Voltage-dependent potassium channels are involved in glutamate-induced apoptosis of rat hippocampal neurons

Astrocytes Exhibit a Protective Role in Neuronal Firing Patterns under Chemically Induced Seizures in Neuron-Astrocyte Co-Cultures

Astrocytes and neurons respond to each other by releasing transmitters, such as γ-aminobutyric acid (GABA) and glutamate, that modulate the synaptic transmission and electrochemical behavior of both cell types. Astrocytes also maintain neuronal homeostasis by clearing neurotransmitters from the extracellular space. These astrocytic actions are altered in diseases involving malfunction of neurons, e.g., in epilepsy, Alzheimer's disease, and Parkinson's disease. Convulsant drugs such as 4-aminopyridine (4-AP) and gabazine are commonly used to study epilepsy in vitro. In this study, we aim to assess the modulatory roles of astrocytes during epileptic-like conditions and in compensating drug-elicited hyperactivity. We plated rat cortical neurons and astrocytes with different ratios on microelectrode arrays, induced seizures with 4-AP and gabazine, and recorded the evoked neuronal activity. Our results indicated that astrocytes effectively counteracted the effect of 4-AP during stimulation. Gabazine, instead, induced neuronal hyperactivity and synchronicity in all cultures. Furthermore, our results showed that the response time to the drugs increased with an increasing number of astrocytes in the co-cultures. To the best of our knowledge, our study is the first that shows the critical modulatory role of astrocytes in 4-AP and gabazine-induced discharges and highlights the importance of considering different proportions of cells in the cultures.

Anticancer effects of ginsenoside Rk3 on non-small cell lung cancer cells: in vitro and in vivo

Ginsenoside Rk3 (Rk3) is present in the roots of processed Panax notoginseng herbs and it exerts anti-platelet aggregation, pro-immunogenic and cardioprotective effects. However, little is known regarding the anticancer activities of this compound, especially in lung cancer. This study was designed to investigate the anticancer effects of Rk3 on non-small cell lung cancer (NSCLC) cells and in an H460 xenograft tumor model. Our results showed that Rk3 reduced cell viability, inhibited both cell proliferation and colony formation, and induced G1 phase cell cycle arrest by downregulating the expression of cyclin D1 and CDK4 and upregulating the expression of P21. Rk3 also induced apoptosis in a concentration-dependent manner in H460 and A549 cells by Annexin V/PI staining, TUNEL assay and JC-1 staining, resulting in a change in the nuclear morphology. Moreover, Rk3 induced the activation of caspase-8, -9, and -3, promoted changes in mitochondrial membrane potential, decreased the expression of Bcl-2, increased the expression of Bax, and caused the release of cytochrome c, which indicated that the apoptosis-inducing effects of Rk3 were triggered via death receptor-mediated mitochondria-dependent pathways. Furthermore, Rk3 significantly inhibited the growth of H460 xenograft tumors without an obvious effect on the body weight of the treated mice. Histological analysis indicated that Rk3 inhibited tumor growth by altering the proliferation and morphology of tumor cells. In addition, we confirmed that Rk3 inhibited angiogenesis via CD34 staining and chick embryo chorioallantoic membrane (CAM) assay in vivo. Taken together, our findings revealed not only the anticancer effect of Rk3 on NSCLC cells but also a new promising therapeutic agent for human NSCLC.

Mitochondrial BK Channel Openers CGS7181 and CGS7184 Exhibit Cytotoxic Properties

