Modulation of N-methyl-D-aspartate receptor-mediated increases in cytosolic calcium in cultured rat cerebellar granule cells

Key proteins of activating cell death can be predicted through a kainic acid-induced excitotoxic stress

Epilepsy is a major neurological disorder characterized by spontaneous seizures accompanied by neurophysiological changes. Repeated seizures can damage the brain as neuronal death occurs. A better understanding of the mechanisms of brain cell death could facilitate the discovery of novel treatments for neurological disorders such as epilepsy. In this study, a model of kainic acid- (KA-) induced neuronal death was established to investigate the early protein markers associated with apoptotic cell death due to excitotoxic damage in the rat cortex. The results indicated that KA induces both apoptotic and necrotic cell death in the cortex. Incubation with high concentrations (5 and 500 μM, >75%) and low concentrations (0.5 pM: 95% and 50 nM: 8%) of KA for 180 min led to necrotic and apoptotic cell death, respectively. Moreover, proteomic analysis using two-dimensional gel electrophoresis and mass spectrometry demonstrated that antiapoptotic proteins, including heat shock protein 70, 3-mercaptopyruvate sulfurtransferase, tubulin-B-5, and pyruvate dehydrogenase E1 component subunit beta, were significantly higher in apoptosis than in necrosis induced by KA. Our findings provide direct evidence that several proteins are associated with apoptotic and necrotic cell death in excitotoxicity model. The results indicate that these proteins can be apoptotic biomarkers from the early stages of cell death.

Synthesis, chiral resolution and pharmacological evaluation of a 2,3-benzodiazepine-derived noncompetitive AMPA receptor antagonist

The resolution of 1-(4-aminophenyl)-3,5-dihydro-3-N-ethylcarbamoyl-5-methyl-7,8-methylenedioxy-4H-2,3-benzodiazepin-4-one (R,S)-(+/-)-5 by chiral HPLC and assignment of the absolute configuration of the two enantiomers was carried out. Compound (R,S)-(+/-)-5 and its enantiomers were tested in a binding assay to evaluate their affinity for AMPA receptors. Enantiomer (S)-(-)-5 appears to be more potent than its optical antipode (R)-(+)-5. In a primary culture of rat cerebellar granule cells, which express AMPA receptors, (R,S)-(+/-)-5 and (S)-(-)-5 inhibited kainate- induced [Ca(2+)](i) increase, thus confirming the antagonism at the AMPA receptor.

Measurements with fluorescent probes in primary neural cultures; improved multiwell techniques

INTRODUCTION

Fluorescence imaging techniques are valuable tools for the pharmacological characterization of CNS drugs. Dissected cerebellar granule neurons (CGN) are an important model system in the study of mechanisms of excitotoxicity, glutamate receptors and transporters. Widely applied techniques use fluorescent probes loaded in neural cells cultured on glass supports. CGN, however, require at least 7 days for differentiation and over time cells tend to cluster and loose adherence to the glass substrate. This problem is accentuated in small wells (e.g. 96-well plates).

METHODS

CGN were grown on large coverslips (60 x 24 mm) and measurements made with a designed mountable multiwell in 48 regions on 4 coverslips at a time. The UV ratiometric probe fura-2 was used to measure glutamatergic calcium ([Ca(2+)](i)) responses induced by NMDA. The IC(50) of NMDA receptor antagonists was determined from inhibition curves with 6 doses and 8 parallels per experiment.

RESULTS

The method was validated by comparing with published data for the dose response to NMDA and glycine and IC(50) values for ion-channel block by Mg(2+) and MK-801.

DISCUSSION

Resolution is enhanced with the new technique since it allows measurement of multiple doses on cells from the same batch. It has advantages to cuvette techniques because cells have intact dendritic tree and synaptic function and it is a convenient method to obtain reliable dose-response curves for NMDA channel modulators on differentiated neural cells.

Maturation-dependent reduced responsiveness of intracellular free Ca2+ ions to repeated stimulation by N-methyl-d-aspartate in cultured rat cortical neurons

In contrast to other ionotropic glutamate receptors, N-methyl-d-aspartate (NMDA) receptor channels are rather stable after the simulation. Brief exposure to NMDA at 50 microM rapidly increased the fluorescence intensity for increased intracellular free Ca(2+) levels in a reversible- and concentration-dependent manner in rat cortical neurons cultured for 3-15 days in vitro (DIV), while EC(50) values were significantly decreased in proportion to cellular maturation from 3 to 15 DIV. Although a constant increase was persistently seen in the fluorescence throughout the sustained exposure to NMDA for 60 min irrespective of the cell maturation from 3 to 15 DIV, the second brief exposure for 5 min resulted in a less efficient increase in the fluorescence than that found after the first brief exposure for 5 min in a manner dependent on intervals between the two repetitive brief exposures. In vitro maturation significantly shortened the interval required for the reduced responsiveness to the second brief exposure, while in immature neurons prolonged intervals were required for the reduced responsiveness to the second brief exposure to NMDA. Moreover, brief exposure to NMDA led to a marked decrease in immunoreactivity to extracellular loop of NR1 subunit in cultured neurons not permeabilized in proportion to the time after washing. These results suggest that cellular maturation would facilitate the desensitization process to repeated stimulation by NMDA, without markedly affecting that to sustained stimulation, through a mechanism related to the decreased number of NMDA receptors expressed at cell surfaces in cultured rat cortical neurons.

New 7,8-ethylenedioxy-2,3-benzodiazepines as noncompetitive AMPA receptor antagonists

A series of 1-aryl-3,5-dihydro-7,8-ethylenedioxy-4H-2,3-benzodiazepin-4-ones 2a-f, were synthesized and screened as anticonvulsant agents in DBA/2 mice against sound-induced seizures. The new compounds display anticonvulsant properties although the ED(50) values are higher than those of prototypes 1-aryl-3,5-dihydro-7,8-methylenedioxy-4H-2,3-benzodiazepin-4-ones (1) and GYKI 52466, well-known noncompetitive AMPA receptor antagonists. Functional tests were performed to evaluate the antagonistic activity at the AMPA and kainate receptors.

Synthesis, Chiral Resolution, and Enantiopharmacology of a Potent 2,3-Benzodiazepine Derivative as Noncompetitive AMPA Receptor Antagonist

This paper describes the synthesis of racemic 3,5-dihydro-5-methyl-7,8-methylenedioxy-4H-2,3-benzodiazepin-4-one (+/-)-5, attempted stereoselective synthesis of its enantiomers, chiral HPLC resolution of the racemate, and assignment of the absolute configuration. Enantiomer (5S)-(-)-5 is provided with an in vivo anticonvulsant activity 8 times higher than its enantiomer (5R)-(+)-5. This result is confirmed in the in vitro test by the ability to inhibit the kainate-induced increase of the [Ca(2+)](i) in a primary culture of rat cerebellar granule cells which express alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. Binding affinity of compound (+/-)-5 at the AMPA and N-methyl-d-aspartic acid (NMDA) receptors was also evaluated.

