Systemic administration of domoic acid-induced spinal cord lesions in neonatal rats

Uptake of β-N-methylamino-L-alanine (BMAA) into glutamate-specific synaptic vesicles: Exploring the validity of the excitotoxicity mechanism of BMAA

The first mechanism of toxicity proposed for the cyanobacterial neurotoxin β-N-methylamino-L-alanine (BMAA) was excitotoxicity, and this was supported by numerous in vitro studies in which overactivation of both ionotropic and metabotropic glutamate receptors was reported. However, the excitotoxicity of BMAA is weak in comparison with other known excitotoxins and on par with that of glutamate, implying that to achieve sufficient synaptic concentrations of BMAA to cause classical in vivo excitotoxicity, BMAA must either accumulate in synapses to allow persistent glutamate receptor activation or it must be released in sufficiently high concentrations into synapses to cause the overexcitation. Since it has been shown that BMAA can be readily removed from synapses, release of high concentrations of BMAA into synapses must be shown to confirm its role as an excitotoxin in in vivo systems. This study therefore sought to evaluate the uptake of BMAA into synaptic vesicles and to determine if BMAA affects the uptake of glutamate into synaptic vesicles. There was no evidence to support uptake of BMAA into glutamate-specific synaptic vesicles but there was some indication that BMAA may affect the uptake of glutamate into synaptic vesicles. The uptake of BMAA into synaptic vesicles isolated from areas other than the cerebral cortex should be investigated before definite conclusions can be drawn about the role of BMAA as an excitotoxin.

Developmental Exposure to Domoic Acid Disrupts Startle Response Behavior and Circuitry in Zebrafish

Harmful algal blooms produce potent neurotoxins that accumulate in seafood and are hazardous to human health. Developmental exposure to the harmful algal bloom toxin, domoic acid (DomA), has behavioral consequences well into adulthood, but the cellular and molecular mechanisms of DomA developmental neurotoxicity are largely unknown. To assess these, we exposed zebrafish embryos to DomA during the previously identified window of susceptibility and used the well-known startle response circuit as a tool to identify specific neuronal components that are targeted by exposure to DomA. Exposure to DomA reduced startle responsiveness to both auditory/vibrational and electrical stimuli, and even at the highest stimulus intensities tested, led to a dramatic reduction of one type of startle (short-latency c-starts). Furthermore, DomA-exposed larvae had altered kinematics for both types of startle responses tested, exhibiting shallower bend angles and slower maximal angular velocities. Using vital dye staining, immunolabeling, and live imaging of transgenic lines, we determined that although the sensory inputs were intact, the reticulospinal neurons required for short-latency c-starts were absent in most DomA-exposed larvae. Furthermore, axon tracing revealed that DomA-treated larvae also showed significantly reduced primary motor neuron axon collaterals. Overall, these results show that developmental exposure to DomA targets large reticulospinal neurons and motor neuron axon collaterals, resulting in measurable deficits in startle behavior. They further provide a framework for using the startle response circuit to identify specific neural populations disrupted by toxins or toxicants and to link these disruptions to functional consequences for neural circuit function and behavior.

Public health risks associated with chronic, low-level domoic acid exposure: A review of the evidence

Domoic acid (DA), the causative agent for the human syndrome Amnesic Shellfish Poisoning (ASP), is a potent, naturally occurring neurotoxin produced by common marine algae. DA accumulates in seafood, and humans and wildlife alike can subsequently be exposed when consuming DA-contaminated shellfish or finfish. While strong regulatory limits protect people from the acute effects associated with ASP, DA is an increasingly significant public health concern, particularly for coastal dwelling populations, and there is a growing body of evidence suggesting that there are significant health consequences following repeated exposures to levels of the toxin below current safety guidelines. However, gaps in scientific knowledge make it difficult to precisely determine the risks of contemporary low-level exposure scenarios. The present review characterizes the toxicokinetics and neurotoxicology of DA, discussing results from clinical and preclinical studies after both adult and developmental DA exposure. The review also highlights crucial areas for future DA research and makes the case that DA safety limits need to be reassessed to best protect public health from deleterious effects of this widespread marine toxin.

