N-methyl-D-aspartate raises cytosolic calcium concentration in rat cerebellar granule cells in culture

Neuroprotective properties of the marine carotenoid astaxanthin and omega-3 fatty acids, and perspectives for the natural combination of both in krill oil

The consumption of marine fishes and general seafood has long been recommended by several medical authorities as a long-term nutritional intervention to preserve mental health, hinder neurodegenerative processes, and sustain cognitive capacities in humans. Most of the neurological benefits provided by frequent seafood consumption comes from adequate uptake of omega-3 and omega-6 polyunsaturated fatty acids, n-3/n-6 PUFAs, and antioxidants. Optimal n-3/n-6 PUFAs ratios allow efficient inflammatory responses that prevent the initiation and progression of many neurological disorders. Moreover, interesting in vivo and clinical studies with the marine antioxidant carotenoid astaxanthin (present in salmon, shrimp, and lobster) have shown promising results against free radical-promoted neurodegenerative processes and cognition loss. This review presents the state-of-the-art applications of n-3/n-6 PUFAs and astaxanthin as nutraceuticals against neurodegenerative diseases associated with exacerbated oxidative stress in CNS. The fundamental “neurohormesis” principle is discussed throughout this paper. Finally, new perspectives for the application of a natural combination of the aforementioned anti-inflammatory and antioxidant agents (found in krill oil) are also presented herewith.

Effects of disrupting calcium homeostasis on neuronal maturation: early inhibition and later recovery

It has become increasingly clear that agents that disrupt calcium homeostasis may also be toxic to developing neurons. Using isolated primary neurons, we sought to understand the neurotoxicity of agents such as MK801 (which blocks ligand-gated calcium entry), BAPTA (which chelates intracellular calcium), and thapsigargin (TG; which inhibits the endoplasmic reticulum Ca(2+)-ATPase pump). Thus, E18 rat cortical neurons were grown for 1 day in vitro (DIV) and then exposed to vehicle (0.1% DMSO), MK801 (0.01-20 microM), BAPTA (0.1-20 microM), or TG (0.001-1 microM) for 24 h. We found that all three agents could profoundly influence early neuronal maturation (growth cone expansion, neurite length, neurite complexity), with the order of potency being MK801 < BAPTA < TG. We next asked if cultures exposed to these agents were able to re-establish their developmental program once the agent was removed. When we examined network maturity at 4 and 7 DIV, the order of recovery was MK801 > BAPTA > TG. Thus, mechanistically distinct ways of disrupting calcium homeostasis differentially influenced both short-term and long-term neuronal maturation. These observations suggest that agents that act by altering intracellular calcium and are used in obstetrics or neonatology may be quite harmful to the still-developing human 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.

Age-related differences in NMDA responses in cultured rat hippocampal neurons

The N-methyl-D-aspartate receptor (NMDAR) is expressed in the cerebral cortex and hippocampus and is important in learning and memory. NMDARs are influenced by aging and implicated in neurodegenerative disorders. We investigated age-related differences in NMDAR ionic currents and intracellular calcium in embryonic (E18), middle-age (9-10 month) and old (26 month) rat hippocampal neurons cultured in serum-free medium for 7-12 days. Responses to 200 microM NMDA with 50 microM glycine were measured using whole cell voltage clamp and fura-2 fluorescence. Embryonic neurons exhibited significantly larger and faster NMDA responses than adults. Old rats had 1.5 fold greater normalized NMDA peak current compared to middle-age rats, while intracellular calcium rose 1.3 fold higher. Differences in regression slopes generated from the integral of NMDA current versus normalized NMDA current indicate age-related differences are not exclusively due to changes in receptor density but likely influenced by changes in receptor function. Corresponding age-related measures of intracellular calcium by fura-2 fluorescence in response to NMDA showed a strong correlation with peak current (r(2)=0.996). Our data support the hypothesis that NMDAR responsiveness is altered during aging with an enhanced NMDA peak current in both old and embryonic neurons compared to middle-age neurons.

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.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

Association of c-fos mRNA expression and excitotoxicity in primary cultures of mouse neocortical and cerebellar neurons

The effect of excitatory amino acids (EAAs) on c-fos mRNA expression was studied in primary cultures of mouse cerebellar granule cells and in neocortical neurons after 2 and 7 days in vitro (div). In cultured granule cells at 2 and 7 div, and in cortical neurons at 2 div, exposure to low levels (< or = 10 microM) of a variety of EAAs (viz. glutamate [Glu], S-sulpho-L-cysteine [SC], N-methyl-D-aspartate [NMDA], alpha-amino-3-hydroxy-5-methyl-4-isoxazole [AMPA], and kainate [KA]) resulted in a transient increase in the level of c-fos mRNA which peaked at 30 min but returned to a basal level by 120 min. However, exposure of granule cells (7 div) to high levels (250 microM) of Glu, NMDA, KA, SC and of cortical neurons (7 div) to high levels (250 microM) of Glu, NMDA, KA, SC, or AMPA and to low levels (< or = 10 microM) of Glu and AMPA resulted in a delay in c-fos mRNA induction but a subsequent, progressive increase that was sustained for at least 240 min. Furthermore, this effect was accompanied by a dose-related increase in the release of the cytosolic enzyme, lactate dehydrogenase, used as an indicator of excitotoxicity. A ratio (Q240/30) for the steady-state levels of c-fos mRNA after 30 min and 240 min of exposure to EAAs was determined which showed that Q240/30 >2 correlated reproducibly with excitotoxic cell death, whereas a ratio of < or = 1 correlated with a nonexcitotoxic event. In both cell types at 7 div, coadministration of the selective NMDA receptor antagonist, DL(+/-)-2-amino-5-phosphonopentanoic acid (APV) with cytotoxic levels of Glu 1) protected against EAA-induced neurotoxicity and 2) exhibited a transient c-fos mRNA expression (Q240/30 values approximately 1). In contrast, the AMPA/KA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), provided no protection against excitotoxicity and had no significant effect on the Glu-induced delay in c-fos mRNA expression. These results suggest that the Q240/30 c-fos mRNA ratio may 1) be used as a predictive index for excitotoxic neuronal death, 2) provide information on the identity of the receptor subtype mediating excitotoxicity in different brain cell types, and 3) aid in establishing the role of excitotoxicity during the development of neurons in vitro.

