Interaction of domoic acid and several derivatives with kainic acid and AMPA binding sites in rat brain

Accumulation, transformation and tissue distribution of domoic acid, the amnesic shellfish poisoning toxin, in the common cuttlefish, Sepia officinalis

Domoic acid (DA) is a phycotoxin produced by some diatoms, mainly from the Pseudo-nitzschia genus, and has been detected throughout the marine food web. Although DA has been frequently found in cephalopod prey such as crustaceans and fish, little is known about DA accumulation in these molluscs. This study presents the first data showing relevant concentrations of DA detected in the common cuttlefish, Sepia officinalis, which is one of the most studied cephalopod species in the world. Domoic acid was consistently found throughout 2003 and 2004 in the digestive gland of cuttlefish reaching concentrations of 241.7 microg DA g(-1). The highest DA values were detected during spring and summer months, periods when Pseudo-nitzschia occur in the plankton. In fact, Pseudo-nitzschia blooms preceded the highest DA concentrations in cuttlefish. Evaluation of DA tissue distribution showed elevated DA concentrations in the digestive gland and branchial hearts. Further, DA isomers comprised a relevant percentage of the toxin profile, indicating degradation and biotransformation of the toxin in the branchial hearts. The common cuttlefish, like other cephalopod species, plays a central position in the food web and might be a new DA vector to top predators like marine mammals. Human intoxications are not expected since DA was only seldom detected in the mantle and even then in very low levels (max 0.7 microg DA g(-1)). However, in some countries whole juvenile animals are consumed (i.e. without evisceration) and in this case they might represent a risk to human health. This study contributes to understanding the occurrence of phycotoxins in cephalopods and reveals a new member of the marine food web able to accumulate DA.

Reduction in functional potency of the neurotoxin domoic acid in the presence of cadmium and zinc ions

The tricarboxylic neurotoxin domoic acid (DA) binds trace metals such as iron and copper. In vitro brain slice recording (area CA1 of rat hippocampal slices) was used to assess changes in DA potency in the presence of cadmium and zinc. Cadmium or zinc alone had little or no effect on CA1 responses. DA alone produced hyperexcitability and, with prolonged administration, a robust suppression of CA1 responses. Coadministration of DA with either 2 or 4μM Cd(2+) produced significant reductions in the potency of DA; less striking effects were seen in the presence of 4μM Zn(2+). These findings suggest that interactions of Cd(2+) and Zn(2+) with DA result in the formation of trace metal-neurotoxin complexes which are either unavailable for binding to ionotropic glutamate receptors, or bind without producing full agonist activity.

Diet-brain connections: role of neurotoxicants

In certain cases, the consumption of food or beverages can lead to intoxication and disease. Such food-induced intoxications may be due to microbial toxins, to toxic substances naturally occurring in some foods, or to contaminants or residues of various kinds. Some of these agents have neurotoxic properties and may contribute to the etiology of certain psychiatric disorders or neurodegenerative diseases. This paper reviews a selected number of dietary neurotoxicants that naturally, or as a result of human interventions, find their way into food or beverages, and have been associated with neurotoxic outcomes in humans. Chosen examples include domoic acid, a phycotoxin associated with amnesic shellfish poisoning; β-N-oxalylamine-l-alanine (l-BOAA), present in chickling peas and believed to be responsible for neurolathyrism; and two alcohols, methanol and ethanol, which can cause severe neurotoxic effects in adults and the developing fetus.

