Excitotoxins: Possible Mechanisms of Action

REVIEW ■ : Neuroprotection in Brain Ischemia: An Update (Part I

Ischemic brain injury produced by stroke or cardiac arrest is a major cause of human neurological disability. Steady advances in the neurosciences have elucidated pathophysiological mechanisms of brain ischemia and have suggested many therapeutic approaches directed at specific injury mechanisms to achieve neuroprotection of the acutely ischemic brain. The first portion of this two-part review highlights the differentiating features and pathological mechanisms of focal and global cerebral ischemic injury and summarizes a wealth of recent evidence as to how antagonism of excitatory amino acid neurotoxicity, mediated via NMDA as well as non-NMDA receptors, may offer a means of diminishing the extent of ischemic injury. The Neuroscientist 1:95-103, 1995

Pathogenic Mechanisms in Sporadic Amyotrophic Lateral Sclerosis

In recognition of the 100th anniversary of Charcot’s death we have reviewed possible pathogenic mechanisms in amyotrophic lateral sclerosis (ALS). Advances in the last 5 years in molecular biology and genetics have identified mutations in the cytosolic dismutase (SODI) gene in some patients with familial ALS raising the possibility that oxidative stress may be involved in the pathogenesis. An excitotoxic pathogenesis has been implicated based on elevated plasma and CSF levels of amino acids and altered contents of amino acids in the nervous system of ALS patients and changes in the number of excitatory amino acid receptors. ALS sera containing antibodies to L-type calcium channels and the development of immune mediated lower and upper and lower motor neuron models have revitalized research efforts focusing on an immune basis for ALS. Other pathogenic mechanisms which have been the subject of recent research include elemental toxicity, apoptosis and programmed cell death and possibly a deficiency or abnormality in growth factors. Pathogenic processes for ALS must account for an increasing incidence of ALS, male preponderance, and the selective vulnerability of the corticomotoneuronal system.

Resuscitative mild hypothermia as a protective tool in brain damage: is there evidence?

Resuscitative mild hypothermia is and will increasingly be used in the emergency department as protection for the brain after an ischaemic insult. The clinical application of resuscitative mild hypothermia and its limitations will be summarized in this paper. The evidence for each application and its underlying mechanism will also be reviewed.

Roles of mitochondria in health and disease

Mitochondria play a central role in cell life and cell death. An increasing number of studies place mitochondrial dysfunction at the heart of disease, most notably in the heart and the central nervous system. In this article, I review some of the key features of mitochondrial biology and focus on the pathways of mitochondrial calcium accumulation. Substantial evidence now suggests that the accumulation of calcium into mitochondria may play a key role as a trigger to mitochondrial pathology, especially when that calcium uptake is accompanied by another stressor, in particular nitrosative or oxidative stress. The major process involved is the opening of the mitochondrial permeability transition pore, a large conductance pore that causes a collapse of the mitochondrial membrane potential, leading to ATP depletion and necrotic cell death or to cytochrome c release and apoptosis, depending on the rate of ATP consumption. I discuss two models in particular in which these processes have been characterized. The first is a model of oxidative stress in cardiomyocytes, in which reperfusion after ischemia causes mitochondrial calcium overload, and oxidative stress. Recent experiments suggest that cardioprotection by hypoxic preconditioning or exposure to the ATP-dependent K(+) channel opener diazoxide increases mitochondrial resistance to oxidative injury. In a second model, of calcium overload in neurons, the neurotoxicity of glutamate depends on mitochondrial calcium uptake, but the toxicity to mitochondria also requires the generation of nitric oxide. Glutamate toxicity after activation of N-methyl-D-aspartate (NMDA) receptors results from the colocalization of NMDA receptors with neuronal nitric oxide synthase (nNOS). The calcium increase mediated by NMDA receptor activation is thus associated with nitric oxide generation, and the combination leads to the collapse of mitochondrial membrane potential followed by cell death.

Alkaloid fraction of Uncaria rhynchophylla protects against N-methyl-D-aspartate-induced apoptosis in rat hippocampal slices

Uncaria rhynchophylla is a medicinal herb which has sedative and anticonvulsive effects and has been applied in the treatment of epilepsy in Oriental medicine. In this study, the effect of alkaloid fraction of U. rhynchophylla against N-methyl-D-aspartate (NMDA)-induced neuronal cell death was investigated. Pretreatment with an alkaloid fraction of U. rhynchophylla for 1 h decreased the degree of neuronal damage induced by NMDA exposure in cultured hippocampal slices and also inhibited NMDA-induced enhanced expressions of apoptosis-related genes such as c-jun, p53, and bax. In the present study, the alkaloid fraction of U. rhynchophylla was shown to have a protective property against NMDA-induced cytotoxicity by suppressing the NMDA-induced apoptosis in rat hippocampal slices.

