The potassium channel opener (-)-cromakalim prevents glutamate-induced cell death in hippocampal neurons

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

NeuroAiD: properties for neuroprotection and neurorepair

BACKGROUND

Treatments for stroke and other brain injuries are limited. NeuroAiD has been shown to be beneficial in clinical studies. We reviewed the pharmacological effects of NeuroAiD on the normal and ischemic brain and neurons.

METHODS

In vivo and in vitro experiments using mouse model of stroke (focal ischemia), rat model of cardiac arrest (global ischemia) and cortical neurons in culture were reviewed and summarized.

RESULTS

NeuroAiD improved survival, attenuated infarct size, improved functional recovery in the model of focal ischemia, and protected neurons against glutamate-induced injury. Furthermore, it enhanced cognitive recovery by reducing hippocampal CA1 cell degeneration, DNA fragmentation, Bax expression and ma-londialdehyde release in the model of global ischemia. Activation of the Akt survival pathway and opening of KATP channels may contribute to the neuroprotective properties of NeuroAiD. NeuroAiD increased BDNF expression and induced proliferation of cells which differentiate and mature into neurons. It enhanced rosette formation of human embryonic stem cells. NeuroAiD-treated embryonic cortical neurons developed into neurons with longer neurites, denser outgrowths and networks, and more synaptic release sites.

CONCLUSIONS

NeuroAiD demonstrated both neuroprotective and neuroregenerative properties in rodent models of focal and global ischemia and in cortical cell cultures. These properties would be important for developing a treatment strategy in reducing the long-term disability of stroke, cardiac arrest and other brain injuries.

Activation of ATP-sensitive potassium channels as an element of the neuroprotective effects of the Traditional Chinese Medicine MLC901 against oxygen glucose deprivation

NeuroAid (MLC601 and MLC901), a Traditional Medicine used in China for patients after stroke has been reported in preclinical models of ischemia to induce neuroprotection and neuroplasticity. This work shows the effects of MLC901 on an in vitro model of oxygen glucose deprivation (OGD). MLC901 prevents neuronal death induced by 120 min OGD and decreases the exaggerated Ca²⁺ entry in mature cortical neurons exposed to 120 min OGD. The neuroprotective effect of MLC901 is associated with a large hyperpolarization of ∼20 mV which is antagonized by glibenclamide, the specific inhibitor of K(ATP) channels. In addition MLC901 strengthens the activation of K(ATP) channels. MLC901 has been directly shown to act as an activator of K(ATP) channels as potent as the classical K(ATP) channel opener. The capacity of MLC901 to produce a large hyperpolarization, particularly in neurons that have suffered from energy deprivation probably plays an important role in the neuroprotective effects of this traditional medicine that comes in addition to its previously demonstrated neuroregenerative properties.

Suppression of c-Fos protein and mRNA expression in pentylenetetrazole-induced kindled mouse brain by isoxylitones

An early immediate gene c-fos has been proposed as the gene responsible for turning on molecular events that might underlie the long-term neural changes occurring during kindling. We have evaluated the effects of novel anticonvulsant isomeric compounds isoxylitones [(E/Z)-2-propanone-1,3,5,5-trimethyl-2-cyclohexen-1-ylidine] on the c-Fos protein and mRNA expression in the brain samples of kindled mice and compared it with the normal and untreated kindled groups. Kindling was induced in male NMRI mice by repeated administration of sub-convulsive dose (50 mg/kg) of pentylenetetrazole (PTZ) until a seizure score of 4-5 was achieved. The c-Fos expression was quantified by combination of immunohistochemistry and RT-PCR protocols. Both the immunohistochemical and RT-PCR analysis revealed a marked increase in the expression of c-fos mRNA and protein in the brain regions tested in case of PTZ-kindled control group compared to normal control. In contrast, the isoxylitone (30 mg/kg)-treated group demonstrated significant reduction of c-Fos expression compared to PTZ-kindled control animals. However, low expression of c-fos mRNA was only detected in the thalamus of the isoxylitone-treated brain samples. Based on these observations, we suggest that isoxylitones may have the capacity to control the seizure pattern by mechanism such as the suppression of c-Fos protein and mRNA levels in different regions of the brain. Further investigations to explore the mechanism of action of these compounds are under process.

