Hippocampal neurons exhibit both persistent Ca2+ influx and impairment of Ca2+ sequestration/extrusion mechanisms following excitotoxic glutamate exposure

Impaired Glutamate Receptor Function Underlies Early Activity Loss of Ipsilesional Motor Cortex after Closed-Head Mild Traumatic Brain Injury

Although mild traumatic brain injury (mTBI) accounts for the majority of TBI patients, the effects and cellular and molecular mechanisms of mTBI on cortical neural circuits are still not well understood. Given the transient and non-specific functional deficits after mTBI, it is important to understand whether mTBI causes functional deficits of the brain and the underlying mechanism, particularly during the early stage after injury. Here, we used in vivo optogenetic motor mapping to determine longitudinal changes in cortical motor map and in vitro calcium imaging to study how changes in cortical excitability and calcium signals may contribute to the motor deficits in a closed-head mTBI model. In channelrhodopsin 2 (ChR2)-expressing transgenic mice, we recorded electromyograms (EMGs) from bicep muscles induced by scanning blue laser on the motor cortex. There were significant decreases in the size and response amplitude of motor maps of the injured cortex at 2 h post-mTBI, but an increase in motor map size of the contralateral cortex in 12 h post-mTBI, both of which recovered to baseline level in 24 h. Calcium imaging of cortical slices prepared from green fluorescent calmodulin proteins-expressing transgenic mice showed a lower amplitude, but longer duration, of calcium transients of the injured cortex in 2 h post-mTBI. Blockade of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid or N-methyl-d-aspartate receptors resulted in smaller amplitude of calcium transients, suggesting impaired function of both receptor types. Imaging of calcium transients evoked by glutamate uncaging revealed reduced response amplitudes and longer duration in 2, 12, and 24 h after mTBI. Higher percentages of neurons of the injured cortex had a longer latency period after uncaging than that of the uninjured neurons. The results suggest that impaired glutamate neurotransmission contributes to functional deficits of the motor cortex in vivo, which supports enhancing glutamate neurotransmission as a potential therapeutic approach for the treatment of mTBI.

Neuronal-Specific Inhibition of Endoplasmic Reticulum Mg2+/Ca2+ ATPase Ca2+ Uptake in a Mixed Primary Hippocampal Culture Model of Status Epilepticus

Loss of intracellular calcium homeostasis is an established mechanism associated with neuronal dysfunction and status epilepticus. Sequestration of free cytosolic calcium into endoplasmic reticulum by Mg²⁺/Ca²⁺ adenosinetriphosphatase (ATPase) is critical for maintenance of intracellular calcium homeostasis. Exposing hippocampal cultures to low-magnesium media is a well-accepted in vitro model of status epilepticus. Using this model, it was shown that endoplasmic reticulum Ca²⁺ uptake was significantly inhibited in homogenates from cultures demonstrating electrophysiological seizure phenotypes. Calcium uptake was mainly neuronal. However, glial Ca²⁺ uptake was also significantly inhibited. Viability of neurons exposed to low magnesium was similar to neurons exposed to control solutions. Finally, it was demonstrated that Ca²⁺ uptake inhibition and intracellular free Ca²⁺ levels increased in parallel with increasing incubation in low magnesium. The results suggest that inhibition of Mg²⁺/Ca²⁺ ATPase-mediated endoplasmic reticulum Ca²⁺ sequestration contributes to loss of intracellular Ca²⁺ homeostasis associated with status epilepticus. This study describes for the first time inhibition of endoplasmic reticulum Mg²⁺/Ca²⁺ ATPase in a mixed primary hippocampal model of status epilepticus. In combination with animal models of status epilepticus, the cell culture model provides a powerful tool to further elucidate mechanisms that result in inhibition of Mg²⁺/Ca²⁺ ATPase and downstream consequences of decreased enzyme activity.

Otoprotective effects of ethosuximide in NOD/LtJ mice with age-related hearing loss

Despite long-term efforts to elucidate the mechanisms responsible for age-related hearing loss (AHL), there is currently no available treatment strategy able to provide a cure. Apoptotic cell death, including that of hair cells and spiral ganglion neurons (SGNs) in the cochlea has been proposed to be the classic theory behind the development of AHL. As calcium signaling plays key roles in signal transduction in apoptosis, in this study, we selected ethosuximide, which is able to block T-type calcium (Ca²⁺ion) channels, suppressing Ca²⁺. We hypothesized that the apoptotic pathway may be blocked through the inhibition of T-type Ca²⁺ channels in cochlear cells in NOD/LtJ mice. NOD/LtJ mice were divided into 2 groups as follows: the ethosuximide-treated and untreated (control) groups. Ethosuximide was administered by intraperitoneal injection every other day from post-natal day seven (P7) until the mice were 8 weeks of age. Following treatment, auditory-evoked brainstem response (ABR) thresholds and distortion product oto-acoustic emission (DPOAE) of the mice in the 2 groups were measured at different time points. Morphometric analysis and the expression of genes involved in the T-type Ca²⁺-mediated apoptotic pathway were monitored. The ABR and DPOAE results revealed that the NOD/LtJ mice exhibited early-onset and rapidly progressive AHL. A histological examination revealed that hair cell degeneration coincided with the progression of hearing loss. Hair cell and SGN was were significantly lower and auditory function was significantly improved in the ethosuximide-treated group compared to the untreated group. Our data thus indicate that ethosuximide prevents the degeneration of cochlear cells by regulating the expression of genes in apoptotic pathways. Our findings suggest that activating the T-type Ca²⁺ channel and downstream genes may be key pathological mechanisms responsible for AHL in NOD/LtJ mice.