Genetic basis of kainate-induced excitotoxicity in mice: phenotypic modulation of seizure-induced cell death

Excitotoxicity, a process in which excessive excitation of glutamate receptors results in cell death, has been implicated in a number of neurological disorders. However, the genetic characteristics and molecular mechanisms that can modulate the extent of cell death are unclear. Previously, we had reported that the extent of excitotoxic cell death is conferred by differences in the genetic background of several mouse strains. As a first step in the identification of loci that can modulate the extent of excitotoxin-induced cell death, we tested C57BL/6 and FVB/N mice, their F1 hybrids and backcross progeny for differences in apparent excitotoxic cell death induced by kainic acid (KA). While no strain dependent differences in seizure duration were observed, phenotypic analysis of cell death indicated that C57BL/6 mice showed no seizure-induced cell death, while FVB/N mice exhibited extensive cell death. Studies of seizure-induced cell death in hybrid and backcross progeny revealed an association between seizure-induced cell death and genotype. Mice from the F1 cross exhibited little to no seizure-induced cell death, indicative that the extent of cell death is conferred as a dominant genetic trait. Phenotypic assessment of cell death in backcross progeny suggests that differences in apparent cell death are conferred by a single gene locus. These findings implicate genetic factors in individual differences in excitotoxin-induced cell death.

Protection provided by cyclosporin A against excitotoxic neuronal death is genotype dependent

PURPOSE

Previous studies have shown that the immunosuppressant cyclosporin A (CsA), a specific blocker of the mitochondrial permeability transition (MPT) pore, can dramatically ameliorate the selective neuronal necrosis resulting from ischemia-reperfusion, traumatic brain injury, and N-methyl-d-aspartate (NMDA)-evoked neurotoxicity. The purpose of this study was to determine whether two different immunosuppressants, CsA and FK-506, could ameliorate the neuronal damage observed after kainate-induced seizures in strains that are differentially susceptible to excitotoxin-induced cell death.

METHODS

Excitotoxin-resistant (C57BL/6) or -susceptible (FVB/N) mice were administered kainate alone (30 mg/kg), CsA alone (5, 10, or 20 mg/kg), or one of the immunosuppressants (CsA, 5 mg/kg or 10 mg/kg; FK-506, 0.5 mg/kg) followed by kainate. After drug administration, mice were monitored continuously for the onset and extent of seizure activity. After a survival of 7 days, animals were assessed for hippocampal damage.

RESULTS

Whereas CsA alone induced no epileptogenic effects and both immunosuppressants were without effect on the induction of kainate-induced seizures, administration of CsA to excitotoxin-susceptible mice (FVB/N) virtually eliminated neuronal cell death. In contrast, induction of neuronal cell death was evident when CsA was administered to excitotoxin-resistant mice (C57BL/6). Administration of FK-506, another commonly used immunosuppressant, which lacks an effect on the MPT, had no effect on modification of susceptibility to kainate-induced cell death in either strain.

CONCLUSIONS

As our data show differential protection of hippocampal neurons against excitotoxic cell death by pretreatment with CsA, these results suggest that strain-dependent differences in mitochondrial integrity and function may exist.

Why do neuroprotective drugs work in animals but not humans?

Many neuroprotective agents that seemed promising in animal studies of ischemic brain injury prove to have no effect when tested in clinical trials, suggesting that fundamental elements of translational research require better definition. A number of modifications have led to improvements in preclinical and human studies since the earliest controlled trials failed to confirm hypotheses suggested by animal data. Continued re-evaluation and sharing of information derived from the laboratory bench or the patient's bedside should eventually lead to effective neuroprotection in acute stroke. Experimental data should be carefully studied to improve the quality of agents coming to clinical trials and to design trial phasing that effectively determines drug safety and efficacy. This article will examine preclinical modeling and its translation to prospective studies of acute stroke therapy and will focus on some potential solutions directed at clinical trial design.