Chronic lithium administration ameliorates 2,4,6-trinitrobenzene sulfonic acid-induced colitis in rats; potential role for adenosine triphosphate sensitive potassium channels

BACKGROUND AND AIM

Inflammatory bowel disease (IBD) is a multi-factorial disease with an unknown etiology characterized by oxidative stress, leukocyte infiltration and a rise in inflammatory cytokines. This study was conducted to investigate lithium in 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced chronic model of experimental IBD, and the contribution of potassium channels as a possible underlying mechanism.

METHODS

Experimental IBD was induced in rats by a single colonic administration of 10 mg of TNBS. Lithium, Glibenclamide (a potassium channel blocker), Lithium + Glibenclamide, Cromakalim or Lithium+Glibenclamide+ Cromakalim were given twice daily for 7 successive days. At the end of the experiment, macroscopic and histopathologic scores, colonic malondialdehyde (MDA), tumor necrosis factor-α (TNF-α) level, and myeloperoxidase (MPO) activity as well as plasma lithium level were assessed.

RESULTS

Both macroscopic and histological features of colonic injury were markedly ameliorated by lithium. Likewise, the elevated amounts of MPO and MDA were diminished as well as those of TNF-α (P < 0.05). Glibenclamide reversed the effect of lithium on these markers, Addition of cromakalim abrogated the effects mediated by glibenclamide and markedly decreased MPO activity, MDA level and TNF-α content (P < 0.0.05). Macroscopic and microscopic scores and biochemical markers were significantly decreased in Cromakalim-treated animals. No significant difference was observed between TNBS and Glibenclamide groups.

CONCLUSION

Lithium exerts prominent anti-inflammatory effects on TNBS-induced colitis in rats. Potassium channels contribute to these beneficial properties.

Aging redistributes medial prefrontal neuronal excitability and impedes extinction of trace fear conditioning

Cognitive flexibility is critical for survival and reflects the malleability of the central nervous system (CNS) in response to changing environmental demands. Normal aging results in difficulties modifying established behaviors, which may involve medial prefrontal cortex (mPFC) dysfunction. Using extinction of conditioned fear in rats to assay cognitive flexibility, we demonstrate that extinction deficits reminiscent of mPFC dysfunction first appear during middle age, in the absence of hippocampus-dependent context deficits. Emergence of aging-related extinction deficits paralleled a redistribution of neuronal excitability across two critical mPFC regions via two distinct mechanisms. First, excitability decreased in regular spiking neurons of infralimbic-mPFC (IL), a region whose activity is required for extinction. Second, excitability increased in burst spiking neurons of prelimbic-mPFC (PL), a region whose activity hinders extinction. Experiments using synaptic blockers revealed that these aging-related differences were intrinsic. Thus, changes in IL and PL intrinsic excitability may contribute to cognitive flexibility impairments observed during normal aging.

Effect of recombinant divercin RV41, structural variants and the activators of potassium channels on Listeria monocytogenes EGDe

The effect of recombinant divercin RV41 (DvnRV41) and its structural variants on the K-channel formation was determined. The growth of Listeria monocytogenes EGDe (sensitive phenotype) and its isogenic strain (resistant phenotype) was assessed in the presence of DvnRV41 combined or not with pinacidil, NS1619, cromakalim (as K-channel activators), iberiotoxin and glipizide (as K-channel blockers). The combined action of DvnRV41 and K activators permitted formation of ATP-dependent pores. The combination of DvnRV41 and ATP-dependent pore activator cromakalim inhibited the growth of sensitive strain. The antilisterial activity of structural variants was less important than that of DvnRV41 but their mode of action remained overall similar.

From the background to the spotlight: TASK channels in pathological conditions

TWIK-related acid-sensitive potassium channels (TASK1-3) belong to the family of two-pore domain (K(2P) ) potassium channels. Emerging knowledge about an involvement of TASK channels in cancer development, inflammation, ischemia and epilepsy puts the spotlight on a leading role of TASK channels under these conditions. TASK3 has been especially linked to cancer development. The pro-oncogenic potential of TASK3 could be shown in cell lines and in various tumor entities. Pathophysiological hallmarks in solid tumors (e.g. low pH and oxygen deprivation) regulate TASK3 channels. These conditions can also be found in (autoimmune) inflammation. Inhibition of TASK1,2,3 leads to a reduction of T cell effector function. It could be demonstrated that TASK1(-/-) mice are protected from experimental autoimmune inflammation while the same animals display increased infarct volumes after cerebral ischemia. Furthermore, TASK channels have both an anti-epileptic as well as a pro-epileptic potential. The relative contribution of these opposing influences depends on their cell type-specific expression and the conditions of the cellular environment. This indicates that TASK channels are per se neither protective nor detrimental but their functional impact depends on the "pathophysiological" scenario. Based on these findings TASK channels have evolved from "mere background" channels to key modulators in pathophysiological conditions.