1-Aryl-6,7-methylenedioxy-3 H -quinazolin-4-ones as anticonvulsant agents

A set of novel 1-aryl-6,7-methylenedioxy-3H-quinazolin-4-(thi)ones (3a-f) has been designed and screened as anticonvulsant agents in DBA/2 mice. The new compounds are provided with anticonvulsant properties comparable to those of GYKI 52466. To clarify the mode of action, their affinity for the quinazolinone/2,3-benzodiazepine site of the AMPA receptor complex has been assayed.

Characterization of the mechanism of anticonvulsant activity for a selected set of putative AMPA receptor antagonists

A selected set of 1-aryl-7,8-methylenedioxy-2,3-benzodiazepin-4-ones and their analogues were evaluated for their ability to bind the competitive and noncompetitive sites of the AMPA receptors complex as well as to the glycine site of the NMDA receptors. The results put in evidence that most of the test compounds, despite a close structural similarity with GYKI 52466, possess a significantly different pharmacological profile.

Akt mediates the anti-apoptotic effect of NMDA but not that induced by potassium depolarization in cultured cerebellar granule cells

Apoptosis of cultured cerebellar granule neurons (CGNs) deprived of serum is prevented by K+ depolarization or moderate concentrations of N-methyl-d-aspartate (NMDA). Here, we have examined the role of the serine/threonine kinase Akt in these protective effects. The exposure of mouse CGNs to NMDA or K+ depolarization increased the phosphorylation of Akt, compared with that measured in cells incubated in a physiological K+ concentration. Only the NMDA-evoked response was reduced by inhibitors of phosphatidylinositol 3-kinase (wortmannin and LY294002) and mitogen-activated protein kinase (PD98059 and U0126). Similarly, the capacity of NMDA to inhibit apoptosis of CGNs deprived of serum was greatly reduced by these inhibitors as well as by the transfection of neurons with a catalytically inactive mutant of Akt, whereas the protective effect of K+ depolarization remained unaffected. These findings indicate that K+ depolarization and NMDA activate Akt through different signalling pathways in CGNs. Moreover, Akt mediates the anti-apoptotic effect of NMDA, but not that evoked by K+ depolarization.

Blockade by ferrous iron of Ca2+ influx through N-methyl-D-aspartate receptor channels in immature cultured rat cortical neurons

Rat cortical neurons cultured for 3 days in vitro were loaded with the fluorescent indicator fluo-3 for assessment of intracellular free calcium ion (Ca2+) concentrations with the aid of a confocal laser-scanning microscope. In the absence of added MgCl2, the addition of NMDA induced a rapid but sustained increase in the number of fluorescent neurons in a concentration-dependent manner at a concentration range of 1-100 micro m with the increase by KCl being transient. The addition of FeCl2, but not FeCl3, markedly inhibited the increase by NMDA in a reversible manner at concentrations of 10-200 micro m, without affecting that by KCl. Extensive analyses revealed clear differentiation between inhibitions by ferrous iron and other channel blockers known to date. The inhibition by FeCl2 was completely prevented by the addition of two different iron chelators. Exposure to NMDA alone did not lead to cell death in immature cultured neurons, however, while further addition of FeCl2 invariably induced neuronal cell death 24 h after exposure. These results give support to our previous proposal that NMDA receptor complex may contain a novel site sensitive to blockade by ferrous iron in rat brain.

Relationship Between Intracellular Free Calcium Concentration and NMDA-induced Cerebellar Granule Cell Survival In Vitro

The survival of cerebellar granule cells in culture is stimulated by activation of the N-methyl-d-aspartate (NMDA) class of glutamate receptors. Activation of these receptors at the key period for cell survival in vitro (3 days; 3DIV) resulted in a sustained elevation of intracellular free calcium concentration [Ca2+]i over the same concentration range of NMDA that led to granule cell survival. Agents that release Ca2+ from intracellular stores led to only small, transient elevations of [Ca2+]i and were unable to stimulate granule cell survival. Addition of the Ca2+ ionophore ionomycin to granule cell cultures at 3DIV resulted in increased granule cell number at 7DIV. The ability of ionomycin to stimulate granule cell survival was related to the [Ca2+]i elicited, indicating that a rise in [Ca2+]i is sufficient to activate the processes leading to granule cell survival and that the extent of the elevation in [Ca2+]i is crucially important in determining granule cell fate.

Potentiation of nuclear activator protein-1 DNA binding following brief exposure to N-methyl-D-aspartate in immature cultured rat hippocampal neurons

Similar potentiation was seen with the nuclear transcription factor activator protein-1 (AP1) binding in rat hippocampal neurons cultured for 3 and 9 DIV, when determined immediately after exposure to 500 microM N-methyl-D-aspartate (NMDA) for 60-120 min. Growth-associated protein-43 was markedly expressed in hippocampal neurons cultured for 3-5 DIV, with a decline up to 9 DIV. In immature neurons cultured for 3 DIV, NMDA was effective in significantly potentiating AP1 binding even in the presence of Mg(2+) with less potency than in the absence of Mg(2+) when determined immediately after sustained exposure for 120 min. When determined 120 min after brief exposure for 5 min, by contrast, NMDA significantly potentiated AP1 binding at a range of 100-500 microM only in the absence of Mg(2+) in immature neurons cultured for 3 DIV. At least 60 min was required for significant potentiation of AP1 binding as an interval between brief exposure and subsequent cell harvest. Dizocilpine abolished the potentiation determined 120 min after brief exposure to 500 microM NMDA, and both dantrolene and nifedipine were similarly effective in significantly preventing the potentiation at 10-50 microM. These results suggest that NMDA may potentiate AP1 binding following a sustained increase in intracellular free Ca(2+) concentrations through influxes across NMDA-operated and L-type voltage-sensitive Ca(2+) channels, in addition to release from intracellular Ca(2+) stores, in immature cultured rat hippocampal neurons.

