Histopathological and molecular changes produced by hippocampal microinjection of domoic acid

Effect of different charges of modified electroconvulsive seizure on the cognitive behavior in stressed rats: Effects of GluR1 phosphorylation and CaMKIIα activity

Electroconvulsive therapy (ECT) is an efficient therapy for major depression and modern ECT requires anesthesia to enhance safety. However, the commonly used anesthetic, propofol, may weaken the treatment efficacy. A recent study confirmed that ketamine rapidly reduced the symptoms of depression in affected patients. A previous study found that electroconvulsive seizure (ECS), the animal model for ECT, under anesthesia of low-dose ketamine combined with propofol could enhance the antidepressant efficacy and improve the cognitive performance. The present study aimed to investigate the responses to different charges (0, 60, 120, 180 or 240 mC) of ECS under compound anesthetics, ketamine combined with propofol, in stressed rats and the underlying mechanisms to aid in optimization of treatment regimens. The results indicated that ECS exhibited an improved antidepressant effects at 120 mC compared with 60 mC, however, no significant differences in antidepressant effects were identified among the 120, 180 and 240 mC groups. Furthermore, rats subjected to ECS at 120 mC exhibited the best cognitive performance. The phosphorylation levels of calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) at Thr286, glutamate receptor 1 (GluR1) at Ser831 and cAMP-response element-binding protein (CREB) at the Ser133 were higher in the 120-mC group compared with all other groups. These results indicated that the ECS at medium intensity (120 mC) with administration of compound anesthetics may exert an improved therapeutic effect on depression compared with other intensities (0, 60, 180 and 240 mC). The results also suggested that the improvement in cognitive function in stressed rats may be attributed to the phosphorylation of CaMKIIα (Thr286), GluR1 (Ser831) and CREB (Ser133).

Sex differences in effects of low level domoic acid exposure

Consumption of seafood containing the phytoplankton-derived toxin domoic acid (DOM) causes neurotoxicity in humans and in animals. It has been reported that DOM-induced symptoms may be more severe in men than women, but to date the effect of sex on DOM-induced effects in adults is not known. We investigated sex differences in DOM-induced effects in adult rats. Since low level exposure is of greatest relevance to human health (due to DOM regulatory limit), we examined the effects of low level exposure. Adult male and female Sprague Dawley rats were administered a single intraperitoneal injection of DOM (0, 1.0, 1.8 mg/kg). Behaviour was monitored for 3h and immunohistochemistry in the dorsal hippocampus and olfactory bulb was also examined. DOM increased locomotor and grooming activity, compared to vehicle group. DOM exposure also significantly increased stereotypic behaviours and decreased phosphorylated cAMP response element-binding protein immunoreactivity (pCREB-IR). There was no effect of sex on the magnitude of the behavioural responses, but the onset of DOM-induced locomotor activity and ear scratches was quicker in females than in males. Mixed effect modelling revealed the predicted peak in locomotor activity in response to DOM was also quicker in females than in males. Severe toxicity was evident in 2/7 male rats and 0/8 female rats dosed with 1.8 mg/kg DOM. These data suggest that males exposed to low level DOM may be more susceptible to severe neurotoxicity, whereas females are affected more quickly. Understanding sex differences in DOM-induced neurotoxicity may contribute to future protective strategies and treatments.

Enhanced neurogenesis in organotypic cultures of rat hippocampus after transient subfield-selective excitotoxic insult induced by domoic acid

New neurons are continuously generated in the hippocampus and may play an important role in many physiological and pathological conditions. Here we present evidence of cell proliferation and neurogenesis after a selective and transient excitotoxic injury to the hippocampal cornu ammonis 1 (CA1) area induced by low concentrations of domoic acid (DOM) in rat organotypic hippocampal slice cultures (OHSC). DOM is an excitatory amino acid analog to kainic acid that acts through glutamate receptors to elicit a rapid and potent excitotoxic response. Exposure of slice cultures to varying concentrations of DOM for 24 h induced dose-dependent neuronal toxicity that was independent of activation of classic apoptotic markers. Treatment with 2 μM DOM for 24 h caused a selective yet transient neurotoxic injury in the CA1 subfield of the hippocampus that appeared recovered after 7 days of incubation in a DOM-free medium and showed significant microgliosis but no sign of astrogliosis. The DOM insult (2 μM, 24 h) resulted in a significant upregulation of cell proliferation, as assessed by 5-bromo-2-deoxyuridine (BrdU) incorporation, and a concurrent increase of the neuronal precursor cell marker doublecortin (DCX) within the subgranular zone of the dentate gyrus and area CA1. Neurogenesis occurred primarily during the first week after termination of the DOM exposure. Our study shows that exposure of OHSC to concentrations of DOM below those required to induce permanent neurotoxicity can induce proliferation and differentiation of neural progenitor cells that may contribute to recovery from mild injury and to develop abnormal circuits relevant to disease.