Potassium channel openers (KCOs) have been shown to play a role in cytoprotection through the activation of mitochondrial potassium channels. Recently, in several reports, a number of data has been described as off-target actions for KCOs. In the present study, we investigated the effects of BK_Ca channel openers CGS7181, CGS7184, NS1619, and NS004 in neuronal cells. For the purpose of this research, we used a rat brain, the mouse hippocampal HT22 cells, and the human astrocytoma U-87 MG cell line. We showed that CGS7184 activated the mitochondrial BK_Ca (mitoBK_Ca) channel in single-channel recordings performed on astrocytoma mitoplasts. Moreover, when applied to the rat brain homogenate or isolated rat brain mitochondria, CGS7184 increased the oxygen consumption rate, and can thus be considered a potentially cytoprotective agent. However, experiments on intact neuronal HT22 cells revealed that both CGS7181 and CGS7184 induced HT22 cell death in a concentration- and time-dependent manner. By contrast, we did not observe cell death when NS1619 or NS004 was applied. CGS7184 toxicity was not abolished by BK_Ca channel inhibitors, suggesting that the observed effects were independent of a BK_Ca-type channel activity. CGS7184 treatment resulted in an increase of cytoplasmic Ca²⁺ concentration that likely involved efflux from internal calcium stores and the activation of calpains (calcium-dependent proteases). The cytotoxic effect of the channel opener was partially reversed by a calpain inhibitor. Our data show that KCOs under study not only activate mitoBK_Ca channels from brain tissue, but also induce cell death when used in cellular models.

ZD7288, a selective hyperpolarization-activated cyclic nucleotide-gated channel blocker, inhibits hippocampal synaptic plasticity

The selective hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD7288) blocks the induction of long-term potentiation in the perforant path–CA3 region in rat hippocampus in vivo. To explore the mechanisms underlying the action of ZD7288, we recorded excitatory postsynaptic potentials in perforant path–CA3 synapses in male Sprague-Dawley rats. We measured glutamate content in the hippocampus and in cultured hippocampal neurons using high performance liquid chromatography, and determined intracellular Ca²⁺ concentration [Ca²⁺]_i) using Fura-2. ZD7288 inhibited the induction and maintenance of long-term potentiation, and these effects were mirrored by the nonspecific HCN channel blocker cesium. ZD7288 also decreased glutamate release in hippocampal tissue and in cultured hippocampal neurons. Furthermore, ZD7288 attenuated glutamate-induced rises in [Ca²⁺]_i in a concentration-dependent manner and reversed 8-Br-cAMP-mediated facilitation of these glutamate-induced [Ca²⁺]_i rises. Our results suggest that ZD7288 inhibits hippocampal synaptic plasticity both glutamate release and resultant [Ca²⁺]_i increases in rat hippocampal neurons.

Piracetam ameliorated oxygen and glucose deprivation-induced injury in rat cortical neurons via inhibition of oxidative stress, excitatory amino acids release and P53/Bax

Our previous work has demonstrated that piracetam inhibited the decrease in amino acid content induced by chronic hypoperfusion, ameliorated the dysfunction of learning and memory in a hypoperfusion rat model, down-regulated P53, and BAX protein, facilitated the synaptic plasticity, and may be helpful in the treatment of vascular dementia. To explore the precise mechanism, the present study further evaluated effects of piracetam on Oxygen and glucose deprivation (OGD)-induced neuronal damage in rat primary cortical cells. The addition of piracetam to the cultured cells 12 h before OGD for 4 h significantly reduced neuronal damage as determined by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and lactate dehydrogenase release experiments. Piracetam also lowered the levels of malondialdehyde, nitrogen monoxidum, and xanthine oxidase which was increased in the OGD cells, and enhanced the activities of superoxide dismutase and glutathione peroxidase, which were decreased in the OGD cells. We also demonstrated that piracetam could decrease glutamate and aspartate release when cortical cells were subjected to OGD. Furthermore, Western blot study demonstrated that piracetam attenuated the increased expression of P53 and BAX protein in OGD cells. These observations demonstrated that piracetam reduced OGD-induced neuronal damage by inhibiting the oxidative stress and decreasing excitatory amino acids release and lowering P53/Bax protein expression in OGD cells.

Effect of methamphetamine on the microglial damage: role of potassium channel Kv1.3