Developmental Neurotoxicity of the Harmful Algal Bloom Toxin Domoic Acid: Cellular and Molecular Mechanisms Underlying Altered Behavior in the Zebrafish Model

BACKGROUND

Harmful algal blooms (HABs) produce potent neurotoxins that threaten human health, but current regulations may not be protective of sensitive populations. Early life exposure to low levels of the HAB toxin domoic acid (DomA) produces long-lasting behavioral deficits in rodent and primate models; however, the mechanisms involved are unknown. The zebrafish is a powerful in vivo vertebrate model system for exploring cellular processes during development and thus may help to elucidate mechanisms of DomA developmental neurotoxicity.

OBJECTIVES

We used the zebrafish model to investigate how low doses of DomA affect the developing nervous system, including windows of susceptibility to DomA exposure, structural and molecular changes in the nervous system, and the link to behavioral alterations.

METHODS

To identify potential windows of susceptibility, DomA (0.09-0.18 ng) was delivered to zebrafish through caudal vein microinjection during distinct periods in early neurodevelopment. Following exposure, structural and molecular targets were identified using live imaging of transgenic fish and RNA sequencing. To assess the functional consequences of exposures, we quantified startle behavior in response to acoustic/vibrational stimuli.

RESULTS

Larvae exposed to DomA at 2 d postfertilization (dpf), but not at 1 or 4 dpf, showed consistent deficits in startle behavior at 7 dpf, including lower responsiveness and altered kinematics. Similarly, myelination in the spinal cord was disorganized after exposure at 2 dpf but not 1 or 4 dpf. Time-lapse imaging revealed disruption of the initial stages of myelination. DomA exposure at 2 dpf down-regulated genes required for maintaining myelin structure and the axonal cytoskeleton.

DISCUSSION

These results in zebrafish reveal a developmental window of susceptibility to DomA-induced behavioral deficits and identify altered gene expression and disrupted myelin structure as possible mechanisms. The results establish a zebrafish model for investigating the mechanisms of developmental DomA toxicity, including effects with potential relevance to exposed sensitive human populations. https://doi.org/10.1289/EHP6652.

Neurobehavioral assessment of mice following repeated oral exposures to domoic acid during prenatal development

Domoic acid (DA) is an algal toxin which has been associated with significant neurotoxicity in humans, non-human primates, rodents, and marine mammals. Developmental exposure to DA is believed to result in neurotoxicity that may persist into adulthood. DA is produced by harmful algal blooms of Pseudo-nitzschia, raising concerns about the consumption of contaminated seafood. We evaluated oral exposures to DA during pregnancy in mice. Doses of 0 (vehicle), 1 or 3mg/kg/d of DA were administered by gavage to C57BL/6J mice on gestational days 10 to 17. The offspring were tested for persistent neurobehavioral consequences during early development, adolescence and adulthood. Neurobehavioral tests revealed both dose- and gender-related differences in several neurobehavioral measures, including motor coordination in the rotarod test, behavior in the elevated plus maze, circadian patterns of activity in Phenotyper cages, gait as assessed in the Catwalk, and exploratory activity in the Morris water maze. This study demonstrated significant gender-specific and persistent neurobehavioral effects of repeated prenatal oral exposures to DA at low-dose levels that did not induce toxicity in dams.

Pathology of Domoic Acid Toxicity in California Sea Lions (Zalophus californianus)

Over 100 free-ranging adult California sea lions ( Zalophus californianus) and one Northern fur seal ( Callorhinus ursinus), predominantly adult females, were intoxicated by domoic acid (DA) during three harmful algal blooms between 1998 and 2000 in central and northern California coastal waters. The vector prey item was Northern anchovy ( Engraulis mordax) and the primary DA-producing algal diatom was Psuedonitzschia australis. Postmortem examination revealed gross and histologic findings that were distinctive and aided in diagnosis. A total of 109 sea lions were examined, dying between 1 day and 10 months after admission to a marine mammal rehabilitation center. Persistent seizures with obtundation were the main clinical findings. Frequent gross findings in animals dying acutely consisted of piriform lobe malacia, myocardial pallor, bronchopneumonia, and complications related to pregnancy. Gross findings in animals dying months after intoxication included bilateral hippocampal atrophy. Histologic observations implicated limbic system seizure injury consistent with excitotoxin exposure. Peracutely, there was microvesicular hydropic degeneration within the neuropil of the hippocampus, amygdala, pyriform lobe, and other limbic structures. Acutely, there was ischemic neuronal necrosis, particularly apparent in the granular cells of the dentate gyrus and the pyramidal cells within the hippocampus cornu ammonis (CA) sectors CA4, CA3, and CA1. Dentate granular cell necrosis has not been reported in human or experimental animal DA toxicity and may be unique to sea lions. Chronically, there was gliosis, mild nonsuppurative inflammation, and loss of laminar organization in affected areas.