Domoic acid-induced neurotoxicity in the hippocampus of adult rats

Domoic acid (DA), an agonist of non-N-methyl-D-aspartate (non-NMDA) receptor subtype including kainate receptor, was identified as a potent neurotoxin showing involvement in neuropathological processes like neuronal degeneration and atrophy. In the past decade evidence indicating a role for excitatory amino acids in association with neurological disorders has been accumulating. Although the mechanisms underlying the neuronal damage induced by DA are not yet fully understood, many intracellular processes are thought to contribute towards DA-induced excitotoxic injury, acting in combination leading to cell death. In this review article, we report the leading hypotheses in the understanding of DA-induced neurotoxicity, which focus on the role of DA in neuropathological manifestations, the formation of the retrograde messenger molecule nitric oxide (NO) for the production of free radicals in the development of neuronal damage, the activation of glial cells (microglia and astrocytes) in response to DA-induced neuronal damage and the neuroprotective role of melatonin as a free radical scavenger or antioxidant in DA-induced neurotoxicity. The possible implications of molecular mechanism underlying the neurotoxicity in association with necrosis, apoptosis, nitric oxide synthases (nNos and iNOS) and glutamate receptors (NMDAR1 and GluR2) related genes and their expression in DA-induced neuronal damage in the hippocampus have been discussed.

Domoic acid enhances Bcl-2-calcineurin-inositol-1,4,5-trisphosphate receptor interactions and delayed neuronal death in rat brain slices

Mechanisms of neuronal death following neuronal damage due to domoic acid are not completely defined. Bcl-2, a survival protein, protects neurons from ischemia and excitotoxin-induced damage. We previously demonstrated that Bcl-2 shuttles calcineurin to its substrates and may regulate calcium release from internal stores during neuronal ischemia. We now confirm that during excitotoxicity induced by domoic acid, calcineurin-Bcl-2 and calcineurin-1,4,5-inositol-trisphosphate receptor (IP3-R) interactions increase. Furthermore, we now show that calcineurin-IP3-R interactions are mediated by Bcl-2 in brain slices following short-term treatment with domoic acid (10 microM). Domoic acid induced late neuronal death and caspase-3-like activity in organotypic cortical and hippocampal cultures. These experiments further define the mechanisms by which neurons respond to excitotoxic insults, and suggest that interactions between calcineurin and its target proteins may influence cellular responses to injury.

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.

Domoic acid-induced hippocampal CA1 hyperexcitability independent of region CA3 activity

Domoic acid (DOM) is a potent agonist of AMPA and kainic acid (KA) receptors in the CNS and is known to produce seizures acutely, and lasting excitotoxic damage in several brain regions. While the excitotoxic effects of DOM are well documented, its seizurogenic properties are less clear. In this study, we assessed the acute effects of DOM and KA in region CA1 of intact rat hippocampal slices (CA3-on) and in slices lacking region CA3 (CA3-off). Orthodromic Schaffer collateral-evoked CA1 field potentials (population spikes and somal EPSP's) were monitored during DOM and KA (10-500 nM) administration. In CA3-off slices both KA and DOM produced immediate increases in CA1 population spike amplitude. With prolonged exposure, lasting dose-dependent reductions in spike amplitude and EPSP slope were observed, possibly due to depolarising conduction block following excessive AMPA/KA receptor activation; DOM was several-fold more potent than KA in this regard. Population spike threshold did not vary with DOM, but in CA3-on slices a dose-dependent steepening of the I/O curve and increase in maximum spike amplitude was seen. CA1 hyperexcitability, as evidenced by the appearance of prominent second and third population spikes, was equivalently increased across a range of DOM concentrations in both CA3-on and CA3-off slices and, in general, DOM-induced CA1 hyperexcitability was not enhanced by the presence of CA3 for any of the other variables assessed in this study. These findings show that DOM directly promotes neuronal hyperactivity in region CA1, presumably due to tonic AMPA and/or KA-receptor mediated depolarization, and further suggests that DOM-induced hyperactivity in the recurrently networked, AMPA/KA-receptor rich region CA3 does not contribute to the onset and spread of limbic seizures during relatively mild DOM intoxication.

