Neurologic sequelae of domoic acid intoxication due to the ingestion of contaminated mussels

Glutamate as a neurotransmitter in the brain: review of physiology and pathology

Glutamate is the principal excitatory neurotransmitter in brain. Our knowledge of the glutamatergic synapse has advanced enormously in the last 10 years, primarily through application of molecular biological techniques to the study of glutamate receptors and transporters. There are three families of ionotropic receptors with intrinsic cation permeable channels [N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate]. There are three groups of metabotropic, G protein-coupled glutamate receptors (mGluR) that modify neuronal and glial excitability through G protein subunits acting on membrane ion channels and second messengers such as diacylglycerol and cAMP. There are also two glial glutamate transporters and three neuronal transporters in the brain. Glutamate is the most abundant amino acid in the diet. There is no evidence for brain damage in humans resulting from dietary glutamate. A kainate analog, domoate, is sometimes ingested accidentally in blue mussels; this potent toxin causes limbic seizures, which can lead to hippocampal and related pathology and amnesia. Endogenous glutamate, by activating NMDA, AMPA or mGluR1 receptors, may contribute to the brain damage occurring acutely after status epilepticus, cerebral ischemia or traumatic brain injury. It may also contribute to chronic neurodegeneration in such disorders as amyotrophic lateral sclerosis and Huntington's chorea. In animal models of cerebral ischemia and traumatic brain injury, NMDA and AMPA receptor antagonists protect against acute brain damage and delayed behavioral deficits. Such compounds are undergoing testing in humans, but therapeutic efficacy has yet to be established. Other clinical conditions that may respond to drugs acting on glutamatergic transmission include epilepsy, amnesia, anxiety, hyperalgesia and psychosis.

Age-dependent increase in infarct volume following photochemically induced cerebral infarction: putative role of astroglia

This study demonstrates that the photochemically induced model of stroke is an extremely viable method of inducing cerebral infarction in old animals. The lesions are reproducible both in terms of location and size and compatible with long-term survival of the animal. With this model we demonstrated, one week following surgery, a significantly larger infarct in rats 20 and 24 months of age compared to 4-month-old rats. The older rats also sustained greater neurologic deficits as assessed on a rotarod task. Older rats also were characterized by a glial response that was far less intense than in young animals. While the precise relationship between glia activation and cerebral damage remains to be determined, it would appear that a better understanding of those factors that contribute to the astrocytic response in the aged rat may be of particular benefit in designing therapeutic strategies aimed at reducing the pathologic consequences of cerebral infarction in elderly humans.

AMPA exposures induce mitochondrial Ca(2+) overload and ROS generation in spinal motor neurons in vitro

The reason for the selective vulnerability of motor neurons in amyotrophic lateral sclerosis (ALS) is primarily unknown. A possible factor is the expression by motor neurons of Ca(2+)-permeable AMPA/kainate channels, which may permit rapid Ca(2+) influx in response to synaptic receptor activation. However, other subpopulations of central neurons, most notably forebrain GABAergic interneurons, consistently express large numbers of these channels but do not degenerate in ALS. Indeed, when subjected to identical excitotoxic exposures, motor neurons were more susceptible than GABAergic neurons to AMPA/kainate receptor-mediated neurotoxicity. Microfluorimetric studies were performed to examine the basis for the difference in vulnerability. First, AMPA or kainate exposures appeared to trigger substantial mitochondrial Ca(2+) loading in motor neurons, as indicated by a sharp increase in intracellular Ca(2+) after addition of the mitochondrial uncoupler carbonyl cyanide p-(trifluoromethoxy)phenyl hydrazone (FCCP) after the agonist exposure. The same exposures caused little mitochondrial Ca(2+) accumulation in GABAergic cortical neurons. Subsequent experiments examined other measures of mitochondrial function to compare sequelae of AMPA/kainate receptor activation between these populations. Brief exposure to either AMPA or kainate caused mitochondrial depolarization, assessed using tetramethylrhodamine ethylester, and reactive oxygen species (ROS) generation, assessed using hydroethidine, in motor neurons. However, these effects were only seen in the GABAergic neurons after exposure to the nondesensitizing AMPA receptor agonist kainate. Finally, addition of either antioxidants or toxins (FCCP or CN(-)) that block mitochondrial Ca(2+) uptake attenuated AMPA/kainate receptor-mediated motor neuron injury, suggesting that the mitochondrial Ca(2+) uptake and consequent ROS generation are central to the injury process.

Plasticity in the enteric nervous system

Enteric ganglia can maintain integrated functions, such as the peristaltic reflex, in the absence of input from the central nervous system, which has a modulatory role. Several clinical and experimental observations suggest that homeostatic control of gut function in a changing environment may be achieved through adaptive changes occurring in the enteric ganglia. A distinctive feature of enteric ganglia, which may be crucial during the development of adaptive responses, is the vicinity of the final effector cells, which are an important source of mediators regulating cell growth. The aim of this review is to focus on the possible mechanisms underlying neuronal plasticity in the enteric nervous system and to consider approaches to the study of plasticity in this model. These include investigations of neuronal connectivity during development, adaptive mechanisms that maintain function after suppression of a specific neural input, and the possible occurrence of activity-dependent modifications of synaptic efficacy, which are thought to be important in storage of information in the brain. One of the applied aspects of the study of plasticity in the enteric nervous system is that knowledge of the underlying mechanisms may eventually enable us to develop strategies to correct neuronal alterations described in several diseases.

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.

Preparation and characterization of domoic acid-protein conjugates using small amount of toxin in a reversed micellar medium: application in a competitive enzyme-linked immunosorbent assay

With the aim of producing novel antibodies to domoic acid (DA), an original, rapid, and simple procedure for preparing minute amount of hapten-protein conjugates was developed. The amide-bond-generating mixed anhydride method of Erlanger was performed using 0.32-0.64 micromol of DA in a reversed micellar medium allowing strong carrier haptenization as determined by spectrophotometric measurement. Bovine serum albumin (BSA) and ovalbumin (OVA) conjugates were, respectively, used for immunization of BALB/c mice and antibody screening by enzyme-linked immunosorbent assay (ELISA). Specific polyclonal antibodies were produced upon multiple injections of (DA)(17)-BSA conjugate administered by three different routes: (i) intraperitoneal (i.p.), (ii) intraperitoneal + subcutaneous (i.p. + s.c.), (iii) footpad (f.p.). The i.p. route induced antisera of higher titer (1:350000) than did the other protocols (approximately 1:72900) and was selected throughout further experiments. Using a competitive ELISA format with a peroxidase immunoconjugate and a chromogenic substrate, no significant cross-reactivity was observed with glutamic acid, aspartic acid and kainic acid (KA), a structural analogue of DA. The sensitivity of this assay could be enhanced by 1 order of magnitude by using a beta-galactosidase immunoconjugate with a fluorogenic substrate while preserving DA specificity. The calculated dissociation constant (K(D)) for the interaction of the antibodies with free DA was 5 x 10(-)(7) M (chromogenic assay) and 5 x 10(-)(8) M (fluorogenic assay). Using the optimized assay the limit of detection (LOD) and the limit of quantitation (LOQ) in the ELISA buffer were 1.4 and 3 ng/mL, respectively. Moreover this assay was found applicable for measuring DA levels in spiked mussel extracts pre-cleaned through a solid-phase extraction column, as a very good correlation (r(2) = 0.96) was observed between the actual amounts of DA added and amounts detected by ELISA. Thus, accurate determinations of DA in clean extracts could be achieved between 2 and 180 ng/mL in spiked samples which corresponds to 0.02-1.8 microg/g of original mussel tissue. Owing to the regulation limits of 20 microg DA/g of shellfish tissue, these extraction and assay procedures should provide a useful complement to the standard HPLC analytical technique currently employed in monitoring DA in shellfish tissue.

