Human Heart Glutamate Receptors—Implications for Toxicology, Food Safety, and Drug Discovery

The Emerging Role of N-Methyl-D-Aspartate (NMDA) Receptors in the Cardiovascular System: Physiological Implications, Pathological Consequences, and Therapeutic Perspectives

N-methyl-D-aspartate receptors (NMDARs) are ligand-gated ion channels that are activated by the neurotransmitter glutamate, mediate the slow component of excitatory neurotransmission in the central nervous system (CNS), and induce long-term changes in synaptic plasticity. NMDARs are non-selective cation channels that allow the influx of extracellular Na⁺ and Ca²⁺ and control cellular activity via both membrane depolarization and an increase in intracellular Ca²⁺ concentration. The distribution, structure, and role of neuronal NMDARs have been extensively investigated and it is now known that they also regulate crucial functions in the non-neuronal cellular component of the CNS, i.e., astrocytes and cerebrovascular endothelial cells. In addition, NMDARs are expressed in multiple peripheral organs, including heart and systemic and pulmonary circulations. Herein, we survey the most recent information available regarding the distribution and function of NMDARs within the cardiovascular system. We describe the involvement of NMDARs in the modulation of heart rate and cardiac rhythm, in the regulation of arterial blood pressure, in the regulation of cerebral blood flow, and in the blood-brain barrier (BBB) permeability. In parallel, we describe how enhanced NMDAR activity could promote ventricular arrhythmias, heart failure, pulmonary artery hypertension (PAH), and BBB dysfunction. Targeting NMDARs could represent an unexpected pharmacological strategy to reduce the growing burden of several life-threatening cardiovascular disorders.

The low glutamate diet reduces blood pressure in veterans with Gulf War Illness: A CONSORT randomized clinical trial

BACKGROUND

Gulf War Illness is a multi-symptom condition affecting veterans of the 1990 to 1991 Gulf War, which often presents with comorbid hypertension. The purpose of this study was to analyze the effects of the low glutamate diet, as well as an acute challenge of monosodium glutamate (MSG)/placebo, on resting heart rate, blood oxygenation level, and blood pressure (BP) in this population.

METHODS

These data were measured at 4 time points: baseline, after 1 month on the low glutamate diet, and during each challenge week, where subjects were randomized into a double-blind, placebo-controlled, crossover challenge with MSG/placebo over 3 days each week. Pre-post diet changes were analyzed using paired t tests, change in the percentage of veterans meeting the criteria for hypertension was compared using chi-square or Fisher exact tests, and crossover challenge results were analyzed using general linear modeling in SAS® 9.4.

RESULTS

There was a significant reduction in systolic BP (sitting and recumbent; both P < .001) and diastolic BP (sitting; P = .02) after 1 month on the diet. The percentage meeting the criteria for hypertension was also significantly reduced (P < .05). Challenge with MSG/placebo did not demonstrate an acute effect of glutamate on blood pressure.

CONCLUSION

Overall, these findings suggest that the low glutamate diet may be an effective treatment for lowering blood pressure in veterans with Gulf War Illness. This dietary effect does not appear to be driven by reduced consumption of free glutamate, but rather, by an increase in consumption of non-processed foods.

The generation of a lactate-rich environment stimulates cell cycle progression and modulates gene expression on neonatal and hiPSC-derived cardiomyocytes

In situ tissue engineering strategies are a promising approach to activate the endogenous regenerative potential of the cardiac tissue helping the heart to heal itself after an injury. However, the current use of complex reprogramming vectors for the activation of reparative pathways challenges the easy translation of these therapies into the clinic. Here, we evaluated the response of mouse neonatal and human induced pluripotent stem cell-derived cardiomyocytes to the presence of exogenous lactate, thus mimicking the metabolic environment of the fetal heart. An increase in cardiomyocyte cell cycle activity was observed in the presence of lactate, as determined through Ki67 and Aurora-B kinase. Gene expression and RNA-sequencing data revealed that cardiomyocytes incubated with lactate showed upregulation of BMP10, LIN28 or TCIM in tandem with downregulation of GRIK1 or DGKK among others. Lactate also demonstrated a capability to modulate the production of inflammatory cytokines on cardiac fibroblasts, reducing the production of Fas, Fraktalkine or IL-12p40, while stimulating IL-13 and SDF1a. In addition, the generation of a lactate-rich environment improved ex vivo neonatal heart culture, by affecting the contractile activity and sarcomeric structures and inhibiting epicardial cell spreading. Our results also suggested a common link between the effect of lactate and the activation of hypoxia signaling pathways. These findings support a novel use of lactate in cardiac tissue engineering, modulating the metabolic environment of the heart and thus paving the way to the development of lactate-releasing platforms for in situ cardiac regeneration.

