Differential coupling of N- and P/Q-type calcium channels to glutamate exocytosis in the rat cerebral cortex

Albiflorin Decreases Glutamate Release from Rat Cerebral Cortex Nerve Terminals (Synaptosomes) through Depressing P/Q-Type Calcium Channels and Protein Kinase A Activity

The purpose of this study was to investigate whether and how albiflorin, a natural monoterpene glycoside, affects the release of glutamate, one of the most important neurotransmitters involved in neurotoxicity, from cerebrocortical nerve terminals (synaptosomes) in rats. The results showed that albiflorin reduced 4-aminopyridine (4-AP)-elicited glutamate release from synaptosomes, which was abrogated in the absence of extracellular Ca2+ or in the presence of the vesicular glutamate transporter inhibitor or a P/Q-type Ca2+ channel inhibitor, indicating a mechanism of action involving Ca2+-dependent depression of vesicular exocytotic glutamate release. Albiflorin failed to alter the increase in the fluorescence intensity of 3,3-diethylthiacarbocyanine iodide (DiSC3(5)), a membrane-potential-sensitive dye. In addition, the suppression of protein kinase A (PKA) abolished the effect of albiflorin on glutamate release. Albiflorin also reduced the phosphorylation of PKA and synaptosomal-associated protein of 25 kDa (SNAP-25) and synapsin I at PKA-specific residues, which correlated with decreased available synaptic vesicles. The results of transmission electron microscopy (TEM) also observed that albiflorin reduces the release competence of synaptic vesicles evoked by 4-AP in synaptosomes. In conclusion, by studying synaptosomally released glutamate, we suggested that albiflorin reduces vesicular exocytotic glutamate release by decreasing extracellular Ca2+ entry via P/Q-type Ca2+ channels and reducing PKA-mediated synapsin I and SNAP-25 phosphorylation.

Gypenoside XVII Reduces Synaptic Glutamate Release and Protects against Excitotoxic Injury in Rats

Excitotoxicity is a common pathological process in neurological diseases caused by excess glutamate. The purpose of this study was to evaluate the effect of gypenoside XVII (GP-17), a gypenoside monomer, on the glutamatergic system. In vitro, in rat cortical nerve terminals (synaptosomes), GP-17 dose-dependently decreased glutamate release with an IC50 value of 16 μM. The removal of extracellular Ca2+ or blockade of N-and P/Q-type Ca2+ channels and protein kinase A (PKA) abolished the inhibitory effect of GP-17 on glutamate release from cortical synaptosomes. GP-17 also significantly reduced the phosphorylation of PKA, SNAP-25, and synapsin I in cortical synaptosomes. In an in vivo rat model of glutamate excitotoxicity induced by kainic acid (KA), GP-17 pretreatment significantly prevented seizures and rescued neuronal cell injury and glutamate elevation in the cortex. GP-17 pretreatment decreased the expression levels of sodium-coupled neutral amino acid transporter 1, glutamate synthesis enzyme glutaminase and vesicular glutamate transporter 1 but increased the expression level of glutamate metabolism enzyme glutamate dehydrogenase in the cortex of KA-treated rats. In addition, the KA-induced alterations in the N-methyl-D-aspartate receptor subunits GluN2A and GluN2B in the cortex were prevented by GP-17 pretreatment. GP-17 also prevented the KA-induced decrease in cerebral blood flow and arginase II expression. These results suggest that (i) GP-17, through the suppression of N- and P/Q-type Ca2+ channels and consequent PKA-mediated SNAP-25 and synapsin I phosphorylation, reduces glutamate exocytosis from cortical synaptosomes; and (ii) GP-17 has a neuroprotective effect on KA-induced glutamate excitotoxicity in rats through regulating synaptic glutamate release and cerebral blood flow.

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.

Photons Induce Vesicular Exocytotic Release of Glutamate in a Power-Dependent Way

Increasing evidence indicates that photobiomodulation, based on tissue irradiation with photons in the red to near-infrared spectrum, may be an effective therapeutic approach to central nervous system disorders. Although nervous system functionality has been shown to be affected by photons in animal models, as well as in preliminary evidence in healthy subjects or in patients with neuropsychiatric disorders, the mechanisms involved in the photobiomodulation effects have not yet been clarified. We previously observed that photobiomodulation could stimulate glutamate release. Here, we investigate mechanisms potentially involved in the glutamate-releasing effect of photons from adult mouse cerebrocortical nerve terminals. We report evidence of photon ability to induce an exocytotic vesicular release of glutamate from the terminals of glutamatergic neurons in a power-dependent way. It can be hypothesized that photobiomodulation, depending on the potency, can release glutamate in a potentially neurotoxic or physiological range.

