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

NMDA receptors inhibit axonal outgrowth by inactivating Akt and activating GSK-3β via calcineurin in cultured immature hippocampal neurons

Neurons are highly polarized cells with an axon and dendritic arbors. It is still not well studied that how formation and elaboration of axon and dendrites is controlled by diffusible signaling factors such as glutamate via specific receptors. We found that N-methyl-D-aspartate (NMDA) receptors were enriched (stage 2-3) but decreased expression (stage 4-5) at tip of axon of cultured hippocampal neurons during distinct development stages. Inhibition of NMDA receptor activity by competitive antagonist DL-2-amino-5-phosphonovalerate (APV) or channel blocker MK801 promoted axonal outgrowth at the early stages, whereas inhibited dendritic development in later stages. Meanwhile, knockdown of NMDA receptors also promoted axonal outgrowth and branch in immature neurons. Furthermore, GluN2B but not GluN2A subunit inhibited axonal outgrowth in immature hippocampal neurons. Finally, we found that NMDA receptors inhibited axonal outgrowth by inactivating Akt and activating GSK-3β signaling in a calcineurin-dependent manner. Taken together, our results demonstrate that stabilization GSK-3β activation in the axon growth cone by Ca²⁺ influx through NMDA receptors may be involved in regulation of axon formation in immature neurons at early stages.

The effects of verapamil and its combinations with glutamate and glycine on cardiodynamics, coronary flow and oxidative stress in isolated rat heart

The role of N-methyl-D-aspartate receptor (NMDA-R) in heart is still unclear. For these ionotropic glutamate receptors is characteristic the necessity of both co-agonists, glutamate and glycine, for their activation, which primarily allows influx of calcium. The aim of the present study was to examine the effects of verapamil, as a calcium channel blocker, alone and its combination with glycine and/or glutamate on cardiac function, coronary flow, and oxidative stress in isolated rat heart or to examine the effects of potential activation of NMDA-R in isolated rat heart. The hearts of male Wistar albino rats were excised and perfused according to Langendorff technique, and cardiodynamic parameters and coronary flow were determined during the administration of verapamil and its combinations with glutamate and/or glycine. The oxidative stress biomarkers, including thiobarbituric acid-reactive substances, nitrites, superoxide anion radical, and hydrogen peroxide, were each determined spectrophotometrically from coronary venous effluent. The greatest decline in parameters of cardiac contractility and systolic pressure was in the group that was treated with verapamil only, while minimal changes were observed in group treated with all three tested substances. Also, the largest changes in coronary flow were in the group treated only with verapamil, and at least in the group that received all three tested substances, as well as the largest increase in oxidative stress parameters. Based on the obtained results, it can be concluded that NMDA-R activation allows sufficient influx of calcium to increase myocardial contractility and systolic pressure, as well as short-term increase of oxidative stress.

Sulforaphane Prevents Methylmercury-Induced Oxidative Damage and Excitotoxicity Through Activation of the Nrf2-ARE Pathway

Methylmercury (MeHg) is a prominent environmental neurotoxicant, which induces oxidative damage and an indirect excitotoxicity caused by altered glutamate (Glu) metabolism. However, the interaction between oxidative damage and excitotoxicity in MeHg-exposed rats has not been fully recognized. Here, we explored the interaction between oxidative damage and excitotoxicity and evaluated the preventive effects of sulforaphane (SFN) on MeHg-induced neurotoxicity in rat cerebral cortex. Seventy-two rats were randomly assigned to four groups: control group, MeHg-treated groups (4 and 12 μmol/kg), and SFN pretreatment group. After treatment (28 days), the rats were killed and the cerebral cortex was analyzed. Then, Hg, glutathione (GSH), malondialdehyde (MDA), protein sulfhydryl, protein carbonyl, 8-hydroxy-2-deoxyguanosine (8-OHdG), and the levels of reactive oxygen species (ROS) and apoptosis were examined. Glu and glutamine (Gln) levels, glutamine synthetase (GS), phosphate-activated glutaminase (PAG), superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), Na⁺-K⁺-ATPase and Ca²⁺-ATPase activities, intracellular Ca²⁺ levels, and the mRNA and protein expressions of Nrf2, Nrf2-regulated gene products, and N-methyl-D-aspartate receptors (NMDARs) were investigated in rat cerebral cortex. In our study, MeHg exposure not only induced Hg accumulation, apoptosis, ROS formation, GSH depletion, inhibition of antioxidant enzyme activities, and activation of Nrf2-ARE pathway signaling but also caused lipid, protein, and DNA peroxidative damage in a dose-dependent manner in rat cerebral cortex. Moreover, MeHg treatment significantly altered Gln/Glu cycling and NMDAR expression and resulted in calcium overloading. Furthermore, the present study also indicated that SFN pretreatment significantly reinforced the activation of the Nrf2-ARE pathway, which could prevent the toxic effects of MeHg exposure. Collectively, MeHg initiates multiple additive or synergistic disruptive mechanisms that lead to oxidative damage and excitotoxicity in rat cerebral cortex; pretreatment with SFN might prevent the MeHg-induced neurotoxicity by reinforcing the activation of the Nrf2-ARE pathway and then downregulating the interaction between oxidative damage and excitotoxicity pathways.

