Localized Ca2+ entry preferentially effects protein dephosphorylation, phosphorylation, and glutamate release

Ursolic acid inhibits the synaptic release of glutamate and prevents glutamate excitotoxicity in rats

The present study evaluated the effect of ursolic acid, a natural pentacyclic triterpenoid, on glutamate release in rat cortical nerve terminals (synaptosomes) and its neuroprotection in a kainic acid-induced excitotoxicity rat model. In cortical synaptosomes, ursolic acid produced a concentration-dependent inhibition of evoked glutamate release with a half-maximum inhibition of release value of 9.5 μM, and calcium-free medium and the P/Q -type Ca2+ channel blocker, ω-agatoxin IVA, but not ω-conotoxin GVIA, an N-type Ca2+ channel blocker, prevented the ursoloic acid effect. The molecular docking study indicated that ursolic acid interacted with P/Q-type Ca2+ channels. Ursolic acid also significantly decreased the depolarization-induced activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and the subsequent phosphorylation of synapsin I, and the ursolic acid effect on evoked glutamate release was inhibited by the CaMKII inhibitor KN 62 in synaptosomes. In addition, in rats that were intraperitoneally injected with ursolic acid 30 min before kainic acid intraperitoneal injection, cortical neuronal degeneration was attenuated. This effect of ursolic acid in the improvement of kainic acid-induced neuronal damage was associated with the reduction of kainic acid-induced glutamate increase in the cortex of rats; this was characterized by the reduction of glutamate and glutaminase levels and elevation of glutamate dehydrogenase, glutamate transporter 1, glutamate-aspartate transporter, and glutamine synthetase protein levels. These results suggest that ursolic acid inhibits glutamate release from cortical synaptosomes by decreasing P/Q-type Ca2+ channel activity and subsequently suppressing CaMKII and exerts a preventive effect against glutamate neurotoxicity by controlling glutamate levels.

Inhibition of Synaptic Glutamate Exocytosis and Prevention of Glutamate Neurotoxicity by Eupatilin from Artemisia argyi in the Rat Cortex

The inhibition of synaptic glutamate release to maintain glutamate homeostasis contributes to the alleviation of neuronal cell injury, and accumulating evidence suggests that natural products can repress glutamate levels and associated excitotoxicity. In this study, we investigated whether eupatilin, a constituent of Artemisia argyi, affected glutamate release in rat cortical nerve terminals (synaptosomes). Additionally, we evaluated the effect of eupatilin in an animal model of kainic acid (KA) excitotoxicity, particularly on the levels of glutamate and N-methyl-D-aspartate (NMDA) receptor subunits (GluN2A and GluN2B). We found that eupatilin decreased depolarization-evoked glutamate release from rat cortical synaptosomes and that this effect was accompanied by a reduction in cytosolic Ca2+ elevation, inhibition of P/Q-type Ca2+ channels, decreased synapsin I Ca2+-dependent phosphorylation and no detectable effect on the membrane potential. In a KA-induced glutamate excitotoxicity rat model, the administration of eupatilin before KA administration prevented neuronal cell degeneration, glutamate elevation, glutamate-generating enzyme glutaminase increase, excitatory amino acid transporter (EAAT) decrease, GluN2A protein decrease and GluN2B protein increase in the rat cortex. Taken together, the results suggest that eupatilin depresses glutamate exocytosis from cerebrocortical synaptosomes by decreasing P/Q-type Ca2+ channels and synapsin I phosphorylation and alleviates glutamate excitotoxicity caused by KA by preventing glutamatergic alterations in the rat cortex. Thus, this study suggests that eupatilin can be considered a potential therapeutic agent in the treatment of brain impairment associated with glutamate excitotoxicity.

The Effect of Isosaponarin Derived from Wasabi Leaves on Glutamate Release in Rat Synaptosomes and Its Underlying Mechanism

Excessive glutamate release is known to be involved in the pathogenesis of neurological diseases, and suppression of glutamate release from nerve terminals is considered to be a treatment strategy. In this study, we investigated whether isosaponarin, a flavone glycoside isolated from wasabi leaves, could affect glutamate release in rat cerebral cortex nerve terminals (synaptosomes). The release of glutamate was evoked by the K⁺ channel blocker 4-aminopyridine (4-AP) and measured by an online enzyme-coupled fluorimetric assay. Isosaponarin produced a concentration-dependent inhibition of 4-AP-evoked glutamate release with a half-maximum inhibition of release value of 22 μM. The inhibition caused by isosaponarin was prevented by eliminating extracellular Ca²⁺ or by using bafilomycin A1, an inhibitor of synaptic vesicle exocytosis. Isosaponarin decreased intrasynaptosomal rises in Ca²⁺ levels that were induced by 4-AP, without affecting the synaptosomal membrane potential. The isosaponarin-induced inhibition of glutamate release was significantly prevented in synaptosomes that were pretreated with a combination of the calcium channel blockers ω-conotoxin GVIA (N-type) and ω-agatoxin IVA (P/Q-types). The protein kinase C (PKC) pan-inhibitor GF109203X and the Ca²⁺-dependent PKC inhibitor Go6976 abolished the inhibition of glutamate release by isosaponarin, while the Ca²⁺-independent PKC inhibitor rottlerin did not show any effect. The results from immunoblotting assays also showed that isosaponarin lowered PKC, PKCα, synaptosomal-associated protein of 25 kDa (SNAP-25), and myristoylated alanine-rich C-kinase substrate (MARCKS) phosphorylation induced by 4-AP. In addition, FM1-43-labeled synaptic vesicles in synaptosomes showed that treatment with isosaponarin resulted in an attenuation of the 4-AP-induced decrease in fluorescence intensity that is consistent with glutamate release. Transmission electron microscopy of synaptosomes also provided evidence that isosaponarin altered the number of synaptic vesicles. These results indicate that isosaponarin suppresses the Ca²⁺-dependent PKC/SNAP-25 and MARCKS pathways in synaptosomes, causing a decrease in the number of available synaptic vesicles, which inhibits vesicular glutamate release from synaptosomes.

Kaempferol 3-Rhamnoside on Glutamate Release from Rat Cerebrocortical Nerve Terminals Involves P/Q-Type Ca2+ Channel and Ca2+/Calmodulin-Dependent Protein Kinase II-Dependent Pathway Suppression

Excess synaptic glutamate release has pathological consequences, and the inhibition of glutamate release is crucial for neuroprotection. Kaempferol 3-rhamnoside (KR) is a flavonoid isolated from Schima superba with neuroprotective properties, and its effecton the release of glutamate from rat cerebrocortical nerve terminals was investigated. KR produced a concentration-dependent inhibition of 4-aminopyridine (4-AP)-evoked glutamate release with half-maximal inhibitory concentration value of 17 µM. The inhibition of glutamate release by KR was completely abolished by the omission of external Ca²⁺ or the depletion of glutamate in synaptic vesicles, and it was unaffected by blocking carrier-mediated release. In addition, KR reduced the 4-AP-evoked increase in Ca²⁺ concentration, while it did not affect 4-AP-evoked membrane potential depolarization. The application of selective antagonists of voltage-dependent Ca²⁺ channels revealed that the KR-mediated inhibition of glutamate release involved the suppression of P/Q-type Ca²⁺ channel activity. Furthermore, the inhibition of release was abolished by the calmodulin antagonist, W7, and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) inhibitor, KN62, but not by the protein kinase A (PKA) inhibitor, H89, or the protein kinase C (PKC) inhibitor, GF109203X. We also found that KR reduced the 4-AP-induced increase in phosphorylation of CaMKII and its substrate synapsin I. Thus, the effect of KR on evoked glutamate release is likely linked to a decrease in P/Q-type Ca²⁺ channel activity, as well as to the consequent reduction in the CaMKII/synapsin I pathway.