Methamphetamine (Meth) abusing represents a major public health problem worldwide. Meth has long been known to induce neurotoxicity. However, the mechanism is still remained poorly understood. Growing evidences indicated that the voltage-gated potassium channels (Kv) were participated in neuronal damage and microglia function. With the whole cell patch clamp, we found that Meth significantly increased the outward K⁺ currents, therefore, we explored whether Kv1.3, one of the major K⁺ channels expressed in microglia, was involved in Meth-induced microglia damage. Our study showed that Meth significantly increased the cell viability in a dose dependent manner, while the Kv blocker, tetraethylamine (TEA), 4-Aminopyridine (4-AP) and Kv1.3 specific antagonist margatoxin (MgTx), prevented against the damage mediated by Meth. Interestingly, treatment of cells with Meth resulted in increasing expression of Kv1.3 rather than Kv1.5, at both mRNA and protein level, which is partially blocked by MgTx. Furthermore, Meth also stimulated a significant increased expression of IL-6 and TNF-α at protein level, which was significantly inhibited by MgTx. Taken together, these results demonstrated that Kv1.3 was involved in Meth-mediated microglial damage, providing the potential target for the development of therapeutic strategies for Meth abuse.

Voltage-gated potassium channels at the crossroads of neuronal function, ischemic tolerance, and neurodegeneration

Voltage-gated potassium (Kv) channels are widely expressed in the central and peripheral nervous system and are crucial mediators of neuronal excitability. Importantly, these channels also actively participate in cellular and molecular signaling pathways that regulate the life and death of neurons. Injury-mediated increased K(+) efflux through Kv2.1 channels promotes neuronal apoptosis, contributing to widespread neuronal loss in neurodegenerative disorders such as Alzheimer's disease and stroke. In contrast, some forms of neuronal activity can dramatically alter Kv2.1 channel phosphorylation levels and influence their localization. These changes are normally accompanied by modifications in channel voltage dependence, which may be neuroprotective within the context of ischemic injury. Kv1 and Kv7 channel dysfunction leads to neuronal hyperexcitability that critically contributes to the pathophysiology of human clinical disorders such as episodic ataxia and epilepsy. This review summarizes the neurotoxic, neuroprotective, and neuroregulatory roles of Kv channels and highlights the consequences of Kv channel dysfunction on neuronal physiology. The studies described in this review thus underscore the importance of normal Kv channel function in neurons and emphasize the therapeutic potential of targeting Kv channels in the treatment of a wide range of neurological diseases.

Distinct modifications in Kv2.1 channel via chemokine receptor CXCR4 regulate neuronal survival-death dynamics

The chemokine stromal cell-derived factor-1α (SDF-1α) has multiple effects on neuronal activity, survival, and death under conditions that generate a proinflammatory microenvironment within the brain, via signaling through C-X-C-type chemokine receptor 4 (CXCR4), although the underlying cellular/molecular mechanisms are unclear. Using rat hippocampal neurons, we investigated distinct modifications in the voltage-gated K⁺ (Kv) channel Kv2.1 in response to short- and long-term SDF-1α/CXCR4-mediated signaling as an underlying mechanism for CXCR4-dependent regulation of neuronal survival and death. Acute exposure of neurons to SDF-1α led to dynamic dephosphorylation and altered localization of Kv2.1 channel, resulting in enhanced voltage-dependent activation of Kv2.1-based delayed-rectifier Kv currents (I(DR)). These changes were dependent on CXCR4- and/or NMDA receptor-mediated activation of calcineurin and provide neuroprotection. However, prolonged SDF-1α treatment leads to CXCR4-mediated activation of p38 mitogen-activated protein kinase, resulting in phosphorylation of Kv2.1 at S800 and enhanced surface trafficking of the channel protein, resulting in increased I(DR)/Kv2.1 current density. This, in combination with sustained dephosphorylation-induced enhancement of the voltage-dependent activation of I(DR)/Kv2.1, predisposed neurons to excessive K⁺ efflux, a vital step for the neuronal apoptotic program. Such apoptotic death was dependent on CXCR4 and Kv2.1 function and was absent in cells expressing the Kv2.1-S800A mutant channel. Furthermore, similar modifications in Kv2.1 and CXCR4/Kv2.1-dependent apoptosis were observed following treatment of neurons with the human immunodeficiency virus-1 (HIV-1) glycoprotein gp120. Therefore, distinct modifications in Kv2.1 in response to short- and long-term CXCR4-mediated signaling could provide a basis for neuroprotection or apoptosis in neuropathologies, such as neuroinflammation, stroke, brain tumors, and HIV-associated neurodegeneration.

