Effects of algal-produced neurotoxins on metabolic activity in telencephalon, optic tectum and cerebellum of Atlantic salmon (Salmo salar)

Sublethal neurotoxicity of saxitoxin in early zebrafish development: Impact on sensorimotor function and neurotransmission systems

Saxitoxin (STX) represents a marine toxin of significant concern due to its deleterious implications for aquatic ecosystems and public food safety. As a potent paralytic agent, the role of STX in obstructing voltage-gated sodium channels (VGSCs) is well-characterized. Yet, the mechanistic details underlying its low-dose toxicity remain largely enigmatic. In the current study, zebrafish embryos and larvae were subjected to subchronic exposure of graded STX concentrations (0, 1, 10, and 100 μg/L) until the 7th day post-fertilization. A tactile stimulus-based assay was employed to evaluate potential behavioral perturbations resulting from STX exposure. Both behavioral and transcription level analyses unveiled a compromised tactile response, which was found to be associated with a notable upregulation in the mRNA of two distinct VGSC isoforms, specifically the scn8aa/ab and scn1Laa/ab transcripts, even at the minimal STX dose. Notably, exposure to this lowest STX concentration also resulted in alterations in the transcriptional patterns of pivotal genes for cholinergic and GABAergic pathways, including ache and gabra1. Furthermore, STX induced a marked decrease in the levels of the neurotransmitter GABA. Our findings underscore that prolonged low-dose STX exposure during early development can significantly compromise the tactile response behavior in zebrafish. This study reveals that chronic low-dose STX exposure of developing zebrafish alters neurotransmission pathways that converge on altered tactile behavior.

A New Bioassay for the Detection of Paralytic and Amnesic Biotoxins Based on Motor Behavior Impairments of Zebrafish Larvae

The global concern about the increase of harmful algal bloom events and the possible impacts on food safety and aquatic ecosystems presents the necessity for the development of more accessible techniques for biotoxin detection for screening purposes. Considering the numerous advantages that zebrafish present as a biological model and particularly as a toxicants sentinel, we designed a sensitive and accessible test to determine the activity of paralytic and amnesic biotoxins using zebrafish larvae immersion. The ZebraBioTox bioassay is based on the automated recording of larval locomotor activity using an IR microbeam locomotion detector, and manual assessment of four complementary responses under a simple stereoscope: survival, periocular edema, body balance, and touch response. This 24 h acute static bioassay was set up in 96-well microplates using 5 dpf zebrafish larvae. For paralytic toxins, a significant decrease in locomotor activity and touch response of the larvae was detected, allowing a detection threshold of 0.1-0.2 µg/mL STXeq. In the case of the amnesic toxin the effect was reversed, detecting hyperactivity with a detection threshold of 10 µg/mL domoic acid. We propose that this assay might be used as a complementary tool for environmental safety monitoring.

Chronic exposure to sublethal concentrations of saxitoxin reduces antioxidant activity and immunity in zebrafish but does not affect reproductive parameters

Saxitoxin produced by dinoflagellates and cyanobacteria can be transferred to humans through intoxicated organisms such as fish, but limited research has addressed the adverse effects of this toxin on aquatic organisms. In this study, we measured the potential effects of a 90-day exposure to saxitoxin (0.1 or 1 µg·L ^- 1) on body weight and length, antioxidant defense system, immunity, sex hormones, and genes involved in associated key metabolic pathways in zebrafish (Danio rerio). Significant impairments in body weight and length were observed in response to 1 µg·L ^- 1 saxitoxin in both male and female zebrafish. A significant increase in the levels of malondialdehyde, together with decreased enzymatic activities of catalase and superoxide dismutase, was observed in fish of both sexes exposed to 1 µg·L ^- 1 saxitoxin, indicating the occurrence of lipid peroxidation and oxidative stress. Immune parameters such as alternative complement activity, lysozyme activity, and immunoglobulin content were also significantly reduced. However, exposure of male and female zebrafish to saxitoxin for 90 days did not significantly affect reproductive parameters such as the gonadosomatic index and plasma concentrations of vitellogenin, estradiol, and 11-keto testosterone. Transcriptional responses showed similar trends to those of the biochemical parameters, as genes involved in the antioxidant defense system and immunity were downregulated, whereas the transcription of genes related to reproductive metabolism showed no significant change upon treatment with 1 µg·L ^- 1 saxitoxin. Our findings indicate that long-term exposure to a sublethal concentration of saxitoxin can inhibit growth through induction of oxidative stress and immunosuppression, while the reproductive parameters of zebrafish are not a main target of this toxin at sublethal concentrations.

