Glutamate receptor activation in cultured cerebellar granule cells increases cytosolic free Ca2+ by mobilization of cellular Ca2+ and activation of Ca2+ influx

Neurobiological mechanisms of mood disorders: Stress vulnerability and resilience

Stress is an important factor in the development of several human pathologies. The response of rodents and humans to stress depends on many factors; some people and rodents develop stress-related mood disorders, such as depression and anxiety in humans, depression-like and anxiety-like behavior in mice and rats, while others report no new psychological symptoms in response to chronic or acute stress, and are considered susceptible and resilient to stress, respectively. Resilience is defined as the ability to thrive in the face of adversity and is a learned process that can help protect against occupational stressors and mental illnesses. There is growing interest in the underlying mechanisms involved in resilience and vulnerability to depression caused by stress, and some studies have demonstrated that individual variability in the way animals and humans respond to stress depends on several mechanisms, such as oxidative stress, neuronal plasticity, immunology and genetic factors, among others not discussed in this review, this review provides a general overview about this mechanism.

Ryanodine receptors: physiological function and deregulation in Alzheimer disease

Perturbed Endoplasmic Reticulum (ER) calcium (Ca2+) homeostasis emerges as a central player in Alzheimer disease (AD). Accordingly, different studies have reported alterations of the expression and the function of Ryanodine Receptors (RyR) in human AD-affected brains, in cells expressing familial AD-linked mutations on the β amyloid precursor protein (βAPP) and presenilins (the catalytic core in γ-secretase complexes cleaving the βAPP, thereby generating amyloid β (Aβ) peptides), as well as in the brain of various transgenic AD mice models. Data converge to suggest that RyR expression and function alteration are associated to AD pathogenesis through the control of: i) βAPP processing and Aβ peptide production, ii) neuronal death; iii) synaptic function; and iv) memory and learning abilities. In this review, we document the network of evidences suggesting that RyR could play a complex dual "compensatory/protective versus pathogenic" role contributing to the setting of histopathological lesions and synaptic deficits that are associated with the disease stages. We also discuss the possible mechanisms underlying RyR expression and function alterations in AD. Finally, we review recent publications showing that drug-targeting blockade of RyR and genetic manipulation of RyR reduces Aβ production, stabilizes synaptic transmission, and prevents learning and memory deficits in various AD mouse models. Chemically-designed RyR "modulators" could therefore be envisioned as new therapeutic compounds able to delay or block the progression of AD.

Homeostatic regulation of glutamate neurotransmission in primary neuronal cultures

Glutamate is the mayor excitatory neurotransmitter in vertebrate nervous system. It has a crucial role in most brain functions under physiological conditions through the activation of both ionotropic and metabotropic glutamate receptors. In addition, extracellular glutamate concentration is tightly regulated through different excitatory amino acid transporters (EAAT). Glutamate neurotransmission is also involved in the neurotoxic effects of many environmental chemicals and drugs. Furthermore, homeostatic changes in glutamate neurotransmission appear in response to prolonged block/enhancement of electrical activity. Here, we describe different approaches to evaluate alterations in glutamate neurotransmission regarding glutamate receptors and glutamate transporters by using primary cultures of neurons and astrocytes. The methods are based on the increased fluorescence of calcium-sensitive probes in response to glutamate agonists, on radioligand binding to glutamate receptors and transport sites, and on inmunocytochemistry visualization of glutamate receptors.

Dantrolene: mechanisms of neuroprotection and possible clinical applications in the neurointensive care unit

Calcium plays a central role in neuronal function and injury. Dantrolene, an inhibitor of the ryanodine receptor, inhibits intracellular calcium release from the sarco-endoplasmic reticulum. We review the available data of dantrolene as a potential neuroprotective agent and briefly summarize its other pharmacologic effects that may have potential applications for patients in the neurointensive care unit (NICU). Areas with the need for continued research are identified.

Disruption of endoplasmic reticulum calcium stores is involved in neuronal death induced by glycolysis inhibition in cultured hippocampal neurons

Disturbances in neuronal calcium homeostasis have been implicated in a variety of neuropathological conditions, including cerebral ischemia, hypoglycemia, and epilepsy, and possibly constitute part of the cell death process associated with chronic neurodegenerative disorders. We investigated if endoplasmic reticulum (ER) calcium stores participate in neuronal death triggered by moderate glycolysis inhibition induced by iodoacetate, an inhibitor of glyceraldehyde-3-phosphate dehydrogenase, in cultured hippocampal neurons. Results show that exposure to iodoacetate leads to a slow partial decrease in cell survival, which is significantly prevented in the absence of Ca(2+) or in the presence of the calcium chelator BAPTA-AM. Treatment with caffeine and a low (1 microM) concentration of ryanodine, which activates the ryanodine receptor (RyR), exacerbates neuronal death, whereas dantrolene and 25 microM ryanodine, which antagonizes RyR, prevents damage. Xestospongin C (XeC), an antagonist of the inositol-3-phosphate (IP(3)) receptor (IP(3)R) also prevents neuronal damage. Inhibitors of the ER calcium ATPase (sarcoendoplasmic reticulum Ca(2+) ATPase; SERCA) have no effect. The decrease in ATP levels induced by iodoacetate is potentiated by caffeine and prevented by dantrolene. Although only a slight increase in glutamate extracellular levels is observed 3.5 hr after iodoacetate exposure, the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, MK-801, efficiently prevents neuronal damage. Taken together, the data suggest that neuronal death induced during moderate glycolysis inhibition involves calcium influx through NMDA receptors and calcium release from intracellular ER stores. These results might be relevant to the understanding the mechanisms involved in neuronal damage related to aging and chronic neurodegenerative diseases, which have been associated with decreased glucose metabolism.

Neurotoxic effects of trimethyltin and triethyltin on human fetal neuron and astrocyte cultures: a comparative study with rat neuronal cultures and human cell lines

Trimethyltin (TMT) and triethyltin (TET) caused cell death in cultures of primary human neurons and astrocytes, rat neurons and human neuroblastoma cell lines. Human neurons and astrocytes showed a delayed response to TMT cytotoxicity. After 24h of TMT exposure, LC50 values were 148.1, 335.5 and 609.7 microM for SK-N-MC neuroblastoma cell line, neurons and astrocytes, respectively. Over 5 days of exposure, the cytotoxic potency of TMT increased about 70-fold in human cortical neurons. Rat hippocampal neurons were the most vulnerable cells to TMT cytotoxicity, exhibiting an LC50 value 30-fold lower (1.4 microM) than that of rat cerebellar granule cells (44.28 microM). With the exception of rat hippocampal neurons, TET was more potent than TMT in inducing cell death (LC50 values of 3.5-16.9 microM). Moreover, TET was more effective than TMT in increasing intracellular free Ca2+ concentration in human and rat neurons. This work shows that human fetal neuron and astrocyte cultures are a useful model for studying the neurotoxic effects of these environmental contaminants and, thus, predicting their impact on human health.