Differential effects of domoic acid and E. coli lipopolysaccharide on tumor necrosis factor-alpha, transforming growth factor-beta1 and matrix metalloproteinase-9 release by rat neonatal microglia: evaluation of the direct activation hypothesis

The excitatory amino acid domoic acid is the causative agent of amnesic shellfish poisoning in humans. The in vitro effects of domoic acid on rat neonatal brain microglia were compared with E. coli lipopolysaccharide (LPS), a known activator of microglia mediator release over a 4 to 24 hour observation period. LPS [3 ng/mL] but not domoic acid [1mM] stimulated a statistically significant increase in TNF-α mRNA and protein generation. Furthermore, both LPS and domoic acid did not significantly affect TGF-β1 gene expression and protein release. Finally, an in vitro exposure of microglia to LPS resulted in statistically significant MMP-9 expression and release, thus extending and confirming our previous observations. However, in contrast, no statistically significant increase in MMP-9 expression and release was observed after domoic acid treatment. Taken together our observations do not support the hypothesis that a short term (4 to 24 hours) in vitro exposure to domoic acid, at a concentration toxic to neuronal cells, activates rat neonatal microglia and the concomitant release of the pro-inflammatory mediators tumor necrosis factor-α (TNF-α) and matrix metalloproteinases-9 (MMP-9), as well as the anti-inflammatory cytokine transforming growth factor β1 (TGF-β1).

Domoic acid as a developmental neurotoxin

Domoic acid (DomA) is an excitatory amino acid which can accumulate in shellfish and finfish under certain environmental conditions. DomA is a potent neurotoxin. In humans and in non-human primates, oral exposure to a few mg/kg DomA elicits gastrointestinal effects, while slightly higher doses cause neurological symptoms, seizures, memory impairment, and limbic system degeneration. In rodents, which appear to be less sensitive than humans or non-human primates, oral doses cause behavioral abnormalities (e.g. hindlimb scratching), followed by seizures and hippocampal degeneration. Similar effects are also seen in other species (from sea lions to zebrafish), indicating that DomA exerts similar neurotoxic effects across species. The neurotoxicity of DomA is ascribed to its ability to interact and activate the AMPA/KA receptors, a subfamily of receptors for the neuroexcitatory neurotransmitter glutamate. Studies exploring the neurotoxic effects of DomA on the developing nervous system indicate that DomA elicits similar behavioral, biochemical and morphological effects as in adult animals. However, most importantly, developmental neurotoxicity is seen at doses of DomA that are one to two orders of magnitude lower than those exerting neurotoxicity in adults. This difference may be due to toxicokinetic and/or toxicodynamic differences. Estimated safe doses may be exceeded in adults by high consumption of shellfish contaminated with DomA at the current limit of 20 microg/g. Given the potential higher susceptibility of the young to DomA neurotoxicity, additional studies investigating exposure to, and effects of this neurotoxin during brain development are warranted.