An integrated systems-level model of the toxicity of brevetoxin based on high-resolution magic-angle spinning nuclear magnetic resonance (HRMAS NMR) metabolic profiling of zebrafish embryos

Brevetoxins (PbTx) are a well-recognized group of neurotoxins associated with harmful algal blooms, and specifically recurrent "Florida Red Tides," in marine waters that are linked to impacts on both human and ecosystem health including well-documented "fish kills" and marine mammal mortalities in affected coastal waters. Understanding mechanisms and pathways of PbTx toxicity enables identification of relevant biomarkers to better understand these environmental impacts, and improve monitoring efforts, in relation to this toxin. Toward a systems-level understanding of toxicity, and identification of potential biomarkers, high-resolution magic angle spinning nuclear magnetic resonance (HRMAS NMR) was utilized for metabolic profiling of zebrafish (Danio rerio) embryos, as an established toxicological model, exposed to PbTx-2 (the most common congener in marine waters). Metabolomics studies were, furthermore, complemented by an assessment of the toxicity of PbTx-2 in embryonic stages of zebrafish and mahi-mahi (Coryphaena hippurus), the latter representing an ecologically and geographically relevant marine species of fish, which identified acute embryotoxicity at environmentally relevant (i.e., parts-per-billion) concentrations in both species. HRMAS NMR analysis of intact zebrafish embryos exposed to sub-lethal concentrations of PbTx-2 afforded well-resolved spectra, and in turn, identification of 38 metabolites of which 28 were found to be significantly altered, relative to controls. Metabolites altered by PbTx-2 exposure specifically included those associated with (1) neuronal excitotoxicity, as well as associated neural homeostasis, and (2) interrelated pathways of carbohydrate and energy metabolism. Metabolomics studies, thereby, enabled a systems-level model of PbTx toxicity which integrated multiple metabolic, molecular and cellular pathways, in relation to environmentally relevant concentrations of the toxin, providing insight to not only targets and mechanisms, but potential biomarkers pertinent to environmental risk assessment and monitoring strategies.

Effects of paralytic shellfish toxins on the middle intestine of Oncorhynchus mykiss: Glutathione metabolism, oxidative status, lysosomal function and ATP-binding cassette class C (ABCC) proteins activity

We studied the absorption, cytotoxicity and oxidative stress markers of Paralytic Shellfish Toxins (PST) from three extracts from Alexandrium catenella and A. ostenfeldii, in middle Oncorhynchus mykiss intestine in vitro and ex vivo preparations. We measured glutathione (GSH) content, glutathione-S transferase (GST), glutathione reductase (GR) and catalase (CAT) enzymatic activity, and lipid peroxidation in isolated epithelium exposed to 0.13 and 1.3 μM PST. ROS production and lysosomal membrane stability (as neutral red retention time 50%, NRRT50) were analyzed in isolated enterocytes exposed to PST alone or plus 3 μM of the ABCC transport inhibitor MK571. In addition, the concentration-dependent effects of PST on NRRT50 were assayed in a concentration range from 0 to 1.3 μM PST. We studied the effects of three different PST extracts on the transport rate of the ABCC substrate DNP-SG by isolated epithelium. The extract with highest inhibition capacity was selected for studying polarized DNP-SG transport in everted and non-everted intestinal segments. We registered lower GSH content and GST activity, and higher GR activity, with no significant changes in CAT activity, lipid peroxidation or ROS level. PST exposure decreased NRRT50 in a concentration-depend manner (IC50 = 0.0045 μM), but PST effects were not augmented by addition of MK571. All the three PST extracts inhibited ABCC transport activity, but this inhibition was effective only when the toxins were applied to the apical side of the intestine and DNP-SG transport was measured at the basolateral side. Our results indicate that PST are absorbed by the enterocytes from the intestine lumen. Inside the enterocytes, these toxins decrease GSH content and inhibit the basolateral ABCC transporters affecting the normal functions of the cell. Furthermore, PST produce a strong cytotoxic effect to the enterocytes by damaging the lysosomal membrane, even at low, non-neurotoxic concentrations.