NMDAR1 mRNA expression and glutamate receptor stimulated increase in cytosolic calcium concentration in rat and mouse cerebellar granule cells

We have previously reported that, unlike their rat counterparts, the survival of mouse cerebellar granule cells is independent of chronic stimulation whether owing to elevated K(+)-induced depolarization or NMDA (N-methyl-D-aspartate) receptor activation. One explanation could be that during the critical period mouse granule cells are very sensitive to NMDA receptor stimulation by endogenous glutamate released in the cultures. If so, this might be reflected by an increased expression of NMDA receptors or an increased response to their activation. We tested this hypothesis by measuring (a) the concentration of mRNA for the obligatory NMDA receptor subunit, NMDAR1, and (b) the glutamate/NMDA stimulated increase in cytosolic Ca(2+)-ion concentration in cultures at physiological or elevated K(+)-ion concentration. The expression of NMDAR1 mRNA was measured by competitive PCR of reversely transcribed mRNA and was normalized to that of the constitutively expressed H3.3 histone mRNA. The glutamate and NMDA stimulated increase in cytosolic Ca(2+)-ion concentration was measured using the fluorescent Ca(2+)-chelator Fluo3. In contrast to the hypothesis, we found NMDAR1 mRNA expression to be lower in mouse than in rat granule cells cultured for 4 days at physiological K(+)-ion concentration. However, the NMDA stimulated increase in cytosolic Ca(2+)-ion concentration did not differ in 4-day rat and mouse cultures. Although the glutamate-stimulated increase in cytosolic Ca(2+)-ion concentration in 2-day cultures was higher in mouse granule cells than in rat granule cells, the developmental profile of the glutamate-stimulated increase in cytosolic Ca(2+)-ion concentration was the same in both cases. In conclusion, we found no obvious evidence for increased NMDA receptor activity in mouse cerebellar granule cells cultured at physiological K(+)-ion concentration.

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.

Developmental expression of N-methyl-D-aspartate (NMDA)-induced neurotoxicity, NMDA receptor function, and the NMDAR1 and glutamate-binding protein subunits in cerebellar granule cells in primary cultures

Cerebellar granule cells maintained in vitro as primary cultures are a relatively homogeneous neuronal population that can be used to evaluate the developmental expression of neurotransmitter receptors and to assess their role in cell survival and degeneration. The toxicity induced by N-methyl-D-aspartate (NMDA) in granule cells maintained under partially depolarizing conditions and in the presence of physiologic extracellular concentrations of Mg2+ was greatest for the neurons maintained for 14 days in vitro (DIV). However, following NMDA receptor activation neurons as young as 5 DIV exhibited increases in the concentration of intracellular free Ca2+ which were as large as those achieved with cells at 8-9 or 13-14 DIV. The less mature neurons exhibited a "down-regulation" of responses to increasing concentrations of NMDA and the more mature cells maintained elevated intracellular Ca2+ levels during the inter-stimulus periods. Immunochemical analyses of the expression of the NMDA receptor-associated proteins NMDAR1 and glutamate-binding protein (GBP) in granule cells indicated a developmental increase in both proteins, albeit the pattern of expression of NMDAR1 was the more complex. No definite correlation has yet been established between toxicity induced by NMDA and the expression of these two proteins. Finally, although the developmental expression of nitric oxide synthase, an enzyme that catalyzes the formation of the potentially neurotoxic radicals nitric oxide and superoxide anion, increased progressively with the maturation of neurons in culture, an inhibitor of this enzyme did not protect neurons from NMDA-induced toxicity. Therefore, the developmental changes in granule cells that lead to increased vulnerability following excessive activation of NMDA receptors are not yet completely defined.

Chronic mild acidosis specifically reduces functional expression of N-methyl-D-aspartate receptors and increases long-term survival in primary cultures of cerebellar granule cells

Previous studies suggest that chronic depolarization by addition of 25 mM KCl or N-methyl-D-aspartate to primary cultures of cerebellar granule cells promotes expression of the N-methyl-D-aspartate subtype of glutamate receptor, as determined by electrophysiological responsiveness and susceptibility to excitotoxicity. Recent studies have demonstrated that acute mild acidosis reduces N-methyl-D-aspartate receptor channel activity by a non-competitive action of H+ on an extracellular site of the receptor channel complex. Since the level of N-methyl-D-aspartate receptor expression in granule cell cultures is activity-dependent, we examined whether chronic mildly acidotic culture conditions would selectively diminish the level of N-methyl-D-aspartate responsiveness in granule cells, in effect producing a functional level of expression more comparable to that observed in vivo. To test this, cerebellar granule cells from eight-day neonatal rats were grown in an HCO3-buffered medium containing elevated K+ (25 mM KCl) either under standard conditions (95% air/5% CO2, pH 7.4), or under chronic mildly acidotic conditions (90% air/10% CO2, estimated pH of 7.1). Glutamate receptor subtype expression was subsequently assessed using standard neurotoxicity assays, a quantitative immunoblotting assay for N-methyl-D-aspartate receptors and whole cell patch clamp recordings. Cells grown in the 10% CO2 environment exhibited a significant reduction in susceptibility to L-glutamate neurotoxicity (at least 10-fold), but not kainate-induced neurotoxicity, relative to cells grown in 5% CO2. In both culture conditions, L-glutamate- and kainate-induced toxicity were mediated by activation of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, respectively, as determined by the sensitivity of agonist-induced toxicity to specific receptor antagonists. Using polyclonal antibodies generated against a peptide sequence recognizing five of eight splice variants in the common "R1" subunit of N-methyl-D-aspartate receptors, a 31% reduction in the amount of immunoreactive protein was observed in membrane preparations from cells grown in 10% CO2, relative to the amount detected in cells grown in 5% CO2. Moreover, perfusion of cells with glutamate (50 microM) in a nominally Mg(2+)-free solution containing glycine (2 microM) elicited N-methyl-D-aspartate antagonist-sensitive inward currents in proportionately fewer cells cultured in 10% CO2, relative to cells cultured in 5% CO2. Long-term survival was also significantly enhanced in cells exposed chronically to mild acidotic culture conditions, relative to cells grown under standard pH conditions (22 days, 10% CO2 vs 16 days, 5% CO2).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Glutamate-mediated injury in focal cerebral ischemia: the excitotoxin hypothesis revised

