Glutamate neurotoxicity, calcium, and zinc

Amitriptyline protects afferent synapses in the cochlea against excitotoxic trauma in vitro

Afferent synapses between inner hair cells (IHCs) and the type I spiral ganglion neurons (SGNs) in the cochlea provide over 95% of sensory signals for auditory perception in the brain. However, these afferent synapses are particularly vulnerable to damage, for example from excitotoxicity, and exposure to noise in the environment which often leads to noise-induced cochlear synaptopathy (NICS). In this study, we simulated excitotoxic trauma by incubating kainic acid, a non-desensitizing agonist for AMPA type glutamate receptors on cultured cochleae. The possible protective effects of amitriptyline against NICS were examined. We found that, in IHCs, amitriptyline reversed the decrease of Ca2+ current and exocytosis caused by excitotoxic trauma. In SGNs, amitriptyline promoted the recovery of neurite loss caused by excitotoxic trauma. Furthermore, we found that the protective effects of amitriptyline are likely mediated by suppressing apoptosis factors that were upregulated during excitotoxic trauma. In conclusion, our results suggest that amitriptyline could protect afferent synapses in the cochlea from NICS, making it a potential drug candidate for hearing protection.

Zinc: Multidimensional Effects on Living Organisms

Zinc is a redox-inert trace element that is second only to iron in abundance in biological systems. In cells, zinc is typically buffered and bound to metalloproteins, but it may also exist in a labile or chelatable (free ion) form. Zinc plays a critical role in prokaryotes and eukaryotes, ranging from structural to catalytic to replication to demise. This review discusses the influential properties of zinc on various mechanisms of bacterial proliferation and synergistic action as an antimicrobial element. We also touch upon the significance of zinc among eukaryotic cells and how it may modulate their survival and death through its inhibitory or modulatory effect on certain receptors, enzymes, and signaling proteins. A brief discussion on zinc chelators is also presented, and chelating agents may be used with or against zinc to affect therapeutics against human diseases. Overall, the multidimensional effects of zinc in cells attest to the growing number of scientific research that reveal the consequential prominence of this remarkable transition metal in human health and disease.

Zinc Therapy in Early Alzheimer's Disease: Safety and Potential Therapeutic Efficacy

Zinc therapy is normally utilized for treatment of Wilson disease (WD), an inherited condition that is characterized by increased levels of non-ceruloplasmin bound ('free') copper in serum and urine. A subset of patients with Alzheimer's disease (AD) or its prodromal form, known as Mild Cognitive Impairment (MCI), fail to maintain a normal copper metabolic balance and exhibit higher than normal values of non-ceruloplasmin copper. Zinc's action mechanism involves the induction of intestinal cell metallothionein, which blocks copper absorption from the intestinal tract, thus restoring physiological levels of non-ceruloplasmin copper in the body. On this basis, it is employed in WD. Zinc therapy has shown potential beneficial effects in preliminary AD clinical trials, even though the studies have missed their primary endpoints, since they have study design and other important weaknesses. Nevertheless, in the studied AD patients, zinc effectively decreased non-ceruloplasmin copper levels and showed potential for improved cognitive performances with no major side effects. This review discusses zinc therapy safety and the potential therapeutic effects that might be expected on a subset of individuals showing both cognitive complaints and signs of copper imbalance.

Delirium pathophysiology: An updated hypothesis of the etiology of acute brain failure

BACKGROUND

Delirium is the most common neuropsychiatric syndrome encountered by clinicians dealing with older adults and the medically ill and is best characterized by 5 core domains: cognitive deficits, attentional deficits, circadian rhythm dysregulation, emotional dysregulation, and alteration in psychomotor functioning.

DESIGN

An extensive literature review and consolidation of published data into a novel interpretation of known pathophysiological causes of delirium.

RESULTS

Available data suggest that numerous pathological factors may serve as precipitants for delirium, each having differential effects depending on patient-specific patient physiological characteristics (substrate). On the basis of an extensive literature search, a newly proposed theory, the systems integration failure hypothesis, was developed to bring together the most salient previously described theories, by describing the various contributions from each into a complex web of pathways-highlighting areas of intersection and commonalities and explaining how the variable contribution of these may lead to the development of various cognitive and behavioral dysfunctions characteristic of delirium. The specific cognitive and behavioral manifestations of the specific delirium picture result from a combination of neurotransmitter function and availability, variability in integration and processing of sensory information, motor responses to both external and internal cues, and the degree of breakdown in neuronal network connectivity, hence the term acute brain failure.

CONCLUSIONS

The systems integration failure hypothesis attempts to explain how the various proposed delirium pathophysiologic theories interact with each other, causing various clinically observed delirium phenotypes. A better understanding of the underlying pathophysiology of delirium may eventually assist in designing better prevention and management approaches.