Characterization and photodynamic activity of a new phthalocyanine nanoparticles

Nanoparticles of a hydrophobic photosensitizer, tetrakis (3-trifluoromethylphenoxy) zinc phthalocyanine (FPcZn) have been synthesized by using a simple reprecipitation technique. The resulting drug nanoparticles (FPcZn-NP) were spherical, highly monodispersed and stable in aqueous system, without an additional stabilizer. Comparative studies with FPcZn-NP and FPcZn indicated that after the formation of nanoparticles, FPcZn-NP maintained the efficiency of (1)O(2) generation. Further more, the in vitro studies demonstrated that such nanoparticles can be efficiently taken up by Hela cells, which might be resulted to cell death by light irradiation. These properties could make the FPcZn-NP to be a promising candidate in clinical photodynamic therapy.

Delta-opioid receptor activation prolongs respiratory motor output during oxygen-glucose deprivation in neonatal rat spinal cord in vitro

Delta opioid receptor (DOR) activation protects the adult mammalian brain during oxygen-glucose deprivation (OGD), but it is not known whether neonatal spinal motor circuits are also protected. Also, it is unclear whether the timing of spinal DOR activation relative to spinal OGD is important for neuroprotection. Thus, a split-bath in vitro neonatal rat brainstem/spinal cord preparation was used to record spontaneous respiratory motor output from cervical (C4-C5) and thoracic (T5-T6) ventral spinal roots while exposing only the spinal cord to OGD solution (0 mM glucose, bubbled with 95% N(2)/5% CO(2)) or DOR agonist drugs (DADLE, DPDPE). Spinal OGD solution application caused respiratory motor output frequency and amplitude to decrease until all activity was abolished (i.e. end-point times) after 25.9±1.4 min (cervical) and 25.2±1.4 min (thoracic). Spinal DOR activation via DPDPE (1.0 μM) prior-to and during spinal OGD increased cervical and thoracic end-point times to 35-48 min. Spinal DADLE or DPDPE (1.0 μM) application 15 min following spinal OGD onset increased cervical and thoracic end-point times to 36-45 min. Brief spinal DPDPE (1.0 μM) application for 10 min at 25 min before spinal OGD onset increased cervical and thoracic end-point times to 41-46 min. Overall, the selective DOR agonist, DPDPE, was more effective at increasing end-point times than DADLE. Naltrindole (DOR antagonist; 10 μM) pretreatment blocked DPDPE-dependent increase in end-point times, suggesting that DOR activation was required. Spinal naloxone (1.0 μM) application before and during spinal OGD also increased end-point times to 31-33 min, but end-point times were not altered by Mu opioid receptor (MOR) activation or DOR activation/MOR blockade, indicating that there are complex interactions between OGD and opioid signaling pathways. These data suggest DOR activation before, during, and after spinal OGD protects central motor networks and may provide neuroprotection during unpredictable perinatal ischemic events.

Purification and neuroprotective effects of polysaccharides from Opuntia Milpa Alta in cultured cortical neurons

Opuntia is a traditional plant from China with medicinal applications. In this experiment, polysaccharides from Opuntia Milpa Alta (MAPs) were analyzed using gas chromatograph-mass spectrometer (GC-MS) method and result showed that MAPs contained mannose (6.37%), rhamnose (14.94%), xylose (1.99%), arabinose (24.07%), galactose (38.25%), ribose (2.63%) and glucose (11.48%). The neuroprotective effects of MAPs were evaluated at the mechanistic level in vitro models of cerebral ischemic injury. In vitro oxygen/glucose deprivation (OGD) model, MAPs (0.5 μg/ml, 5 μg/ml, 50 μg/ml) effectively increased cell viability by methyl thiazolyl tetrazolium (MTT) assay, inhibited cell cytotoxicity by lactate dehydrogenase (LDH) assay, reduced neuronal cell death, suppressed the production of intracellular reactive oxygen species (ROS), decreased of intracellular free Ca(2+) concentrations ([Ca(2+)](i)), and reduced extracellular glutamate level. Therefore, MAPs might prevent intracellular calcium overload and decreased glutamate excitotoxicity, both of which can cause neuronal injury and death in vitro models of cerebral ischemic injury.