Neuronal injury following focal cerebral ischemia is widely attributed to the excitotoxic effects of glutamate. However, critical analysis of published data on glutamate toxicity in vitro and the comparison of these data with in vivo release of glutamate and the therapeutic effect of glutamate antagonists raises doubts about a neurotoxic mechanism. An alternative explanation for glutamate-mediated injury is hypoxia due to peri-infarct spreading depression-like depolarizations. These depolarizations are triggered in the core of the ischemic infarct and spread at irregular intervals into the peri-infarct surrounding. In ischemically uncompromised tissue, the metabolic workload associated with spreading depression is coupled to an increase in blood flow and oxygen supply, assuring maintenance of oxidative respiration. In the penumbra region of focal ischemia, the hemodynamic constraints of collateral blood circulation prevail the adequate adjustment of oxygen delivery, leading to transient episodes of relative tissue hypoxia. The hypoxic episodes cause a suppression of protein synthesis, a gradual deterioration of energy metabolism and a progression of irreversibly damaged tissue into the penumbra zone. The generation of peri-infarct spreading depressions and the associated metabolic workload can be suppressed by NMDA and non-NMDA antagonists. As a result, the penumbral inhibition of protein synthesis and the progressing energy failure is also prevented, and the volume of ischemic infarct decreases. Interventions to improve ischemic resistance should therefore aim at improving the oxygen supply or reducing the metabolic workload in the penumbra region.

The relationship between synaptogenesis and expression of voltage-dependent currents in cerebellar granule cells in situ

In this work we consider the ontogenetic changes of membrane currents and their relationship with synaptogenesis in cerebellar granule cells. Recordings were performed in whole-cell patch-clamp configuration from cerebellar slices obtained from 4 to 31-day-old rats. Granule cells in the external granular layer, and non-connected granule cells in the internal granular layer expressed outward currents, and inconstantly also small Ca2+ currents, but no fast Na+ currents. Most connected granule cells expressed Ca2+ and Na+ currents. These data indicate that Ca2+ and Na+ current development occurs after synapse formation, while outward (K+) currents begin their development before. Mixed NMDA/non-NMDA synaptic currents were observed at all stages, while synaptic currents with a prominent NMDA component were observed exclusively at immature stages. At P4, ie 1-2 days after the arrival of the first granule cells in the internal granular layer, some granule cells already expressed mature synaptic and voltage-dependent currents, suggesting that establishment of mossy fibre synapses and development of membrane properties takes just 1-2 days to complete. Starting at P4, the probability of activating mossy fibre currents, and sizeable Ca2+ and Na+ currents increased at a similar rate, attaining a plateau level around P20. Average amplitude of Na+ and outward currents decreased until P10 and then increased attaining plateau soon beyond P20. Average amplitude of Ca2+ currents increased monotonically. The time courses of probability and average current amplitude curves are likely explained by changes in the rate of accumulation of migrating granule cells in the internal granular layer, and by changes in granule cell membrane surface extension. These data suggest a relevant role for the process of synapse formation in inducing the expression of new channels in the developing granule cells, which may involve Ca2+ influx through the NMDA channel.

Conantokin-G antagonism of the NMDA receptor subtype expressed in cultured cerebellar granule cells

Conantokins are peptide antagonists of the N-methyl-D-aspartate (NMDA) subclass of excitatory amino acid receptors. We compared conantokin-G and AP5 antagonism of NMDA receptor activity expressed in cultures of neonatal rat cerebellar granule cells, using the fluorescent calcium indicator dye fura-2. The results were consistent with the binding of two molecules of agonist (NMDA) for channel activation and one antagonist molecule (AP5) for inhibition. However, conantokin-G antagonism was more complex: the peptide inhibited only approximately 70% of the elevation of intracellular free calcium produced by NMDA. These results, when combined with previous ones [8], suggest that conantokin-G may have different affinities for, and functional effects on, different subtypes of NMDA receptor complexes expressed in the mammalian CNS.

Vulnerability of cerebellar granule cells to alcohol-induced cell death diminishes with time in culture

This study examined the effects of alcohol exposure on the viability of cerebellar granule cells in culture. Continuous alcohol exposure, starting 1 day after the cultures were established, significantly reduced granule cell numbers, even with a single day of exposure to an alcohol concentration as low as 100 mg/dl. The depletion of cerebellar granule cells by alcohol was concentration-dependent (greater loss of cells at higher alcohol concentrations) and duration-dependent (greater loss of cells at longer exposure durations). The loss of granule cells also depended on the number of days the granule cells were in culture before alcohol exposure. Alcohol was significantly more effective in reducing the cell numbers of newly established granule cell cultures (1 day in vitro) compared with older cultures (4 or 7 days in vitro). Cell cycle analysis established that the cerebellar granule cells did not proliferate in culture, indicating that alcohol exposure did not reduce cell numbers by interfering with cell proliferation in this system. Instead, alcohol-induced killing of the granule cells was the most likely mechanism to account for the depletion of granule cells in vitro. Granule cell cultures are a useful in vitro model system to study the cellular and molecular aspects of neuronal cell depletion associated with fetal alcohol exposure. The potential role of the N-methyl-D-aspartate receptor in this alcohol-induced neuronal cell death is discussed.