Dantrolene inhibits NMDA-induced 45Ca uptake in cultured cerebellar granule neurons

Dantrolene is an inhibitor of a skeletal muscle subtype of ryanodine receptors that stabilizes intracellular calcium concentrations and exerts neuroprotective effects in neurons submitted to excitotoxic challenges. The mechanisms of dantrolene-induced neuroprotection are not clear. In this study, using a model of cultured rat cerebellar granule neurons, we demonstrated that dantrolene inhibits NMDA-evoked 45Ca uptake, indicating that this drug may inhibit the activity of NMDA receptor channels. Primary neuronal cultures were incubated for 10 min in Mg(2+)-free ionic medium with NMDA and 45Ca in the presence of different concentrations of dantrolene, then radioactivity in neurons was measured by liquid scintillation spectroscopy. The results demonstrated that dantrolene, applied at micromolar concentrations, inhibits NMDA-evoked 45Ca uptake in neurons in a dose-dependent manner. DMSO, a vehicle to dantrolene, in concentrations used in this study had no effect on NMDA-evoked 45Ca uptake. These results, indicating that dantrolene inhibits activation of the NMDA receptors, might at least partially explain the mechanisms of a dantrolene-evoked protection of neurons against excitotoxicity mediated by agonists of NMDA receptors.

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.

High sensitivity bioassay of paralytic (PSP) and amnesic (ASP) algal toxins based on the fluorimetric detection of [Ca(2+)](i) in rat cortical primary cultures

A high sensitivity bioassay able to recognise small amounts of paralytic and amnesic toxins in algal acetic extracts is described. The method is based on the measure of intracellular [Ca(2+)](i) in primary cultures of rat cortical neurones preloaded with Fura-2 and submitted to electrical field stimulation. Under normal conditions the basal [Ca(2+)](i) level was about 50-100 nM and was nearly doubled during the peaks induced by trains of electrical pulses at 10 Hz for 10 s. Saxitoxin (STX) 3.5 nM and tetrodotoxin (TTX) 24 nM halved the peaks height without affecting basal [Ca(2+)](i). Conversely, domoic acid increased the basal [Ca(2+)](i) (EC(50)=3. 7 microM) and decreased the calcium peaks (EC(50)=7.3 microM). CNQX (a competitive antagonist of AMPA/KA receptors) at 10 microM shifted to the right by a factor of 3 the concentration-response curves of domoic acid. The extracts of non-toxic algae were well tolerated by up to 10 microg protein/ml, whereas extracts of Alexandrium lusitanicum at 1-4 microg protein/ml reduced [Ca(2+)](i) peaks and increased basal calcium levels. This toxic effect of A. lusitanicum was unexpected since parallel HPLC analysis showed only the presence of gonyautoxins, known to act like saxitoxin. Therefore, the bioassay on rat cortical neurones revealed a complex composition of the toxins present in A. lusitanicum. The relevance of fluorimetric detection of [Ca(2+)](i) in primary neuronal cultures in the evaluation of algal risk is stressed.

Short-term block of Na+/K+-ATPase in neuro-glial cell cultures of cerebellum induces glutamate dependent damage of granule cells

Granule cells in a dissociated neuro-glial cell culture of cerebellum when exposed to ouabain (10(-3) M) for 25 min apparently swell, increase their [Ca2+]i with obvious depolarization of the mitochondrial membrane. In 3 h after ouabain was omitted from the solution, 62 +/- 3% of granule cells had pycnotic nuclei. The supplement of a solution with competitive specific antagonist of NMDA receptors, L-2-amino-7-phosphonoheptanoate (10(-4) M, APH) together with ouabain prevented cells from swelling, mitochondrial deenergization, neuronal death and increase of [Ca2+]i. These data suggest that cellular Na+/K+-ATPase inactivation in neuro-glial cell cultures of cerebellum leads to glutamate (Glu) accumulation, hyperstimulation of glutamate receptors, higher Ca2+ and Na+ influxes into the cells through the channels activated by Glu. This process leads to cell swelling, mitochondrial deenergization and death of granule cells. Possibly, the decrease of Na+/K+-ATPase activity in brain cells can lead to the onset of at least some chronic neurological disorders.

Early postischemic dantrolene-induced amelioration of poly(ADP-ribose) polymerase-related bioenergetic failure in neonatal rat brain slices

In the infant brain, ischemia-induced ionic and enzyme mechanisms may independently lead to cell death by energy depletion: resequestration of calcium mobilized from intracellular stores consumes ATP, and activated poly(ADP-ribose) polymerase (PARP) uses oxidized nicotinamide adenine dinucleotide to form polyADP-ribosyl nuclear proteins associated with DNA damage. Using 31P nuclear magnetic resonance spectroscopy, we have monitored intracellular pH and cellular energy metabolites in ex vivo neonatal rat cerebral cortex before, during, and after substrate and oxygen deprivation. In an insult that exhibited secondary energy failure and apoptosis we identified a relative 25% augmentation of high-energy phosphates at the end of recovery when the ryanodine-receptor antagonist, dantrolene, was introduced in the early (0- to 40-minute) but not late (40- to 120-minute) stage of recovery (P < 0.05). In contrast to the absence of a late dantrolene-sensitive effect, inhibition of PARP with 3-methoxybenzamide was as effective (P < 0.05) as early dantrolene, even when introduced after a 40-minute delay. The dantrolene and 3-methoxybenzamide effects on high-energy phosphates were not additive, rather the early dantrolene-sensitive effect nullified the potential 3-methoxybenzamide effect. Therefore, in this vascular-independent neonatal preparation, postischemic mobilization of calcium from intracellular stores is associated with PARP-related energy depletion. Inhibition of either of these processes confers improved postischemic bioenergetic recovery in the developing brain.

Piracetam prevents pentylenetetrazol kindling-induced neuronal loss and learning deficits

The effect of the nootropic drug piracetam (100 mg/kg) on kindled seizures, kindling-induced learning deficits, and histological alterations due to changes in central excitability was investigated in Wistar rats. The animals were kindled by repeated i.p. injections of an initially subconvulsive dose of pentylenetetrazol (PTZ). As a control, piracetam or physiological saline was given 60 minutes before PTZ. Twenty-four hours after completion of kindling the rats were tested in a shuttle-box paradigm. Seven days after the final kindling injection, the animals received a challenge dose of PTZ. Finally, the brains of the rats were processed for histological investigation. Pentylenetetrazol-kindled animals showed increasing seizure scores, and a learning deficit in the shuttle-box. Piracetam had no effect either on kindling development or on the reaction to a challenge dose of PTZ, but it protected the animals against the kindling-induced reduction of learning performance. The substance had no effect on learning performance in control animals. In distinct hippocampal structures, a neuronal cell loss was found in kindled rats. Interestingly, piracetam counteracted this damage efficaciously. The effects of piracetam are discussed in terms of its cytoprotective action. It is suggested that a coadministration of piracetam with clinically used antiepileptic drugs might be useful in antiepileptic therapy.