Memantine and its benefits for cancer, cardiovascular and neurological disorders

Memantine is a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist that was initially indicated for the treatment of moderate to severe Alzheimer's disease. It is now also considered for a variety of other pathologies in which activation of NMDA receptors apparently contributes to the pathogenesis and progression of disease. In addition to the central nervous system (CNS), NMDA receptors can be found in non-neuronal cells and tissues that recently have become an interesting research focus. Some studies have shown that glutamate signaling plays a role in cell transformation and cancer progression. In addition, these receptors may play a role in cardiovascular disorders. In this review, we focus on the most recent findings for memantine with respect to its pharmacological effects in a range of diseases, including inflammatory disorders, cardiovascular diseases, cancer, neuropathy, as well as retinopathy.

Transcriptomic Profiling of Canine Atrial Fibrillation Models After One Week of Sustained Arrhythmia

Supplemental Digital Content is available in the text.

Atrial fibrillation (AF), the most common sustained arrhythmia, is associated with increased morbidity, mortality, and health care costs. AF develops over many years and is often related to substantial atrial structural and electrophysiological remodeling. AF may lack symptoms at onset, and atrial biopsy samples are generally obtained in subjects with advanced disease, so it is difficult to study earlier stage pathophysiology in humans.

Gut microbe-derived metabolite trimethylamine N-oxide activates the cardiac autonomic nervous system and facilitates ischemia-induced ventricular arrhythmia via two different pathways

BACKGROUND

We previously demonstrated the gut microbes-derived metabolite trimethylamine N-oxide (TMAO) could activate the atrial autonomic ganglion plexus and promote atrial arrhythmia. The cardiac sympathetic nervous system (CSNS) play important roles in modulating ventricular arrhythmia (VA).

METHODS

Part 1: To test whether TMAO can directly activate the CSNS, we performed local injection of TMAO into the left stellate ganglion (LSG). Part 2: To test whether TMAO can indirectly activate the CSNS through the central nervous system, we performed intravenous injection of TMAO. Ventricular electrophysiology and LSG function and neural activity were measured before and after TMAO administration. Then, the left anterior descending coronary artery was ligated, and electrocardiograms were recorded for 1 h. At the end of the experiment, LSG and paraventricular nucleus (PVN) tissues were excised for molecular analyses.

FINDINGS

Compared with the control, both intravenous and local TMAO administration significantly increased LSG function and activity, shortened effective refractory period, and aggravated ischemia-induced VA. Proinflammatory markers and c-fos in the LSG were also significantly upregulated in both TMAO-treated groups. Particularly, c-fos expression in PVN was significantly increased in the systemic TMAO administration group but not the local TMAO administration group.

INTERPRETATION

The gut microbe-derived metabolite TMAO can activate the CSNS and aggravate ischemia-induced VA via the direct pathway through the LSG and the indirect pathway through central autonomic activation. FUND: This work was supported by the National Key R&D Program of China [2017YFC1307800], and the National Natural Science Foundation of China [81530011, 81770364, 81570463, 81871486, 81600395, 81600367 and 81700444].

Memantine, an NMDA Receptor Antagonist, Prevents Thyroxin-induced Hypertension, but Not Cardiac Remodeling

Stimulation of glutamatergic tone has been causally linked to myocardial pathogenesis and amplified systemic blood pressure (BP). Memantine, a noncompetitive N-methyl-D-aspartate glutamatergic receptor (NMDA-R) antagonist, has been proposed to be an active cardioprotective drug. However, the efficacy of memantine and subsequently the possible involvement of the NMDA-R in the thyroxin (T4)-induced cardiovascular complications have never been investigated. We examined the effect of memantine (30 mg·kg·d) on the T4 (500 μg·kg·d)-provoked increase in mouse BP, cardiac hypertrophy indicated by enlarged overall myocardial mass, and reformed reactions of the contractile myocardium both in vivo and ex vivo after 2 weeks of treatment. Memantine alone did not result in any cardiovascular pathology in mice. Instead, memantine significantly prevented the T4-triggered systemic hypertension. But, it did not reverse cardiac hypertrophy, coupled in vivo left ventricular dysfunction (LV) or ex vivo right ventricular (RV) papillary muscle contractile alterations of the T4-treated mice. Our results openly direct the cardiovascular safety and tolerability of memantine therapy. Yet, extra research is necessary to endorse these prospective advantageous outcomes. Also, we believe that this is the first study to inspect the possible role of NMDA-R in the T4-stimulated cardiovascular disorders and concluded that NMDA-R could play a key role in the T4-induced hypertension.