What can we learn about stroke from retinal ischemia models?

Retinal ischemia is a very useful model to study the impact of various cell death pathways, such as apoptosis and necrosis, in the ischemic retina. However, it is important to note that the retina is formed as an outpouching of the diencephalon and is part of the central nervous system. As such, the cell death pathways initiated in response to ischemic damage in the retina reflect those found in other areas of the central nervous system undergoing similar trauma. The retina is also more accessible than other areas of the central nervous system, thus making it a simpler model to work with and study. By utilizing the retinal model, we can greatly increase our knowledge of the cell death processes initiated by ischemia which lead to degeneration in the central nervous system. This paper examines work that has been done so far to characterize various aspects of cell death in the retinal ischemia model, such as various pathways which are activated, and the role neurotrophic factors, and discusses how these are relevant to the treatment of ischemic damage in both the retina and the greater central nervous system.

Altered calcium signaling following traumatic brain injury

Cell death and dysfunction after traumatic brain injury (TBI) is caused by a primary phase, related to direct mechanical disruption of the brain, and a secondary phase which consists of delayed events initiated at the time of the physical insult. Arguably, the calcium ion contributes greatly to the delayed cell damage and death after TBI. A large, sustained influx of calcium into cells can initiate cell death signaling cascades, through activation of several degradative enzymes, such as proteases and endonucleases. However, a sustained level of intracellular free calcium is not necessarily lethal, but the specific route of calcium entry may couple calcium directly to cell death pathways. Other sources of calcium, such as intracellular calcium stores, can also contribute to cell damage. In addition, calcium-mediated signal transduction pathways in neurons may be perturbed following injury. These latter types of alterations may contribute to abnormal physiology in neurons that do not necessarily die after a traumatic episode. This review provides an overview of experimental evidence that has led to our current understanding of the role of calcium signaling in death and dysfunction following TBI.

Determination of transmembrane water fluxes in neurons elicited by glutamate ionotropic receptors and by the cotransporters KCC2 and NKCC1: a digital holographic microscopy study

Digital holographic microscopy (DHM) is a noninvasive optical imaging technique that provides quantitative phase images of living cells. In a recent study, we showed that the quantitative monitoring of the phase signal by DHM was a simple label-free method to study the effects of glutamate on neuronal optical responses (Pavillon et al., 2010). Here, we refine these observations and show that glutamate produces the following three distinct optical responses in mouse primary cortical neurons in culture, predominantly mediated by NMDA receptors: biphasic, reversible decrease (RD) and irreversible decrease (ID) responses. The shape and amplitude of the optical signal were not associated with a particular cellular phenotype but reflected the physiopathological status of neurons linked to the degree of NMDA activity. Thus, the biphasic, RD, and ID responses indicated, respectively, a low-level, a high-level, and an "excitotoxic" level of NMDA activation. Moreover, furosemide and bumetanide, two inhibitors of sodium-coupled and/or potassium-coupled chloride movement strongly modified the phase shift, suggesting an involvement of two neuronal cotransporters, NKCC1 (Na-K-Cl) and KCC2 (K-Cl) in the genesis of the optical signal. This observation is of particular interest since it shows that DHM is the first imaging technique able to monitor dynamically and in situ the activity of these cotransporters during physiological and/or pathological neuronal conditions.