Neuroprotection and the ischemic cascade

Brain ischemia is a process of delayed neuronal cell death, not an instantaneous event. The concept of neuroprotection is based on this principle. Diminished cerebral blood flow initiates a series of events (the "ischemic cascade") that lead to cell destruction. This ischemic cascade is akin to a spreading epidemic starting from a hypothesized core of ischemia and radiating outward. If intervention occurs early, the process may be halted. Interventions have been directed toward salvaging the ischemic penumbra. Hypothermia decreases the size of the ischemic insult in both anecdotal clinical and laboratory reports. In addition, a wide variety of agents have been shown to reduce infarct volume in animal models. Pharmacologic interventions that involve thrombolysis, calcium channel blockade, and cell membrane receptor antagonism have been studied and have been found to be beneficial in animal cortical stroke models. Human trials of neuroprotective therapies have been disappointing. Other than thrombolytics, no agents have shown an unequivocal benefit. The future of neuroprotection will require a logical extension of what has been learned in the laboratory and previous human trials. A sensible approach to the use of multiple-agent cocktails used in combination with thrombolytics is likely to offer the highest chance for benefit.

Disparity of cell swelling and rapid neuronal death by excitotoxic insults in rat hippocampal slice cultures

The rapidly (< 1 h) developing neuronal death induced by a 15-min-exposure to N-methyl-D-aspartate (NMDA) in rat hippocampal slice cultures is associated with cell swelling. We examined whether the swelling directly leads to neuronal death. The rapid neuronal death assayed by propidium iodide was Cl(-)-dependent, as reported for the cell swelling. However, the dose-dependence for NMDA-induced neuronal death differed from that for the cell swelling. Also, cell swelling alone induced by hypotonic insults led to neuronal death only when the cell size increased far more than the extent achieved by NMDA insults. Moreover, contrary to the previous notion, the rapid neuronal death was Ca2+-dependent. Thus, the primary cause of the rapid neuronal death induced by NMDA cannot be attributed to cell swelling.

Interpretation of intrinsic optical signals and calcein fluorescence during acute excitotoxic insult in the hippocampal slice

Immediate (acute) neuronal damage in response to overstimulation of glutamate receptors results from toxic exposure to food poisons acting as glutamate analogues. Glutamate agonist application evokes dramatic intrinsic optical signals (IOSs) in the rat hippocampal slice preparation, particularly in the CA1 region. Theoretically IOSs are generated by alterations to neuronal and glial structure that change light transmittance (LT) in live brain tissue. To better understand such signals, IOSs evoked by the glutamate agonist N-methyl-D-aspartate were imaged in the rat hippocampal slice. We correlated these excitotoxic signals with: (1) biophysical principles governing light transport, (2) tissue volume changes as measured using a free intracellular fluorophore (calcein), (3) dendritic morphology visualized by dye injection, and (4) standard histopathology. In theory LT elevation evoked during acute excitotoxic swelling is generated by change to subcellular structure that reduces light scattering during cell swelling. However, in responsive dendritic regions, initial LT elevation caused by cell swelling was overridden by the formation of dendritic beads, a conformation that increased light scattering (thereby reducing LT) even as the calcein signal demonstrated that the tissue continued to swell. Thus IOS imaging reveals acute somatic and dendritic damage during excitotoxic stress that can be monitored across slices of brain tissue in real time.

Novel glycineB antagonists show neuroprotective activity in vivo

The degeneration or dysfunction of cholinergic neurons within the basal forebrain of patients with Alzheimer's disease (AD) may be related to the vulnerability of these cells to endogenous glutamate (Beal, 1995; Greenamyre and Young, 1989). The administration of drugs that attenuate the toxic actions of glutamate in the early stages of the disease might significantly delay its rate of progression. Two approaches to neuroprotection from endogenous glutamatergic function were investigated and found to be effective: blockade of voltage-dependent, NMDA-type glutamate receptor channels and antagonism of an NMDA-receptor related glycineB modulatory site.

Dopaminergic modulation of early signs of excitotoxicity in visualized rat neostriatal neurons

Cell swelling induced by activation of excitatory amino acid receptors is presumably the first step in a toxic cascade that may ultimately lead to cell death. Previously we showed that bath application of N-methyl-D-aspartate (NMDA) or kainate (KA) produces swelling of neostriatal cells. The present experiments examined modulation of NMDA and KA-induced cell swelling by dopamine (DA) and its receptor agonists. Nomarski optics and infra-red videomicroscopy were utilized to visualize neostriatal medium-sized neurons in thick slices from rat pups (12-18 postnatal days). Increase in somatic cross-sectional area served as the indicator of swelling induced by bath application of glutamate receptor agonists. NMDA induced cell swelling in a dose-dependent manner. Activation of DA receptors in the absence of NMDA did not produce swelling. DA and the D1 receptor agonist SKF 38393, increased the magnitude of swelling produced by NMDA. This effect was reduced in the presence of the D1 receptor antagonist, SCH 23390. In contrast, activation of D2 receptors by quinpirole decreased the magnitude of NMDA-induced cell swelling. DA slightly attenuated cell swelling induced by activation of KA receptors. Quinpirole produced a significant concentration-dependent reduction in KA-induced swelling while SKF38393 increased KA-induced swelling, but only at a low concentration of KA. Together, these results provide additional support for the hypothesis that the direction of DA modulation depends on the glutamate receptor subtype, as well as the DA receptor subtype activated. One possible consequence of these observations is that endogenous DA may be an important contributing factor in the mechanisms of cell death in Huntington's disease.

