Glutamate release by an Na+ load and oxidative stress in nerve terminals: relevance to ischemia/reperfusion

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

A plethora of neurological disorders shares a final common deadly pathway known as excitotoxicity. Among these disorders, ischemic injury is a prominent cause of death and disability worldwide. Brain ischemia stems from cardiac arrest or stroke, both responsible for insufficient blood supply to the brain parenchyma. Glucose and oxygen deficiency disrupts oxidative phosphorylation, which results in energy depletion and ionic imbalance, followed by cell membrane depolarization, calcium (Ca²⁺) overload, and extracellular accumulation of excitatory amino acid glutamate. If tight physiological regulation fails to clear the surplus of this neurotransmitter, subsequent prolonged activation of glutamate receptors forms a vicious circle between elevated concentrations of intracellular Ca²⁺ ions and aberrant glutamate release, aggravating the effect of this ischemic pathway. The activation of downstream Ca²⁺-dependent enzymes has a catastrophic impact on nervous tissue leading to cell death, accompanied by the formation of free radicals, edema, and inflammation. After decades of “neuron-centric” approaches, recent research has also finally shed some light on the role of glial cells in neurological diseases. It is becoming more and more evident that neurons and glia depend on each other. Neuronal cells, astrocytes, microglia, NG2 glia, and oligodendrocytes all have their roles in what is known as glutamate excitotoxicity. However, who is the main contributor to the ischemic pathway, and who is the unsuspecting victim? In this review article, we summarize the so-far-revealed roles of cells in the central nervous system, with particular attention to glial cells in ischemia-induced glutamate excitotoxicity, its origins, and consequences.

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.

H₂S attenuates cognitive deficits through Akt1/JNK3 signaling pathway in ischemic stroke

Neuronal damage in the hippocampal formation which is more sensitive to ischemic stimulation and easily injured will cause severe learning and memory impairment. Therefore, inhibiting hippocampal neuron injuries is the main contributor for learning and memory impairment during cerebral ischemia. Hydrogen sulfide (H2S) is a new type of neurotransmitter that regulates the nervous, circulatory and immune systems as well as various adverse factors that can reduce cerebral vascular or brain parenchyma injury. During an ischemic stroke, H2S inhibits hippocampal neuronal damage, reducing learning and memory impairment. However, this molecular mechanism has not been elucidated clearly. In this study, we established four-vessel occlusion model in rats with cerebral ischemia. We found that NaHS (28 mmol/kg, intraperitoneally, for 7 days before ischemia), donor of H2S, significantly shortened the distance and time of loading onto the hidden platform in the positioning navigation process, decreased the latency in the space exploration process when cognitive testing with Morris water maze was performed during ischemic stroke in rats. NaHS also significantly shortened latency and reduced the number of errors in the platform diving experiment. The survival rate of neurons in the CA1 area of the hippocampus and the phosphorylation of Akt in the neurons were increased, the phosphorylation ASK1 and JNK3 were inhibited by NaHS. After an intracerebroventricular injection of LY294002 (inhibitor of PI3K/Akt, 10 μL, 100 nmol in 25% DMSO in PBS), the above effects of NaHS were attenuated. These findings suggest that H2S may improve the survival rate of hippocampal neurons and reduce the impairment of learning and memory by increasing the phosphorylation of Akt, inhibiting the phosphorylation of ASK1 and JNK3 in rats with induced ischemic stroke.

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.

Ionic dysregulations typical of ischemia provoke release of glycine and GABA by multiple mechanisms

Energy deprivation during ischemia causes dysregulations of ions, particularly sodium, potassium and calcium. Under these conditions, release of neurotransmitters is often enhanced and can occur by multiple mechanisms. The aim of this work was to characterize the modes of exit of glycine and GABA from nerve endings exposed to stimuli known to reproduce some of the ionic changes typical of ischemic conditions. Their approach was chosen instead of application of ischemic conditions because the release evoked during ischemia is mechanistically too heterogeneous. Mouse hippocampus and spinal cord synaptosomes, pre-labeled with [(3)H]glycine or [(3)H]GABA, were exposed in superfusion to 50 mM KCl or to 10 microM veratridine. The evoked overflows differed greatly between the two transmitters and between the two regions examined. Significant portions of the K(+)- and the veratridine-evoked overflows occurred by classical exocytosis. Carrier-mediated release of GABA, but not of glycine, was evoked by high K(+); GABA and, less so, glycine were released through transporter reversal by veratridine. External calcium-dependent overflows were only in part sensitive to omega-conotoxins; significant portions occurred following reversal of the plasmalemmal Na(+)/Ca(2+) exchanger. Finally, a relevant contribution to the overall transmitter overflows came from cytosolic calcium originating through the mitochondrial Na(+)/Ca(2+) exchanger. To conclude, ionic dysregulations typical of ischemia cause neurotransmitter release by heterogeneous mechanisms that differ depending on the transmitters and the CNS regions examined.