Characterization of metabotropic glutamate receptor-mediated nitric oxide production in vivo

We tested the hypothesis that stimulation of metabotropic glutamate receptors (mGluRs) increases nitric oxide (NO) production in the hippocampus in vivo. Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 h with artificial cerebrospinal fluid (CSF) containing 3 microM [14C]-L-arginine. Recovery of [14C]-L-citrulline in the effluent was used as a marker of NO production. In nine groups of rats, increases in [14C]-L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of the selective mGluR agonist, trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD); the mGluR antagonist, (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG); the NO synthase inhibitor, N-nitro-L-arginine (LNNA); the ryanodine sensitive calcium-release channel inhibitor dantrolene, the non-N-methyl-D-aspartate (NMDA); receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX); the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d] cyclohepten-5,10-imine (MK-801); and the Na+ channel blocker, tetrodotoxin. Recovery of [14C]-L-citrulline during perfusion with artificial CSF progressively increased to 90 +/- 21 fmol/min (+/-SD) over 5 h. Perfusion in the contralateral hippocampus with 1 mM ACPD augmented [14C]-L-citrulline recovery to 250 +/- 81 fmol/min. Perfusion of 1 mM nitroarginine + ACPD inhibited [14C]-L-citrulline recovery compared to that with ACPD alone. Perfusion with 1 mM MCPG + ACPD attenuated ACPD enhanced [14C]-L-citrulline recovery. Perfusion of 1 mM dantrolene + ACPD inhibited the ACPD-evoked increase in [14C]-L-citrulline recovery. Perfusion of 1 mM MCPG or dantrolene without ACPD did not decrease [14C]-L-citrulline recovery as compared to CSF alone. ACPD-enhanced [14C]-L-citrulline recovery was not attenuated by CNQX, MK-801, or tetrodotoxin (TTX). Using an indirect method of assessing NO production in vivo, these data demonstrate that mGluR stimulation enhances NO production in rat hippocampus. Inhibition with dantrolene suggests that calcium-induced calcium release amplifies the inositol triphosphate-mediated calcium signal associated with mGluR stimulation, thereby resulting in augmented calcium-dependent NO production.

Intracellular inositol 1,3,4,5-tetrakisphosphate enhances the calcium current in hippocampal CA1 neurones of the gerbil after ischaemia

1. To examine the role of the phosphoinositide cascade triggered by disturbed Ca2+ homeostasis in ischaemic neurones, inositol 1,3,4,5-tetrakisphosphate (InsP4) was applied to the cytoplasmic face of membrane patches isolated from CA1 pyramidal neurones in the gerbil hippocampus. 2. In outside-out recordings, InsP4 induced an inward current which was increased by raising the extracellular [Ca2+]. In contrast, no clear channel openings could be observed in patches from neurones of sham-operated gerbils. 3. Open probabilities of InsP4-activated channels were significantly decreased upon application of omega-conotoxin but were not affected by omega-agatoxin or nifedipine. 4. In inside-out patches using high concentrations of Ca2+, Ba2+ or Sr2+ in the pipette solution, InsP4 enhanced inward currents. 5. Application of the isomers of InsP4 slightly enhanced the currents, but inositol 1,4,5-trisphosphate (InsP3) had no effect. 6. In the absence of InsP4 there was a single main Ba2+ current peak of 4.0 pA in amplitude, whereas upon its application two main peaks of 3.0 and 7.2 pA were present. 7. The open probabilities of these channels were apparently increased by InsP4. 8. These findings support the view that a disturbed phosphoinositide cascade occurs in the hippocampal pyramidal neurones after ischaemia and the InsP4 thus formed plays an important role in promoting the Ca2+ accumulation which results in neuronal death.

A flow cytometric study of N-methyl-D-aspartate effects on dissociated cerebellar cells

The effects of N-methyl-D-aspartate (NMDA) on the generation of intracellular reactive oxygen species (ROS) and intracellular calcium in rat dissociated cerebellar cells were examined by flow cytometry. Flow cytometry allows the selection of a specific viable neuronal population with high sensitivity. We used 2',7'-dichlorofluorescin diacetate (DCFH-DA) as a marker of intracellular oxidative stress, and intracellular calcium was measured using Indo-1 as a calcium-sensitive indicator. The cerebellar cell population was isolated by size, granularity and NMDA-sensitivity by cell-sorting. In this cerebellar cell preparation, in which no glial cells were found, NMDA induced a concentration-dependent increase in ROS and intracellular calcium levels. These effects were inhibited by the non-competitive NMDA antagonist (+)MK-801. These results indicate that flow cytometry could be a useful tool to study the effect of neuroprotective drugs on NMDA receptor in isolated cerebellar neurons. Moreover, due to its high speed of analysis and the possibility to detect simultaneously a variety of fluorescent markers, we stated the utility of this technique in the pharmacology and physiology of the central nervous system.

Mechanisms for synchronous calcium oscillations in cultured rat cerebellar neurons

Removal of Mg2+ caused oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential in cultured cerebellar granule neurons. Oscillations of [Ca2+]i were synchronous in all the cells, and were restricted to the neurons (immunocytochemically identified) that responded to exogenous N-methyl-D-aspartate (NMDA). Oscillations were blocked by Ca2+ removal, nickel, NMDA receptor antagonists, omega-agatoxin IVA, tetrodotoxin, sodium removal and gamma-aminobutyric acid, but not by dihydropyridines, omega-conotoxin M VIIA or by emptying the intracellular Ca2+ stores with thapsigargin or ionomycin. The upstroke of the [Ca2+]i oscillations coincided in time with an increase in manganese permeability of the plasma membrane. Propagation of the [Ca2+]i wave followed more than one pathway and the spatiotemporal pattern changed with time. Membrane potential oscillations consisted of transient slow depolarizations of approximately 20 mV with faster phasic activity superimposed. We propose that the synchronous [Ca2+]i oscillations are the expression of irradiation of random excitation through a neuronal network requiring generation of action potentials and functional glutamatergic synapses. Oscillations of -Ca2+-i are due to cyclic Ca2+ entry through NMDA receptor channels activated by synaptic release of glutamate, which requires Ca2+ entry through P-type Ca2+ channels activated by action potentials at the presynaptic terminal.

Kainic acid-induced seizures and brain damage in the rat: role of calcium homeostasis

Seizure activity induced by kainic acid (KA) and subsequent neuronal death are thought to be associated with an increase in cytoplasmic free calcium ([Ca2+]i) and can be prevented by N-methyl-D-aspartate (NMDA) antagonists. In addition to influx through receptor operated Ca2+ channels the increase in [Ca2+]i may be the result of an increased influx through voltage-operated calcium channels and/or release from intracellular deposits. It was therefore investigated whether compounds other than NMDA antagonists with known actions on the intracellular Ca2+ homeostasis had any protective effect against KA-induced neuronal death. Voltage-operated calcium channels in the cell membrane were blocked with the L-type ion channel antagonist, Nimodipine (1.0 mg/kg), and release of Ca2+ from internal stores was prevented with Dantrolene (10 mg/kg). Animals from two control groups injected with kainate (8 mg/kg) exhibited a survival rate of 67 and 53%, respectively. Countings of neurons in dorsal hippocampus showed subtotal or total loss in the CA1 and CA3 subregions. There were no significant differences concerning seizure and survival rates in the groups injected with kainate and treated with Dantrolene or Nimodipine and the control groups. The group treated with Dantrolene showed no neuropathological changes in the hippocampal CA3 region and only slight changes in the Ca1 region, while the neuron loss in the Nimodipine group did not differ from that of its control group. The results emphasize the importance of Dantrolene-sensitive Ca2+ release from intracellular stores for the development of seizure-induced neuronal death.

N-methyl-D-aspartate-evoked release of cyclo-oxygenase products in rabbit hippocampus: an in vivo microdialysis study

In vivo microdialysis of the rabbit hippocampus was used to study the effects of N-methyl-D-aspartate (NMDA) receptor stimulation on dialysate concentrations of thromboxane B2 (Tx B2)- and 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha)-immunoreactive materials that are stable metabolites of biologically active thromboxane A2 and prostacyclin. All pharmacological substances were applied in the dialysis medium. The application of 1 mM NMDA for 20 min resulted in five- and eightfold increases in Tx B2 and 6-keto PGF1 alpha concentrations, respectively. An increase in NMDA concentration to 2.5 mM did not potentiate a peak eicosanoid release, but significantly prolonged this effect. Either 10 microM MK-801 or the extrusion of Ca2+ from the dialysis medium inhibited the release by about 50%. Quinacrine, a phospholipase A2 inhibitor (250 microM), decreased the NMDA-evoked eicosanoid release by 30%, whereas 10 microM indomethacin, a cyclo-oxygenase inhibitor, completely suppressed the release. One hundred micromolar furegrelate, an inhibitor of thromboxane synthase, reduced by 75% Tx B2 release with concomitant 100% increase in 6-keto PGF1 alpha formation. Thus, stimulation of NMDA receptors induces calcium-dependent formation of thromboxane A2 and prostacyclin in the hippocampus, which may have pathophysiological implications. The neuronal site of their formation seems probable, although a transcellular mechanism of their synthesis should be also considered.