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.

Neuroprotective Role of the B Vitamins in the Modulation of the Central Glutamatergic Neurotransmission

BACKGROUND

Regulation of glutamate release is crucial for maintaining normal brain function, but excess glutamate release is implicated in many neuropathological conditions. Therefore, the minimum glutamate release from presynaptic nerve terminals is an important neuroprotective mechanism.

OBJECTIVE

In this mini-review, we analyze the three B vitamins, namely vitamin B2 (riboflavin), vitamin B6 (pyridoxine), and vitamin B12 (cyanocobalamin), that affect the 4-aminopyridine (4- AP)-evoked glutamate release from presynaptic nerve terminal in rat and discuss their neuroprotective role.

METHODS

In this study, the measurements include glutamate release, DiSC3(5), and Fura-2.

RESULTS

The riboflavin, pyridoxine, and cyanocobalamin produced significant inhibitory effects on 4-aminopyridine-evoked glutamate release from rat cerebrocortical nerve terminals (synaptosomes) in a dose-dependent relationship. These presynaptic inhibitory actions of glutamate release are attributed to inhibition of physiologic Ca2+-dependent vesicular exocytosis but not Ca2+-independent nonvesicular release. These effects also did not affect membrane excitability, while diminished cytosolic (Ca2+)c through a reduction of direct Ca2+ influx via Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels, rather than through indirect Ca2+induced Ca2+ release from ryanodine-sensitive intracellular stores. Furthermore, their effects were attenuated by GF109203X and Ro318220, two protein kinase C (PKC) inhibitors, suggesting suppression of PKC activity. Taken together, these results suggest that riboflavin, pyridoxine, and cyanocobalamin inhibit presynaptic vesicular glutamate release from rat cerebrocortical synaptosomes, through the depression Ca2+ influx via voltage- dependent Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels, and PKC signaling cascade.

CONCLUSION

Therefore, these B vitamins may reduce the strength of glutamatergic synaptic transmission and is of considerable importance as potential targets for therapeutic agents in glutamate- induced excitation-related diseases.

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.

Inhibition of Glutamate Release from Rat Cortical Nerve Terminals by Dehydrocorydaline, an Alkaloid from Corydalis yanhusuo

Excessive release of glutamate induces excitotoxicity and causes neuronal damage in several neurodegenerative diseases. Natural products have emerged as potential neuroprotective agents for preventing and treating neurological disorders. Dehydrocorydaline (DHC), an active alkaloid compound isolated from Corydalis yanhusuo, possesses neuroprotective capacity. The present study investigated the effect of DHC on glutamate release using a rat brain cortical synaptosome model. Our results indicate that DHC inhibited 4-aminopyridine (4-AP)-evoked glutamate release and elevated intrasynaptosomal calcium levels. The inhibitory effect of DHC on 4-AP-evoked glutamate release was prevented in the presence of the vesicular transporter inhibitor bafilomycin A1 and the N- and P/Q-type Ca²⁺ channel blocker ω-conotoxin MVIIC but not the intracellular inhibitor of Ca²⁺ release dantrolene or the mitochondrial Na⁺/Ca²⁺ exchanger inhibitor CGP37157. Moreover, the inhibitory effect of DHC on evoked glutamate release was prevented by the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) inhibitor PD98059. Western blotting data in synaptosomes also showed that DHC significantly decreased the level of ERK1/2 phosphorylation and synaptic vesicle-associated protein synapsin I, the main presynaptic target of ERK. Together, these results suggest that DHC inhibits presynaptic glutamate release from cerebrocortical synaptosomes by suppressing presynaptic voltage-dependent Ca²⁺ entry and the MAPK/ERK/synapsin I signaling pathway.

