Ca(2+)- and Cl(-)-dependent, NMDA receptor-mediated neuronal death induced by depolarization in rat hippocampal organotypic cultures

Optimization of Near-Infrared Fluorescence Voltage-Sensitive Dye Imaging for Neuronal Activity Monitoring in the Rodent Brain

Many currently employed clinical brain functional imaging technologies rely on indirect measures of activity such as hemodynamics resulting in low temporal and spatial resolutions. To improve upon this, optical systems were developed in conjunction with methods to deliver near-IR voltage-sensitive dye (VSD) to provide activity-dependent optical contrast to establish a clinical tool to facilitate direct monitoring of neuron depolarization through the intact skull. Following the previously developed VSD delivery protocol through the blood-brain barrier, IR-780 perchlorate VSD concentrations in the brain were varied and stimulus-evoked responses were observed. In this paper, a range of optimal VSD tissue concentrations was established that maximized fluorescence fractional change for detection of membrane potential responses to external stimuli through a series of phantom, in vitro, ex vivo, and in vivo experiments in mouse models.

Transcranial Recording of Electrophysiological Neural Activity in the Rodent Brain in vivo Using Functional Photoacoustic Imaging of Near-Infrared Voltage-Sensitive Dye

Minimally-invasive monitoring of electrophysiological neural activities in real-time-that enables quantification of neural functions without a need for invasive craniotomy and the longer time constants of fMRI and PET-presents a very challenging yet significant task for neuroimaging. In this paper, we present in vivo functional PA (fPA) imaging of chemoconvulsant rat seizure model with intact scalp using a fluorescence quenching-based cyanine voltage-sensitive dye (VSD) characterized by a lipid vesicle model mimicking different levels of membrane potential variation. The framework also involves use of a near-infrared VSD delivered through the blood-brain barrier (BBB), opened by pharmacological modulation of adenosine receptor signaling. Our normalized time-frequency analysis presented in vivo VSD response in the seizure group significantly distinguishable from those of the control groups at sub-mm spatial resolution. Electroencephalogram (EEG) recording confirmed the changes of severity and frequency of brain activities, induced by chemoconvulsant seizures of the rat brain. The findings demonstrate that the near-infrared fPA VSD imaging is a promising tool for in vivo recording of brain activities through intact scalp, which would pave a way to its future translation in real time human brain imaging.

Ultrasensitive Graphene Optoelectronic Probes for Recording Electrical Activities of Individual Synapses

The complex neuronal circuitry connected by submicron synapses in our brain calls for technologies that can map neural networks with ultrahigh spatiotemporal resolution to decipher the underlying mechanisms for multiple aspects of neuroscience. Here we show that, through combining graphene transistor arrays with scanning photocurrent microscopy, we can detect the electrical activities of individual synapses of primary hippocampal neurons. Through measuring the local conductance change of graphene optoelectronic probes directly underneath neuronal processes, we are able to estimate millivolt extracellular potential variations of individual synapses during depolarization. The ultrafast nature of graphene photocurrent response allows for decoding of activity patterns of individual synapses with a sub-millisecond temporal resolution. This new neurotechnology provides promising potentials for recording of electrophysiological outcomes of individual synapses in neural networks.

Electroosmotic perfusion of tissue: sampling the extracellular space and quantitative assessment of membrane-bound enzyme activity in organotypic hippocampal slice cultures

This review covers recent advances in sampling fluid from the extracellular space of brain tissue by electroosmosis (EO). Two techniques, EO sampling with a single fused-silica capillary and EO push-pull perfusion, have been developed. These tools were used to investigate the function of membrane-bound enzymes with outward-facing active sites, or ectoenzymes, in modulating the activity of the neuropeptides leu-enkephalin and galanin in organotypic-hippocampal-slice cultures (OHSCs). In addition, the approach was used to determine the endogenous concentration of a thiol, cysteamine, in OHSCs. We have also investigated the degradation of coenzyme A in the extracellular space. The approach provides information on ectoenzyme activity, including Michaelis constants, in tissue, which, as far as we are aware, has not been done before. On the basis of computational evidence, EO push-pull perfusion can distinguish ectoenzyme activity with a ~100 μm spatial resolution, which is important for studies of enzyme kinetics in adjacent regions of the rat hippocampus.