Natural Product Isoliquiritigenin Activates GABAB Receptors to Decrease Voltage-Gate Ca2+ Channels and Glutamate Release in Rat Cerebrocortical Nerve Terminals

Reduction in glutamate release is a key mechanism for neuroprotection and we investigated the effect of isoliquiritigenin (ISL), an active ingredient of Glycyrrhiza with neuroprotective activities, on glutamate release in rat cerebrocortical nerve terminals (synaptosomes). ISL produced a concentration-dependent inhibition of glutamate release and reduced the intraterminal [Ca²⁺] increase. The inhibition of glutamate release by ISL was prevented after removing extracellular Ca²⁺ or blocking P/Q-type Ca²⁺ channels. This inhibition was mediated through the γ-aminobutyric acid type B (GABA_B) receptors because ISL was unable to inhibit glutamate release in the presence of baclofen (an GABA_B agonist) or CGP3548 (an GABA_B antagonist) and docking data revealed that ISL interacted with GABA_B receptors. Furthermore, the ISL inhibition of glutamate release was abolished through the inhibition of G_i/o-mediated responses or G_βγ subunits, but not by 8-bromoadenosine 3′,5′-cyclic monophosphate or adenylate cyclase inhibition. The ISL inhibition of glutamate release was also abolished through the inhibition of protein kinase C (PKC), and ISL decreased the phosphorylation of PKC. Thus, we inferred that ISL, through GABA_B receptor activation and G_βγ-coupled inhibition of P/Q-type Ca²⁺ channels, suppressed the PKC phosphorylation to cause a decrease in evoked glutamate release at rat cerebrocortical nerve terminals.

TCD, a triterpenoid isolated from wild bitter gourd, reduces synaptosomal release of glutamate and protects against kainic acid-induced neuronal death

3β,7β,25-Trihydroxycucurbita-5,23(E)-dien-19-al (TCD) is a triterpenoid isolated from wild bitter gourd that is a common tropical vegetable with neuroprotective effects. Because excessive glutamate release is a major cause of neuronal damage in various neurological disorders, the aims of this study were to examine the effect of TCD on glutamate release in vitro and to examine the effect of TCD in vivo. In rat cerebrocortical synaptosomes, TCD reduced 4-aminopyridine (4-AP)-stimulated glutamate release and Ca2+ concentration elevation, but had no effect on plasma membrane potential. TCD-mediated inhibition of 4-AP-induced glutamate release was dependent on the presence of extracellular calcium; persisted in the presence of the glutamate transporter inhibitor dl-TBOA, P/Q-type Ca2+ channel blocker ω-agatoxin IVA, and intracellular Ca2+-releasing inhibitors dantrolene and CGP37157; and was blocked by the vesicular transporter inhibitor bafilomycin A1 and the N-type Ca2+ channel blocker ω-conotoxin GVIA. Molecular docking studies have demonstrated that TCD binds to N-type Ca2+ channels. TCD-mediated inhibition of 4-AP-induced glutamate release was abolished by the Ca2+-dependent protein kinase C (PKC) inhibitor Go6976, but was unaffected by the Ca2+-independent PKC inhibitor rottlerin. Furthermore, TCD considerably reduced the phosphorylation of PKC, PKCα, and myristoylated alanine-rich C kinase substrate, a major presynaptic substrate for PKC. In a rat model of kainic acid (KA)-induced excitotoxicity, TCD pretreatment substantially attenuated KA-induced neuronal death in the CA3 hippocampal region. These results suggest that TCD inhibits synaptosomal glutamate release by suppressing N-type Ca2+ channels and PKC activity and exerts protective effects against KA-induced excitotoxicity in vivo.

Typhaneoside Suppresses Glutamate Release Through Inhibition of Voltage-Dependent Calcium Entry in Rat Cerebrocortical Nerve Terminals

Glutamate is the major excitatory neurotransmitter in the brain and is involved in many brain functions. In this study, we investigated whether typhaneoside, a flavonoid from Typhae angustifolia pollen, affects endogenous glutamate release from rat cortical synaptosomes. Using a one-line enzyme-coupled fluorometric assay, glutamate release stimulated by the K⁺ channel blocker 4-aminopyridine was monitored to explore the possible underlying mechanisms. The vesicular transporter inhibitor bafilomycin A1 and chelation of extracellular Ca²⁺ ions with EGTA suppressed the effect of typhaneoside on the induced glutamate release. Nevertheless, the typhaneoside activity has not been affected by the glutamate transporter inhibitor dl-threo-beta-benzyloxyaspartate. The synaptosomal plasma membrane potential was assayed using a membrane potential-sensitive dye DiSC₃(5), and cytosolic Ca²⁺ concentrations ([Ca²⁺]_C) was monitored by a Ca²⁺ indicator Fura-2. Results showed that typhaneoside did not alter the synaptosomal membrane potential but lowered 4-aminopyridine-induced increases in [Ca²⁺]_C. Furthermore, the Ca_v2.2 (N-type) channel blocker ω-conotoxin GVIA blocked Ca²⁺ entry and inhibited the effect of typhaneoside on 4-aminopyridine-induced glutamate release. However, the inhibitor of intracellular Ca²⁺ release dantrolene and the mitochondrial Na⁺/Ca²⁺ exchanger blocker 7-chloro-5-(2-chloropheny)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one have no effect on the suppression of glutamate release mediated by typhaneoside. Moreover, inhibition of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) prevented the inhibitory effect of typhaneoside on induced glutamate release. Typhaneoside reduced 4-aminopyridine-induced phosphorylation of ERK1/2 and the major presynaptic ERK target synapsin I, which is a synaptic vesicle-associated protein. In conclusion, these findings suggest a role for typhaneoside in modulating glutamate release by suppressing voltage-dependent Ca²⁺ channel mediated presynaptic Ca²⁺ influx and the MAPK/ERK/synapsin I signaling cascade.

Protein Phosphorylation in Depolarized Synaptosomes: Dissecting Primary Effects of Calcium from Synaptic Vesicle Cycling

Synaptic transmission is mediated by the regulated exocytosis of synaptic vesicles. When the presynaptic membrane is depolarized by an incoming action potential, voltage-gated calcium channels open, resulting in the influx of calcium ions that triggers the fusion of synaptic vesicles (SVs) with the plasma membrane. SVs are recycled by endocytosis. Phosphorylation of synaptic proteins plays a major role in these processes, and several studies have shown that the synaptic phosphoproteome changes rapidly in response to depolarization. However, it is unclear which of these changes are directly linked to SV cycling and which might regulate other presynaptic functions that are also controlled by calcium-dependent kinases and phosphatases. To address this question, we analyzed changes in the phosphoproteome using rat synaptosomes in which exocytosis was blocked with botulinum neurotoxins (BoNTs) while depolarization-induced calcium influx remained unchanged. BoNT-treatment significantly alters the response of the synaptic phoshoproteome to depolarization and results in reduced phosphorylation levels when compared with stimulation of synaptosomes by depolarization with KCl alone. We dissect the primary Ca2+-dependent phosphorylation from SV-cycling-dependent phosphorylation and confirm an effect of such SV-cycling-dependent phosphorylation events on syntaxin-1a-T21/T23, synaptobrevin-S75, and cannabinoid receptor-1-S314/T322 on exo- and endocytosis in cultured hippocampal neurons.