Preventive effects of dextromethorphan on methylmercury-induced glutamate dyshomeostasis and oxidative damage in rat cerebral cortex

Methylmercury (MeHg) is a well-known environmental pollutant leading to neurotoxicant associated with aberrant central nervous system (CNS) functions, but its toxic mechanisms have not yet been fully recognized. In the present study, we tested the hypothesis that MeHg induces neuronal injury via glutamate (Glu) dyshomeostasis and oxidative damage mechanisms and that these effects are attenuated by dextromethorphan (DM), a low-affinity and noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonist. Seventy-two rats were randomly divided into four groups of 18 animals in each group: control group, MeHg-treated group (4 and 12 μmol/kg), and DM-pretreated group. After the 4-week treatment, we observed that the administration of MeHg at a dose of 12 μmol/kg significantly increased in total mercury (Hg) levels, disrupted Glu metabolism, overexcited NMDARs, and led to intracellular calcium overload in the cerebral cortex. We also found that MeHg reduced nonenzymatic and enzymatic antioxidants, enhanced neurocyte apoptosis, induced reactive oxygen species (ROS), and caused lipid, protein, and DNA peroxidative damage in the cerebral cortex. Moreover, glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) appeared to be inhibited by MeHg exposure. These alterations were significantly prevented by the pretreatment with DM at a dose of 13.5 μmol/kg. In conclusion, these findings strongly implicate that DM has potential to protect the brain from Glu dyshomeostasis and oxidative damage resulting from MeHg-induced neurotoxicity in rat.

Possible protection by notoginsenoside R1 against glutamate neurotoxicity mediated by N-methyl-D-aspartate receptors composed of an NR1/NR2B subunit assembly

Notoginsenoside R1 (NTR1) is the main active ingredient in Panax notoginseng, a herbal medicine widely used in Asia for years. The purpose of this study was to investigate pharmacological properties of NTR1 on neurotoxicity of glutamate (Glu) in primary cultured mouse cortical neurons along with its possible mechanism of action. We found that NTR1 significantly protected neurons from the loss of cellular viability caused by brief exposure to 10 microM Glu for 1 hr in a dose-dependent manner at concentrations from 0.1 to 10 microM, without affecting the viability alone. NTR1 significantly inhibited the increased number of cells positive to propidium iodide (PI) staining, increase of intracellular free Ca(2+) ions, overproduction of intracellular reactive oxygen species, and depolarization of mitochondrial membrane potential in cultured neurons exposed to Glu, in addition to blocking decreased Bcl-2 and increased Bax expression levels. We further evaluated the target site at which NTR1 protects neurons from Glu toxicity by using the acquired expression strategy of N-methyl-D-aspartate (NMDA) receptor subunits in human embryonic kidney 293 cells. We found that 10 microM NTR1 protected NR1/NR2B subunit expressing cells from cell death by 100 microM NMDA, but not cells expressing NR1/NR2A subunits, when determined by PI staining. These results suggest that NTR1 may preferentially protect neurons from Glu excitotoxicity mediated by NMDA receptor composed of an NR1/NR2B subunit assembly in the brain.