Electroosmotic push-pull perfusion: description and application to qualitative analysis of the hydrolysis of exogenous galanin in organotypic hippocampal slice cultures

We demonstrate here a method that perfuses a small region of an organotypic hippocampal culture with a solution containing an enzyme substrate, a neuropeptide. Perfusate containing hydrolysis products is continually collected and subsequently analyzed for the products of the enzymatic degradation of the peptide substrate. The driving force for perfusion is an electric field. The fused silica capillaries used as "push" and "pull" or "source" and "collection" capillaries have a ζ-potential that is negative and greater in magnitude than the tissue's ζ-potential. Thus, depending on the magnitudes of particular dimensions, the electroosmotic flow in the capillaries augments the fluid velocity in the tissue. The flow rate is not directly measured; however, we determine it using a finite-element approach. We have determined the collection efficiency of the system using an all d-amino acid internal standard. The flow rates are low, in the nL/min range, and adjustable by controlling the current or voltage in the system. The collection efficiency of the d-amino acid peptide internal standard is variable, increasing with increased current and thus electroosmotic flow rate. The collection efficiency can be rationalized in the context of a Peclet number. Electroosmotic push-pull perfusion of the neuropeptide galanin (gal1-29) through the extracellular space of an organotypic hippocampal culture results in its hydrolysis by ectopeptidase reactions occurring in the extracellular space. The products of hydrolysis were identified by MALDI-MS. Experiments at two levels of current (8-12 μA and 19-40 μA) show that the probability of seeing hydrolysis products (apparently from aminopeptidases) is greater in the Cornu Ammonis area 3 (CA3) than in the Cornu Ammonis area 1 (CA1) in the higher current experiments. In the lower current experiments, shorter peptide products of aminopeptidases (gal13-29 to gal20-19) are seen with greater frequency in CA3 than in CA1 but there is no statistically significant difference for longer peptides (gal3-29 to gal12-29).

Assessment of tissue viability following electroosmotic push-pull perfusion from organotypic hippocampal slice cultures

We have developed a novel sampling technique that allows both introduction and removal of fluid from the extracellular space of living tissue. This method is based on the fluidics of push-pull perfusion but flow is driven by electroosmosis. We have applied this method to organotypic hippocampal cultures. A source capillary is inserted into the tissue and a collection capillary is in contact with the tissue surface through a thin layer of fluid. A voltage is applied across the proximal ends of source and collection capillary. In the applied field, fluid will move from source, into the tissue, and then be collected. In this process, damage to cells may occur. To understand better what sampling conditions influence damage most, we tested various sampling geometries and applied voltages, quantifying damage 16-24 h later using propidium iodide as a cell death marker. We found that damage correlates with both voltage drop and power dissipated in the tissue, but that voltage drop is a better indicator of damage when comparing models in which capillary arrangement and length are different.

A simple method for measuring organotypic tissue slice culture thickness

This paper presents a simple method to measure tissue slice thicknesses using an ohmmeter. The circuit described here is composed of a metal probe, an ohmmeter, a counter electrode, culture medium or physiological buffer, and tissue slice. The probe and the electrode are on opposite interfaces of an organotypic hippocampal slice culture. The circuit closes when the metal probe makes contact with the surface of the tissue slice. The probe position is recorded and compared to its position when it makes contact with the insert membrane on which the tissue grows, thus yielding a thickness measurement. The method does not reduce the viability of slice cultures. Thicknesses of the slice cultures were measured under a number of culturing protocols. An initial drop in thickness occurred between 0 and 4 days in culture. Thicknesses are rather constant thereafter. The type of culture medium and the initial thickness of the tissue explant influence the thickness. Slice thicknesses were compared to a known technique by using optical measurements of slice cross-sections to obtain thicknesses. In contrast to this known technique, the proposed method does not sacrifice the slice culture for measurement purposes. The proposed measurement technique described is straightforward and rapid, about 1 min per culture.

Alterations in discrete glutamate receptor subunits in adult mouse dentate gyrus granule cells following perforant path transection

Custom-designed microarray analysis was utilized to evaluate expression levels of glutamate receptors (GluRs) and GluR-interacting protein genes within isolated dentate gyrus granule cells following axotomy of the principal input, the perforant path (PP). Dentate gyrus granule cells were evaluated by microdissection via laser capture microdissection, terminal continuation RNA amplification, and microarray analysis following unilateral PP transections at seven time points. Expression profiles garnered from granule cells on the side ipsilateral to PP transections were compared and contrasted with naive subjects and mice subjected to unilateral occipital cortex lesions. Selected microarray observations were validated by real-time quantitative PCR analysis. Postlesion time-dependent alterations in specific alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors, kainate receptors, N-methyl-D-aspartate (NMDA) receptors, and GluR-interacting protein genes were found across the time course of the study, suggesting a neuroplasticity response associated with the transsynaptic granule cell alterations following axotomy of incoming PP terminals.

Neuroprotective effects of an extract from the inflamed skin of rabbits inoculated with vaccinia virus on glutamate-induced neurotoxicity in cultured neuronal cell line

Objective: Protein-free extracts from the inflamed skin of rabbits inoculated with vaccinia virus (Rosemorgen^® and Neurotropin^®) are widely employed to combat chronic pain and treat allergic conditions in human subjects in Japan. However, the pharmacologic mechanisms of Rosemorgen^® and Neurotropin^® remain unclear.