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.

Glutamatergic receptors regulate expression, phosphorylation and accumulation of neurofilaments in spinal cord neurons

Glutamatergic regulation of neurofilament expression, phosphorylation and accumulation in cultured spinal cord neurons was studied. At seven days in culture, 0.15% of the neurons were immunoreactive for non-phosphorylated neurofilaments, but essentially no cells immunoreactive for phosphorylated neurofilaments were seen. The number and size of the immunoreactive cells in culture corresponded well to those of rat and human spinal cord neurons in vivo. In spinal cord cultures, sublethal, long-lasting stimulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate or metabotrophic receptors, but not N-methyl-D-aspartate receptors, dose-dependently increased the number of non-phosphorylated neurofilament-immunoreactive cells, which was blocked by nifedipine, an antagonist of voltage-sensitive Ca2+ channels. Stimulation of kainate or all non-N-methyl-D-aspartate receptors decreased the expression of medium-molecular-weight neurofilament messenger RNA. Blockade of AMPA/kainate receptors, but not of N-methyl-D-aspartate receptors, increased the amount of phosphorylated neurofilament protein and the number of phosphorylated neurofilament-immunoreactive cell bodies. The phosphorylated neurofilament-immunoreactive cell population was different from the non-phosphorylated neurofilament-immunoreactive neurons, which lost their axonal non-phosphorylated neurofilament immunoreactivity but showed intense cytoplasmic labeling in response to the blockade of AMPA/ kainate receptors. Immunoreactivity for phosphoserine did not change upon glutamate receptor stimulation and blockade. The results show that activation of AMPA/kainate receptors decreases the expression of neurofilament messenger RNA and neurofilament phosphorylation in spinal cord neurons by a mechanism involving active voltage-sensitive Ca2+ channels. Blockade of these receptors seems to disturb axonal neurofilament transport. Because AMPA/kainate receptors mediate chronic glutamatergic death of spinal motor neurons and these receptors have been suggested to be involved in the pathogenesis of amyotrophic lateral sclerosis, the observed alteration in neurofilament phosphorylation and distribution may contribute to the pathogenesis of chronic motor neuron diseases.

Selective neurotoxins, chemical tools to probe the mind: the first thirty years and beyond

For centuries, starting with the advent of the microscope, cytotoxins have been known to non-selectively destroy nerves and other tissue cells. However, neurotoxins restricted in effect to one kind of neuron are an invention of the 20th century. One might reasonably trace the origins of this field to 1960 when the Nobel Laureates, R. Levi- Montalcini and S Cohen, showed that an antibody to nerve growth factor effectively prevented development of sympathetic nerves in the absence of overt changes in dorsal root ganglia and other neural and non-neural tissues. The year 1967 marks discovery of 6-hydroxydopamine, the first of dozens of chemically-selective neurotoxins. As stated by the physiologist W.B. Cannon, neural function can be deduced by denoting absence-deficits. A wealth of knowledge in neuroscience has been realized through use of neurotoxins. In the 21st century we foresee neurotoxins for virtually all neurochemically-identifiable or receptor-specific neurons, acting at/via functional proteins or characteristic DNA sites. These tools will provide us with a better means to probe the mind and thereby lead to a fuller understanding of the intricate roles of identifiable neuronal systems in integrative neuroscience.

Food toxins, ampa receptors, and motor neuron diseases

Environmental chemicals involved in the etiology of human neurodegenerative disorders are challenging to identify. Described here is research designed to determine the etiology and molecular pathogenesis of nerve cell degeneration in two little known corticomotoneuronal diseases with established environmental triggers. Both conditions are toxic-nutritional disorders dominated by persistent spastic weakness of the legs and degeneration of corresponding corticospinal pathways. Lathyrism, a disease caused by dietary dependence on grass pea (Lathyrus sativus), is mediated by a stereospecific plant amino acid (beta-N-oxalylamino-L-alanine) that serves as a potent agonist at the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) subclass of neuronal glutamate receptors. A neurologically similar disorder, konzo ("tied legs"), is found among protein-poor African communities that rely for food on cyanogen-containing cassava roots. Thiocyanate, the principal metabolite of cyanide, is an attractive etiologic candidate for konzo because it selectively promotes the action of glutamate at AMPA receptors. Studies are urgently needed to assess the health effects of cassava and other cyanogenic plants, components of which are widely used as food.

Status epilepticus-induced neuronal injury and network reorganization

It is now evident that prolonged febrile seizures in childhood, or an episode of status epilepticus at any age, can produce the highly characteristic pattern of hippocampal cell loss and shrinkage that is seen later in life, when patients develop temporal lobe epilepsy. Seizure-induced and presumably excitotoxic pathology includes neuronal loss, reactive gliosis, aberrant synaptic reorganization of surviving cells, and hippocampal tissue shrinkage that may alter extracellular space and affect ionic homeostasis. Whether any of these pathological effects of prolonged excitation play a causal role in the epileptogenic process that ultimately leads to spontaneous afebrile seizures remains a subject of intense interest. Two hypotheses have been suggested to explain how seizure-induced neuronal loss might initiate the epileptogenic process. One hypothesis suggests that normal inhibition and excitability is maintained by vulnerable non-principal cells, and that their loss deactivates inhibitory neurons, rendering principal cells disinhibited and hyperexcitable. The other hypothesis regards the initial loss as a stimulus for normally unconnected principal cells to form aberrant recurrent excitatory connections. Additional influences undoubtedly include a "kindling" process that gradually overcomes polysynaptic inhibition, and changes in extracellular space that may facilitate synaptic and ephaptic depolarization. Identification of the suspected substrates of epileptogenesis will serve as a stimulus for future progress and provide direction for new experimental designs.

Radiofrequency current density imaging of kainate-evoked depolarization

The purpose of this study was to examine whether radiofrequency current density imaging (RF-CDI) can quantitatively monitor depolarizations evoked by excitatory amino acids in a rat's brain. To evoke depolarization, a glutamate receptor agonist, kainate, was administered into the right lateral ventricle. First, electroencephalographic activity was recorded in a basal condition and after the application of kainate. Complex behavioral patterns were observed. Second, impedance measurements were performed to assess the change in conductivity of the brain due to kainate at the Larmor frequency of the imager. Calculated changes were about 17%. Third, a set of current density images was obtained with RF-CDI before and after the administration of kainate. Kainate-induced excitatory changes were observed on current density images as brighter regions, mainly in the hippocampal area compared with the same area in the basal condition.