Taurine release evoked by NMDA receptor activation is largely dependent on calcium mobilization from intracellular stores

It is known that the activation of N-methyl-D-aspartate (NMDA) receptors leads to an increase in extracellular taurine concentration in different brain regions. The mechanism that mediates this effect is not totally understood. In this study, rat hippocampal slices were used to determine the dependence of NMDA-induced taurine release on extracellular calcium and/or on calcium mobilization from intracellular stores. NMDA was administered through a microdialysis probe inserted into the slice, at the level of CA1 stratum radiatum, which was also used to collect amino acids from the extracellular space. Field potentials evoked by stimulation of the Schaffer collaterals and recorded in the stratum pyramidale of CA1 were used as a control of NMDA receptor activation. NMDA induced a marked increase in extracellular taurine levels and a decrease in field potential amplitude, and both effects were suppressed in the presence of MK-801, a blocker of the NMDA receptor-linked channel. Dantrolene, an inhibitor of calcium release from intracellular stores, partially inhibited the extracellular taurine increase, while 2-nitro-4-carboxyphenyl-N,N-diphenyl carbamate (NCDC), an inhibitor of phosphatidylinositol-specific phospholipase C activation, had no effect. Removal of extracellular calcium diminished, but did not abolish, the extracellular taurine increase caused by NMDA. The remaining taurine response was totally suppressed by dantrolene, and also by NCDC. These results demonstrate that the release of taurine induced by NMDA receptor activation is triggered by the increase in cytoplasmic calcium concentration. We suggest that, under physiological conditions, calcium influx provides the signal for NMDA-induced taurine release, which is amplified by calcium-dependent calcium mobilization from intracellular stores.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotrophic effects of NMDA receptor activation on developing cerebellar granule cells

Glutamate acting on N-methyl-D-aspartate (NMDA) receptors controls a variety of aspects of neuronal plasticity in the adult and developing brain. This review summarizes its effects on developing cerebellar granule cells. The glutamatergic mossy fibre input to cerebellar granule cells exerts a neurotrophic effect on these cells during development. The investigation of potential neurotrophic agents can be carried out using enriched granule cell cultures. Considerable evidence now indicates that glutamate acting on N-methyl-D-aspartate receptors is an important neurotrophic factor that regulates granule cell development. In culture, neurite growth, differentiation and cell survival are all stimulated by N-methyl-D-aspartate receptor activation. The intracellular pathways involved following Ca2+ entry through the N-methyl-D-aspartate receptor channel are beginning to be elucidated. The cerebellar granule cell culture system may provide an ideal model to investigate the molecular mechanisms involved in long term N-methyl-D-aspartate receptor-mediated changes in neuronal function.

Phosphoproteins of cultured cerebellar granule cells and response to the differentiation-promoting stimuli NMDA, high K+ and ionomycin

In order to investigate signalling pathways involved in the control of granule cell differentiation, survival and other functions by depolarization or activation of NMDA receptors we have characterized protein phosphorylation in cerebellar granule cells. Cultures of cerebellar granule cells were incubated with 32P orthophosphate and then challenged with NMDA, K+ or the Ca2+ ionophore ionomycin, agents which raise [Ca2+]i and stimulate differentiation and survival. Upon separation of labelled phosphoproteins by two-dimensional gel electrophoresis three differences were found in response to all of these agents. These were an increase in acidity of two phosphoproteins of 87 and 48 kDa (p87 and p48) and increased 32P-incorporation into a phosphoprotein of 120 kDa (p120). Treatment with PMA which stimulates neurite outgrowth but not survival affected p87 (increased its acidity) but not p48. The acidic shift of p87, therefore, is not sufficient to stimulate granule cell survival. The identification of p87 as the actin-binding MARCKS protein and the demonstration of its presence in neurites and growth cones of granule cells suggests that it may be involved in NMDA-stimulated neurite outgrowth. The phosphoproteins p120 and p48 may potentially be involved in events linking the rise in [Ca2+]i to increased granule cell survival or other aspects of granule cell differentiation.

Modulation of neuronal migration by NMDA receptors

The N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor is essential for neuronal differentiation and establishment or elimination of synapses in a developing brain. The activity of the NMDA receptor has now been shown to also regulate the migration of granule cells in slice preparations of the developing mouse cerebellum. First, blockade of NMDA receptors by specific antagonists resulted in the curtailment of cell migration. Second, enhancement of NMDA receptor activity by the removal of magnesium or by the application of glycine increased the rate of cell movement. Third, increase of endogenous extracellular glutamate by inhibition of its uptake accelerated the rate of cell migration. These results suggest that NMDA receptors may play an early role in the regulation of calcium-dependent cell migration before neurons reach their targets and form synaptic contacts.

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.

N-methyl-D-aspartate responses in rat cerebellar granule cells are modified by chronic depolarisation in culture

Following culture in high (25 mM) K+ conditions cerebellar granule cells only respond with a rise in cytosolic free calcium concentration ([Ca2+]i after removal of external Mg2+. When granule cells are grown in low (5 mM) K+ N-methyl-D-aspartate (NMDA) exerts a neurotrophic effect. We show that at the critical time for this effect NMDA will elicit a rise in [Ca2+]i in 5 mM K+ cultures even in the presence of Mg2+ and that growth in 25 mM K+ induces the rapid appearance of a Mg2+ block of NMDA receptors in granule cells. This suggests firstly, that a rise in [Ca2+]i could be involved in the neurotrophic effect of NMDA and secondly, that the characteristics of the NMDA responses in granule cells are modified as a result of growth under depolarising conditions.

N-methyl-D-aspartate receptor-mediated neuroprotection in cerebellar granule cells requires new RNA and protein synthesis

Cerebellar granule cells are susceptible to the excitotoxin glutamate, which acts at N-methyl-D-aspartate (NMDA) receptors, as well as the neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+), the active cytotoxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Paradoxically, preincubation of cultured cerebellar granule cells with low concentrations of NMDA or glutamate markedly antagonizes the neurotoxicity resulting from subsequent exposure to toxic concentrations of either MPP+ or glutamate. The neuroprotective effects of NMDA and glutamate against MPP+ toxicity are observed at agonist concentrations as low as 1 microM, are blocked by specific NMDA receptor antagonists, and require at least 30 min to develop fully. Moreover, NMDA receptor-mediated neuroprotection is prevented by the RNA synthesis inhibitor actinomycin D or the protein synthesis inhibitor cycloheximide. Thus, in cerebellar granule cells activation of NMDA receptors by glutamate can result in either neurotoxicity or neuroprotection, depending on the apparent degree of receptor stimulation. NMDA receptor-mediated neuroprotection requires new RNA and protein synthesis and therefore appears to be mediated by the expression of a neuroprotective protein(s). These data demonstrate the presence of an active NMDA receptor-mediated and transcriptionally directed neuroprotective mechanism in cerebellar granule cells.