Eupafolin Suppresses P/Q-Type Ca2+ Channels to Inhibit Ca2+/ Calmodulin-Dependent Protein Kinase II and Glutamate Release at Rat Cerebrocortical Nerve Terminals

Eupafolin, a constituent of the aerial parts of Phyla nodiflora, has neuroprotective property. Because reducing the synaptic release of glutamate is crucial to achieving pharmacotherapeutic effects of neuroprotectants, we investigated the effect of eupafolin on glutamate release in rat cerebrocortical synaptosomes and explored the possible mechanism. We discovered that eupafolin depressed 4-aminopyridine (4-AP)-induced glutamate release, and this phenomenon was prevented in the absence of extracellular calcium. Eupafolin inhibition of glutamate release from synaptic vesicles was confirmed through measurement of the release of the fluorescent dye FM 1-43. Eupafolin decreased 4-AP-induced [Ca²⁺]_i elevation and had no effect on synaptosomal membrane potential. The inhibition of P/Q-type Ca²⁺ channels reduced the decrease in glutamate release that was caused by eupafolin, and docking data revealed that eupafolin interacted with P/Q-type Ca²⁺ channels. Additionally, the inhibition of calcium/calmodulin-dependent protein kinase II (CaMKII) prevented the effect of eupafolin on evoked glutamate release. Eupafolin also reduced the 4-AP-induced activation of CaMK II and the subsequent phosphorylation of synapsin I, which is the main presynaptic target of CaMKII. Therefore, eupafolin suppresses P/Q-type Ca²⁺ channels and thereby inhibits CaMKII/synapsin I pathways and the release of glutamate from rat cerebrocortical synaptosomes.

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.

Rosmarinic Acid, a Bioactive Phenolic Compound, Inhibits Glutamate Release from Rat Cerebrocortical Synaptosomes through GABAA Receptor Activation

Rosmarinic acid, a major component of rosemary, is a polyphenolic compound with potential neuroprotective effects. Asreducing the synaptic release of glutamate is crucial to achieving neuroprotectant's pharmacotherapeutic effects, the effect of rosmarinic acid on glutamate release was investigated in rat cerebrocortical nerve terminals (synaptosomes). Rosmarinic acid depressed the 4-aminopyridine (4-AP)-induced glutamate release in a concentration-dependent manner. The removal of extracellular calcium and the blockade of vesicular transporters prevented the inhibition of glutamate release by rosmarinic acid. Rosmarinic acid reduced 4-AP-induced intrasynaptosomal Ca2+ elevation. The inhibition of N-, P/Q-type Ca2+ channels and the calcium/calmodulin-dependent kinase II (CaMKII) prevented rosmarinic acid from having effects on glutamate release. Rosmarinic acid also reduced the 4-AP-induced activation of CaMKII and the subsequent phosphorylation of synapsin I, the main presynaptic target of CaMKII. In addition, immunocytochemistry confirmed the presence of GABAA receptors. GABAA receptor agonist and antagonist blocked the inhibitory effect of rosmarinic acid on 4-AP-evoked glutamate release. Docking data also revealed that rosmarinic acid formed a hydrogen bond with the amino acid residues of GABAA receptor. These results suggested that rosmarinic acid activates GABAA receptors in cerebrocortical synaptosomes to decrease Ca2+ influx and CaMKII/synapsin I pathway to inhibit the evoked glutamate release.

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.

Fingolimod inhibits glutamate release through activation of S1P1 receptors and the G protein βγ subunit-dependent pathway in rat cerebrocortical nerve terminals

Fingolimod, a sphingosine-1-phosphate (S1P) receptor modulator approved for treating multiple sclerosis, is reported to prevent excitotoxic insult. Because excessive glutamate release is a major cause of neuronal damage in various neurological disorders, the effect of fingolimod on glutamate release in rat cerebrocortical nerve terminals (synaptosomes) was investigated in the current study. Fingolimod decreased 4-aminopyridine (4-AP)-stimulated glutamate release and calcium concentration elevation. Fingolimod-mediated inhibition of 4-AP-induced glutamate release was dependent on extracellular calcium, persisted in the presence of the glutamate transporter inhibitor DL-TBOA or intracellular Ca²⁺-releasing inhibitors dantrolene and CGP37157, and was prevented by blocking vesicular transporters or N- and P/Q-type channels. Western blot and immunocytochemical analysis revealed the presence of S1P1 receptor proteins in presynaptic terminals. Fingolimod-mediated inhibition of 4-AP-induced glutamate release was also abolished by the sphingosine kinase inhibitor DMS, selective S1P1 receptor antagonist W146, Gi/o protein inhibitor pertussis toxin, and G protein βγ subunit inhibitor gallein; however, it was unaffected by the adenylyl cyclase inhibitor SQ22536, protein kinase A inhibitor H89, and phospholipase C inhibitor U73122. These data indicate that fingolimod decreases glutamate release from rat cerebrocortical synaptosomes by suppressing N- and P/Q-type Ca²⁺ channel activity; additionally, the activation of presynaptic S1P1 receptors and the G protein βγ subunit participates in achieving the effect.