Working memory deficits induced by single but not repeated exposures to domoic acid

Single injections of domoic acid, given either intraperitoneally to mice or directly into the hippocampal formation of rats, have been shown to impair learning on the place version of the Morris water maze task and the eight arm radial maze task. The present study was designed to test whether both single and repeated exposures of intraperitoneally administered domoic acid (1.0 or 2.0 mg/kg) impair spatial working memory in mice on a delayed matching-to-sample task. DBA strain mice were given a series of four injections over a 7-day period consisting of either saline or one of two doses of domoic acid. During the 18 days of testing, each subject was given one trial per day consisting of one information run, followed by three test runs. On non-alternation days (days in which the correct response was the same as the preceding day) the saline injected group significantly outperformed the single injection 2.0 mg/kg domoic acid group. This indicates that domoic acid-treated animals were incapable of forming a memory that persisted for 24 h and hence were less able to utilize the prior day's experience. However, the repeated exposure groups did not perform as poorly on non-alternation days than the single exposure groups, indicating that domoic acid may affect multiple mechanisms involved in memory consolidation.

Domoic Acid Attenuates the Adenosine-5'-Triphosphate-Induced Increase in

BACKGROUND: Although domoic acid (DA), a shellfish neurotoxin, carries a negative surface charge at physiological pH like that of adenosine-5'-triphosphate (ATP), very little is known about its cellular effects. In view of the potentially significant role of extracellular ATP as a signaling molecule for increasing the intracellular concentration of Ca(2+) ([Ca(2+)](i)), we examined the possibility that DA may interfere with this signal transduction mechanism in the myocardium. METHODS AND RESULTS: Cardiomyocytes were isolated from rat heart and loaded with Fura-2 to measure the [Ca(2+)](i). ATP produced a gradual rise in [Ca(2+)](i), reaching a peak level in 25-30 seconds and declining thereafter. DA did not affect the [Ca(2+)](i) in cardiomyocytes; however, it diminished the ATP-induced elevation in [Ca(2+)](i) in the concentration-dependent manner. Kainic acid, an analogue of DA, had a similar effect but at a 25-fold higher concentration, whereas glutamate and aspartate did not modify the action of ATP. Well-known inhibitors of L-type voltage-sensitive Ca(2+) channels, nifedipine and nicardipine, depressed the ATP-induced increase in [Ca(2+)](i), but DA did not produce additive effects with either of these agents. On the other hand, DA potentiated the KCl-induced increase in [Ca(2+)](i) in quiescent cardiomyocytes and augmented the nicardipine-sensitive Ca(2+) transients in electrically stimulated cardiomyocytes. CONCLUSIONS: These results suggest that DA may diminish the ATP-induced increase in [Ca(2+)](i) by inhibiting the ATP interaction with cardiomyocytes in a specific manner.

Biological markers in the diagnosis and treatment of ALS

The care of patients with amyotrophic lateral sclerosis (ALS), which has classically focused on treatment of symptomatology, has now entered an encouraging new era of therapy targeted at the pathophysiology of the disease. However, an objective measure of disease progression and therapeutic response is sorely needed. Quantitative neuromuscular examinations, measurement of pulmonary function, disability scales, and even survival, are limited by variability due to a number of poorly controlled factors. Quantitative electromyography, positron emission tomography scanning, and magnetic cortical stimulation, provide potential objective indicators of disease progression, but require a large number of patients and a long observation period for adequate statistical power. We have examined the role of magnetic resonance spectroscopic imaging in detecting acute changes in motor cortical metabolism in response to riluzole therapy. N-acetylaspartate (NAA), the most prominent signal in proton spectra of normal brain, is a neuron-specific molecule. ALS patients were found to experience a significant increase in the NAA/creatine ratio within 3 weeks of initiation of riluzole therapy. As glutamate can trigger the generation of reactive oxygen species in neurons, we speculate that acute changes in NAA levels may reflect oxidative injury to mitochondria where NAA is synthesised. The advent of a useful test for upper motor neuron metabolic compromise may provide an objective, non-invasive, short duration measure with which to screen the efficacy of potential therapeutic agents for ALS.

Status epilepticus: new concepts

Generalized convulsive status epilepticus is a neurological emergency characterized by abnormally prolonged seizures. This review emphasizes recent developments that bear on our understanding of the pathophysiology and management of status epilepticus. Topics include GABAA receptor modulation during prolonged seizures, the role of genetics in susceptibility to status epilepticus, neuron-specific enolas, the Veterans Administration Cooperative Study Group trial comparison of various drug regimens, utility of the electroencephalogram in patient monitoring, emerging drug therapies and patient management in out-of-hospital settings.

The role of excitotoxicity in neurodegenerative disease: implications for therapy

Glutamic acid is the principal excitatory neurotransmitter in the mammalian central nervous system. Glutamic acid binds to a variety of excitatory amino acid receptors, which are ligand-gated ion channels. It is activation of these receptors that leads to depolarisation and neuronal excitation. In normal synaptic functioning, activation of excitatory amino acid receptors is transitory. However, if, for any reason, receptor activation becomes excessive or prolonged, the target neurones become damaged and eventually die. This process of neuronal death is called excitotoxicity and appears to involve sustained elevations of intracellular calcium levels. Impairment of neuronal energy metabolism may sensitise neurones to excitotoxic cell death. The principle of excitotoxicity has been well-established experimentally, both in in vitro systems and in vivo, following administration of excitatory amino acids into the nervous system. A role for excitotoxicity in the aetiology or progression of several human neurodegenerative diseases has been proposed, which has stimulated much research recently. This has led to the hope that compounds that interfere with glutamatergic neurotransmission may be of clinical benefit in treating such diseases. However, except in the case of a few very rare conditions, direct evidence for a pathogenic role for excitotoxicity in neurological disease is missing. Much attention has been directed at obtaining evidence for a role for excitotoxicity in the neurological sequelae of stroke, and there now seems to be little doubt that such a process is indeed a determining factor in the extent of the lesions observed. Several clinical trials have evaluated the potential of antiglutamate drugs to improve outcome following acute ischaemic stroke, but to date, the results of these have been disappointing. In amyotrophic lateral sclerosis, neurolathyrism, and human immunodeficiency virus dementia complex, several lines of circumstantial evidence suggest that excitotoxicity may contribute to the pathogenic process. An antiglutamate drug, riluzole, recently has been shown to provide some therapeutic benefit in the treatment of amyotrophic lateral sclerosis. Parkinson's disease and Huntington's disease are examples of neurodegenerative diseases where mitochondrial dysfunction may sensitise specific populations of neurones to excitotoxicity from synaptic glutamic acid. The first clinical trials aimed at providing neuroprotection with antiglutamate drugs are currently in progress for these two diseases.