Synaptic Plasticity in Cardiac Innervation and Its Potential Role in Atrial Fibrillation

Synaptic plasticity is defined as the ability of synapses to change their strength of transmission. Plasticity of synaptic connections in the brain is a major focus of neuroscience research, as it is the primary mechanism underpinning learning and memory. Beyond the brain however, plasticity in peripheral neurons is less well understood, particularly in the neurons innervating the heart. The atria receive rich innervation from the autonomic branch of the peripheral nervous system. Sympathetic neurons are clustered in stellate and cervical ganglia alongside the spinal cord and extend fibers to the heart directly innervating the myocardium. These neurons are major drivers of hyperactive sympathetic activity observed in heart disease, ventricular arrhythmias, and sudden cardiac death. Both pre- and postsynaptic changes have been observed to occur at synapses formed by sympathetic ganglion neurons, suggesting that plasticity at sympathetic neuro-cardiac synapses is a major contributor to arrhythmias. Less is known about the plasticity in parasympathetic neurons located in clusters on the heart surface. These neuronal clusters, termed ganglionated plexi, or “little brains,” can independently modulate neural control of the heart and stimulation that enhances their excitability can induce arrhythmia such as atrial fibrillation. The ability of these neurons to alter parasympathetic activity suggests that plasticity may indeed occur at the synapses formed on and by ganglionated plexi neurons. Such changes may not only fine-tune autonomic innervation of the heart, but could also be a source of maladaptive plasticity during atrial fibrillation.

Can we spice up our Christmas dinner? : Busting the myth of the 'Chinese restaurant syndrome'

Background

Monosodium glutamate (MSG), also referred to as Vetsin or E621, is a flavour enhancer frequently used in Asian cuisine and abundantly present in the famous Chinese dish Peking duck. MSG is notorious for triggering the onset of the so-called ‘Chinese restaurant syndrome’ (CRS), a complex of unpleasant symptoms, which might include flushing, sweating and the onset of atrial fibrillation (AF). This study aims to determine the effects of MSG on the occurrence of AF.

Methods

We conducted a placebo self-controlled single-arm study in the Academic Medical Centre in Amsterdam. We included paroxysmal AF patients who reported a consistent onset of AF upon MSG intake. During three admissions, participants were subsequently administered: placebo, 1.5 g and 3 g MSG. If AF was recorded after the dose of 1.5 g MSG, patients were given another placebo instead of 3 g MSG. The primary outcome was the onset of AF registered by 24-hour Holter monitoring. The secondary outcomes were any other arrhythmia and the onset of CRS defined as two or more symptoms of CRS after MSG intake.

Results

Six men participated in the study. Both 1.5 g and 3 g MSG were unrelated to CRS, arrhythmias or AF occurrence.

Conclusion

Peking duck can be put on the Christmas menu without risking guests to be admitted to the emergency department with new episodes of AF.

Electronic supplementary material

The online version of this article (10.1007/s12471-017-1053-5) contains study inlcusion and exclusion criteria and recipe of Peking duck, which is available to authorized users.

High Concentrations of Tranexamic Acid Inhibit Ionotropic Glutamate Receptors

BACKGROUND

The antifibrinolytic drug tranexamic acid is structurally similar to the amino acid glycine and may cause seizures and myoclonus by acting as a competitive antagonist of glycine receptors. Glycine is an obligatory co-agonist of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. Thus, it is plausible that tranexamic acid inhibits NMDA receptors by acting as a competitive antagonist at the glycine binding site. The aim of this study was to determine whether tranexamic acid inhibits NMDA receptors, as well as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate subtypes of ionotropic glutamate receptors.

METHODS

Tranexamic acid modulation of NMDA, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainate receptors was studied using whole cell voltage-clamp recordings of current from cultured mouse hippocampal neurons.