Glutamate-induced intracellular calcium changes and neurotoxicity in cortical neurons in vitro: effect of chemical ischemia

To study the role of calcium in neuronal death during ischemia, we examined the characteristics of intracellular Ca2+ ([Ca2+]i) changes in single rat forebrain neurons exposed for 5 min to glutamate (3 microM + 1 microM glycine), NMDA (30 microM + 1 microM glycine), kainate (100 microM) or high K+ (50 mM), under both normal and ischemic conditions. The parameters of [Ca2+]i change measured included peak [Ca2+]i level, plateau [Ca2+]i level, area under the [Ca2+]i response curve and time taken by [Ca2+]i to recover to 10% of the peak response. Under normal conditions, all the agonists studied produced similar [Ca2+]i changes. Chemical ischemia simulated by application of 5 mM KCN in glucose-free buffer had no effect on the basal level of [Ca2+]i, but significantly enhanced and prolonged the [Ca2+]i changes produced by all the agonists. However, in toxicity studies, chemical ischemia significantly potentiated the toxicity of only glutamate and N-methyl-D-aspartate. In correlation studies, all the neurons which died displayed an irreversible secondary [Ca2+]i load prior to loss of viability. These studies suggest that while Ca2+ entry may play a critical role in neuronal death, the magnitude of initial [Ca2+]i change does not predict the toxicity of an agonist in cortical neurons.

Inositol 1,3,4,5-tetrakisphosphate as a mediator of neuronal death in ischemic hippocampus

Selective death of CA1 pyramidal neurons after transient forebrain ischemia has attracted interest for its possible relation to the pathogenesis of memory deficits and dementia. Using whole cell patch-clamp recording from CA1 pyramidal neurons in hippocampal slices of gerbils after ischemia we studied the intracellular signaling mechanisms related to the phosphoinositide cycle. Intracellular application of an antibody against phosphatidylinositol 4,5-bisphosphate rescued ischemic neurons from stimulus-induced irreversible depolarization. Furthermore, application of inositol 1,3,4,5-tetrakisphosphate in normal cells caused an irreversible depolarization in response to synaptic input, which mimicked the deterioration of ischemic neurons. Depolarization of both ischemic and normal neurons in the presence of inositol 1,3,4,5-tetrakisphosphate was prevented by the addition of the Ca2+ chelator, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetate. Application of antibody against inositol 1,4,5-triphosphate 3-kinase, which blocks formation of inositol 1,3,4,5-tetrakisphosphate, also protected against cell deterioration. Our results suggest that the vulnerability of hippocampal pyramidal neurons following ischemia is caused by a disturbed phosphoinositide cascade, with one metabolite, inositol 1,3,4,5-tetrakisphosphate, playing a key role in the induction of Ca2+ accumulation, which leads to neuronal death.

Glutamate-stimulated phosphatidylinositol metabolism in the avian cochlear nucleus

This study examined the ability of the excitatory amino acid glutamate and its analogs to stimulate phosphatidylinositol metabolism in isolated cochlear nucleus tissue from young chicks. In the presence of lithium chloride, glutamate and (+/-)-1-aminocyclopentyl-trans-1,3-dicarboxylate (ACPD) stimulated the formation of inositol phosphates to levels significantly above unstimulated control levels. Unexpectedly, quisqualate did not stimulate inositol phosphates formation. The N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV), the ionotropic kainate/quisqualate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the putative metabotropic glutamate receptor antagonist 2-amino-3-phosphonopropionate (AP3) had no effect on the glutamate stimulated formation of inositol phosphates. We conclude that a metabotropic glutamate receptor is present on cochlear nucleus neurons of posthatch chicks and is able to stimulate formation of inositol phosphates.

Fluorimetric determination of electrically evoked increase in intracellular calcium in cultured cerebellar granule cells

A technique is described to measure the electrically evoked increase in intracellular calcium in cerebellar granule cells cultured on glass coverslips and preloaded with FURA-2. To minimize light scattering, the coverslip containing the granules was placed in the fluorimeter cuvette at a 30 degrees angle to the exciting light beam. The cuvette was provided with 2 platinum electrodes so as to stimulate the neurons with a tangential field. The [Ca2+]i transients were maximized by omitting Mg2+. The fluorescence peaks were directly related to the pulse (1 ms, 100 mA) frequency and to the train length. The responses were completely tetrodotoxin- and [Ca2+]o-dependent and could be replicated 5-6 times at 5-min intervals. At the stimulation rate of 20 Hz for 5 s, a condition ensuring submaximal peaks, the [Ca2+]i rose from the basal levels of 41 +/- 2.7 nmol/l to 89.6 +/- 5.8 nmol/l. The participation of various membrane channels in the electrically induced [Ca2+]i increase was demonstrated. 4-Aminopyridine (1 mM) increased the height of the peaks to 240%. Both nifedipine (10 microM) and omega-conotoxin (1 microM) reduced the transients by about 25%. The residual response (in the absence of Mg2+) depended mostly on the release of endogenous glutamate as it proved sensitive to NMDA, AMPA and t-ACPD receptor antagonists. Since a technique to measure the electrically evoked release of D-[3H]aspartate is presently available, the parallel determination of release and of [Ca2+]i in twin populations of cultured granule cells is possible.

On the origin of a sustained increase in cytosolic Ca2+ concentration after a toxic glutamate treatment of the nerve cell culture

A sustained increase of cytosolic Ca2+ concentration, [Ca2+]i, (Ca2+ plateau) was induced by a 15-min treatment with 50 microM glutamate of cultured cerebellar granule cells and hippocampal neurons in a Mg(2+)-free solution. Plateau proved to be insensitive to inhibition of Na+o/Ca2+i exchange caused by removal external Na+ in the post-glutamate period. A approximately 10(5)-fold reduction of [Ca2+]o (from 1.5 mM to 20 nM) in the post-glutamate period caused in most cells only a slow and small decrease in [Ca2+]i, although the same low-Ca2+ trial before glutamate treatment caused in hippocampal cells very quick blockade of spontaneous [Ca2+]i oscillation and a decrease in the basal [Ca2+]i. The results suggest that the Ca2+ plateau is due to a suppression of the Ca2+ extrusion from the cell (in particular via Na+/Ca2+ exchange) rather than from a persistent increase in Ca2+ permeability of neuronal membrane.