Elevated glutamate and NMDA disrupt production of the second messenger cyclic GMP in the early postnatal mouse cortex

The second messenger cyclic guanosine monophosphate (cGMP) plays many roles during nervous system development. Consequently, cGMP production shows complex patterns of regulation throughout early development. Elevated glutamate levels are known to increase cGMP levels in the mature nervous system. A number of clinical conditions including ischemia and perinatal asphyxia can result in elevated glutamate levels in the developing brain. To investigate the effects of elevated glutamate levels on cGMP in the developing cortex we exposed mouse brain slices to glutamate or N-methyl D-aspartate (NMDA). We find that at early postnatal stages when the endogenous production of cGMP is high, glutamate or NMDA exposure results in a significant lowering of the overall production of cGMP in the cortex, unlike the situation in the mature brain. However, this response pattern is complex with regional and cell-type specific exceptions to the overall lowered cGMP production. These data emphasize that the response of the developing brain to physiological disturbances can be different from that of the mature brain, and must be considered in the context of the developmental events occurring at the time of disturbance.

Multifunction of myricetin on A beta: neuroprotection via a conformational change of A beta and reduction of A beta via the interference of secretases

Myricetin (3,3',4',5,5',7-hexahydroxyflavone) is classified as a flavonoid with strong antioxidant effects. Oxidative stress plays a key role in various neurological diseases such as ischemia and Alzheimer's disease (AD). To elucidate whether myricetin could counter the progress of AD, we examined the effects of myricetin on neurotoxicity induced by beta-amyloid (A beta), a component of senile plaques in the AD brain. We found that cultured rat primary cortical neurons treated for 48 hr with A beta1-42 (1 microM) induced significant neuronal injury. Conformationally altered A beta1-42 caused apoptotic changes, such as nuclear fragmentation, as shown by DAPI staining. Pre- and simultaneous administration of myricetin and A beta1-42 reduced A beta neurotoxicity in a concentration-dependent manner. By using circular dichroism spectroscopy and a thioflavin T binding assay, we show that myricetin (10 microM, 48 hr) prevented structural changes in A beta1-42 from a random coil to a beta-sheet-rich structure. A beta1-42-induced apoptotic changes and caspase-3 activation were reduced by myricetin treatment. Furthermore, we determined that administration of myricetin significantly decreased A beta1-40 and A beta1-42 levels in culture media. These effects were based on two mechanisms: the activation and up-regulation of alpha-secretase (ADAM10) protein levels as indicated by fluorescence resonance energy transfer (FRET) assay and immunoblot analysis and the direct binding and inhibition of beta-secretase (BACE-1) indicated by cell-free FRET assays. Evidently, myricetin has multiple functions to counter the progress of AD by the reduction of A beta production and the detoxification of A beta through a structural change.

Insensitivity to glutamate neurotoxicity mediated by NMDA receptors in association with delayed mitochondrial membrane potential disruption in cultured rat cortical neurons

We have attempted to elucidate mechanisms underlying differential vulnerability to glutamate (Glu) using cultured neurons prepared from discrete structures of embryonic rat brains. Brief exposure to Glu led to a significant decrease in the mitochondrial activity in hippocampal neurons cultured for 9 or 12 days at 10 muM to 1 mM with an apoptosis-like profile, without markedly affecting that in cortical neurons. Brief exposure to Glu also increased lactate dehydrogenase release along with a marked decrease in the number of cells immunoreactive for a neuronal marker protein in hippocampal, but not cortical, neurons. Similar insensitivity was seen to the cytotoxicity by NMDA, but not to that by tunicamycin, 2,4-dinitrophenol, hydrogen peroxide or A23187, in cortical neurons. However, NMDA was more efficient in increasing intracellular free Ca2+ levels in cortical neurons than in hippocampal neurons. Antagonists for neuroprotective metabotropic Glu receptors failed to significantly affect the insensitivity to Glu, while NMDA was more effective in disrupting mitochondrial membrane potentials in hippocampal than cortical neurons. These results suggest that cortical neurons would be insensitive to the apoptotic neurotoxicity mediated by NMDA receptors through a mechanism related to mitochondrial membrane potentials, rather than intracellular free Ca2+ levels, in the rat brain.