Methods: In this study, we examined the effects of Rosemorgen^® on L-glutamic acid (Glu)-induced cell death in N18-RE-105 neural cell line, which only possessed non-N-methyl-D-aspartate (NMDA)-type receptors.

Results: There were many large cytoplasmic cells and elongation of fivers in phosphate-buffered saline (PBS) additional group without Glu. In PBS and Glu simultaneous additional group, the survival ratio was decrease significantly compared with PBS alone group. Moreover, there were dead cells which did not have cytoplasm and aggregated nucleus. The Glu-induced cell death of N18-RE-105 cells was inhibited by both pre-treatment (24 hours before Glu treatment) and simultaneous treatment with Rosemorgen^®. There were many large cytoplasmic cells and elongation of fivers in Rosemorgen^® group.

Discussion: From this finding in N18-RE-105 cells, Rosemorgen^® was concluded to inhibit Glu-induced cell death via non-NMDA type receptors. One of the pharmacologic mechanisms of Rosemorgen^® has been clear. These results suggest that Rosemorgen^® depresses allodynia and chronic pain through interaction with non-NMDA type receptors.

Vesicular release of glutamate from hippocampal neurons in culture: an immunocytochemical assay

Glutamate, the main excitatory neurotransmitter in the brain, may cause excitotoxic damage through excessive release during a number of pathological conditions. We have developed an immunocytochemical assay to investigate the mechanisms and regulation of glutamate release from intact, cultured neurons. Our results indicate that cultured hippocampal neurons have a large surplus of glutamate available for release upon chemically induced depolarization. Long incubations with high K(+)-concentrations, and induction of repetitive action potentials with the K(+)-channel blocker 4-aminopyridine (4-AP), caused a significant reduction in glutamate labeling in a subset of boutons, demonstrating that transmitter release exceeded the capacity for replenishment. The number of boutons where release exceeded replenishment increased continuously with time of stimulation. This depletion was Ca(2+)-dependent and sensitive to bafilomycin A1 (baf), indicating that it was dominated by vesicular release mechanisms. The depletion of glutamate from cell bodies and dendrites was also Ca(2+)-dependent. Thus, under the present conditions, cytosolic glutamate is taken up in vesicles prior to release, and the main escape route for the amino acid is through vesicular exocytosis. Depolarization with lower concentrations of K(+) caused sustainable release of glutamate, i.e., without full depletion.

Role of GABAergic antagonism in the neuroprotective effects of bilobalide

Bilobalide, a constituent of Ginkgo biloba, has neuroprotective properties. Its mechanism of action is unknown but it was recently found to block GABA(A) receptors. The goal of this study was to test the potential role of a GABAergic mechanism for the neuroprotective activity of bilobalide. In rat hippocampal slices exposed to NMDA, release of choline indicates breakdown of membrane phospholipids. NMDA-induced choline release was almost completely blocked in the presence of bilobalide (10 microM) and under low-chloride conditions. Bicuculline (100 microM), a competitive antagonist at GABA(A) receptors, reduced NMDA-induced choline release to a small extent (-23%). GABA (100 microM) partially antagonized the inhibitory action of bilobalide. Exposure of hippocampal slices to NMDA also caused edema formation as measured by increases of tissue water content. NMDA-induced edema formation was suppressed by bilobalide and by low-chloride conditions. Bicuculline exerted partial protection (by 30%) while GABA reduced bilobalide's effect by about one third. To investigate bilobalide's interaction with GABA(A) receptors directly, we measured binding of [(35)S]-TBPS to rat cortical membranes. TBPS binding was competitively inhibited by bilobalide in the low micromolar range (IC(50)=3.7 microM). As a functional test, we determined (36)chloride flux in rat corticohippocampal synaptoneurosomes. GABA (100 microM) significantly increased (36)chloride flux (+65%), and this increase was blocked by bilobalide, but with low potency (IC(50): 39 microM). We conclude that, while antagonism of GABA(A) receptors may contribute to bilobalide's neuroprotective effects, additional mechanisms must be postulated to fully explain bilobalide's actions.

Effects of chloride flux modulators in an in vitro model of brain edema formation

Brain edema is a serious consequence of hemispheric stroke and traumatic brain injury and contributes significantly to patient mortality. In the present study, we measured water contents in hippocampal slices as an in vitro model of edema formation. Excitotoxic conditions induced by N-methyl-D-aspartate (NMDA, 300 microM), as well as ischemia induced by oxygen-glucose deprivation (OGD), caused cellular edema formation as indicated by an increase of slice water contents. In the presence of furosemide, an inhibitor of the Na,K,Cl-cotransporter, NMDA-induced edema were reduced by 64% while OGD-induced edema were unaffected. The same observation, i.e., reduction of excitotoxic edema formation but no effect on ischemia-induced edema, was made with chloride transport inhibitors such as DIDS and niflumic acid. Under ischemic conditions, modulation of GABAA receptors by bicuculline, a GABA antagonist, or by diazepam, a GABAergic agonist, did not significantly affect edema formation. Further experiments demonstrated that low chloride conditions prevented NMDA-induced, but not OGD-induced, water influx. Omission of calcium ions had no effect. Our results show that NMDA-induced edema formation is highly dependent on chloride influx as it was prevented by low-chloride conditions and by various compounds that interfere with chloride influx. In contrast, OGD-induced edema observed in brain slices was not affected by modulators of chloride fluxes. The results are discussed with reference to ionic changes occurring during tissue ischemia.