Copper and Zinc Dysregulation in Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common forms of dementia. Despite a wealth of knowledge on the molecular mechanisms involved in AD, current treatments have mainly focused on targeting amyloid β (Aβ) production, but have failed to show significant effects and efficacy. Therefore, a critical reconsideration of the multifactorial nature of the disease is needed. AD is a complex multifactorial disorder in which, along with Aβ and tau, the convergence of polygenic, epigenetic, environmental, vascular, and metabolic factors increases the global susceptibility to the disease and shapes its course. One of the cofactors converging on AD is the dysregulation of brain metals. In this review, we focus on the role of AD-related neurodegeneration and cognitive decline triggered by the imbalance of two endogenous metals: copper and zinc.

Neuropathogenesis of delirium: review of current etiologic theories and common pathways

Delirium is a neurobehavioral syndrome caused by dysregulation of neuronal activity secondary to systemic disturbances. Over time, a number of theories have been proposed in an attempt to explain the processes leading to the development of delirium. Each proposed theory has focused on a specific mechanism or pathologic process (e.g., dopamine excess or acetylcholine deficiency theories), observational and experiential evidence (e.g., sleep deprivation, aging), or empirical data (e.g., specific pharmacologic agents' association with postoperative delirium, intraoperative hypoxia). This article represents a review of published literature and summarizes the top seven proposed theories and their interrelation. This review includes the "neuroinflammatory," "neuronal aging," "oxidative stress," "neurotransmitter deficiency," "neuroendocrine," "diurnal dysregulation," and "network disconnectivity" hypotheses. Most of these theories are complementary, rather than competing, with many areas of intersection and reciprocal influence. The literature suggests that many factors or mechanisms included in these theories lead to a final common outcome associated with an alteration in neurotransmitter synthesis, function, and/or availability that mediates the complex behavioral and cognitive changes observed in delirium. In general, the most commonly described neurotransmitter changes associated with delirium include deficiencies in acetylcholine and/or melatonin availability; excess in dopamine, norepinephrine, and/or glutamate release; and variable alterations (e.g., either a decreased or increased activity, depending on delirium presentation and cause) in serotonin, histamine, and/or γ-aminobutyric acid. In the end, it is unlikely that any one of these theories is fully capable of explaining the etiology or phenomenologic manifestations of delirium but rather that two or more of these, if not all, act together to lead to the biochemical derangement and, ultimately, to the complex cognitive and behavioral changes characteristic of delirium.

Intravenous administration of achyranthes bidentata polypeptides supports recovery from experimental ischemic stroke in vivo

Background

Achyranthes bidentata Blume (A. bidentata) is a commonly prescribed Chinese medicinal herb. A. bidentata polypeptides (ABPP) is an active composite constituent, separated from the aqueous extract of A. bidentata. Our previous studies have found that ABPP have the neuroprotective function in vitro and in rat middle cerebral artery occlusion (MCAO) model in attenuating the brain infract area induced by focal ischemia-reperfusion. However, the ultimate goal of the stroke treatment is the restoration of behavioral function. Identifying behavioral deficits and therapeutic treatments in animal models of ischemic stroke is essential for potential translational applications.

Methodology and Principal Findings

The effect of ABPP on motor, sensory, and cognitive function in an ischemic stroke model with MCAO was investigated up to day 30. The function recovery monitored by the neurological deficit score, grip test, body asymmetry, beam-balancing task, and the Morris Water Maze. In this study, systemic administration of ABPP by i.v after MCAO decreased the neurological deficit score, ameliorated the forepaw muscle strength, and diminished the motor and sensory asymmetry on 7^th and 30^th day after MCAO. MCAO has been observed to cause prolonged disturbance of spatial learning and memory in rats using the MWM, and ABPP treatment could improve the spatial learning and memory function, which is impaired by MCAO in rats, on 30^th day after MCAO. Then, the viable cells in CA1 region of hippocampus were counted by Nissl staining, and the neuronal cell death were significantly suppressed in the ABPP treated group.

Conclusion

ABPP could improve the recovery of sensory, motor and coordination, and cognitive function in MCAO-induced ischemic rats. And this recovery had a good correlation to the less of neuronal injury in brain.

Hurdles to clear before clinical translation of ischemic postconditioning against stroke

Ischemic postconditioning has been established for its protective effects against stroke in animal models. It is performed after post-stroke reperfusion and refers to a series of induced ischemia or a single brief one. This review article addresses major hurdles in clinical translation of ischemic postconditioning to stroke patients, including potential hazards, the lack of well-defined protective paradigms, and the paucity of deeply-understood protective mechanisms. A hormetic model, often used in toxicology to describe a dose-dependent response to a toxic agent, is suggested to study both beneficial and detrimental effects of ischemic postconditioning. Experimental strategies are discussed, including how to define the hazards of ischemic (homologous) postconditioning and the possibility of employing non-ischemic (heterologous) postconditioning to facilitate clinical translation. This review concludes that a more detailed assessment of ischemic postconditioning and studies of a broad range of heterologous postconditioning models are warranted for future clinical translation.