Impact of global cerebral ischemia on K+ channel expression and membrane properties of glial cells in the rat hippocampus

Astrocytes and NG2 glia respond to CNS injury by the formation of a glial scar. Since the changes in K(+) currents in astrocytes and NG2 glia that accompany glial scar formation might influence tissue outcome by altering K(+) ion homeostasis, we aimed to characterize the changes in K(+) currents in hippocampal astrocytes and NG2 glia during an extended time window of reperfusion after ischemic injury. Global cerebral ischemia was induced in adult rats by bilateral, 15-min common carotid artery occlusion combined with low-pressure oxygen ventilation. Using the patch-clamp technique, we investigated the membrane properties of hippocampal astrocytes and NG2 glia in situ 2 hours, 6 hours, 1 day, 3 days, 7 days or 5 weeks after ischemia. Astrocytes in the CA1 region of the hippocampus progressively depolarized starting 3 days after ischemia, which coincided with decreased Kir4.1 protein expression in the gliotic tissue. Other K(+) channels described previously in astrocytes, such as Kir2.1, Kir5.1 and TREK1, did not show any changes in their protein content in the hippocampus after ischemia; however, their expression switched from neurons to reactive astrocytes, as visualized by immunohistochemistry. NG2 glia displayed increased input resistance, decreased membrane capacitance, increased delayed outwardly rectifying and A-type K(+) currents and decreased inward K(+) currents 3 days after ischemia, accompanied by their proliferation. Our results show that the membrane properties of astrocytes after ischemia undergo complex alterations, which might profoundly influence the maintenance of K(+) homeostasis in the damaged tissue, while NG2 glia display membrane currents typical of proliferating cells.

Prolonged Abeta treatment leads to impairment in the ability of primary cortical neurons to maintain K+ and Ca2+ homeostasis

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disease, characterised by the formation of insoluble amyloidogenic plaques and neurofibrillary tangles. Beta amyloid (Abeta) peptide is one of the main constituents in Abeta plaques, and is thought to be a primary causative agent in AD. Neurons are likely to be exposed to chronic, sublethal doses of Abeta over an extended time during the pathogenesis of AD, however most studies published to date using in vitro models have focussed on acute studies. To experimentally model the progressive pathogenesis of AD, we exposed primary cortical neurons daily to 1 muM of Abeta1-40 over 7 days and compared their survival with age-similar untreated cells. We also investigated whether chronic Abeta exposure affects neuronal susceptibility to the subsequent acute excitotoxicity induced by 10 muM glutamate and assessed how Ca2+ and K+ homeostasis were affected by either treatment.

RESULTS

We show that continuous exposure to 1 muM Abeta1-40 for seven days decreased survival of cultured cortical neurons by 20%. This decrease in survival correlated with increased K+ efflux from the cells. One day treatment with 1 muM Abeta followed by glutamate led to a substantially higher K+ efflux than in the age-similar untreated control. This difference further increased with the duration of the treatment. K+ efflux also remained higher in Abeta treated cells 20 min after glutamate application leading to 2.8-fold higher total K+ effluxed from the cells compared to controls. Ca2+ uptake was significantly higher only after prolonged Abeta treatment with 2.5-fold increase in total Ca2+ uptake over 20 min post glutamate application after six days of Abeta treatment or longer (P < 0.05).

CONCLUSIONS

Our data suggest that long term exposure to Abeta is detrimental because it reduces the ability of cortical neurons to maintain K+ and Ca2+ homeostasis in response to glutamate challenge, a response that might underlie the early symptoms of AD. The observed inability to maintain K+ homeostasis might furthermore be useful in future studies as an early indicator of pathological changes in response to Abeta.