NMDA receptor stimulation in the absence of extracellular Ca2+ potentiates Ca2+ influx-dependent cell death system

The meaning of Ca2+ influx in the time course of glutamate stimulation of neuronal cells was addressed. We demonstrated that Ca2+ influx did not work straightforward in the determination of the fate of neuronal cells. There appears to be a critical period for Ca2+ influx to work efficiently in glutamate-induced neuronal cell death. When Ca2+ influx for 5 min from the beginning of glutamate stimulation was allowed in the whole stimulation period for 15 min, potent neuronal cell death could not be attained. On the other hand, when neuronal cells had been pre-treated with glutamate or NMDA for 5-10 min in the absence of extracellular Ca2+ following Ca2+ influx for 5 min fully induced neuronal cell death. APV inhibited this pre-treatment effect. It appears that the pre-treatment of neuronal cells with glutamate or NMDA in the absence of extracellular Ca2+ promotes the Ca2+ influx-dependent process executing cell death. The pre-treatment itself did not change the pattern of intracellular Ca2+ elevation by the activation of NMDA receptors. These results imply that glutamate activation of NMDA receptors consists of two different categories of pathways relating to neuronal cell death, i.e., Ca2+ influx independent and dependent, and that the former facilitates the latter to drive neuronal cells to death. This study clarified a mechanism by which glutamate quickly determines cell fate.

Altered morphological and electrophysiological properties of Cajal-Retzius cells in cerebral cortex of embryonic Presenilin-1 knockout mice

Mutations of Presenilin-1 are the major cause of familial Alzheimer's disease. Presenilin-1 knockout (PS1-/-) mice develop severe cortical dysplasia related to human type 2 lissencephaly. This overmigration syndrome has been attributed to the premature loss of Cajal-Retzius cells (CRcs), pioneer neurons required for the termination of radial neuronal migration. To elucidate the potential cellular mechanisms responsible for this premature neuronal loss, we investigated the morphological and electrophysiological properties of visually identified CRcs of wild-type (WT) and PS1-/- mouse brains at embryonic day 16.5. The density of CRcs was substantially reduced in the cerebral cortex of PS1-/-. In PS1-/- CRcs the number of axonal branches was significantly increased to 12.5 +/- 4.9 (n = 8; WT, 4.0 +/- 1.4, n = 12), while no differences in dendritic branching and total length of dendritic and axonal compartments were observed. Additionally, the resting membrane potential of PS1-/- CRcs was significantly depolarized (-48.3 +/- 1.7 mV; n = 23) in contrast to WT CRcs (-57.9 +/- 2.1 mV; n = 38). Active membrane properties were not affected by PS1 deficiency. CRcs of both genotypes showed spontaneous postsynaptic currents that could be completely blocked by 100 microM bicuculline and were unaffected by glutamatergic antagonists, suggesting that they were mediated by GABAA receptors. These results demonstrate that axonal branching and resting membrane potential of CRcs was affected in embryonic cerebral cortex of PS1-/- mice. The depolarized membrane potential observed in PS1-/- CRcs may increase the susceptibility to neuronal death, thus facilitating the premature loss of CRcs in PS1-/- mice.

Effects of AMPA-receptor and voltage-sensitive sodium channel blockade on high potassium-induced glutamate release and neuronal death in vivo

High extracellular potassium induces spreading depression-like depolarizations and elevations of extracellular glutamate. Both occur in the penumbra of a focal ischemic infarct, and may be responsible for the spread of cell death from the infarct core to the penumbra. We have modeled this situation with microdialysis of an isotonic high-potassium solution into the normal rat amygdala for 70 min. This elevates extracellular glutamate up to 8-fold or more and produces irreversibly damaged, acidophilic neurons. NMDA-receptor blockade protects neurons and reduces the elevation of extracellular glutamate. Here we investigated the effects of sodium channel blockade with the voltage-sensitive sodium channel blocker tetrodotoxin and the AMPA receptor antagonist 2,3-dihydroxy-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulfonamide disodium (NBQX disodium) on high potassium-induced neuronal death and extracellular glutamate elevations. The acidophilic neurons produced are necrotic by ultrastructural examination. Tetrodotoxin, at dialysate concentrations of 33, 330 and 3300 microM (only a small fraction is extracted by tissue), markedly reduced the elevations of glutamate in rat amygdala at nearly all time points during high-potassium perfusion, but it reduced tissue edema only at the highest concentration, and it was neuroprotective only if dialyzed prior to high-potassium microdialysis (at 330 microM concentration). Although both 250 microM (6.2% is extracted by tissue) and 500 microM NBQX reduced elevations of glutamate, neither was neuroprotective, and neuropil edema was not reduced by either concentration. Our results suggest that in vivo, sodium influx through voltage-sensitive sodium channels but not through ligand-gated AMPA receptor channels contributes to high potassium-induced neuronal necrosis.