Two pore domain potassium channels in cerebral ischemia: a focus on K2P9.1 (TASK3, KCNK9)

Background

Recently, members of the two-pore domain potassium channel family (K_2P channels) could be shown to be involved in mechanisms contributing to neuronal damage after cerebral ischemia. K_2P3.1^-/- animals showed larger infarct volumes and a worse functional outcome following experimentally induced ischemic stroke. Here, we question the role of the closely related K_2P channel K_2P9.1.

Methods

We combine electrophysiological recordings in brain-slice preparations of wildtype and K_2P9.1^-/- mice with an in vivo model of cerebral ischemia (transient middle cerebral artery occlusion (tMCAO)) to depict a functional impact of K_2P9.1 in stroke formation.

Results

Patch-clamp recordings reveal that currents mediated through K_2P9.1 can be obtained in slice preparations of the dorsal lateral geniculate nucleus (dLGN) as a model of central nervous relay neurons. Current characteristics are indicative of K_2P9.1 as they display an increase upon removal of extracellular divalent cations, an outward rectification and a reversal potential close to the potassium equilibrium potential. Lowering extracellular pH values from 7.35 to 6.0 showed comparable current reductions in neurons from wildtype and K_2P9.1^-/- mice (68.31 ± 9.80% and 69.92 ± 11.65%, respectively). These results could be translated in an in vivo model of cerebral ischemia where infarct volumes and functional outcomes showed a none significant tendency towards smaller infarct volumes in K_2P9.1^-/- animals compared to wildtype mice 24 hours after 60 min of tMCAO induction (60.50 ± 17.31 mm³ and 47.10 ± 19.26 mm³, respectively).

Conclusions

Together with findings from earlier studies on K_2P2.1^-/- and K_2P3.1^-/- mice, the results of the present study on K_2P9.1^-/- mice indicate a differential contribution of K_2P channel subtypes to the diverse and complex in vivo effects in rodent models of cerebral ischemia.

Hydrogen sulfide protects neurons against hypoxic injury via stimulation of ATP-sensitive potassium channel/protein kinase C/extracellular signal-regulated kinase/heat shock protein 90 pathway

Cerebral hypoxia is one of the main causes of cerebral injury. This study was conducted to investigate the potential protective effect of H(2)S in in vitro hypoxic models by subjecting SH-SY5Y cells to either oxygen-glucose deprivation or Na(2)S(2)O(4) (an oxygen scavenger) treatment. We found that treatment with NaHS (an H(2)S donor, 10-100 microM) 15 min prior to hypoxia increased cell viability in a concentration-dependent manner. Time-course study showed that NaHS was able to exert its protective effect even when added 8 h before or less than 4 h after hypoxia induction. Interestingly, endogenous H(2)S level was markedly reduced by hypoxia induction. Over-expression of cystathionine-beta-synthase prevented hypoxia induced cell apoptosis. Blockade of ATP-sensitive K(+) (K(ATP)) channels with glibenclamide and HMR-1098, protein kinase C (PKC) with its three specific inhibitors (chelerythrine, bisindolylmaleide I and calphostin C), extracellular signal-regulated kinase 1/2 (ERK1/2) with PD98059 and heat shock protein 90 (Hsp90) with geldanamycin and radicicol significantly attenuated the protective effects of NaHS. Western blots showed that NaHS significantly stimulated ERK1/2 activation and Hsp90 expression. In conclusion, H(2)S exerts a protective effect against cerebral hypoxia induced neuronal cell death via K(ATP)/PKC/ERK1/2/Hsp90 pathway. Our findings emphasize the important neuroprotective role of H(2)S in the brain during cerebral hypoxia.

Bioelectric mechanisms in regeneration: Unique aspects and future perspectives

Regenerative biology has focused largely on chemical factors and transcriptional networks. However, endogenous ion flows serve as key epigenetic regulators of cell behavior. Bioelectric signaling involves feedback loops, long-range communication, polarity, and information transfer over multiple size scales. Understanding the roles of endogenous voltage gradients, ion flows, and electric fields will contribute to the basic understanding of numerous morphogenetic processes and the means by which they can robustly restore pattern after perturbation. By learning to modulate the bioelectrical signals that control cell proliferation, migration, and differentiation, we gain a powerful set of new techniques with which to manipulate growth and patterning in biomedical contexts. This chapter reviews the unique properties of bioelectric signaling, surveys molecular strategies and reagents for its investigation, and discusses the opportunities made available for regenerative medicine.

