Delayed neurotoxicity of excitatory amino acids in vitro

In Vitro Testing of Neurotoxicity

Many of the toxic compounds that are at large in the environment represent a risk to our neuronal functions. Chemicals may have a direct or indirect effect on the nervous system and they may interfere with general biochemical properties or specific neuronal structures and processes. In this review, a brief presentation of the major neurotoxicological targets is given, together with a discussion of some aspects of the use of different in vitro models for screening purposes and mechanistic studies. It is believed that in vitro methods offer special opportunities for the development of new neurotoxicological assays, and that this development will mainly involve cultured model systems. Therefore, a presentation of nerve and glia tissue culture methods is given, followed by an overview of how information on the action of mercury and mercurials, excitotoxins and acrylamide has been obtained through the use of cultured cell models. It is concluded that the developmental potential in cell neurotoxicology lies within the areas of separation and identification of cells representative for different structures in the nervous system, co-cultivation of different cell types, in vivo/in vitro (ex vivo) procedures, chemically defined media, metabolic competent cultures of human cells and improved physiological conditions for cultivation and exposure.

Merits and Limitations of Studying Neuronal Depolarization-Dependent Processes Using Elevated External Potassium

Elevated extracellular potassium chloride is widely used to achieve membrane depolarization of cultured neurons. This technique has illuminated mechanisms of calcium influx through L-type voltage sensitive calcium channels, activity-regulated signaling, downstream transcriptional events, and many other intracellular responses to depolarization. However, there is enormous variability in these treatments, including durations from seconds to days and concentrations from 3mM to 150 mM KCl. Differential effects of these variable protocols on neuronal activity and transcriptional programs are underexplored. Furthermore, potassium chloride treatments in vitro are criticized for being poor representatives of in vivo phenomena and are questioned for their effects on cell viability. In this review, we discuss the intracellular consequences of elevated extracellular potassium chloride treatment in vitro, the variability of such treatments in the literature, the strengths and limitations of this tool, and relevance of these studies to brain functions and dysfunctions.

Neurotransmitter alterations in the anterior cingulate cortex in Crohn's disease patients with abdominal pain: A preliminary MR spectroscopy study

Purpose

Crohn's disease (CD) has been known to cause both abdominal pain alongside functional and structural alterations in the central nervous system (CNS) in affected patients. This study seeks to determine the alternations of metabolites in the bilateral anterior cingulate cortex (ACC) of CD patients with abdominal pain by using proton magnetic resonance spectroscopy (¹H-MRS) to further explore the neural mechanism.

Methods

Sixteen CD patients with abdominal pain and 13 CD patients without abdominal pain, were recruited alongside 20 healthy controls (HCs) for this study. Clinical evaluations, including the 0–10 Visual Analogue Scale (VAS) of pain, Hospital Anxiety and Depression Scale (HADS) and Crohn's Disease Activity Index (CDAI), were evaluated prior to MR scanning. This study selected the bilateral ACC as the region of interest (ROI). The metabolites of the bilateral ACC were quantitatively analyzed by LCModel and Gannet. A independent sample t-test and one-way analysis of variance (ANOVA) were performed for statistical analysis. Spearman correlation analyses were performed to examine the relationship between the metabolite levels and clinical evaluations.

Results

The results indicated that CD patients with abdominal pain exhibited significantly higher levels of Glutamate (Glu)/(creatine + phosphocreatine, total creatine, tCr) over CD patients without abdominal pain, and HCs (p = 0.003, 0.009, respectively) in the bilateral ACC. The level of (Glutamate + Glutamine, Glx)/tCr of pain CD group was higher than non-pain CD group (p = 0.022). Moreover, within the pain CD group, Glu/tCr and Glx/tCr levels correlated strongly with the VAS scores of pain (ρ = 0.86, 0.59 respectively, p < 0.05). Meanwhile, the results indicates that CD patients with abdominal pain have significantly lower levels of γ-aminobutyric acid plus (GABA+)/tCr (p = 0.002) than HCs. To some extent, CDAI demonstrated a trend of negative correlation with GABA+/tCr levels (p = 0.088, ρ = −0.60).

Conclusion

The neural mechanism of CD patients with abdominal pain in pain processing is tightly associated with neurochemical metabolites. An imbalance in Glu and GABA may play a key role in abdominal pain processing for patients with CD. This mechanism of pain may associate with the intestinal microbiota on the brain-gut axis.

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The brain metabolite in CD patients with abdominal pain was firstly investigated.

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The study was conducted in vivo by using ¹H-MRS.

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Glu and GABA levels altered in ACC of CD patients with abdominal pain.

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CD patients with abdominal pain in pain processing implicated neurotransmitters.

Time-lapse imaging of p65 and IκBα translocation kinetics following Ca2+-induced neuronal injury reveals biphasic translocation kinetics in surviving neurons

The transcription factor nuclear factor-κB (NF-κB) regulates neuronal differentiation, plasticity and survival. It is well established that excitatory neurotransmitters such as glutamate control NF-κB activity. Glutamate receptor overactivation is also involved in ischemic- and seizure-induced neuronal injury and neurodegeneration. However, little is known at the single cell-level how NF-κB signaling relates to neuronal survival during excitotoxic injury. We found that silencing of p65/NF-κB delayed N-methyl-d-aspartate (NMDA)-induced excitotoxic injury in hippocampal neurons, suggesting a functional role of p65 in excitotoxicity. Time-lapse imaging of p65 and its inhibitor IκBα using GFP and Cerulean fusion proteins revealed specific patterns of excitotoxic NF-κB activation. Nuclear translocation of p65 began on average 8±3min following 15min of NMDA treatment and was observed in up to two thirds of hippocampal neurons. Nuclear translocation of IκBα preceded that of p65 suggesting independent translocation processes. In surviving neurons, the onset of p65 nuclear export correlated with mitochondrial membrane potential recovery. Dying neurons exhibited persistent nuclear accumulation of p65-eGFP until plasma membrane permeabilization. Our data demonstrate an important role for p65 activation kinetics in neuronal cell death decisions following excitotoxic injury.