Syndromics: a bioinformatics approach for neurotrauma research

Substantial scientific progress has been made in the past 50 years in delineating many of the biological mechanisms involved in the primary and secondary injuries following trauma to the spinal cord and brain. These advances have highlighted numerous potential therapeutic approaches that may help restore function after injury. Despite these advances, bench-to-bedside translation has remained elusive. Translational testing of novel therapies requires standardized measures of function for comparison across different laboratories, paradigms, and species. Although numerous functional assessments have been developed in animal models, it remains unclear how to best integrate this information to describe the complete translational "syndrome" produced by neurotrauma. The present paper describes a multivariate statistical framework for integrating diverse neurotrauma data and reviews the few papers to date that have taken an information-intensive approach for basic neurotrauma research. We argue that these papers can be described as the seminal works of a new field that we call "syndromics", which aim to apply informatics tools to disease models to characterize the full set of mechanistic inter-relationships from multi-scale data. In the future, centralized databases of raw neurotrauma data will enable better syndromic approaches and aid future translational research, leading to more efficient testing regimens and more clinically relevant findings.

Kv7-type channel currents in spiral ganglion neurons: involvement in sensorineural hearing loss

Alterations in K(v)7-mediated currents in excitable cells result in several diseased conditions. A case in DFNA2, an autosomal dominant version of progressive hearing loss, involves degeneration of hair cells and spiral ganglion neurons (SGNs) from basal to apical cochlea, manifesting as high-to-low frequency hearing loss, and has been ascribed to mutations in K(v)7.4 channels. Analyses of the cellular mechanisms of K(v)7.4 mutations and progressive degeneration of SGNs have been hampered by the paucity of functional data on the role K(v)7 channels play in young and adult neurons. To understand the cellular mechanisms of the disease in SGNs, we examined temporal (young, 0.5 months old, and senescent, 17 months old) and spatial (apical and basal) roles of K(v)7-mediated currents. We report that differential contribution of K(v)7 currents in mice SGNs results in distinct and profound variations of the membrane properties of basal versus apical neurons. The current produces a major impact on the resting membrane potential of basal neurons. Inhibition of the current promotes membrane depolarization, resulting in activation of Ca(2+) currents and a sustained rise in intracellular Ca(2+). Using TUNEL assay, we demonstrate that a sustained increase in intracellular Ca(2+) mediated by inhibition of K(v)7 current results in significant SGN apoptotic death. Thus, this study provides evidence of the cellular etiology and mechanisms of SGN degeneration in DFNA2.

Pathoetiological model of delirium: a comprehensive understanding of the neurobiology of delirium and an evidence-based approach to prevention and treatment

Delirium is the most common complication found in the general hospital setting. Yet, we know relatively little about its actual pathophysiology. This article contains a summary of what we know to date and how different proposed intrinsic and external factors may work together or by themselves to elicit the cascade of neurochemical events that leads to the development delirium. Given how devastating delirium can be, it is imperative that we better understand the causes and underlying pathophysiology. Elaborating a pathoetiology-based cohesive model to better grasp the basic mechanisms that mediate this syndrome will serve clinicians well in aspiring to find ways to correct these cascades, instituting rational treatment modalities, and developing effective preventive techniques.

Neuroprotective strategies to avert seizure-induced neurodegeneration in epilepsy

Neurodegeneration in limbic circuits is a hallmark feature of chronic temporal lobe epilepsy (TLE). Studies in experimental animal models and human patients indicate that seizure-induced neuronal injury involves some active, as well as passive cell death processes. Experimental approaches that inhibit active steps in cell death programs have been shown to reduce neuronal cell death and sclerosis, but not to prevent epileptogenesis in animal models of TLE. These findings suggest that we need additional research using both animal models and brain slices from human patients to understand the pathological mechanisms underlying seizure generation. Such comparative studies will also aid in evaluating the potential therapeutic value of inhibiting cell death in seizure disorders.

Aromatase expression and cell proliferation following injury of the adult zebra finch hippocampus

Estrogens can be neuroprotective following traumatic brain injury. Immediately after trauma to the zebra finch hippocampus, the estrogen-synthetic enzyme aromatase is rapidly upregulated in astrocytes and radial glia around the lesion site. Brain injury also induces high levels of cell proliferation. Estrogens promote neuronal differentiation, migration, and survival naturally in the avian brain. We suspect that glia are a source of estrogens promoting cell proliferation after neural injury. To explore this hypothesis, we examined the spatial and temporal relationship between glial aromatase expression and cell proliferation after neural injury in adult female zebra finches. Birds were ovariectomized and given a blank implant or one filled with estradiol; some birds were also administered an aromatase inhibitor or vehicle. All birds received penetrating injuries to the right hippocampus. Twenty-four hours after lesioning, birds were injected once with BrdU to label mitotically active cells and euthanized 2 h, 24 h, or 7 days later. The brains were processed for double-label BrdU and aromatase immunocytochemistry. Injury-induced glial aromatase expression was unaffected by survival time and aromatase inhibition. BrdU labeling was significantly reduced at 24 h by ovariectomy and by aromatase inhibition; effects were partially reversed by E2 replacement. Irrespective of ovariectomy, the densities of aromatase immunoreactive astrocytes and BrdU-labeled cells at known distances from the lesion site were highly correlated. These data suggest that injury-induced glial aromatization may influence the reorganization of injured tissue by providing a rich estrogenic environment available to influence cellular incorporation.