RESULTS

Tranexamic acid rapidly and reversibly inhibited NMDA receptors (half maximal inhibitory concentration = 241 ± 45 mM, mean ± SD; 95% CI, 200 to 281; n = 5) and shifted the glycine concentration-response curve for NMDA-evoked current to the right. Tranexamic acid also inhibited α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (half maximal inhibitory concentration = 231 ± 91 mM; 95% CI, 148 to 314; n = 5 to 6) and kainate receptors (half maximal inhibitory concentration = 90 ± 24 mM; 95% CI, 68 to 112; n = 5).

CONCLUSIONS

Tranexamic acid inhibits NMDA receptors likely by reducing the binding of the co-agonist glycine and also inhibits α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and kainate receptors. Receptor blockade occurs at high millimolar concentrations of tranexamic acid, similar to the concentrations that occur after topical application to peripheral tissues. Glutamate receptors in tissues including bone, heart, and nerves play various physiologic roles, and tranexamic acid inhibition of these receptors may contribute to adverse drug effects.

Activation of N-methyl-d-aspartate receptors reduces heart rate variability and facilitates atrial fibrillation in rats

AIMS

The goal of this study was to assess the effects of N-methyl-d-aspartate (NMDA) receptors activation on heart rate variability (HRV) and susceptibility to atrial fibrillation (AF).

METHODS AND RESULTS

Rats were randomized for treatment with saline, NMDA (agonist of NMDA receptors), or NMDA plus MK-801 (antagonist of NMDA receptors) for 2 weeks. Heart rate variability was evaluated by using implantable electrocardiogram telemeters. Atrial fibrillation susceptibility was assessed with programmed stimulation in isolated hearts. Compared with the controls, the NMDA-treated rats displayed a decrease in the standard deviation of normal RR intervals, the standard deviation of the average RR intervals, the mean of the 5-min standard deviations of RR intervals, the root mean square of successive differences, and high frequency (HF); and an increase in low frequency (LF) and LF/HF (all P< 0.01). Additionally, the NMDA-treated rats showed prolonged activation latency and reduced effective refractory period (all P< 0.01). Importantly, AF was induced in all NMDA-treated rats. While atrial fibrosis developed, connexin40 downgraded and metalloproteinase 9 upgraded in the NMDA-treated rats (all P< 0.01). Most of the above alterations were mitigated by co-administering with MK-801.

CONCLUSION

These results indicate that NMDA receptors activation reduces HRV and enhances AF inducibility, with cardiac autonomic imbalance, atrial fibrosis, and degradation of gap junction protein identified as potential mechanistic contributors.

Hyperhomocysteinemia Alters Sinoatrial and Atrioventricular Nodal Function: Role of Magnesium in Attenuating These Effects

Patients with hyperhomocysteinemia (HHcy), or elevated plasma homocysteine (Hcy), are at higher risk of developing arrhythmias and sudden cardiac death; however, the mechanisms are unknown. In this study, the effects of HHcy on sinus node function, atrioventricular conduction, and ventricular vulnerability were investigated by electrophysiological (EP) analysis, and the role of magnesium (Mg(2+)), an endogenous N-methyl-D-aspartate (NMDA) receptor antagonist, in attenuating EP changes due to HHcy was explored. Wild-type mice (WT) and mice receiving Hcy in the drinking water for 12 weeks (DW) were subjected to electrocardiographic and EP studies. DW compared to WT had significantly shorter RR, PR, QT, and HV intervals, corrected sinus node recovery times (CSNRT), Wenckebach periodicity (WP), atrioventricular nodal effective refractory periods (AVNERP), and right ventricular effective refractory periods (RVERP). To examine the role of Mg(2+) in mitigating conduction changes in HHcy, WT, DW, and heterozygous cystathionine-β-synthase knockout mice (CBS (+/-) ) were subjected to repeat EP studies before and after administration of low-dose magnesium sulfate (20 mg/kg). Mg(2+) had no effect on EP variables in WT, but significantly slowed CSNRT, WP, and AVNERP in DW, as well as WP and AVNERP in CBS (+/-) . These findings suggest that ionic channels modulated by Mg(2+) may contribute to HHcy-induced conduction abnormalities.