Hypothermia reduces calcium entry via the N-methyl-D-aspartate and ryanodine receptors in cultured hippocampal neurons

Hypothermia is a powerful neuroprotective method when induced following cardiac arrest, stroke, and traumatic brain injury. The physiological effects of hypothermia are multifaceted and therefore a better knowledge of its therapeutic targets will be central to developing innovative combination therapies to augment the protective benefits of hypothermia. Altered neuronal calcium dynamics have been implicated following stroke, status epilepticus and traumatic brain injury. This study was therefore initiated to evaluate the effect of hypothermia on various modes of calcium entry into a neuron. Here, we utilized various pharmacological agents to stimulate major routes of calcium entry in primary cultured hippocampal neurons. Fluorescent calcium indicator Fura-2AM was used to compare calcium ratio under normothermic (37 °C) and hypothermic (31 °C) conditions. The results of this study indicate that hypothermia preferentially reduces calcium entry through N-methyl-D-aspartate receptors and ryanodine receptors. Hypothermia, on the other hand, did not have a significant effect on calcium entry through the voltage-dependent calcium channels or the inositol tri-phosphate receptors. The ability of hypothermia to selectively affect both N-methyl-D-aspartate receptors and ryanodine receptors-mediated calcium systems makes it an attractive intervention for alleviating calcium elevations that are present following many neurological injuries.

Memantine: a treatment for Alzheimer’s disease with a new formulation

In nearly 20 years, aside from cholinesterase inhibitors, memantine is the only drug approved for the treatment of Alzheimer’s disease (AD). Memantine is an uncompetitive N-methyl-D-aspartate receptor antagonist that blocks pathological glutamate activity while permitting normal physiological function, thus preventing glutamate-induced excitotoxicity. Three Phase III pivotal trials demonstrated memantine’s efficacy in treating moderate-to-severe AD, which led to its initial approval by the EMA in 2002 and US FDA in 2003. The recommended target dose is 10 mg twice daily. The US FDA recently approved an extended-release (ER) formulation of memantine for once-daily 28-mg dosing. Memantine ER was evaluated in a 24-week placebo-controlled trial of patients with moderate-to-severe AD, which found significant benefits for cognition, global assessment, behavior and caregiver burden, but not function. The most common adverse events were headache, dizziness, diarrhea, hypertension, anxiety and influenza. Overall, memantine in all formulations has a favorable safety/tolerability profile and is safe to use with cholinesterase inhibitors. Memantine ER has yet to be evaluated against conventionally dosed immediate-release memantine.

Regulated expression of surface AMPA receptors reduces excitotoxicity in auditory neurons

Dynamic regulation of the expression of surface AMPA receptors (AMPARs) is a key mechanism to modulate synaptic strength and efficacy in the CNS and also to regulate auditory sensitivity. Here we address the role of surface AMPAR expression in excitotoxicity by blocking clathrin-mediated AMPAR endocytosis in auditory neurons. We used a membrane-permeable, dynamin-derived, myristoylated peptide (myr-Dyn) to inhibit surface AMPAR endocytosis induced by glutamate receptor agonists in culture and by noise exposure in vivo. Myr-Dyn infused into the mouse cochlea induced excitotoxic responses to acoustic stimuli that were normally not excitotoxic. These included vacuolization in the nerve terminals and spiral ganglion as well as irreversible auditory brain stem response threshold shifts. In cultured spiral ganglion neuronal cells, blockade of the reduction of surface AMPARs exacerbated neuronal death by incubation with N-methyl-d-aspartate and AMPA. This excitotoxic neuronal death could be prevented by calpeptin, a calpain-specific inhibitor. These results suggest that the reduction of surface AMPAR by endocytosis during excitatory stimulation plays an important role in limiting the excitotoxic damage to the neuron.

N-methyl-D-aspartate receptor antagonists have variable affect in 3-nitropropionic acid toxicity

There is accumulating evidence that excitotoxicity and oxidative stress resulting from excessive activation of glutamate (N-methyl-D-aspartate) NMDA receptors are major participants in striatal degeneration associated with 3-nitropropionic acid (3NP) administration. Although excitotoxic and oxidative mechanisms are implicated in 3NP toxicity, there are conflicting reports as to whether NMDA receptor antagonists attenuate or exacerbate the 3NP-induced neurodegeneration. In the present study, we investigated the involvement of NMDA receptors in striatal degeneration, protein oxidation and motor impairment following systemic 3NP administration. We examined whether NMDA receptor antagonists, memantine and ifenprodil, influence the neurotoxicity of 3NP. The development of striatal lesion and protein oxidation following 3NP administration is delayed by memantine but not affected by ifenprodil. However, in behavioral experiments, memantine failed to improve and ifenprodil exacerbated the motor deficits associated with 3NP toxicity. Together, these findings suggest caution in the application of NMDA receptor antagonists as a neuroprotective agent in neurodegenerative disorders associated with metabolic impairment.

Elevation of Intracellular Ca2+ Concentration Induced by Hypoxia in Retinal Ganglion Cells

PURPOSE

To analyze the mechanism of hypoxia-induced changes of the intracellular Ca(2+) concentration ([Ca(2+)](i)) in retinal ganglion cells (RGCs).

METHODS

Fluo-3 was applied to the cut edge of the optic nerve of 6-week-old rats. The retina was sliced, and the Ca images were captured. A hypoxic condition was created by superfusing the retinal slice with an oxygen/glucose-deprived solution.

RESULTS

The retrograde staining method filled the RGCs selectively. Fifteen minutes of hypoxic conditions induced an increase in [Ca(2+)](i) in the RGCs (Delta0.13 +/- 0.03, n = 23). Application of 60 microM DL-2-amino-5-phosphonovaleric acid partially blocked the hypoxia-induced [Ca(2+)](i) increase in dendrites (Delta0.03 +/- 0.02, n = 4, P < 0.05) but not in the somata (Delta0.12 +/- 0.02, n = 9). The RGC dendrites showed a further increase in [Ca(2+)](i) after being switched back to an oxygenated solution (Delta0.14 +/- 0.04, n = 4). Neither 6-cyano-7-nitroquinoxaline-2,3-dione disodium, DL: -threo-beta-benzyloxyaspartate, nifedipine, nor bepridil inhibited the hypoxia-induced [Ca(2+)](i) increase. A Ca(2+)-free superfusion prevented the hypoxia-induced [Ca(2+)](i) increase in the somata (Delta0.07 +/- 0.02, n = 5, P < 0.05) but not in the dendrites (Delta0.16 +/- 0.005, n = 4).

CONCLUSIONS

The mechanism of the hypoxia-induced increase in [Ca(2+)](i) differs between somata and dendrites. The N-methyl-D-aspartate channel of dendrites seems to be the main route of Ca(2+) influx.