Levetiracetam inhibits both ryanodine and IP3 receptor activated calcium induced calcium release in hippocampal neurons in culture

Epilepsy affects approximately 1% of the population worldwide, and there is a pressing need to develop new anti-epileptic drugs (AEDs) and understand their mechanisms of action. Levetiracetam (LEV) is a novel AED and despite its increasingly widespread clinical use, its mechanism of action is as yet undetermined. Intracellular calcium ([Ca2+]i) regulation by both inositol 1,4,5-triphosphate receptors (IP3R) and ryanodine receptors (RyR) has been implicated in epileptogenesis and the maintenance of epilepsy. To this end, we investigated the effect of LEV on RyR and IP3R activated calcium-induced calcium release (CICR) in hippocampal neuronal cultures. RyR-mediated CICR was stimulated using the well-characterized RyR activator, caffeine. Caffeine (10mM) caused a significant increase in [Ca2+]i in hippocampal neurons. Treatment with LEV (33 microM) prior to stimulation of RyR-mediated CICR by caffeine led to a 61% decrease in the caffeine induced peak height of [Ca2+]i when compared to the control. Bradykinin stimulates IP3R-activated CICR-to test the effect of LEV on IP3R-mediated CICR, bradykinin (1 microM) was used to stimulate cells pre-treated with LEV (100 microM). The data showed that LEV caused a 74% decrease in IP3R-mediated CICR compared to the control. In previous studies we have shown that altered Ca2+ homeostatic mechanisms play a role in seizure activity and the development of spontaneous recurrent epileptiform discharges (SREDs). Elevations in [Ca2+]i mediated by CICR systems have been associated with neurotoxicity, changes in neuronal plasticity, and the development of AE. Thus, the ability of LEV to modulate the two major CICR systems demonstrates an important molecular effect of this agent on a major second messenger system in neurons.

Expansion of the calcium hypothesis of brain aging and Alzheimer's disease: minding the store

Evidence accumulated over more than two decades has implicated Ca²⁺ dysregulation in brain aging and Alzheimer's disease (AD), giving rise to the Ca²⁺ hypothesis of brain aging and dementia. Electrophysiological, imaging, and behavioral studies in hippocampal or cortical neurons of rodents and rabbits have revealed aging-related increases in the slow afterhyperpolarization, Ca²⁺ spikes and currents, Ca²⁺ transients, and L-type voltage-gated Ca²⁺ channel (L-VGCC) activity. Several of these changes have been associated with age-related deficits in learning or memory. Consequently, one version of the Ca²⁺ hypothesis has been that increased L-VGCC activity drives many of the other Ca²⁺-related biomarkers of hippocampal aging. In addition, other studies have reported aging- or AD model-related alterations in Ca²⁺ release from ryanodine receptors (RyR) on intracellular stores. The Ca²⁺-sensitive RyR channels amplify plasmalemmal Ca²⁺ influx by the mechanism of Ca²⁺-induced Ca²⁺ release (CICR). Considerable evidence indicates that a preferred functional link is present between L-VGCCs and RyRs which operate in series in heart and some brain cells. Here, we review studies implicating RyRs in altered Ca²⁺ regulation in cell toxicity, aging, and AD. A recent study from our laboratory showed that increased CICR plays a necessary role in the emergence of Ca²⁺-related biomarkers of aging. Consequently, we propose an expanded L-VGCC/Ca²⁺ hypothesis, in which aging/pathological changes occur in both L-type Ca²⁺ channels and RyRs, and interact to abnormally amplify Ca²⁺ transients. In turn, the increased transients result in dysregulation of multiple Ca²⁺-dependent processes and, through somewhat different pathways, in accelerated functional decline during aging and AD.

Aging is associated with elevated intracellular calcium levels and altered calcium homeostatic mechanisms in hippocampal neurons

Aging is associated with increased vulnerability to neurodegenerative conditions such as Parkinson's and Alzheimer's disease and greater neuronal deficits after stroke and epilepsy. Emerging studies have implicated increased levels of intracellular calcium ([Ca(2+)](i)) for the neuronal loss associated with aging related disorders. Recent evidence demonstrates increased expression of voltage gated Ca(2+) channel proteins and associated Ca(2+) currents with aging. However, a direct comparison of [Ca(2+)](i) levels and Ca(2+) homeostatic mechanisms in hippocampal neurons acutely isolated from young and mid-age adult animals has not been performed. In this study, Fura-2 was used to determine [Ca(2+)](i) levels in CA1 hippocampal neurons acutely isolated from young (4-5 months) and mid-age (12-16 months) Sprague-Dawley rats. Our data provide the first direct demonstration that mid-age neurons in comparison to young neurons manifest significant elevations in basal [Ca(2+)](i) levels. Upon glutamate stimulation and a subsequent [Ca(2+)](i) load, mid-age neurons took longer to remove the excess [Ca(2+)](i) in comparison to young neurons, providing direct evidence that altered Ca(2+) homeostasis may be present in animals at significantly younger ages than those that are commonly considered aged (> or =24 months). These alterations in Ca(2+) dynamics may render aging neurons more vulnerable to neuronal death following stroke, seizures or head trauma. Elucidating the functionality of Ca(2+) homeostatic mechanisms may offer an understanding of the increased neuronal loss that occurs with aging, and allow for the development of novel therapeutic agents targeted towards decreasing [Ca(2+)](i) levels thereby restoring the systems that maintain normal Ca(2+) homeostasis in aged neurons.