Excitatory amino acid-mediated cytotoxicity and calcium homeostasis in cultured neurons

A large body of evidence suggests that disturbances of Ca2+ homeostasis may be a causative factor in the neurotoxicity induced by excitatory amino acids (EAAs). The route or routes by which an increase in intracellular calcium concentration ([Ca2+]i) is mediated in vivo are presently not clarified. This may partly reflect the complexity of intact nervous tissue in combination with the relative unspecific action of the available "calcium antagonists," e.g., blockers of voltage-sensitive calcium channels. By using primary cultures of cortical neurons as a model system, it has been found that all EAAs stimulate increases in [Ca2+]i but via different mechanisms. By using the drug dantrolene, it has been shown that 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionate (AMPA) apparently exclusively stimulates Ca2+ influx through agonist-operated calcium channels and voltage-operated calcium channels. Increased [Ca2+]i due to exposure to kainate (KA) is for the major part caused by influx, as in the case of AMPA, but a small part of the increase in [Ca2+]i may be attributed to a release of Ca2+ from intracellular stores. Quisqualate (QA) stimulates Ca2+ release from an intracellular store that is independent of Ca2+ influx; presumably this store is activated by inositol phosphates. The increase in [Ca2+]i due to exposure to glutamate or N-methyl-D-aspartate (NMDA) may be compartmentalized into three components, one of which is related to influx and the other two to Ca2+ release from internal stores. Only one of the latter stores is dependent on Ca2+ influx with regard to release of Ca2+, whereas the other is activated by some other second messengers or, alternatively, directly coupled to the receptor. In muscles dantrolene is known to inhibit Ca2+ release from the sarcoplasmic reticulum, and also in neurons dantrolene inhibits an equivalent release from one or more hitherto unidentified internal Ca2+ pool(s). By using this drug it has been possible to show to what extent these Ca2+ stores are involved in the toxicity observed subsequent to exposure to the EAAs. It turned out that dantrolene, even under conditions allowing Ca2+ influx, inhibited toxicity induced by QA, NMDA, and glutamate, whereas that induced by AMPA or KA was unaffected. In combination with the findings that dantrolene inhibited release from the intracellular stores activated by QA, NMDA, and glutamate, it may be concluded that Ca2+ influx per se is not the primary event causing toxicity following exposure to these EAAs in these neurons. However, it may certainly be involved in the cases of toxicity induced by AMPA and KA.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurons respond to hyposmotic conditions by an increase in intracellular free calcium

The effect of hyposmotic conditions on the concentration of intracellular free calcium ([Ca2+]i) was studied in cultured cerebellar granule cells and cerebral cortical neurons after loading of the cells with the fluorescent Ca2+ chelator Fluo-3. It was found that in both types of neurons exposure to media with a decrease in osmolarity of 20 to 50% of the osmolarity in the isosmotic medium (320 mOsm) led to a dose dependent increase in [Ca2+]i with a time course showing the highest value at the earliest measured time point, i.e. 40 s after exposure to the hyposmotic media and a subsequent decline towards the basal level during the following 320 s. The response in the cortical neurons was larger than in the granule cells but both types of neurons exhibited a similar increase in [Ca2+]i after exposure to 50 mM K+ which was of the same magnitude as the increase in [Ca2+]i observed in the cortical neurons exposed for 40 s to a medium with a 50% reduction in osmolarity. In both types of neurons the blocker of voltage gated Ca2+ channels verapamil had no effect on the hyposmolarity induced increase in [Ca2+]i. On the contrary, this increase in [Ca2+]i was dependent upon external calcium and could be inhibited partly or completely by the inorganic blockers of Ca2+ channels Mg2+ and La3+. Dantrolene which prevents release of Ca2+ from internal stores had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Origin of intracellular Ca2+ elevation induced by in vitro ischemia-like condition in hippocampal slices

Microfluorometry was used to investigate the origin of intracellular Ca2+ ([Ca2+]i) elevation in field CA1 of gerbil hippocampal slices perfused with a glucose-free physiological medium equilibrated with a 95% N2/5% CO2 gas mixture (standard in vitro ischemia-like condition). Large [Ca2+]i elevation was detected about 4 min after the beginning of standard in vitro ischemia-like condition, which was accompanied with a negative shift of extracellular DC potential. When slices were perfused with Ca(2+)-free in vitro ischemia-like medium, large [Ca2+]i elevation was observed about 3.5 min after the beginning of Ca(2+)-free in vitro ischemia-like condition, however, the increase in [Ca2+]i was more gradual and of a lesser extent compared with that detected in the slices perfused with the standard in vitro ischemia-like medium that contained Ca2+. When slices were perfused with the Ca(2+)-free in vitro ischemia-like medium that contained dantrolene (50 microM) which is known to prevent Ca(2+)-induced Ca2+ release from intracellular Ca2+ stores, the increase in [Ca2+]i was more gradual and of a lesser extent compared with that detected in the slices perfused with the Ca(2+)-free in vitro ischemia-like medium that did not contain dantrolene. These results indicate that large [Ca2+]i elevation induced by in vitro ischemia-like condition in field CA1 of the hippocampus was caused by both Ca2+ influx from extracellular space and Ca2+ release from intracellular Ca2+ stores, and that a part of the Ca2+ release was due to Ca(2+)-induced Ca2+ release from intracellular Ca2+ stores.

35 mM K(+)-stimulated 45Ca2+ uptake in cerebellar granule cell cultures mainly results from NMDA receptor activation

In primary cultures of cerebellar granule cells, the Ca2+ influx resulting from K+ depolarization (35 mM) was equal to one-third of that observed with 100 microM N-methyl-D-aspartate (NMDA) and was reduced in a major part (90%) by NMDA receptor antagonists. The rank order of potency of these competitive and non-competitive NMDA receptor antagonists was very close to their affinity for the NMDA and phencyclidine sites respectively. Granular cell depolarization with 35 mM K+ also induced a large increase in the extracellular glutamate concentration. Repeated washes of the culture wells, addition of glutamate pyruvate transaminase (+2 mM pyruvate), or pretreatment of the cells with tetanus toxin resulted in a parallel reduction of the extracellular glutamate concentration and 45Ca2+ uptake measured after a 35 mM K+ stimulation. Dihydropyridine (BAY K-8644) stimulated the release of glutamate in a nifedipine-sensitive manner in the presence of 15 mM K+. However, nifedipine (1 microM), which decreased by 60% the K(+)-induced 45Ca2+ uptake, did not reduce the 35 mM K(+)-evoked glutamate release. Taken together, these results demonstrated that in cerebellar granule cell cultures, 90% of the 35 mM K(+)-stimulated 45Ca2+ influx resulted from the release of glutamate and the consecutive activation of NMDA receptors. Activation of these glutamate receptors then allows Ca2+ influx to occur through L-type voltage-operated Ca2+ channels.