Memantine in the treatment of mild-to-moderate Alzheimer's disease

Memantine is the first and only medication that has been approved by European, US and Canadian regulatory agencies for the treatment of moderate-to-severe Alzheimer's disease (AD). It is an NMDA receptor antagonist that works to prevent excitotoxicity and cell death, which are mediated by the excessive influx of calcium during a sustained release of glutamate. Preclinical studies of memantine reveal that it has the potential to improve memory and learning processes after impairment has occurred, as well as to prevent further neuronal damage. Although memantine has been considered for the treatment of earlier AD, it has not yet been approved for this. Randomized controlled trials of memantine in the treatment of mild-to-moderate AD have demonstrated small treatment effects in measures of cognition, global assessment and behavior favoring the use of memantine. However, the differences between treatment groups were not consistently significant. Two ongoing long-term trials are further investigating the efficacy of memantine in the treatment of mild-to-moderate AD.

Memantine in the treatment of Alzheimer’s disease

Memantine is a moderate-affinity glutamate antagonist that primarily takes action at the N-methyl-D-aspartate receptor site. It has US FDA and European Medicines Agency approval for the treatment of moderate-to-severe Alzheimer’s disease. Memantine replaces Mg2+ at the N-methyl-D-aspartate receptor, blocking pathological glutamate activity but allowing normal glutamate action at this site. Consequently, calcium homeostasis is better maintained, reducing slow after hyperpolarization and preventing neuronal excitotoxicity and cell death. Clinical trials have shown that memantine is generally safe and well tolerated, and have provided evidence for its efficacy as assessed by cognitive, behavioral, functional and global measures. It has also been shown to be well tolerated and effective in the treatment of moderate-to-severe Alzheimer’s disease when patients received previous and ongoing treatment with donepezil. The tolerability and efficacy of memantine is under continued investigation in milder Alzheimer’s disease and other forms of dementia.

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.

Electrophysiological mechanisms of delayed excitotoxicity: positive feedback loop between NMDA receptor current and depolarization-mediated glutamate release

Delayed excitotoxic neuronal death after insult from exposure to high glutamate concentrations appears important in several CNS disorders. Although delayed excitotoxicity is known to depend on NMDA receptor (NMDAR) activity and Ca(2+) elevation, the electrophysiological mechanisms underlying postinsult persistence of NMDAR activation are not well understood. Membrane depolarization and nonspecific cationic current in the postinsult period were reported previously, but were not sensitive to NMDAR antagonists. Here, we analyzed mechanisms of the postinsult period using parallel current- and voltage-clamp recording and Ca(2+) imaging in primary hippocampal cultured neurons. We also compared more vulnerable older neurons [about 22 days in vitro (DIV)] to more resistant younger (about 15 DIV) neurons, to identify processes selectively associated with cell death in older neurons. During exposure to a modest glutamate insult (20 microM, 5 min), similar degrees of Ca(2+) elevation, membrane depolarization, action potential block, and increased inward current occurred in younger and older neurons. However, after glutamate withdrawal, these processes recovered rapidly in younger but not in older neurons. The latter also exhibited a concurrent postinsult increase in spontaneous miniature excitatory postsynaptic currents, reflecting glutamate release. Importantly, postinsult NMDAR antagonist administration reversed all of these persisting responses in older cells. Conversely, repolarization of the membrane by voltage clamp immediately after glutamate exposure reversed the NMDAR-dependent Ca(2+) elevation. Together, these data suggest that, in vulnerable neurons, excitotoxic insult induces a sustained positive feedback loop between NMDAR-dependent current and depolarization-mediated glutamate release, which persists after withdrawal of exogenous glutamate and drives Ca(2+) elevation and delayed excitotoxicity.

Changes in secondary glutamate release underlie the developmental regulation of excitotoxic neuronal cell death

Vulnerability to excitotoxicity increases during development in vivo and in vitro. To determine whether the mere presence of mature N-methyl-D-aspartate (NMDA) receptors coincides with the emergence of excitotoxicity or whether post-receptor signaling processes may also contribute, we examined the temporal relationship of NMDA receptor expression, function and toxicity using cortical cell cultures. Surface expression of all NMDA receptor subunits increased with time in culture. This correlated with NMDA receptor function, assessed both biochemically and electrophysiologically, but not with the appearance of excitotoxicity. Specifically, cells at day in vitro (DIV) 10 were less susceptible to NMDA receptor-induced neurotoxicity than those cultured for 14 days, even though receptor expression/function was identical. In addition, cell-attached single channel recordings revealed that NMDA receptor conductance, open probability, and frequency of channel openings were not significantly different between the two days. Intriguingly, depolarization-induced release of glutamate from cultures grown for 10 days was significantly lower than that released from cultures grown for 14 days. Further, exogenous addition of glutamate receptor agonists immediately after removal of NMDA rendered cultures at DIV 10 susceptible to excitotoxicity, while toxicity was significantly reduced by addition of an NMDA receptor antagonist immediately after exposure to NMDA at DIV 14. These data are the first to demonstrate that the subsequent, secondary release of glutamate plays an equal, if not more important, role than NMDA receptor development per se, in mediating the enhanced vulnerability of neurons to excitotoxicity that occurs with age.

Cleavage of the plasma membrane Na+/Ca2+ exchanger in excitotoxicity

In brain ischemia, gating of postsynaptic glutamate receptors and other membrane channels triggers intracellular Ca2+ overload and cell death. In excitotoxic settings, the initial Ca2+ influx through glutamate receptors is followed by a second uncontrolled Ca2+ increase that leads to neuronal demise. Here we report that the major plasma membrane Ca2+ extruding system, the Na+/Ca2+ exchanger (NCX), is cleaved during brain ischemia and in neurons undergoing excitotoxicity. Inhibition of Ca2+-activated proteases (calpains) by overexpressing their endogenous inhibitor protein, calpastatin or the expression of an NCX isoform not cleaved by calpains, prevented Ca2+ overload and rescued neurons from excitotoxic death. Conversely, down-regulation of NCX by siRNA compromised neuronal Ca2+ handling, transforming the Ca2+ transient elicited by non-excitotoxic glutamate concentrations into a lethal Ca2+overload. Thus, proteolytic inactivation of NCX-driven neuronal Ca2+ extrusion is responsible for the delayed excitotoxic Ca2+ deregulation and neuronal death.