Epidermal growth factor selectively enhances NMDA receptor-mediated increase of intracellular Ca2+ concentration in rat hippocampal neurons

We have previously reported that recombinant human epidermal growth factor (hEGF) facilitates induction of hippocampal long-term potentiation (LTP). In order to clarify the mechanism underlying the LTP-facilitating effect of hEGF, the influence of hEGF on intracellular Ca2+ concentration ([Ca2+]i) of hippocampal neurons was investigated using dissociated cell cultures. Changes in [Ca2+]i were measured by microfluorometrically monitoring the fluorescence intensities from individual neurons loaded with fura-2. Application of hEGF (0.6-20 ng/ml) alone did not affect the basal level of [Ca2+]i in cultured hippocampal neurons, but significantly enhanced the [Ca2+]i increase induced by L-glutamate (3 x 10(-6) M). The N-methyl-D-aspartate (NMDA) (10(-5) and 3 x 10(-5) M)-induced [Ca2+]i increase was also enhanced by hEGF, but the quisqualate (10(-7) and 3 x 10(-7) M)-induced response was not affected by the presence of hEGF. These results suggest that hEGF selectively enhances the NMDA receptor-mediated responses in hippocampal neurons. This action of hEGF may underlie the facilitation of hippocampal LTP.

N-methyl-D-aspartate effects on the growth, morphology and cytoskeleton of individual neurons in vitro

Short-term (up to 5 h post-plating) cerebellar granule cell cultures were prepared from the week-old rat and maintained in a micro-incubator during time lapse video microscopy to examine normal and N-methyl-D-aspartate (NMDA)-evoked neurite extension. In untreated cultures growth of neurites was stochastic but proceeded at an average rate of 12.0 +/- 1.4 microns/h. Growth cone morphology was variable. The classical filopodia and lamellipodia possessing tips were motile or non-motile, while those processes ending in a club shape were rarely seen to extend. Individual growth cones passed through several different morphologies during growth. New processes extended more rapidly (13.0 +/- 1.7 microns/h) than those already present (9.0 +/- 0.5 microns/h). Addition of the NMDA receptor antagonist, aminophosphonovalerate (APV), caused a marked retraction of pre-existing processes. Stimulation of the receptor with 50 microM NMDA caused a marked increase in growth rate compared to controls (15.0 microns/h and 1.7 microns/h, respectively). When the presence of actin-rich structures was examined using rhodamine-phalloidin labelling it was found that NMDA increased the proportion of neuronal processes that possessed a growth cone by 28%. Conversely, inhibition of NMDA receptor activity with APV reduced the formation of lamellipodia from neuronal cell bodies.

Regulation of intracellular free calcium in normal and dystrophic mouse cerebellar neurons

We measured free intracellular calcium ([Ca2+]i) in cultured cerebellar granule cells from normal and mdx mice. Resting levels of ([Ca2+]i) were 24% higher in the dystrophic neurons (normal: 61.2 +/- 1.5 nM calcium, n = 104; dystrophic: 76.1 +/- 2.4 nM calcium, n = 136, P less than 0.01). Dystrophic neurons showed a significantly greater increase in ([Ca2+]i) in the presence of elevated (18 mM) extracellular calcium levels. Resting sodium levels ([Na+]i), however, were found to be similar in normal and dystrophic granule neurons. In addition, sodium influx rates after ouabain inhibition of the Na+/K+ ATPase were also identical. Therefore, the increased permeability of granule neurons was specific to calcium, and did not result from a non-selective cation-permeable conductance. Unlike granule cells, astrocytes do not express dystrophin. Glial cells from normal and dystrophic mice showed no difference in their resting free calcium levels or their response to a high calcium load. Thus, cerebellar granule neurons from mdx mice show a calcium-specific regulatory defect similar to that found in dystrophic muscle fibers, while cerebellar glial cells, which do not normally express dystrophin, have no such defect.

Intracellular Ca2+ and N-methyl-D-aspartate-stimulated neuritogenesis in rat cerebellar granule cell cultures

Week-old rat cerebellar granule cells were grown in the presence of the cell-permeable calcium chelating agent BAPTA-acetoxy methyl ester (BAPTA-AM) for the first 8 h in vitro. There was a dose-dependent inhibition of process outgrowth with an IC50 of approximately 5 microM. Neurite outgrowth could be partially recovered by the addition of N-methyl-D-aspartate (NMDA; 50 microM) to BAPTA-AM-treated cells. Phorbol ester stimulation of treated cells evoked a profound inhibition of neuritogenesis compared to a stimulatory effect on control cultures. The inhibition of growth caused by phorbol esters could not be reversed by NMDA co-addition. Neurites extended by BAPTA-AM-treated granule cells were thinner than in control cultures and did not form elaborate growth cones even when growth was stimulated by NMDA. The distribution of tyrosinated and acetylated alpha-tubulin in the processes of BAPTA-AM-treated cells appeared similar to that in controls. However, rhodamine-phalloidin labelling of microfilaments in the cell cultures emphasised the loss of an elaborate actin-rich growth cone in BAPTA-AM-treated cells even when neurite formation was partially recovered. These results indicate the importance of [Ca2+]i in the production of neurites from cerebellar granule cells in vitro.

Nitric oxide does not mediate the neurotrophic effects of excitatory amino acids in cultured cerebellar granule neurons

Excitatory amino acids, such as N-methyl-D-aspartate (NMDA) and kainate, promote neuritogenesis and viability in primary cultures of cerebellar granule cells. In view of the recent demonstration that excitatory amino acids activate the synthesis of nitric oxide, the present study examined a potential role of nitric oxide in mediating the neurotrophic effects of excitatory amino acids. NMDA enhanced the viability of 8-day-old cerebellar granule cell cultures in a concentration-dependent fashion, whereas kainate showed a concentration-dependent biphasic effect. A specific inhibitor of nitric oxide synthase, NG-nitroarginine (0.5 mM), did not antagonize the neurotrophic effects of NMDA (0.5 mM) or kainate (0.05 mM). The concentration of NG-nitroarginine was sufficient to inhibit NMDA or kainate stimulated nitric oxide synthesis and was stable in the culture media throughout the 8-day culture period. Using a specific chemiluminescence detection method, endogenous nitric oxide was directly measured in the headspace gas phase of homogenized cultured cerebellar granule cells, and was not detected when homogenates were incubated with NG-nitroarginine (0.5 mM). Furthermore, addition of a nitric oxide precursor, S-nitroso-N-acetylpenicillamine (1-200 microM), was not neurotrophic, but rather, was neurotoxic in granule cells. These findings indicate that nitric oxide is not a neurotrophic factor in primary cultures of cerebellar granule cells.