3H-D-aspartate release from cerebellar granule neurons is differentially regulated by glutamate- and K+-stimulation

Neurotransmitter release in response to either 55 mM K+ or 25 microM glutamate as well as its dependency on Ca2+ from different sources was compared in cultured glutamatergic cerebellar granule cells from rat brain. The intracellular Ca2+ concentration was monitored at the single cell level in neurites as well as cell bodies employing the fluorescent Ca2+ indicator fura-2. Transmitter release was assayed using 3H-D-aspartate to label the exogenously accessible glutamate pools, which in these neurons is believed to also include the transmitter pool. In an attempt to distinguish whether transmitter release was dependent on an intact cytoskeleton or not, the colchicine-like drug Nocodazole, which also blocks transport of vesicles, was used. K(+)-stimulated transmitter release consisted for the major part (around 70%) of a Ca(2+)-dependent, Nocodazole sensitive release component and this K(+)-induced release appeared to be almost exclusively dependent on N-type Ca2+ channels. In contrast, 50% of the glutamate-induced Ca(2+)-dependent release was triggered by Ca2+ from a Dantrolene sensitive intracellular Ca2+ pool. Since these neurons undergo a pronounced maturational change in which neurotransmitter vesicles become increasingly prominent, the Ca2+ responses and transmitter release evoked by the two different stimuli were investigated as a function of the culture period. K+ and glutamate were found to increase intracellular [Ca2+] differentially. In 1-day-old cultures K+ elicited a small albeit significant increase in [Ca2+]i while glutamate was completely without effect. In 7-day-old neurons both agents induced a large increase in [Ca2+].(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between Ca2+ mobilizing mechanisms in cultured rat cerebellar granule cells

1. The interactions between IP3 receptor-mediated and Ca(2+)-induced Ca2+ release were investigated in cerebellar granule cell bodies, using the techniques of microfluorimetry and image analysis. 2. The IP3-sensitive Ca2+ release mechanism was activated using acetylcholine (ACh) and the selective metabotropic glutamate receptor agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (ACPD). Caffeine was used to activate, and ryanodine to inhibit, the Ca(2+)-induced Ca2+ release process. Thapsigargin was used to deplete intracellular Ca2+ stores. 3. Transient applications of caffeine (5-50 mM), ACPD (50-500 microM) and ACh (0.05-1 microM) mobilized intracellular Ca2+ ([Ca2+]i). Ca2+ mobilizing responses to 50 mM caffeine and 1 microM ACh increased with time in culture until day 4. However, beyond this period the responsiveness of cells to caffeine, but not to ACh, declined markedly. 4. Responses induced by ACPD and ACh were inhibited in the presence of caffeine at concentrations below those which mobilized Ca2+ (1-5 mM). This effect was not due to Ca2+ pool depletion, elevation of cAMP or inhibition of phosphodiesterases. 5. Prior challenge with ACh or ACPD inhibited Ca2+ mobilization induced by caffeine (50 mM). Transient exposure to caffeine inhibited subsequent responses to ACh through a mechanism which involved store depletion. 6. Thapsigargin (0.1-1 microM) inhibited, to a similar extent, Ca2+ mobilization induced by caffeine, ACPD and ACh. 7. Ryanodine (10 microM) antagonized Ca2+ mobilization induced by caffeine, ACh and ACPD. However, the ability of ryanodine to block inositol 1,4,5-trisphosphate-linked agonist responses varied considerably between cells. The sensitivity of ACh-induced responses to ryanodine correlated with the sensitivity of the cells to caffeine. 8. The possible explanations for the pronounced interactions between IP3 receptor-mediated and Ca(2+)-induced Ca2+ release processes in cerebellar granule cells are discussed.

Interactions between phospholipase C-coupled and N-methyl-D-aspartate receptors in cultured cerebellar granule cells: protein kinase C mediated inhibition of N-methyl-D-aspartate responses

The N-methyl-D-aspartate (NMDA) receptor of rat cerebellar granule cells in primary culture is inhibited by phospholipase C-coupled receptor activation. In the absence of ionotropic agonist, cells modulate their cytoplasmic free Ca2+, [Ca2+]c, in response to stimulation of M3 muscarinic receptors, metabotropic glutamate receptors, and endothelin receptors by the respective agonists carbachol, trans-1-amino-1,3-cyclopentanedicarboxylic acid, and endothelin-1. The response is consistent with the ability of phospholipase C-coupled receptors to release a pool of intracellular Ca2+ and induce a subsequent Ca2+ entry into the cell; both of these responses can be abolished by discharge of internal Ca2+ stores with low concentrations of ionomycin or thapsigargin. In the case of cells stimulated with NMDA, the [Ca2+]c response to the phospholipase C-coupled agonists is complex and agonist dependent; however, in the presence of ionomycin each agonist produces a partial inhibition of the NMDA component of the [Ca2+]c signal. This inhibition can be mimicked by the protein kinase C activator 4 beta-phorbol 12,13-dibutyrate. It is concluded that NMDA receptors on cerebellar granule cells are inhibited by phospholipase C-coupled muscarinic M3, glutamatergic, and endothelin receptors via activation of protein kinase C.

Inositol phosphate regulation of voltage-dependent calcium channels in cerebellar granule neurons

The effects of intracellularly applied inositol phosphates on voltage-dependent calcium channel currents were assessed in rat cerebellar neurons using the whole-cell recording configuration of the patch-clamp technique. Intraneuronal perfusion of 10 microM inositol 1,4,5-trisphosphate (IP3) increased the amplitude of currents elicited by depolarization from a holding potential of -40 mV. IP3 did not modify current activation, but shifted the steady-state inactivation curve toward more positive values. The dose-response curve indicated an EC50 of 0.5 microM for IP3. Inositol 1,3,4,5-tetrakisphosphate (IP4), but not inositol 4,5,-bisphosphate, mimicked the effect of IP3. The effect of IP3 persisted in the presence of 100 micrograms/ml heparin and did not depend on intracellular calcium mobilization, as similar responses were not produced by 10 mM caffeine or by intrapipette calcium buffering at pCa 6 instead of pCa 7.7. Preincubation with omega-conotoxin led to a 55% inhibition of barium current; however, inhibition was reversed by IP3, which reestablished the control current amplitude. These results imply that IP3 and IP4 can elicit calcium entry by modifying both the gating characteristics and the pharmacological properties of voltage-dependent calcium channels.

5-(N-ethyl-N-isopropyl)amiloride and mild acidosis protect cultured cerebellar granule cells against glutamate-induced delayed neuronal death

In the experiments on the primary cerebellar granule cell cultures, delayed neuronal death was induced by 15 min treatment of the cells with 50 microM glutamate. 5-(N-ethyl-N-isopropyl)amiloride (10 microM) known as a potent inhibitor of the Na+/H+ exchanger, when added to the glutamate-containing Mg(2+)-free solution caused a considerable (approximately by 40%) decrease in the number of dead cells counted 4 h after the termination of glutamate treatment. Patch-clamp experiments with freshly isolated rat hippocampal neurons have shown that the neuroprotective effect of 5-(N-ethyl-N-isopropyl)amiloride can be explained by its ability to block N-methyl-D-aspartate channels (receptors) at micromolar concentrations. A similar mechanism apparently underlies neuroprotective effect of external acidosis (reduction of pH from 7.6-7.8 to 6.7-6.8) during glutamate application. 5-(N-ethyl-N-isopropyl)amiloride (10 microM) and low pH (6.7) also proved capable of exhibiting neuroprotective effects upon application during the post-glutamate period. In this instance, however, the number of dead cells was decreased by no more than 20%. This neuroprotective effect of 5-(N-ethyl-N-isopropyl)amiloride and low pH is interpreted as resulting from inhibition of Na+/H+ exchange, since a direct blockade of N-methyl-D-aspartate receptors by 1 mM DL-2-amino-5-phosphonovalerate after termination of glutamate treatment did not attenuate the delayed neuronal death. Finally, we have established that the addition of 10 microM 5-(N-ethyl-N-isopropyl)amiloride to the cultures both during glutamate treatment and after its termination results in a complete protection of cultured cerebellar granule cells.