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.

Partial involvement of group I metabotropic glutamate receptors in the neurotoxicity of 3-N-oxalyl-L-2,3-diaminopropanoic acid (L-beta-ODAP)

Neurolathyrism is a human motoneuron disease caused by the overconsumption of grass pea (Lathyrus sativus) that contains a toxic non-protein amino acid, 3-N-oxalyl-L-2,3-diaminopropanoic acid (L-beta-ODAP). The preventive activities of various glutamatergic agents from acute neuronal death caused by L-beta-ODAP were studied using rat primary cortical neuron/glia culture. Nearly 80% of the rat primary cortical neurons were killed by 300 microM L-beta-ODAP within 24 h. Though antagonists acting on the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor prevented most of the toxicity, antagonists acting on group I metabotropic glutamatergic receptors (mGluRs), including (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), and 2-methyl-6-(2-phenylethenyl)pyridine (SIB1893) partially and significantly prevented neuronal death due to L-beta-ODAP. These antagonists, within limited concentrations, did not have any inhibitory effects on the currents through AMPA receptors expressed in Xenopus oocytes. L-beta-ODAP itself did not induce the currents through group I mGluRs expressed in Xenopus oocytes. These results suggest that the neurotoxicity induced by L-beta-ODAP is partially mediated by the activation of group I mGluRs by an indirect mechanisms.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

The neuropharmacological basis for the use of memantine in the treatment of Alzheimer's disease

Memantine has been demonstrated to be safe and effective in the symptomatic treatment of Alzheimer's disease (AD). While the neurobiological basis for the therapeutic activity of memantine is not fully understood, the drug is not a cholinesterase inhibitor and, therefore, acts differently from current AD therapies. Memantine can interact with a variety of ligand-gated ion channels. However, NMDA receptors appear to be a key target of memantine at therapeutic concentrations. Memantine is an uncompetitive (channel blocking) NMDA receptor antagonist. Like other NMDA receptor antagonists, memantine at high concentrations can inhibit mechanisms of synaptic plasticity that are believed to underlie learning and memory. However, at lower, clinically relevant concentrations memantine can under some circumstances promote synaptic plasticity and preserve or enhance memory in animal models of AD. In addition, memantine can protect against the excitotoxic destruction of cholinergic neurons. Blockade of NMDA receptors by memantine could theoretically confer disease-modifying activity in AD by inhibiting the "weak" NMDA receptor-dependent excitotoxicity that has been hypothesized to play a role in the progressive neuronal loss that underlies the evolving dementia. Moreover, recent in vitro studies suggest that memantine abrogates beta-amyloid (Abeta) toxicity and possibly inhibits Abeta production. Considerable attention has focused on the investigation of theories to explain the better tolerability of memantine over other NMDA receptor antagonists, particularly those that act by a similar channel blocking mechanism such as dissociative anesthetic-like agents (phencyclidine, ketamine, MK-801). A variety of channel-level factors could be relevant, including fast channel-blocking kinetics and strong voltage-dependence (allowing rapid relief of block during synaptic activity), as well as reduced trapping (permitting egress from closed channels). These factors may allow memantine to block channel activity induced by low, tonic levels of glutamate--an action that might contribute to symptomatic improvement and could theoretically protect against weak excitotoxicity--while sparing synaptic responses required for normal behavioral functioning, cognition and memory.

Chloride influx aggravates Ca2+-dependent AMPA receptor-mediated motoneuron death

AMPA receptor-mediated excitotoxicity has been implicated in the pathogenesis of stroke, neurotrauma, epilepsy, and many neurodegenerative diseases such as motoneuron disease. We studied the role of Cl- in AMPA receptor-mediated Ca2+-dependent excitotoxicity in cultured rat spinal motoneurons. Using the gramicidin perforated patch-clamp technique, the intracellular Cl- concentration could be calculated from the reversal potential of the GABA-induced current. The membrane depolarization caused by AMPA receptor stimulation resulted in Cl- influx through 5-nitro-2(3-phenylpropyl-amino) benzoic acid- and niflumic acid-sensitive Cl- channels. Cl- influx during AMPA receptor stimulation aggravated excitotoxic motoneuron death by two mechanisms: an increase of AMPA receptor conductance and an elevation of the Ca2+ driving force through a partial repolarization. The Cl- influx during AMPA receptor stimulation was enhanced by coadministration of GABA. This resulted in an increased Ca2+ influx and an enhanced cell death, suggesting that concomitant GABAergic stimulation may aggravate excitotoxic motoneuron death.

Current Treatment for Alzheimer Disease and Future Prospects

A cascade of pathophysiological events is triggered in Alzheimer disease (AD) that ultimately involves common cellular signaling pathways and leads to cellular and network dysfunction, failure of neurotransmission, cell death, and a common clinical outcome. The process is asynchronous, meaning that viable neurons remain as targets for therapy even in the diseased state, and each stage of the cascade affords the possibility for therapeutic intervention. Cholinesterase inhibitors are the only available treatment in the United States for patients with mild to moderate AD, helping maintain cognitive and functional abilities in most patients and conferring beneficial behavioral effects in some. Memantine is an NMDA receptor antagonist that has recently been approved in Europe for treatment of moderately severe to severe AD and is under investigation in the United States. Its mechanism of action may include enhanced neurotransmission in several systems as well as antiexcitotoxic effects. There are data regarding the effectiveness of the combination of memantine with cholinesterase inhibitors that will be useful for the practicing clinician. Other agents have shown some benefit in clinical trials, including the antioxidants vitamin E, selegiline, and Ginkgo biloba extracts, although the weight of evidence regarding their effects is not sufficient to define clinical practice. Potential future therapies currently are in development that target multiple aspects of the illness cascade, including aberrant inflammation, neurotrophic function, and processing of beta amyloid and tau proteins. These newer approaches hold promise for disease modification but are as yet unproven. Whether or not disease-modifying or preventive therapies become a reality, clinicians will be faced with AD patients who require treatment at all stages of illness for the indefinite future. Cholinergic and emerging noncholinergic medications will likely prevail as the standards of treatment for years to come.