Axonal transection in adult rat brain induces transsynaptic apoptosis and persistent atrophy of target neurons

We used the fimbria-fornix (FF) transection model of axonal injury to test the hypothesis that transneuronal degeneration occurs in the adult central nervous system in response to deafferentation. The medial mammillary nucleus, pars medialis (MMNm) was analyzed by light and electron microscopy at 3, 7, 14, and 30 days, and 6 months after unilateral FF transection in adult rat to identify the time course of neuronal responses in a remote target. Presynaptic terminals and neuronal cell bodies degenerated in the MMNm ipsilateral to FF transection. Terminal degeneration occurred predominantly at 3 and 7 days postlesion. Between 14 and 30 days postlesion, neuronal number in the MMNm decreased (approximately 20%). Two forms of neuronal degeneration were found in the MMNm after deafferentation. Some neurons died apoptotically. Other neurons underwent vacuolar degeneration. In these latter neurons, somatodendritic pathology occurred at 14 and 30 days and 6 months postlesion. The ultrastructure of this vacuolar degeneration was characterized by disorganization of the cytoplasm, formation of membrane-bound vacuolar cisternae and membranous inclusions, loss of organelles, cytoplasmic pallor, and chromatin alterations. This study shows that both anterograde axonal degeneration and transneuronal degeneration occur in a fornical target after FF axon transection. This transneuronal degeneration can be classified as sustained neuronal atrophy or transsynaptic apoptosis.

Cl(-)-dependent excitotoxicity is associated with 3H2O influx in chick embryonic retina

The aim of this study was to show that Cl(-)-dependent excitotoxicity, with its characteristic cell swelling, involves actual water influx into the intracellular compartment. Taking advantage of the Ca2+ omission paradigm of Cl(-)-dependent excitotoxicity, in the chick embryonic neural retina ex vivo, which is associated with toxicity levels (lactate dehydrogenase (LDH) release) considerably higher than those seen after simple exposure of the retinas to glutamate agonists, we have demonstrated that an intracellular water intake of 4.2 microl into retinal cells is associated with 13.3% total retinal LDH release. The fact that mannitol blocks both water inflow and LDH release appears to link both events from a pathogenic point of view.

Cytotoxicity of tributyltin in rat hippocampal slice cultures

The neurotoxic effects of tributyltin (TBT), an endocrine-disrupting chemical, were evaluated in organotypic slice cultures of immature rat hippocampus. Confocal microscopy study with propidium iodide showed that TBT induced severe neuronal death in a concentration- and time-dependent manner with CA3 > CA1 > dentate gyrus ranking of vulnerability of the hippocampal subfields. Dead or damaged neurons exhibited chromatin condensation, which is one of the morphological characteristics of apoptosis, as revealed by acridine orange staining. TBT neurotoxicity was alleviated by application of free radical scavengers or antioxidants, such as catalase, superoxide dismutase, Trolox and alpha-tocopherol but not by ascorbic acid or N-acetyl-L-cysteine, which suggests an involvement of free radicals, particularly reactive oxygen species. Neurons displayed a long-lasting increase in intracellular Ca2+ concentrations after TBT treatment. Although neither N-methyl-D-aspartate (NMDA) receptor inhibitors nor voltage-sensitive Ca2+ channel blockers protected hippocampal neurons against TBT neurotoxicity, non-NMDA receptor antagonist completely prevented TBT-induced neurodegeneration. These data suggest that TBT provokes apoptosis-like neuronal cell death, which might be mediated by intracellular Ca2+ elevation and free radical generation via non-NMDA receptor activation.

Dendrite loss is a characteristic early indicator of toxin-induced neurodegeneration in rat midbrain slices