Pharmacological and functional characterization of excitatory amino acid mediated cytotoxicity in cerebral cortical neurons

The cytotoxic action of the excitatory amino acids (EAAs) glutamate, N-methyl-D-aspartate (NMDA), quisqualate (QA), kainate (KA) and (RS)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate (AMPA) was studied in cerebral cortical neurons in culture. The pharmacological profile of these actions was characterized using the NMDA selective antagonist D-(-)-2-amino-5-phosphonopentanoate (APV) and the non-NMDA selective antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX), 2-amino-3[3-(carboxymethoxy)-5-methylisoxazol-4-yl]-propionate (AMOA) and 2-amino-3-[2-(3-hydroxy-5-methylisoxazol-4-yl)methyl-5-methyl-3- oxoisoxazolin-4-yl]propionate (AMNH). The role of intracellular Ca++ homeostasis and cGMP production for development of EAA mediated cytotoxicity was assessed by measurements of changes in [Ca++]i using the fluorescent Ca++ chelator Fluo-3 and in cGMP concentrations using a conventional radioimmune assay. It was found that glutamate toxicity involves both NMDA and non-NMDA receptor activation and that aberrations in Ca++ homeostasis brought about by Ca++ influx and/or liberation of Ca++ from internal stores are important for development of toxicity. The drug dantrolene which prevents release of Ca++ from such stores can prevent toxicity induced by glutamate, NMDA and QA completely but has no effect on KA and AMPA toxicity. Changes in cGMP levels appear to play a role for development of glutamate, NMDA and KA toxicity but does not seem to be involved in that triggered by QA and AMPA.

Mobilization of dantrolene-sensitive intracellular calcium pools is involved in the cytotoxicity induced by quisqualate and N-methyl-D-aspartate but not by 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate and kainate in cultured cerebral cortical neurons

By using primary cultures of cerebral cortical neurons, it has been demonstrated that the antihyperthermia drug dantrolene protects against cytotoxicity induced by the excitatory amino acids quisqualate (QA) and N-methyl-D-aspartate (NMDA), whereas no effect was observed on cell damage mediated by kainate (KA) or 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate (AMPA). In parallel it was shown that KA and AMPA increased the concentration of intracellular free calcium ([Ca2+]i) mainly by influx, whereas the increase in [Ca2+]i stimulated by NMDA and QA predominantly was caused by release of Ca2+ from intracellular stores, which for NMDA seemed to be mediated at least partly by Ca2+ influx. In accordance with the effects on cytotoxicity, dantrolene blocked the increase in [Ca2+]i elicited by QA and NMDA leaving the increase induced by KA and AMPA unaffected. The finding that 2-amino-3-[3-(carboxymethoxy)-5-methylisoxazol-4-yl]propionate, which regarding toxicity is a selective KA antagonist, only reduced the KA-stimulated increase in [Ca2+]i by 30% may suggest that the elevation of [Ca2+]i is not the only element in KA-induced cytotoxicity. On the other hand, the present study underlines the importance of Ca2+ for cytotoxicity induced by some excitatory amino acids (glutamate, NMDA, and QA) and supports the current proposal that multiple mechanisms are operating, even concerning calcium homeostasis. Because excitatory amino acid-induced cytotoxicity is thought to be involved in neuropathological conditions such as ischemia, it is possible that dantrolene might be of therapeutic interest.

L-glutamate and acetylcholine mobilise Ca2+ from the same intracellular pool in cerebellar granule cells using transduction mechanisms with different Ca2+ sensitivities

Ca2+ mobilisation induced by L-glutamate (Glu) and acetylcholine (ACh) has been studied in cultured cerebellar granule cells using digital fluorescence microscopy. The ability of Glu-receptor activation to mobilise Ca2+ was decreased when [Ca2+]o was lowered to 10 microM (from 1.8 mM). It was enhanced when [Ca2+]i was raised using 25 mM external K+ or by N-methyl-D-aspartate (NMDA), which selectively activates a distinct Glu-receptor subtype. The enhancement was dependent on entry of external Ca2+. In contrast, the ability of ACh receptor activation to mobilise Ca2+ was not affected by these conditions. Furthermore, pretreatment with pertussis toxin inhibited Ca2+ mobilisation in response to Glu-receptor activation without affecting mobilisation in response to ACh. However, activation of both receptors mobilised Ca2+ from a common, thapsigargin-sensitive pool. We conclude that there are differences in the Ca2+ mobilization pathways for the two receptor systems in cerebellar granule cells. The Ca(2+)-sensitivity of this Ca2+ mobilizing Glu receptor may have implications for its function in neuronal synaptogenesis and plasticity.

First direct electron microscopic visualization of a tight spatial coupling between GABAA-receptors and voltage-sensitive calcium channels

Using cerebellar granule neurons in culture it was demonstrated that exposure of the cells to the GABAA receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) leads to an increase in the number of voltage-gated calcium channels as revealed by quantitative preembedding indirect immunogold labelling using a monoclonal antibody specific for phenylalkylamine and dihydropyridine sensitive Ca2+ channels. Using the same technique and a monoclonal antibody (bd-17) to the beta 2/beta 3-subunit of the GABAA-receptor, double labelling of Ca2+ channels and GABAA-receptors with gold particles of different and well defined sizes were performed. This showed that in THIP-treated cultures 20% of GABAA-receptors in cell processes were located in close proximity (i.e. within 40 nm) of Ca2+ channels in the plasma membrane. This was not observed in non-treated cultures nor was it observed in cell bodies of THIP-treated cultures. This suggests that primarily low affinity GABAA-receptors are closely associated with Ca2+ channels and this may be important for the ability of these receptors to mediate an inhibitory action on transmitter release even under extreme depolarizing conditions.

Possible role of cGMP in excitatory amino acid induced cytotoxicity in cultured cerebral cortical neurons

Using cultured cerebral cortical neurons at mature stages (9 days in culture, d.i.c.) it was demonstrated that glutamate, NMDA (N-methyl-D-aspartate) and to a lesser extent KA (kainate) increase the intracellular cGMP concentration ([cGMP]i) whereas no such effect was observed after exposure of the cells to QA (quisqualate) and AMPA (2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionate). No effect of glutamate, NMDA and KA was observed in immature neurons (2 d.i.c.). The pharmacology of these cGMP responses was investigated using the glutamate antagonists APV (2-amino-5-phosphonovalerate) with selectivity for NMDA receptors, CNQX (6-cyano-7-nitro-quinoxaline-2,3-dione) with selectivity for non-NMDA receptors and the novel KA selective antagonists AMOA (2-amino-3-[3-(carboxymethoxy)-5-methylisoxazol-4-yl]propionate) and AMNH (2-amino-3-[2-(3-hydroxy-5-methylisoxazol-4-yl)methyl-5-methyl-3- oxoisoxazolin-4-yl]propionate). In addition, the cytotoxicity of glutamate, NMDA and KA was studied and found to be enhanced by addition of the non-metabolizable cGMP analogue 8-Br-cGMP. On the contrary, the toxicity of QA and AMPA was not affected by 8-Br-cGMP. Pertussis toxin augmented the toxicity elicited by glutamate, NMDA, KA and QA but not that induced by AMPA. On the other hand, only glutamate and KA induced toxicity was potentiated by cholera toxin, which also enhanced the stimulatory effect of glutamate and NMDA but not that of KA on the cellular cGMP content. The toxicity as well as the effects on intracellular cGMP levels could be antagonized by the specific excitatory amino acid (EAA) antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of gamma-[3H]aminobutyric acid release from cultured mouse cerebral cortex neurons by sulphur-containing excitatory amino acid transmitter candidates: receptor activation mediates two distinct mechanisms of release