Plasma membrane ion channels in suicidal cell death

The machinery leading to apoptosis includes altered activity of ion channels. The channels contribute to apoptotic cell shrinkage and modify intracellular ion composition. Cl(-) channels allow the exit of Cl(-), osmolytes and HCO(3)(-) leading to cell shrinkage and cytosolic acidification. K(+) exit through K(+) channels contributes to cell shrinkage and decreases intracellular K(+) concentration, which in turn favours apoptotic cell death. K(+) channel activity further determines the cell membrane potential, a driving force for Ca(2+) entry through Ca(2+) channels. Ca(2+) may enter through unselective cation channels. An increase of cytosolic Ca(2+) may stimulate several enzymes executing apoptosis. Specific ion channel blockers may either promote or counteract suicidal cell death. The present brief review addresses the role of ion channels in the regulation of suicidal cell death with special emphasis on the role of channels in CD95 induced apoptosis of lymphocytes and suicidal death of erythrocytes or eryptosis.

Cell volume regulatory ion channels in cell proliferation and cell death

Alterations of cell volume are key events during both cell proliferation and apoptotic cell death. Cell proliferation eventually requires an increase of cell volume, and apoptosis is typically paralleled by cell shrinkage. Alterations of cell volume require the participation of ion transport across the cell membrane, including appropriate activity of Cl(-) and K(+) channels. Cl(-) channels modify cytosolic Cl(-) activity and mediate osmolyte flux, and thus influence cell volume. Most Cl(-) channels allow exit of HCO(3)(-), leading to cytosolic acidification, which in turn inhibits cell proliferation and favors apoptosis. K(+) exit through K(+) channels decreases cytosolic K(+) concentration, which may sensitize the cell for apoptotic cell death. K(+) channel activity further maintains the cell membrane potential, a critical determinant of Ca(2+) entry through Ca(2+) channels. Ca(2+) may, in addition, enter through Ca(2+)-permeable cation channels, which, in some cells, are activated by hyperosmotic shock. Increases of cytosolic Ca(2+) activity may trigger both mechanisms required for cell proliferation and mechanisms, leading to apoptosis. Thereby cell proliferation and apoptosis depend on magnitude and temporal organization of Ca(2+) entry, as well as activity of other signaling pathways. Accordingly, the same ion channels may participate in the stimulation of both cell proliferation and apoptosis. Specific ion channel blockers may thus abrogate both cellular mechanisms, depending on cell type and condition.

Hydrogen sulfide protects HT22 neuronal cells from oxidative stress

Hydrogen sulfide (H2S) is a neuromodulator in the brain and a relaxant for smooth muscle. H2S protects primary cortical neurons from oxidative stress by increasing the intracellular concentrations of glutathione, the major antioxidant in cells. However, changes in glutathione alone are not sufficient to account for full protection in all types of nerve cells. H2S is here shown to protect an immortalized mouse hippocampal cell line from oxidative glutamate toxicity by activating ATP-dependent K+ (KATP) and Cl- channels, in addition to increasing the levels of glutathione. The present study therefore identifies a novel pathway for H2S protection from oxidative stress.

Ion channels in cell proliferation and apoptotic cell death

Cell proliferation and apoptosis are paralleled by altered regulation of ion channels that play an active part in the signaling of those fundamental cellular mechanisms. Cell proliferation must--at some time point--increase cell volume and apoptosis is typically paralleled by cell shrinkage. Cell volume changes require the participation of ion transport across the cell membrane, including appropriate activity of Cl- and K+ channels. Besides regulating cytosolic Cl- activity, osmolyte flux and, thus, cell volume, most Cl- channels allow HCO3- exit and cytosolic acidification, which inhibits cell proliferation and favors apoptosis. K+ exit through K+ channels may decrease intracellular K+ concentration, which in turn favors apoptotic cell death. K+ channel activity further maintains the cell membrane potential, a critical determinant of Ca2+ entry through Ca2+ channels. Cytosolic Ca2+ may trigger mechanisms required for cell proliferation and stimulate enzymes executing apoptosis. The switch between cell proliferation and apoptosis apparently depends on the magnitude and temporal organization of Ca2+ entry and on the functional state of the cell. Due to complex interaction with other signaling pathways, a given ion channel may play a dual role in both cell proliferation and apoptosis. Thus, specific ion channel blockers may abrogate both fundamental cellular mechanisms, depending on cell type, regulatory environment and condition of the cell. Clearly, considerable further experimental effort is required to fully understand the complex interplay between ion channels, cell proliferation and apoptosis.