Vasopressin and oxytocin decrease excitatory amino acid release in adult rat supraoptic nucleus

Oxytocin and vasopressin reduce the amplitude of excitatory postsynaptic responses in magnocellular neuroendocrine cells of the supraoptic nucleus (SON). To test whether synaptic glutamate release is modulated by these neuropeptides, we examined the combined effect of vasopressin and oxytocin on depolarization-induced glutamate and aspartate release from acutely dissected rat SON or fronto-parietal cortex punches. Glutamate release was stimulated with 60 mm K+ for 5-10 min and measured using ion exchange chromatography or high-performance liquid chromatography. During depolarization with high K+, extracellular glutamate levels increased, on average, to 204% of control values. In the presence of vasopressin/oxytocin, K+-stimulated glutamate and aspartate release were significantly reduced by 34% and 62%, respectively, in the SON. Treatment with the aminopeptidase inhibitor amastatin did not mimic the effects of exogenous vasopressin/oxytocin on glutamate or aspartate release, suggesting that, under the conditions tested here, amastatin treatment may produce more complex effects. The effects of exogenous neuropeptides are likely mediated by oxytocin and/or vasopressin receptors, as the oxytocin- and V1a-receptor antagonist, Manning Compound (10-100 micro m), partially reversed the effects of vasopressin/oxytocin on SON glutamate release. In contrast, in cortical punches, glutamate release was enhanced by high K+, but vasopressin/oxytocin did not significantly reduce glutamate/aspartate release, consistent with the relatively sparse distribution of vasopressin/oxytocin receptors in fronto-parietal cortex. These findings suggest that locally released oxytocin and vasopressin may autoregulate SON magnocellular neuroendocrine cell activity in part by modulating the release of excitatory amino acids from afferent terminals targeting these cells and/or from other cellular sources.

Effect of anoxia and pharmacological anoxia on whole-cell NMDA receptor currents in cortical neurons from the western painted turtle

The mammalian brain undergoes rapid cell death during anoxia that is characterized by uncontrolled Ca(2+) entry via N-methyl-D-aspartate receptors (NMDARs). In contrast, the western painted turtle is extremely anoxia tolerant and maintains close-to-normal [Ca(2+)](i) during periods of anoxia lasting from days to months. A plausible mechanism of anoxic survival in turtle neurons is the regulation of NMDARs to prevent excitotoxic Ca(2+) injury. However, studies using metabolic inhibitors such as cyanide (NaCN) as a convenient method to induce anoxia may not represent a true anoxic stress. This study was undertaken to determine whether turtle cortical neuron whole-cell NMDAR currents respond similarly to true anoxia with N(2) and to NaCN-induced anoxia. Whole-cell NMDAR currents were measured during a control N(2)-induced anoxic transition and a control NaCN-induced transition. During anoxia with N(2) normalized, NMDAR currents decreased to 35.3%+/-10.8% of control values. Two different NMDAR current responses were observed during NaCN-induced anoxia: one resulted in a 172%+/-51% increase in NMDAR currents, and the other was a decrease to 48%+/-14% of control. When responses were correlated to the two major neuronal subtypes under study, we found that stellate neurons responded to NaCN treatment with a decrease in NMDAR current, while pyramidal neurons exhibited both increases and decreases. Our results show that whole-cell NMDAR currents respond differently to NaCN-induced anoxia than to the more physiologically relevant anoxia with N(2).

Glycine and neuroprotective effect of hypothermia in hypoxic-ischemic brain damage

The beneficial effects of hypothermia have long been known in non-traditional medicine but it is only in the past few decades that studies on the neuroprotective effects of hypothermia in hypoxic-ischemic brain injury have begun. Different mechanisms have been put forward to explain hypothermic neuroprotection including reduction of the excessive release of the excitatory amino acid neurotransmitter, glutamate. Recent experiments have questioned the key role of this neurotoxin in hypoxic-ischemic neuropathogenesis. In contrast, a mediatory role for another neurotransmitter, glycine in the neuroprotective effects of hypothermia has become more attractive, along with an indication of its role in the pathogenesis of ischemic neuronal damage. Thus, on the basis of reviewing relevant literature the hypothesis of a glycine-related mechanism of hypothermic neuroprotection in ischemia-induced neuronal injury has been put forward.

Protection Against N-methyl-D-aspartate Receptor-Mediated Neuronal Degeneration In Rat Brain by 7-chlorokynurenate and 3-amino-1-hydroxypyrrolid-2-one, Antagonists at The Allosteric Site for Glycine

7-Chlorokynurenate (7-Cl KYNA) and 3-amino-1-hydroxypyrrolid-2-one (HA-966), two selective antagonists of the glycine site on the N-methyl-D-aspartate (NMDA) receptor, have been used to assess the involvement of this site in the neurodegeneration resulting from injection of excitotoxins in the rat brain. In the rat striatum, reductions in the enzymes choline acetyltransferase (CAT) and glutamate decarboxylase (GAD), occurring 7 days after a unilateral, intrastriatal injection of quinolinate (200 nmol), were prevented in a dose-dependent manner by intrastriatal administration of 7-Cl KYNA (10 - 50 nmol) and HA-966 (200 - 500 nmol) 1 h after the excitotoxin. In the rat hippocampus, degeneration of pyramidal and granule neurons caused by direct injection of quinolinate (60 nmol) was completely prevented by 7-Cl KYNA (50 nmol) and partially by HA-966 (500 nmol) injected intrahippocampally 1 h after the excitotoxin. In the rat striatum, 7-Cl KYNA (50 nmol) and HA-966 (500 nmol) also reduced neurotoxicity caused by intrastriatal injection of NMDA (200 nmol), but not that caused by the 'non-NMDA' receptor agonists DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) or kainate. The time course of protective effects of 7-Cl KYNA and HA-966 in the striatum was similar to that previously observed with the uncompetitive NMDA receptor antagonist MK-801, indicating that activation of the glycine site contributes to the delayed degeneration of neurons which occurs over the first 5 h following quinolinate injection. The neuroprotective effects of both 7-Cl KYNA and HA-966 in the rat striatum appear to be mediated via the glycine site on the NMDA receptor as they were completely reversed by D-serine, but not L-serine. These results indicate that activation of the glycine site is essential for the expression of the delayed degeneration of neurons resulting from intracerebral injection of an NMDA receptor agonist, a process which bears similarities to the delayed neurodegeneration which results from a period of cerebral ischaemia.