Silymarin Inhibits Glutamate Release and Prevents against Kainic Acid-Induced Excitotoxic Injury in Rats

Silymarin, a polyphenoic flavonoid derived from the seeds of milk thistle (Silybum marianum), exhibits neuroprotective effects. In this study, we used a model of rat cerebrocortical synaptosomes to investigate whether silymarin affects the release of glutamate, an essential neurotransmitter involved in excitotoxicity. Its possible neuroprotective effect on a rat model of kainic acid (KA)-induced excitotoxicity was also investigated. In rat cortical synaptosomes, silymarin reduced glutamate release and calcium elevation evoked by the K⁺ channel blocker 4-aminopyridine but did not affect glutamate release caused by the Na⁺ channel activator veratridine or the synaptosomal membrane potential. Decreased glutamate release by silymarin was prevented by removal of extracellular calcium and blocking of N- and P/Q-type Ca²⁺ channel or extracellular signal-regulated kinase 1/2 (ERK1/2) but not by blocking of intracellular Ca²⁺ release. Immunoblotting assay results revealed that silymarin reduced 4-aminopyridine-induced phosphorylation of ERK1/2. Moreover, systemic treatment of rats with silymarin (50 or 100 mg/kg) 30 min before systemic KA (15 mg/kg) administration attenuated KA-induced seizures, glutamate concentration elevation, neuronal damage, glial activation, and heat shock protein 70 expression as well as upregulated KA-induced decrease in Akt phosphorylation in the rat hippocampus. Taken together, the present study demonstrated that silymarin depressed synaptosomal glutamate release by suppressing voltage-dependent Ca²⁺ entry and ERK1/2 activity and effectively prevented KA-induced in vivo excitotoxicity.

Gyroxin, a toxin from Crotalus durissus terrificus snake venom, induces a calcium dependent increase in glutamate release in mice brain cortical synaptosomes

Gyroxin is a thrombin-like toxin obtained from the venom of the South American rattlesnake, Crotalus durissus terrificus. Literature has reported "gyroxin syndrome" characterized, in mice, as series of aberrant motor behavior, known as barrel rotation, mainly after intraperitoneal administration. Despites several studies, a physiological mechanism of "gyroxin syndrome" are still not completely understood. In this context, alterations on the central nervous system (CNS), especially causing neurotoxic events, are pointed out as likely candidates. Then, we decided to investigate whether gyroxin induces alterations in glutamate release, one of the most important neurotransmitter involved in neurotoxicity. For that, we performed all experiments, in vitro, using a model of mice brain cortical synaptosomes. Notably, our results indicate that the administration of gyroxin on purified presynaptic brain cortical terminals resulted in an extracellular Ca²⁺- dependent raise in glutamate release. Indeed, our results also showed that gyroxin increases intrasynaptosomal calcium (Ca²⁺) levels through acting on voltage gated calcium channels (VGCC), specifically N and P/Q subtypes. Moreover, our data show that gyroxin increases exocytosis rate. Interestingly, these data suggest that gyroxin might induce neurotoxicity by increasing glutamate levels. However, future investigations are needed in order to elucidate the nature of the following events.