Biologically-based dose-response model for neurotoxicity risk assessment

Domoic acid is a tricarboxylic amino acid that is structurally-related to kainic acid and glutamic acid. It is produced by phytoplankton that may contaminate seafood. To determine domoate's toxicological effects and their pathogenesis, cynomolgus monkeys were dosed intravenously at one of a range of bolus doses from 0.25 to 4.0 mg/kg. Histochemical staining, using silver methods, revealed degenerating axons and cell bodies. Doses in the range of 0.5-1.0 mg/kg produced a small area of silver grains restricted to axons of the hippocampal CA2 stratum lucidum, the most sensitive brain area identified. Quantitation of the abundance of these silver grains yielded continuous dose-response data. A four step quantitative risk estimation approach was used: (1) determination of a dose-response model; (2) determination of the distribution of measurements (variability) about the model; (3) determination of an adverse or abnormal level with the use of the control data; and (4) estimation of the probability that a measure is beyond the abnormal level as a function of dose. The currently used safety-factor (S-F) approach, the benchmark (BM) approach and this quantitative (Q) approach was used to assess the same data set. Assuming a 5% oral absorption of domoic acid, acceptable doses would be achieved if subjects ate 200 g of seafood containing 12, 6 and 10 ppm domoic acid for the S-F, BM and Q approaches, respectively. This quantitative approach uses all the available data, takes into account the variability of the data and provides an actual risk at a given dose of domoic acid.

Selective reduction of GluR2 protein in adult hippocampal CA3 neurons following status epilepticus but prior to cell loss

Kainic acid (KA) induces status epilepticus and delayed neurodegeneration of CA3 hippocampal neurons. Downregulation of glutamate receptor 2 (GluR2) subunit mRNA [the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) subunit that limits Ca2+ permeability] is thought to a play role in this neurodegeneration, possibly by increased formation of Ca2+ permeable AMPA receptors. The present study examined early hippocampal decreases in GluR2 mRNA and protein following kainate-induced status epilepticus and correlated expression changes with the appearance of dead or dying cells by several histological procedures. At 12 h, in situ hybridization followed by emulsion dipping showed nonuniform decreases in GluR2 mRNA hybridization grains overlying morphologically healthy-appearing CA3 neurons. GluR1 and N-methyl-D-aspartate receptor mRNAs were unchanged. At 12-16 h, when little argyrophilia or cells with some features of apoptosis were detected by silver impregnation or electron microscopy, single immunohistochemistry with GluR2 and GluR2/3 subunit-specific antibodies demonstrated a pattern of decreased GluR2 receptor protein within CA3 neurons that appeared to predict a pattern of damage, similar to the mRNA observations. Double immunolabeling showed that GluR2 immunofluorescence was depleted and that GluR1 immunofluorescence was sustained in clusters of the same CA3 neurons. Quantitation of Western blots showed increased GluR1:GluR2 ratios in CA3 but not in CA1 or dentate gyrus subfields. Findings indicate that the GluR1:GluR2 protein ratio is increased in a population of CA3 neurons prior to significant cell loss. Data are consistent with the "GluR2 hypothesis" that reduced expression of GluR2 subunits will increase formation of AMPA receptors permeable to Ca2+ and predict vulnerability to a particular subset of pyramidal neurons following status epilepticus.

Diagnosis of amyotrophic lateral sclerosis

This review of the differential diagnosis of amyotrophic lateral sclerosis focuses on two themes. The first is practical, how to establish the diagnosis based primarily on clinical findings buttressed by electrodiagnosis. The main considerations are multifocal motor neuropathy and cervical spondylotic myelopathy. The second theme is the relationship of motor neuron disease to other conditions, including benign fasciculation (Denny-Brown, Foley syndrome), paraneoplastic syndromes, lymphoproliferative disease, radiation damage, monomelic amyotrophy (Hirayama syndrome), as well as an association with parkinsonism, dementia and multisystem disorders of the central nervous system.

Learning and memory difficulties after environmental exposure to waterways containing toxin-producing Pfiesteria or Pfiesteria-like dinoflagellates

BACKGROUND

At the beginning of autumn, 1996, fish with "punched-out" skin lesions and erratic behaviour associated with exposure to toxins produced by Pfiesteria piscicida or Pfiesteria-like dinoflagellate species were seen in the Pocomoke River and adjacent waterways on the eastern shore of the Chesapeake Bay in Maryland, USA. In August, 1997, fish kills associated with Pfiesteria occurred in these same areas. People who had had contact with affected waterways reported symptoms, including memory difficulties, which raises questions about the human-health impact of environmental exposure to Pfiesteria toxins.

METHODS

We assessed 24 people who had been exposed. We collected data on exposure history and symptoms, did a complete medical and laboratory assessment (13 people), and carried out a neuropsychological screening battery. Performance on neuropsychological measures was compared with a matched control group.

RESULTS

People with high exposure were significantly more likely than occupationally matched controls to complain of neuropsychological symptoms (including new or increased forgetfulness); headache; and skin lesions or a burning sensation of skin on contact with water. No consistent physical or laboratory abnormalities were found. However, exposed people had significantly reduced scores on the Rey Auditory Verbal Learning and Stroop Color-Word tests (indicative of difficulties with learning and higher cognitive function), and the Grooved Pegboard task. There was a dose-response effect with the lowest scores among people with the highest exposure. By 3-6 months after cessation of exposure, all those assessed had test scores that had returned to within normal ranges.

INTERPRETATION

People with environmental exposure to waterways in which Pfiesteria toxins are present are at risk of developing a reversible clinical syndrome characterised by difficulties with learning and higher cognitive functions. Risk of illness is directly related to degree of exposure, with the most prominent symptoms and signs occurring among people with chronic daily exposure to affected waterways.

The role of glutamate in dementia

Glutamate is an excitatory neurotransmitter, but may also act as an endogenous neurotoxin. There is good evidence for an involvement of the glutamatergic system in the pathophysiology of dementia. The glutamatergic transmission machinery is quite complex and provides a gallery of possible drug targets. There are good arguments both for an agonist and an antagonist strategy. When following the antagonist strategy, the goal is to provide neuroprotective effects via glutamate receptor antagonisms without inhibiting the physiological transmission that is required for learning and memory formation. When following the agonist strategy, the goal is to activate glutamatergic transmission without neurotoxic side effects. Several available antidementia drugs may modulate the glutamatergic transmission.

Molecular and immunochemical characterization of the ionotropic glutamate receptors in the rat heart

Excitatory amino acids (EAA) and glutamate receptors (GluRs) play a fundamental role in the central nervous system (CNS). Ionotropic glutamate receptors (iGluRs) are coupled to ion channels, which are classified according to their most selective agonists. These ligand-gated channels are permeable to Na+, K+, and Ca+. Interaction of EAA receptor is linked to Ca+2/Na+ influx. Influx changes lead to an action potential, which in the heart is transmitted along the cardiocyte membrane. Furthermore, the heart has a rich innervation and specialized conduction system for rapid conduction and regulation of cardiac rhythmicity. Availability of EAA receptors in the heart might be important for cardiac function. The following GluRs were cloned by isoform-specific RT-PCR from rat heart ribonucleic acid (RNA): GluR 1, GluR 3, GluR 4, GIuR 7, Ka 1, and Ka 2. Expression in cardiac tissue was confirmed by western (for anti-GluR 2/3) and northern blots (for GluR 3, NMDAR 1, and Ka 2). The anatomical distribution was investigated by immunohistochemistry. Antibodies to GluR 2/3, GluR 5/6/7, Ka 2, and NMDAR 1 showed the strongest signals. These signals were specifically localized to cardiac nerve terminals, ganglia, conducting fibers, and some to myocardiocytes particularly in the atrium. Each antibody had a specific pattern of distribution. This anatomical localization suggests that they might play a role in cardiac electrophysiology and pathology.