Functional characterization of CP-465,022, a selective, noncompetitive AMPA receptor antagonist

The hypothesis that aberrant alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor activity contributes to epileptogenesis and neurodegeneration has prompted the search for AMPA receptor antagonists as potential therapeutics to treat these conditions. We describe the functional characterization of a novel quinazolin-4-one AMPA receptor antagonist, 3-(2-chloro-phenyl)-2-[2-(6-diethylaminomethyl-pyridin-2-yl)-vinyl]-6-fluoro-3H-quinazolin-4-one (CP-465,022). This compound inhibits AMPA receptor-mediated currents in rat cortical neurons with an IC(50) of 25 nM. Inhibition is noncompetitive with agonist concentration and is not use- or voltage-dependent. CP-465,022 is selective for AMPA over kainate and N-methyl-D-aspartate receptors. However, the compound is found to be equipotent for AMPA receptors composed of different AMPA receptor subunit combinations. This is indicated by the finding that CP-465,022 is equivalently potent for inhibition of AMPA receptor-mediated responses in different types of neurons that express different AMPA receptor subunits. Thus, CP-465,022 provides a new tool to investigate the role of AMPA receptors in physiological and pathophysiological processes.

Dual mechanisms of Ca(2+) increases elicited by N-methyl-D-aspartate in immature and mature cultured cortical neurons

Cortical primary cultures were loaded with the fluorescent indicator fluo-3 for assessment of intracellular-free Ca(2+) ions with the aid of a confocal laser-scanning microscope. The addition of N-methyl-D-aspartic acid (NMDA) markedly increased the number of fluorescent cells in a manner sensitive to prevention by both an NMDA channel blocker and MgCl(2). In the absence of added MgCl(2), NMDA induced a sustained increase in the number of fluorescent cells with a transient increase by KCl in cells cultured for 3 days in vitro (DIV). Both nifedipine and dantrolene were more potent in preventing the increase by NMDA in cortical preparations cultured for 9 DIV than those for 3 DIV. These results suggest that activation of NMDA receptors may lead to a sustained increase in intracellular-free Ca(2+) concentrations in immature cultured neurons, in a manner less dependent on the influx through L-type voltage-dependent channels as well as the release from intracellular stores than in mature neurons.

Characterization of the binding site for a novel class of noncompetitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonists

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor is an ionotropic glutamate receptor that mediates fast excitatory synaptic transmission throughout the central nervous system. In addition to the glutamate binding site, allosteric modulatory sites on the receptor are inferred from the ability of synthetic compounds to affect channel function without interaction with the glutamate binding site. We have identified a novel class of potent, noncompetitive AMPA receptor antagonists typified by CP-465, 022 and CP-526,427. The latter compound was radiolabeled and used to elucidate the pharmacology of one allosteric modulatory site. [(3)H]CP-526,427 labels a single binding site in rat forebrain membranes with a K(d) value of 3.3 nM and a B(max) of 7.0 pmol/mg of protein. The [(3)H]CP-526,427 binding site does not seem to interact directly with the glutamate binding site but overlaps with that for another class of AMPA receptor antagonists, the 2,3-benzodiazepines. This binding site is distinct from that for the antagonist Evans blue and for several classes of compounds that modulate AMPA receptor desensitization. These results indicate the existence of at least two physically distinct allosteric sites on the AMPA receptor through which channel activity or desensitization is modulated.

Chiral synthesis and pharmacological evaluation of NPS 1407: a potent, stereoselective NMDA receptor antagonist

The stereoselective synthesis and biological activity of NPS 1407 (4a), (S)-(-)-3-amino-1,1-bis(3-fluorophenyl)butane, a potent, stereoselective antagonist of the NMDA receptor, are described. The racemate (4) was found to be active at the NMDA receptor in an in vitro assay, prompting the synthesis of the individual stereoisomers. The S isomer (4a) was found to be 12 times more potent than the R isomer (4b). Compound 4a demonstrated in vivo pharmacological activity in neuroprotection and anticonvulsant assays.

Topiramate blocks kainate-evoked cobalt influx into cultured neurons

PURPOSE

This study evaluated topiramate (TPM) antagonism of glutamate receptors activated by kainate.

METHODS

The ability of TPM (3-30 microM) to attenuate kainate (300 microM)-activated cobalt (Co2+) flux through nonselective cation channels permeable to Co2+, Mn2+, and Ca2+ into cultured cerebellar granule neurons [9-14 days in vitro (div)] was investigated. Results were compared with those obtained with the non-N-methyl-D-aspartate (non-NMDA) antagonist 6,7-dinitroquinoxalone-2,3-dione (DNQX) (10 microM).

RESULTS

Topiramate produced a concentration- and time-dependent inhibition of Co2+ uptake into cerebellar granule cells cultured 9-11 div. Inhibition was evident at 10 microM, and complete inhibition was observed at 30 microM. Maximal inhibition of Co2+ uptake required pretreatment with TPM for > or =30 minutes before stimulation by kainate. The effect of 30 microM TPM on Co2+ uptake was similar to that of 10 microM DNQX. However, TPM, unlike DNQX, did not affect kainate-evoked Co2+ uptake into older neurons (i.e., 13-14 div).

CONCLUSIONS

These results provide additional support for an antagonistic effect of TPM on some types of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and/or kainate receptors, and specifically suggest that TPM interacts with a Ca2+-permeable non-NMDA receptor that is developmentally regulated. This observation may provide insight into the molecular biology underlying the pathophysiology of seizure disorders and antiepileptic drug resistance.

Intracellular survival pathways against glutamate receptor agonist excitotoxicity in cultured neurons. Intracellular calcium responses

Cultured rat cerebellar granule cells are resistant to the excitotoxic effects of N-methyl-D-aspartate (NMDA) and non-NMDA receptor agonists under three conditions: 1) prior to day seven in vitro when cultured in depolarizing concentrations of potassium [25 mM]; 2) at any time in vitro when cultured in non-depolarizing concentrations of potassium 5 mM[; and 3) when neurons, cultured in depolarizing concentrations of potassium 25 mM[ for eight days in vitro, are pretreated with a subtoxic concentration of NMDA. The focus of this paper is to determine: a) whether the resistance to excitotoxicity by NMDA and non-NMDA receptor agonists is due to a decreased intracellular calcium Ca++[i response to glutamate receptor agonists in cultured rat cerebellar granule cells; or b) whether Ca++[i levels induced by the agonists are similar to those observed under excitotoxic conditions. Granule cells, matured in non-depolarizing growth medium, treated with glutamate resulted in an increase in Ca++[i followed by a plateau that remained above baseline in virtually all neurons that responded to glutamate. The response was rapid in onset (< 10 sec) and the pattern of response heterogeneous in that cells responsive to glutamate increased their Ca++[i to different extents; some cells did not respond to glutamate. Kainate also produced significant elevations in Ca++[i. The Ca++[i response to glutamate in neurons matured in depolarizing (25 mM K+) growth medium for three days was rapid, transient and heterogeneous, which reached a plateau that was elevated above baseline levels; removing the glutamate markedly reduced the Ca++[i concentration. Activation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors by kainic acid produced similar changes in Ca++[i responses. At a time when cultured cerebellar granule cells become susceptible to the excitotoxic effects of glutamate acting at NMDA receptors (day in vitro (DIV) 8) in depolarizing growth medium, glutamate elicited Ca++[i responses similar to those observed at a culture time when the neurons are not susceptible to the excitotoxic effects of glutamate (DIV 3). Pretreatment of the cultured neurons with a subtoxic concentration of NMDA, which protects all neurons against the excitotoxic effects of glutamate, did not alter the maximal Ca++[i elicited by an excitotoxic concentration of glutamate.