The temporal profile of activity-dependent presynaptic phospho-signalling reveals long-lasting patterns of poststimulus regulation

Depolarization of presynaptic terminals stimulates calcium influx, which evokes neurotransmitter release and activates phosphorylation-based signalling. Here, we present the first global temporal profile of presynaptic activity-dependent phospho-signalling, which includes two KCl stimulation levels and analysis of the poststimulus period. We profiled 1,917 regulated phosphopeptides and bioinformatically identified six temporal patterns of co-regulated proteins. The presynaptic proteins with large changes in phospho-status were again prominently regulated in the analysis of 7,070 activity-dependent phosphopeptides from KCl-stimulated cultured hippocampal neurons. Active zone scaffold proteins showed a high level of activity-dependent phospho-regulation that far exceeded the response from postsynaptic density scaffold proteins. Accordingly, bassoon was identified as the major target of neuronal phospho-signalling. We developed a probabilistic computational method, KinSwing, which matched protein kinase substrate motifs to regulated phosphorylation sites to reveal underlying protein kinase activity. This approach allowed us to link protein kinases to profiles of co-regulated presynaptic protein networks. Ca²⁺- and calmodulin-dependent protein kinase IIα (CaMKIIα) responded rapidly, scaled with stimulus strength, and had long-lasting activity. Mitogen-activated protein kinase (MAPK)/extracellular signal–regulated kinase (ERK) was the main protein kinase predicted to control a distinct and significant pattern of poststimulus up-regulation of phosphorylation. This work provides a unique resource of activity-dependent phosphorylation sites of synaptosomes and neurons, the vast majority of which have not been investigated with regard to their functional impact. This resource will enable detailed characterization of the phospho-regulated mechanisms impacting the plasticity of neurotransmitter release.

Analysis of activity-dependent phosphorylation-based signalling in synaptosomes revealed six patterns of long-lasting presynaptic regulation from 1,917 phosphopeptides. The authors identified patterns most likely to be regulated by CamKII and MAPK/ERK and showed the active zone scaffold protein bassoon to be a major signalling target.

Neurobiological processes are altered by linking neuronal activity to regulated changes in protein phosphorylation levels that influence protein function. Although some of the major targets of activity-dependent phospho-signalling have been identified, a large number of substrates remain unknown. Here, we have screened systematically for these substrates and extended the list from hundreds to thousands of phosphorylation sites, thereby providing a new depth of understanding. We monitored phospho-signalling for 15 min after the stimulation, which to our knowledge had not been attempted at a large scale. We focused on presynaptic protein substrates of phospho-signalling by isolating the presynaptic terminal. We also stimulated hippocampal neurons but did not monitor the poststimulus. Although the phospho-signalling is immensely complex, the findings could be simplified through data exploration. We identified distinct patterns of presynaptic phospho-regulation across the time course that may constitute co-regulated protein networks. In addition, we found a subset of proteins that had many more phosphorylation sites than the average and high-magnitude responses, implying major signalling or functional roles for these proteins. We also determined the likely protein kinases with the strongest responses to the stimulus at different times using KinSwing, a computational tool that we developed. This resource reveals a new depth of activity-dependent phospho-signalling and identifies major signalling targets, major protein kinases, and co-regulated phosphoprotein networks.

Xanthohumol-induced presynaptic reduction of glutamate release in the rat hippocampus

This study examined whether xanthohumol, a hop-derived prenylated flavonoid present in beer, affects glutamate release in the rat hippocampus. In the rat hippocampal nerve terminals (synaptosomes), xanthohumol inhibited the release of 4-aminopyridine (4-AP)-evoked glutamate and the elevation of cytosolic Ca(2+) concentration, whereas it had no effect on 4-AP-mediated depolarization. The inhibitory effect of xanthohumol on the evoked glutamate release was prevented by removing extracellular Ca(2+), using the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-CgTX MVIIC, the calmodulin antagonists W7 and calmidazolium, and the protein kinase A inhibitor H89; however, no such effect was observed when the G-protein inhibitor N-ethylmaleimide was used. In addition, immunocytochemical data demonstrated that GABAA receptors are present in the hippocampal synaptosomes and that the xanthohumol effect on evoked glutamate release was antagonized by the GABAA receptor antagonist SR95531. Furthermore, in slice preparations, xanthohumol reduced the frequency of miniature excitatory postsynaptic currents without affecting their amplitude. We conclude that xanthohumol acts at GABAA receptors present in the hippocampal nerve terminals to decrease the Ca(2+) influx through N- and P/Q-type Ca(2+) channels, which subsequently suppresses the Ca(2+)-calmodulin/PKA cascade to decrease the evoked glutamate release.

Neuroimmunomodulation in the Gut: Focus on Inflammatory Bowel Disease

Intestinal immunity is finely regulated by several concomitant and overlapping mechanisms, in order to efficiently sense external stimuli and mount an adequate response of either tolerance or defense. In this context, a complex interplay between immune and nonimmune cells is responsible for the maintenance of normal homeostasis. However, in certain conditions, the disruption of such an intricate network may result in intestinal inflammation, including inflammatory bowel disease (IBD). IBD is believed to result from a combination of genetic and environmental factors acting in concert with an inappropriate immune response, which in turn interacts with nonimmune cells, including nervous system components. Currently, evidence shows that the interaction between the immune and the nervous system is bidirectional and plays a critical role in the regulation of intestinal inflammation. Recently, the maintenance of intestinal homeostasis has been shown to be under the reciprocal control of the microbiota by immune mechanisms, whereas intestinal microorganisms can modulate mucosal immunity. Therefore, in addition to presenting the mechanisms underlying the interaction between immune and nervous systems in the gut, here we discuss the role of the microbiota also in the regulation of neuroimmune crosstalk involved in intestinal homeostasis and inflammation, with potential implications to IBD pathogenesis.

Echinacoside Inhibits Glutamate Release by Suppressing Voltage-Dependent Ca(2+) Entry and Protein Kinase C in Rat Cerebrocortical Nerve Terminals

The glutamatergic system may be involved in the effects of neuroprotectant therapies. Echinacoside, a phenylethanoid glycoside extracted from the medicinal Chinese herb HerbaCistanche, has neuroprotective effects. This study investigated the effects of echinacoside on 4-aminopyridine-evoked glutamate release in rat cerebrocortical nerve terminals (synaptosomes). Echinacoside inhibited Ca²⁺-dependent, but not Ca²⁺-independent, 4-aminopyridine-evoked glutamate release in a concentration-dependent manner. Echinacoside also reduced the 4-aminopyridine-evoked increase in cytoplasmic free Ca²⁺ concentration but did not alter the synaptosomal membrane potential. The inhibitory effect of echinacoside on 4-aminopyridine-evoked glutamate release was prevented by ω-conotoxin MVIIC, a wide-spectrum blocker of Cav2.2 (N-type) and Cav2.1 (P/Q-type) channels, but was insensitive to the intracellular Ca²⁺ release-inhibitors dantrolene and 7-chloro-5-(2-chloropheny)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157). Furthermore, echinacoside decreased the 4-aminopyridine-induced phosphorylation of protein kinase C, and protein kinase C inhibitors abolished the effect of echinacoside on glutamate release. According to these results, we suggest that the inhibitory effect of echinacoside on evoked glutamate release is associated with reduced voltage-dependent Ca²⁺ entry and subsequent suppression of protein kinase C activity.