Effects of acute waterborne exposure to harmful algal toxin domoic acid on foraging and swimming behaviours of fish early stages

Domoic acid (DA) is a neurotoxin naturally produced by Pseudo-nitzschia diatoms that may be transferred through the marine food web and cause mass mortality events at higher trophic levels. Yet, the effects of the dissolved marine toxin on foraging responses and swimming performances of fish early stages are poorly known. Here we evaluated the effects of short-term exposure (24 h) to a single dose of domoic acid (136 μg DA L^-1) on larvae (15-20 days post-hatch) of three commercially important fish species (the sea breams Diplodus sargus and Sparus aurata and the meagre Argyrosomus regius). Although DA exposure did not elicit significant effects on larval survival (p > 0.05) and swimming performance (p > 0.05), the toxin significantly affected the fish capture success (p < 0.001). Our findings suggest that toxigenic Pseudo-nitzschia blooms may compromise fish early stages, in particular larvae feeding behaviours, leading to complications in the development and increasing fish vulnerability and mortality.

UV/persulfate preoxidation to improve coagulation efficiency of Microcystis aeruginosa

The performance of UV-activated persulfate (UV/PS) technology as preoxidation process to enhance Microcystis aeruginosa removal by subsequent coagulation-sedimentation was firstly evaluated. The results demonstrate that UV/PS preoxidation could successfully promote coagulation of algae cells through the effective neutralization of zeta potential, which was caused by the changes of cell morphology, size distribution and surface properties after simultaneous UV irradiation and formed reactive species (i.e. SO₄^- and HO) oxidation. Since excessive oxidation would cause cell rupture along with the release of organics, which could deteriorate coagulation efficiency, optimal PS dose (60mg/L) and UV dose (375mJ/cm²) were proposed to exist in this study. The concentrations of extracellular algal organic matter (AOM) sharply increased by 48.2% during the preoxidation period, while gradually decreased in the following coagulation and sedimentation. Most of the concerned disinfection by-products (DBPs) monotonically decreased or followed fluctuant reduction with increasing PS doses, whereas the trichloromethane, trichloroacetic acid and dichloroacetonitrile persistently increased, which was inferred to be related to the variation of AOM. This study suggests that UV/PS might be a potential pretreatment process to assist coagulation on the removal of algae.

Exposure to bloom-like concentrations of two marine Synechococcus cyanobacteria (strains CC9311 and CC9902) differentially alters fish behaviour

Coastal California experiences large-scale blooms of Synechococcus cyanobacteria, which are predicted to become more prevalent by the end of the 21st century as a result of global climate change. This study investigated whether exposure to bloom-like concentrations of two Synechococcus strains, CC9311 and CC9902, alters fish behaviour. Black perch (Embiotoca jacksoni) were exposed to Synechococcus strain CC9311 or CC9902 (1.5 × 10(6) cells ml(-1)) or to control seawater in experimental aquaria for 3 days. Fish movement inside a testing arena was then recorded and analysed using video camera-based motion-tracking software. Compared with control fish, fish exposed to CC9311 demonstrated a significant preference for the dark zone of the tank in the light-dark test, which is an indication of increased anxiety. Furthermore, fish exposed to CC9311 also had a statistically significant decrease in velocity and increase in immobility and they meandered more in comparison to control fish. There was a similar trend in velocity, immobility and meandering in fish exposed to CC9902, but there were no significant differences in behaviour or locomotion between this group and control fish. Identical results were obtained with a second batch of fish. Additionally, in this second trial we also investigated whether fish would recover after a 3 day period in seawater without cyanobacteria. Indeed, there were no longer any significant differences in behaviour among treatments, demonstrating that the sp. CC9311-induced alteration of behaviour is reversible. These results demonstrate that blooms of specific marine Synechococcus strains can induce differential sublethal effects in fish, namely alterations light-dark preference behaviour and motility.