NMDA-induced increase in [Ca2+]i and45Ca2+ uptake in acutely dissociated brain cells derived from adult rats

A preparation of acutely dissociated brain cells derived from adult (3-month-old) rat has been developed under conditions preserving the metabolic integrity of the cells and the function of N-methyl-D-aspartate (NMDA) receptors. The effects of glutamate and NMDA on [Ca2+]i measured with fluo3 and 45Ca2+ uptake have been studied on preparations derived from hippocampus and cerebral cortex. Glutamate (100 microM) and N-methyl-DL-aspartate (200 microM) increased [Ca2+]i by 26-12 nM and 23-9 nM after 90 s in cerebral cortex and hippocampus, and stimulated 45Ca2+ uptake about 16-10% in the same regions. The increases in [Ca2+]i and 45Ca2+ uptake were inhibited by 40% in the presence of 1 mM MgCl2 and by 90-50% in the presence of MK-801. The results indicate (a) that a large fraction of the [Ca2+]i response to glutamate in freshly dissociated brain cells from the adult rat involves NMDA receptors, (b) when compared with results in newborn rats, there is a substantial blunting of the [Ca2+]i increase in adult age.

N-methyl-D-aspartate stimulation of the survival of rat cerebellar granule cells in culture is not dependent upon increased c-fos expression and is not mimicked by protein kinase C activation

The role of c-fos expression and protein kinase C in the survival of cultures of rat cerebellar granule cells was investigated. Results from immunocytochemistry and immunoblotting suggest that increased c-fos expression is not essential for the survival of cells grown in low K+ media in the presence of N-methyl-D-aspartate (NMDA) at the critical time point when sensitivity to survival requirements develops. In addition the phorbol ester, phorbol 12-myristate 13-acetate failed to bring about survival of cells cultured in low K+ media in the absence of NMDA when given chronically, suggesting that protein kinase C activation alone is not sufficient to maintain granule cell survival in culture.

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.

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

The concentration of intracellular free Ca2+ ([Ca2+]i) was measured in rat cerebellar granule cells using the fluorescent indicator fura-2. Culturing the cells as monolayers on plastic squares which could be placed into cuvettes allowed measurements of [Ca2+]i to be performed on large and homogeneous populations of CNS neurons. Granule cells so cultured maintained low levels of [Ca2+]i (around 90 nM) which increased promptly upon the addition of various excitatory amino acids including N-methyl-D-aspartate (NMDA). Increases in [Ca2+]i elicited by NMDA were inhibited by Mg2+ (1 mM) and often potentiated by glycine (1 microM). The addition of TTX or strychnine (5 microM each) did not alter responses to NMDA or NMDA plus glycine. Cytosolic Ca2+ responses to NMDA/glycine were dependent on the presence of extracellular Ca2+ and were unaffected by concentrations of nifedipine or verapamil that blocked increases in [Ca2+]i elicited by K+ depolarization. Responses elicited by NMDA/glycine were inhibited competitively by 2-amino-5-phosphonovalerate or 3-((+-)-2-carboxypiperazin-4-yl)-propyl-1- phosphonic acid and non-competitively by MK-801 or Mg2+. HA-966 and 7-chlorokynurenate inhibited responses to NMDA alone and blocked competitively the potentiating effects of glycine. The results demonstrate NMDA-mediated increases in [Ca2+]i in cerebellar granule cells that arise solely from influx of extracellular Ca2+ through dihydropyridine-insensitive channels. The strict dependence of the NMDA-evoked response on extracellular Ca2+ provides little evidence for a coupling of NMDA receptors to inositol phosphate metabolism and mobilization of intracellular Ca2+. The effect of various agents on NMDA/glycine-induced increases in [Ca2+]i parallels their effects on ligand binding to or current flow through the NMDA receptor-channel complex. The measurement of cytosolic Ca2+ in this preparation of neuronal cells thus appears especially well suited for assessing, on a functional level, the regulation of NMDA receptors in the CNS.

Excitatory amino acid receptors in normal and abnormal vestibular function

Although excitatory amino acid (EAA) receptors have been investigated extensively in the limbic system and neocortex, less is known of the function of EAA receptors in the brainstem. A number of biochemical and electrophysiological studies suggest that the synapse between the ipsilateral vestibular (VIIIth) nerve and the brainstem vestibular nucleus (VN) is mediated by an EAA acting predominantly on kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. In addition, there is electrophysiological evidence that input from the contralateral vestibular nerve via the contralateral VN is partially mediated by N-methyl-D-aspartate (NMDA) receptors. Input to the VN from the spinal cord may also be partially mediated by NMDA receptors. All of the electrophysiological studies conducted so far have used in vitro preparations, and it is possible that denervation of the VN during the preparation of an explant or slice causes changes in EAA receptor function. Nonetheless, these results suggest that EAA receptors may be important in many different parts of the vestibular reflex pathways. Studies of the peripheral vestibular system have also shown that EAAs are involved in transmission between the receptor hair cells and the vestibular nerve fibers. A number of recent studies in the area of vestibular plasticity have reported that antagonists for the NMDA receptor subtype disrupt the behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (vestibular compensation). It has been suggested that vestibular compensation may be owing to an upregulation or increased affinity of NMDA receptors in the VN ipsilateral to the peripheral deafferentation; however; at present, there is no clear evidence to support this hypothesis.