Defining and modeling known adverse outcome pathways: Domoic acid and neuronal signaling as a case study

An adverse outcome pathway (AOP) is a sequence of key events from a molecular-level initiating event and an ensuing cascade of steps to an adverse outcome with population-level significance. To implement a predictive strategy for ecotoxicology, the multiscale nature of an AOP requires computational models to link salient processes (e.g., in chemical uptake, toxicokinetics, toxicodynamics, and population dynamics). A case study with domoic acid was used to demonstrate strategies and enable generic recommendations for developing computational models in an effort to move toward a toxicity testing paradigm focused on toxicity pathway perturbations applicable to ecological risk assessment. Domoic acid, an algal toxin with adverse effects on both wildlife and humans, is a potent agonist for kainate receptors (ionotropic glutamate receptors whose activation leads to the influx of Na(+) and Ca²(+)). Increased Ca²(+) concentrations result in neuronal excitotoxicity and cell death, primarily in the hippocampus, which produces seizures, impairs learning and memory, and alters behavior in some species. Altered neuronal Ca²(+) is a key process in domoic acid toxicity, which can be evaluated in vitro. Furthermore, results of these assays would be amenable to mechanistic modeling for identifying domoic acid concentrations and Ca²(+) perturbations that are normal, adaptive, or clearly toxic. In vitro assays with outputs amenable to measurement in exposed populations can link in vitro to in vivo conditions, and toxicokinetic information will aid in linking in vitro results to the individual organism. Development of an AOP required an iterative process with three important outcomes: a critically reviewed, stressor-specific AOP; identification of key processes suitable for evaluation with in vitro assays; and strategies for model development.

In vivo seizure induction and affinity studies of domoic acid and isodomoic acids-D, -E and -F

Domoic acid and its isomers are produced via algal blooms and are found in high concentrations in shellfish. Here, we assessed the acute seizurogenic potencies of isomers-D, -E and -F and their binding affinities at heterogeneous populations of KA receptors from rat cerebrum. In addition, binding affinities of all six isomers (Iso-A through -F) were assessed at AMPA receptors. Radioligand displacement studies indicated that the seizurogenic potency of Iso-F (E-configuration) closely correlates with its affinities at both KA and AMPA receptors, whereas isomers-D (Z) and -E (E), which exhibit distinctly lower seizurogenic potencies, are quite weak displacers. Previously observed functional potencies for isomers-A, -B and -C (Sawant et al., 2008) correlated with AMPA receptor affinities observed here. Taken together, these findings call into question previous structure-activity rules. Significantly, in our hands, Iso-D was ten-fold less potent than Iso-F. To further explain observed links between structural conformation and functional potency, molecular modeling was employed. Modeling results closely matched the rank order of potency and binding data observed. We further assessed the efficacy of isomers-D, -E and -F as pharmacological preconditioning agents. Acute preconditioning with low-dose Iso-D, -E or -F, before high-dose DA failed to impart behavioural tolerance. This study has shed new light on structural conformations affecting non-NMDA ionotropic glutamate receptor binding and functional potency, and provides a foundation for future work in areas of AMPA and KA receptor modeling.

Domoic acid induces a long-lasting enhancement of CA1 field responses and impairs tetanus-induced long-term potentiation in rat hippocampal slices

Domoic acid (DOM) is known to cause hippocampal neuronal damage and produces amnesic effects. We examined synaptic plasticity changes induced by DOM exposure in rat hippocampal CA1 region. Brief bath application of DOM to hippocampal slices produces a chemical form of long-term potentiation (LTP) of CA1 field synaptic potentials. The potentiation cannot be blocked by NMDA receptor antagonist MK-801 but can be blocked by the calcium-calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-62 or cAMP-dependent protein kinase (PKA) inhibitor H-89. DOM-potentiated slices show decreased autophosphorylated CaMKII (p-Thr286), an effect that is also dependent on the activity of CaMKII and PKA. Increased phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit GluR1 (p-Ser831) was seen in DOM-potentiated slices. Therefore, aberrant regulation of CaMKII and GluR1 phosphorylation occurs after DOM application. In addition, tetanus-induced LTP as well as the increase of phosphorylation of CaMKII (p-Thr286) were reduced in DOM-potentiated slices. Compared with brief exposure, slices recovering from prolonged exposure did not show potentiation or altered levels of CaMKII (p-Thr286) or GluR (p-Ser831). However, decreased phosphorylation of GluR1 at Ser845 was seen. These results describe a new chemical form of LTP and uncover novel molecular changes induced by DOM. The observed impairment of tetanus LTP and misregulation of CaMKII and GluR1 phosphorylation may partially account for DOM neurotoxicity and underlie the molecular basis for DOM-induced memory deficit.