Cytoprotective properties of phenolic antidiarrheic ingredients in cultured astrocytes and neurons of rat brains

We have previously shown that particular phenolic antidiarrheic ingredients, including 2-methoxy-4-methylphenol (2M4MP) and 2-methoxy-4-ethyphenol (2M4EP), but not 2-methoxyphenol (2MP), significantly inhibit cellular maturation and differentiation of the bone-resorbing osteoclasts with concomitant protection of the bone-forming osteoblasts against oxidative stress by hydrogen peroxide (H(2)O(2)). In the present study, we evaluated the pharmacological actions of these three major phenolic antidiarrheic ingredients on the cellular viability in cultured astrocytes and neurons of the rat brain in vitro. Both 2M4MP and 2M4EP induced more efficient prevention of cell death induced by the brief exposure to 0.1 mM H(2)O(2) for 2 h than 2MP upon simultaneous exposure in cultured rat cortical astrocytes. Similarly, both 2M4MP and 2M4EP were more effective than 2MP in significantly protecting the cytotoxicity by brief exposure to 0.1 mM H(2)O(2) for 6 h in cultured rat hippocampal neurons, with concomitant suppression of the generation of intracellular reactive oxygen species in neurons exposed to H(2)O(2). Moreover, the three ingredients not only significantly prevented cell death in hippocampal neurons exposed to 0.1 mM glutamate for 1 h when determined 48 h after the brief exposure, but also inhibited the generation of intracellular reactive oxygen species and the elevation of intracellular Ca(2+) ions in neurons exposed to glutamate. These results suggest that particular phenolic antidiarrheic ingredients may prevent cell death through a mechanism related to diminution of the neurotoxicity of glutamate in neurons, in addition to eliciting cytoprotection against oxidative stress in astrocytes and neurons, in the rat brain.

Activator protein-1 responsive to the group II metabotropic glutamate receptor subtype in association with intracellular calcium in cultured rat cortical neurons

Activation of ionotropic glutamate (Glu) receptors, such as N-methyl-d-aspartate receptors, is shown to modulate the gene transcription mediated by the transcription factor activator protein-1 (AP1) composed of Fos and Jun family proteins in the brain, while little attention has been paid to the modulation of AP1 expression by metabotropic Glu receptors (mGluRs). In cultured rat cortical neurons, where constitutive expression was seen with all groups I, II and III mGluR subtypes, a significant and selective increase was seen in the DNA binding activity of AP1 120 min after the brief exposure to the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) for 5 min. In cultured rat cortical astrocytes, by contrast, a significant increase was induced by a group I mGluR agonist, but not by either a group II or III mGluR agonist. The increase by DCG-IV was significantly prevented by a group II mGluR antagonist as well as by either an intracellular Ca(2+) chelator or a voltage-sensitive Ca(2+) channel blocker, but not by an intracellular Ca(2+) store inhibitor. Moreover, DCG-IV significantly prevented the increase of cAMP formation by forskolin in cultured neurons. Western blot analysis revealed differential expression profiles of Fos family members in neurons briefly exposed to DCG-IV and NMDA. Prior or simultaneous exposure to DCG-IV led to significant protection against neuronal cell death by NMDA. These results suggest that activation of the group II mGluR subtype would modulate the gene expression mediated by AP1 through increased intracellular Ca(2+) levels in cultured rat cortical neurons.

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.

Functional expression of A glutamine transporter responsive to down-regulation by lipopolysaccharide through reduced promoter activity in cultured rat neocortical astrocytes

The prevailing view is that the glutamine (Gln) transporter (GlnT/ATA1/SAT1/SNAT1) is a member of the system A transporter superfamily with the ability to fuel the glutamate/Gln cycle at nerve terminals in glutamatergic neurons. Semiquantitative reverse transcription-polymerase chain reaction revealed similarly high expression of mRNA for GlnT by rat brain neocortical astrocytes as well as neurons, with progressively lower expression by cerebellar astrocytes, hippocampal astrocytes, and whole-brain microglia in culture. [(3)H]Gln was accumulated in a temperature-dependent manner with a saturable profile in both cultured neocortical neurons and astrocytes, whereas biochemical and pharmacological analyses on [(3)H]Gln accumulation revealed the expression of both system A and system L transporters by cultured neocortical neurons and astrocytes. Exposure to lipopolysaccharide (LPS) for 24 hr resulted in a significant decrease in both GlnT mRNA expression and [(3)H]Gln accumulation, with a concomitant drastic increase in nitrite formation in cultured neocortical astrocytes. Moreover, LPS significantly inhibited the promoter activity of GlnT in the astrocytic cell line C6 glioma cells as well as primary rat neocortical astrocytes in culture. These results suggest that activation by LPS would lead to down-regulation of the expression of GlnT responsible for the incorporation of extracellular Gln into intracellular spaces across plasma membranes through the inhibition of its promoter activity in cultured rat neocortical astrocytes.