Erratum to "Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintenance of epilepsy." [Pharmacol. Ther. 105(3) (2005) 229-266]

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury [central nervous system (CNS) insult]. (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels ([Ca(2+)](i)) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but the share a common molecular mechanism for producing brain damage--an increase in extracellular glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Inhibition of cyclooxygenase-2 expression by zinc-chelator in retinal ischemia

The zinc ion (Zn2+) is abundant in neurons. However, excessive Zn2+ can induce neuronal cell death. This study examined the role of Zn2+ in transient retinal ischemia in adult male rats. The rats were sacrificed 4-24 h after retinal ischemia by high intra-ocular pressure, and the retinas were prepared for microscopic examination of retinal cell degeneration, and fluorescence microscopy using zinquin ethyl ester as the zinc ion-specific probe. Moreover, COX-2 expression was observed by Western blotting. In control retinas, there was a low Zn2+ concentration in the inner plexiform layer (IPL), a high Zn2+ concentration in the outer plexiform layer (OPL), and no detectable Zn2+ in either the ganglion cell layer (GCL) or the inner nuclear layer (INL). In contrast, in the retinas exposed to ischemia without the administration of the zinc ion chelators (Ca2+-EDTA and TPEN), Zn2+ deposits were found in the IPL and INL beginning 4 h after ischemia and degeneration of neurons was found in the GCL and INL. Less Zn2+ accumulation in the IPL and INL and less neuronal degeneration in the GCL and INL were found in the retinas treated with Ca2+-EDTA or TPEN before ischemia. Furthermore, the COX-2 protein levels increased 4-8 h after retinal ischemia, and chelation of zinc ion inhibited this effect. These results suggest that the accumulation of Zn2+ following an ischemic insult can cause retinal degeneration and induce abnormal COX-2 expression.

DNA damage and nonhomologous end joining in excitotoxicity: neuroprotective role of DNA-PKcs in kainic acid-induced seizures

DNA repair plays a critical, but imprecisely defined role in excitotoxic injury and neuronal survival throughout adulthood. We utilized an excitotoxic injury model to compare the location and phenotype of degenerating neurons in mice (strain 129-C57BL) deficient in the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), an enzyme required for nonhomologous end joining (NHEJ). Brains from untreated adult heterozygous and DNA-PKcs null mice displayed comparable cytoarchitecture and undetectable levels of cell death. By day 1, and extending through 4 days following kainic acid-induced seizures, brains from DNA-PKcs null mice showed widespread neurodegeneration that encompassed the entire hippocampal CA1-CA3 pyramidal cell layer, entorhinal cortex, and lateral septum, with relative sparing of the dentate gyrus granule cell layer and hilus, as judged by toluidine blue, Fluoro-Jade B, and terminal dUTP nick end labeling staining. In contrast, seizure-related neurodegeneration in heterozygous littermates was limited to the CA3 region of the hippocampus. NeuN and calbindin staining revealed a selective decrease in the number and density of NeuN-positive neurons in the pyramidal layers of degenerating regions in both heterozygous and DNA-PKcs null mice. To elucidate the mechanisms leading to cell death, we examined an involvement of the p53 pathway, known to be induced by DNA damage. Addition of pifithrin-alpha, a p53 inhibitor, or expression of a dominant-negative p53 rescued neurons from kainate-induced excitotoxic cell death in primary cortical cultures derived from wildtype, DNA-PKcs heterozygous, or DNA-PKcs null neonatal mice. Moreover, pifithrin-alpha prevented kainate-induced loss of mitochondrial membrane potential, dendrite degeneration, and cell death. Results suggest that NHEJ plays a neuroprotective role in excitotoxicity, within the perforant, Schaffer collateral, hippocampal-septal, and temperoammonic pathways, in part by repairing DNA damage that would otherwise result in activation of a p53-dependent apoptotic cascade.

Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintainance of epilepsy

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury (central nervous system [CNS] insult), (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels [Ca(2+)](i) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but they share a common molecular mechanism for producing brain damage-an increase in extracellular glutamate concentration that causes increased intracellular neuronal calcium, leading to neuronal injury and/or death. Neurons that survive the injury induced by glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Novel treatment of excitotoxicity: targeted disruption of intracellular signalling from glutamate receptors

Glutamate signalling plays key physiological roles in excitatory neurotransmission and CNS plasticity, but also mediates excitotoxicity, the process responsible for triggering neurodegeneration through glutamate receptor overactivation. Excitotoxicity is thought to be a key neurotoxic mechanism in neurological disorders, including brain ischemia, CNS trauma and epilepsy. However, treating excitotoxicity using glutamate receptor antagonists has not proven clinically viable, necessitating more sophisticated approaches. Increasing knowledge of the composition of the postsynaptic density at glutamatergic synapses has allowed us to extend our understanding of the molecular mechanisms of excitotoxicity and to dissect out the distinct signalling pathways responsible for excitotoxic damage. Key molecules in these pathways are physically linked to the cytoplasmic face of glutamate receptors by scaffolding proteins that exhibit binding specificity for some receptors over others. This imparts specificity to physiological and pathological glutamatergic signalling. Recently, we have capitalized on this knowledge and, using targeted peptides to selectively disrupt intracellular interactions linked to glutamate receptors, have blocked excitotoxic signalling in neurones. This therapeutic approach circumvents the negative consequences of blocking glutamate receptors, and may be a practical strategy for treating neurological disorders that involve excitotoxicity.

Zinc-induced inhibition of protein synthesis and reduction of connexin-43 expression and intercellular communication in mouse cortical astrocytes

Zinc released from a subpopulation of glutamatergic synapses, mainly localized in the cerebral cortex and the hippocampus, facilitates or reduces glutamatergic transmission by acting on neuronal AMPA and NMDA receptors, respectively. However, neurons are not the only targets of zinc. In the present study, we provide evidence that zinc inhibits protein synthesis in cultured astrocytes from the cerebral cortex of embryonic mice. This inhibition, which reached 85% in the presence of 100 micro m zinc, was partially and slowly reversible and resulted from the successive inhibition of the elongation and the initiation steps of the protein translation process. This was assessed by measuring the phosphorylation level of the elongation factor eEF-2 and of the alpha subunit of the initiation factor eIF-2. Due to the rapid turnover of connexin-43 that forms junction channels in cultured astrocytes, the zinc-induced decrease of protein synthesis led to a partial disappearance of connexin-43, which was associated with an inhibition of the cellular coupling in the astrocytic syncitium. In conclusion, zinc not only inhibits protein synthesis in neurons, as previously demonstrated, but also in astrocytes. The resulting decrease in the intercellular communication between astrocytes should alter the function of surrounding neurons as well as their survival.

A comparison of some metabolic effects of N-methylaspartate stereoisomers, glutamate and depolarization: a multinuclear MRS study

Exposure of guinea pig brain slices to low concentrations (10 microM) of NMDA caused decreases in PCr and ATP within 30 min, with a slower decrease in NAA and increase in lactate, both detectable after 1 h. Exposure to NMDA for over 1 h or at higher concentrations caused further increases in lactate and decreases in NAA, with no further change in PCr or ATP. The L-isomer, NMLA, and the racemic mixture, NMDLA, caused similar changes in lactate and NAA, but both produced greater decreases in the energy state than NMDA, similar to those caused by prolonged exposure to glutamate. MK-801 prevented the changes in the energy state caused by NMDA, but not those caused by NMLA or by glutamate. The results are compared to previous studies on depolarization and discussed in terms of the role of the NMDA sub-type of glutamate receptor in the excitotoxic hypothesis of neuronal degeneration.

Delirium

BACKGROUND

Delirium is a serious and often undetected neuropsychiatric syndrome. Failure to recognize and manage delirium can lead to longer hospital stays and increased morbidity and mortality, especially among the elderly.

REVIEW SUMMARY

This article reviews definitions and diagnosis. The Diagnostic and Statistical Manual of Mental Disorders, 4th edition, and the International Statistical Classification of Diseases and Related Health Problems, 10th edition, criteria are quite similar in their diagnostic criteria. Risk factors include advanced age, preexisting brain disease or cognitive impairment, multiple medications, and severe medical problems. Delirium in the elderly can be more subtle and recovery more prolonged. Diagnosis is more complex if there is already an underlying dementia. An organized approach should be used to discover etiology and in ordering appropriate laboratory studies. At the cellular level, delirium is considered to be a reversible disregulation of neuronal membrane function. This involves a selective vulnerability of certain populations of neurons and neurotransmitter dysfunction. Practical treatment issues are reviewed.

CONCLUSIONS

Despite advances, delirium is usually still diagnosed at the bedside. Having an organized approach to diagnosis and understanding the underlying pathophysiology should help with overall evaluation and treatment.