Glutamate receptors in the mammalian central nervous system

Glutamate receptors (GluRs) mediate most of the excitatory neurotransmission in the mammalian central nervous system (CNS). In addition, they are involved in plastic changes in synaptic transmission as well as excitotoxic neuronal cell death that occurs in a variety of acute and chronic neurological disorders. The GluRs are divided into two distinct groups, ionotropic and metabotropic receptors. The ionotropic receptors (iGluRs) are further subdivided into three groups: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) receptor channels. The metabotropic receptors (mGluRs) are coupled to GTP-binding proteins (G-proteins), and regulate the production of intracellular messengers. The application of molecular cloning technology has greatly advanced our understanding of the GluR system. To date, at least 14 cDNAs of subunit proteins constituting iGluRs and 8 cDNAs of proteins constituting mGluRs have been cloned in the mammalian CNS, and the molecular structure, distribution and developmental change in the CNS, functional and pharmacological properties of each receptor subunit have been elucidated. Furthermore, the obtained clones have provided valuable tools for conducting studies to clarify the physiological and pathophysiological significances of each subunit. For example, the generation of gene knockout mice has disclosed critical roles of some GluR subunits in brain functions. In this article, we review recent progress in the research for GluRs with special emphasis on the molecular diversity of the GluR system and its implications for physiology and pathology of the CNS.

Glutamate in neurologic diseases

Excitotoxicity has been implicated as a mechanism of neuronal death in acute and chronic neurologic diseases. Cerebral ischemia, head and spinal cord injury, and prolonged seizure activity are associated with excessive release of glutamate into the extracellular space and subsequent neurotoxicity. Accumulating evidence suggests that impairment of intracellular energy metabolism increases neuronal vulnerability to glutamate which, even when present at physiologic concentrations, can damage neurons. This mechanism of slow excitotoxicity may be involved in neuronal death in chronic neurodegenerative diseases such as the mitochondrial encephalomyopathies, Huntington's disease, spinocerebellar degeneration syndromes, and motor neuron diseases. If so, glutamate antagonists in combination with agents that selectively inhibit the multiple steps downstream of the excitotoxic cascade or help improve intracellular energy metabolism may slow the neurodegenerative process and offer a therapeutic approach to treat these disorders.

AMPA- and kainate-receptors differentially mediate excitatory amino acid-induced dopamine and acetylcholine release from rat striatal slices

Rat striatal slices, preincubated with [3H]dopamine (DA) and [14C]choline, were superfused continuously. Detection of radioactivity was used to monitor the release of the neurotransmitters DA and acetylcholine (ACh). 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX) caused a concentration-dependent decrease in 100 microM alpha-amino-3-hydroxy-5-methylisoxazol-4-propionate (AMPA)-, 100 microM kainate- or 100 mM glutamate-induced release of DA and ACh. The IC50 of NBQX is 3-fold higher (for ACh release) and is 2-fold lower (for DA release) than that of CNQX. This is in agreement with the IC50 ratio of NBQX and CNQX on kainate- and AMPA-receptor binding. These two antagonists, at doses that produce an equivalent blockade of kainate-receptor binding (5 microM for NBQX and 1.56 microM for CNQX), caused an approximately equal decrease in ACh- but not DA-release induced by 100 microM kainate or AMPA. At doses that produce an equivalent blockade of AMPA-receptor binding (5 microM for NBQX and 10 microM for CNQX), they caused an approximately equal decrease in DA but not ACh release induced by 100 microM AMPA or kainate. Moreover, concanavalin A (0.3 and 0.5 mg/ml), which selectively potentiates kainate-receptor responses, markedly enhanced 100 microM kainate-induced release of ACh but not DA. Cyclothiazide (10 microM), which selectively potentiates AMPA-receptor responses, significantly increased 100 microM AMPA- or kainate-induced release of DA but not ACh. In summary, these results indicate that AMPA-and kainate-receptor activation, respectively, are predominantly involved in excitatory amino acid (EAA)-induced DA and ACh release in the striatum.

Excitotoxic mechanisms of neurodegeneration in transmissible spongiform encephalopathies

Endogenous excitatory amino acids (EAAs) such as glutamic or aspartic acids have been proposed to mediate the brain damage to EAA receptor-rich brain sites that is caused by a variety of external toxic agents (glutamic acid, domoic acid, kainic acid, ibogaine, trimethyltin (TMT), 3-nitropropionic acid (3-NPA)), as well as from such naturally-occurring age-related neurodegenerative diseases as Alzheimer's disease, Huntington's chorea, and Parkinson's disease. Sites often damaged include the hypothalamus (glutamate), the hippocampal and neocortical pyramidal neurons (domoic acid), the cerebellar Purkinje neurons (ibogaine) and the corpus striatum (3-NPA, amphetamine). The excitotoxic damage occurs to neuronal cell bodies and their dendrites, resulting in a characteristics appearance of pyknotic neurons surrounded by their vacuolated, swollen dendrites. Axons passing through the region that lack EAA receptors are completely spared. However, astrocytes with swollen perikarya and nuclei (Alzheimer's type II "reactive" astrocytes) are often observed in the vicinity of the lesions. Animal and human "Prion Diseases" or "Transmissible Spongiform Encephalopathies" (TSEs) result (after a period of months to years) in a neurodegenerative picture characterized by pyknotic neurons surrounded by vacuoles with numerous reactive astrocytes in the vicinity of the damage. In addition, amyloid deposits composed of a protease-resistant protein (PrPSc) characteristic of the particular host species with the disease are found near the degenerating neurons. By using different strains of the scrapies TSE agent to inoculate hamsters and mice, reproducible models of hypothalamic, hippocampal, or cerebellar damage resulting in the appropriate functional deficits may be obtained. Because of the close similarity in the appearance, localization, and functional consequences from TSE neuropathology compared to some of the well-known EAA syndromes, we propose that excitotoxic mechanisms may play a role in the pathogenesis of TSE neurodegenerative diseases. The similarity in pathogenesis of the neurodegenerative processes in excitotoxicity compared to TSE diseases also implies that neuroprotective strategies against excitotoxicity may also be effective against TSEs.

Corticomotor threshold is reduced in early sporadic amyotrophic lateral sclerosis

The pathogenesis of idiopathic amyotrophic lateral sclerosis (ALS) remains unknown, but accumulating evidence suggests a neuroexcitotoxic mechanism may have some credence. Glutamate-induced hyperexcitability of cortical or spinal motoneurons may be expected to manifest itself as a reduced threshold for activation of these structures. We have measured corticomotor threshold to the first dorsal interosseous (FDI) muscles of 48 patients with sporadic ALS using magnetic brain stimulation and have correlated the findings with physical signs of upper and/or lower motor neuron degeneration. We find that if FDI in patients with ALS shows no weakness, wasting, or signs of an upper motor neuron lesion, mean corticomotor threshold is significantly lower than in 102 healthy control FDI muscles (P = 0.02). In contrast, FDI muscles showing signs of lower motor neuron degeneration only or mixed upper and lower motor neuron signs are associated with a raised corticomotor threshold (P = 0.008, P < 0.001, respectively). We conclude that early in ALS, at a time when hand muscle function is normal, corticomotor threshold is reduced and suggest that this may be a manifestation of abnormal excitability of cortical or spinal motoneurons to neurotransmitters, whose action will ultimately lead to cell death.