Ion Dependence and Receptor Mediation of Glutamate Toxicity in the Immature Rat Hippocampal Slice

Glutamate (glu) is a major excitatory transmitter and a toxin in the brain. In the present study, the immature rat hippocampal slice was used to determine the morphology, topography, ionic mediation and receptor specificity of glu toxicity. Slices were exposed to glu for 30 min, and the damage was evaluated after 3 h of recovery in regular medium. The effects on glu toxicity of changes of [Ca2+], [Cl-] and [Na+] were determined. The receptor preference of glu was assessed by using the N-methyl-D-aspartate (NMDA) antagonist MK-801 and the kainate (KA)/quisqualate (QA) antagonist DNQX, alone or in combination. Further, to see whether glu produces cytotoxicity via osmolysis, the effects of hyperosmolal sucrose on glu toxicity were studied. Glu toxicity was similar to the previously described NMDA toxicity with regard to cytopathology, but differed in some aspects from that caused by KA and QA. The severity of the lesion was determined by the proximity of neurons to the incubation fluid, probably as a consequence of cellular accumulation of the amino acid. Omission of Ca2+ abolished glu toxicity in all neurons except the granule cells of the outer blade. This population was completely protected when Ca2+ was omitted and [Cl-] was reduced. Elevation of [Ca2+] markedly aggravated the lesion caused by glu. Substitution of isethionate for Cl- worsened the glu-induced damage, whilst the amino acid produced qualitatively different neuropathology when choline substituted for Na+. Apparently glu did not damage hippocampal nerve cells through an osmolytic mechanism as medium supplemented with 100 mM sucrose increased the toxicity of glu. Since the lesion produced by glu was more widespread in the presence of high [Ca2+], the effects of receptor antagonists were studied under this condition. MK-801 inhibited glu toxicity whereas DNQX had no effect. Combination of MK-801 and DNQX did not offer better protection than did MK-801 alone. The results suggest that Ca2+ is the main (but not single) determinant of glu toxicity in the immature hippocampal slice. The ionic requirements of glu neurotoxicity are identical to those of NMDA, but differ from those of KA and QA. The notion that glu is a selective NMDA agonist in the present model was confirmed by the protection of MK-801, and by the lack of an effect of DNQX. This is the first report demonstrating that the toxicity of glu is mediated by NMDA receptors in brain tissue which has developed normally. The findings indicate that specific blockade of NMDA receptors may be the most rational strategy in the prevention of glu-related neuronal death occurring in certain neurological anomalies.

7-Chlorokynurenate Blocks NMDA Receptor-Mediated Neurotoxicity in Murine Cortical Culture

We examined the neuroprotective actions of the glycine site N-methyl-D-aspartate (NMDA) antagonist, 7-chlorokynurenate, in murine neocortical cell cultures. Cultures exposed for 5 min to 100 - 500 microM NMDA in the absence of added glycine developed substantial neuronal degeneration over the next 24 h. The addition of 10 microM glycine did not increase submaximal NMDA-induced neuronal injury, suggesting that endogenous glycine levels were sufficient to saturate its receptor sites on NMDA receptor complexes. Addition of 3 - 300 microM 7-chlorokynurenate produced concentration-dependent reduction in this neuronal damage with an IC50 of approximately 30 microM. Some injury reduction was seen even if the drug was added after completion of the NMDA exposure. The protective effect of 100 microM 7-chlorokynurenate could be overcome by adding 10 - 1000 microM glycine (glycine median effective concentration (EC50) approximately 100 microM) or 1 mM D-serine. As predicted by its ability to block NMDA receptor-mediated injury, 10 - 300 microM 7-chlorokynurenate also produced concentration-dependent reduction in the neuronal loss induced by 50 - 60 min exposure to combined glucose and oxygen deprivation. These data support the suggestion that pharmacologic interference with the binding of glycine to the NMDA receptor complex represents a potentially effective approach to blocking NMDA receptor-induced neurotoxicity in ischemia.

Modification of glutamate binding sites in newborn brain during hypoglycemia

We have shown that acute insulin-induced hypoglycemia leads to specific changes in the cerebral NMDA receptor-associated ion channel in the newborn piglet. The present study tests the hypothesis that exposure to acute hypoglycemia in the newborn will alter the glutamate binding site of both NMDA and kainate receptors. Studies were performed in 3-6 days-old piglets randomized to control (n=6) or hypoglycemic (n=6) groups. Hypoglycemia was maintained for 120 min using insulin infusion. Saturation binding assays were performed in cerebral cell membranes using (3)H-glutamate or (3)H-kainate to determine the characteristics of the glutamate binding sites of the NMDA and kainate receptors, respectively. The concentration of glucose in cerebral cortex was 10-fold less in hypoglycemic piglets than in controls (P<0.05). Brain ATP was not significantly decreased during hypoglycemia, but phosphocreatine decreased from control of 6.6 +/- 1.3 micromoles/g brain to 3.2 +/- 1.9 micromoles/g brain in hypoglycemic piglets. The B(max) for NMDA-displaceable (3)H-glutamate binding was 992 +/- 64 fmol/mg protein in hypoglycemic animals, significantly higher than the control value of 746 +/- 42 fmol/mg protein. However, the dissociation constant for glutamate was unchanged during hypoglycemia. The (3)H-kainate binding studies demonstrated no change in B(max) of high-affinity kainate receptors during hypoglycemia. In contrast, the affinity of the kainate receptor glutamate binding site significantly increased compared to control. Thus, acute hypoglycemia in the newborn piglet had specific effects on the glutamate binding sites of the NMDA and kainate receptors that could be due to alterations in cell membrane lipids or modification of receptor proteins.

Evidence that the medial amygdala projects to the anterior/ventromedial hypothalamic nuclei to inhibit maternal behavior in rats