Reversible and irreversible damage to cochlear afferent neurons by kainic acid excitotoxicity

Kainic acid (KA) selectively damages afferent synapses that innervate, in chickens, mainly tall hair cells. To better understand the nature of KA-induced excitotoxic damage to the cochlear afferent neurons, KA, at two different concentrations (0.3 or 5 mM), was injected directly into the inner ear of adult chickens. Pathologic changes in the afferent nerve ending and cell body were evaluated with light and transmission electron microscopy at various time points after KA application. The compound action potential (CAP) and cochlear microphonic (CM) potential were recorded to monitor the physiologic status of the afferent neurons and hair cells, respectively. Hair cell morphology and function were essentially normal after KA treatment. However, afferent synapses beneath tall hair cells were swollen within 30 minutes after KA at both low (KA-L) and high (KA-H) doses. In the KA-L group, the swelling disappeared within 1 day and the morphology of the postsynaptic region returned to near normal condition. In the KA-H group, by contrast, the vacant region beneath tall hair cells remained evident even 20 weeks after KA. The number of cochlear ganglion neurons in the KA-H group decreased progressively from 1 to 8-20 weeks, whereas hair cells in the basilar papilla remained morphologically intact out to 20 weeks after KA. There was no significant change in neuron number in the KA-L group. Temporal changes in the CAP amplitude paralleled the anatomic changes, although the CAP only partially recovered. These results suggest that KA induces partially reversible damage to cochlear afferent neurons with low KA concentration; above this level, KA triggers irreversible, progressive neurodegeneration.

Zinc-enriched (ZEN) terminals in mouse olfactory bulb

The present study was designed to localize zinc-enriched (ZEN) terminals in mouse olfactory bulb by means of ZnT3 immunocytochemistry (ICC) and zinc autometallography (AMG). The immunocytochemical staining of ZnT3 was closely correlated with the AMG pattern. ZEN terminals were defined as terminals showing both ZnT3 immunoreactivities and AMG granules. At the light microscopic level, dense staining patterns for ZnT3 immunoreactivity were seen in the granule cell layer and the olfactory glomerular layer. At the ultrastructural level, ZEN terminals were restricted to presynaptic terminals with single or multiple postsynaptic thickenings. The postsynaptic profiles contacting ZEN terminals appeared to be dendrites or somata of granule cells in the granule cell layer and periglomerular cells and mitral/tufted (M/T) cells in the olfactory glomerular layer. This suggests that two main sources of ZEN terminals are present in mouse olfactory bulb: (1) centrifugal fibres making asymmetrical synapses with granule cells and periglomerular cells, and (2) olfactory receptor terminals contacting dendritic profiles of M/T cells or periglomerular cells. The close correlation between ZEN terminals and the glutamatergic system is discussed.

Magnetic resonance spectroscopy studies on changes in cerebral calcium and zinc and the energy state caused by excitotoxic amino acids

Under control conditions, superfused hippocampal slices exhibited a significantly higher phosphocreatine (PCr)/ATP ratio than cortical slices; the evidence suggests that this is due to lower concentrations of ATP, rather than higher concentrations of PCr. Glutamate caused relatively rapid decreases in PCr and ATP levels to approximately 45%, accompanied or immediately followed by an increased free intracellular calcium concentration ([Ca2+]i) and the release of Zn2+ in the cortex. In the hippocampus PCr and ATP decreased further to approximately 20% of control values, but the changes in [Ca2+]i and Zn2+ content were slower. This is in contrast to the effects of depolarisation, which produced the same rapid changes in the energy state and [Ca2+]i, with no detectable Zn2+, in both tissues. NMDA causes effects similar to those of glutamate in the cortex (decreases in the energy state, increased [Ca2+]i, and release of Zn2+). Pretreatment of the cortex for 1 h with the NMDA blocker MK-801 prevented all of the observed effects of NMDA. In contrast, pretreatment with MK-801 had no detectable effect on the increase in [Ca2+]i or the decreases in PCr and ATP caused by glutamate, although it prevented the release of zinc. The results are discussed in relation to the function of the NMDA subtype of glutamate receptor in excitotoxicity.

Zn2+ entry produces oxidative neuronal necrosis in cortical cell cultures

Evidence has accumulated that Zn2+ plays a central role in neurodegenerative processes following brain injuries including ischaemia or epilepsy. In the present study, we examined patterns and possible mechanisms of Zn2+ neurotoxicity. Inclusion of 30-300 microM Zn2+ for 30 min caused neuronal necrosis apparent by cell body and mitochondrial swelling in cortical cell cultures. This Zn2+ neurotoxicity was not attenuated by antiapoptosis agents, inhibitors of protein synthesis or caspase. Blockade of glutamate receptors or nitric oxide synthase showed no beneficial effect against Zn2+ neurotoxicity. Interestingly, antioxidants, trolox or SKF38393, attenuated Zn(2+)-induced neuronal necrosis. Pretreatment with insulin or brain-derived neurotrophic factor increased the Zn(2+)-induced free radical injury. Kainate or AMPA facilitated Zn2+ entry and potentiated Zn2+ neurotoxicity in a way sensitive to trolox. Reactive oxygen species and lipid peroxidation were generated in the early phase of Zn2+ neurotoxicity. These findings indicate that entry and accumulation of Zn2+ result in generation of toxic free radicals and then cause necrotic neuronal degeneration under certain pathological conditions in the brain.