Ion transport in a human lens epithelial cell line exposed to hyposmotic and apoptotic stress

Membrane transport changes in human lens epithelial (HLE-B3) cells under hyposmotic and apoptotic stress were compared. Cell potassium content, K(i), uptake of the K congener rubidium, Rb(i), and water content were measured after hyposmotic stress induced by hypotonicity, and apoptotic stress by the protein-kinase inhibitor staurosporine (STP). Cell water increased in hyposmotic (150 mOsm) as compared to isosmotic (300 mOsm) balanced salt solution (BSS) by >2-fold at 5 min and decreased within 15 min to baseline values accompanied by a 40% K(i) loss commensurate with cell swelling and subsequent cell shrinkage likely due to regulatory volume decrease (RVD). Loss of K(i), and accompanying water, and Rb(i) uptake in hyposmotic BSS were prevented by clotrimazole (CTZ) suggesting water shifts associated with K and Rb flux via intermediate conductance K (IK) channels, also detected at the mRNA and protein level. In contrast, 2 h after 2 microM STP exposure, the cells lost approximately 40% water and approximately 60% K(i), respectively, consistent with apoptotic volume decrease (AVD). Indeed, water and K(i) loss was at least fivefold greater after hyposmotic than after apoptotic stress. High extracellular K and 2 mM 4-aminopyridine (4-AP) but not CTZ significantly reduced apoptosis. Annexin labeling phosphatidylserine (PS) at 15 min suggested loss of lipid asymmetry. Quantitative PCR revealed significant IK channel expression during prolonged hyposmotic stress. Results suggest in HLE-B3 cells, IK channels likely partook in and were down regulated after RVD, whereas pro-apoptotic STP-activation of 4-AP-sensitive voltage-gated K channels preceded or accompanied PS externalization before subsequent apoptosis.

Polycomb group proteins as epigenetic mediators of neuroprotection in ischemic tolerance

Exposing the brain to sublethal ischemia affects the response to a subsequent, otherwise injurious ischemia, resulting in transcriptional suppression and neuroprotection, a response called ischemic tolerance. Here, we show that the proteomic signature of the ischemic-tolerant brain is characterized by increased abundance of transcriptional repressors, particularly polycomb group (PcG) proteins. Knocking down PcG proteins precluded the induction of ischemic tolerance, whereas in an in vitro model, overexpressing the PcG proteins SCMH1 or BMI1 induced tolerance to ischemia without preconditioning. We found that PcG proteins are associated with the promoter regions of genes encoding two potassium channel proteins that show decreased abundance in ischemic-tolerant brains. Furthermore, PcG proteins decreased potassium currents in cultured neuronal cells, and knocking down potassium channels elicited tolerance without preconditioning. These findings reveal a previously unknown mechanism of neuroprotection that involves gene repressors of the PcG family.

Involvement of the 4-aminopyridine-sensitive transient A-type K+ current in macrophage-induced neuronal injury

Through their capacity to secrete, upon activation, a variety of bioactive molecules, brain macrophages (and resident microglia) play an important role in brain immune and inflammatory responses. To test our hypothesis that activated macrophages induce neuronal injury by enhancing neuronal outward K(+) current, we studied the effects of lipopolysaccharide (LPS)-stimulated human monocyte-derived macrophage (MDM) on neuronal transient A-type K(+) current (I(A)) and resultant neuronal injury in primary rat hippocampal neuronal cultures. Bath application of LPS-stimulated MDM-conditioned media (MCM+) enhanced neuronal I(A) in a concentration-dependent manner. Non-stimulated MCM (MCM-) failed to alter I(A). The enhancement of neuronal I(A) was recapitulated in neurons co-cultured with macrophages. The link of MCM(+)-induced enhancement of I(A) to MCM(+)-associated neuronal injury, as detected by propidium iodide and 4'',6-diamidino-2-phenylindol staining (DAPI) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay, was demonstrated by experimental results showing that addition of I(A) blocker 4-aminopyridine to the cultures protected hippocampal neurons from MCM(+)-induced neuronal injury. Further investigation revealed that glutamate was involved in MCM(+)-induced enhancement of neuronal I(A). These results suggest that during brain inflammation macrophages (and microglia) might mediate neuronal injury via enhancement of neuronal I(A), and that neuronal K(v) channel might be a potential target for the development of therapeutic strategies for some neurodegenerative disorders by which immune and inflammatory responses are believed to be involved in the pathogenesis.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Contribution of Kv channel subunits to glutamate-induced apoptosis in cultured rat hippocampal neurons