Activation of a novel injury-induced calcium-permeable channel that plays a key role in causing extended neuronal depolarization and initiating neuronal death in excitotoxic neuronal injury

Protracted elevation in intracellular calcium caused by the activation of the N-methyl-d-aspartate receptor is the main cause of glutamate excitotoxic injury in stroke. However, upon excitotoxic injury, despite the presence of calcium entry antagonists, calcium unexpectedly continues to enter the neuron, causing extended neuronal depolarization and culminating in neuronal death. This phenomenon is known as the calcium paradox of neuronal death in stroke, and it represents a major problem in developing effective therapies for the treatment of stroke. To investigate this calcium paradox and to determine the source of this unexpected calcium entry after neuronal injury, we evaluated whether glutamate excitotoxicity activates an injury-induced calcium-permeable channel responsible for conducting a calcium current that underlies neuronal death. We used a combination of whole-cell and single-channel patch-clamp recordings, fluorescent calcium imaging, and neuronal cell death assays in a well characterized primary hippocampal neuronal culture model of glutamate excitotoxicity/stroke. Here, we report activation of a novel calcium-permeable channel upon excitotoxic glutamate injury that carries calcium current even in the presence of calcium entry inhibitors. Blocking this injury-induced calcium-permeable channel for a significant time period after the initial injury is still effective in preventing calcium entry, extended neuronal depolarization, and delayed neuronal death, thereby accounting for the calcium paradox. This injury-induced calcium-permeable channel represents a major source for the initial calcium entry following stroke, and it offers a new target for extending the therapeutic window for preventing neuronal death after the initial excitotoxic (stroke) injury.

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.

Tf-lipoplex-mediated NGF gene transfer to the CNS: neuronal protection and recovery in an excitotoxic model of brain injury

The development of efficient systems for in vivo gene transfer to the central nervous system (CNS) may provide a useful therapeutic strategy for the alleviation of several neurological disorders. In this study, we evaluated the feasibility of nonviral gene therapy to the CNS mediated by cationic liposomes. We present evidence of the successful delivery and expression of both a reporter and a therapeutic gene in the rodent brain, as evaluated by immunohistochemical assays. Our results indicate that transferrin-associated cationic liposome/DNA complexes (Tf-lipoplexes) allow a significant enhancement of transfection activity as compared to plain complexes, and that 8/1 (+/-) Tf-lipoplexes constitute the best formulation to mediate in vivo gene transfer. We demonstrated that Tf-lipoplex-mediated nerve growth factor transgene expression attenuates the morphological damages of the kainic acid-induced lesion as assessed by 2,3,5-triphenyltetrazolium chloride (TTC) vital staining. These findings suggest the usefulness of these lipid-based vectors in mediating the delivery of therapeutic genes to the CNS.

Calcium, mitochondria and oxidative stress in neuronal pathology. Novel aspects of an enduring theme

The interplay among reactive oxygen species (ROS) formation, elevated intracellular calcium concentration and mitochondrial demise is a recurring theme in research focusing on brain pathology, both for acute and chronic neurodegenerative states. However, causality, extent of contribution or the sequence of these events prior to cell death is not yet firmly established. Here we review the role of the alpha-ketoglutarate dehydrogenase complex as a newly identified source of mitochondrial ROS production. Furthermore, based on contemporary reports we examine novel concepts as potential mediators of neuronal injury connecting mitochondria, increased [Ca2+]c and ROS/reactive nitrogen species (RNS) formation; specifically: (a) the possibility that plasmalemmal nonselective cationic channels contribute to the latent [Ca2+]c rise in the context of glutamate-induced delayed calcium deregulation; (b) the likelihood of the involvement of the channels in the phenomenon of 'Ca2+ paradox' that might be implicated in ischemia/reperfusion injury; and (c) how ROS/RNS and mitochondrial status could influence the activity of these channels leading to loss of ionic homeostasis and cell death.

TRPMs and neuronal cell death

Death of CNS neurons during acute injury occurs as a result of a complex combination of excitotoxicity, necrosis, apoptosis, oedema and inflammatory reactions. Neuroprotection via glutamate receptor blockade or antioxidant or anti-inflammatory therapy have not proven effective in the clinical treatment of brain damage due to narrow therapeutic windows, poor pharmacokinetics or blockade of the signalling essential for normal excitatory neurotransmission and neuronal survival. Recent work in neuronal biochemistry, genomics and proteomics has increased understanding of the molecular organization of the excitatory synapse and the neuronal postsynaptic density. Transient receptor potential (TRP) channels are an exciting new family of cation channels that are highly expressed in the brain. Several members can be induced by oxidative stress and oxygen free radicals, both of which play important roles in neurodegeneration. Recent work has indicated that members of the melastatin subfamily (TRPM) of TRP proteins, particularly TRPM7 and TRPM2, may play key roles in neuronal death that is activated by oxidative stress and downstream from excitotoxic signal pathways. This discovery provides an exiting new avenue for research into the pathophysiology and treatment of acute neurodegeneration.