Phosphorus-31 magnetic resonance spectroscopy studies of pig spinal cord injury. Myelin changes, intracellular pH, and bioenergetics

RATIONALE AND OBJECTIVES

Phosphorus-31 (31P) nuclear magnetic resonance (NMR) spectroscopy was used to monitor changes in phosphocreatine (PCr), adenosine triphosphate (ATP), inorganic phosphate (Pi), intracellular pH (pHi), and free magnesium in the in vivo pig spinal cord after injury.

METHODS

Phosphorus-31 NMR spectra were acquired from healthy (n = 4) and injured pig spinal cords (n = 8) under in vivo conditions using a 4.7-tesla spectrometer. Spinal cords were injured by dropping a 20-g weight from 20 cm onto the surgically exposed cord surface.

RESULTS

In vivo spectra of injured cords revealed a reduction in ATP, PCr, pHi, and an increase in Pi. In addition, a broad resonance that is likely to arise from myelin phospholipids was reduced significantly after injury.

CONCLUSIONS

Phosphorus-31 NMR can be used to follow in vivo changes in high energy phosphates after injury and may have the potential to follow changes in myelin structure. This technique may prove important in the study of myelin breakdown after secondary, nonreversible spinal cord injury. Changes in high energy phosphates and pHi did not seem to parallel these putative changes in myelin structure.

In vitro effects of arachidonic and L-glutamic acids on the high-affinity choline transport in rat hippocampus

A second messenger role for arachidonic acid (AA) in the regulation of the high-affinity choline uptake (HACU) was suggested. It was reported that micromolar concentrations of AA applied in vitro decreased the HACU values and increased the specific binding of [3H]hemicholinium-3 ([3H]HCh-3). It was published that L-glutamic acid (GA) applied in vivo produced a fall in the HACU values. In addition, GA liberates free AA. In this study, an ability of GA to influence in vitro the activity of presynaptic cholinergic nerve terminals via its effect on the release of AA is investigated in hippocampal synaptosomes of young Wistar rats. Millimolar concentrations of GA decrease both the high- and low-affinity choline uptake, the specific as well as nonspecific binding of [3H]HCh-3 and the activity of Na+, K(+)-ATPase. Kinetic analysis (Lineweaver-Burk and Scatchard plots) reveals a change in Vmax and Bmax, but not in KM and KD. It appears very likely that under normal conditions GA applied in vitro is not able to change markedly the choline transport via its effect on the release of AA. Results confirm the hypothesis about an indirect inhibitory role for glutamatergic receptors on cholinergic cells.

Dose-dependent effects of acute in vivo ethanol exposure on extracellular glutamate concentration in the cerebral cortex of the near-term fetal sheep

The cerebral cortex is a target site of ethanol teratogenesis. L-Glutamate is a major excitatory neurotransmitter that plays an important neurotrophic role in brain development. It has been proposed that optimal function of the glutamate neuronal system is required for normal brain development; overactivation could lead to excitotoxic-induced neuronal injury, whereas underactivation could delay/restrict brain development. The objective of this study was to test the hypothesis that acute in vivo ethanol exposure alters basal glutamate release in the fetal cerebral cortex. The experimental approach involved measuring fetal cortical extracellular glutamate concentration using the technique of in vivo microdialysis. Near-term fetal sheep were chronically instrumented with a microdialysis probe placed in the parasagittal cortex. At 124 +/- 3 days of gestation, the effects of maternal intravenous infusion of 2 g or 4 g ethanol/kg maternal body weight or an equivalent volume of saline, given as four equally divided doses over 5 hr, on fetal cerebral cortical extracellular glutamate concentration were determined. None of the three treatment regimens produced fetal or maternal demise during the time course of the study. There was an ethanol dose-dependent increase, p = 0.005, in extracellular glutamate concentration in the fetal cerebral cortex. This increase was paroxysmal in nature and was not directly related to the fetal blood ethanol concentration. In view of the proposed role for glutamate in neuronal development, this apparent ethanol-induced increase in glutamate release may be important in the pathogenesis of ethanol teratogenesis involving the cerebral cortex.

High-field MRS studies in brain slices

We are applying multi-nuclear high-field (500 MHz) MR spectroscopy of metabolising whole tissue preparations of the mammalian brain to studies on individual components of convulsions, which include prolonged depolarization, metabolic deprivation, and the effects of excitotoxins. The responses of glial cells and neurones can be partially distinguished by following labelling patterns of metabolic intermediates from 13C-labelled glucose or acetate (which enters only glial cells). This approach clearly confirmed our earlier indications that the metabolic response to depolarization (40 mM extracellular K+) occurs essentially in glial cells. Some evidence for metabolic shuttling between glia and neurones was obtained from the changes in C3/C4 ratios of glutamate and glutamine, and the C2/C3 of GABA. Mechanisms for metabolic support of neurones by glia may be of importance in neuronal protection under such metabolic stress as occurs in epilepsy. Changes in free intracellular divalent cations ([Ca2+]i and [Zn2+]i) were monitored using the 19F-MRS indicator, 5FBAPTA. Large increases in [Ca2+]i and decreases in PCr were produced by excitotoxins (glutamate and NMDA), depolarization or ischemia, but intracellular Zn2+ appeared only after exposure to the excitotoxins. The NMDA receptor blocker, MK801, removed all of the responses to NMDA, but only prevented the appearance of Zn2+ observed with glutamate. These results indicate that the damage caused to neurones by such insults as convulsions is not due simply to the presence of excessive excitotoxic glutamate.