Heart Alterations after Domoic Acid Administration in Rats

Domoic acid (DA) is one of the best known marine toxins, causative of important neurotoxic alterations. DA effects are documented both in wildlife and experimental assays, showing that this toxin causes severe injuries principally in the hippocampal area. In the present study we have addressed the long-term toxicological effects (30 days) of DA intraperitoneal administration in rats. Different histological techniques were employed in order to study DA toxicity in heart, an organ which has not been thoroughly studied after DA intoxication to date. The presence of DA was detected by immunohistochemical assays, and cellular alterations were observed both by optical and transmission electron microscopy. Although histological staining methods did not provide any observable tissue damage, transmission electron microscopy showed several injuries: a moderate lysis of myofibrils and loss of mitochondrial conformation. This is the first time the association between heart damage and the presence of the toxin has been observed.

Lymphocyte Subpopulations and Cytokine Levels in Experimental Hyperhomocysteinemia

Moderate exogenous hyperhomocysteinemia is associated with an increase in lymphocyte count (mainly at the expense of cytotoxic T cells), a 2-fold drop of the immunoregulatory index, and increase in TNF-α, IFN-γ, and IL-17α concentrations in the serum and in mononuclear cells. Homocysteine concentration in the mononuclears remains unchanged.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Gap junction modulation and its implications for heart function

Gap junction communication (GJC) mediated by connexins is critical for heart function. To gain insight into the causal relationship of molecular mechanisms of disease pathology, it is important to understand which mechanisms contribute to impairment of gap junctional communication. Here, we present an update on the known modulators of connexins, including various interaction partners, kinases, and signaling cascades. This gap junction network (GJN) can serve as a blueprint for data mining approaches exploring the growing number of publicly available data sets from experimental and clinical studies.

Ameliorative effect of α-tocopherol on monosodium glutamate-induced cardiac histological alterations and oxidative stress

BACKGROUND

Chronic oral intake of high doses of monosodium glutamate (MSG) causes oxidative stress. Oxidative stress plays an important role in the development of cardiac dysfunction and injury. Supplementation with α-tocopherol protects the body against oxidative stress and its related complications. This study was proposed to examine the protective effect of α-tocopherol against MSG-induced biochemical and histological alterations in blood and cardiac tissue of rats for a period of 180 days.

RESULTS

Chronic oral administration of MSG (4 g kg(-1)) caused oxidative stress that was manifested by significant increase (P < 0.05) in malondialdehyde, conjugated dienes and by the decrease in the activities of superoxide dismutase, catalase, reduced glutathione, glutathione peroxidase and glutathione S-transferase in cardiac tissue. The significantly increased (P < 0.05) activities of aspartate transaminase, creatine phosphokinase and lactate dehydrogenase in serum suggested a cardiac functional disorder. Moreover, heart muscle fibers showed cloudy swelling, fiber separation and vascular congestion. Administration of α-tocopherol (200 mg kg(-1)) significantly (P < 0.05) attenuated the MSG-induced biochemical alterations in serum and cardiac tissue. α-Tocopherol also prevented the pathological changes in cardiac tissue when compared with the MSG-treated group.

CONCLUSION

Our findings suggest that α-tocopherol may have a protective effect against MSG-induced cardiotoxicity, possibly through its antioxidant activity.

Ischemic cardiomyopathy following seizure induction by domoic Acid

Exposure to the excitotoxin domoic acid (DOM) has been shown to produce cardiac lesions in both clinical and animal studies. We have previously shown that DOM failed to directly affect cardiomyocyte viability and energetics, but the development of this cardiomyopathy has remained unexplained. The present study compared effects of high-level seizure induction obtained by intraperitoneal (2 mg/kg) or intrahippocampal (100 pmol) bolus administration of DOM on development of cardiac pathologies in a rat model. Assessment of cardiac pressure derivatives and coronary flow rates revealed a significant time-dependent decrease in combined left ventricular (LV) systolic and diastolic function at 1, 3, 7, and 14 days after intraperitoneal administration and at 7 and 14 days after intrahippocampal DOM administration. LV dysfunction was matched by a similar time-dependent decrease in mitochondrial respiratory control, associated with increased proton leakage, and in mitochondrial enzyme activities. Microscopic examination of the LV midplane revealed evidence of progressive multifocal ischemic damage within the subendocardial, septal, and papillary regions. Lesions ranged from reversible early damage (vacuolization) to hypercontracture and inflammatory necrosis progressing to fibrotic scarring. Plasma proinflammatory IL-1α, IL-1β, and TNF-α cytokine levels were also increased from 3 days after seizure induction. The observed cardiomyopathies did not differ between intraperitoneal and intrahippocampal groups, providing strong evidence that cardiac damage after DOM exposure is a consequence of a seizure-evoked autonomic response.