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.

Neuronal-specific endoplasmic reticulum Mg2+/Ca2+ ATPase Ca2+ sequestration in mixed primary hippocampal culture homogenates

Endoplasmic reticulum Mg(2+)/Ca(2+) ATPase Ca(2+) sequestration is crucial for maintenance of neuronal Ca(2+) homeostasis. The use of cell culture in conjunction with modern Ca(2+) imaging techniques has been invaluable in elucidating these mechanisms. While imaging protocols evaluate endoplasmic reticulum Ca(2+) loads, measurement of Mg(2+)/Ca(2+) ATPase activity is indirect, comparing cytosolic Ca(2+) levels in the presence or absence of the Mg(2+)/Ca(2+) ATPase inhibitor thapsigargin. Direct measurement of Mg(2+)/Ca(2+) ATPase by isolation of microsomes is impossible due to the minuscule amounts of protein yielded from cultures used for imaging. In the current study, endoplasmic reticulum Mg(2+)/Ca(2+) ATPase Ca(2+) sequestration was measured in mixed homogenates of neurons and glia from primary hippocampal cultures. It was demonstrated that Ca(2+) uptake was mediated by the endoplasmic reticulum Mg(2+)/Ca(2+) ATPase due to its dependence on ATP and Mg(2+), enhancement by oxalate, and inhibition by thapsigargin. It was also shown that neuronal Ca(2+) uptake, mediated by the type 2 sarco(endo)plasmic reticulum Ca(2+) ATPase isoform, could be distinguished from glial Ca(2+) uptake in homogenates composed of neurons and glia. Finally, it was revealed that Ca(2+) uptake was sensitive to incubation on ice, extremely labile in the absence of protease inhibitors, and significantly more stable under storage conditions at -80 degrees C.

Reduction of intracellular calcium removal rate can explain changes in seizure dynamics: studies in neuronal network models

Complex partial seizures originating from mesial temporal structures are characterized by relatively short durations of organized rhythmic activity (ORA) of 5-8 Hz, typically lasting less than 60s. Previous investigations into seizure dynamics have revealed that this ORA undergoes a monotonic decline prior to seizure evolution into intermittent bursting and subsequent seizure termination. Large neural network models of simplified single-compartment neurons were employed to address the hypothesis that changes in the free intracellular calcium ([Ca2+]i) removal rate during network bursting can result in the alterations of rhythmic seizure activity similar to that observed in recordings from humans. Both exponential and linear models of decreasing calcium removal rates resulted in changes in the predominant frequency of network bursting very similar in frequency and time course to those seen in human intracranial recordings. This supports the concept that changes in [Ca2+]i removal can explain this important network behavior, while not excluding alternative hypotheses. Identifying potential mechanisms underlying the dynamic changes seen in epileptogenic activity in large neural networks can provide important insights into seizure evolution and termination. Model neural network ensembles are attractive systems to address these questions that are difficult to investigate in biological preparations.

Retinal ischemia: mechanisms of damage and potential therapeutic strategies

Retinal ischemia is a common cause of visual impairment and blindness. At the cellular level, ischemic retinal injury consists of a self-reinforcing destructive cascade involving neuronal depolarisation, calcium influx and oxidative stress initiated by energy failure and increased glutamatergic stimulation. There is a cell-specific sensitivity to ischemic injury which may reflect variability in the balance of excitatory and inhibitory neurotransmitter receptors on a given cell. A number of animal models and analytical techniques have been used to study retinal ischemia, and an increasing number of treatments have been shown to interrupt the "ischemic cascade" and attenuate the detrimental effects of retinal ischemia. Thus far, however, success in the laboratory has not been translated to the clinic. Difficulties with the route of administration, dosage, and adverse effects may render certain experimental treatments clinically unusable. Furthermore, neuroprotection-based treatment strategies for stroke have so far been disappointing. However, compared to the brain, the retina exhibits a remarkable natural resistance to ischemic injury, which may reflect its peculiar metabolism and unique environment. Given the increasing understanding of the events involved in ischemic neuronal injury it is hoped that clinically effective treatments for retinal ischemia will soon be available.