Excitotoxic amino acids cause appearance of magnetic resonance spectroscopy-observable zinc in superfused cortical slices

(1) The effects of glutamate and NMDA on the free intracellular calcium concentration ([Ca2+]i) have been followed in superfused cortical slices using the 19F-magnetic resonance indicator 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBAPTA). (2) Glutamate (0.5 or 1 mM) caused a 75-100% increase in [Ca2+]i, and a new resonance was attributed to zinc-5FBAPTA, which was confirmed from its disappearance in the presence of a high-affinity chelator of heavy metals, N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine. The appearance of zinc occurred with or just after the rise in [Ca2+]i and was independent of Mg2+. (3) NMDA, N-methyl-DL-aspartate, or N-methyl-L-aspartate (10-200 microM) caused a slower increase in [Ca2+]i, and zinc was observed in some but not all experiments. When present, zinc appeared later than the increase in [Ca2+]i. These changes were also independent of Mg2+. (4) Decreases in both phosphocreatine and ATP were observed in all of these studies. (5) The results are discussed in terms of the proposed role of zinc as a modulator of excitotoxicity. Observations of zinc after exposure to glutamate or more slowly to NMDA, but not after depolarisation or deprivation of glucose and O2 (where increases also occur in [Ca2+]i), suggest that the cellular damage caused by the latter insults (depolarisation and fuel deprivation as in ischaemia) involves mechanisms not solely attributable to release of excitotoxins.

Effects of hypoxia on oligodendrocyte signal transduction

We have previously established that 21-day-old postnatal rat oligodendrocytes, maintained in monolayer culture and subjected to 6 h of hypoxia, show reversible inhibition of synthesis of alpha-hydroxy fatty acid and myelin basic protein but a dramatic induction of a 22-kDa protein, suggesting that this is a good model to study the mechanism of CNS demyelination caused by hypoxic injury. We now report that hypoxia also dramatically inhibits the basal protein kinase C-mediated phosphorylation of myelin basic protein and myelin 2',3'-cyclic nucleotide phosphohydrolase by 80%, but that the inhibition of phosphorylation can be reversed by addition of a protein kinase C activator, phorbol 12-myristate 13-acetate. The mechanism of action appears to involve the uncoupling of signal transduction at a site before phospholipase C, because hypoxia did not affect protein kinase C activity or its translocation to the membrane fraction. The most potent activator of phospholipase C (as measured by inositol phosphate release) was carbachol (muscarinic M1 receptor agonist), followed by L-phenylephrine (alpha 1-adrenergic receptor agonist) in normal oligodendrocytes. Excitatory amino acids and histamine were ineffective. Hypoxia for 6 h completely inhibited both muscarinic and alpha 1-adrenergic receptor-mediated inositol monophosphate release but did not affect phospholipase D-coupled phosphatidylethanol production in response to carbachol. We therefore conclude from this and earlier work that early, reversible changes in oligodendrocyte metabolism result not simply from ATP depletion, but may specifically target GTP binding protein-mediated processes.

The mechanism of cerebral hypoxic-ischemic damage

The four most prominent hypotheses on the cellular processes leading to hypoxic-ischemic neuronal damage or death are (1) the lactacidosis hypothesis, (2) the calcium overload hypothesis, (3) the excitotoxic hypothesis, and (4) the oxygen-free radical hypothesis. The authors comment on the evidence in favor of and against each in an attempt to select the one hypothesis that best explains the mechanism of cerebral hypoxic-ischemic damage while withstanding the scrutiny of scientific testing. A major part of this inquiry is derived from in vitro studies that are suited to mechanistic exploration. They conclude that the calcium overload hypothesis is the best qualified in this respect. It is important to note, however, that some of the other hypothetical mechanisms may play a secondary role in exacerbating neuronal damage by accelerating calcium influx and overload.

Does modulation of glutamatergic function represent a viable therapeutic strategy in Alzheimer's disease?

Although glutamate dysfunction has been implicated in the pathogenesis of Alzheimer's disease (AD), it is unclear which direction a glutamatergic strategy should take in this illness. Increasing glutamate function may enhance excitotoxicity and neuronal death, whereas decreasing activity in this excitatory amino acid pathway may impair memory processes. Pharmacological modulation of the different NMDA and nonNMDA receptor sites, together with the concept of an agonist versus antagonist approach, are discussed in this review. It would appear that a glutamatergic approach may represent a new and exciting option to pursue in the experimental pharmacotherapeutics of AD.