Synthesis, biological activity, and absolute stereochemical assignment of NPS 1392: a potent and stereoselective NMDA receptor antagonist

The synthesis, biological activity, and single crystal X-ray structure of NPS 1392, (R)-(-)-3,3-bis(3-fluorophenyl)-2-methylpropan-1-amine (3a), a potent, stereoselective antagonist of the NMDA receptor, are described. The NMDA receptor selectively bound the levo isomer (3a) over its enantiomer (3b), which prompted a rigorous absolute configuration assignment. NPS 1392 has the R configuration based on the single-crystal X-ray diffraction analysis of the hydroiodide salt of NPS 1392. This compound is a potential neuroprotective agent for use in the treatment of ischemic stroke.

Effects of depolarizing stimuli on calcium homeostasis in cultured rat motoneurones

Intracellular calcium concentrations in individual rat motoneurones in enriched primary cultures were measured by Indo-1 fluorimetry. Motoneurones in the cultures were characterized morphometrically and by cholineacetyltransferase immunocytochemistry. Depolarization of the cells with glutamic acid or veratridine increased intracellular calcium levels, which returned to baseline only slowly after removal of the depolarizing agent. The use of selective agonists (N-methyl-D-aspartic acid, AMPA, kainic acid, quisqualic acid and 1R-3S-ACPD) and antagonists (MK 801 and CNQX) showed that the excitatory amino acid-evoked responses were mediated by AMPA/kainate receptors rather than by NMDA receptors. Depolarization-evoked calcium transients in motoneurones are blocked by the neuroprotective drug riluzole Calcium transients reflected entry of calcium from without the cell, and their blockade by nitrendipine and lanthanum chloride suggested that this entry took place primarily through voltage-dependent calcium channels. These findings may be relevant for understanding the selective vulnerability of motoneurones to excitotoxicity in amyotrophic lateral sclerosis, and the therapeutic activity of riluzole in the treatment of this disease.

Design, synthesis, and biological evaluation of spider toxin (argiotoxin-636) analogs as NMDA receptor antagonists

PURPOSE

Twelve synthetic spider toxin analogs were prepared in an effort to better understand the structure-activity relationships of the polyamine portion of argiotoxin-636 (Arg-636), a noncompetitive NMDA receptor (NMDAR) antagonist.

METHODS

The 1,13-diamino-4,8-diazatridecane portion of the side chain of Arg-636 was systematically modified in an effort to further our knowledge of the structural requirements for the alkyl linker spacing between the amine nitrogens. Systematic isosteric replacement of each of the amine nitrogens in the polyamine moiety with either oxygen or carbon provided a series of compounds which were evaluated in vitro for NMDAR antagonist activity.

RESULTS

One-half of the heteroatoms found in Arg-636 were removed to provide analogs which maintained in vitro potency below 1 microM. However, these simplified analogs produced similar or more pronounced effects on the cardiovascular system than Arg-636 in vivo.

CONCLUSIONS

In this set of analogs, a minimum of three basic nitrogens in the side chain was required for maximum potency as NMDAR antagonists. Isosteric nitrogen substitutions in the polyamine chain reduced the in vitro potency of these analogs. An analog binding-conformation model was proposed to rationalize the inactivity of these isosterically substituted analogs.

New isoforms of the chick glutamate receptor subunit GluR4: molecular cloning, regional expression and developmental analysis

To identify chick GluR4 isoforms, we used PCR to amplify a C-terminal region that is the site of alternative splicing in rat. We report here the cloning of three novel chick GluR4 isoforms. GluR4c has a 113-bp insert in the C-terminus, is expressed in flip and flop isoforms, is most strongly expressed in the cerebellum, midbrain and forebrain, and appears from embryonic day (E) 2.5 through at least post-hatching day (P) 2, with a peak of expression at E17. GluR4d has a 184-bp segment inserted at the 4c splice site, occurs as flip and flop isoforms, is expressed most strongly in cerebellum, hindbrain and forebrain, and is present from E11 through P2, with peak expression at E17. GluR4s is a shortened form that lacks the nominal 4th transmembrane and flip/flop domains and shares a common C-terminal region with GluR4. GluR4s is expressed most strongly in the hindbrain and cerebellum and its expression increases from E11 through P2. Experiments on purified cerebellar cells show that glia express GluR4c and GluR4d at combined levels nearly twice that of GluR4 and that flip isoforms predominate. In contrast, granule cells express GluR4c and GluR4d at a level comparable to GluR4 and express GluR4s at a level less than half that in cerebellar glia. Thus, the independence of alternative splicing at the flip/flop and C-terminal splice sites allows seven alternatively spliced forms of GluR4 to exist in chick CNS. This structural diversity increases the potential for functional diversity in neuronal and glial GluRs incorporating GluR4.

Topiramate enhances GABA-mediated chloride flux and GABA-evoked chloride currents in murine brain neurons and increases seizure threshold

The anticonvulsant topiramate is effective in laboratory animals against maximal electroshock seizures, amygdala kindling, and spike-wave discharges and has demonstrated efficacy in humans for the treatment of complex partial seizures. However, its mechanism of action has yet to be clearly elucidated. When the chloride-sensitive fluorescent probe N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) was used as a tool for estimating the effect of anticonvulsant drugs on GABA receptor function, topiramate was observed to enhance GABA-stimulated chloride (Cl-) flux. At a therapeutic concentration, topiramate (10 microM) enhanced GABA-stimulated (10 microM) Cl- influx into cerebellar granule neurons but did not significantly increase Cl- influx alone. Phenytoin (10 microM) and acetazolamide (300 microM) did not enhance GABA-stimulated Cl- influx. In patch-clamp electrophysiological studies, topiramate also enhanced GABA-evoked whole cell Cl- currents in mouse cerebral cortical neurons in culture. In vivo anticonvulsant studies confirmed that topiramate, like phenytoin, is primarily effective against tonic extension seizures induced by maximal electroshock and is ineffective against clonic seizures induced by the subcutaneously administered chemoconvulsants pentylenetetrazol (PTZ), bicuculline (Bic), and picrotoxin (Pic). In contrast to phenytoin, topiramate, at a dose equivalent to the MES median effective dose (ED50), was found to elevate seizure threshold as estimated by the intravenous PTZ seizure threshold test. Taken together these results support the conclusion that enhancement of GABA-mediated Cl- flux may represent one mechanism that contributes to the anticonvulsant activity of topiramate.

