Effects of domoic acid on serum levels of TSH and thyroid hormones

Potential endocrine correlation with exposure to domoic acid in Southern Right Whale (Eubalaena australis) at the Península Valdés breeding ground

In waters off Península Valdés (PV), Argentina, southern right whales (SRW, Eubalaena australis) are occasionally exposed to domoic acid (DA), a neurotoxin produced by diatoms of the genus Pseudo-nitzschia. Domoic acid toxicity in marine mammals can cause gastrointestinal and neurological clinical signs, alterations in hematologic and endocrine variables, and can be fatal in extreme cases. In this study, we validated an enzyme immunoassay to quantify fecal glucocorticoid metabolites (fGCm) in 16 SRW fecal samples from live and dead stranded whales in PV from 2013 to 2018 and assessed fGCm levels associated with DA exposure. Overall, fGCm levels were significantly lower in SRWs with detectable fecal DA (n = 3) as compared to SRWs with undetectable fecal DA levels (n = 13). The highest fecal DA was observed in a live lactating female, which had low fGCm compared to the other lactating females studied. The highest fGCm was observed in a lactating female with undetectable DA; interestingly, at the time of sample collection, this female was sighted with two calves, an extremely unusual occurrence in this species. Though the sample size of these exceptionally rare breeding-season fecal samples was unavoidably small, our study provides evidence of potential adrenal alterations in whales exposed to an environmental neurotoxin such as DA.

Direct effects of the algal toxin, domoic acid, on ovarian function: Bovine granulosa and theca cells as an in vitro model

Domoic acid (DA) is a potent neurotoxin produced by alga Pseudo-nitzschia spp. and has been associated with reproductive disorders in mammals. The aim of this study was to investigate if DA can affect the reproductive system via direct action on ovarian function. Bovine granulosa and theca cells were used as in vitro models for evaluating DA effects on ovarian cell proliferation and steroid production. In small-follicle granulosa cells (SMGC), cell proliferation and estradiol (E2) production was not affected (P>0.05) while progesterone (P4) production was inhibited (P<0.05) by DA at all doses tested. In large-follicle granulosa cells (LGGC), DA had no effect (P>0.05) on cell proliferation or P4 production while E2 production was stimulated by 1 and 5 µg/ml DA (P<0.05). DA (1 µg/ml) attenuated (P<0.05) insulin-like growth factor 1 (IGF-1)-induced P4 production by large-follicle theca cells (LGTC), but did not affect androstenedione (A4) production or proliferation of LGTC. In glutamate-free medium, DA inhibited (P<0.05) SMGC E2 production and this inhibition was similar to inhibition of E2 by trans-(±)-1-amino-1,3-cyclopentanedicarboxylic acid monohydrate (ACPD; a selective metabotropic glutamate receptor subtype agonist) while kainic acid (KA; an ionotropic glutamate receptor subtype agonist) had no effect (P>0.10) on E2 production. Collectively, these results show for the first time that DA has direct effects on ovarian GC and TC steroidogenesis. Because DA inhibited E2 and P4 production, DA has the potential to be an endocrine disruptor.

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.

Changes in plasma thyroid hormones and cortisol levels in crucian carp (Carassius auratus) exposed to the extracted microcystins

The endocrine response of crucian carp injected intraperitoneally with extracted microcystins (MC) was investigated in this study. Fish were injected intraperitoneally either with 0.75% NaCl (control) and Microcystis extract corresponding to 150 and 600 microg microcystins per kg body weight. The plasma levels of triiodothyronine (T(3)), thyroxine (T(4)), free triiodothyronine (FT(3)), free thyroxine (FT(4)), and cortisol were determined at 0, 1, 3, 12, 24, and 48h post-administration of MC-containing extract. Treated fish displayed abnormal behaviors, such as a startle response and disoriented swimming, as well as changes in ventilation rates. Plasma cortisol concentrations of fish in both dose groups significantly increased after administration of extracted MC and remained high throughout the experiment, which suggested that MC elicited a stress response in treated fish. The profiles of cortisol changes in treated fish appeared to be dose dependent, indicating that fish in the high dose group experienced greater MC-induced disturbance. Mortality occurred after 12h in the high dose group. Plasma levels of T(4), T(3), FT(4), and FT(3) did not vary significantly between the control fish. In contrast to this, fish exposed to MC-containing extract showed significant declines in T(3), FT(4), and FT(3) levels in a dose-dependent manner throughout the experiment. Plasma T(4) levels, however, did not vary significantly in the low dose group, whereas they decreased significantly at 48h post injection in the high dose group. This study demonstrates that administration of microcystins-containing extract causes a stress response and reduces the plasma levels of thyroid hormones in crucian carp. These results illustrate that microcystins exerted potent effects on the endocrine system of crucian carp, through activating their hypothalamus-pituitary- interrenal axis and disturbing thyroid function.

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.

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.