The maternal behaviors shown by a rat that has given birth are not shown by a virgin female rat when she is first presented with young. This absence of maternal behavior in virgins has been attributed to the activity of a neural circuit that inhibits maternal behavior in nulliparae. The medial amygdala and regions of the medial hypothalamus such as the anterior and ventromedial hypothalamic nuclei have previously been shown to inhibit maternal behavior, in that lesions to these regions promote maternal responding. Furthermore, we have recently shown that these and other regions, such as the principal bed nucleus of the stria terminalis, the ventral lateral septum, and the dorsal premammillary nucleus, show higher pup-induced Fos-immunoreactivity in non-maternal rats exposed to pups than during the performance of maternal behavior, indicating that they too could be involved in preventing maternal responsiveness. The current study tested whether the medial amygdala projects to the anterior/ventromedial hypothalamic nuclei in a neural circuit that inhibits maternal behavior, as well as to see what other brain regions could participate in this circuit. Bilateral excitotoxic lesions of the medial amygdala, or of the anterior/ventromedial hypothalamic nuclei, promoted maternal behavior. Unilateral medial amygdala lesions caused a reduction of pup-induced Fos-immunoreactivity in the anterior/ventromedial hypothalamic nuclei in non-maternal rats ipsilateral to the lesion, as well as in the principal bed nucleus of the stria terminalis, ventral lateral septum, and dorsal premammillary nucleus. Finally, unilateral medial amygdala lesions paired with contralateral anterior/ventromedial hypothalamic nuclei lesions promoted maternal behavior, although ipsilateral lesion placements were also effective.Together, these results indicate that the medial amygdala projects to the anterior/ventromedial hypothalamic nuclei in a neural circuit that inhibits maternal behavior, and that the principal bed nucleus of the stria terminalis, ventral lateral septum, and dorsal premammillary nucleus could also be involved in this circuit.

Neurotoxic and neuroprotective effects of glutamate are enhanced by introduction of amyloid precursor protein cDNA

The physiological role of amyloid precursor protein (APP), whose anomalous metabolite is a putative pathogen for Alzheimer disease, remains unclear. From the enhanced responsiveness to glutamate in cultured hippocampal neurons after the introduction of cDNA of APP695 (an isoform of APP dominant in human brain) using an adenovirus vector, we have recently raised the hypothesis that APP modulates neuronal sensitivity to glutamate. To test this hypothesis, we utilized here the unique effects of glutamate on the survival of different types of neurons. It is known that hippocampal neurons undergo deterioration in 24 h after application of glutamate in a dose-dependent manner. This vulnerability was increased in the cells transfected with adenovirus carrying cDNA of APP695. By contrast, it is known that cerebellar granule neurons require for their survival the supplementation of NMDA to the medium. The dose of NMDA required for survival was reduced after the transfection of the APP-adenovirus to cerebellar granule neurons. These enhancing effects of APP on the glutamate-induced vulnerability in hippocampal neurons and the glutamate (NMDA)-dependent survival in cerebellar neurons were blocked by glutamate receptor inhibitors, and were not seen after application of a control adenovirus carrying cDNA of beta-galactosidase. Since the effects of glutamate were enhanced in both directions, the hypothesis became more likely that one of the physiological functions of cellular APP is the regulation of glutamate receptors.

Oxidative glutamate toxicity can be a component of the excitotoxicity cascade

Along with ionotropic and metabotropic glutamate receptors, the cystine/glutamate antiporter x(c)(-) may play a critical role in CNS pathology. High levels of extracellular glutamate inhibit the import of cystine, resulting in the depletion of glutathione and a form of cell injury called oxidative glutamate toxicity. Here we show that a portion of the cell death associated with NMDA receptor-initiated excitotoxicity can be caused by oxidative glutamate toxicity. In primary mouse cortical neurons the cell death resulting from the short-term application of 10 microm glutamate can be divided into NMDA and NMDA receptor-independent phases. The NMDA receptor-independent component is associated with high extracellular glutamate and is inhibited by a variety of reagents that block oxidative glutamate toxicity. These results suggest that oxidative glutamate toxicity toward neurons lacking functional NMDA receptors can be a component of the excitotoxicity-initiated cell death pathway.

Establishment of CHO cell lines expressing four N-methyl-D-aspartate receptor subtypes and characterization of a novel antagonist PPDC

To develop an assay system that allows the N-methyl-D-aspartate (NMDA) receptor subtype-selective antagonistic potency of drugs, we have established Chinese hamster ovary cell lines expressing the four NMDA receptor subtypes (GluRepsilon1/zeta1-GluRepsilon4/zeta1) heat-indelibly. Using these clonal cells, we found that a novel antagonist, (1S,2R)-1-phenyl-2[(S)-1-aminopropyl]-N,N-diethylcyclopropanecarboxamide, was less selective for the GluRepsilon1/zeta1: the IC(50) values for the GluRepsilon1/zeta1-GluRepsilon4/zeta1 were 41.7, 13.3, 12.6 and 11.5 microM, respectively, while two well-known antagonists, DL-2-amino-5-phosphonovaleric acid and ifenprodil, showed the known potency and selectivity for each subtype. Thus, the established clonal cells are of use in characterizing the pharmacological properties of drugs that act on NMDA receptors.

Calbindin overexpression buffers hippocampal cultures from the energetic impairments caused by glutamate

A dramatic rise in free cytosolic calcium concentration is thought to be a central event in the pathogenesis of glutamate excitotoxicity in neurons. We have previously demonstrated that gene transfer of the calcium-binding protein calbindin D28k via a Herpes simplex amplicon vector decreases the rise in intracellular calcium and promotes cell survival following glutamatergic challenge. This study explores the effect of calbindin transgene expression on cellular metabolism following glutamate excitotoxicity. Because excitotoxic insults are often energetic in nature, and because calcium sequestering and extrusion place heavy energy demands on a cell, we hypothesized that calbindin overexpression may help preserve cellular energy levels during an insult. We overexpressed calbindin in primary hippocampal cultures, using a Herpes simplex amplicon vector system. We found that calbindin overexpression protected neurons from the decline in ATP levels, mitochondrial potential and metabolic rate following a glutamatergic insult. These results indicate that calbindin expression helps preserve cellular energy state following glutamate excitotoxicity. This illustrates the energetic load placed on neurons by increased free cytosolic calcium and may help explain the neuroprotective effects of calbindin.

Long-term enhancement of synaptic transmission induced by veratridine in rat CA3 hippocampal neurons

Veratridine is a neurotoxin that induces persistent activation of sodium channels in excitable cells. We investigated the effects of this toxin on excitatory synaptic transmission in CA3 neurons of juvenile rat hippocampus using whole-cell patch-clamp and field-potential recordings. The population spikes evoked by electrical stimulation of the mossy fiber were gradually enhanced after washout of veratridine (0.3 microM), but they were not enhanced by the co-application of veratridine and an N-methyl-D-aspartate (NMDA) receptor antagonist (D-APV, 30 microM). When a pipette solution contained QX-314 that antagonized the effect of veratridine in the recorded neuron, oscillatory membrane depolarization appeared in the early stage during bath-application of veratridine and gradually decreased in the late stage. After washout of veratridine, however, the oscillatory depolarization was gradually restored and maintained for at least 3 h. This oscillatory depolarization was also abolished by D-APV. We suggest that the activation of NMDA receptors is involved in the veratridine-induced long-lasting enhancement in the excitatory synaptic transmission in rat CA3 hippocampal neurons.