CP-101,606, a potent neuroprotectant selective for forebrain neurons

The neuroprotective activity of (1S,2S)-1-(4-hydroxyphenyl)-2-(4-hydroxy-4-phenylpiperidino)-1-propanol (CP-101,606), an N-methyl-D-aspartate (NMDA) receptor antagonist structurally similar to ((+/-)-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)-1-++ +piperidineethanol (ifenprodil), was investigated in neurons in primary culture. CP-101,606 potently and efficaciously protected hippocampal neurons from glutamate toxicity but was > 900-fold less effective for cerebellar granule neurons. The neuroprotective activity in the hippocampal neurons is mediated through a high affinity binding site distinct from the agonist and thienylcyclohexylpiperidine (TCP) binding sites of the NMDA receptor. Autoradiography indicates the CP-101,606 binding site is localized in forebrain, most notably in hippocampus and the outer layers of cortex. The functional selectivity for hippocampal neurons, forebrain localization of binding sites, and structural relation to ifenprodil suggest that CP-101,606 is an NMDA antagonist highly selective for NR2B subunit containing receptors.

Routes of NMDA- and K(+)-stimulated calcium entry in rat cerebellar granule cells

The routes of Ca2+ entry in response to N-methyl-D-aspartate (NMDA) and K+ depolarisation in cerebellar granule cells have been investigated using fura-2 fluorescence to measure intracellular Ca2+ concentrations ([Ca2+]i) and Mn2+ quench of fura-2 fluorescence as an index of Ca2+ entry. Removal of extracellular Na+ did not affect the [Ca2+]i elevation or the rate of Mn2+ quench of fura-2 fluorescence in response to NMDA (100 microM). K+ (25 mM) produced a [Ca2+]i increase which showed a 27% reduction in the presence of the NMDA channel blocker MK-801 (10 microM), whereas no reduction was detected in 50 mM K+ stimulated [Ca2+]i increases. K+ (25 and 50 mM)-stimulated Mn2+ quench rates were not significantly reduced by MK-801. These results demonstrate that NMDA primarily stimulates Ca2+ entry directly through the NMDA receptor without a major component of Ca2+ entry through voltage-gated Ca2+ channels (VGCCs). Under conditions which minimise the accumulation of endogenous glutamate, K+ depolarisation elicits a Ca2+ influx resulting mainly from activation of VGCCs. Additionally, these results show Mn2+ quench of fura-2 fluorescence to be a sensitive and definitive assay of Ca2+ entry through the NMDA receptor and VGCCs.

Regulation of motility of cells from marine sponges by calcium ions

Sponges are known not to contain muscle and nerve cells. Since sponge cells are characterized by high motility we determined the effect of intracellular calcium ion concentration ([Ca2+]i) on their motility. Addition of the Ca2+ ionophore ionomycin to dissociated cells from the marine sponge Suberites domuncula caused in Ca(2+)-containing artificial seawater (ASW) an increase in motility from 0.2 micron/min (absence of the ionophore) to 3.7 microns/min (presence of ionomycin). When the experiments were performed in Ca(2+)-free medium, no effect of ionomycin could be observed. In parallel experiments the changes of [Ca2+]i using the dye Fura-2 were measured. The experiments revealed that ionomycin causes an influx of Ca2+ into the cytosol of cells suspended in Ca(2+)-containing artificial seawater. In contrast, if cells were suspended in Ca(2+)-free artificial seawater, no increase of [Ca2+]i occurred. Incubation of cells in the presence of inhibitors, specific for endoplasmatic Ca(2+)-ATPase in mammals such as thapsigargin, cyclopiazonic acid, or 2,5 di-t-butylhydrochinone, did not influence the [Ca2+]i if cells were suspended in Ca(2+)-free artificial seawater. From these data we conclude that the [Ca2+]i is primarily regulated through channels in the plasma membrane. In addition we summarize experimental evidence indicating that the [Ca2+]i is involved in the control of cell motility. From the marine sponge Geodia cydonium a partial sequence of the myosin cDNA has been cloned. The deduced amino acid sequence comprises highest homology to nonmuscle myosin type II found in higher invertebrates and vertebrates. Taken together, these data show that the [Ca2+]i level in sponge cells can be modulated by incubation with ionomycin. An increase of the Ca2+ level parallels with higher motility of cells, suggesting an activation of Ca(2+)-dependent protein kinases of myosin type II. Investigations on the ionomycin-activated influx of Ca2+ into the cytosol revealed that predominantly the Ca2+ channels in plasma membrane control the level of [Ca2+]i.

Vasodilation Effects and Action Mechanisms of TMB-8 on Basilar Artery in Rabbits

BACKGROUND: 8-(N,N'-diethylamino)-n-octyl-3,4,5-trimethoxybenzoate (TMB-8) is a potent Ca(2+)-antagonist that can prevent/treat ischemic stroke and inhibit the contractility of smooth, skeletal, and cardiac muscles. Further studies are warranted to elucidate the efficacy of TMB-8 on rabbit basilar artery preparation and its action mechanisms on vascular smooth muscle cell cultures. METHODS AND RESULTS: Effects of TMB-8 on the contractility of rabbit's basilar artery in vitro and those on intracellular free Ca(2+) concentrations, [Ca(2+)](i), were studies with isolated organ bath and Fura-2 methods. Histamine-induced concentration-response curves were shifted by TMB-8 in a mixed manner whereas those of norepinephrine and KCl were shifted in a non-competitive manner. In the presence of nifedipine or in a Ca(2+)-free medium, 2,5-di(tert-butyl)-1,4-benzohydroquinone (BHQ) (10 µM) induced an immediate transient contraction in rabbit basilar artery, whereas ryanodine showed a slow, weak, sustained contraction, followed by a weak, sustained relaxation. TMB-8 (30 µM) significantly inhibited these contractions of BHQ and ryanodine. Further, aminophylline enhanced the inhibitory action of TMB-8 on vasocontractions, suggesting that TMB-8's inhibitory actions may be related to the increase of cAMP level. The muscle contraction induced by BHQ was enhanced by pretreatment of the artery ring with TMB-8 for 15 minutes and then TMB-8 was rinsed out. These results indicate that TMB-8 pretreatment can increase Ca(2+) sequestration into sarcoplasmic reticulum, which leads to a larger subsequent Ca(2+) release by BHQ. KCl-induced increase of [Ca(2+)](i) in vascular smooth muscle cells was reduced when the cells were bathed in the medium containing nifedipine. TMB-8 made further reduction on KCl-induced [Ca(2+)](i) increase in nifedipine-containing solution, which had already blocked the voltage-operated Ca(2+) entry. CONCLUSION: These results indicate that (a) TMB-8 can enhance Ca(2+) sequestration into sarcoplasmic reticulum, which leads to a larger amount of Ca(2+) that can be released by BHQ; (b) TMB-8 can inhibit KCl-induced muscle contraction caused by the reduction of [Ca(2+)](i) through saturation of Ca(2+) inside the sarcoplasmic reticulum rather than a direct blockade of Ca(2+)-influx at cell membrane site; and (c) TMB-8 increases cAMP, which enhances Ca(2+) uptake into the sarcoplasmic reticulum.