HDT-1, a new synthetic compound, inhibits glutamate release in rat cerebral cortex nerve terminals (synaptosomes)

AIM

Excessive glutamate release has been proposed to be involved in the pathogenesis of several neurological diseases. In this study, we investigated the effect of HDT-1 (3, 4, 4a, 5, 8, 8a-hexahydro-6,7-dimethyl-4a-(phenylsulfonyl)- 2-tosylisoquinolin-1(2H)-one), a novel synthetic compound, on glutamate release in rat cerebrocortical nerve terminals and explored the possible mechanism.

METHODS

The release of glutamate was evoked by the K+ channel blocker 4-aminopyridine (4-AP) or the high external [K+] and measured by one-line enzyme-coupled fluorometric assay. We also determined the loci at which HDT-1 impinges on cerebrocortical nerve terminals by using membrane potentialsensitive dye to assay nerve terminal excitability and depolarization, and Ca2+ indicator Fura-2 to monitor Ca2+ influx.

RESULTS

HDT-1 inhibited the release of glutamate evoked by 4-AP and KCl in a concentration-dependent manner. HDT-1 did not alter the resting synaptosomal membrane potential or 4-APevoked depolarization. Examination of the effect of HDT-1 on cytosolic [Ca2+] revealed that the diminution of glutamate release could be attributed to reduction in voltage-dependent Ca2+ influx. Consistent with this, the HDT-1-mediated inhibition of glutamate release was significantly prevented in synaptosomes pretreated with the N- and P/Q-type Ca2+ channel blocker omega-conotoxin MVIIC.

CONCLUSION

In rat cerebrocortical nerve terminals, HDT-1 inhibits glutamate release through a reduction of voltage-dependent Ca2+ channel activity and subsequent decrease of Ca2+ influx into nerve terminals, rather than any upstream effect on nerve terminal excitability.

Peroxiredoxin 2 overexpression protects cortical neuronal cultures from ischemic and oxidative injury but not glutamate excitotoxicity, whereas Cu/Zn superoxide dismutase 1 overexpression protects only against oxidative injury

We previously reported that peroxiredoxin 2 (PRDX2) and Cu/Zn superoxide dismutase 1 (SOD1) proteins are up-regulated in rat primary neuronal cultures following erythropoietin (EPO) preconditioning. In the present study, we have demonstrated that adenovirally mediated overexpression of PRDX2 in cortical neuronal cultures can protect neurons from in vitro ischemia (oxygen-glucose deprivation) and an oxidative insult (cumene hydroperoxide) but not glutamate excitotoxicity. We have also demonstrated that adenovirally mediated overexpression of SOD1 in cortical neuronal cultures protected neurons only against the oxidative insult. Interestingly, we did not detect up-regulation of PRDX2 or SOD1 protein in the rat hippocampus following exposure to either 3 min or 8 min of global cerebral ischemia. Further characterization of PRDX2's neuroprotective mechanisms may aid in the development of a neuroprotective therapy.

Hypertonic shrinking but not hypotonic swelling increases sodium concentration in rat brain synaptosomes

Neurotransmitter release is dependent on both calcium and sodium influx. Hypotonic swelling and hypertonic shrinking of neurons evokes calcium-independent exocytosis of neurotransmitters into the synaptic cleft. To date, there are not too much data available on relationship between extracellular osmolarity and sodium concentration in presynaptic endings. In the present study we investigated the effects of hypotonic swelling and hypertonic shrinking on sodium levels, as measured using fluorescent dyes SBFI-AM and Sodium Green in rat brain synaptosomes. Reduction of incubation medium osmolarity from 310 to 230 mOsm did not raise the intrasynaptosomal sodium concentration. An increase of osmolarity from 310 to 810 mOsm is accompanied by a dose-dependent elevation of sodium concentration from 8.1+/-0.5 to 46.5+/-2.8mM, respectively. This effect was insensitive to several channel inhibitors such as: tetrodotoxin, an inhibitor of voltage-gated sodium channels, bumetanide, an inhibitor of Na(+)/K(+)/2Cl(-) cotransport, gadolinium, an inhibitor of nonselective mechanosensitive channels, ruthenium red, an inhibitor of transient receptor potential channel and amiloride, an inhibitor of epithelial sodium channel/degenerin. Additionally, using the fluorescent dye BCECF-AM, we have shown that hypertonic shrinking caused a dose-dependent acidification of intrasynaptosomal cytosol, which suggests that the Na(+)/H(+) exchanger is not involved in the effect of increased osmolarity on cytosolic sodium levels. The increase in intrasynaptosomal sodium concentrations following increases in osmolarity is probably due to sodium influx through another sodium channels.