Domoic acid-induced neuronal damage in the rat hippocampus: changes in apoptosis related genes (bcl-2, bax, caspase-3) and microglial response

Domoic acid (DA), a potent neurotoxin, administered intravenously (0.75 mg/kg body weight) in adult rats evoked seizures accompanied by nerve cell damage in the present study. Neuronal degeneration and microglial reaction in the hippocampus were investigated, and the temporal profile of bcl-2, bax, and caspase-3 genes in cell death or survival was assessed following the administration of DA. Nissl staining showed darkly stained degenerating neurons in the hippocampus following the administration of DA at 1-21 days, the degeneration being most severe at 5 days. Ultrastructural study in CA1 and CA3 regions of hippocampus revealed two types of neuronal degeneration, cells that exhibited swollen morphology and shrunken electron-dense cells. Immunoreactivity of Bcl-2 and Bax was increased considerably at 16 hr and 24 hr in the neurons of the hippocampus following DA administration. No significant change was observed in the immunoreactivity of caspase-3 in the controls and DA-treated rats at any time interval. Microglial cells in the hippocampus showed intense immunoreaction with the antibodies OX-42 and OX-6 at 1-21 days after DA administration, indicating the up-regulation of complement type 3 receptors and major histocompatibility complex type II antigens for increased phagocytic activity and antigen presentation, respectively. Terminal deoxynucleotidyl-transferase-mediated dUTP nick end labeling (TUNEL) showed occasional positive neurons in the CA1 and CA3 regions at 5 days after DA administration, with no positive cells in the controls. RT-PCR analysis revealed that bcl-2 and bax mRNA transcripts in the hippocampus were significantly increased at 16 hr and gradually decreased at 24 hr following the administration of DA. Although bax and bcl-2 mRNA expression is rapidly induced at early stages, in situ hybridization analysis revealed complete loss of bcl-2, bax, and caspase-3 mRNA at 24 hr after DA administration in the region of neuronal degeneration in the hippocampus. These results indicate that the pattern of neuronal degeneration observed during DA-induced excitotoxic damage is mostly necrotic.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Domoic acid accumulation in French shellfish in relation to toxic species of Pseudo-nitzschia multiseries and P. pseudodelicatissima

Within the French phytoplankton monitoring network (REPHY), domoic acid (DA), the toxin responsible for amnesic shellfish poisoning, was first detected in samples collected in 1998. Toxin analysis by the official method [liquid chromatography with diode array detection (LC/DAD)] was performed when Pseudo-nitzschia cell concentration was greater than 1.0 x 10(5) cells/l. LC/DAD results obtained in 1999 and 2000 showed increased DA accumulation in bivalves sampled at different sites along French coasts. The toxin maximum in 1999 was 3.2 microg DA/g of whole tissue, whereas the levels in 2000 (53 microg) were above the sanitary threshold (20 microg DA/g tissue). Phytoplankton samples collected during blooms were observed by both light and scanning electron microscopy (SEM). Identification of phytoplankton species by SEM analyses confirmed the presence of two known DA-producing species, P. pseudodelicatissima and P. multiseries. LC/DAD results for a mass culture of P. multiseries indicated that this species was involved in DA accumulation in French shellfish.

Status epilepticus-induced neuronal loss in humans without systemic complications or epilepsy

PURPOSE

To determine the regional distribution of neuronal damage caused strictly by status epilepticus (SE) without systemic complications, underlying brain pathology, or a history of preexisting epilepsy.

METHODS

The medical records and electroencephalograms (EEGs) of three deceased patients who developed SE in the hospital were reviewed. Their brains were formalin-fixed, and 17 brain regions were selected, embedded in paraffin, and sectioned. Alternate sections were stained with either hematoxylin and eosin and cresyl violet to determine the extent of neuronal loss and gliosis or glial fibrillary astrocytic protein to confirm the extent of astrocytic proliferation.

RESULTS

The three patients died 11 to 27 days after the onset of focal motor SE; none had hypotension, hypoxemia, hypoglycemia, or significant hyperthermia. Two patients had no prior seizures and no underlying brain pathology. The third patient, who had leptomeningeal carcinomatosis, had one seizure 2 months before the onset of SE. The duration of SE was 8.8 hours to 3 days. EEGs showed unilateral temporal lobe sharp-wave discharges in one patient and independent temporal lobe sharp-wave discharges bilaterally in the other two patients. In addition to widespread neuronal loss and reactive gliosis in the hippocampus, amygdala, dorsomedial thalamic nucleus, and Purkinje cell layer of the cerebellum, we report for the first time periamygdaloid (piriform) and entorhinal cortical damage occurring acutely after SE in humans.