Long-term effect of early discharge on sEPSC and [Ca2+]i in developing neurons

To study the long-term changes induced in immature rat cortical neuronal cultures by transient exposure to an Mg(2+)-free treatment, at cultured day 6, cells were assigned into three groups, based on the mediums they were transiently exposed to as follows: control group 1 (CONT1) was exposed to Dulbecco's Modified Eagle's Medium (DMEM), control group 2 was exposed to a physiological solution (PS), and the magnesium-free physiological solution group (MGFPS) was exposed to the same medium as CONT2 except for the removal of magnesium. Following a 3-h exposure, the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSC) were recorded and intracellular calcium concentrations ([Ca2+]i) were measured. Compared to the CONT1 and CONT2 groups, the MGFPS group displayed a significantly greater amplitude (at d6, d7, d9, and d12) and frequency (at d6, d7, and d9) of sEPSC (p<0.05). Also, both the resting and peak intracellular calcium levels were significantly greater in the MGFPS group at days 6, 7, 9, 12 and 17 (p<0.05). The rise time (time from resting level to peak level of intracellular calcium following NMDA application) was significantly shorter in the MGFPS group at culture days 7 and 17 and significantly longer at culture day 12 (p<0.05). Finally, we compared the percentage of cortical neurons expressing neuron-specific enolase (NSE) and found that there were no significant differences in the number of NSE positive neurons among three groups at days 7, 12, and 17. Our results suggests that there are long-term changes in sEPSCs and [Ca2+]i in cultured rat cortical neurons following exposure to Mg2+-free environment without cell loss.

Protective effect of donepezil in primary-cultured rat cortical neurons exposed to N-methyl-d-aspartate (NMDA) toxicity

Donepezil has a neuroprotective effect against oxygen-glucose deprivation injury and glutamate toxicity in cultured cortical neurons. In this study, we further characterized the neuroprotective properties of donepezil in rat cortical cell cultures using glutamate receptor-specific agonists (N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) and kainate). Pretreatment with donepezil (1 microM) for 12 h significantly decreased the lactate dehydrogenase (LDH) release in response to NMDA (100 microM) by 43.8%, and reduced the LDH release in response to kainate (100 microM) and AMPA (100 microM) by 11.9% and 7.5% (without statistical significance), respectively. Donepezil appeared to inhibit LDH release in a concentration-dependent manner at 0.1-10 microM. Cortical neurons exposed to NMDA retained a normal morphological appearance in the presence of 10 microM donepezil. In binding assay for glutamate receptors, donepezil at 100 microM only slightly inhibited binding to the glycine and polyamine sites on NMDA receptor complex. On the other hand, 12 h pretreatment with donepezil at 10 and 100 microM significantly decreased the NMDA-induced increase of intracellular calcium concentration ([Ca2+]i). In conclusion, our results show that donepezil has protective activity against NMDA toxicity in cortical neurons, and this neuroprotection seems to be partially mediated by inhibition of the increase of [Ca2+]i.

Effect of NMDA on staurosporine-induced activation of caspase-3 and LDH release in mouse neocortical and hippocampal cells

To achieve a better understanding of developmentally regulated NMDA- and staurosporine-induced apoptotic processes, we investigated the concerted action of these agents on caspase-3 activity and LDH release in neocortical and hippocampal cell cultures at different stages in vitro (DIV). Hoechst 33342 and MAP-2 stainings were additionally employed to visualize apoptotic changes and cell damage. The vulnerability of neocortical cells to NMDA was more prominent at later culture stages, whereas hippocampal neurons were more susceptible to NMDA treatment at earlier stages. A persistent activation of caspase-3 by staurosporine was found at all experimental stages. Despite of certain differences in susceptibility to NMDA and staurosporine, both tissues responded to regulatory action of NMDA towards staurosporine-activated caspase-3 in a similar way. Combined treatment with NMDA and staurosporine resulted in a substantial increase in caspase-3 activity in neocortical and hippocampal neurons on 2 DIV. Additive effects were also observed in neocortical cultures on 12 DIV. In contrast, NMDA substantially inhibited staurosporine-induced caspase-3 activity on 7 DIV in neocortical and hippocampal cultures. Additionally, pro-apoptotic effects of 17beta-estradiol were attenuated by NMDA on 7 DIV. Changes in vulnerability to NMDA- and staurosporine-mediated activation of caspase-3 were not strictly related to LDH release. Our data revealed that NMDA can both enhance and inhibit the staurosporine-induced neuronal cell apoptosis. The pro-apoptotic effect of NMDA was exhibited at early and late culture stages, whereas the anti-apoptotic effect was transient occurring on 7 DIV only.