Long-term changes of ionotropic glutamate and GABA receptors after unilateral permanent focal cerebral ischemia in the mouse brain

Long-term hyperexcitability was found after unilateral, permanent middle cerebral artery occlusion in exofocal neocortical areas of the adult mouse [Mittmann et al. (1998) Neuroscience 85, 15-27]. The aim of the present study was to test the hypothesis in an identical paradigm of ischemia. whether alterations in the densities of both excitatory and inhibitory amino acid receptors may underlie these pathophysiological changes. Alterations in densities of [3H]dizocilpine, [3H]D,L-amino-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, [3H]kainate and [3H]muscimol binding sites were demonstrated with quantitative in vitro receptor autoradiography. All binding sites were severely reduced in the core of the ischemic lesion. A completely different reaction was found in the exofocal, histologically inconspicuous parts of the somatosensory cortex and the more remote neocortical areas of both hemispheres. The [3H]muscimol binding sites were significantly reduced four weeks after ischemia in the motor cortex, hindlimb representation area and exofocal parts of the primary and secondary somatosensory cortices of both hemispheres. The focus of the reduction in [3H]muscimol binding sites was found in lower layer V and upper layer VI. Contrastingly, the densities of [3H]dizocilpine binding sites were found to be increased in these areas, whereas those of [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and [3H]kainate binding sites did not show significant changes. The [3H]dizocilpine binding site density increased predominantly in layers III and IV. All binding sites were also reduced in the retrogradely reacting, gliotic part of the ipsilateral ventroposterior thalamic nucleus, whereas the [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites were increased in the surround of the ipsilateral nucleus and no changes in binding sites were seen in the whole contralateral nucleus. We conclude that permanent local ischemia leads to a long-term and widespread impairment of the normal balance between binding sites of excitatory and inhibitory neurotransmitter receptors in neocortical areas far away from the focus of the post-ischemic tissue damage. The imbalance comprises an up-regulation of the [3H]dizocilpine binding sites in the ion channels of N-methyl-D-aspartate receptors and a down-regulation of [3H]muscimol binding sites of the GABA(A) receptors in the ipsi- and contralateral neocortex. These changes at the receptor level explain the previously observed hyperexcitability with the appearance of epileptiform field potentials and the long duration of excitatory postsynaptic potentials four weeks after ischemia.

Activity-dependent regulation of [Ca2+]i in avian cochlear nucleus neurons: roles of protein kinases A and C and relation to cell death

Neurons of the cochlear nucleus, nucleus magnocellularis (NM), of young chicks require excitatory afferent input from the eighth nerve for maintenance and survival. One of the earliest changes seen in NM neurons after deafferentation is an increase in intracellular calcium concentration ([Ca2+]i). This increase in [Ca2+]i is due to loss of activation of metabotropic glutamate receptors (mGluR) that activate second-messenger cascades involved in [Ca2+]i regulation. Because mGluRs are known to act via the phospholipase C and adenylate cyclase signal transduction pathways, the goal of this study was to determine the roles of protein kinases A (PKA) and C (PKC) activities in the regulation of NM neuron [Ca2+]i by eighth nerve stimulation. Additionally, we sought to determine the relationship between increased [Ca2+]i and cell death as measured by propidium iodide incorporation. [Ca2+]i of individual NM neurons in brain stem slices was monitored using fura-2 ratiometric fluorescence imaging. NM field potentials were monitored in experiments in which the eighth nerve was stimulated. Five hertz orthodromic stimulation maintained NM neuron [Ca2+]i at approximately 110 nM for 180 min. In the absence of stimulation, NM neuron [Ca2+]i increased steadily to a mean of 265 nM by 120 min. This increase was attenuated by superfusion of PKC activators phorbol-12,13-myristate acetate (100 nM) or dioctanoylglycerol (50 microM) and by activators of PKA: 1 mM 8-bromoadenosine-3',5'-cyclophosphate sodium (8-Br-cAMP), 50 microM forskolin or 100 microM Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine. Inhibition of PKA (100 microM Rp-cAMPS) or PKC (50 nM bisindolymaleimide or 10 microM U73122) during continuous orthodromic stimulation resulted in an increase in NM neuron [Ca2+]i that exceeded 170 and 180 nM, respectively, by 120 min. Nonspecific kinase inhibition with 1 microM staurosporine during stimulation resulted in an [Ca2+]i increase that was greater in magnitude than that seen with either PKA or PKC inhibition alone, equal to that seen in the absence of stimulation, but much smaller than that seen with inhibition of mGluRs. In addition, manipulations that resulted in a [Ca2+]i increase >/=250 nM resulted in an increase in number and percentage of propidium iodide-labeled NM neurons. These results suggest that eighth nerve activity maintains [Ca2+]i of NM neurons at physiological levels in part via mGluR-mediated activation of PKA and PKC and that increases in [Ca2+]i due to activity deprivation or interruption of the PKA and PKC [Ca2+]i regulatory mechanisms are predictive of subsequent cell death.