Immunomodulatory effects of domoic acid differ between in vivo and in vitro exposure in mice

The immunotoxic potential of domoic acid (DA), a well-characterized neurotoxin, has not been fully investigated. Phagocytosis and lymphocyte proliferation were evaluated following in vitro and in vivo exposure to assay direct vs indirect effects. Mice were injected intraperitoneally with a single dose of DA (2.5 μg/g b.w.) and sampled after 12, 24, or 48 hr. In a separate experiment, leukocytes and splenocytes were exposed in vitro to 0, 1, 10, or 100 μM DA. In vivo exposure resulted in a significant increase in monocyte phagocytosis (12-hr), a significant decrease in neutrophil phagocytosis (24-hr), a significant decrease in monocyte phagocytosis (48-hr), and a significant reduction in T-cell mitogen-induced lymphocyte proliferation (24-hr). In vitro exposure significantly reduced neutrophil and monocyte phagocytosis at 1 μM. B- and T-cell mitogen-induced lymphocyte proliferation were both significantly increased at 1 and 10 μM, and significantly decreased at 100 μM. Differences between in vitro and in vivo results suggest that DA may exert its immunotoxic effects both directly and indirectly. Modulation of cytosolic calcium suggests that DA exerts its effects through ionotropic glutamate subtype surface receptors at least on monocytes. This study is the first to identify DA as an immunotoxic chemical in a mammalian species.

Gene expression profiles in zebrafish brain after acute exposure to domoic acid at symptomatic and asymptomatic doses

Domoic acid (DA) is a neuroexcitatory amino acid that is naturally produced by some marine diatom species of the genus Pseudo-nitzschia. Ingestion of DA-contaminated seafood by humans results in a severe neurotoxic disease known as amnesic shellfish poisoning (ASP). Clinical signs of ASP include seizures and neuronal damage from activation of ionotropic glutamate receptors. However, the impacts of DA exposure at levels below those known to induce outward signs of neurobehavioral exicitotoxicity have not been well characterized. To further understand the mechanisms of neurotoxic injury associated with DA exposure, we examined the transcriptome of whole brains from zebrafish (Danio rerio) receiving intracoelomic (IC) injection of DA at both symptomatic and asymptomatic doses. A majority of zebrafish exposed to high-dose DA (1.2 microg DA/g) exhibited clinical signs of neuroexcitotoxicity (EC(50) of 0.86 microg DA/g) within 5-20 min of IC injection. All zebrafish receiving low-dose DA (0.47 microg DA/g) or vehicle only maintained normal behavior. Microarray analysis of symptomatic and asymptomatic exposures collectively yielded 306 differentially expressed genes (1.5-fold, p </= 0.05) predominately represented by signal transduction, ion transport, and transcription factor functional categories. Transcriptional profiles were suggestive of neuronal apoptosis following an overwhelming of protective adaptive pathways. Further, potential molecular biomarkers of neuropathic injury, including the zebrafish homolog of human NDRG4, were identified and may be relevant to DA exposure levels below that causing neurobehavioral injury. In general, DA-modulated gene expression was consistent with other model species thereby validating zebrafish as an appropriate vertebrate model to study mechanisms of DA neurotoxicity. These data provide a basis for identifying pathways of DA-induced injury as well as biomarkers of asymptomatic and symptomatic DA exposure levels.

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.

Isodomoic acids A and C exhibit low KA receptor affinity and reduced in vitro potency relative to domoic acid in region CA1 of rat hippocampus

Several natural isomers of the seizurogenic neurotoxin domoic acid (DA) have been found to occur at up to mg/kg levels in shellfish. The aim of the current study was to assess the neurotoxic potency of isodomoic acids A and C (Iso-A and Iso-C), recently isolated from commercial shellfish. Hippocampal slices were obtained from young adult rats and maintained in a tissue recording chamber. Synaptically evoked population spikes were recorded in region CA1 before and after exposure to DA or its isomers. Both Iso-A and Iso-C produced transient neuronal hyperexcitability followed by a dose-dependent suppression of population spikes, but were, respectively, 4- and 20-fold less potent than DA (spike area: EC50 DA=237 nM; Iso-A=939 nM; Iso-C=4.6 microM). In the hippocampus, DA preconditioning induces tolerance to subsequent DA toxicity. However, in the present study neither Iso-A nor Iso-C were effective as preconditioning agents. Competitive binding studies using homomeric GluR6 kainate (kainic acid, KA) receptors showed the affinity of Iso-A to be 40-fold lower than DA (Ki DA=3.35 nM; Iso-A=130 nM). Together with earlier work showing Iso-C affinity at GluR6 receptors to be 240-fold lower than DA, our results suggest that neuroexcitatory effects of Iso-A in CA1 may involve both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and KA receptors, while Iso-C likely involves the activation of AMPA receptors alone.