Counteraction by repetitive daily exposure to static magnetism against sustained blockade of N-methyl-D-aspartate receptor channels in cultured rat hippocampal neurons

In rat hippocampal neurons cultured with the antagonist for N-methyl-D-aspartate (NMDA) receptors dizocilpine (MK-801) for 8 days in vitro (DIV), a significant decrease was seen in the expression of microtubule-associated protein-2 (MAP-2) as well as mRNA for both brain-derived neurotrophic factor (BDNF) and growth-associated protein-43 (GAP-43), in addition to decreased viability. MK-801 not only decreased the expression of the NR1 subunit of NMDA receptors but also increased NR2A expression, without affecting NR2B expression. Repetitive daily exposure to static magnetic fields at 100 mT for 15 min led to a decrease in the expression of MAP-2, without significantly affecting cell viability or the expression of neuronal nuclei (NeuN) and GAP-43. However, the repetitive magnetism prevented decreases in both BDNF mRNA and MAP-2 and additionally increased the expression of NR2A subunit, without altering NR1 expression in neurons cultured in the presence of MK-801. Repetitive magnetism was also effective in preventing the decrease by MK-801 in the ability of NMDA to increase intracellular free Ca2+ ions, without affecting the decrease in the maximal response. These results suggest that repetitive magnetism may at least in part counteract the neurotoxicity of MK-801 through modulation of the expression of particular NMDA receptor subunits in cultured rat hippocampal neurons.

An increase in intracellular free calcium ions by nicotinic acetylcholine receptors in a single cultured rat cortical astrocyte

Neuronal nicotinic acetylcholine receptors (nAChRs) are composed of an assembly between at least seven alpha (alpha2-alpha7, alpha9) and three beta (beta2-beta4) subunits in mammals. The addition of 50 mM KCl or 1 mM nicotine immediately increased the number of cells with high fluorescence intensity in rat cortical astrocytes on fluo-3 fluorescence measurement. Nicotine was effective at increasing the fluorescence intensity in astrocytes cultured for 2 days after replating, but not in those used 1 or 5 days after replating, without markedly affecting the cellular viability irrespective of the exposure period. Nicotine markedly increased the fluorescence intensity in a concentration-dependent manner at a concentration range of 10-100 microM in cultured astrocytes when analyzed on a responsive single cell. In these responsive single cells, the increase by nicotine was significantly prevented by the heteromeric alpha4/beta2 subtype antagonist dihydro-beta-erythroidine and the homomeric alpha7 subtype antagonist methyllycaconitine, as well as by nifedipine and EGTA but not thapsigargin. Methyllycaconitine failed to inhibit further the increase by nicotine in the presence of nifedipine, however, whereas the expression of mRNA was seen for all mammalian neuronal nAChR subunits in cultured rat cortical astrocytes as well as neurons. These results suggest that nicotine may increase intracellular free Ca2+ through the influx of extracellular Ca2+ across L-type voltage-gated Ca2+ channels rather than Ca2+ release from intracellular stores, in a manner related to the alpha4/beta2 and/or alpha7 nAChR channels functionally expressed in cultured rat cortical astrocytes.

Functional proteins involved in regulation of intracellular Ca(2+) for drug development: desensitization of N-methyl-D-aspartate receptor channels

N-Methyl-D-aspartate (NMDA) receptors are a principal subtype of excitatory ligand-gated ion channels with crucial roles in a variety of physiological and pathological processes in the mammalian central nervous system (CNS). In contrast to synaptic non-NMDA receptors that rapidly internalize, synaptic NMDA receptors have been shown to be rather static. However, recent accumulating evidence gives rise to the possibility that NMDA receptors may also undergo internalization. In our studies, repeated but not sustained stimulation by an agonist indeed induced desensitization of NMDA-receptor channels, which may be due to a decrease in the number of NMDA receptors expressed on cellular surfaces by internalization in a manner similar to the clathrin-induced endocytosis. Desensitization of NMDA-receptor channels provides a clue for the elucidation of fundamental mechanisms underlying dynamic regulation of the number of NMDA receptors at synapses, which is undoubtedly critical for the maintenance of both integrity and functionality in the CNS.