Exciting times beyond the brain: metabotropic glutamate receptors in peripheral and non-neural tissues

Metabotropic glutamate (mGlu) receptors are G-protein-coupled receptors expressed primarily on neurons and glial cells, where they are located in the proximity of the synaptic cleft. In the central nervous system (CNS), mGlu receptors modulate the effects of l-glutamate neurotransmission in addition to that of a variety of other neurotransmitters. However, mGlu receptors also have a widespread distribution outside the CNS that has been somewhat neglected to date. Based on this expression, diverse roles of mGlu receptors have been suggested in a variety of processes in health and disease including controlling hormone production in the adrenal gland and pancreas, regulating mineralization in the developing cartilage, modulating lymphocyte cytokine production, directing the state of differentiation in embryonic stem cells, and modulating gastrointestinal secretory function. Understanding the role of mGlu receptors in the periphery will also provide a better insight into potential side effects of drugs currently being developed for neurological and psychiatric conditions. This review summarizes the new potential roles of mGlu receptors and raises the possibility of novel pharmacological targets for various disorders.

Functional NMDA receptors in rat erythrocytes

N-methyl-d-aspartate (NMDA) receptors are ligand-gated nonselective cation channels mediating fast neuronal transmission and long-term potentiation in the central nervous system. These channels have a 10-fold higher permeability for Ca2+compared with Na+or K+and binding of the agonists (glutamate, homocysteine, homocysteic acid, NMDA) triggers Ca2+uptake. The present study demonstrates the presence of NMDA receptors in rat erythrocytes. The receptors are most abundant in both erythroid precursor cells and immature red blood cells, reticulocytes. Treatment of erythrocytes with NMDA receptor agonists leads to a rapid increase in intracellular Ca2+resulting in a transient shrinkage via Gardos channel activation. Additionally, the exposure of erythrocytes to NMDA receptor agonists causes activation of the nitric oxide (NO) synthase facilitating either NO production in l-arginine-containing medium or superoxide anion (O2·−) generation in the absence of l-arginine. Conversely, treatment with an NMDA receptor antagonist MK-80, or the removal of Ca2+from the incubation medium causes suppression of Ca2+accumulation and prevents attendant changes in cell volume and NO/O2·−production. These results suggest that the NMDA receptor activity in circulating erythrocytes is regulated by the plasma concentrations of homocysteine and homocysteic acid. Moreover, receptor hyperactivation may contribute to an increased incidence of thrombosis during hyperhomocysteinemia.

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

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

Characterization of a degenerative cardiomyopathy associated with domoic acid toxicity in California sea lions (Zalophus californianus)

Domoic acid, produced by marine algae, can cause acute and chronic neurologic sequela in California sea lions (Zalophus californianus) from acute toxicity or sublethal exposure. Eight sea lions, representing acute and chronic cases, both sexes, and all age classes, were selected to demonstrate a concurrent degenerative cardiomyopathy. Critical aspects of characterizing the cardiomyopathy by lesion distribution and morphology were the development of a heart dissection and tissue-trimming protocol and the delineation of the cardiac conducting system by histomorphology and immunohistochemistry for neuron-specific protein gene product 9.5. Histopathologic features and progression of the cardiomyopathy are described, varying from acute to chronic active and mild to severe. The cardiomyopathy is distinguished from other heart lesions in pinnipeds. Based on histopathologic features, immunopositive staining for cleaved caspase-3, and comparison with known, similar-appearing cardiomyopathies, the proposed pathogenesis for the degenerative cardiomyopathy is the primary or at least initial direct interaction of domoic acid with receptors that are suspected to exist in the heart. l-Carnitine, measured in the heart and skeletal muscle, and troponin-I, measured in serum collected at the time of death from additional animals (n = 58), were not predictive of the domoic acid-associated cardiomyopathy. This degenerative cardiomyopathy in California sea lions represents another syndrome beyond central neurologic disease associated with exposure to domoic acid and may contribute to morbidity and mortality.