In rat brain substantia nigra catecholamine neurons in vitro, a sensitive indicator of excitatory amino-acid-induced damage is dendritic degeneration that precedes the loss of the cell body. The present study has shown that dendritic loss is not specific for excitatory amino acids and is an early indicator of neurodegeneration produced by numerous agents that initiate damage by different primary cellular actions. Rats were anesthetised by fluothane inhalation and killed, and the brain was rapidly removed. Three-hundred-micrometer-thick slices containing substantia nigra were incubated for 2 h at 35 degrees C in the presence or absence of kainic acid (50 microM), 1-methyl-4-phenylpyridinium ion (10 or 50 microM), ouabain (10 or 30 microM), 6-hydroxydopamine (10 or 100 microM), potassium cyanide (100 microM or 1 mM), or elevated extracellular potassium chloride (25, 50, or 100 mM). The slices were fixed and recut into thin sections (30 micrometer) and substantia nigra dopamine neurons were immunolabeled for tyrosine hydroxylase coupled to diaminobenzidine. Both the cell body and the extensive dendritic projections were immunolabeled. Each agent caused a similar pattern of toxicity including loss of tyrosine-hydroxylase-immunolabeled dendrites at lower concentrations and damage to, or disintegration of, the cell bodies at higher concentrations. For example, 100 microM potassium cyanide reduced the proportion of substantia nigra neurons which exhibited dendrites from 66 +/- 4% (SEM) in controls to 54 +/- 7%, without obvious changes in cell bodies. After 1 mM potassium cyanide, only 13 +/- 2% of substantia nigra neurons retained dendrites and cell bodies were shrunken or disintegrated. Loss of dendrites was also evident in substantia nigra neurons stained with cresyl violet or immunolabeled for microtubule-associated protein 2. The findings suggest that disruption of the dendritic arbor is an early indicator of neurodegeneration, irrespective of how this is initiated. The approach that we have developed may therefore prove valuable in investigating the mechanisms of degeneration of catecholamine neurons.

Fimbria-fornix transection and excitotoxicity produce similar neurodegeneration in the septum

Fimbria-fornix transection produces neuronal injury in the septum. This cellular pathology is characterized by somatodendritic vacuolar abnormalities in neurons. Because these cellular changes are reminiscent of some of the morphological abnormalities seen with glutamate receptor-mediated excitoxicity, we tested whether excitotoxic injury to the septal complex of adult rats mimics the degeneration observed within the dorsolateral septal nucleus and medial septal nucleus following fimbria-fornix transection. The septal complex was evaluated at various time-points (6 h to 14 days) by light and electron microscopy following unilateral injection of the N-methyl-D-aspartate receptor agonist quinolinate or the non-N-methyl-D-aspartate receptor agonist kainate, and the morphological changes observed were compared to those abnormalities in the medial septal nucleus and dorsolateral septal nucleus at three to 14 days after fimbria-fornix transection. The patterns of cytoplasmic abnormalities and vacuolar injury were morphologically similar in the somatodendritic compartment of neurons following excitotoxicity and axotomy paradigms. These similarities were most evident when comparing the persistently injured neurons in the penumbral regions of the excitotoxic lesions at one to 14 days recovery to neurons in the medial septal nucleus and dorsolateral septal nucleus at seven and 14 days after fimbria-fornix transection. Pretreatment with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate prior to unilateral fimbria-fornix transection attenuated the somatodentritic vacuolar damage found within the ipsilateral dorsolateral and medial septal nuclei at 14 days recovery. Because glutamate is the principal transmitter of hippocampal efferents within the fimbria-fornix, we conclude that postsynaptic glutamate receptor activation participates in the evolution of septal neuron injury following fimbria-fornix transection. Thus, excitotoxicity is a possible mechanism for transneuronal degeneration following central nervous system axotomy.

Role of sodium ion influx in depolarization-induced neuronal cell death by high KCI or veratridine

Intracellular Na+ concentration plays an important role in the regulation of cellular energy metabolism; i.e., increased intracellular Na+ concentration stimulates glucose utilization both in cultured neurons and astrocytes. Both high KCI and veratridine, which have been known to cause neuronal damage, elicit increased glucose utilization, presumably via increased intracellular Na+ concentration. In the present study, we examined the role of intracellular Na+ influx in the mechanisms of neuronal cell damage induced by high KCl or veratridine assayed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric method. Rat primary cultures of striatal neurons were incubated with high KCl (final concentrations: 25, 50 mM) or veratridine (0.1-100 microM) with or without various inhibitors. High KCl depolarizes cell membrane, thus, leading to Na+ influx through an activation of voltage-sensitive Na+ channels, while veratridine elicits Na+ influx by directly opening these channels. After 24-h incubation with elevated [K+]o or veratridine, glucose contents in the medium decreased significantly (approximately by 7 mM), but remained higher than 18 mM. High [K+]o reduced percent cell viability significantly (approximately 50% at 25 mM, approximately 40% at 50 mM [K+]o, P<0.01), but tetrodotoxin (100 nM) had no protective effect, indicating that Na+ influx was not essential to high K+ -induced cell death. DL-2-Amino-5-phosponovaleric acid (APV) (1 mM) completely blocked cell death induced by elevated [K+]o, while 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) did not. In contrast, veratridine (>10 microM) caused cell damage in a dose-dependent and tetrodotoxin-sensitive manner, but none of APV, CNQX, or bepridil (Na+ -Ca2+ exchanger blocker) had any protective effect. Nifedipine (50 approximately 100 microM), however, reduced percent cell damage induced by veratridine.