Characterization of Ca2(+)-mobilizing excitatory amino acid receptors in cultured chick cortical cells

The effects of glutamate and other more selective excitatory amino acid (EAA) analogs on intracellular free calcium concentration ( [Ca2+]i) were examined in Fura 2-loaded cultured chick embryo cortical cells (90% neuronal). Four EAA receptors were evident in these studies: an N-methyl-D-aspartate (NMDA) receptor, a kainate receptor, and two quisqualate receptors. The [Ca2+]i response to NMDA was blocked or reversed by selective antagonists such as 2-amino-5-phosphonovalerate (APV), MK801 and ketamine, as well as by desmethylimipramine and dextromethorphan. Glycine potentiated the [Ca2+]i response to NMDA, and high concentrations of glycine selectively overcame blockade by kynurenic acid, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and cis-piperidine-2,3-dicarboxylic acid (PDA). The [(Ca2+]i response to kainate was generally larger than the NMDA response, and the kainate response desensitized slightly over the first minute. CNQX was more potent as an antagonist of the kainate response than of the NMDA response, even in the absence of added glycine; kynurenic acid and PDA conversely had little effect on the kainate response in these cells at concentrations which blocked the NMDA response. The desensitization of the [Ca2+]i response to kainate was greatly augmented by quisqualate and by the putative ionotropic quisqualate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). In the absence of kainate, both quisqualate and AMPA increased [Ca2+]i though less so than did NMDA or kainate. Quisqualate (and AMPA and glutamate) were not acting as partial agonists at the kainate receptor, since the potency of these agonists in reversing the kainate [Ca2+]i response was independent of kainate concentration. Quisqualate, but not AMPA, also produced a small increase in [Ca2+]i which preceded the negative effect of this agonist on the kainate response. This increase in [Ca2+]i could also be evoked by quisqualate or glutamate after inhibition of the kainate response by AMPA. Quisqualate and glutamate, but not the other EAA agonists, also increased [Ca2+]i after chelation of extracellular calcium with EGTA. This effect appears to be mediated by the metabotropic quisqualate receptor. These cells should provide a useful system for studying regulation and interactions of EAA receptors, and for screening drugs which might act at these receptors.

The expression of excitatory amino acid binding sites during neuritogenesis in the developing rat cerebellum

The present study has examined excitatory amino acid transmitter binding sites as measured autoradiographically in cryostat sections prepared from developing rat cerebella during the period of granule cell neuritogenesis. The external germinal layer (EGL) and molecular layer (ML), which during development contain granule cells at early stages of axon growth, contained only low levels of NMDA-displaceable L-[3H]glutamate binding sites. Similarly, [3H]glycine binding to the NMDA receptor linked binding site was not enriched in the EGL. Radioligand binding to the NMDA receptor was always greater in the granular layer (GL) than in the ML. The developmental increases in NMDA-displaceable L-[3H]glutamate and in [3H]glycine binding to the GL were similar but NMDA displaceable L-[3H]glutamate binding density increased before [3H]glycine binding sites. Glycine increased NMDA-displaceable L-[3H]glutamate binding only in the adult cerebellum. These results suggest that NMDA stimulation of neuritogenesis in granule cell cultures may reflect stimulation of dendritogenesis in the developing glomerulus rather than a stimulation of axon growth in the EGL. Also, NMDA receptors may be present in an immature form during cerebellar development and have different properties to the adult receptor. Binding sites for [3H]kainate and [3H]AMPA were present in both the GL and ML and increased during development. At all times the amount of binding sites for [3H]kainate were highest in the GL whereas those for [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate were highest in the ML.

Neuritogenesis in cerebellar granule cells in vitro: a role for protein kinase C

We have used short-term (8 h) cultures of week-old rat cerebellar granule cells to examine the effects on neuritogenesis of activation and down-regulation of protein kinase C by phorbol esters. We have previously demonstrated that endogenously released glutamate promoted neurite outgrowth in the same system acting via N-methyl-D-aspartate receptors. Low levels (0.1-1 nM) of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) evoked increases in the number of granule cells which extended neurites; higher levels (10-250 nM) which caused a down-regulation of total protein kinase C, inhibited outgrowth in a dose-dependent manner. N-Methyl-D-aspartate by itself also stimulated process outgrowth but could not reverse the inhibition evoked by either TPA or the protein kinase C inhibitor sphingosine. Stimulation of protein kinase C with 0.1 nM TPA resulted in a general increase in the incorporation of 32P-labelled inorganic orthophosphate into granule cell polypeptides. The results indicate that the activation of protein kinase C is involved in neuritogenesis in granule cells and are consistent with the idea that N-methyl-D-aspartate receptor activation may exert its effect on neuritogenesis through protein kinase C.

Localized calcium influx orients axon formation in embryonic hippocampal pyramidal neurons

A fundamental property of neurons is their polarization into distinct axonal and dendritic compartments which have characteristic structural and functional properties. The mechanisms regulating the formation of neuronal polarity are unknown. We used cultured embryonic rat hippocampal pyramidal neurons to test the hypothesis that a localized calcium influx can orient axon formation, and thereby direct the establishment of neuronal polarity. Transection of an initial axon, or focal application of A23187 or K+ to the initial axon, caused a new axon to form at a site distant from the initial axon. Fura-2 measurements of intracellular calcium revealed a localized calcium influx at the site of axon transection or focal application of A23187 or K+, and a calcium gradient spreading into the soma. New axon formation was inhibited when axons were transected in medium lacking calcium or containing calcium-elevating agents (conditions which prevented the formation of a calcium gradient). When calcium ionophore A23187 was applied focally to neurons which had not yet established an axon, the axon always formed at a site distant from the site of ionophore application; bath exposure to A23187 prevented axon formation. Taken together, these data demonstrate that a localized influx of calcium can suppress axon formation at the site of influx, and can thereby influence where the axon forms. These data suggest that gradients of intracellular calcium may be involved in orienting neuronal polarity.

Apparent desensitization of NMDA responses in Xenopus oocytes involves calcium-dependent chloride current

N-Methyl-D-aspartate (NMDA) receptors were expressed and studied in Xenopus oocytes injected with rat brain RNA. NMDA application elicits a rapid inward current that decays in several seconds to a relatively stable level. This decay is reportedly due to desensitization. However, we found the early transient component could be evoked more than once during a single application of NMDA, suggesting that the receptor did not actually desensitize. Removal of external Ca2+, replacement of Ca2+ with Ba2+, or intracellular injection of EGTA abolished the transient component. Furthermore, a variety of Cl- channel blockers nearly eliminated the transient component and inhibited the plateau current as well. We propose that a significant portion of the NMDA current recorded in oocytes is carried by a transient inward Cl- current triggered by Ca2+ influx through the NMDA receptor/channel.