[1-(4-chloro-3-nitrobenzenesulfonyl)-1H-indol-3-yl]-methanol, an indole-3-carbinol derivative, inhibits glutamate release in rat cerebrocortical nerve terminals by suppressing the P/Q-type Ca2+ channels and Ca2+/calmodulin/protein kinase A pathway

Indole-3-carbinol (I3C), found in cruciferous vegetables, has been proposed to exhibit neuroprotective effects. This study aimed to investigate the effect of the I3C derivative [1(4-chloro-3-nitrobenzenesulfonyl)-1H-indol-3-yl]-methanol (CIM), which has superior pharmacokinetic properties to I3C, on glutamate release in rat cerebrocortical nerve terminals (synaptosomes). We observed that CIM dose-dependently inhibited glutamate release evoked by the potassium channel blocker 4-aminopyridine (4-AP). CIM-mediated inhibition of glutamate release was attributed to reduced exocytosis, as it correlated with the removal of extracellular calcium and blocking of the vesicular glutamate transporter but not the glutamate transporter. In addition, CIM decreased 4-AP-evoked intrasynaptosomal Ca²⁺ elevation; however, it did not alter the synaptosomal membrane potential. The inhibition of P/Q-typeCa²⁺ channels abolished the effect of CIM on 4-AP-evoked glutamate release, and the effect was not prevented by intracellular Ca²⁺ release inhibitors. Moreover, the molecular docking study showed that CIM exhibited the highest binding affinity with the P/Q-type Ca²⁺channels. Finally, the CIM-mediated inhibition of glutamate release was sensitive to calmodulin, adenylate cyclase (AC), and protein kinase A (PKA) inhibitors. Based on these results, we propose that CIM, through the direct suppression of P/Q-type Ca²⁺ channels, decreases Ca²⁺ influx and the activation of Ca²⁺/calmodulin/AC/PKA signaling, thereby inhibiting glutamate release. This finding is crucial for understanding the role of CIM in the central nervous system and for exploiting its potential in therapeutic interventions.

Asiatic acid, an active substance of Centella asiatica, presynaptically depresses glutamate release in the rat hippocampus

Inhibiting glutamate release can reduce neuronal excitability and is recognized as a key mechanism of anti-epileptic drugs. In this study, by using isolated nerve terminal (synaptosome) and slice preparations, we investigated the effect of asiatic acid, a triterpene isolated from Centella asiatica with antiepileptic activity, on glutamate release in the hippocampus of rats. In hippocampal synaptosomes, application of asiatic acid resulted in a concentration-dependent inhibition of 4-aminopyridine-evoked glutamate release. This inhibitory action was dependent on extracellular calcium, blocked by inhibiting the vesicular transporter, but was unaffected by inhibiting the glutamate transporter. In addition, asiatic acid decreased the 4-aminopyridine-induced increase in the intraterminal calcium and failed to alter the synaptosomal potential. Furthermore, the asiatic acid-mediated release inhibition was significantly suppressed by the N- and P/Q-type calcium channel inhibitor ω-conotoxin MVIIC or protein kinase C inhibitor GF109203X. Western blotting data in synaptosomes also revealed that asiatic acid reduced 4-aminopyridine-induced phosphorylation of protein kinase C. In hippocampal slices, asiatic acid decreased the frequencies of spontaneous excitatory postsynaptic currents without changing their amplitudes and glutamate-activated currents in CA3 pyramidal neurons. We also observed that asiatic acid significantly suppressed 4-aminopyridine-induced burst firing. These data suggest that, in rat hippocampal nerve terminals, asiatic acid attenuates the calcium influx via N- and P/Q-type calcium channels, subsequently suppressing protein kinase C activity and decreasing glutamate release.

Cycloheterophyllin Inhibits the Release of Glutamate from Nerve Terminals of the Rat Hippocampus

The effect of cycloheterophyllin, a prenylflavone isolated from Artocarpus heteophyllus, on glutamate release was studied in the rat hippocampus using synaptosome and slice preparations. In rat hippocampal synaptosomes, cycloheterophyllin inhibited 4-aminopyridine (4-AP)-evoked glutamate release and elevation of intrasynaptosomal calcium levels. The inhibitory effect of cycloheterophyllin on 4-AP-evoked glutamate release was prevented in the presence of the vesicular transporter inhibitor, the N- and P/Q-type calcium channel blocker, and the protein kinase C (PKC) inhibitor but was insensitive to the intracellular Ca²⁺ release inhibitors, the protein kinase A inhibitor, and the mitogen-activated/extracellular signal-regulated kinase inhibitor. Western blotting data in synaptosomes also showed that cycloheterophyllin significantly decreased the level of phosphorylation of PKC. In addition, cycloheterophyllin decreased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) without influencing the amplitude of sEPSCs and glutamate-activated currents in hippocampal slices, supporting a presynaptic action. Together, these results suggest that cycloheterophyllin inhibits presynaptic glutamate release by suppressing N- and P/Q-type calcium channel and PKC activity in the rat hippocampus.