Domoic acid: neurobehavioral consequences of exposure to a prevalent marine biotoxin

Domoic acid (DA), the cause of Amnesic Shellfish Poisoning, is a naturally occurring marine biotoxin that is usually produced by the microscopic algae Pseudo-nitzschia. As is the case for other types of toxic algae, Pseudo-nitzschia outbreaks are becoming more frequent. Acute high-dose symptomology in humans includes vomiting, cramping, coma and death as well as neurological effects such as hallucinations, confusion and memory loss. Experimental studies and medical reports have collectively shown that DA exposure primarily affects the hippocampal regions of the brain and is associated with seizures and the disruption of cognitive processes. The neurobehavioral signature of DA is unique in that it includes transient and permanent changes in memory function that resemble human antegrade amnesia. Experimental studies with adult nonhuman primates have established that DA is a dose-dependent emetic that produces clinical and neuropathological changes consistent with excitotoxicity. Behavioral evaluations of treated rodents have shown that hyperactivity and stereotypical scratching are the first functional markers of toxicity. Mid-dose treatment is associated with memory impairment and behavioral hyperreactivity, suggesting changes in arousal and/or emotionality. At higher doses, DA treatment results in frank neurotoxicity that is characterized by seizures, status epilepticus and death in treated animals. The route of DA exposure is important and influences the severity of effects; intraperitoneal and intravenous treatments produce classic signs of poisoning at significantly lower doses than oral exposure. While developmental studies are few, DA readily crosses the placenta and enters the fetal brain. Domoic acid is not associated with congenital dysmorphia but is linked to persistent changes in motor behavior and cognition in exposed offspring. Comparative research suggests that functional losses associated with DA can be persistent and injuries to the CNS can be progressive. Long-term studies will be necessary to accurately track the expression of DA-related injury, in health and behavior, over the lifespan.

Domoic acid toxicologic pathology: a review

Domoic acid was identified as the toxin responsible for an outbreak of human poisoning that occurred in Canada in 1987 following consumption of contaminated blue mussels [Mytilus edulis]. The poisoning was characterized by a constellation of clinical symptoms and signs. Among the most prominent features described was memory impairment which led to the name Amnesic Shellfish Poisoning [ASP]. Domoic acid is produced by certain marine organisms, such as the red alga Chondria armata and planktonic diatom of the genus Pseudo-nitzschia. Since 1987, monitoring programs have been successful in preventing other human incidents of ASP. However, there are documented cases of domoic acid intoxication in wild animals and outbreaks of coastal water contamination in many regions world-wide. Hence domoic acid continues to pose a global risk to the health and safety of humans and wildlife. Several mechanisms have been implicated as mediators for the effects of domoic acid. Of particular importance is the role played by glutamate receptors as mediators of excitatory neurotransmission and the demonstration of a wide distribution of these receptors outside the central nervous system, prompting the attention to other tissues as potential target sites. The aim of this document is to provide a comprehensive review of ASP, DOM induced pathology including ultrastructural changes associated to subchronic oral exposure, and discussion of key proposed mechanisms of cell/tissue injury involved in DOM induced brain pathology and considerations relevant to food safety and human health.

Domoic acid induced spinal cord lesions in adult mice: evidence for the possible molecular pathways of excitatory amino acids in spinal cord lesions

Domoic acid (DA) is an excitatory amino acids (EAAs) analog which induced excitotoxicity lesion to central nervous system, but whether induced adult animal spinal cord is not known, furthermore, previous studies have shown that EAAs play an important role in spinal cord lesion, however, the molecular pathways in spinal cord lesion are not fully known. Therefore, a motor neuron-like cell culture system and a DA-induced spinal cord lesioned mice model were used to study the effect of DA on spinal cord in adult mice and the possible molecular pathways of EAAs in spinal cord lesions. Exposure of motor neuron-like cells NSC34 to DA dramatically increased reactive oxygen species (ROS) production by the DCF fluorescent oxidation assay, reduced mitochondrial function by MTT assay, cell viability by trypan blue exclusion assay, and was accompanied by an increase of cell apoptosis by histone protein release assay. In DA-induced spinal cord lesioned mice model, we showed that the decrease of proteasome activity, increase of UCP4 expression by immunohistochemistry and neural cell apoptosis by TUNEL staining, and was accompanied by an decrease of motor disturbance grade during the different stages of DA treatment. Taken together, the in vitro and in vivo data presented in the current report demonstrated that DA induces spinal cord lesions in adult mice, and the multiple molecular pathways promoted by EAAs in spinal cord lesions, at least partially was associated with ROS generation increase, mitochondrial dysfunction, proteasome activity decrease and UCP4 expression increase.