Hesperidin inhibits glutamate release and exerts neuroprotection against excitotoxicity induced by kainic acid in the hippocampus of rats

The citrus flavonoid hesperidin exerts neuroprotective effects and could cross the blood-brain barrier. Given the involvement of glutamate neurotoxicity in the pathogenesis of neurodegenerative disorders, this study was conducted to evaluate the potential role of hesperidin in glutamate release and glutamate neurotoxicity in the hippocampus of rats. In rat hippocampal nerve terminals (synaptosomes), hesperidin inhibited the release of glutamate and elevation of cytosolic free Ca(2+) concentration evoked by 4-aminopyridine (4-AP), but did not alter 4-AP-mediated depolarization. The inhibitory effect of hesperidin on evoked glutamate release was prevented by chelating the extracellular Ca(2+) ions and blocking the activity of Cav2.2 (N-type) and Cav2.1 (P/Q-type) channels or protein kinase C. In hippocampal slice preparations, whole-cell patch clamp experiments showed that hesperidin reduced the frequency of spontaneous excitatory postsynaptic currents without affecting their amplitude, indicating the involvement of a presynaptic mechanism. In addition, intraperitoneal (i.p.) injection of kainic acid (KA, 15 mg/kg) elevated the extracellular glutamate levels and caused considerable neuronal loss in the hippocampal CA3 area. These KA-induced alterations were attenuated by pretreatment with hesperidin (10 or 50 mg/kg, i.p.) before administering the KA. These results demonstrate that hesperidin inhibits evoked glutamate release in vitro and attenuates in vivo KA-induced neuronal death in the hippocampus. Our findings indicate that hesperidin may be a promising candidate for preventing or treating glutamate excitotoxicity related brain disorders such as neurodegenerative diseases.

Apigenin, a natural flavonoid, inhibits glutamate release in the rat hippocampus

The purpose of this study was to examine the effect and mechanism of apigenin, a natural flavonoid, on glutamate release in the rat hippocampus. In rat hippocampal nerve terminals (synaptosomes), apigenin inhibited glutamate release and the elevation of the cytosolic free Ca(2+) concentration evoked by 4-aminopyridine, whereas it had no effect on 4-aminopyridine-mediated depolarization and Na(+) influx. The apigenin-mediated inhibition of evoked glutamate release was prevented by chelating the extracellular Ca(2+) ions and blocking Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel activity. Furthermore, we determined that gamma-aminobutyric acid type A (GABAA) receptors are present in the hippocampal nerve terminals because they are colocalized with the presynaptic marker synaptophysin. However, the effect of apigenin on 4-aminopyridine-evoked glutamate release from synaptosomes was unaffected by the GABAA receptor antagonists SR95531 and bicuculline. Furthermore, in slice preparations, whole-cell patch-clamp experiments showed that apigenin reduced the frequency of spontaneous excitatory postsynaptic currents without affecting their amplitude, suggesting a presynaptic mechanism. On the basis of these results, we suggested that apigenin exerts its presynaptic inhibition probably by reducing Ca(2+) entry mediated by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channels, thereby inhibiting glutamate release from the rat hippocampal nerve terminals.

Palmitoylethanolamide inhibits glutamate release in rat cerebrocortical nerve terminals

The effect of palmitoylethanolamide (PEA), an endogenous fatty acid amide displaying neuroprotective actions, on glutamate release from rat cerebrocortical nerve terminals (synaptosomes) was investigated. PEA inhibited the Ca²⁺-dependent release of glutamate, which was triggered by exposing synaptosomes to the potassium channel blocker 4-aminopyridine. This release inhibition was concentration dependent, associated with a reduction in cytosolic Ca²⁺ concentration, and not due to a change in synaptosomal membrane potential. The glutamate release-inhibiting effect of PEA was prevented by the Cav2.1 (P/Q-type) channel blocker ω-agatoxin IVA or the protein kinase A inhibitor H89, not affected by the intracellular Ca²⁺ release inhibitors dantrolene and CGP37157, and partially antagonized by the cannabinoid CB1 receptor antagonist AM281. Based on these results, we suggest that PEA exerts its presynaptic inhibition, likely through a reduction in the Ca²⁺ influx mediated by Cav2.1 (P/Q-type) channels, thereby inhibiting the release of glutamate from rat cortical nerve terminals. This release inhibition might be linked to the activation of presynaptic cannabinoid CB1 receptors and the suppression of the protein kinase A pathway.

Myricetin inhibits the release of glutamate in rat cerebrocortical nerve terminals

The excessive release of glutamate is a critical element in the neuropathology of acute and chronic brain disorders. The purpose of the present study was to investigate the effect and possible mechanism of myricetin, a naturally occurring flavonoid with a neuroprotective profile, on endogenous glutamate release in the nerve terminals (synaptosomes) of the rat cerebral cortex. The release of glutamate was evoked by the K(+) channel blocker 4-aminopyridine (4-AP) and measured by one-line enzyme-coupled fluorometric assay. We also used a membrane potential-sensitive dye to assay the synaptosomal plasma membrane potential, and a Ca(2+) indicator Fura-2 to monitor cytosolic Ca(2+) concentrations ([Ca(2+)]C). Results show that myricetin inhibited 4-AP-evoked glutamate release, and this effect was prevented by chelating extracellular Ca(2+) ions and the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor dl-threo-beta-benzyl-oxyaspartate had no effect on myricetin action. Myricetin did not alter the synaptosomal membrane potential, but decreased 4-AP-induced increases in the cytosolic free Ca(2+) concentration. Furthermore, the myricetin effect on 4-AP-evoked glutamate release was prevented by blocking the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channels, but not by blocking intracellular Ca(2+) release. These results suggest that myricetin inhibits glutamate release from cerebrocortical synaptosomes by attenuating voltage-dependent Ca(2+) entry. This implies that the inhibition of glutamate release is an important pharmacological activity of myricetin that may play a critical role in the apparent clinical efficacy of this compound.

Cyclooxygenase 2 inhibitor celecoxib inhibits glutamate release by attenuating the PGE2/EP2 pathway in rat cerebral cortex endings

The excitotoxicity caused by excessive glutamate is a critical element in the neuropathology of acute and chronic brain disorders. Therefore, inhibition of glutamate release is a potentially valuable therapeutic strategy for treating these diseases. In this study, we investigated the effect of celecoxib, a selective cyclooxygenase-2 (COX-2) inhibitor that reduces the level of prostaglandin E2 (PGE2), on endogenous glutamate release in rat cerebral cortex nerve terminals (synaptosomes). Celecoxib substantially inhibited the release of glutamate induced by the K(+) channel blocker 4-aminopyridine (4-AP), and this phenomenon was prevented by chelating the extracellular Ca(2+) ions and by the vesicular transporter inhibitor bafilomycin A1. Celecoxib inhibited a 4-AP-induced increase in cytosolic-free Ca(2+) concentration, and the celecoxib-mediated inhibition of glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC. However, celecoxib did not alter 4-AP-mediated depolarization and Na(+) influx. In addition, this glutamate release-inhibiting effect of celecoxib was mediated through the PGE2 subtype 2 receptor (EP2) because it was not observed in the presence of butaprost (an EP2 agonist) or PF04418948 [1-(4-fluorobenzoyl)-3-[[6-methoxy-2-naphthalenyl)methyl]-3-azetidinecarboxylic acid; an EP2 antagonist]. The celecoxib effect on 4-AP-induced glutamate release was prevented by the inhibition or activation of protein kinase A (PKA), and celecoxib decreased the 4-AP-induced phosphorylation of PKA. We also determined that COX-2 and the EP2 receptor are present in presynaptic terminals because they are colocalized with synaptophysin, a presynaptic marker. These results collectively indicate that celecoxib inhibits glutamate release from nerve terminals by reducing voltage-dependent Ca(2+) entry through a signaling cascade involving EP2 and PKA.

