement. The present study was designed to differentiate how much TRPM8 alone and combined TRPA1 and TRPM8 contribute to cold sensing. We analyzed behavior in the thermal ring track assay adjusted between 30 and 5°C and found a large reduction in cold avoidance of the double knockout mice as compared to the TRPM8-deficient mice. We also revisited skin-nerve recordings from saphenous-nerve skin preparations with regard to nociceptors and thermoreceptors. We compared the frequency and characteristics of the cold responses of TRPM8-expressing and TRPM8-negative C-fiber nociceptors in C57BL/6J mice with nociceptors of TRPM8-deficient and DKO mice and found that TRPM8 enables nociceptors to encode cold temperatures with higher firing rates and larger responses with sustained, static component. In TRPM8^−/−, C-fiber cold nociceptors were markedly reduced and appeared further reduced in DKO. Nevertheless, the remaining cold responses in both knockout strains were similar in their characteristics and they were indifferent from the TRPM8-negative cold responses found in C57BL/6J mice. TRPM8 had a comparably essential role for encoding cold in thermoreceptors and lack of TRPM8 reduced response magnitude, peak and mean firing rates and the incidence of thermoreceptors. The encoding deficits were similar in the DKO strain. Our data illustrate that lack of TRPA1 in TRPM8-deficient mice results in a disproportionately large reduction in cold avoidance behavior and also affects the incidence of cold encoding fiber types. Presumably TRPA1 compensates for lack of TRPM8 to a certain extent and both channels cooperate to cover the entire cold temperature range, making cold-temperature encoding by TRPA1—although less powerful—synergistic to TRPM8.

Brain mechanisms of abnormal temperature perception in cold allodynia induced by ciguatoxin

OBJECTIVE

Cold allodynia occurs as a major symptom of neuropathic pain states. It remains poorly treated with current analgesics. Ciguatoxins (CTXs), ichthyosarcotoxins that cause ciguatera, produce a large peripheral sensitization to dynamic cold stimuli in Aδ-fibers by activating sodium channels without producing heat or mechanical allodynia. We used CTXs as a surrogate model of cold allodynia to dissect the framework of cold allodynia-activated central pain pathways.

METHODS

Reversible cold allodynia was induced in healthy male volunteers by shallow intracutaneous injection of low millimolar concentrations of CTX into the dorsal skin of the forefoot. Cold and warm stimuli were delivered to the treated and the control site using a Peltier-driven thermotest device. Functional magnetic resonance imaging (fMRI) scans were acquired with a 3T MRI scanner using a blood oxygen level-dependent (BOLD) protocol.

RESULTS

The CTX-induced substantial peripheral sensitization to cooling stimuli in Aδ-fibers is particularly retrieved in BOLD changes due to dynamic temperature changes and less during constant cooling. Brain areas that responded during cold allodynia were almost always located bilaterally and appeared in the medial insula, medial cingulate cortex, secondary somatosensory cortex, frontal areas, and cerebellum. Whereas these areas also produced changes in BOLD signal during the dynamic warming stimulus on the control site, they remained silent during the warming stimuli on the injected site.

INTERPRETATION

We describe the defining feature of the cold allodynia pain percept in the human brain and illustrate why ciguatera sufferers often report a perceptual temperature reversal. ANN NEUROL 2017;81:104-116.