Neuro-bioenergetic concepts in cancer prevention and treatment

Cancer remains one of the most difficult and elusive disorders to prevent and treat, despite great efforts in research and treatment over the last 30 years. Researchers have tried to understand the pathogenesis of cancer by discovering the single cellular mechanism or pathway derived from a genetic mutation. There are limited efforts made toward discovering a unified concept of cancer. We propose a neuro-bioenergetic concept of cancer pathogenesis based on the central mechanism of cellular hyperexcitability via inducible overexpression of voltage-gated ion channels, ligand-gated channels and neurotransmitters. Exploration of this concept could lead to a better understanding of the cause of cancer as well as developing more effective and specific strategies toward cancer prevention and treatment.

Neuroprotective effect of the novel Na+/Ca2+ channel blocker NS-7 on rat retinal ganglion cells

PURPOSE

To investigate whether NS-7, 4-(4-fluorophenyl)-2-methyl-6-(5-piperidinopentyloxy) pyrimidine hydrochloride, a novel Na(+)/Ca(2+) channel blocker, can protect the rat retina subjected to ischemia-reperfusion insult.

METHODS

To evaluate the protective effect of NS-7 against retinal damage, the drug was administered before and after ischemia-reperfusion. Damage to the retina was assessed by measuring the thickness of the inner plexiform layer (IPL) and the outer nuclear layer (ONL) of each eye. In a subsequent experiment, electroretinographic (ERG) evaluation was also used.

RESULTS

In histopathologic evaluation, ischemia-reperfusion injury caused a significant reduction of IPL thickness (measured as the IPL/ONL ratio). In the NS-7-treated group, retinal damage was partially prevented by a concentration of 0.25 mg/kg per day. In the ERG evaluation, ischemia-reperfusion injury caused a reduction of A- and B-wave amplitudes. NS-7 treatment significantly prevented the reduction of the B wave at a concentration of 0.1 or 0.3 mg/kg, while the reduction of the A wave was not significantly affected.

CONCLUSIONS

NS-7 has neuroprotective effects against retinal damage resulting from subjection to ischemia. In addition, NS-7 can be used as an agent for treating acute ischemic retinopathy, including diseases associated with very high intraocular pressure, such as acute angle-closure glaucoma.

Endogenous glutamate contributes to the maintenance of glutathione level under oxidative stress in isolated nerve terminals

Exposure of isolated nerve terminals to hydrogen peroxide (25-500 microM) for 10 min produced a partially reversible decrease in the total and reduced glutathione level. No release and resynthesis of glutathione by the oxidant was involved in this effect. Loss of reduced glutathione was associated with elimination of H(2)O(2), which was very quick with >70% of the oxidant eliminated within 5 min. Recovery of both total and reduced glutathione was pronounced after 10 min when the majority of H(2)O(2) was eliminated. Previously we have reported that glutamate metabolism under oxidative stress contributes to the operation of the Krebs cycle, thus to the production of NAD(P)H [J. Neurosci. 20 (2000) 8972]. In the present study we addressed whether metabolism of endogenous glutamate plays a role in the maintenance of glutathione level in nerve terminals. Glutamine and beta-hydroxybutyrate (5mM), alternative metabolites in synaptosomes, were able to decrease the loss of total and reduced glutathione induced by hydrogen peroxide. Metabolic consumption of glutamate was reduced at the same time. In addition an increased demand on the glutathione system by the catalase inhibitor aminotriazole augmented the metabolic consumption of glutamate. It is concluded that under oxidative stress glutamate metabolism contributes to the maintenance of glutathione level, thus to the antioxidant capacity of nerve terminals.