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.

Inhibitory effects of tetrandrine on intracellular free Ca2+ increase induced by glutamate, serotonin and histamine in dissociated retina cells

The effects of tetrandrine (Tet) on the elevation of intracellular free Ca2+ concentration ([Ca2+]i) induced by glutamate, serotonin and histamine in dissociated rabbit retina cells were studied. The changes of [Ca2+]i were reflected by the fluorescent indicator, Fura-2/AM, employed. In the presence of extracellular Ca2+ (1.3 mM), glutamate, serotonin and histamine significantly increased the [Ca2+]i in a dose-dependent manner. Glutamate (100 microM), serotonin (100 microM) and histamine (200 microM) markedly increased the [Ca2+]i of retina cells by 165%, 126% and 58%, respectively. Tet 30 microM significantly inhibited the increase of [Ca2+]i induced by glutamate (100 microM), serotonin (100 microM) and histamine (200 microM) by 28.0%, 46.8% and 29.0%, respectively. A lower concentration (10 microM) of Tet also produced an inhibitory effect on the increase of [Ca2+]i but was less effective than the Tet 30 microM. In Ca(2+)-free Hank's solution, Tet did not produce a significant inhibitory effect on the increase of [Ca2+]i caused by serotonin and histamine. These results indicate that Tet exercises blocking Ca2+ influx from the extracellular site via NMDA, 5-HT2 and H1-receptor operated Ca2+ channels and has no obvious effect on the Ca2+ release from intracellular Ca2+ stores.

SB 201823-A antagonizes calcium currents in central neurons and reduces the effects of focal ischemia in rats and mice

BACKGROUND AND PURPOSE

Excessive calcium entry into depolarized neurons contributes significantly to cerebral tissue damage after ischemia. We evaluated the ability of a novel neuronal calcium channel blocker, SB 201823-A, to block central neuronal calcium influx in vitro and to reduce ischemic injury in two rodent models of focal stroke.

METHODS

Patch-clamp electrophysiology and intracellular Ca2+ imaging in rat hippocampal and cerebellar neurons were used to determine effects on neuronal calcium channel activity. Middle cerebral artery occlusion was performed in Fisher 344 rats and CD-1 mice to determine the effects on rodent focal ischemic injury and neurological deficits. Cardiovascular monitoring in conscious rats was conducted to determine cardiovascular liabilities of the compound.

RESULTS

In cultured rat hippocampal cells, calcium current measured at plateau was reduced by 36 +/- 8% and 89 +/- 4% after 5 and 20 mumol/L SB 201823-A, respectively. In cerebellar granule cells in culture, pretreatment with 2.5 mumol/L SB 201823-A totally prevented initial calcium influx and reduced later calcium influx by 50 +/- 2.5% after N-methyl-D-aspartate/glycine stimulation (P < .01). KCl depolarization-induced calcium influx also was reduced by more than 95%. In rats, a single treatment with 10 mg/kg IV SB 201823-A beginning 30 minutes after focal ischemia decreased (P < .05) hemispheric infarct by 30.4% and infarct volume by 29.3% and reduced (P < .05) forelimb deficits by 47.8% and hindlimb deficits by 36.3%. In mice, treatments with 10 mg/kg IP SB 201823-A beginning 30 minutes after focal ischemia significantly reduced infarct volume by 41.5% (P < .01). No blood pressure effects were observed with the therapeutic dose of the compound.

CONCLUSIONS

These results indicate that the new neuronal calcium channel blocker SB 201823-A can block stimulated calcium influx into central neurons and can provide neuroprotection in two models of focal cerebral ischemia without affecting blood pressure. Data from several different studies now indicate that the neuronal calcium channel antagonists are a promising therapy for the postischemic treatment of stroke.

Divalent cation entry in cultured rat cerebellar granule cells measured using Mn2+ quench of fura 2 fluorescence

In this study the rate of Mn2+ quench of fura-2 fluorescence evoked by glutamatergic and cholinergic agonists, depolarization and Ca2+ store modulators was measured in cultured cerebellar granule cells, in order to study their effects on Ca2+ entry in isolation from effects on Ca2+ store release. The rate of fluorescence quench by 0.1 mM Mn2+ was markedly increased by 25 mM K(+)-evoked depolarization or by 200 microM N-methyl-D-aspartate (NMDA), with a significantly greater increase occurring during the rapid-onset peak phase compared to the plateau phase of the K(+)- or NMDA-evoked [Ca2+]i response. The stimulatory effect of NMDA on Mn2+ quench was abolished by dizocilpine (10 microM), but nitrendipine (2 microM), while decreasing the rate of basal quench, did not affect NMDA-stimulated Mn2+ entry. This suggests that nitrendipine may not act on NMDA channels in granule cells, at least under these conditions, and that voltage-operated Ca2+ channels are involved in control quench whereas the NMDA-evoked quench is dependent on entry through the receptor channel. The t1/2 of quench was unaffected by alpha-amino-hydroxyisoxazole propionic acid (200 microM) and carbamyl choline (1 mM). Neither thapsigargin (10 microM) nor dantrolene (30 microM) significantly affected the rate of quench under control or NMDA- or K(+)-stimulated conditions, which confirms that the previously reported inhibitory effects on [Ca2+]i elevations evoked by these agents are due to actions on Ca2+ stores. However, thapsigargin elevated [Ca2+]i in the presence of normal [Ca2+]o but not in nominally Ca(2+)-free medium, indicating that it evokes Ca2+ entry in cerebellar granule cells, probably subsequent to store depletion, which appears to be either too small to be detected by Mn2+ quench or to occur via Mn(2+)-impermeant channels.