Protective preconditioning by transient global ischemia in the rat: components of delayed injury progression and lasting protection distinguished by comparisons of depolarization thresholds for cell loss at long survival times

Robust ischemic preconditioning has been shown in rodent brain, but there are concerns regarding the persistence of neuron protection. This issue was examined in rat hippocampus following 4-vessel occlusion (4-VO) ischemia, using DC shifts characteristic of ischemic depolarization to reproducibly define insult severity. Preconditioning ischemia producing 2 to 3.5 mins depolarization was followed at intervals of 2, 5, or 7 days by test insults of varied duration, after which CA1 counts were obtained at 1, 2, 4, or 12 weeks. Neuron loss in naive animals increased with depolarization time longer than 4 mins regardless of postischemic survival interval. Preconditioning 2, 5, or 7 days before test insults prolonged the injury threshold evaluated at 1 week survival to 15, 9, or 6 mins, respectively, showing robust protection and a rapid decay of the protected state. However, by 2 weeks survival after preconditioning at a 2-day interval, the injury threshold dramatically regressed from 15 to 9 mins. Thereafter protection remained relatively stable through 1 month, but slight progression of neuron injury was evident at 3 months. Inflammatory responses were seen in both naive and preconditioned hippocampi throughout this interval, appropriate to the extent of neuron injury. These studies show distinct components of transient and lasting protection after ischemic preconditioning. Finally, it was found that ischemic depolarization was delayed by approximately 1 min in optimally preconditioned rat hippocampus, in contrast to previous results in the gerbil, identifying one specific mechanism by which insult severity is reduced in this model.

Mitochondria in health and disease: perspectives on a new mitochondrial biology

The integrity of mitochondrial function is fundamental to cell life. It follows that disturbances of mitochondrial function will lead to disruption of cell function, expressed as disease or even death. In this review, I consider recent developments in our knowledge of basic aspects of mitochondrial biology as an essential step in developing our understanding of the contributions of mitochondria to disease. The identification of novel mechanisms that govern mitochondrial biogenesis and replication, and the delicately poised signalling pathways that coordinate the mitochondrial and nuclear genomes are discussed. As fluorescence imaging has made the study of mitochondrial function within cells accessible, the application of that technology to the exploration of mitochondrial bioenergetics is reviewed. Mitochondrial calcium uptake plays a major role in influencing cell signalling and in the regulation of mitochondrial function, while excessive mitochondrial calcium accumulation has been extensively implicated in disease. Mitochondria are major producers of free radical species, possibly also of nitric oxide, and are also major targets of oxidative damage. Mechanisms of mitochondrial radical generation, targets of oxidative injury and the potential role of uncoupling proteins as regulators of radical generation are discussed. The role of mitochondria in apoptotic and necrotic cell death is seminal and is briefly reviewed. This background leads to a discussion of ways in which these processes combine to cause illness in the neurodegenerative diseases and in cardiac reperfusion injury. The demands of mitochondria and their complex integration into cell biology extends far beyond the provision of ATP, prompting a radical change in our perception of mitochondria and placing these organelles centre stage in many aspects of cell biology and medicine.

NMDA receptor stimulation in the absence of extracellular Ca2+ potentiates Ca2+ influx-dependent cell death system

The meaning of Ca2+ influx in the time course of glutamate stimulation of neuronal cells was addressed. We demonstrated that Ca2+ influx did not work straightforward in the determination of the fate of neuronal cells. There appears to be a critical period for Ca2+ influx to work efficiently in glutamate-induced neuronal cell death. When Ca2+ influx for 5 min from the beginning of glutamate stimulation was allowed in the whole stimulation period for 15 min, potent neuronal cell death could not be attained. On the other hand, when neuronal cells had been pre-treated with glutamate or NMDA for 5-10 min in the absence of extracellular Ca2+ following Ca2+ influx for 5 min fully induced neuronal cell death. APV inhibited this pre-treatment effect. It appears that the pre-treatment of neuronal cells with glutamate or NMDA in the absence of extracellular Ca2+ promotes the Ca2+ influx-dependent process executing cell death. The pre-treatment itself did not change the pattern of intracellular Ca2+ elevation by the activation of NMDA receptors. These results imply that glutamate activation of NMDA receptors consists of two different categories of pathways relating to neuronal cell death, i.e., Ca2+ influx independent and dependent, and that the former facilitates the latter to drive neuronal cells to death. This study clarified a mechanism by which glutamate quickly determines cell fate.