3-Nitropropionic acid exacerbates N-methyl-D-aspartate toxicity in striatal culture by multiple mechanisms

We examined the effects of 3-nitropropionic acid-induced succinate dehydrogenase inhibition on neuronal ATP content, N-methyl-D-aspartate-induced neuronal death, resting membrane potential, and N-methyl-D-aspartate-induced changes in cytosolic calcium concentration ([Ca2+]c) in cultured rat striatal neurons. Exposure of cultures to 3 mM 3-nitropropionic acid for 3 h did not cause overt toxicity, but reduced ATP content by 35%. Treatment with 3-nitropropionic, or removal of Mg2+ from the medium, enhanced subsequent N-methyl-D-aspartate toxicity, reducing the LC50 from 250 microM to 12 microM or 30 microM, respectively. Even after Mg2+ removal, enhancement of N-methyl-D-aspartate toxicity by 3-nitropropionic acid remained pronounced, with the LC50 further decreasing to 3 microM. The mean resting membrane potential of neurons treated with 3-nitropropionic acid was -37 mV, while that in control neurons was -61 mV. Treatment with 3-nitropropionic did not affect baseline [Ca2+]c as determined by fura-2 microfluorimetry. N-methyl-D-aspartate (30 microM) caused a rapid rise in [Ca2+]c, the initial magnitude of which was not affected by 3-nitropropionic acid. However, after a 1-h treatment, [Ca2+]c was dramatically higher in 3-nitropropionic acid-treated neurons. This increased calcium load was washed out slowly and only partially, although calcium in control neurons washed out rapidly and almost completely. These results suggest that in striatal neurons, the enhancement of N-methyl-D-aspartate toxicity caused by succinate dehydrogenase inhibition may be due to synergism between partial relief of the Mg2+ blockade of the N-methyl-D-aspartate receptor and other mechanisms, including disruption of neuronal calcium regulation. This synergism may be relevant to the neuronal death observed in neurodegenerative disorders.

Neuroprotection by novel antagonists at the NMDA receptor channel and glycineB sites

Glutamate may act via an N-methyl-D-Aspartate (NMDA)-sensitive receptor site to destroy cholinergic neurons within the nucleus basalis magnocellularis in age-associated neurodegenerative diseases. Multiple interesting properties of the NMDA receptor are relevant to its excitotoxic actions, e.g., glutamate is ineffective unless a glycine (gly) modulatory site is also occupied. Thus, the antagonism of glutamate receptor-related toxicity by blockade of either the NMDA-sensitive recognition site or the gly binding site may therefore have therapeutic applications. The current study investigated the ability of four novel noncompetitive antagonists at these two sites: one NMDA open channel antagonist (MRZ 2/579: 1-amino-1,3,3,5,5-pentamethyl-cyclohexane hydrochloride), and three glyB receptor antagonists (MRZ 2/570: 8-bromo-4-hydroxy-1-oxo-1,2-dihydropyridaziono [4,5-beta] quinoline-5-oxide choline salt; MRZ 2/57: 8-fluoro-4-hydroxy-1-oxo-1,2-dihydropyridaziono [4,5-beta] quinoline-5-oxide choline; MRZ 2/576: 8-chloro-4-hydroxy-1-oxo-1,2-dihydropyridaziono [4,5-beta] quinoline-5-oxide choline) administered acutely, to provide neuroprotection from a NMDA receptor agonist within the nucleus basalis magnocellularis of young rats. Injection of NMDA into the nucleus basalis magnocellularis significantly decreased cortical choline acetyltransferase activity. Acute administration (i.p.) of MRZ 2/579, 2/570, 2/571 and 2/576 provided significant neuroprotection from NMDA.