CONCLUSIONS

In the absence of systemic complications or preexisting epilepsy, SE produces neuronal loss in a distribution similar to that from domoic acid-induced SE in humans and from kainic acid- and pilocarpine-induced SE in rats.

The activation of glutamate receptors by kainic acid and domoic acid

The neurotoxins kainic acid and domoic acid are potent agonists at the kainate and alphaamino-5-methyl-3-hydroxyisoxazolone-4-propionate (AMPA) subclasses of ionotropic glutamate receptors. Although it is well established that AMPA receptors mediate fast excitatory synaptic transmission at most excitatory synapses in the central nervous system, the role of the high affinity kainate receptors in synaptic transmission and neurotoxicity is not entirely clear. Kainate and domoate differ from the natural transmitter, L-glutamate, in their mode of activation of glutamate receptors; glutamate elicits rapidly desensitizing responses while the two neurotoxins elicit non-desensitizing or slowly desensitizing responses at AMPA receptors and some kainate receptors. The inability to produce desensitizing currents and the high affinity for AMPA and kainate receptors are undoubtedly important factors in kainate and domoate-mediated neurotoxicity. Mutagenesis studies on cloned glutamate receptors have provided insight into the molecular mechanisms responsible for these unique properties of kainate and domoate.

The marine toxin domoic acid may affect the developing brain by activation of neonatal brain microglia and subsequent neurotoxic mediator generation

Amnesic shellfish poisoning, one of the shellfish poisoning syndromes, is caused by the marine diatom toxin domoic acid (DOM). While in adult rats, mice, monkeys and humans DOM poorly penetrates the blood-brain barrier, DOM has been shown to be very toxic to fetal in newborn mice, because the blood-brain barrier is incomplete during neurodevelopment. This fact may explain why neonates show a higher sensitivity to neurotoxins like DOM as compared to adult animals. Mechanistic studies on DOM's neurotoxicity have mainly concentrated on the investigation of DOM's effect on neuronal tissue. Recent studies have shown that glia is also involved in DOM's neurotoxicity to the adult as well as the developing nervous system. The scientific literature strongly supports the hypothesis that the microglia may play a critical role in mediating DOM's neurotoxic effects. However, the effect of DOM on microglia has not been systematically investigated. The literature supporting our hypothesis is presented and discussed.

Marine algal toxins: origins, health effects, and their increased occurrence

Certain marine algae produce potent toxins that impact human health through the consumption of contaminated shellfish and finfish and through water or aerosol exposure. Over the past three decades, the frequency and global distribution of toxic algal incidents appear to have increased, and human intoxications from novel algal sources have occurred. This increase is of particular concern, since it parallels recent evidence of large-scale ecologic disturbances that coincide with trends in global warming. The extent to which human activities have contributed to their increase therefore comes into question. This review summarizes the origins and health effects of marine algal toxins, as well as changes in their current global distribution, and examines possible causes for the recent increase in their occurrence.

A neurophysiological method of rapid detection and analysis of marine algal toxins

We have examined the effectiveness of the in vitro rat hippocampal slice preparation as a means of rapidly and specifically detecting the marine algal toxins saxitoxin, brevetoxin, and domoic acid and have identified toxin-specific electrophysiological signatures for each. Brevetoxin (PbTX3, 50-200 nM) produced a significant reduction in orthodromic population spike amplitude which was quick to reverse during a 50 min wash-out, while antidromic population spikes and field EPSPs exhibited only slight reductions, and fibre spiof orthodrokes showed no change at all. Domoic acid (100 nM) produced a robust, reversible increase in amplitude mic spikes, and the appearance of multiple spikes (i.e., epileptiform activity) within minutes of toxin wash-in. Other notable features of the domoic acid signature included a significant decrease in amplitude of the field EPSPs, and a complete absence of effect on either antidromic or fibre spikes. Fifty nanomolar saxitoxin (PSP) abolished all responses in all slices. Only antidromic spikes showed any recovery during wash-out. Field EPSP and fiber spike analysis further demonstrated that the preparation is capable of reliably detecting saxitoxin in a linearly responsive fashion at toxin concentrations of 25-200 nM, and tests of naturally contaminated shellfish confirmed the utility of this assay as a screening method for PSP. Our findings suggest that the in vitro hippocampal slice preparation has potential in the detection and analysis of three marine algal toxins important to the shellfish industry.