Glutamate, excitotoxicity and amyotrophic lateral sclerosis

The "glutamate hypothesis" is one of three major pathophysiological mechanisms of motor neurone injury towards which current research effort into amyotrophic lateral sclerosis (ALS) is directed. There is great structural and functional diversity in the glutamate receptor family which results from combinations of 14 known gene products and their splice variants, with or without additional RNA editing. It is possible that motor neurones express a unique molecular profile of glutamate receptors. Abnormal activation of glutamate receptors is one of five main candidates as a final common pathway to neuronal death. In classical acute excitotoxicity, there is influx of Na+ and CI-, and destabilisation of intracellular Ca2+ homeostasis, which activates a cascade of harmful biochemical events. The concept of secondary excitotoxicity, where cellular injury by glutamate is triggered by disturbances in neuronal energy status, may be particularly relevant to a chronic neurodegenerative disease such as ALS. Data are now beginning to emerge on the fine molecular structure of the glutamate receptors present on human motor neurones, which have a distinct profile of AMPA receptors. Two important molecular features of motor neurones have been identified that may contribute to their vulnerability to neurodegeneration. The low expression of calcium binding proteins and the low expression of the GluR2 AMPA receptor subunit by vulnerable motor neurone groups may render them unduly susceptible to calcium-mediated toxic events following glutamate receptor activation. Eight lines of evidence that indicate a disturbance of glutamatergic neurotransmission in ALS patients are reviewed. The links between abnormal activation of glutamate receptors and other potential mechanisms of neuronal injury, including activation of calcium-mediated second messenger systems and free radical mechanisms, are emphasised. Riluzole, which modulates the glutamate neurotransmitter system, has been shown to prolong survival in patients with ALS. Further research may allow the development of subunit-specific therapeutic targeting of glutamate receptors and modulation of "downstream" events within motor neurones, aimed at protecting vulnerable molecular targets in specific populations of ALS patients.

Sequelae of parenteral domoic acid administration in rats: comparison of effects on different metabolic markers in brain

Parenterally administered domoic acid, a structural analog of the excitatory amino acids glutamic acid and kainic acid, has specific effects on brain histology in rats, as measured using different anatomic markers. Domoic acid-induced convulsions affects limbic structures such as hippocampus and entorhinal cortex, and different anatomic markers can detect these neurotoxic effects to varying degrees. Here we report effects of domoic acid administration on quantitative indicators of brain metabolism and gliosis. Domoic acid, 2.25 mg/kg i.p., caused stereotyped behavior and convulsions in approximately 60% of rats which received it. Six to eight days after domoic acid or vehicle administration, the animals were processed to measure regional brain incorporation of the long-chain fatty acids [1-(14)C]arachidonic acid ([14C]AA) and [9,10-(3)H]palmitic acid ([3H]PA), or regional cerebral glucose utilization (rCMRglc) using 2-[1-(14)C]deoxy-D-glucose, by quantitative autoradiography. Others rats were processed to measure brain glial fibrillary acidic protein (GFAP) by enzyme-linked immunosorbent assay. Domoic acid increased GFAP in the anterior portion of cerebral cortex, the caudate putamen and thalamus compared with vehicle. However, in rats that convulsed after domoic acid GFAP was significantly increased throughout the cerebral cortex, as well as in the hippocampus, septum, caudate putamen, and thalamus. Domoic acid, in the absence of convulsions, decreased relative [14C]AA incorporation in the claustrum and pyramidal cell layer of the hippocampus compared with vehicle-injected controls. In the presence of convulsions, relative [14C]AA incorporation was decreased in hippocampus regions CA1 and CA2. Uptake of [3H]PA into brain was unaffected. Relative rCMRglc decreased in entorhinal cortex following domoic acid administration with or without convulsions. These results suggest that acute domoic acid exposure affects discrete brain circuits by inducing convulsions, and that domoic acid-induced convulsions cause chronic effects on brain function that are reflected in altered fatty acid metabolism and gliosis.

Inhibition of [3H] gamma-aminobutyric acid release by kainate receptor activation in rat hippocampal synaptosomes

We studied the modulation of gamma-aminobutyric acid (GABA) release by activation of kainate receptor in rat whole hippocampal synaptosomes. Kainate (10-300 microM) inhibited [3H]GABA release in a concentration-dependent manner with an EC50 of 25 microM. This effect of kainate (30 microM) was prevented by the ionotropic non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and by the selective kainate receptor antagonist 5-nitro-6,7,8,9-tetrahydrobenzo(g)indole-2,3-dione-3-oxime (NS-102, 10 microM), but not by the selective non-competitive AMPA receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5 H-2,3-benzodiazepine (GYKI 52466, 100 microM). Other kainate receptor agonists, such as domoic acid (0.3-10 microM) and (2S,4R)-4-methylglutamic acid (MGA, 0.3-3 microM), also inhibited [3H]GABA release in a concentration-dependent manner with EC50 values of 4.0 microM and 0.90 microM, respectively, whereas alpha-amino-3-hydroxy-5-methyl-4-oxazolepropionate (AMPA, 10-100 microM) was devoid of effect. These inhibitory effects of both domoic acid (3 microM) and MGA (1 microM) were antagonized by CNQX (10 microM). These results indicate that GABA release can be modulated directly by presynaptic high-affinity kainate heteroreceptors.

Sequelae of parenteral domoic acid administration in rats: comparison of effects on different anatomical markers in brain

Brain damage following administration of domoic acid, a structural analog of the excitatory amino acids glutamic acid and kainic acid, was compared using different anatomic markers in adult rats. Seven days after administration of domoic acid (2.25 mg/kg i.p.) or vehicle, brains were collected and sectioned and stained to visualize Nissl substance using thionin, argyrophilia using a cupric silver staining method, astroglia using immunohistochemistry to detect glial fibrillary acidic protein-like immunoreactivity (GFAP-ir), and activated microglia using lectin histochemistry to detect Griffonia simplicifolia I-B4 isolectin (GSI-B4) binding in adjacent sections. In approximately 60% of rats to which it was administered, domoic acid caused stereotyped behavior within 60 min, followed by convulsions within 2-3 h. Brains of domoic acid-administered rats that did not manifest stereotyped behavior or convulsions did not differ from brains from vehicle-administered controls. In animals that had manifested stereotyped behavior and convulsions, Nissl staining was mostly unremarkable in brain sections. In contrast, there was intense argyrophilia in anterior olfactory nucleus, CA1 hippocampus, lateral septum, parietal (layer IV), piriform, and entorhinal cortices, ventral posterolateral thalamus, and amygdala. This pattern was reminiscent of that seen in postmortem specimens from humans who consumed domoic acid-tainted mussels and in experimental animals after kainic acid administration. Adjacent sections displayed astrogliosis, evidenced by increased GFAP-ir, which was more diffuse than the argyrophilic reaction. Activated microglia were revealed using GSI-B4 histochemistry. These data suggest activation of discrete brain circuits in rats that convulse following domoic acid administration and subsequent pathological alterations. The data strongly suggest that neuropathology following domoic acid occurs only in animals manifesting domoic acid-induced sterotypy and convulsions. The data do not rule out more insidious damage in behaviorally normal rats that receive domoic acid.