Intrinsic optical signaling denoting neuronal damage in response to acute excitotoxic insult by domoic acid in the hippocampal slice

Using the seafood contaminant domoic acid (an AMPA/kainate receptor agonist), we demonstrate a distinct excitotoxic sequence of events leading to acute neuronal damage in the hippocampal slice as measured by (1) loss of the evoked CA1 field potential, (2) irreversible changes in light transmittance, (3) histopathology, and (4) lucifer yellow injection of single CA1 pyramidal neurons. Change in light transmittance (LT) through the submerged slice indirectly measures altered cell volume, both neuronal and glial. At 37 degrees C, a 1-min superfusion of 10 mu M domoate induced a prolonged reversible increase in LT, primarily in the dendritic regions of CA1 and dentate granule cells (GC), but not in the CA3 region. Spectral analysis (400-800 nm) revealed a wide-band transmittance increase, indicating cell swelling as a major source of the intrinsic signal. The evoked field potential recorded in the CA1 cell body region (PYR) was lost as LT peaked, but completely recovered upon return to the baseline LT level. Increasing domoate exposure to 10 min elicited a different and distinct LT sequence in CA1 and dentate regions. An initial LT increase in dendritic regions evolved in an irreversible decrease in LT. At the same time, LT irreversibly increased in cell body regions (CA1 PYR and GC) and the evoked field potential was irretrievably lost. Also, there was histological damage to cell body and dendritic regions of CA1 and granule cells. Injection of lucifer yellow into single CA1 neurons in slices displaying the irreversible LT sequence revealed extensive dendritic beading, whereas CA1 cells in control slices displayed a smoothly contoured arbor. Consistent with acute neuronal damage, the optical changes generated by domoate did not require extracellular Ca2+, and lowering the temperature protected the slice from irreversible damage to CA1 and GC regions. Although glial changes may also occur, we conclude that imaging light transmittance reveals dynamic and compartmentalized excitotoxic changes in neuronal volume. Beading of the dendritic arbor increases light scatter, thereby decreasing LT and highlighting damaged dendritic regions.

Visualization of neuronal form and function in brain slices by infrared videomicroscopy

As a standard preparation for neurophysiological experiments, brain slices were introduced some 20 years ago. Although this technique has greatly advanced our understanding of brain physiology, the utility of this preparation has been limited to some extent by the difficulty of visualizing individual neurons in standard thick slices. The use of infrared videomicroscopy has solved this problem. It is now possible to visualize neurons in slices in great detail, and neuronal processes can be patch-clamped under visual control. Infrared videomicroscopy has also been applied successfully to other fields of neuroscience, such as neuronal development and neurotoxicity. A further development of infrared videomicroscopy allows the visualization of the spread of excitation in slices, making the technique a tool for investigating neuronal function and the pharmacology of synaptic transmission.

Histamine modulates NMDA-dependent swelling in the neostriatum

In the present study, infrared differential interference contrast videomicroscopy was used to examine the effect of histamine on N-methyl-D-aspartate-induced swelling in neostriatal neurons in a brain slice preparation. Histamine caused a concentration-dependent increase in swelling evoked by N-methyl-D-aspartate. By itself, histamine did not cause swelling. Electrical stimulation also caused N-methyl-D-aspartate-dependent swelling which was enhanced by histamine. In addition, histamine was found to enhance N-methyl-D aspartate-induced swelling from postnatal day 7 to 28 but not at postnatal day 3. Finally, this histamine-induced enhancement was prevented by treatment with either the H2 receptor antagonist cimetidine or with the potassium channel blocker tetraethylammonium chloride. Overall, these findings suggest that histamine modulates N-methyl-D-aspartate receptor function in the neostriatum through a H2 receptor-mediated regulation of potassium channels.

Glutamate receptor-induced toxicity in neostriatal cells

Infrared differential interference contrast (IR DIC) videomicroscopy was used to measure and characterize cell swelling induced by activation of glutamate receptors (GluR) in a neostriatal brain slice preparation. This swelling is, in many cases, a prelude to necrotic cell death. Activation of N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic GluRs caused cell swelling. The concentration-response relationships and the time courses of the onset of agonist-induced swelling were very similar for NMDA and kainate (KA). However, cells were able to recover from KA but not NMDA-induced swelling. Results from ion substitution experiments suggest that sodium, chloride and to a lesser extent calcium ions play critical roles in this swelling. Heterogeneity in the response to NMDA occurred within cells of the neostriatum. Approximately 15% of the cells did not swell when exposed to NMDA. The magnitude of the NMDA-induced swelling also varied depending on the region of the nervous system. Swelling was greater in the neostriatum and neocortex than in the hippocampus and it did not occur in the suprachiasmatic nucleus. In conclusion, IR DIC videomicroscopy can be used to follow quantitatively the dynamics of GluR-evoked responses in single cells and should be instrumental in determining the factors capable of modifying excitotoxicity.