Production and characterization of a monoclonal antibody against domoic acid and its application to enzyme immunoassay

For production of monoclonal antibodies against domoic acid, a causative agent of amnesic shellfish poisoning, three immunogens, domoic acid conjugated with bovine serum albumin (BSA), ovalbumin (OVA) and human gamma globulin (HGG), were prepared. The antiserum obtained from BALB/c mice immunized with domoic acid-BSA showed the highest affinity for domoic acid. The monoclonal antibody, DA-3, obtained from the mice was highly specific for domoic acid and showed a minor cross-reactivity with the isomers of domoic acid (isodomoic acids B, E, F, G and H), except for isodomoic acid A. Using DA-3 antibody, an indirect competitive enzyme immunoassay (idc-EIA) was developed for measurement of domoic acid. The working range for quantitative measurement of domoic acid and the quantification limit for domoic acid in shellfish were estimated to be 0.15-10 ng/ml and less than 0.04 microg/g, respectively. The mean recovery of domoic acid added to extracts of shellfish at toxin levels of 0.02 to 0.2 microg/ml was 103% with a coefficient of variation of 4.5%. The newly developed idc-EIA seems to be a useful method for monitoring domoic acid in shellfish.

High-affinity [3H] kainic acid binding to brain membranes: a re-evaluation of ligand potency and selectivity

[3H]Kainic acid ([3H]KA) is a widely used tool for studying the KA class of excitatory amino acid receptors. [3H]KA of significantly higher specific activity has become available permitting use of radioligand concentrations below the dissociation constant (K(D)) of the high-affinity binding site. We employed low radioligand (0.05-0.2 nM) and receptor concentrations (0.01 nM) to gain new insights into the binding characteristics of the high-affinity KA binding site in a standard preparation of lyzed synaptosomal membranes from the cerebral cortex of male Sprague-Dawley rats. Under these conditions, KA binds to a single class of high-affinity sites with a K(D) of 1.0+/- 0.3 nM. The potencies of competing agents are considerably higher than published reports. Specifically, domoic acid, glutamate, and glutamine exhibit IC(50) values for displacing [3H]KA of 0.37+/-0.02, 94+/-13, and 1500+/-500 nM, respectively. Domoate (1 microM) was tested against a panel of 32 central nervous system binding sites and found to be inactive at each, indicating this toxin displays considerable selectivity. This study illustrates the remarkable potency of domoic acid and underlines the importance of performing radioligand binding studies at concentrations of constituents that permit characterization of high-affinity interactions.

Kainic and domoic acids differentially affect NADPH-diaphorase neurons in the mouse hippocampal formation

We investigated changes in numbers of nitric-oxide-producing cells in the hippocampal formation, striatum, and temporal cortex of mice 24 h after intraperitoneal administration of kainic acid (5, 10, 15, and 20 mg/kg) or domoic acid (1, 2, and 4 mg/kg). Nitric-oxide-producing cells were demonstrated histochemically by staining for nicotinamide adenine dinucleotide phosphate diaphorase. Nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in the dentate gyrus and the subiculum did not change in number following administration of kainic acid or domoic acid at any dose. Positive neurons in the CA3 region of mice treated with kainic acid or domoic acid at any dose were significantly fewer than in controls. Although the numbers of positive neurons in the CA1/CA2 regions did not differ from those of controls at any of the four doses of kainic acid, positive cells in the CA1/CA2 were significantly more numerous than in controls at any dose of domoic acid. Although no significant differences in the numbers of positive neurons in the striatum were apparent between controls and any of the four doses of kainic acid, domoic acid significantly decreased the numbers of such cells. These results suggest that systemically administered kainic acid and domoic acid affect differentially nitric-oxide-producing cells in the hippocampal formation.