Activation of iGluR5 kainate receptors inhibits neurogenic dural vasodilatation in an animal model of trigeminovascular activation

BACKGROUND AND PURPOSE

Migraine is a disabling neurological disorder involving activation, or the perception of activation, of trigeminovascular afferents containing calcitonin gene-related peptide (CGRP). Released CGRP from peripheral trigeminal afferents causes dilatation of dural blood vessel, and this is used to measure trigeminal nerve activation. Kainate receptors with the GluR5 subunit (iGluR5, ionotropic glutamate receptor) are present in the trigeminal ganglion and may be involved in nociception. We investigated the possible involvement of prejunctional iGluR5 kainate receptors on CGRP release from trigeminal afferents.

EXPERIMENTAL APPROACH

We used neurogenic dural vasodilatation, which involves reproducible vasodilatation in response to CGRP release after electrical stimulation of the dura mater surrounding the middle meningeal artery. The effects of the specific iGluR5 receptor antagonist UBP 302 and agonist (S)-(-)-5-iodowillardiine were investigated on neurogenic and CGRP-induced dural vasodilatation in rats, by using intravital microscopy.

KEY RESULTS

Administration of 10 and 20 mg.kg(-1) of iodowillardiine inhibited electrically induced dural vessel dilatation, an effect blocked by pretreatment with 50 mg.kg(-1) UBP 302. Administration of the iGluR5 receptor antagonist UBP 302 alone had no significant effect. CGRP (1 mg.kg(-1))-induced dural vasodilatation was not inhibited by the iGluR5 receptor agonist iodowillardiine.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates that activation of the iGluR5 kainate receptors with the selective agonist iodowillardiine is able to inhibit neurogenic dural vasodilatation probably by inhibition of prejunctional release of CGRP from trigeminal afferents. Taken together with recent clinical studies the data reinforce CGRP mechanisms in primary headaches and demonstrate a novel role for kainate receptor modulation of trigeminovascular activation.

Identification of potential pharmacological targets by analysis of the comprehensive G protein-coupled receptor repertoire in the four cardiac chambers

Cardiac function is regulated by many hormones and neurotransmitters that exert their physiological effects through the activation of G protein-coupled receptors (GPCRs). Identification of new GPCRs that might display a specific pattern of expression within the heart and differentially regulate specific cardiac functions represents an important issue for the development of new drugs. Indeed, highly targeted receptors represent only a small percentage of known GPCRs. Here, we quantified the expression of 395 endoGPCRs (all GPCRs excluding taste and odorant receptors) in male mouse right and left atria and ventricles by using high-throughput real-time reverse-transcriptase polymerase chain reaction (RT-PCR) and focused on the 135 most highly expressed transcripts. No cardiac functional data are available for almost half of these receptors; therefore, linking GPCR expression patterns to cardiac function has allowed us to provide new insights into the possible function of some of these receptors. Indeed, ventricles and atria are both contractile; however, the latter, and especially the right atrium, are central to the generation and regulation of cardiac rhythm. Accordingly, the right atrium exhibited the most specific signature, whereas the majority of GPCRs found in ventricles were evenly expressed in both the right and left chambers. RT-PCR data were confirmed at the protein level for six selected transcripts. Furthermore, we provide new data showing that, as suggested by our repertoire, the metabotropic glutamate receptor 1b is expressed and is functional in ventricular cardiac myocytes. This is the first report describing GPCRs in the four cardiac chambers and may assist in the identification of therapeutic targets.

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.

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.

Neuroexcitatory targets in the female reproductive system of the nonhuman primate (Macaca fascicularis)

Glutamate receptors (GluRs) have been implicated in brain function and pathology. Their presence in peripheral tissues suggests a vital role in the pathophysiology of various organ systems. In earlier studies, the authors reported the differential distribution of ionotropic and metabotropic GluRs in neural and nonneural peripheral tissues of the rat. In this study, they investigated the presence and the localization of the GluRs in the reproductive organs of Macaca fascicularis. The data illustrate the presence of the GluR 2/3, metabotropic glutamate receptor 2/3, kainate 2, and N-methyl-D-aspartate receptor 1 (NMDAR 1). These are localized in the different structures of the ovaries, uterine cervix, myometrium, endometrium, and inflammatory cells. Smooth muscle of the myometrium and arterioles showed strong immunolabeling with anti-GluR 2/3 and, to a lesser intensity, with the other ionotropic glutamate receptor antibodies. NMDAR 1 showed the most widespread staining in all the structures. Mast cells showed strong immunolabeling with the anti-NMDA antibody. The demonstration and the differential expression of GluRs in the female reproductive system of nonhuman primate experimental models provide first evidence suggesting excitatory signaling in these tissues.