Potassium channel dysfunction has been implicated in apoptosis in many pathological conditions. However, which Kv channel subunit is involved in glutamate-induced apoptosis remains unknown. In this study, the contributions of nine Kv alpha and three Kv beta subunits to glutamate-induced hippocampal neuronal apoptosis were investigated. Results showed that neuronal apoptosis was not obvious with 12 hr incubation of glutamate but increased markedly after 18 hr, which was attenuated by the Kv channel blocker TEA. Among all the Kv subunits investigated, gene and protein expression of Kv2.1 increased significantly before the appearance of neuronal apoptosis, whereas the Kv1.1 mRNA level decreased quickly, and protein expression was reduced gradually after the insult. Seven other Kv alpha subunits and three Kv beta subunits were not obviously affected over time. In addition, Kv1.1 overexpression could reduce glutamate-induced hippocampal neuronal apoptosis. Therefore, the alterations of Kv1.1 and Kv2.1 might contribute to glutamate-induced toxicity in hippocampal neurons. These findings suggest that these two Kv channel subunits may represent potential therapeutic targets for neuropathological conditions in which glutamate-induced toxicity is thought to contribute to neuronal dysfunction.

Regulation of intrinsic excitability in hippocampal neurons by activity-dependent modulation of the KV2.1 potassium channel

KV2.1 is the prominent somatodendritic sustained or delayed rectifier voltage-gated potassium (KV) channel in mammalian central neurons, and is a target for activity-dependent modulation via calcineurin-dependent dephosphorylation. Using hanatoxin-mediated block of KV2.1 we show that, in cultured rat hippocampal neurons, glutamate stimulation leads to significant hyperpolarizing shifts in the voltage-dependent activation and inactivation gating properties of the KV2.1-component of delayed rectifier K+ (IK) currents. In computer models of hippocampal neurons, these glutamate- stimulated shifts in the gating of the KV2.1-component of IK lead to a dramatic suppression of action potential firing frequency. Current-clamp experiments in cultured rat hippocampal neurons showed glutamate stimulation induced a similar suppression of neuronal firing frequency. Membrane depolarization also resulted in similar hyperpolarizing shifts in the voltage-dependent gating properties of neuronal IK currents, and suppression of neuronal firing. The glutamate-induced effects on neuronal firing were eliminated by hanatoxin, but not by dendrotoxin-K, a blocker of KV1.1-containing channels. These studies together demonstrate a specific contribution of modulation of KV2.1 channels in the activity-dependent regulation of intrinsic neuronal excitability.

DDPH ameliorated oxygen and glucose deprivation-induced injury in rat hippocampal neurons via interrupting Ca2+ overload and glutamate release

Our previous work has demonstrated that DDPH (1-(2, 6-dimethylphenoxy)-2-(3, 4-dimethoxyphenylethylamino) propane hydrochloride), a competitive alpha(1)-adrenoceptor antagonist, could improve cognitive deficits, reduce histopathological damage and facilitate synaptic plasticity in vivo possibly via increasing NR2B (NMDA receptor 2B) expression and antioxidation of DDPH itself. The present study further evaluated effects of DDPH on OGD (Oxygen and glucose deprivation)-induced neuronal damage in rat primary hippocampal cells. The addition of DDPH to the cultured cells 12 h before OGD for 4 h significantly reduced neuronal damage as determined by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and LDH (lactate dehydrogenase) release experiments. The effects of DDPH on intracellular calcium concentration were explored by Fura-2 based calcium imaging techniques and results showed that DDPH at the dosages of 5 microM and 10 microM suppressed the increase of intracellular calcium ([Ca(2+)](i)) stimulated by 50 mM KCl in Ca(2+)-containing extracellular solutions. However, DDPH couldn't suppress the increase of [Ca(2+)](i) induced by both 50 microM glutamate in Ca(2+)-containing extracellular solutions and 20 microM ATP (Adenosine Triphosphate) in Ca(2+)-free solution. These results indicated that DDPH prevented [Ca(2+)](i) overload in hippocampal neurons by blocking Ca(2+) influx (voltage-dependent calcium channel) but not Ca(2+) mobilization from the intracellular Ca(2+) store in endoplasm reticulum (ER). We also demonstrated that DDPH could decrease glutamate release when hippocampal cells were subjected to OGD. These observations demonstrated that DDPH protected hippocampal neurons against OGD-induced damage by preventing the Ca(2+) influx and decreasing glutamate release.