Dimebon, an antihistamine drug, inhibits glutamate release in rat cerebrocortical nerve terminals

The excessive release of glutamate is a critical element in the neuropathology of acute and chronic brain disorders. The purpose of the present study was to investigate the effect and possible mechanism of dimebon, an antihistamine with a neuroprotective profile, on endogenous glutamate release in the nerve terminals (synaptosomes) of the rat cerebral cortex. Dimebon inhibited the release of glutamate that was evoked by exposing the synaptosomes to the K(+) channel blocker 4-aminopyridine, and this effect was prevented by chelating extracellular Ca(2+) ions, and the vesicular transporter inhibitor bafilomycin A1. Dimebon inhibited depolarization-evoked increase in cytosolic free Ca(2+) concentration, and the dimebon-mediated inhibition of glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC. The inhibitory action of dimebon on glutamate release was not due to its decreasing synaptosomal excitability, because dimebon did not alter the resting synaptosomal membrane potential or 4-aminopyridine-mediated depolarization. Furthemore, the dimebon effect on 4-aminopyridine-evoked glutamate release was prevented by the protein kinase C inhibitor, and dimebon substantially reduced the 4-AP-induced phosphorylation of protein kinase C. However, the dimebon-mediated inhibition of glutamate release was unaffected by the N-methyl-d-aspartate receptor agonist or antagonist. These results suggest that dimebon inhibits glutamate release from rat cortical synaptosomes by suppressing presynaptic voltage-dependent Ca(2+) entry and protein kinase C activity. This implies that the inhibition of glutamate release is an additional pharmacological activity of dimebon that may play a critical role in the apparent clinical efficacy of this compound.

Acacetin inhibits glutamate release and prevents kainic acid-induced neurotoxicity in rats

An excessive release of glutamate is considered to be a molecular mechanism associated with several neurological diseases that causes neuronal damage. Therefore, searching for compounds that reduce glutamate neurotoxicity is necessary. In this study, the possibility that the natural flavone acacetin derived from the traditional Chinese medicine Clerodendrum inerme (L.) Gaertn is a neuroprotective agent was investigated. The effect of acacetin on endogenous glutamate release in rat hippocampal nerve terminals (synaptosomes) was also investigated. The results indicated that acacetin inhibited depolarization-evoked glutamate release and cytosolic free Ca²⁺ concentration ([Ca²⁺]C) in the hippocampal nerve terminals. However, acacetin did not alter synaptosomal membrane potential. Furthermore, the inhibitory effect of acacetin on evoked glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker known as ω-conotoxin MVIIC. In a kainic acid (KA) rat model, an animal model used for excitotoxic neurodegeneration experiments, acacetin (10 or 50 mg/kg) was administrated intraperitoneally to the rats 30 min before the KA (15 mg/kg) intraperitoneal injection, and subsequently induced the attenuation of KA-induced neuronal cell death and microglia activation in the CA3 region of the hippocampus. The present study demonstrates that the natural compound, acacetin, inhibits glutamate release from hippocampal synaptosomes by attenuating voltage-dependent Ca²⁺ entry and effectively prevents KA-induced in vivo excitotoxicity. Collectively, these data suggest that acacetin has the therapeutic potential for treating neurological diseases associated with excitotoxicity.

Berberine Inhibits the Release of Glutamate in Nerve Terminals from Rat Cerebral Cortex

Berberine, an isoquinoline plant alkaloid, protects neurons against neurotoxicity. An excessive release of glutamate is considered to be one of the molecular mechanisms of neuronal damage in several neurological diseases. In this study, we investigated whether berberine could affect endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes) and explored the possible mechanism. Berberine inhibited the release of glutamate evoked by the K⁺ channel blocker 4-aminopyridine (4-AP), and this phenomenon was prevented by the chelating extracellular Ca²⁺ ions and the vesicular transporter inhibitor bafilomycin A1, but was insensitive to the glutamate transporter inhibitor DL-threo-beta-benzyl-oxyaspartate. Inhibition of glutamate release by berberine was not due to it decreasing synaptosomal excitability, because berberine did not alter 4-AP-mediated depolarization. The inhibitory effect of berberine on glutamate release was associated with a reduction in the depolarization-induced increase in cytosolic free Ca²⁺ concentration. Involvement of the Ca_v2.1 (P/Q-type) channels in the berberine action was confirmed by blockade of the berberine-mediated inhibition of glutamate release by the Ca_v2.1 (P/Q-type) channel blocker ω-agatoxin IVA. In addition, the inhibitory effect of berberine on evoked glutamate release was prevented by the mitogen-activated/extracellular signal-regulated kinase kinase (MEK) inhibitors. Berberine decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synapsin I, the main presynaptic target of ERK; this decrease was also blocked by the MEK inhibition. Moreover, the inhibitory effect of berberine on evoked glutamate release was prevented in nerve terminals from mice lacking synapsin I. Together, these results indicated that berberine inhibits glutamate release from rats cortical synaptosomes, through the suppression of presynaptic Cav2.1 channels and ERK/synapsin I signaling cascade. This finding may provide further understanding of the mode of berberine action in the brain and highlights the therapeutic potential of this compound in the treatment of a wide range of neurological disorders.

Tanshinone IIA, a constituent of Danshen, inhibits the release of glutamate in rat cerebrocortical nerve terminals

ETHNOPHARMACOLOGICAL RELEVANCE

Danshen is a commonly used traditional Chinese medicine and has received considerable attention due to their beneficial effects on the health, including prevention of cardiovascular disease, and cancer. Tanshinone IIA, a major active constituent of Danshen, has been reported to have a neuroprotective profile.

AIM OF THE STUDY

An excessive release of glutamate is considered to be related to neuropathology of several neurological diseases. In this study, we investigated whether tanshinone IIA could affect endogenous glutamate release and explored the possible mechanism.

MATERIALS AND METHODS

The experimental model was the isolated nerve terminals (synaptosomes) purified from the rat cerebral cortex. The release of glutamate was evoked by the K(+) channel blocker 4-aminopyridine (4-AP) and measured by one-line enzyme-coupled fluorometric assay. We also used a membrane potential-sensitive dye to assay nerve terminal excitability and depolarization, and a Ca(2+) indicator, Fura-2-acetoxymethyl ester, to monitor cytosolic Ca(2+) concentrations ([Ca(2+)]C).

RESULTS

Tanshinone IIA inhibited the release of glutamate evoked by 4-AP in a concentration-dependent manner. Inhibition of glutamate release by tanshinone IIA was prevented by the chelating the extracellular Ca(2+) ions, and by the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor DL-threo-beta-benzyl-oxyaspartate did not have any effect on the action of tanshinone IIA. Tanshinone IIA decreased the depolarization-induced increase in [Ca(2+)]C, whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. Furthermore, the effect of tanshinone IIA on evoked glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC, but not by the ryanodine receptor blocker dantrolene or the mitochondrial Na(+)/Ca(2+) exchanger blocker CGP37157. Mitogen-activated protein kinase (MEK) inhibition also prevented the inhibitory effect of tanshinone IIA on evoked glutamate release.

CONCLUSION

These results show that tanshinone IIA inhibits glutamate release from cortical synaptosomes in rats through the suppression of presynaptic voltage-dependent Ca(2+) entry and MEK signaling cascade.

Presynaptic kainate receptor-mediated facilitation of glutamate release involves Ca2+-calmodulin and PKA in cerebrocortical synaptosomes

We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors (KARs) in the cortex using isolated nerve terminals (synaptosomes). Kainate (KA) produced an increase on glutamate release at 100 μM. The effect of KA was antagonized by NBQX (with AMPA receptors blocked by GYKI53655). This facilitation was suppressed by the inhibition of PKA activation by Rp-Br-cAMP and H-89. Moreover, the facilitation of glutamate release mediated by KAR requires the mobilization of intrasynaptosomal Ca(2+) stores and the formation of a Ca(2+)-calmodulin complex. We conclude that KARs present on presynaptic terminals in the neocortex mediate the facilitation of glutamate release through a mechanism involving an increase in cytosolic Ca(2+) to activate a Ca(2+)-calmodulin-AC/cAMP/PKA signaling cascade.