Piperine-mediated suppression of voltage-dependent Ca2+ influx and glutamate release in rat hippocampal nerve terminals involves 5HT1A receptors and G protein βγ activation

Piperine is the crucial alkaloid component of black pepper (Piper nigrum Linn.) and has neuroprotective effects.

P/Q Type Calcium Channel Cav2.1 Defines a Unique Subset of Glomeruli in the Mouse Olfactory Bulb

Voltage-gated calcium (Cav) channels are a prerequisite for signal transmission at the first olfactory sensory neuron (OSN) synapse within the glomeruli of the main olfactory bulb (MOB). We showed previously that the N-type Cav channel subunit Cav2.2 is present in the vast majority of glomeruli and plays a central role in presynaptic transmitter release. Here, we identify a distinct subset of glomeruli in the MOB of adult mice that is characterized by expression of the P/Q-type channel subunit Cav2.1. Immunolocalization shows that Cav2.1+ glomeruli reside predominantly in the medial and dorsal MOB, and in the vicinity of the necklace glomerular region close to the accessory olfactory bulb. Few glomeruli are detected on the ventral and lateral MOB. Cav2.1 labeling in glomeruli colocalizes with the presynaptic marker vGlut2 in the axon terminals of OSNs. Electron microscopy shows that Cav2.1+ presynaptic boutons establish characteristic asymmetrical synapses with the dendrites of second-order neurons in the glomerular neuropil. Cav2.1+ glomeruli receive axonal input from OSNs that express molecules of canonical OSNs: olfactory marker protein, the ion channel Cnga2, and the phosphodiesterase Pde4a. In the main olfactory epithelium, Cav2.1 labels a distinct subpopulation of OSNs whose distribution mirrors the topography of the MOB glomeruli, that shows the same molecular signature, and is already present at birth. Together, these experiments identify a unique Cav2.1+ multiglomerular domain in the MOB that may form a previously unrecognized olfactory subsystem distinct from other groups of necklace glomeruli that rely on cGMP signaling mechanisms.

5-HT1B receptor agonist CGS12066 presynaptically inhibits glutamate release in rat hippocampus

CGS12066, a 5-hydroxytryptamine 1B (5-HT_1B) receptor agonist, has been reported to exhibit antidepressant activity. Considering that glutamatergic dysfunction is implicated in depression, the effect of CGS12066 on glutamate release in rat hippocampal nerve terminals and possible underlying mechanism were investigated. We observed that CGS12066 inhibited 4-aminopyridine (4-AP)-evoked glutamate release, and that a 5-HT_1B receptor antagonist blocked this inhibition. Western blot analysis and immunocytochemistry confirmed the presence of presynaptic 5-HT_1B receptor proteins. CGS12066-mediated inhibition of 4-AP-evoked glutamate release was completely abolished in the synaptosomes pretreated with inhibitors of Gi/Go-protein, adenylate cyclase (AC), and protein kinase A (PKA), namely pertussis toxin, MDL12330A, and H89, respectively. CGS12066 reduced the elevation of 4-AP-evoked intrasynaptosomal Ca²⁺ and cyclic AMP (cAMP) levels, but did not affect the synaptosomal membrane potential. Furthermore, in the presence of ω-conotoxin MVIIC, a N- and P/Q-type channel blocker, CGS12066-mediated inhibition of 4-AP-evoked glutamate release was markedly reduced; however, the intracellular Ca²⁺-release inhibitors dantrolene and CGP37157 did not affect the CGS12066 effect. Furthermore, CGS12066 reduced glutamatergic miniature excitatory postsynaptic current (mEPSC) frequency but did not affect mEPSC amplitude or glutamate-activated currents in hippocampal slices. Our data are the first to suggest that CGS12066 reduces AC/cAMP/PKA activation, through the activation of Gi/Go protein-coupled 5-HT_1B receptors present on hippocampal nerve terminals, subsequently reducing Ca²⁺ entry through voltage-dependent Ca²⁺ channels and reducing 4-AP-evoked glutamate release. This investigation into the role of 5-HT_1B receptors in glutamate release provides crucial information regarding the potential therapeutic role of 5-HT_1B receptors for treating depression.