In primary cultures of mouse cerebral cortex neurons, sulphur-containing excitatory amino acids (SAAs; namely, L-cysteine sulphinate, L-cysteate, L-homocysteine sulphinate, L-homocysteate, S-sulphocysteine) at concentrations ranging from 0.1 microM to 1 mM evoked a saturable release of gamma-[3H]aminobutyric acid ([3H]GABA) in the absence of any other depolarizing agent. All SAAs exhibited essentially similar potency (EC50, 100-150 microM) in releasing [3H]GABA although a variable profile of maximal stimulatory effect was observed when compared with basal release. The intracellular accumulation of the lipophilic cation, [3H]tetraphenylphosphonium, was significantly reduced in the presence of all SAAs, thus verifying a depolarization of the neuronal plasma membrane. SAA-stimulated release of [3H]GABA was shown to comprise two distinct components, calcium-dependent and calcium-independent, which occur after activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Thus, all SAA-evoked responses were antagonized by the selective, competitive NMDA-receptor antagonist, 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (IC50 range, greater than 50 microM) and the non-NMDA-receptor antagonist, 6,7-dinitroquinoxalinedione (IC50 range, 5-50 microM). Removal of magnesium ions from the superfusion medium caused a significant potentiation of SAA-evoked responses without having any effect on basal levels of [3H]GABA efflux, a result consistent with an involvement of NMDA-receptor activation. Calcium-independent release (i.e., that release remaining in the presence of 1 mM cobalt ions) was a distinct component but of smaller magnitude. Using 500 microM excitatory amino acid agonist concentrations, this component of release was (1) markedly attenuated by 15 microM SKF-89976-A, a non-transportable inhibitor of the GABA carrier, and (2) abolished when choline ions replaced sodium ions in the superfusion medium or when in the presence of excitatory amino acid receptor antagonists. These observations are clearly consistent with a receptor-mediated, depolarization-induced reversal of the GABA carrier.

L-glutamate diethyl ester and deaminated analogues as excitatory amino acid antagonists in rat cerebral cortex

1. The effects of L-glutamate diethyl ester (GDEE) HCl, glutarate diethyl ester (GlrDEE) and glutarate dimethyl ester (GlrDME) on depolarizing responses to alpha-amino-3-hydroxy-5- methyl-4-isoxazolepropionate (AMPA), kainate (Kain), N-methyl-D-aspartate (NMDA) and quisqualate (Quis), and spontaneous paroxysmal discharges (SPDs) were examined. Experiments were performed on slices of rat cingulate cortex using the in vitro grease gap recording technique in nominally Mg(2+)-free Krebs medium. 2. GDEE HCl (3-14 mM) caused a concentration-dependent depolarization of the d.c. baseline potential. L-Glutamate (0.1-0.5 mM), HCl (15 mM) and sucrose (30 mM) also depolarized the baseline. GlrDEE (3-12 mM) and GlrDME (4-26 mM) had no consistent effect on baseline potential. 3. GDEE HCl (10 mM) had no effect on depolarizing responses to AMPA, Kain and NMDA, but caused potentiation of those to Quis with a dose-ratio of 0.53 (0.44-0.63) (n = 4). In two other experiments, where the depolarization of the baseline induced by GDEE HCl was large, a depression of Quis response amplitude was observed. 4. GlrDEE (10 mM) antagonized depolarizing responses to Kain, and to a lesser extent NMDA, with dose-ratios of 2.14 (1.92-2.38) and 1.61 (1.39-1.87), respectively. This concentration of GlrDEE had no effect on AMPA responses, but potentiated Quis responses, with a dose-ratio of 0.64 (0.58-0.71). 5. GlrDME (10 mM) antagonized depolarizing responses to Kain and to Quis, with dose-ratios of 1.66 (1.48-1.85) and 1.22 (1.15-1.29), respectively, and had no effect on responses to NMDA. 6. The SPDs were inhibited by GDEE HCI (IC50 6.7 +/- 0.37mM), GlrDEE (IC50 5.6 +/- 0.38 mM) and GlrDME (IC50 10.4 +/- 0.73 mM). 7. In conclusion, there is little evidence that GDEE HCI is an antagonist of the postsynaptic excitatory amino acid receptors in the rat neocortex, and its effects may result from its contamination with Lglutamate and increased osmolarity of the bathing medium at high concentrations. The deaminated analogues of GDEE are very weak Kain antagonists.

Inositol 1,4,5-triphosphate-stimulated calcium release from permeabilized cerebellar granule cells

1. Muscarinic cholinoceptor stimulation of phosphoinositide hydrolysis in rat cultured cerebellar granule cells results in a rapid, transient accumulation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), which has been implicated in the release of non-mitochondrial intracellular Ca2+ stores. In the present study, the release of Ca2+ from intracellular stores and the Ins(1,4,5)P3 receptor responsible for this process have been investigated. 2. Monolayers of saponin-permeabilized granule cells accumulate 45Ca2+ in an ATP-dependent manner and the sequestered 45Ca2+ can be concentration-dependently released by Ins(1,4,5)P3 by a stereospecific and heparin-sensitive mechanism. The EC50 for Ins(1,4,5)P3-stimulated 45Ca2+ release was 80 +/- 3 nM. 3. Radioligand binding studies performed on a crude granule cell membrane fraction indicated the presence of an apparently homogeneous population of stereo-specific Ins(1,4,5)P3 receptors (KD 54.7 +/- 2.0 nM; Bmax 1.37 +/- 0.29 pmol mg-1 protein). 4. This study provides evidence for Ins(1,4,5)P3-sensitive intracellular Ca2+ stores in primary cultures of cerebellar granule cells and suggest that these cells provide an excellent model neuronal system in which to study the relative functional roles of Ca2+ release from intracellular stores and Ca(2+)-entry in neuronal Ca2+ homeostasis.

Inhibition of sodium-potassium-ATPase: a potentially ubiquitous mechanism contributing to central nervous system neuropathology

Direct and indirect evidence suggests that Na+/K(+)-ATPase activity is reduced or insufficient to maintain ionic balances during and immediately after episodes of ischemia, hypoglycemia, epilepsy, and after administration of excitotoxins (glutamate agonists). Recent results show that inhibition of this enzyme results in neuronal death, and thus a hypothesis is proposed that a reduction and/or inhibition of this enzyme contributes to producing the central neuropathy found in the above disorders, and identifies potential mechanisms involved. While the extent of inhibition of Na+/K(+)-ATPase during ischemia, hypoglycemia and epilepsy may be insufficient to cause neuronal death by itself, unless the inhibition is severe and prolonged, there are a number of interactions which can lead to a potentiation of the neurotoxic actions of glutamate, a prime candidate for causing part of the damage following trauma. Presynaptically, inhibition of the Na+/K(+)-ATPase destroys the sodium gradient which drives the uptake of acidic amino acids and a number of other neurotransmitters. This results in both a block of reuptake and a stimulation of the release not only of glutamate but also of other neurotransmitters which modulate the neurotoxicity of glutamate. An exocytotic release of glutamate can also occur as inhibition of the enzyme causes depolarization of the membrane, but exocytosis is only possible when ATP levels are sufficiently high. Postsynaptically, the depolarization could alleviate the magnesium block of NMDA receptors, a major mechanism for glutamate-induced neurotoxicity, while massive depolarization results in seizure activity. With less severe inhibition, the retention of sodium results in osmotic swelling and possible cellular lysis. A build-up of intracellular calcium also occurs via voltage-gated calcium channels following depolarization and as a consequence of a failure of the sodium-calcium exchange system, maintained by the sodium gradient.