Cytosolic Ca2+ changes during in vitro ischemia in rat hippocampal slices: major roles for glutamate and Na+-dependent Ca2+ release from mitochondria

This work determined Ca2+ transport processes that contribute to the rise in cytosolic Ca2+ during in vitro ischemia (deprivation of oxygen and glucose) in the hippocampus. The CA1 striatum radiatum of rat hippocampal slices was monitored by confocal microscopy of calcium green-1. There was a 50-60% increase in fluorescence during 10 min of ischemia after a 3 min lag period. During the first 5 min of ischemia the major contribution was from Ca2+ entering via NMDA receptors; most of the fluorescence increase was blocked by MK-801. Approximately one-half of the sustained increase in fluorescence during 10 min of ischemia was caused by activation of Ca2+ release from mitochondria via the mitochondrial 2Na+-Ca2+ exchanger. Inhibition of Na+ influx across the plasmalemma using lidocaine, low extracellular Na+, or the AMPA/kainate receptor blocker CNQX reduced the fluorescence increase by 50%. The 2Na+-Ca2+ exchange blocker CGP37157 also blocked the increase, and this effect was not additive with the effects of blocking Na+ influx. When added together, CNQX and lidocaine inhibited the fluorescence increase more than CGP37157 did. Thus, during ischemia, Ca2+ entry via NMDA receptors accounts for the earliest rise in cytosolic Ca2+. Approximately 50% of the sustained rise is attributable to Na+ entry and subsequent Ca2+ release from the mitochondria via the 2Na+-Ca2+ exchanger. Sodium entry is also hypothesized to compromise clearance of cytosolic Ca2+ by routes other than mitochondrial uptake, probably by enhancing ATP depletion, accounting for the large inhibition of the Ca2+ increase by the combination of CNQX and lidocaine.

Ultrastructural analysis of the progression of neurodegeneration in the septum following fimbria-fornix transection

The fimbria-fornix transection paradigm has been used as a model of retrograde neurodegeneration within the medial septal nucleus and anterograde degeneration of axon terminals within the lateral septal nucleus. Because the maintenance and survival of neurons may depend on the integrity of both efferents and afferents, the ultrastructure of neurons in the medial septal nucleus and dorsolateral septal nucleus was analysed at three, seven, 14, 30 days, and six months following unilateral transection of the fimbria-fornix in adult rats. Degeneration of axonal and somatodendritic compartments occurred in both nuclei on the side ipsilateral to fimbria-fornix transection. Degeneration of axons and terminals was present by three days and dissipated thereafter, although degenerating axodendritic and axosomatic terminals were still detected at 14-30 days postlesion. Dendrosomal alterations in both septal nuclei manifested as redistribution of organelles, dispersion and loss of rough endoplasmic reticulum, formation of membrane-bound vacuolar cisternae and membranous inclusions, loss of cytoplasmic matrix, and dispersion of chromatin throughout the nucleoplasmic matrix. These changes occurred in the absence of apparent ultrastructural damage to mitochondria and condensation of the nucleus. Dendritic pathology in both the medial and dorsolateral septal nuclei was most prominent at 14-30 days postlesion, but the neuropil recovered to control appearance by six months postlesion. In contrast, the cytoplasmic rarefaction and vacuolation of neuronal cell bodies were persistent in both the medial septal nucleus and the dorsolateral septal nucleus. We conclude that, following disconnection from the hippocampus, ultrastructural abnormalities occur within neurons in both the medial and lateral septal nuclei. The characteristics and time-course for these changes are similar in both nuclei. The neuropilar degeneration was transient, in contrast to the neuronal cell body injury which was persistent and was morphologically consistent with long-term neuronal atrophy.

Tamoxifen, a chloride channel blocker, reduces glutamate and aspartate release from the ischemic cerebral cortex

The effects of the anti-estrogen, anion channel blocker, tamoxifen on amino acid release from the ischemic rat cerebral cortex was investigated using a cortical cup technique. Tamoxifen (20 microM in artificial cerebrospinal fluid), applied topically, inhibited the ischemia-evoked efflux of aspartate, glutamate, taurine and phosphoethanolamine. Reductions in the ischemia-evoked levels of these amino acids suggest that tamoxifen may attenuate chloride-related osmotic cell swelling and the associated regulatory volume decrease (RVD) release of amino acids.