Regulation of N-methyl-D-aspartate-induced toxicity in the neostriatum: a role for metabotropic glutamate receptors?

Glutamate release activates multiple receptors that interact with each other and thus determine the response of the cell. Exploring these interactions is critical to developing an understanding of the functional consequences of synaptic transmission. Activation of metabotropic glutamate receptors (mGluRs) inhibits N-methyl-D-aspartate (NMDA)-evoked responses measured electrophysiologically in neostriatal slices. The present study examines the functional consequences of this regulation using infrared differential interference contrast videomicroscopy to measure and characterize glutamate receptor-induced cell swelling in a neostriatal brain slice preparation. This swelling is, in many cases, a prelude to necrotic cell death and the dye trypan blue was used to confirm that swelling can result in the death of neostriatal cells. Activation of mGluRs by the agonist 1-aminocyclopentane-1,3-dicarboxylic acid (tACPD) inhibited NMDA but not amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-induced swelling. This regulation was cell-type specific as tACPD did not alter NMDA-induced swelling in pyramidal cells of the hippocampus. Importantly, these findings could be extended to in vivo preparations. Pretreatment with tACPD limited the size of lesions and associated behavioral deficits induced by intrastriatal administration of the NMDA receptor agonist quinolinic acid.

Differential effects of excitatory amino acids on mesencephalic neurons expressing either calretinin or tyrosine hydroxylase in primary cultures

In mesencephalic primary cultures derived from E14 rat embryos, calretinin- and tyrosine hydroxylase-immunoreactive neurons comprised 2% and 5% of the total cell population, respectively, at 6-7 days in vitro. The number of calretinin-immunoreactive neurons was unchanged after a 12- or 24-h exposure to 500 microM kainic acid (KA), but a 50% cell loss was detected after a 48-h exposure to KA. Tyrosine hydroxylase-immunoreactive neurons demonstrated a 50% and 67% cell loss at 24- and 48-h exposures to 500 microM KA. A 500 microM N-methyl-D-aspartic acid (NMDA) incubation for 24 h had no effect on calretinin-immunoreactive cell number, but did significantly reduce tyrosine hydroxylase-immunoreactive cell numbers by 26%. In tyrosine hydroxylase-immunoreactive cells, exposure to KA appeared to stimulate the retraction of the neuritic tree and to cause somatic swelling. In contrast, calretinin-immunoreactive neurons developed larger and more complex neuritic trees after a 24-h exposure to 500 microM KA but not NMDA. Immunohistochemical colocalization studies revealed that all tyrosine hydroxylase-immunoreactive and the majority of calretinin-immunoreactive neurons expressed the glutamate receptor subunits GluR2-R3. Very low levels of NMDAR1 receptor subunits were detected on cells in this culture and GluR4 receptor subunits were not detectable. Our experiments showed that glutamate receptors present in both calretinin- and tyrosine hydroxylase-immunoreactive cells were functional, since phosphorylated cAMP/Ca2+ response element-binding protein levels were increased in both cell types after 10 or 30 min exposures to 500 microM KA. The present results indicate that in the mesencephalic cultures tyrosine hydroxylase-immunoreactive cells are more vulnerable to KA excitotoxicity than calretinin-immunoreactive neurons.

Leber hereditary optic neuropathy. Electron microscopy and molecular genetic analysis of a case

BACKGROUND

Leber hereditary optic neuropathy (LHON) is a mitochondrial genetic disorder characterized by bilateral central visual loss typically in early adulthood. Few histopathologic studies, including ultrastructural and molecular genetic analysis, have been reported.

METHODS

Ocular tissue was obtained postmortem from an 81-year-old woman with LHON from the Queensland 1 pedigree characterized by mutations at nucleotide positions 4160 and 14484. Routine histopathologic studies, electron microscopy, electron-probe analysis, and molecular genetic analysis were performed.

RESULTS

Marked atrophy of the nerve fiber and retinal ganglion cell layers and optic nerves was present. Results of electron microscopic examination demonstrated 1.2 microns electron-dense, double-membrane-bound inclusions, consisting of calcium by electron-probe analysis, in retinal ganglion cells. The optic nerve was homoplasmic for mutations 4160 and 14484.