Role of desensitization and subunit expression for kainate receptor-mediated neurotoxicity in murine neocortical cultures

The neurotoxic actions of kainate and domoate were studied in cultured murine neocortical neurons at various days in culture and found to be developmentally regulated involving three components of neurotoxicity: (1) toxicity via indirect activation of N-methyl-D-aspartate (NMDA) receptors, (2) toxicity mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors, and (3) toxicity that can be mediated by kainate receptors when desensitization of the receptors is blocked. The indirect action at NMDA receptors was discovered because (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-im ine (MK-801), an NMDA receptor antagonist, was able to block part of the toxicity. The activation of NMDA receptors is most likely a secondary effect resulting from glutamate release upon kainate or domoate stimulation. 1-(4-Aminophenyl)-3-methylcarbamyl-4-methyl-3,4-dihydro-7,8-ethyle nedioxy-5H-2,3-benzodiazepine (GYKI 53655), a selective AMPA receptor antagonist, abolished the remaining toxicity. These results indicated that kainate- and domoate-mediated toxicity involves both the NMDA and the AMPA receptors. Pretreatment of the cultures with concanavalin A to prevent desensitization of kainate receptors led to an increased neurotoxicity upon stimulation with kainate or domoate. In neurons cultured for 12 days in vitro a small but significant neurotoxic effect was observed when stimulated with agonist in the presence of MK-801 and GYKI 53655. This indicates that the toxicity is produced by kainate receptors in mature cultures. Examining the subunit expression of the kainate receptor subunits GluR6/7 and KA2 did, however, not reveal any major change during development of the cultures.

The regulation of AMPA receptor-binding sites

A wide variety of mechanisms have been identified that can regulate the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-receptor complex. Modulation has been shown to occur at the nucleic acid level via RNA editing and alternative splicing. At the posttranslational level, processes such as phosphorylation, glycosylation, chemical modification of reactive groups on the receptor proteins, interaction with a putative receptor-associated modulatory protein, and changes in the lipid environment have been reported to regulate receptor binding and function. In this review, we discuss general aspects of the cell biology, pharmacology, and function of AMPA receptors. In particular, we focus on some factors shown to modulate agonist binding and discuss possible molecular mechanisms underlying the regulation observed.

Neurotoxin domoic acid produces cytotoxicity via kainate- and AMPA-sensitive receptors in cultured cortical neurones

Domoic acid, a naturally occurring kainoid, has been responsible for several outbreaks of fatal poisoning after shellfish ingestion, and we examined its neurotoxic mechanism in cultured murine cortical neurones. Using observations of neuronal viability and morphology, exposure to domoic acid for 24 h was found to induce substantial concentration-dependent neuronal cell death. Domoic acid-mediated neuronal death was attenuated by the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione and the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-selective antagonist LY293558 ((3S,4aR,6R,8aR)-6-[2-(1H-tetrazol-5-yl)-ethyl]-1,2,3, 4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid), but unaffected by NS-102 (5-nitro-6,7,8,9-tetrahydrobenzo[g]indole-2, 3-dione-3-oxime)--a low-affinity kainate receptor antagonist. Domoic acid was equipotent with (S)-AMPA (EC50 values 3.8 and 3.4 microM respectively); however, (S)-AMPA induced only 50% cell death compared to > 80% cell death induced by domoic acid. Kainate also killed > 80% of cortical neurones; however, domoic acid was about 19 times more potent than kainate (EC50 75 microM). We show the potent neurotoxicity of domoic acid for the first time in a pure neuronal model and indicate that domoic acid acts via high-affinity AMPA- and kainate-sensitive glutamate receptors to produce excitotoxic cell death.