Metabotropic glutamate 7 receptor agonist AMN082 inhibits glutamate release in rat cerebral cortex nerve terminal

AMN082 is a selective metabotropic glutamate mGlu₇ receptor agonist reported to exhibit antidepressant activity. Considering that excessive glutamate release is involved in the pathogenesis of depression, the effect of N,N'-dibenzyhydryl-ethane-1,2-diamine dihydrochloride (AMN082) on glutamate release in rat cerebrocortical nerve terminals and the possible underlying mechanism were investigated. In this study, we observed here that AMN082 inhibited 4-aminopyridine-evoked glutamate release and this phenomenon was blocked by the metabotropic glutamate mGlu₇ receptor antagonist MMPIP. Moreover, western blot analysis and immunocytochemistry confirmed the presence of presynaptic metabotropic glutamate mGlu₇ receptor proteins. The effect of AMN082 on the 4-aminopyridine-evoked release of glutamate was prevented by chelating the extracellular Ca²⁺ ions and the vesicular transporter inhibitor; however, the effect of AMN082 was unaffected by the glutamate transporter inhibitor. AMN082 reduced the elevation of 4-aminopyridine-evoked intrasynaptosomal Ca²⁺ concentration, but did not alter the synaptosomal membrane potential. In the presence of the Ca_v2.2 (N-type) and Ca_v2.1 (P/Q-type) channel blocker, the adenylate cyclase inhibitor, and the protein kinase A inhibitor, the action of AMN082 on the 4-aminopyridine-evoked glutamate release was markedly reduced. These results suggest that the activation of the metabotropic glutamate mGlu₇ receptors by AMN082 reduces adenylate cyclase/protein kinase A activation, which subsequently reduces the entry of Ca²⁺ through voltage-dependent Ca²⁺ channels and decreases evoked glutamate release. Additionally, fluoxetine, a clinically effective antidepressant, completely occluded the inhibitory effect of AMN082 on glutamate release, thus indicating the existence of a common intracellular mechanism for these two compounds to inhibit glutamate release from the cerebrocortical nerve terminals.

Lycopene depresses glutamate release through inhibition of voltage-dependent Ca2+ entry and protein kinase C in rat cerebrocortical nerve terminals

Lycopene is a natural dietary carotenoid that was reported to exhibit a neuroprotective profile. Considering that excitotoxicity and cell death induced by glutamate are involved in many brain disorders, the effect of lycopene on glutamate release in rat cerebrocortical nerve terminals and the possible mechanism involved in such effect was investigated. We observed here that lycopene inhibited 4-aminopyridine (4-AP)-evoked glutamate release and intrasynaptosomal Ca²⁺ concentration elevation. The inhibitory effect of lycopene on 4-AP-evoked glutamate release was markedly reduced in the presence of the Ca_v2.2 (N-type) and Ca_v2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC, but was insensitive to the intracellular Ca²⁺-release inhibitors dantrolene and CGP37157. Furthermore, in the presence of the protein kinase C inhibitors GF109203X and Go6976, the action of lycopene on evoked glutamate release was prevented. These results are the first to suggest that lycopene inhibits glutamate release from rat cortical synaptosomes by suppressing presynaptic Ca²⁺ entry and protein kinase C activity.

Capsaicin presynaptically inhibits glutamate release through the activation of TRPV1 and calcineurin in the hippocampus of rats

Capsaicin is the major ingredient in hot peppers of the plantCapsicum genuswith neuroprotective effects in several preclinical models; its effect on glutamate release has been investigated in the rat hippocampus using isolated nerve terminals (synaptosomes) and brain slices.

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.