Postnatal maturation of gamma-aminobutyric acidA and B-mediated inhibition in the CA3 hippocampal region of the rat

In the adult central nervous system, GABAergic synaptic inhibition is known to play a crucial role in preventing the spread of excitatory glutamatergic activity. This inhibition is achieved by a membrane hyperpolarization through the activation of postsynaptic gamma-aminobutyric acidA (GABAA) and GABAB receptors. In addition, GABA also depress transmitter release acting through presynaptic GABAB receptors. Despite the wealth of data regarding the role of GABA in regulating the degree of synchronous activity in the adult, little is known about GABA transmission during early stages of development. In the following we report that GABA mediates most of the excitatory drive at early stages of development in the hippocampal CA3 region. Activation of GABAA receptors induces a depolarization and excitation of immature CA3 pyramidal neurons and increases intracellular Ca2+ ([Ca2+]i)] during the first postnatal week of life. During the same developmental period, the postsynaptic GABAB-mediated inhibition is poorly developed. In contrast, the presynaptic GABAB-mediated inhibition is well developed at birth and plays a crucial role in modulating the postsynaptic activity by depressing transmitter release at early postnatal stages. We have also shown that GABA plays a trophic role in the neuritic outgrowth of cultured hippocampal neurons.

Felbamate modulates the strychnine-insensitive glycine receptor

Felbamate (2-phenyl-1,3-propanediol dicarbamate) is a novel anticonvulsant substance whose mechanism of action is not clearly understood. The present investigation examined its ability to modulate the strychnine-insensitive glycine receptor associated with the N-methyl-D-aspartate (NMDA) receptor. Felbamate decreased the magnitude of glycine (100 microM)-enhanced NMDA (100 microM)-induced intracellular calcium ([Ca2+]i) transients in mouse cerebellar granule cells which had been loaded with the Ca(2+)-sensitive fluorescent probe indo-1 acetoxymethyl ester (indo-1/AM). This effect of felbamate was concentration dependent, with a maximal effect observed at 300 microM (65 +/- 4% of control). In the Frings audiogenic seizure-susceptible mouse model of reflex epilepsy, the glycine agonist D-serine (150 nmol, i.c.v.) completely blocked the anticonvulsant activity of a maximally effective dose of felbamate (19 mg/kg, i.p.). This effect of D-serine could be reversed by increasing the administered dose of felbamate to 29 mg/kg. Furthermore, administration of D-serine (300 nmol, i.c.v.) to felbamate-treated Frings mice produced a parallel right shift in felbamate's anticonvulsant dose-response curve (ED50s: 9.4 mg/kg for felbamate vs. 17.7 mg/kg for felbamate + D-serine). The results obtained in this investigation suggest that the ability of felbamate to modulate the strychnine-insensitive glycine receptor may be physiologically and behaviorally relevant to its anticonvulsant mechanism of action.

Neuroprotective effects of tetrodotoxin as a Na+ channel modulator and glutamate release inhibitor in cultured rat cerebellar neurons and in gerbil global brain ischemia

BACKGROUND AND PURPOSE

Studies examining the role of tetrodotoxin-sensitive ion channels in hypoxic-ischemic neuronal damage have concluded that sodium influx is an important initiating event. We examined the neuroprotectant effect of tetrodotoxin on both cultured cerebellar neurons and on CA1 hippocampal neurons of gerbils exposed to brain ischemia.

METHODS

We studied neuroprotective mechanisms using cultured rat cerebellar granule cells exposed to veratridine, which induced cytotoxicity, neurotransmitter release, and calcium influx. Survival of gerbil CA1 neurons was examined by direct neuron counts 7 days after 6 minutes of global ischemia with reperfusion.

RESULTS

Tetrodotoxin protected cultured neurons in a dose-dependent manner from veratridine-induced toxicity (protective concentration [PC50] = 22 nmol/L). Veratridine induced [3H]aspartate efflux that was sodium dependent, only 25% calcium dependent, and was inhibited by tetrodotoxin (inhibitory concentration [IC50] = 60 nmol/L). Veratridine initiated increases in intracellular calcium that were also reversed by tetrodotoxin (IC50 = 63 nmol/L); reversal was dependent on the sodium-calcium exchanger and the sodium-potassium pump. Neuroprotection of 90% (n = 10; P = .001 versus vehicle) of gerbil CA1 hippocampal neurons was achieved by pretreatment with 2 ng of tetrodotoxin delivered three times intracerebroventricularly, without causing hypothermia.

CONCLUSIONS

Sodium channel blockers like tetrodotoxin may have utility in treatment of ischemic neuronal injury by preventing excessive neuronal depolarizations, limiting excitotoxic glutamate release through reversal of the sodium-dependent glutamate transporter, preventing intracellular calcium overload, preserving cellular energy stores, and allowing recovery of ionic homeostasis through operation of the sodium-calcium exchanger.

The effect of intracellular Ca2+ on GABA-activated currents in cerebellar granule cells in culture

The patch clamp technique was used to study the effects of intracellular free calcium ([Ca2+]i) on GABAA-evoked whole-cell and single channel currents of cultured cerebellar granule cells. Changes in [Ca2+]i were obtained by adding to the extracellular solution the calcium ionophore A23187 (2 microM). The relationship between [Ca2+]i and [Ca2+]o in the presence or absence of A23187 was assessed using fluorimetric measurements from Fura-2 loaded cells. In 2 mM [Ca2+]o and A23187, [Ca2+]i was about 1.5 microM, whereas in the absence of A23187 it was about 250 nM. In whole-cell experiments (symmetrical chloride concentrations) at -50 mV, GABA (0.5 microM) evoked inward currents that did not desensitize. Bath application of A23187 significantly reduced the steady-state amplitude of GABA currents by 37 +/- 6%. Single channel currents activated by GABA (0.5 microM) were also recorded in the outside-out configuration of the patch clamp technique. Kinetic analysis of single channel events revealed that A23187 significantly increased the long closed time constant (tau c3) without affecting the open time constants (tau o1 and tau o2) or the short and medium closed time constants (tau c1 and tau c2). Moreover, application of A23187 induced a significant reduction of burst duration (tau b). We conclude that a rise in [Ca2+]i by A23187 may decrease the binding affinity of GABA for the GABAA receptor.