Two new isomers of domoic acid from a red alga, Chondria armata

Isodomoic acids G and H, two new isomers of the neurotoxin domoic acid, along with isodomoic acids A, B, E and F, were isolated from a red alga, Chondria armata, collected at the southern tip of Kyushu Island. The structures of two of these were deduced to be (E, E) and (Z, E) isomers of 2-carboxy-4-(5-carboxy-l-methyl-2-hexenylidene)-3-pyrro- lidineacetic acid, based on electrospray ionization mass and [1H]nuclear magnetic resonance spectral analyses including [1H-1H]correlation spectroscopy and nuclear Overhauser effect correlation spectroscopy.

Characterization of the pharmacology and regional distribution of (S)-[3H]-5-fluorowillardiine binding in rat brain

1. This study examined the binding of the new radioligand (S)-[3H]-5-fluorowillardiine to rat brain synaptic membranes. Specific binding represented greater than 80% of the total binding and was increased by 10% in the presence of 100mM potassium thiocyanate (KSCN). 2. In the absence of KSCN, (S)-[3H]-5-fluorowillardiine identified two binding sites with KD1=22.5 nM, Bmax1=1.4 pmol mg(-1) protein and KD2=1.5 microM, Bmax2=10.8 pmol mg(-1) protein. In the presence of 100 mM KSCN the affinities of both the binding sites were increased, yielding values of KD1=6.9 nM and KD2=0.4 microM KSCN was without effect on the Bmax values. 3. (S)-[3H]-5-fluorowillardiine binding was displaced by non-NMDA receptor ligands with the rank order of potency: 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) > domoate > (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionice acid (AMPA) = L-glutamate > 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) > kainate >> (R)-5-fluorowellardiine. In contrast, both N-methyl-D-aspartate (NMDA) and the metabotropic glutamate receptor agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) were inactive. 4. By use of quantitative autoradiography the regional distribution of (S)-[3H]-5-fluorowillardiine binding in rat brain was assessed. The highest levels of binding were in the dentate gyrus and the CA1 region of the hippocampus. Lower levels of binding were detected in the cerebral cortex, olfactory system, lateral septum, caudate putamen and nucleus accumbens. 5. We conclude that the pharmacological profile and regional distribution of (S)-[3H]-5-fluorowillardiine binding is consistent with its specific interaction with AMPA receptors.

Regional differences in the interaction of the excitotoxins domoate and L-beta-oxalyl-amino-alanine with [3H]kainate binding sites in human hippocampus

The excitotoxic amino acid domoate causes anterograde amnesia and memory deficits while the excitotoxin L-beta-oxalyl-amino-alanine (L-BOAA) is considered the causative agent of the motoneurone disorder, neurolathyrism. Employing quantitative autoradiography we investigated the potency of domoate and L-BOAA to inhibit [3H]kainate binding in human hippocampus. Domoate inhibited binding of [3H]kainate with inhibition constants between 5.8 +/- 2.8 nM (deep layers of gyrus parahippocampalis) and 200.9 +/- 247.8 nM (CA1 region of hippocampus). It was about a thousandfold more potent than L-BOAA with inhibition constants between 2.1 +/- 0.5 microM (superficial layers of gyrus parahippocampalis) and 51.0 +/- 41.9 microM (CA2/3 region of hippocampus). Interestingly, L-BOAA showed lowest affinity to [3H]kainate binding sites in those regions in which domoate showed highest affinity (e.g. CA2/3) and vice versa (e.g. CA1). These data further support the notion that the neurological symptoms observed after domoate intoxication are due to an excitotoxic action at kainate receptors and provide evidence for heterogeneity of kainate receptors in human hippocampus.