Ferulic acid suppresses glutamate release through inhibition of voltage-dependent calcium entry in rat cerebrocortical nerve terminals

This study investigated the effects and possible mechanism of ferulic acid, a naturally occurring phenolic compound, on endogenous glutamate release in the nerve terminals of the cerebral cortex in rats. Results show that ferulic acid inhibited the release of glutamate evoked by the K⁺ channel blocker 4-aminopyridine (4-AP). The effect of ferulic acid on the evoked glutamate release was prevented by chelating the extracellular Ca²⁺ ions, but was insensitive to the glutamate transporter inhibitor DL-threo-beta-benzyl-oxyaspartate. Ferulic acid suppressed the depolarization-induced increase in a cytosolic-free Ca²⁺ concentration, but did not alter 4-AP-mediated depolarization. Furthermore, the effect of ferulic acid on evoked glutamate release was abolished by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking ryanodine receptors or mitochondrial Na⁺/Ca²⁺ exchange. These results show that ferulic acid inhibits glutamate release from cortical synaptosomes in rats through the suppression of presynaptic voltage-dependent Ca²⁺ entry.

Coenzyme Q10 inhibits the release of glutamate in rat cerebrocortical nerve terminals by suppression of voltage-dependent calcium influx and mitogen-activated protein kinase signaling pathway

This study investigates the effects and possible mechanism of coenzyme Q10 (CoQ10) on endogenous glutamate release in the cerebral cortex nerve terminals of rats. CoQ10 inhibited the release of glutamate evoked by the K+ channel blocker 4-aminopyridine (4-AP). CoQ10 reduced the depolarization-induced increase in cytosolic [Ca2+]c but did not alter the 4-AP-mediated depolarization. The effect of CoQ10 on evoked glutamate release was abolished by blocking the Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels and mitogen-activated protein kinase kinase (MEK). In addition, CoQ10 decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synaptic vesicle-associated protein synapsin I, a major presynaptic substrate for ERK. Moreover, the inhibition of glutamate release by CoQ10 was strongly attenuated in mice without synapsin I. These results suggest that CoQ10 inhibits glutamate release from cortical synaptosomes in rats through the suppression of the presynaptic voltage-dependent Ca2+ entry and ERK/synapsin I signaling pathway.

Curcumin inhibits glutamate release from rat prefrontal nerve endings by affecting vesicle mobilization

Curcumin, one of the major constituents of Curcuma longa, has been shown to inhibit depolarization-evoked glutamate release from rat prefrontocortical nerve terminals by reducing voltage-dependent Ca²⁺ entry. This study showed that curcumin inhibited ionomycin-induced glutamate release and KCl-evoked FM1-43 release, suggesting that some steps after Ca²⁺ entry are regulated by curcumin. Furthermore, disrupting the cytoskeleton organization using cytochalasin D abolished the inhibitory action of curcumin on ionomycin-induced glutamate release. Mitogen-activated protein kinase kinase (MEK) inhibition also prevented the inhibitory effect of curcumin on ionomycin-induced glutamate release. Western blot analyses showed that curcumin decreased the ionomycin-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synaptic vesicle-associated protein synapsin I, the main presynaptic target of ERK. These results show that curcumin-mediated inhibition of glutamate release involves modulating downstream events by controlling synaptic vesicle recruitment and exocytosis, possibly through a decrease of MAPK/ERK activation and synapsin I phosphorylation, thereby decreasing synaptic vesicle availability for exocytosis.

Hispidulin inhibits the release of glutamate in rat cerebrocortical nerve terminals

Hispidulin, a naturally occurring flavone, has been reported to have an antiepileptic profile. An excessive release of glutamate is considered to be related to neuropathology of epilepsy. We investigated whether hispidulin affected endogenous glutamate release in rat cerebral cortex nerve terminals (synaptosomes) and explored the possible mechanism. Hispidulin inhibited the release of glutamate evoked by the K⁺ channel blocker 4-aminopyridine (4-AP). The effects of hispidulin on the evoked glutamate release were prevented by the chelation of extracellular Ca²⁺ ions and the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor dl-threo-beta-benzyl-oxyaspartate did not have any effect on hispidulin action. Hispidulin reduced the depolarization-induced increase in cytosolic free Ca²⁺ concentration ([Ca²⁺](C)), but did not alter 4-AP-mediated depolarization. Furthermore, the effect of hispidulin on evoked glutamate release was abolished by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking ryanodine receptors or mitochondrial Na⁺/Ca²⁺ exchange. Mitogen-activated protein kinase kinase (MEK) inhibition also prevented the inhibitory effect of hispidulin on evoked glutamate release. Western blot analyses showed that hispidulin decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synaptic vesicle-associated protein synapsin I, a major presynaptic substrate for ERK; this decrease was also blocked by the MEK inhibitor. Moreover, the inhibition of glutamate release by hispidulin was strongly attenuated in mice without synapsin I. These results show that hispidulin inhibits glutamate release from cortical synaptosomes in rats through the suppression of presynaptic voltage-dependent Ca²⁺ entry and ERK/synapsin I signaling pathway.

σ-1 Receptor agonist SKF10047 inhibits glutamate release in rat cerebral cortex nerve endings

σ-1 Receptors are expressed in the brain, and their activation has been shown to prevent neuronal death associated with glutamate toxicity. This study investigates the possible mechanism and effect of [2S-(2α,6α,11R*]-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-(2-propenyl)-2,6-methano-3-benzazocin-8-ol (SKF10047), a σ-1 receptor agonist, on endogenous glutamate release in the nerve terminals of rat cerebral cortex. Results show that SKF10047 inhibited the release of glutamate evoked by the K⁺ channel blocker 4-aminopyridine (4-AP), and the σ-1 receptor antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine (BD1047) blocked this phenomenon. The effects of SKF10047 on the evoked glutamate release were prevented by the chelating extracellular Ca²⁺ions and the vesicular transporter inhibitor bafilomycin A1. However, the glutamate transporter inhibitor DL-threo-β-benzyl-oxyaspartate did not have any effect on the action of SKF10047. SKF10047 decreased the depolarization-induced increase in the cytosolic free Ca²⁺ concentration ([Ca²⁺](C)), but did not alter 4-AP-mediated depolarization. Furthermore, the effects of SKF10047 on evoked glutamate release were prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking the ryanodine receptors or the mitochondrial Na⁺/Ca²⁺ exchange. In addition, conventional protein kinase C (PKC) inhibitors abolished the SKF10047 effect on 4-AP-evoked glutamate release. Western blot analyses showed that SKF10047 decreased the 4-AP-induced phosphorylation of PKC and PKCα. These results show that σ-1 receptor activation inhibits glutamate release from rat cortical nerve terminals. This effect is linked to a decrease in [Ca²⁺](C) caused by Ca²⁺ entry through presynaptic voltage-dependent Ca²⁺ channels and the suppression of the PKC signaling cascade.