Glutamate and amyloid beta-protein rapidly inhibit fast axonal transport in cultured rat hippocampal neurons by different mechanisms

Impairment of axonal transport leads to neurodegeneration and synapse loss. Glutamate and amyloid beta-protein (Abeta) have critical roles in the pathogenesis of Alzheimer's disease (AD). Here we show that both agents rapidly inhibit fast axonal transport in cultured rat hippocampal neurons. The effect of glutamate (100 microm), but not of Abeta25-35 (20 microm), was reversible, was mimicked by NMDA or AMPA, and was blocked by NMDA and AMPA antagonists and by removal of extracellular Ca2+. The effect of Abeta25-35 was progressive and irreversible, was prevented by the actin-depolymerizing agent latrunculin B, and was mimicked by the actin-polymerizing agent jasplakinolide. Abeta25-35 induced intracellular actin aggregation, which was prevented by latrunculin B. Abeta31-35 but not Abeta15-20 exerted effects similar to those of Abeta25-35. Full-length Abeta1-42 incubated for 7 d, which specifically contained 30-100 kDa molecular weight assemblies, also caused an inhibition of axonal transport associated with intracellular actin aggregation, whereas freshly dissolved Abeta1-40, incubated Abeta1-40, and fresh Abeta1-42 had no effect. These results suggest that glutamate inhibits axonal transport via activation of NMDA and AMPA receptors and Ca2+ influx, whereas Abeta exerts its inhibitory effect via actin polymerization and aggregation. The ability of Abeta to inhibit axonal transport seems to require active amino acid residues, which is probably present in the 31-35 sequence. Full-length Abeta may be effective when it represents a structure in which these active residues can access the cell membrane. Our results may provide insight into the early pathogenetic mechanisms of AD.

Protein kinase C-dependent potentiation of intracellular calcium influx by sigma1 receptor agonists in rat hippocampal neurons

Intracellular calcium concentration ([Ca2+]i) plays a major role in neuronal excitability, especially that triggered by the N-methyl-d-aspartate (NMDA)-sensitive glutamatergic receptor. We have previously shown that sigma1 receptor agonists potentiate NMDA receptor-mediated neuronal activity in the hippocampus and recruit Ca2+-dependent second messenger cascades (e.g., protein kinase C; PKC) in brainstem motor structures. The present study therefore assessed whether the potentiating action of sigma1 agonists on the NMDA response observed in the hippocampus involves the regulation of [Ca2+]i and PKC. For this purpose, [Ca2+]i changes after NMDA receptor activation were monitored in primary cultures of embryonic rat hippocampal pyramidal neurons using microspectrofluorometry of the Ca2+-sensitive indicator Fura-2/acetoxymethyl ester in the presence of sigma1 agonists and PKC inhibitors. We show that successive activations of the sigma1 receptor by 1-min pulses of (+)-benzomorphans or (+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethyl-but-3-en-1-ylamine hydrochloride (JO-1784) concomitantly with glutamate time dependently potentiated before inconstantly inhibiting the NMDA receptor-mediated increase of [Ca2+]i, whereas 1,3-di-o-tolyl-guanidine, a mixed sigma1/sigma2 agonist, did not significantly modify the glutamate response. Both potentiation and inhibition were prevented by the selective sigma1 antagonist N,N-dipropyl-2-[4-methoxy-3-(211phenylethoxy) phenyl]-ethylamine monohydrochloride (NE-100). Furthermore, only (+)-benzomorphans could induce [Ca2+]i influx by themselves after a brief pulse of glutamate. A pretreatment with the conventional PKC inhibitor 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo [2,3-a] pyrrolo [3,4-c] carbazole (Gö-6976) prevented the potentiating effect of (+)-benzomorphans on the glutamate response. Our results provide further support for a general mechanism for the intracellular sigma1 receptor to regulate Ca2+-dependent signal transduction and protein phosphorylation.

Interactions between mitochondrial bioenergetics and cytoplasmic calcium in cultured cerebellar granule cells

The mitochondrion has moved to the center stage in the drama of the life and death of the neuron. The mitochondrial membrane potential controls the ability of the organelle to generate ATP, generate reactive oxygen species and sequester Ca(2+) entering the cell. Each of these processes interact, and their deconvolution is far from trivial. The cultured cerebellar granule cell provides a model in which knowledge gained from studies on isolated mitochondria can be applied to study the role played by the organelles in the maintenance of Ca(2+) homeostasis in the cell under resting, stimulated and pathophysiological conditions. In particular, mitochondria play a complex role in the response of the neuron to excitotoxic stimulation of NMDA and AMPA-kainate selective glutamate receptors. One goal of research in this area is to provide clues as to possible ways in which modulators of mitochondrial function may be used as neuroprotective agents, since mitochondrial Ca(2+) accumulation seems to play a key role in glutamate excitotoxicity.