Domoic acid transfer to milk: evaluation of a potential route of neonatal exposure

Domoic acid (DA), produced by the diatom genus Pseudo-nitzschia, is a glutamate analog and a neurotoxin in humans. During diatom blooms, DA can contaminate filter-feeding organisms, such as shellfish, and can be transferred by ingestion to higher trophic levels. Several intoxication events involving both humans and various marine mammals have been attributed to DA. Affected organisms show neurological symptoms such as seizures, ataxia, headweaving, and stereotypic scratching, as well as prolonged deficits in memory and learning. Neonatal animals have been shown to be substantially more sensitive to DA than adults. However, it has not been demonstrated whether DA can be transferred to nursing young from DA-exposed mothers. This study demonstrates transfer of DA from spiked milk (0.3 and 1.0 mg/kg) to the plasma of nursing neonatal rats and an overall longer DA retention in milk than in plasma after 8 hr in exposed dams. DA was detectable in milk up to 24 hr after exposure (1.0 mg/kg) of the mothers, although the amount of DA transferred to milk after exposure was not sufficient to cause acute symptoms in neonates.

Amnesic shellfish poison

Amnesic shellfish poisoning (ASP) is caused by consumption of shellfish that have accumulated domoic acid, a neurotoxin produced by some strains of phytoplankton. The neurotoxic properties of domoic acid result in neuronal degeneration and necrosis in specific regions of the hippocampus. A serious outbreak of ASP occurred in Canada in 1987 and involved 150 reported cases, 19 hospitalisations and 4 deaths after consumption of contaminated mussels. Symptoms ranged from gastrointestinal disturbances, to neurotoxic effects such as hallucinations, memory loss and coma. Monitoring programmes are in place in numerous countries worldwide and closures of shellfish harvesting areas occur when domoic acid concentrations exceed regulatory limits. This paper reviews the chemistry, sources, metabolism and toxicology of domoic acid as well as human case reports of ASP and discusses a possible mechanism of toxicity.

Effect of a short-term in vitro exposure to the marine toxin domoic acid on viability, tumor necrosis factor-alpha, matrix metalloproteinase-9 and superoxide anion release by rat neonatal microglia

BACKGROUND

The excitatory amino acid domoic acid, a glutamate and kainic acid analog, is the causative agent of amnesic shellfish poisoning in humans. No studies to our knowledge have investigated the potential contribution to short-term neurotoxicity of the brain microglia, a cell type that constitutes circa 10% of the total glial population in the brain. We tested the hypothesis that a short-term in vitro exposure to domoic acid, might lead to the activation of rat neonatal microglia and the concomitant release of the putative neurotoxic mediators tumor necrosis factor-alpha (TNF-alpha), matrix metalloproteinases-2 and-9 (MMP-2 and -9) and superoxide anion (O2-).

RESULTS

In vitro, domoic acid [10 microM-1 mM] was significantly neurotoxic to primary cerebellar granule neurons. Although neonatal rat microglia expressed ionotropic glutamate GluR4 receptors, exposure during 6 hours to domoic acid [10 microM-1 mM] had no significant effect on viability. By four hours, LPS (10 ng/mL) stimulated an increase in TNF-alpha mRNA and a 2,233 % increase in TNF-alpha protein In contrast, domoic acid (1 mM) induced a slight rise in TNF-alpha expression and a 53 % increase (p < 0.01) of immunoreactive TNF-alpha protein. Furthermore, though less potent than LPS, a 4-hour treatment with domoic acid (1 mM) yielded a 757% (p < 0.01) increase in MMP-9 release, but had no effect on MMP-2. Finally, while PMA (phorbol 12-myristate 13-acetate) stimulated O2- generation was elevated in 6 hour LPS-primed microglia, a similar pretreatment with domoic acid (1 mM) did not prime O2- release.

CONCLUSIONS

To our knowledge this is the first experimental evidence that domoic acid, at in vitro concentrations that are toxic to neuronal cells, can trigger a release of statistically significant amounts of TNF-alpha and MMP-9 by brain microglia. These observations are of considerable pathophysiological significance because domoic acid activates rat microglia several days after in vivo administration.