The peripheral benzodiazepine receptor is a sensitive indicator of domoic acid neurotoxicity

To evaluate the utility of the peripheral benzodiazepine receptor (PBR) as a biomarker of neurotoxicity, we measured receptor levels after sub-seizure doses of domoic acid (0-3.0 mg/kg) in rats using [3H]PK-11195 autoradiography. PBR expression in limbic structures was significantly increased 5 days, but not 24 or 48 h after injection of 3.0 mg/kg domoic acid. The largest increase in [3H]PK-11195 binding (> 500% above control) was found in the CA3 subfield of the hippocampus. Other limbic structures including the CA1 hippocampal subfield, subiculum, dentate gyrus and amygdala also showed significant increases in PBR expression, as did the striatum and substantia nigra pars reticulata. Smaller but significant increases were also observed 5 days after injection of 1.5 mg/kg, but not in animals treated with 0.75 mg/kg domoic acid. No pathology was observed after routine histological staining of brain tissue. Spatial learning and memory, a process thought to be associated with the hippocampus, was assessed in the Morris water maze. Groups treated with 1.5 and 3.0 mg/kg, but not 0.75 mg/kg domoic acid were significantly impaired in water maze performance. These findings suggest that the PBR could provide a sensitive and specific biomarker of neurotoxicity.

Effects of maternal oral administration of monosodium glutamate at a late stage of pregnancy on developing mouse fetal brain

Monosodium glutamate (MSG) was shown to penetrate placental barrier and to distribute to embryonic tissues using [3H]glutamic acid ([3H]Glu) as a tracer. However, the distribution is not even; the uptake of MSG in the fetal brain was twice as great as that in the maternal brain in Kunming mice. Other maternal mice were given per os MSG (2.5 mg/g or 4.0 mg/g body weight) at 17-21 days of pregnancy, and their offspring behaviors studied. The results showed that maternal oral administration of MSG at a late stage of pregnancy decreased the threshold of convulsion in the litters at 10 days of age. Y-maze discrimination learning was significantly impaired in the 60-day-old filial mice. On the other hand, no significant difference in spatial learning or tail flick latency was measured between the experimental animals and the controls. The filial mice of MSG-treated mothers could either not grasp a rope tightly, or grasped the rope tightly but could not crawl along the rope at the beginning of the training. However, such mice, after training, could grasp and crawl along the rope as well as controls. Obvious neuronal damage was not detected in the periventricular organs or the hypothalamus under a light microscope. The rate of weight gain for experimental animals was greater than for controls throughout the period from 20 to 90 days. Mating of treated males with treated females resulted in pregnancies and normal offspring, indicating that oral administration of MSG at a late stage of pregnancy did not affected the reproductive capacity of the offspring. The possible differences and relationship between MSG-induced damage to developing human and rodent brain are discussed.

Poisonings: food, fish, shellfish

Not every traveler who gets sick away from home has an infection; some are poisoned. This article describes common and dangerous illnesses caused by food-borne toxins. It explores the toxic illnesses acquired from fish or seafood, including scombroid, ciguatera, pufferfish toxicity, and a variety of shellfish poisonings. It also provides a brief overview of plant toxicity. Although gastroenteritis is a common feature of many food poisonings, this article emphasizes those processes associated with neurologic manifestations, as they tend to be more dangerous to patients and less well understood by physicians. It also stresses strategies to prevent food poisoning.

Effect of pH on domoic acid toxicity in mice

Domoic acid is a shellfish toxin which produces gastrointestinal distress, followed by neurological symptoms such as headache, confusion, disorientation and severe deficits in short-term memory. Domoic acid is an amino acid which contains three carboxylic groups, and one imino group, and its solubility, rate of absorption, and elimination would vary depending on the protonation of these groups at different pH's. We propose that domoic acid toxicity varies with pH of administered domoic acid solution. Domoic acid toxicity was measured in mice as the onset times for scratching behaviour, seizure activity, and death, after the intraperitoneal administration of domoic acid at different pH's. Results of the present study show that the scratching behaviour, seizure activity, and death, occurred at 12, 40, and 55 min, after intraperitoneal administration of domoic acid at pH 3.7. Apparently, the onset times for three types of behaviours were relatively long, and well separated from each other. Domoic acid toxicity was lowest at pH 3.7, and highest at pH 7.4, with intermediate toxicity at other pH's. The onset time of scratching behaviour was not influenced by pH of domoic acid solution at three different doses. In contrast, the onset times for seizure activity, and death were significantly affected by pH of domoic acid, toxicity being higher at pH 7.4 than at pH 3.7. The pH effect on domoic acid toxicity diminished as the dose of domoic acid was increased. In fact, at 14.5 mg/kg domoic acid toxicity was similar at both pH's of 3.7 and 7.4. It is concluded that in vivo toxicity of domoic acid varies depending on pH of the administered solution. The differential toxicity of domoic acid at different pH may be related to its solubility, rate of absorption, and elimination, depending on the degree of protonation of domoic acid molecule. Domoic acid toxicity would also vary depending on the age of animal, receptor sensitivity and density in different regions of brain.

Psychotogenicity and N-methyl-D-aspartate receptor antagonism: implications for neuroprotective pharmacotherapy

The development of neuroprotective agents for the prevention of neuronal loss in acute conditions such as stroke and epilepsy or chronic neurodegenerative disorders including Parkinson's disease, Alzheimer's disease, Huntington's chorea, and motor neuron disease is currently focusing on drugs that inhibit excitatory amino acid neurotransmission or exhibit antioxidant properties. Unfortunately, potent antagonists at the N-methyl-D-aspartate (NMDA) type glutamate receptor, which is thought to mediate excitotoxic neuronal injury, e.g., MK-801 or phencyclidine (PCP), share a high probability of inducing psychotomimetic side effects. Further, these drugs have been associated with acute neurotoxicity in vitro and in vivo, precluding their clinical use. In contrast, low affinity NMDA receptor antagonists like amantadine and its dimethyl derivative, memantine, have been administered clinically for the management of Parkinson's disease, dementia, neuroleptic drug-induced side effects, and spasticity. These agents have only rarely induced significant psychotomimetic side effects. Recent pharmacologic advances have helped to elucidate how high drug affinity for the PCP binding site of the NMDA receptor may enhance psychotogenicity. Low affinity and associated fast voltage-dependent channel unblocking kinetics are likely to be responsible for the better tolerance of amantadine and memantine compared with MK-801 and PCP. Further factors apparently modulating psychotogenicity of glutamate receptor antagonists include differential actions on neuronal populations in various brain regions and interactions with neurotransmitter receptors other than the NMDA type glutamate receptor.