Age-related changes in tolerance to the marine algal excitotoxin domoic acid

During an incident of toxic mussel poisoning, the epileptogenic excitotoxin domoic acid (DOM) was associated with lasting neurological deficits mainly in older patients (), suggesting supersensitivity to excitotoxins is a feature of brain aging. Here, hippocampal slices from young (3 months) and aged (26-29 months) Sprague Dawley rats were assessed by CA1 field potential analysis before and after preconditioning with DOM. In naïve slices from young animals, DOM produced initial hyperexcitability followed by significant dose-dependent reductions in population spike amplitude during prolonged application. Following toxin washout, only small changes in neuronal activity were evident during a second application of DOM, suggesting that a resistance to the effects of DOM occurs in hippocampal slices which have undergone prior exposure to DOM. This inducible tolerance was not antagonized by the NMDA receptor blockers APV or MK-801, nor was it diminished by the group I, II or III mGluR blockers AIDA, CPPG and EGLU. Likewise, neither the AMPA/KA blocker CNQX nor the VSCC blocker nifedipine were effective in blocking tolerance induction in young slices. Field potential analysis revealed significant age-related reductions in CA1 EPSP strength, population spike amplitude and paired-pulse inhibition, but aged slices did not differ in sensitivity to DOM relative to young. However, aged CA1 failed to exhibit any tolerance to DOM following preconditioning, suggesting that a loss of inducible neuroprotective mechanisms may account for increased sensitivity to excitotoxins during aging.

Calcium rise in cultured neurons from medial septum elicits calcium waves in surrounding glial cells

One prerequisite for understanding the physiological relevance of intercellular calcium waves in glia is the examination of mechanisms that trigger these waves. Here, we show that stimulation of cultured septal neurons to produce a large and sustained calcium rise in the soma can initiate calcium waves in surrounding glial cells. The initiation of calcium waves is dependent on calcium influx through voltage-gated calcium channels on the neuron. The waves are not due to direct stimulation of the glial cells or to loss of neuronal membrane integrity. Mechanism of wave initiation is distinct from that involved in wave propagation and does not involve glutamate or acetylcholine release. Communication via gap junctions, or nitric oxide production, is not involved in the initial signaling between a stimulated neuron and the surrounding astrocytes. Suramin, a blocker of P2 receptors blocked the waves but failed to abolish the responses in glial cells immediately surrounding the stimulated neuron. Our results suggest that patterns of calcium rises on neurons, like those seen in glutamate cytotoxicity, can cause calcium waves in surrounding glial cells.

Evidence for a physiological role of endocannabinoids in the modulation of seizure threshold and severity

The anticonvulsant effect of cannabinoids has been shown to be mediated through activation of the cannabinoid CB(1) receptor. This study was initiated to evaluate the effects of endogenously occurring cannabinoids (endocannabinoids) on seizure severity and threshold. The anticonvulsant effect of the endocannabinoid, arachidonylethanolamine (anandamide), was evaluated in the maximal electroshock seizure model using male CF-1 mice and was found to be a fully efficacious anticonvulsant (ED(50)=50 mg/kg i.p.). The metabolically stable analog of anandamide, (R)-(20-cyano-16,16-dimetyldocosa-cis-5,8,11,14-tetraenoyl)-1'-hydroxy-2'-propylamine (O-1812), was also determined to be a potent anticonvulsant in the maximal electroshock model (ED(50)=1.5 mg/kg i.p.). Furthermore, pretreatment with the cannabinoid CB(1) receptor specific antagonist N-(piperidin-1-yl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A) completely abolished the anticonvulsant effect of anandamide as well as O-1812 (P< or =0.01, Fisher exact test), indicating a cannabinoid CB(1) receptor-mediated anticonvulsant mechanism for both endocannabinoid compounds. Additionally, the influence of cannabinoid CB(1) receptor endogenous tone on maximal seizure threshold was assessed using SR141716A alone. Our data show that SR141716A (10 mg/kg i.p.) significantly reduced maximal seizure threshold (CC(50)=14.27 mA) compared to vehicle-treated animals (CC(50)=17.57 mA) (potency ratio=1.23, lower confidence limit=1.06, upper confidence limit=1.43), indicating the presence of an endogenous cannabinoid tone that modulates seizure activity. These data demonstrate that anandamide and its analog, O-1812, are anticonvulsant in a whole animal model and further implicate the cannabinoid CB(1) receptor as a major endogenous site of seizure modulation.

Group I metabotropic glutamate receptor inhibition selectively blocks a prolonged Ca(2+) elevation associated with age-dependent excitotoxicity