AMPA/Zn(2+)-induced neurotoxicity in rat primary cortical cultures: involvement of L-type calcium channels

Zn2+ is believed to be an endogenous modulator of glutamatergic excitation. It has been shown to attenuate NMDA receptor-mediated excitation and to increase AMPA-induced excitatory transmission. The dual activity of Zn2+ on ionotropic excitatory neurotransmission suggests that Zn2+ plays a role in the modulation of excitatory neurodegenerative events. Stimulation of rat primary cortical cultures with the combination of 50 microM AMPA and 300 microM Zn2+ for 30 min induced approximately 50% cell death compared with only approximately 20% cell death induced by AMPA alone. The degree of neurotoxicity 48 h after the incubation was reproducible and was attenuated by CNQX, EDTA, EGTA, diltiazem and DHP-type Ca2+ channel blockers but not by MK-801. These findings suggest that an initial depolarization induced by AMPA and a subsequent influx of Ca2+ and Zn2+ ions through voltage-operated L-type Ca2+ channels are crucial events which finally lead to neuronal death. Racemic nimodipine and its (+)- and (-)-enantiomers had remarkable in vitro neuroprotective efficacies, the IC50 values being 4 nM for the racemate, 11 nM for the (+)- and 1 nM for the (-)-enantiomer. This suggests a possible therapeutic role for Ca2+ channel blockers in neurodegenerative diseases which are characterized by a disturbance of cellular Ca2+ homeostasis.

Protein kinase C activation attenuates N-methyl-D-aspartate-induced increases in intracellular calcium in cerebellar granule cells

Activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor increases levels of intracellular calcium and can lead to stimulation of protein kinase C activity. Several reports have demonstrated that stimulation of protein kinase C can, in turn, increase electrophysiological responses to NMDA in certain cells or in oocytes expressing certain NMDA receptor subunits. In the present study, the effects of protein kinase C activation on NMDA receptor-mediated increases in intracellular Ca2+ level were investigated in primary cultures of rat cerebellar granule cells using fura-2 fluorescence spectroscopy. Pretreatment of the cells with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA), but not the inactive analogue 4 alpha-phorbol 12-myristate 13-acetate, inhibited NMDA-induced increases in intracellular Ca2+ levels. Coincubation of cells with PMA and the kinase inhibitor staurosporine or calphostin C blocked the PMA effect. The potency of NMDA was reduced twofold, and the potency of the NMDA receptor co-agonist, glycine, to enhance the response to NMDA was decreased fourfold by pretreatment of cells with PMA. The effect on glycine was mimicked by pretreatment with okadaic acid, a protein phosphatase inhibitor. PMA treatment did not significantly alter Mg2+ inhibition of the NMDA response but decreased the potency of the competitive antagonist CGS-19755. These data suggest that, in cerebellar granule cells, the function of the NMDA receptor may be subject to feed-back inhibition by protein kinase C stimulation. Under physiological conditions, this inhibition may result from a decreased effectiveness of the endogenous co-agonists, glutamate and glycine.

Neuroprotective effects of carvedilol, a new antihypertensive, as a Na+ channel modulator and glutamate transport inhibitor

The potent antioxidant activity of carvedilol could explain part of its protective action in brain ischemia, and interaction as a low-affinity non-competitive (uncompetitive) antagonist of the N-methyl-D-aspartate (NMDA) receptor would provide rapid channel blockade at this subtype of glutamate receptor. We have now found carvedilol to be neuroprotective (PC50 = 306 nM) against 40 microM veratridine which kills cerebellar granule cell neurons in 60 min regardless of energy state. Carvedilol was also a potent inhibitor (IC50 = 1.7 microM) of veratridine-stimulated 3[H]aspartate release from preloaded neurons, caused by reversal of the Na(+)-dependent glutamate transporter. Veratridine caused a sustained 4.3-fold increase in intracellular Ca2+ ([Ca2+]i) up to 368 nM (n = 22). Carvedilol reversed the [Ca2+]i levels by a maximum of 73% with an IC50 of 0.9 microM. Such reversal of [Ca2+]i was facilitated by Na+/Ca2+ exchange since the stoichiometry of exchange could be disrupted by prior treatment with 1 microM ouabain to inhibit the Na+/K+ pump. These data suggest that, in addition to its antihypertensive effects, antioxidant activity and ability to act as a non-competitive inhibitor at the NMDA receptor, carvedilol has additional neuroprotective activity as a Na+ channel modulator and glutamate release inhibitor.

Potentiation of N-methyl-D-aspartate-evoked elevation of intracellular Ca2+ concentrations by exogenous glycine in cerebellar granule cells

The effect of glycine on the intracellular free Ca2+ concentration ([Ca2+])i response to N-methyl-D-aspartate (NMDA) was examined in small groups of cerebellar granule cells loaded with fura 2. NMDA alone evoked a long-lasting monophasic [Ca2+]i plateau, which was abolished by removal of extracellular Ca2+, or addition of the NMDA channel antagonist dizocilpine or the glycine site antagonist 5,7-dichlorokynurenic acid, virtually unaffected by the L-type Ca2+ channel antagonist (-)-PN 202 791, and greatly, though variably, potentiated by addition of glycine. In the presence of glycine the response to NMDA was clearly biphasic. However, there was no consistent relationship between the magnitudes of the peak and plateau phases of the response, and their temporal relationship was also highly variable. The potentiation seen with exogenous glycine was highly dependent on plating density, which may be the result of higher levels of endogenous glycine in more dense cultures. Our results provide an explanation of the inconsistent findings previously reported by different groups on the potentiation of the [Ca2+]i response to NMDA by exogenous glycine.

Autoradiographic characterization of the non-N-methyl-D-aspartate binding sites in human cerebellum using the antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione

Using quantitative autoradiography, we have characterized the binding properties of the non-N-methyl-D-aspartate (NMDA) glutamate receptor antagonist [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in adult human cerebellum. Saturation experiments revealed [3H]CNQX binding to a single class of sites with similar affinity in the molecular and granule cell layer (Kd = 89.0 +/- 6.4 and 83.3 +/- 9.9 nM, respectively). The maximum number of [3H]CNQX binding sites was much higher in the molecular compared to the granule cell layer (Bmax = 16.2 +/- 1.1 and 2.8 +/- 0.5 pmol/mg protein, respectively). Inhibition experiments were performed in order to examine the pharmacological profile of [3H]CNQX binding in the molecular layer. [3H]CNQX labeled sites with high affinity for both non-NMDA agonists, (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate. Dose-response curves for inhibition of [3H]CNQX by AMPA and kainate were biphasic. The potency of AMPA for displacement of [3H]CNQX binding (Ki: 2.8 +/- 0.8 nM and 12.5 +/- 0.8 microM) was 4- to 6-fold greater than the corresponding potency of kainate (Ki: 18.1 +/- 5.7 nM and 48.7 +/- 9.3 microM). In conclusion, the pharmacological analysis of [3H]CNQX binding in the human cerebellar molecular layer reflects the existence of multiple binding sites of the non-NMDA receptor that have different affinities for both AMPA and kainate.