Functional alterations in immature cultured rat hippocampal neurons after sustained exposure to static magnetic fields

In cultured rat hippocampal neurons, gradual increases were seen in the expression of microtubule-associated protein-2 (MAP-2), neuronal nuclei (NeuN) and growth-associated protein-43 (GAP-43), in proportion to increased duration, up to 9 days in vitro (DIV). Sustained exposure to static magnetic fields at 100 mT for up to 9 DIV significantly decreased expression of MAP-2 and NeuN in cultured rat hippocampal neurons without markedly affecting GAP-43 expression. Although a significant increase was seen in the expression of glial fibrillary acidic protein (GFAP) in hippocampal neuronal preparations cultured for 6-9 DIV under sustained magnetism, GFAP and proliferating cell nuclear antigen expression were not affected markedly in cultured astrocytes prepared from rat hippocampus and neocortex, irrespective of cellular maturity. No significant alteration was seen in cell survivability of hippocampal neurons or astrocytes cultured under sustained magnetism. In hippocampal neurons cultured for 3 DIV under sustained magnetism, marked mRNA expression was seen for N-methyl-D-aspartate (NMDA) receptor subunits, NR1, NR2A-2C, NR2D, and NR3A. In addition, significant potentiation of the ability of NMDA to increase intracellular free Ca(2+) ions was observed. Differential display analysis revealed a significant decrease in mRNA expression for the transcription factor ALF1 in response to sustained magnetism for 3 DIV. These results suggest that sustained exposure to static magnetic fields may affect cellular functionality and maturity in immature cultured rat hippocampal neurons through modulation of expression of particular NMDA receptor subunits.

Effect of recurrent epileptiform discharges induced by magnesium-free treatment on developing cortical neurons in vitro

As seizures in infants and children often originate from the neocortex, neocortical epilepsy models may be appropriate for studying epileptiform activity and seizure-induced injury in the developing nervous system. However, the characterization of epileptiform activity or seizure-induced injury in cultured developing cortical neurons has seldom been reported. Therefore, We attempted to establish a cultured developing cortical neuronal epilepsy model, and to study the subsequent effect on neurons. Cultures were exposed to Mg(2+)-free media for 3 h, and then returned to regular media. Using whole-cell patch-clamp intracellular recording techniques, we found that spontaneously recurrent epileptiform discharges for at least 72 h could be induced after transient Mg(2+)-free treatment. Neuron morphology following Mg(2+)-free treatment demonstrated no prominent alterations. At different time points (6, 24 and 72 h) after Mg(2+)-free treatment, neuronal viability, identified by trypan blue staining and LDH activity, and apoptosis, measured by flow cytometry, showed modest but non-significant (P>0.05) changes compared with the age-matched control group after various culture periods (6 and 17 days) in vitro. Mitochondrial metabolic activity, measured by MTT assay, significantly decreased by 15% at 6 h after Mg(2+)-free treatment (P<0.05) in neurons cultured for 6 days, and at 24 h showed a 29% decrease in neurons cultured for 17 days (P<0.05). In conclusion, brief Mg(2+)-free treatment constitutes a cultured developing cortical neuron 'seizure' model, and can induce transient mitochondrial dysfunction without cell loss.

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.

Activator protein-1 complex expressed by magnetism in cultured rat hippocampal neurons

Brief exposure for 15 min to static magnetic filed at 100 mT led to marked but transient potentiation of binding of a radiolabeled probe for activator protein-1 (AP1) in immature cultured rat hippocampal neurons with high expression of growth-associated protein-43. Immunoblotting and supershift analyses revealed that brief exposure to static magnetic field increased AP1 DNA binding through expression of Fra-2, c-Jun, and Jun-D proteins in immature cultured hippocampal neurons. Significantly less potent increases were seen in both intracellular free Ca(2+) concentration and AP1 binding following the addition of N-methyl-d-aspartate in these immature neurons exposed to magnetism 24 h before. These results suggest that brief exposure to weak static magnetic field may lead to desensitization of NMDA receptor channels through modulation of de novo synthesis of particular inducible target proteins at the level of gene transcription by the AP1 complex expressed in the nucleus of immature cultured rat hippocampal neurons.

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.