It has been recognized for some years that a prolonged Ca(2+) elevation that is predictive of impending cell death develops in cultured neurons following excitotoxic insult. In addition, neurons exhibit enhanced sensitivity to excitotoxic insult with increasing age in culture. However, little is known about the processes that selectively regulate the post-insult Ca(2+) elevation and therefore, it remains unclear whether it is associated specifically with age-dependent toxicity.Here, we tested the hypothesis that a group I metabotropic glutamate receptor antagonist selectively modulates the prolonged Ca(2+) elevation in direct association with its protective effects against excitotoxicity. Rat hippocampal cultures of two ages (8-9 and 21-28 days in vitro) were exposed to a 5-min glutamate insult (400 microM in younger and 10 microM in older cultures) sufficient to kill >50% of the neurons, and were treated with vehicle or the specific group I metabotropic glutamate receptor antagonist 1-aminoindan-1,5-dicarboxylic acid (AIDA; 1 mM), throughout and following the insult. Neuronal survival was quantified 24 h after insult. In parallel studies, neurons of similar age in culture were imaged ratiometrically with a confocal microscope during and for 60 min after the glutamate insult. A large post-insult Ca(2+) elevation was present in older but not most younger neurons. The N-methyl-D-aspartate receptor antagonist, MK-801, blocked the Ca(2+) elevation both during and following the insult. In contrast, AIDA blocked only the post-insult prolonged Ca(2+) elevation in older neurons. Moreover, AIDA was neuroprotective in older but not younger cultures. From these results we suggest that the post-insult Ca(2+) elevation is regulated differently from the Ca(2+) elevation during glutamate insult and is modulated by group I metabotropic glutamate receptors. Further, the prolonged Ca(2+) elevation appears to be directly linked to an age-dependent component of vulnerability.

Sustained Ca2+-induced Ca2+-release underlies the post-glutamate lethal Ca2+ plateau in older cultured hippocampal neurons

Several studies have shown that a prolonged Ca(2+) elevation follows a glutamate-mediated excitotoxic insult in cultured neurons, and may be associated with impending cell death. Recently, we showed that the prolonged Ca(2+) elevation that emerges as neurons age in culture is specifically linked to an age-related increase in excitotoxic vulnerability. However, the multiple sources of Ca(2+) that contribute to Ca(2+) elevation during and after glutamate exposure are not well understood. Here, we examined the Ca(2+) sources of the age-related prolonged Ca(2+) elevation in cultured hippocampal neurons. Studies with caffeine showed that the ryanodine receptor-dependent releasable pool of Ca(2+) from intracellular stores was similar in older and younger neurons. Thapsigargin, which inhibits intracellular store refilling, did not mimic the age-related prolonged Ca(2+) elevation and, in fact, partially reduced it. Ryanodine, which blocks Ca(2+)-induced Ca(2+)-release (CICR) from stores, completely blocked the age-related prolonged Ca(2+) elevation following glutamate exposure but did not alter maximal Ca(2+) elevation during the glutamate exposure. Thus, we conclude that sustained CICR plays a selective and key role in generating the lethal, age-related, prolonged Ca(2+) elevation, and is the likely mechanism underlying age-related, enhanced vulnerability to excitotoxicity in neurons.

Epileptogenesis induces long-term alterations in intracellular calcium release and sequestration mechanisms in the hippocampal neuronal culture model of epilepsy

Calcium and calcium-dependent processes have been hypothesized to be involved in the induction of epilepsy. It has been shown that epileptic neurons have altered calcium homeostatic mechanisms following epileptogenesis in the hippocampal neuronal culture (HNC) and pilocarpine models of epilepsy. To investigate the mechanisms causing these alterations in [Ca2+]i homeostatic processes following epileptogenesis, we utilized the HNC model of in vitro 'epilepsy' which produces spontaneous recurrent epileptiform discharges (SREDs). Using [Ca2+]i imaging, studies were initiated to evaluate the mechanisms mediating these changes in [Ca2+]i homeostasis. 'Epileptic' neurons required much longer to restore a glutamate induced [Ca2+]i load to baseline levels than control neurons. Inhibition of Ca2+ entry through voltage and receptor gated Ca2+ channels and stretch activated Ca2+ channels had no effect on the prolonged glutamate induced increase in [Ca2+]i in epileptic neurons. Employing thapsigargin, an inhibitor of the sarco/endoplasmic reticulum calcium ATPase (SERCA), it was shown that thapsigargin inhibited sequestration of [Ca2+]i by SERCA was significantly decreased in 'epileptic' neurons. Using Ca2+ induced Ca2+ release (CICR) cell permeable inhibitors for the ryanodine receptor (dantrolene) and the IP3 receptor (2-amino-ethoxydiphenylborate, 2APB) mediated CICR, we demonstrated that CICR was significantly augmented in the 'epileptic' neurons, and determined that the IP3 receptor mediated CICR was the major release mechanism altered in epileptogenesis. These data indicate that both inhibition of SERCA and augmentation of CICR activity contribute to the alterations accounting for the impaired calcium homeostatic processes observed in 'epileptic' neurons. The results suggest that persistent changes in [Ca2+]i levels following epileptogenesis may contribute to the long-term plasticity changes manifested in epilepsy and that understanding the basic mechanisms mediating these changes may provide an insight into the development of novel therapeutic approaches to treat epilepsy and prevent or reverse epileptogenesis.