Seizures, brain damage and brain development

Recent evidence suggests that hippocampal damage can be both the result of seizure activity and the cause of further chronic epilepsy. A review of current models of status epilepticus-induced brain damage reveals that excitotoxic mechanisms probably mediate the lesions in most brain regions. NMDA receptors appear to play a dominant role, although non-NMDA glutamate receptors are important in several specific neuronal populations. In the immature brain, a number of unique metabolic features determine a different set of vulnerabilities, resulting in a brain which is more resistant than the adult's to certain mechanisms of brain damage, but quite vulnerable to others. The inhibition of growth by severe seizure activity has implications for the developing brain that have not yet been fully explored. The mechanisms by which seizure-induced hippocampal lesions cause chronic epilepsy have been explored in several recent animal models. A rearrangement of hippocampal circuits may result from death of selected populations of inhibitory neurons, or from misdirected regeneration by excitatory neurons. It could lead to chronic epilepsy through loss of normal inhibition, through sprouting of new excitatory connections, through conservation of excitatory connections which in a healthy brain would be pruned during development, or through facilitation of kindling by one of these mechanisms. These recent results are beginning to reconcile the pathology seen in human hippocampi ablated for intractable epilepsy with that observed in experimental animals, and offer the promise of even greater advances in the future. They suggest a mechanism for Gower's dictum that "seizures beget seizures" and highlight the importance of the interneurons of the dentate gyrus in epileptogenesis.

Domoic acid induced release of [3H]GABA in cultured chick retina cells

The effect of the neurotoxin domoic acid (DOM), a structural analogue of kainic acid, on the release of [3H]gamma-aminobutyric acid (GABA) and on the [Ca2+]i was studied in cultured chick retina cells. DOM stimulated dose-dependently the release of [3H]GABA with an EC50 of 2.5 microM. In Ca(2+)-containing medium (1 mM), DOM (5 microM) increased the [Ca2+]i by about 190 nM and evoked the release of 11.8 +/- 1.3% of the intracellular [3H]GABA, while in the absence of extracellular Ca2+ DOM induced the release of only 7.9 +/- 1.4% of the accumulated [3H]GABA. The Ca(2+)-independent release of [3H]GABA was blocked by the non-competitive inhibitor of the GABA carrier 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid hydrochloride (NNC-711), but a component of Ca(2+)-dependent release remains. DOM evoked Ca(2+)-independent release of [3H]GABA was significantly depressed in the absence of external Na+ and completely blocked by the non-selective antagonist of the non-NMDA glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Similarly, CNQX decreased the [Ca2+]i response to DOM, whereas L(+)-2-amino-3-phosphonopropionic acid (L-AP3), an antagonist of the metabotropic glutamate receptors, was without effect. MK-801 did not affect the release of [3H]GABA stimulated by DOM. Taken together our results indicate that DOM evokes both Ca(2+)-dependent and Ca(2+)-independent release of [3H]GABA, most likely by activating kainate receptors.

Properties of a recombinant kainate receptor expressed in baculovirus-infected insect cells

GluR6 is a glutamate receptor of the kainate subtype that is expressed in the mammalian central nervous system. The cDNA coding for the rat brain receptor was subcloned into a baculovirus expression vector and the purified recombinant virus was used to infect Spodoptera frugiperda (Sf9) insect cells. The pharmacological profile and the status of several post-translational modifications of the GluR6 protein were analyzed. Saturation analyses of [3H]kainic acid binding demonstrated that GluR6 expressed in Sf9 cell membranes bound [3H]kainic acid at a single high affinity site with a dissociation constant of 12 nM. Competition studies indicated the inhibitory potencies of various excitatory amino acids, including the potent neurotoxin domoic acid, were comparable to those observed in mammalian brain tissue. Immunoblots of infected Sf9 cell membranes using an anti-GluR6 antibody revealed two immunoreactive bands. Enzymatic deglycosylation indicated that the higher molecular weight form corresponded to the glycosylated receptor whereas the lower molecular weight form corresponded to the unglycosylated protein. The phosphorylation of GluR6 by cyclic AMP-dependent and cyclic GMP-dependent protein kinase was examined in partially purified preparations of the receptor. GluR6 was phosphorylated by cyclic AMP but not by cyclic GMP-dependent protein kinase in vitro. These results indicate that GluR6 expressed in Sf9 cells has similar pharmacological properties and is processed post-translationally in a manner similar to GluR6 expressed in mammalian cells and tissues. The ease of production and the high levels of expression in baculovirus-infected insect cells relative to other expression systems should facilitate further biochemical and pharmacological characterization of this receptor.