Hydroxybenzoate paralytic shellfish toxins induce transient GST activity depletion and chromosomal damage in white seabream (Diplodus sargus)

Fish are routinely exposed to harmful algal blooms that produce noxious compounds and impact the marine food web. This study investigates the role of phase I and II detoxification enzymes on metabolism of the novel paralytic shellfish toxins (PSTs), the hydroxybenzoate analogues recently discovered in Gymnodinium catenatum strains, in the liver of white seabream, assessing ethoxyresorufin-O-deethylase (EROD) and glutathione S-transferase (GST) activities. Additionally, the genotoxic potential of hydroxybenzoate PSTs was examined through the erythrocytic nuclear abnormality (ENA) assay. Fish were injected with hydroxybenzoate PSTs into the coelomic cavity and sacrificed 2 and 6 days later for biochemical and cytogenetic analyses. While the activity of EROD was unresponsive to toxins, a significant GST activity decrease was observed at 2 days after injection indicating an impairment of this line of the detoxification system. The genotoxic potential of PSTs was demonstrated by the induction of clastogenic/aneugenic effects at 2 days, as measured by the ENA assay. Overall, this study contributes to better understand the impact of toxins produced by G. catenatum blooms in fish, revealing effects that, even transitory, point out a risk associated to hydroxybenzoate analogues.

Molecular staging of marine medaka: a model organism for marine ecotoxicity study

Oryzias melastigma, also called O. dancena, is becoming a very useful model for estuarine and marine ecotoxicity studies. With O. melastigma being adopted by ILSI Health and Environmental Science Institute (HESI) for embryo toxicity testing, improved knowledge of biomarker based embryonic development becomes especially important for mechanism-based toxicity evaluations. Using whole mount in situ hybridization and immunostaining techniques together with widely used molecular markers, this study describes the molecular development of marine medaka embryos, focusing on the brain, eye, heart, pectoral fin, pancreas, liver, muscle and neuron system. These organs are targets of environmental pollutants that disrupt normal embryonic development in medaka and other fish.

Kinetic properties of saxitoxin in Atlantic salmon (Salmo salar) and Atlantic cod (Gadus morhua)

The disposition of STX in Atlantic salmon (Salmo salar) and Atlantic cod (Gadus morhua) was studied after intraperitoneal (IP) injection (5 microg STX/kg bm and 3.43 microg (3)H-STXeq/kg bw respectively), intravenous (IV) injection (5 microg STX/kg bm, only salmon) and waterborne exposure (50 microg STXeq/L, only salmon). Plasma concentrations in salmon were quantified using a receptor binding assay and cod tissues were analyzed using scintillation counting of tissue extracts and autoradiography of whole fish slices. The estimated elimination half-life (T(1/2)) after IV administration of STX in salmon was 102.6 min. The volume of distribution (Vz) was observed to be 467.2 mL/kg and the total body clearance (Cl(T)) was 3.2 mL/min/kg. Waterborne exposure clearly showed that salmon absorbed PSP toxins directly from the water. In cod, (3)H-STX was observed in gills, muscle, brain, liver and posterior kidney from 30 to 480 min. The lowest concentrations of (3)H-STX were found in brain and muscle, whereas posterior kidney contained the majority of the toxin. Autoradiograms confirmed the high levels of (3)H-STX in the kidneys, indicating that renal excretion was the main elimination route. Buildup of harmful levels in edible tissue is not very likely due to the low concentrations accumulated in muscle tissue and rapid excretion.