Curcumin inhibits glutamate release in nerve terminals from rat prefrontal cortex: possible relevance to its antidepressant mechanism

There is abundant evidence suggesting the relevance of glutamate to depression and antidepressant mechanisms. Curcumin, a major active compound of Curcuma longa, has been reported to have the biological function of antidepressant. The aim of the present study was to investigate the effect of curcumin on endogenous glutamate release in nerve terminals of rat prefrontal cortex and the underlying mechanisms. The results showed that curcumin inhibited the release of glutamate that was evoked by exposing synaptosomes to the K(+) channel blocker 4-aminopyridine (4-AP). This phenomenon was blocked by the chelating the extracellular Ca(2+) ions, and by the vesicular transporter inhibitor bafilomycin A1, but was insensitive to the glutamate transporter inhibitor DL-threo-β-benzyl-oxyaspartate (DL-TBOA). Further experiments demonstrated that curcumin decreased depolarization-induced increase in [Ca(2+)](C), whereas it did not alter the resting membrane potential or 4-AP-mediated depolarization. Furthermore, the inhibitory effect of curcumin on evoked glutamate release was prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking intracellular Ca(2+) release or Na(+)/Ca(2+) exchange. These results suggest that curcumin inhibits evoked glutamate release from rat prefrontocortical synaptosomes by the suppression of presynaptic Ca(v)2.2 and Ca(v)2.1 channels. Additionally, we also found that the inhibitory effect of curcumin on 4-AP-evoked glutamate release was completely abolished by the clinically effective antidepressant fluoxetine. This suggests that curcumin and fluoxetine use a common intracellular mechanism to inhibit glutamate release from rat prefrontal cortex nerve terminals.

Luteolin inhibits the release of glutamate in rat cerebrocortical nerve terminals

The present study investigated the effect and possible mechanism of luteolin, a food-derived flavonoid, on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Luteolin inhibited the release of glutamate evoked by the K(+) channel blocker 4-aminopyridine (4-AP), and this phenomenon was concentration-dependent. The effect of luteolin on the evoked glutamate release was prevented by the chelation of the extracellular Ca(2+) ions and by the vesicular transporter inhibitor, but was insensitive to the glutamate transporter inhibitor. Luteolin decreased the 4-AP-induced increase in [Ca(2+)](C), whereas it did not alter 4-AP-mediated depolarization. Furthermore, the effect of luteolin on evoked glutamate release was abolished by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking the ryanodine receptors or the mitochondrial Na(+)/Ca(2+) exchange. In addition, the inhibitory effect of luteolin on evoked glutamate release was prevented by the mitogen-activated/extracellular signal-regulated kinase (MEK) inhibitors. Western blot analyses showed that luteolin decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and synapsin I, the main presynaptic target of ERK. Thus, it was concluded that luteolin inhibits glutamate release from rat cortical synaptosomes through the suppression of presynaptic voltage-dependent Ca(2+) entry and MEK/ERK signaling cascade.

HTDP-2, a new synthetic compound, inhibits glutamate release through reduction of voltage-dependent Ca²⁺ influx in rat cerebral cortex nerve terminals

AIM

The present study was aimed at investigating the effect of trans-6-(4-chlorobutyl)-5-hydroxy-4-(phenylthio)-1-tosyl-5,6-dihydropyridine-2(1H)-one (HTDP-2), a novel synthetic compound, on the release of endogenous glutamate in rat cerebrocortical nerve terminals (synaptosomes) and exploring the possible mechanism.

METHODS

The release of glutamate was evoked by the K⁺ channel blocker 4-aminopyridine (4-AP) and measured by an on-line enzyme-coupled fluorimetric assay. We also used a membrane potential-sensitive dye to assay nerve terminal excitability and depolarization, and a Ca²⁺ indicator, Fura-2-acetoxymethyl ester, to monitor cytosolic Ca²⁺ concentrations ([Ca²⁺](c)).

RESULTS

HTDP-2 inhibited the release of glutamate evoked by 4-AP in a concentration-dependent manner. Inhibition of glutamate release by HTDP-2 was prevented by the chelating intraterminal Ca²⁺ ions, and by the vesicular transporter inhibitor bafilomycin A1, but was insensitive to the glutamate transporter inhibitor DL-threo-β-benzyloxyaspartate. HTDP-2 did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization whereas it decreased the 4-AP-induced increase in [Ca²⁺](c). Furthermore, the inhibitory effect of HTDP-2 on the evoked glutamate release was abolished by the N-, and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC, but not by the ryanodine receptor blocker dantrolene, or the mitochondrial Na⁺/Ca²⁺ exchanger blocker CGP37157.

CONCLUSION

Based on these results, we suggest that, in rat cerebrocortical nerve terminals, HTDP-2 decreases voltage-dependent Ca²⁺ channel activity and, in so doing, inhibits the evoked glutamate release.

Idebenone inhibition of glutamate release from rat cerebral cortex nerve endings by suppression of voltage-dependent calcium influx and protein kinase A

The present study was aimed at investigating the effect and the possible mechanism of idebenone on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Idebenone inhibited the release of glutamate that was evoked by exposing synaptosomes to the K(+) channel blocker 4-aminopyridine (4-AP), and this phenomenon was concentration dependent. Inhibition of glutamate release by idebenone was prevented by chelating extracellular Ca(2+), or by the vesicular transporter inhibitor bafilomycin A1, but was insensitive to DL-threo-beta-benzyl-oxyaspartate, a glutamate transporter inhibitor. Idebenone decreased the depolarization-induced increase in the cytosolic free Ca(2+) concentration ([Ca(2+)](C)),whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. The inhibitory effect of idebenone on evoked glutamate release was prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking intracellular Ca(2+) release or Na(+)/Ca(2+) exchange. Furthermore, the idebenone effect on 4-AP-evoked Ca(2+) influx and glutamate release was completely abolished by the protein kinase A (PKA) inhibitors, H89 and KT5720. On the basis of these results, it was concluded that idebenone inhibits glutamate release from rat cortical synaptosomes and this effect is linked to a decrease in [Ca(2+)](C) contributed by Ca(2+) entry through presynaptic voltage-dependent Ca(2+) channels and to the suppression of PKA signaling cascade.

Inhibition of glutamate release by bupropion in rat cerebral cortex nerve terminals

Central glutamate neurotransmission has been postulated to play a role in pathophysiology of depression and in the mechanism of antidepressants. The present study was undertaken to elucidate the effect and the possible mechanism of bupropion, an atypical antidepressant, on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Result showed that bupropion exhibited a dose-dependent inhibition of 4-aminopyridine (4-AP)-evoked release of glutamate. The effect of bupropion on the evoked glutamate release was prevented by the chelating the intrasynaptosomal Ca(2+) ions, and by the vesicular transporter inhibitor, but was insensitive to the glutamate transporter inhibitor. Bupropion decreased depolarization-induced increase in [Ca(2+)](C), whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. The effect of bupropion on evoked glutamate release was abolished by the N-, P- and Q-type Ca(2+) channel blocker, but not by the ryanodine receptor blocker, or the mitochondrial Na(+)/Ca(2+) exchanger blocker. In addition, the inhibitory effect of bupropion on evoked glutamate release was prevented by the mitogen-activated/extracellular signal-regulated kinase kinase (MEK) inhibitors. Western blot analyses showed that bupropion significantly decreased the 4-AP-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2), and this effect also was blocked by MEK inhibitor. These results are the first to suggest that, in rat cerebrocortical nerve terminals, bupropion suppresses voltage-dependent Ca(2+) channel and MEK/ERK activity and in so doing inhibits evoked glutamate release. This finding may provide important information regarding the beneficial effects of bupropion in the brain.