In vivo elevation of extracellular potassium in the rat amygdala increases extracellular glutamate and aspartate and damages neurons

It is well known that high potassium (K+) solutions introduced by microdialysis into normal brain increase the extracellular concentration of the excitatory amino acid glutamate, and in vitro studies suggest that a high exogenously applied glutamate concentration can produce excitotoxic neuronal death. However, only recently were in vivo studies undertaken to determine whether high-K+ exposure damages neurons. We implanted microdialysis probes into rat amygdalae bilaterally, and after a 2-h baseline period exposed one side to a modified Krebs-Ringer-bicarbonate solution containing 100 mmol/l KCl for 30,50 and 70 min, followed by a 2-h recovery period, and 70 min and 3 h without a recovery period. Of 100.9 +/- 2.0 mmol/l KCl, 12.0 +/- 1.0% was extracted by amygdalar tissue in vivo. Election of the extracellular K+ concentration in the amygdala for 70 min or longer without a recovery period produced extensive neuronal damage and edematous-appearing neuropil in the tissue dialysed, as well as loss of normal neurons. Histological evidence of edema subsided in the groups with a 2-h recovery period. Although the number of damaged neurons was not significantly higher in the group with a 70 min high-K+ exposure and 2-h recovery period, the number of normal neurons was reduced, suggesting cell loss. During 70-min high-K+ exposure, the extracellular glutamate concentration increased to 242-377% of baseline during the first 60 min, and extracellular aspartate rose to 162-213% during the first 50 min; extracellular taurine rose even higher, to 316-567% of baseline, and glutamine fell to 14-27% of baseline. Extracellular serine was decreased at 20, 50 and 70 min of high-K+ exposure; extracellular glycine was unchanged. The elevated extracellular glutamate and aspartate concentrations suggest that exposure of the amygdala to high extracellular K+ may produce cell death through an excitotoxic process, and point the way to future studies to define the specific mechanisms involved.

Depolarization with high-K+ causes Ca(2+)-independent but partially Cl(-)-dependent glutamate release in rat hippocampal slice cultures

We studied the neurotoxic glutamate release induced by high-K+ depolarization in rat hippocampal slice cultures. Depolarization with 90 mM K+ for 30 min caused a significant, three-fold increase in glutamate release. This release was not inhibited by removing extracellular Ca2+, but was significantly inhibited by replacement of extracellular Cl- with SO4(2-). These findings suggest that glutamate is released by mechanisms other than conventional vesicular release under the high-K+ condition.

Long-term hippocampal slices: a model system for investigating synaptic mechanisms and pathologic processes

Organotypic cultures provide a unique strategy with which to examine many aspects of brain physiology and pathology. Long-term slice cultures from the hippocampus, a region involved in memory encoding and one that exhibits early degeneration in Alzheimer's disease and ischemia, are particularly valuable in this regard due to their expression of synaptic plasticity mechanisms (e.g., long-term potentiation) and responsiveness to pathological insults (e.g., excitotoxicity). Long-term slices can be prepared from hippocampi at the second or third postnatal week of development and thus incorporate a number of relatively mature features; further signs of maturation and the preservation of adult-like characteristics occur over succeeding weeks. The stability of the cultured slice renders it an appropriate model for studying 1) prolonged regulation/stabilization events linked to synaptogenesis and certain forms of plasticity, 2) temporal patterns of cellular atrophy associated with pathogenic conditions such as ischemia and epilepsia, and 3) slow processes associated with aging and age-related pathologies.

Stable maintenance of glutamate receptors and other synaptic components in long-term hippocampal slices

Cultured hippocampal slices retain many in vivo features with regard to circuitry, synaptic plasticity, and pathological responsiveness, while remaining accessible to a variety of experimental manipulations. The present study used ligand binding, immunostaining, and in situ hybridization assays to determine the stability of AMPA- and NMDA-type glutamate receptors and other synaptic proteins in slice cultures obtained from 11 day postnatal rats and maintained in culture for at least 4 weeks. Binding of the glutamate receptor ligands [3H]AMPA and [3H]MK-801 exhibited a small and transient decrease immediately after slice preparation, but the binding levels recovered by culture day (CD) 5-10 and remained stable for at least 30 days in culture. Autoradiographic analyses with both ligands revealed labeling of dendritic fields similar to adult tissue. In addition, slices at CD 10-20 expressed a low to high affinity [3H]AMPA binding ratio that was comparable with that in the adult hippocampus (10:1). AMPA receptor subunits GluR1 and GluR2/3 and an NMDA receptor subunit (NMDAR1) exhibited similar postcutting decreases as that exhibited by the ligand binding levels, followed by stable recovery. The GluR4 AMPA receptor subunit was not evident during the first 10 CDs but slowly reached detectable levels thereafter in some slices. Immunocytochemistry and in situ hybridization techniques revealed adult-like labeling of subunit proteins in dendritic processes and their mRNAs in neuronal cell body layers. Long-term maintenance was evident for other synapse-related proteins, including synaptophysin, neural cell adhesion molecule isoforms (NCAMs), and an AMPA receptor related antigen (GR53), as well as for certain structural and cytoskeletal components (e.g., myelin basic protein, spectrin, microtubule-associated proteins). In summary, following an initial and brief depression, many synaptic components were expressed at steady-state levels in long-term hippocampal slices, thus allowing the use of such a culture system for investigations into mechanisms of brain synapses.