Toxic effects of domoic acid in the seabream Sparus aurata

Neurotoxicity induced in fish by domoic acid (DA) was assessed with respect to occurrence of neurotoxic signs, lethality, and histopathology by light microscopy. Sparus aurata were exposed to a single dose of DA by intraperitoneal (i.p.) injection of 0, 0.45, 0.9, and 9.0 mg DA kg(-1) bw. Mortality (66.67 ± 16.67%) was only observed in dose of 9.0 mg kg(-1) bw. Signs of neurological toxicity were detected for the doses of 0.9 and 9.0 mg DA kg(-1) bw. Furthermore, the mean concentrations (±SD) of DA detected by HPLC-UV in extracts of brain after exposure to 9.0 mg DA kg(-1) bw were 0.61 ± 0.01, 0.96 ± 0.00, and 0.36 ± 0.01 mg DA kg(-1) tissue at 1, 2, and 4 hours. The lack of major permanent brain damage in S. aurata, and reversibility of neurotoxic signs, suggest that lower susceptibility to DA or neuronal recovery occurs in affected individuals.

Subclinical effects of saxitoxin and domoic acid on aggressive behaviour and monoaminergic turnover in rainbow trout (Oncorhynchus mykiss)

The algal produced neurotoxins saxitoxin and domoic acid may have serious effects on marine life and can be responsible for the intoxication of for instance sea mammals, sea birds and fish. Given that farmed fish cannot escape algal blooms, they may be more susceptible to intoxication than wild stocks. In the present study, subclinical effects of saxitoxin and domoic on aggressive behaviour and monoaminergic systems in the brain of the rainbow trout (Oncorhynchus mykiss) were investigated. The resident-intruder test was used to measure aggression where only the resident fish were subjected to the toxins and analysed for monoamines and their metabolites. The resident-intruder test was carried out on two consecutive days. On day one basal aggression was measured in the four groups. On day two three of the groups were injected with subclinical doses of one of the following: saxitoxin (1.752 microg/kg bw), domoic (0.75 mg/kg bw) or 0.9% saline solution. This was performed 30 min prior to the aggression test. Handling stress and injection affected aggressive behaviour, cortisol and the serotonergic system in telencephalic brain regions. Cortisol levels were elevated in all of the injected groups when compared to the control group. An increase in serotonergic turnover was evident when all injected groups were pooled and compared to the control group. All together this suggests that the handling stress in connection with the injection was similar in all of the three injected groups. In contrast to both the undisturbed control group and the toxin-injected groups, the saline-injected group displayed a reduction in aggressive behaviour which was evident in increased attack latency. Furthermore the domoic injected group displayed more aggressive attacks towards their conspecifics than the saline-injected group. Consequently the two toxins appear to mask the stress induced alteration in aggressive behaviour. Monoamine levels and monoaminergic turnover could not be demonstrated to be directly affected by the two toxins at the given doses in the investigated brain regions (dorsal and ventral parts of telencephalon, optic tectum, locus coeruleus, raphe nucleus, molecular and granular layer of cerebellum). This could indicate that the toxins mediate aggressive behaviour either through other systems than the monoaminergic systems, such as neuroactive amino acids, or that the mediation occurs in other brain regions.

Cloning and characterization of glutamate receptors in Californian sea lions (Zalophus californianus)

Domoic acid produced by marine algae has been shown to cause acute and chronic neurologic sequelae in Californian sea lions following acute or low-dose exposure. Histological findings in affected animals included a degenerative cardiomyopathy that was hypothesized to be caused by over-excitation of the glutamate receptors (GluRs) speculated to be present in the sea lion heart. Thus tissues from five sea lions without lesions associated with domoic acid toxicity and one animal with domoic acid-induced chronic neurologic sequelae and degenerative cardiomyopathy were examined for the presence of GluRs. Immunohistochemistry localized mGluR 2/3, mGluR 5, GluR 2/3 and NMDAR 1 in structures of the conducting system and blood vessels. NMDAR 1 and GluR 2/3 were the most widespread as immunoreactivity was observed within sea lion conducting system structures. PCR analysis, cloning and subsequent sequencing of the seal lion GluRs showed only 80% homology to those from rats, but more than 95% homologous to those from dogs. The cellular distribution and expression of subtypes of GluRs in the sea lion hearts suggests that exposure to domoic acid may induce cardiac damage and functional disturbances.