Astaxanthin inhibits glutamate release in rat cerebral cortex nerve terminals via suppression of voltage-dependent Ca(2+) entry and mitogen-activated protein kinase signaling pathway

The purpose of this study was to examine the effect and mechanism of astaxanthin, a natural carotenoid, on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Results showed that astaxanthin exhibited a dose-dependent inhibition of 4-aminopyridine (4-AP)-evoked release of glutamate. The effect of astaxanthin on the evoked glutamate release was prevented by chelating the intrasynaptosomal Ca(2+) ions and by the vesicular transporter inhibitor, but was insensitive to the glutamate transporter inhibitor. Astaxanthin decreased depolarization-induced increase in [Ca(2+)](C), whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. The effect of astaxanthin on evoked glutamate release was abolished by the N-, P- and Q-type Ca(2+) channel blockers, but not by the ryanodine receptor blocker or the mitochondrial Na(+)/Ca(2+) exchanger blocker. In addition, the inhibitory effect of astaxanthin on evoked glutamate release was prevented by the mitogen-activated protein kinase (MAPK) inhibitors PD98059 and U0126. Western blot analyses showed that astaxanthin significantly decreased the 4-AP-induced phosphorylation of MAPK, and this effect was blocked by PD98059. On the basis of these results, it was concluded that astaxanthin inhibits glutamate release from rat cortical synaptosomes through the suppression of presynaptic voltage-dependent Ca(2+) entry and MAPK signaling cascade.

Presynaptic roles of intracellular Ca(2+) stores in signalling and exocytosis

The signalling roles of Ca(2+)(ic) (intracellular Ca(2+)) stores are well established in non-neuronal and neuronal cells. In neurons, although Ca(2+)(ic) stores have been assigned a pivotal role in postsynaptic responses to G(q)-coupled receptors, or secondarily to extracellular Ca(2+) influx, the functions of dynamic Ca(2+)(ic) stores in presynaptic terminals remain to be fully elucidated. In the present paper, we review some of the recent evidence supporting an involvement of Ca(2+)(ic) in presynaptic function, and discuss loci at which this source of Ca(2+) may impinge. Nerve terminal preparations provide good models for functionally examining putative Ca(2+)(ic) stores under physiological and pathophysiological stimulation paradigms, using Ca(2+)-dependent activation of resident protein kinases as sensors for fine changes in intracellular Ca(2+) levels. We conclude that intraterminal Ca(2+)(ic) stores may, directly or indirectly, enhance neurotransmitter release following nerve terminal depolarization and/or G-protein-coupled receptor activation. During conditions that prevail following neuronal ischaemia, increased glutamate release instigated by Ca(2+)(ic) store activation may thereby contribute to excitotoxicity and eventual synaptopathy.

Aripiprazole and its human metabolite OPC14857 reduce, through a presynaptic mechanism, glutamate release in rat prefrontal cortex: possible relevance to neuroprotective interventions in schizophrenia

Aripiprazole is a novel atypical antipsychotic drug with neuroprotective properties. As excessive glutamate release is now considered to be part of the pathophysiology of schizophrenia, the objective of this study was to use an in vitro assay system to investigate the effect of aripiprazole and its human metabolite OPC14857 on the release of endogenous glutamate from isolated nerve terminals (synaptosomes), freshly prepared from rat prefrontal cortex. Both aripiprazole and OPC13857 potently inhibited 4-aminopyridine (4-AP)-evoked glutamate release in a concentration-dependent manner. Inhibition of glutamate release by aripiprazole and OPC13857 was associated with a reduction of 4AP-evoked Na+ influx and depolarization, as well as downstream elevation of cytoplasmic free calcium concentration mediated via N- and P/Q-type voltage-dependent Ca2+ channels (VDCCs). Release induced by direct Ca2+ entry with Ca2+ ionophore (ionomycin) was unaffected by aripiprazole or OPC13857, indicating that the inhibitory effect of aripiprazole or OPC13857 is not due to directly interfering with the release process at some point subsequent to Ca2+ influx. In addition, the dopamine D2 receptor antagonist haloperidol and the 5-HT 1A receptor antagonist WAY100635 all effectively blocked the aripiprazole or OPC13857-mediated inhibition of 4-AP-evoked glutamate release. Moreover, aripiprazole or OPC13857 modulation of 4-AP-evoked glutamate release appears to involve a protein kinase A (PKA) signaling cascade, insofar as pretreatment of synaptosomes with the PKA inhibitor H89 suppressed the inhibitory effect of aripiprazole or OPC13857. Together, these results suggest that aripiprazole and its human metabolite OPC14857 inhibit glutamate release from rat prefrontocortical nerve terminals, likely by the activation of dopamine D2 and 5-HT 1A receptors, which subsequently results in the reduction of nerve terminal excitability and downstream VDCC activation through a signaling cascade involving PKA. These actions of aripiprazole may contribute to its neuroprotective effect in excitotoxic injury.

Facilitation of glutamate release from rat cerebrocortical glutamatergic nerve terminals (synaptosomes) by phosphatidylserine and phosphatidylcholine

Phosphatidylserine (PS) and phosphatidylcholine (PC) have been shown to enhance cognitive function. Considering that brain glutamatergic system is thought to participate in cognitive processing, our objective was to determine the effect of PS and PC on glutamate release from the nerve terminal (synaptosome) freshly isolated from rat cerebral cortex. Data showed that both PS and PC potently facilitate 4-aminopyridine (4-AP)-evoked Ca(2+)-dependent and Ca(2+)-independent glutamate release. Facilitation of glutamate release by PS or PC was associated with an increase of 4-AP-evoked depolarization and downstream elevation of cytoplasmic free calcium concentration ([Ca(2+)](c)). In addition, glutamate release elicited by direct Ca(2+)-entry with Ca(2+)-ionophore (ionomycin) was also facilitated by PS or PC. Furthermore, PS- or PC-mediated facilitation of 4-AP-evoked glutamate release was superseded or suppressed by protein kinase C (PKC) activator and inhibitor, respectively. Together, these results suggest that PS or PC effects a facilitation of glutamate exocytosis by increasing nerve terminal excitability and Ca(2+) influx into cerebrocortical nerve terminals through a signaling cascade involving PKC.

Regulation of cocaine- and amphetamine-regulated transcript mRNA expression by calcium-mediated signaling in GH3 cells

Cocaine- and amphetamine-regulated-transcript (CART) peptides are associated with multiple physiological processes, including, feeding, body weight, and the response to drugs of abuse. CART mRNA and peptide levels and the expression of the CART gene appears to be under the control of a number of extra- and intra-cellular factors including the transcription factor, cAMP response element binding protein (CREB). Similar to the effects of CART, Ca(2+) signaling leads to the phosphorylation of CREB and has been associated with both feeding and the actions of psychostimulants; therefore, we hypothesized that Ca(2+) may play a role in CART gene regulation. We used real-time PCR (rtPCR) and GH3 cells to examine the effect of ionomycin, which increases intracellular Ca(2+), on CART mRNA levels. Ionomycin increased CART mRNA in a dose- and time-dependent manner. The effect of ionomycin appeared transient as CART mRNA had returned to control levels 3 h following treatment. Calmidazolium and KN93, inhibitors of calmodulin and Ca(2+)-modulated protein (CaM) kinases respectively, attenuated the effect of ionomycin (10 microM) on CART mRNA levels suggesting a calmodulin-dependent mechanism. Western immunoblotting indicated that ionomycin increased phosphorylated cAMP response element binding protein (pCREB) levels and electrophoretic mobility shift assay/supershift assay using antibodies against pCREB demonstrated increased levels of a CART oligo/pCREB protein complex. Finally, we showed that injection of ionomycin into the rat nucleus accumbens increases CART mRNA levels. To our knowledge, this is the first study providing evidence that the CART gene is, in part, regulated by Ca(2+)/CaM/CREB-dependent cell signaling.