Calcium buffering and protection from excitotoxic cell death by exogenous calbindin-D28k in HEK 293 cells

Calbindin-D28k (CaBP) is a calcium-binding protein found in specific neuronal populations in the mammalian brain that, as a result of its proposed calcium-buffering action, may protect neurons against potentially harmful increases in intracellular calcium. We have stably transfected HEK 293 cells with recombinant human CaBP in order to determine the influence of this protein upon transient increases in intracellular ionic calcium concentration ([Ca(2+)](i)) induced either by transient transfection of the NR1 and NR2A subunits of the N-methyl-D-aspartate (NMDA) receptor and brief exposure to glutamate, photolysis of the caged calcium compound NP-EGTA, or exposure to the Ca(2+)]-ionophore 4-Br-A23187. The presence of CaBP did not significantly reduce the peak [Ca(2+)](i)stimulated by glutamate activation of NMDA receptors but significantly prolonged the recovery to baseline values. Flash photolysis of NP-EGTA in control cells resulted in an almost instantaneous increase in [Ca(2+)](i)followed by a bi-exponential recovery to baseline values. In cells stably expressing CaBP, the peak [Ca(2+)](i)levels were not statistically different from the controls, however, there was a significant prolongation of the initial portion of the slow recovery phase. In cells exposed to 4-Br-A23187, the presence of CaBP significantly reduced the rate of rise of [Ca(2+)](i), reduced the peak response, slowed the rate of recovery, and reduced the depolarization of mitochondria. In studies of delayed, Ca(2+)]-dependent cell death, CaBP transfected cells exhibited enhanced survival 24h after a 1-h exposure to 200 microM NMDA. However, necrotic cell death observed after the first 6h was not prevented by the presence of CaBP. These results provide direct evidence for a Ca(2+)-buffering effect of CaBP which serves to limit Ca(2+)entry and the depolarization of mitochondria, thereby protecting cells from death mediated most likely by apoptosis.

A role for sodium and chloride in kainic acid-induced beading of inhibitory interneuron dendrites

Excitotoxic injury of the dendrites of inhibitory interneurons could lead to decreases in their synaptic activation and explain subsequent local circuit hyperexcitability and epilepsy. A hallmark of dendrotoxicity, at least in principal neurons of the hippocampus and cortex, is focal or varicose swellings of dendritic arbors. In experiments reported here, transient (1h) exposure of hippocampal explant cultures to kainic acid produced marked focal swellings of the dendrites of parvalbumin-immunoreactive pyramidal basket cells in a highly reproducible and dose-dependent manner. At 5mM kainic acid, more than half of the immunopositive apical dendrites in area CA(1) had a beaded appearance. However, the somal volumes of these cells were unaltered by the same treatment. The presence of focal swellings was reversible with kainate washout and was not accompanied by interneuronal cell death. In contrast, exposure to much higher concentrations (300mM) of kainic acid resulted in the total loss of parvalbumin-positive interneurons from explants. Surprisingly, kainic acid-induced dendritic beading does not appear to be mediated by extracellular calcium. Beading was unaltered in the presence of N-methyl-D-aspartate receptor antagonists, the L-type calcium channel antagonist, nimodipine, cadmium, or by removing extracellular calcium. However, blockade of voltage-gated sodium channels by either tetrodotoxin or lidocaine abolished dendritic beading, while the activation of existing voltage-gated sodium channels by veratridine mimicked the kainic acid-induced dendritic beading. Finally, the removal of extracellular chloride prevented the kainic acid-induced dendritic beading.Thus, we suggest that the movement of Na(+) and Cl(-), rather than Ca(2+), into cells underlies the focal swellings of interneuron dendrites in hippocampus.

Inhibitors of mitochondrial respiration, iron (II), and hydroxyl radical evoke release and extracellular hydrolysis of glutathione in rat striatum and substantia nigra: potential implications to Parkinson's disease

In this investigation, microdialysis has been used to study the effects of 1-methyl-4-phenylpyridinium (MPP+), an inhibitor of mitochondrial complex I and alpha-ketoglutarate dehydrogenase and the active metabolite of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), on extracellular concentrations of glutathione (GSH) and cysteine (CySH) in the rat striatum and substantia nigra (SN). During perfusion of a neurotoxic concentration of MPP+ (2.5 mM) into the rat striatum or SN, extracellular concentrations of GSH and CySH remain at basal levels (both approximately 2 microM). However, when the perfusion is discontinued, a massive but transient release of GSH occurs, peaking at 5,000% of basal levels in the striatum and 2,000% of basal levels in the SN. The release of GSH is followed by a slightly delayed and smaller elevation of extracellular concentrations of CySH that can be blocked by the gamma-glutamyl transpeptidase (gamma-GT) inhibitor acivicin. Low-molecular-weight iron and extracellular hydroxyl radical (OH*) have been implicated as participants in the mechanism underlying the dopaminergic neurotoxicity of MPTP/MPP+. During perfusion of Fe2+ (OH*) into the rat striatum and SN, extracellular levels of GSH also remain at basal levels. When perfusions of Fe2+ are discontinued, a massive transient release of GSH occurs followed by a delayed, small, but progressive elevation of extracellular CySH level that again can be blocked by acivicin. Previous investigators have noted that extracellular concentrations of the excitatory/excitotoxic amino acid glutamate increase dramatically when perfusions of neurotoxic concentrations of MPP+ are discontinued. This observation and the fact that MPTP/MPP+ causes the loss of nigrostriatal GSH without corresponding increases of glutathione disulfide (GSSG) and the results of the present investigation suggest that the release and gamma-GT/dipeptidase-mediated hydrolysis of GSH to glutamate, glycine, and CySH may be important factors involved with the degeneration of dopamine neurons. It is interesting that a very early event in the pathogenesis of Parkinson's disease is a massive loss of GSH in the SN pars compacta that is not accompanied by corresponding increases of GSSG levels. Based on the results of this and prior investigations, a new hypothesis is proposed that might contribute to an understanding of the mechanisms that underlie the degeneration of dopamine neurons evoked by MPTP/MPP+, other agents that impair neuronal energy metabolism, and Parkinson's disease.