CONCLUSION

Optic nerve and inner retinal atrophy in LHON may be a result of metabolic mitochondrial dysfunction leading to intramitochondrial calcification. Homoplasmy for mitochondrial mutations 4160 and 14484 in the leukocyte/platelet fraction of whole blood may correlate with homoplasmy in the optic nerve.

Infrared videomicroscopy: a new look at neuronal structure and function

Brain slices were introduced as a standard preparation for neurophysiological experiments some 20 years ago. A drawback of this preparation compared with cell culture has been the difficulty to visualize individual neurones in standard thick slices. This problem has been overcome by the use of infrared videomicroscopy. Neurones in slices can now be visualized in great detail, and neuronal processes can be patch-clamped under direct visual control. Infrared video-microscopy has also been applied successfully to other fields of neuroscience such as neuronal development and neurotoxicity. A further development of infrared videomicroscopy enables one to visualize the spread of excitation in slices making the technique a tool for the direct investigation of neuronal function.

The role of intracellular free calcium in motor neuron disease

The intracellular calcium (Ca2+) concentrations of motoneurons can be altered by the influx of Ca2+ into the cell by the opening of voltage-dependent Ca2+ channels and ligand-gated channels linked to Ca2+ influx, especially by the N-methyl-D-aspartate (NMDA) type of excitatory amino acid receptor. Intracellular Ca2+ concentration is also affected by the release of Ca2+ buffered in mitochondria and endoplasmic reticulum. Evidence that motoneurons may be selectively vulnerable to Ca(2+)-induced cell death include the following observations: (i) the presence of excitatory amino acid receptors on the cell membranes of motoneurons, some of which would permit Ca2+ influx (e.g. NMDA receptors); (ii) the availability of the presynaptic terminal for antibody-mediated effects leading to changes in cell permeability and Ca2+ influx; and (iii) the limited amounts of intracellular Ca(2+)-binding proteins such as calbindin D28K and parvalbumin in motoneurons. Elevation of intracellular free Ca2+ may also be a common event in a number of independent mechanisms leading to motoneuron death in motor neuron disease.

Direct observation of neurotoxicity in brain slices with infrared videomicroscopy

We employed the novel technique of infrared videomicroscopy to study the morphological changes induced by the neurotoxicity of high concentrations of L-glutamate and by anoxia. The infrared videomicroscopy system described uses an inverted microscope and employs a combination of infrared illumination, differential interference contrast (DIC) and contrast enhancement by video. With this system, we were able to observe swelling of neurons 50 microns deep in rat neocortical slices after bath application of glutamatergic agonists or during anoxia. By recording in time lapse mode it was possible to visualize the dynamics of cell swelling and to demonstrate neuroprotection by glutamatergic antagonists. The method may be of use in screening of potential neuroprotective drugs for stroke therapy.

Ion-selective microelectrodes and diffusion measurements as tools to explore the brain cell microenvironment

The construction and application of liquid-membrane ion-selective microelectrodes (ISM) are described. Recommendations are provided for the selection of appropriate cocktails containing neutral carriers to form the liquid membrane to sense K+, Ca2+, H+ and Na+. The use of charged carriers to sense Cl- and the cation tetramethylammonium (TMA+) is discussed. A detailed protocol is given for constructing double-barreled electrodes (ion-sensor and reference barrel) with tips of 1 micron diameter or more for extracellular ion measurements. The primary results obtained with ISMs in the brain cell microenvironment are briefly surveyed. The theoretical basis for measuring diffusion properties of extracellular space is described. Such measurements enable the estimation of volume fraction (proportion of tissue that is extracellular space) and tortuosity (hindrance of diffusion due to cellular obstructions). A method is given for using TMA+ ISMs in combination with iontophoresis or pressure ejection of TMA+ from a nearby micropipette to measure diffusion properties.

Excitotoxicity, energy metabolism and neurodegeneration

There is increasing evidence that the neurotoxic effects of excitatory amino acids and their analogues are part of the pathogenesis of neuronal degeneration in acute and chronic neurological disease. Recent studies indicate that activation of excitatory amino acid receptors is also induced in the mechanism of neuronal damage induced by impairment of cellular energy metabolism. This article briefly summarizes the evidence for the presence of such a mechanism and discusses metabolic diseases in which excitatory amino acids alone or in combination with energy deficiency could play a pathogenetic role. In these and other metabolic diseases, antagonists to excitatory amino acid receptors may offer a therapeutic opportunity; however, there are potential limits that may prevent chronic use.