Effect of dexmedetomidine priming on convulsion reaction induced by lidocaine

To study the effect of dexmedetomidine priming on convulsion reaction induced by lidocaine.The New Zealand white rabbits were applied for the mechanism study of dexmedetomidine priming for preventing convulsion reaction induced by lidocaine. The influence of dexmedetomidine priming with different doses on the time for convulsion occurrence and the duration time of convulsion induced by lidocaine, as well as contents of excitatory amino acids (aspartate [Asp], glutamate [Glu]) and inhibitory amino acids (glycine [Gly], γ-aminobutyric acid [GABA]) in the brain tissue were investigated.With 3 and 5 μg/kg dexmedetomidine priming, the occurrence times of convulsion were prolonged from 196 seconds to 349 and 414 seconds, respectively. With dexmedetomidine priming, the contents of excitatory amino acids (Asp, Glu) were much reduced at occurrence time of convulsion comparing with that without dexmedetomidine priming, while content of inhibitory amino acids Gly was much enhanced.The application of dexmedetomidine before local anesthetics can improve intoxication dose threshold of the lidocaine, delay occurrence of the convulsion, and helped for the recovery of convulsion induced by lidocaine. The positive effect of dexmedetomidine on preventing convulsion would owe to not only the inhibition of excitatory amino acids (Asp, Glu), but also the promotion of inhibitory amino acids Gly secretion.

Baicalein, a Constituent of Scutellaria baicalensis, Reduces Glutamate Release and Protects Neuronal Cell Against Kainic Acid-Induced Excitotoxicity in Rats

Interest in the health benefits of flavonoids, particularly their effects on neurodegenerative disease, is increasing. This study evaluated the role of baicalein, a flavonoid compound isolated from the traditional Chinese medicine Scutellaria baicalensis, in glutamate release and glutamate neurotoxicity in the rat hippocampus. In the rat hippocampal nerve terminals (synaptosomes), baicalein inhibits depolarization-induced glutamate release, and this phenomenon is prevented by chelating the extracellular Ca[Formula: see text] ions and blocking presynaptic Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel activity. In slice preparations, whole cell patch-clamp experiments revealed that baicalein reduced the frequency of miniature excitatory postsynaptic currents, without affecting their amplitude. In a kainic acid rat model, intraperitoneally administering baicalein to rats before the kainic acid intraperitoneal injection substantially attenuated kainic acid-induced neuronal cell death, c-Fos expression, and the activation of the mammalian target of rapamycin in the hippocampus. This study is the first to demonstrate that the natural compound baicalein inhibits glutamate release from hippocampal nerve terminals, and executes a protective action against kainic acid-induced excitotoxicity in vivo. The findings enhance the understanding of baicalein’s action in the brain, and suggest that this natural compound is valuable for treating brain disorders related to glutamate excitotoxicity.

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.

A mixed Ca2+ channel blocker, A-1264087, utilizes peripheral and spinal mechanisms to inhibit spinal nociceptive transmission in a rat model of neuropathic pain

N-, T- and P/Q-type voltage-gated Ca(2+) channels are critical for regulating neurotransmitter release and cellular excitability and have been implicated in mediating pathological nociception. A-1264087 is a novel state-dependent blocker of N-, T- and P/Q-type channels. In the present studies, A-1264087 blocked (IC50 = 1.6 μM) rat dorsal root ganglia N-type Ca(2+) in a state-dependent fashion. A-1264087 (1, 3 and 10 mg/kg po) dose-dependently reduced mechanical allodynia in rats with a spinal nerve ligation (SNL) injury. A-1264087 (4 mg/kg iv) inhibited both spontaneous and mechanically evoked activity of spinal wide dynamic range (WDR) neurons in SNL rats but had no effect in uninjured rats. The inhibitory effect on WDR neurons remained in spinally transected SNL rats. Injection of A-1264087 (10 nmol/0.5 μl) into the spinal cord reduced both spontaneous and evoked WDR activity in SNL rats. Application of A-1264087 (300 nmol/20 μl) into the receptive field on the hindpaw attenuated evoked but not spontaneous firing of WDR neurons. Using electrical stimulation, A-1264087 (4 mg/kg iv) inhibited Aδ- and C-fiber evoked responses and after-discharge of WDR neurons in SNL rats. These effects by A-1264087 were not present in uninjured rats. A-1264087 moderately attenuated WDR neuron windup in both uninjured and SNL rats. In summary, these results indicate that A-1264087 selectively inhibited spinal nociceptive transmission in sensitized states through both peripheral and central mechanisms.

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.

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.