Motor neurons are selectively vulnerable to AMPA/kainate receptor-mediated injury in vitro

The nonphosphorylated neurofilament marker SMI-32 stains motor neurons in spinal cord slices and stains a subset of cultured spinal neurons ["large SMI-32(+) neurons"], which have a morphology consistent with motor neurons identified in vitro: large cell body, long axon, and extensive dendritic arborization. They are found preferentially in ventral spinal cord cultures, providing further evidence that large SMI-32(+) neurons are indeed motor neurons, and SMI-32 staining often colocalizes with established motor neuron markers (including acetylcholine, calcitonin gene-related peptide, and peripherin). Additionally, choline acetyltransferase activity (a frequently used index of the motor neuron population) and peripherin(+) neurons share with large SMI-32(+) neurons an unusual vulnerability to AMPA/kainate receptor-mediated injury. Kainate-induced loss of these motor neuron markers is Ca2+-dependent, which supports a critical role of Ca2+ ions in this injury. Raising extracellular Ca2+ exacerbates injury, whereas removal of extracellular Ca2+ is protective. A basis for this vulnerability is provided by the observation that most peripherin(+) neurons, like large SMI-32(+) neurons, are subject to kainate-stimulated Co2+ uptake, a histochemical stain that identifies neurons possessing Ca2+-permeable AMPA/kainate receptor-gated channels. Finally, of possibly greater relevance to the slow motor neuronal degeneration in diseases, both large SMI-32(+) neurons and peripherin(+) neurons are selectively damaged by prolonged (24 hr) low-level exposures to kainate (10 microM) or to the glutamate reuptake blocker L-trans-pyrrolidine-2,4-dicarboxylic acid (100 microM). During these low-level kainate exposures, large SMI-32(+) neurons showed higher intracellular Ca2+ concentrations than most spinal neurons, suggesting that Ca2+ ions are also important in this more slowly evolving injury.

Opioid neurotoxicity: fentanyl dose-response effects in rats

Opioids, when administered in large doses, produce brain damage, primarily in the limbic system and association areas in rats. This investigation examined the relationship between opioid dose and severity and frequency of brain damage in rats. Forty male Sprague-Dawley rats were anesthetized with halothane/N2O and underwent tracheal intubation, mechanical ventilation, arterial/venous cannulation, and insertion of a rectal temperature probe and biparietal electroencephalogram electrodes. After surgery, halothane was discontinued and O2/N2O 30%/70% was administered for 1 h. Rats were then randomly assigned to one of eight groups. The control group received a loading dose (LD) of 4 mL/kg of 0.9% normal saline solution (NSS) and a maintenance dose (MD) of 4 mL.kg-1.h-1 NSS. The other groups were given fentanyl lypophilized and reconstituted in NSS with the LD ranging from 50 to 3200 micrograms/kg and the MD from 2 to 128 micrograms.kg-1.min-1. After 2 h of fentanyl or NSS infusion; all rats received 100% O2 and, when alert, their tracheas were extubated; after 7 days the rats underwent cerebral perfusion fixation, followed by light microscopic evaluation. Histopathologic lesions (primarily eosinophilic neuron degeneration) were subjectively graded by a pathologist unaware of the experimental treatment; the grades were based on the percentage of dead neurons. There were no lesions observed in the brain areas in any of the control or 200-8 (LD, microgram/kg; MD, microgram.kg-1.min-1) groups. Eleven of 20 rats in the 400-16, 800-32, 1600-64, and 3200-18 groups showed evidence of brain damage primarily in limbic system structures and association areas (P < 0.05). Our data confirm that fentanyl produces limbic system brain damage in rats, and that the damage occurs over a broad range of doses.

Effect of age in rodent models of focal and forebrain ischemia

BACKGROUND AND PURPOSE

The majority of animal experiments examining the nature and treatment of stroke have used relatively young animals ranging in age from 2 to 6 months. However, significant morphological, neurochemical, and behavioral changes occur with aging in rodents particularly during the first 24 months of age. This study examines the effect of age in two models of transient ischemia a forebrain and a focal model in male Wistar rats.

METHODS

We induced forebrain ischemia of 12 minutes duration by bilateral carotid artery occlusion with controlled hypotension at a mean blood pressure of 45 mm Hg and using an intraluminal filament technique, induced focal middle cerebral artery occlusion of 100 minutes duration at a mean blood pressure of 60 mm Hg. Physiological parameters were monitored and maintained within normal limits. On day 7 after ischemia, the rats were perfusion-fixed and the brains removed for quantitative histopathology.

RESULTS

After forebrain ischemia, older rats showed significantly less CA1 neuronal necrosis than the younger group (P < .003), whereas both striatal and neocortical injury were significantly greater in the older group (P < .05). Among animals subjected to focal ischemia, the volume of infarcted tissue and the number of necrotic neurons in the area adjacent to the infarction were both greater in older rats (P < .05).

CONCLUSIONS

This study emphasizes the importance of age in models of forebrain and focal ischemia. The interaction between age-related changes in morphology, neurochemistry, and behavior on the ischemic cascade complicates the interpretation of mechanistic data, and pharmacological effects observed in younger animals may not necessarily translate to an older population.

Anatomical studies of DNA fragmentation in rat brain after systemic kainate administration

Rats treated systemically with kainate develop stereotyped epileptic seizures involving mainly limbic structures that may last for hours. This model of limbic status epilepticus has been widely studied using classical neuropathological techniques. We used in situ nick translation histochemistry to examine patterns of DNA fragmentation in this model. We found a stereotyped and reproducible pattern of neuronal populations that demonstrate evidence of DNA fragmentation from 24 h to one week after kainate treatment. Neither blockade of new protein synthesis nor blockade of the N-methyl-D-aspartate-type glutamate receptors significantly altered this response. Moreover, we saw no evidence of the regular internucleosomal cleavage of DNA that produces a characteristic laddered appearance of 180-200 bp DNA fragments after gel electrophoresis in samples obtained from microdissected affected regions. These studies suggest that DNA fragmentation after systemic kainate-induced seizures is not the result of programmed cell death. This assay may be useful for quantitative testing of both neuroprotective agents and mechanistic hypotheses.

Sepsis and the systemic inflammatory response syndrome: neuromuscular manifestations

OBJECTIVE

To describe the various conditions of peripheral nerve, neuromuscular junction, and muscle associated with the systemic inflammatory response syndrome (SIRS).

DATA SOURCES

Publications in the scientific literature and personal observations during the last 15 yrs.

DATA EXTRACTION

Computer search of the literature and review of patient records relating to polyneuropathy, neuromuscular transmission defects, and myopathies associated with sepsis, the septic syndrome, and SIRS.

SYNTHESIS

SIRS is a new concept in which infection and trauma induce a systemic inflammatory response affecting the microcirculation to organs throughout the body. The nervous system is commonly affected in the forms of septic encephalopathy and critical illness polyneuropathy. Neuromuscular blocking agents and corticosteroids may have additional toxic effects on the neuromuscular system that are manifest as transient neuromuscular blockade, an axonal motor neuropathy, or a thick filament myopathy. Clinical examination in the critical care unit is often unreliable and electrophysiologic studies, at times accompanied by magnetic resonance imaging of the spinal cord, measurement of the circulating creatine phosphokinase concentration, and muscle biopsy, are often necessary to establish the diagnosis. Variants of critical illness polyneuropathy may occur outside the critical care unit. The precise mechanism of these neuromuscular conditions is not known, and further basic research is needed.

CONCLUSIONS

A variety of neuromuscular conditions complicates SIRS. The identification of these conditions is important in patient management and in rendering a prognosis.