Protective effects of TREK-1 against oxidative injury induced by SNP and H2O2

AIM

TREK-1 (TWIK-related K+ channel-1) is a 2-pore-domain K+ channel subtype. The present study investigated the role of TREK-1 in cell death induced by oxidative stress.

METHODS

The cell viability of wild-type Chinese hamster ovary (CHO) and TREK-1-transfected CHO cells (TREK-1/CHO cells) was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in the presence of sodium nitroprusside (SNP) or hydrogen peroxide (H2O2). Apoptosis of wild-type CHO and TREK-1/CHO cells was detected using Hoechst33342 staining.

RESULTS

Both SNP and H2O2 caused dose- and time-dependent growth inhibition of wild-type CHO and TREK-1/ CHO cells. Following a 12 h exposure to SNP, the 50% inhibition (IC(50)) values for wild-type CHO and TREK-1/CHO cells were calculated as 0.69 mmol/L and 1.14 mmol/L, respectively. The IC(50) values were 0.07 mmol/L and 0.09 mmol/L in H2O2-treated wild-type CHO and TREK-1/CHO cells, respectively, following 12 h exposure to H2O2. Moreover, SNP/H2O2 induced less apoptosis in TREK-1/ CHO cells than that in wild-type CHO cells (P<0.05).

CONCLUSION

The results demonstrated that TREK-1 played a protective role against oxidative injury.

Curcumin induces apoptosis through mitochondrial hyperpolarization and mtDNA damage in human hepatoma G2 cells

Curcumin, a major pigment of turmeric, is a natural antioxidant possessing a variety of pharmacological activities and therapeutic properties. But its mechanisms are unknown. In our previous study, we found that a 2-h exposure to curcumin induced DNA damage to both the mitochondrial DNA (mtDNA) and the nuclear DNA (nDNA) in HepG2 cells and that mtDNA damage was more extensive than nDNA damage. Therefore, experiments were initiated to evaluate the role of mtDNA damage in curcumin-induced apoptosis. The results demonstrated that HepG2 cells challenged with curcumin for 1 h showed a transient elevation of the mitochondrial membrane potential (DeltaPsim), followed by cytochrome c release into the cytosol and disruption of DeltaPsim after 6 h exposure to curcumin. Apoptosis was detected by Hoechst 33342 and annexin V/PI assay after 10 h treatment. Interestingly, the expression of Bcl-2 remained unchanged. A resistance to apoptosis for the corresponding rho0 counterparts confirmed a critical dependency for mitochondria during the induction of apoptosis in HepG2 cells mediated by curcumin. The effects of PEG-SOD in protecting against curcumin-induced cytotoxicity suggest that curcumin-induced cytotoxicity is directly dependent on superoxide anion O2- production. These data suggest that mitochondrial hyperpolarization is a prerequisite for curcumin-induced apoptosis and that mtDNA damage is the initial event triggering a chain of events leading to apoptosis in HepG2 cells.

Cell shrinkage and monovalent cation fluxes: role in apoptosis

The loss of cell volume or cell shrinkage has been a morphological hallmark of the programmed cell death process known as apoptosis. This isotonic loss of cell volume has recently been term apoptotic volume decrease or AVD to distinguish it from inherent volume regulatory responses that occurs in cells under anisotonic conditions. Recent studies examining the intracellular signaling pathways that result in this unique cellular characteristic have determined that a fundamental movement of ions, particularly monovalent ions, underlie the AVD process and plays an important role on controlling the cell death process. An efflux of intracellular potassium was shown to be a critical aspect of the AVD process, as preventing this ion loss could protect cells from apoptosis. However, potassium plays a complex role as a loss of intracellular potassium has also been shown to be beneficial to the health of the cell. Additionally, the mechanisms that a cell employs to achieve this loss of intracellular potassium vary depending on the cell type and stimulus used to induce apoptosis, suggesting multiple ways exist to accomplish the same goal of AVD. Additionally, sodium and chloride have been shown to play a vital role during cell death in both the signaling and control of AVD in various apoptotic model systems. This review examines the relationship between this morphological change and intracellular monovalent ions during apoptosis.