Cell-type specific effects of N-methyl-D-aspartate on biochemical differentiation of subcortical neurons in culture

The possible involvement of N-methyl-D-aspartate (NMDA) receptors in the biochemical differentiation of cultured neurons derived from the medial frontal part of the forebrain containing the septum-diagonal band region was studied in terms of the activities of enzymes important in the synthesis of neurotransmitter compounds. The activity of choline acetyltransferase (ChAT) was used as a marker for cholinergic neurons, glutamate decarboxylase (GAD) for GABAergic neurons and phosphate-activated glutaminase (GLNase) and aspartate aminotransferase (ASP-AT) for glutamatergic neurons, while lactate dehydrogenase (LDH) was included as an ubiquitous enzyme. The exposure of cultures to a depolarizing concentration of K+ (40 mM) for the last 3 days (i.e. between 2 and 5 days in vitro) significantly enhanced the expression of ChAT, GAD and GLNase activities, but high K+ caused little alteration in the activities of ASP-AT and LDH. On the other hand, treatment with NMDA markedly elevated the specific activities of GAD and GLNase only, and the compound had no significant effects on the activities of ChAT, ASP-AT and LDH enzymes. The enhancements of the specific activities of GAD and GLNase were completely blocked by the NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid, and by the NMDA receptor-linked Ca2+ ion channel blocker, MK-801. On the basis of the present findings it is concluded that, (a) contrary to an earlier proposal, ASP-AT does not appear to be a good marker for the glutamatergic neurons, (b) the failure of the subcortical cholinergic neurons to respond by an increase in ChAT activity to NMDA may indicate that these nerve cells lack NMDA subtype excitatory amino acid receptors, and (c) as the septal GABAergic input in the hippocampus is involved in the modulation of long-term potentiation, the presence of NMDA receptors on these neurons would now suggest that NMDA receptors are linked to both the initiation and the modulation of hippocampal plasticity in the mammalian brain.

Selective stimulation of excitatory amino acid receptor subtypes and the survival of cerebellar granule cells in culture: effect of kainic acid

Our previous studies showed that the survival of cerebellar granule cells in culture is promoted by treatment with N-methyl-D-aspartate. Here we report on the influence of another glutamate analogue, kainic acid, which, in contrast to N-methyl-D-aspartate, is believed to stimulate transmitter receptors mediating fast excitatory postsynaptic potentials. The kainate effect was complex: increased survival at low concentrations (the maximum, at 25-50 microM, was about 50% promotion), whereas concentrations exceeding 50 microM resulted first in a loss of the effect, and then at concentrations of 2-5 x 10(-4) M cells became vulnerable to kainate. The trophic influence of kainate is mediated through receptors other than the N-methyl-D-aspartate preferring subtype. In contrast to the effect of N-methyl-D-aspartate, that of kainate did not depend on the medium K+ level and was potently blocked by dinitroquinoxalinedione, which--at the concentration used here--did not counteract the promotion of cell survival evoked by N-methyl-D-aspartate. Quisqualate was a potent inhibitor of the rescue by kainate. Furthermore, blockade of N-methyl-D-aspartate receptors with the selective antagonists MK-801 or aminophosphonovalerate did not inhibit, but rather potentiated the trophic effect of kainate. Possible mechanisms underlying the trophic effect of chronic depolarization or treatment with excitatory amino acids are discussed, and it is proposed that they involve elevated free cytoplasmic calcium activity following increased influx through voltage-sensitive Ca2+ channels (high K+ and kainate) or receptorgated channels (N-methyl-D-aspartate).

Functional expression of a subtype of the Ca2+ channels in the embryonic rat hippocampus as detected by dual-fluorescence flow cytometric analysis

We have analyzed the expression of the Ca2+ channels in hippocampal cell suspensions from the 18- to 20-day-old rat embryo using dual-fluorescence flow cytometry. A high concentration of K+ induced elevation of the cytoplasmic free Ca2+ concentration ([Ca2+]i) as well as membrane depolarization. The high K+-evoked [Ca2+]i increase was inhibited by phenytoin, but not by either nifedipine or nicardipine. These agents had no effect on the high K+-induced membrane depolarization. These findings suggest that a subtype corresponding to the low voltage-activated Ca2+ channel is expressed in the embryonic rat hippocampal cells.

2-chloroadenosine attenuates kainic acid-induced toxicity within the rat straitum: relationship to release of glutamate and Ca2+ influx

1. The mechanism by which 2-chloroadenosine (2-chloroado) exerts a neuroprotective action against the excitotoxic effect of kainic acid (KA) when injected into the rat striatum was investigated. 2. Histological examination two weeks after a single injection of KA (2.2 nmol) into rat striatum revealed widespread neuronal damage. Co-injection of 2-chloroado (6-25 nmol) with the neurotoxin afforded dose-dependent neuroprotection. This effect was reversed by administration of an equimolar concentration of the adenosine receptor antagonist theophylline. 3. Both K+ (30 mM) and KA (1 mM) enhanced the release of endogenous glutamate from guinea-pig purified cerebrocortical synaptosomes in a predominantly (approximately 70%) Ca2+-dependent manner. 2-Chloroado (10 nM-1 microM) inhibited the release of glutamate evoked by both KA and K+. These effects were partially reversed by the selective A1-adenosine receptor antagonist 8-cyclopentyltheophylline (CPT) (1 microM). 4. Crude rat cortical synaptosomes were loaded with the fluorescent calcium indicator quin-2 and Ca2+ influx monitored following two successive depolarising stimuli (30 mM K+; 'S1' and 'S2'). 2-Chloroado (10 nM-1 microM) produced a dose-dependent reduction in the S2:S1 ratio when added before the S2 period of stimulation. This effect was reversed by 1 microM theophylline. However, KA (1 mM) failed to enhance Ca2+ influx in the same preparation. 5. These results suggest that the anti-excitotoxic action of 2-chloroado is mediated primarily through a specific presynaptic receptor mechanism involving reduction of transmitter glutamate release, possibly occurring through an inhibition of Ca2+ influx.