Tetraethylammonium exacerbates ischemic neuronal injury in rat cerebrocortical slice cultures

We investigated potential contribution of K+ channel activity to regulation of ischemia-induced neuronal injury, using cerebrocortical slice cultures. Exposure of cultures to a glucose-free conditioning solution containing sodium azide and 2-deoxyglucose caused neuronal cell death as assessed by cellular uptake of propidium iodide, which was prevented by MK-801, an N-methyl-D-aspartate (NMDA) receptor antagonist. Application of tetraethylammonium markedly exacerbated ischemic neuronal injury. Charybdotoxin, a blocker of large-conductance Ca(2+)-activated K+ (BK(Ca)) channels, also augmented ischemic injury, whereas AM 92016, a blocker of delayed rectifier K+ channels, and dequalinium, a blocker of small-conductance Ca(2+)-activated K+ channels, had no significant effect. In addition, tetraethylammonium and charybdotoxin were effective in augmenting NMDA-induced neuronal injury. These results present unprecedented evidence for the ability of tetraethylammonium to enhance ischemic neuronal death, and suggest that BK(Ca) channels constitute an endogenous system to protect cortical neurons from ischemic injury, via prevention of NMDA receptor over-activation.

Role of ionic fluxes in the apoptotic cell death of cultured cerebellar granule neurons

Cultured cerebellar granule neurons (CGC) increase survival in a medium containing 25 mM KCl (K25), and they die apoptotically when cultures are treated with staurosporine (St) or are transferred to a 5-mM KCl containing medium (K5). Apoptotic CGC show nuclear condensation and caspase-3 activation. Cell death induced by these conditions was partially prevented when cultures were maintained under alkaline conditions, which also induced a marked reduction of the caspase-3 activation. The acidification of the medium further increased cell death induced by both stimuli. Cultures transferred to K5 suffered an immediate intracellular alkalinization that remained constant during the time K5 was present. In contrast, St did not modify cytosolic pH at any of the evaluated times. On the other hand, DIDS, furosemide, and bumetanide prevented CGC death induced by K5 and St. Other drugs such as amiloride, EIPA, tamoxifen, NEM, or NPPB did not modify cell death induced by these conditions. Both DIDS and bumetanide markedly inhibited the processing and activation of caspase-3, and DIDS prevented the nuclear condensation induced by K5 and St. These findings suggest that pH is a condition that could contribute to the modulation of cell death induced by some stimuli and that other ions, such as potassium, could have a role in the initial phase of apoptotic death of CGC.

Targeting Ischemic Stroke with a Novel Opener of ATP-Sensitive Potassium Channels in the Brain

During cerebral ischemia, the opening of neuronal ATP-sensitive potassium channels (K(ATP) channels) affords intrinsic protection by regulating membrane potential. To augment this endogenous mechanism, we have synthesized iptakalim, a K(ATP) opener. Through K(ATP) channel activation, iptakalim affected multiple pathways of the glutamatergic system, limiting glutamate release and receptor actions, which are involved in excitotoxicity during ischemic damage. The molecule readily penetrated the brain and showed low toxicity in animal experiments. In different animal models of stroke as well as in cell cultures, iptakalim provided significant neuroprotection, not only in promoting behavioral recovery but also in protecting neurons against necrosis and apoptosis. This compound thus has promise as a neuroprotective drug for the treatment of stroke and other forms of neuronal damage.

Diazoxide preconditioning protects against neuronal cell death by attenuation of oxidative stress upon glutamate stimulation

We examined the effects of diazoxide, the putative mitochondrial adenosine triphosphate-sensitive potassium (mitoK(ATP)) channel opener, against glutamate excitotoxicity in primary cultures of rat cortical neurons. Cells were treated with diazoxide for 24 hr and then exposed to 200 microM glutamate. Cell viability was measured 24 hr after glutamate exposure. We found that treatment 24 hr before glutamate exposure with 250 and 500 microM diazoxide but not with another mitoK(ATP) channel opener, nicorandil, increased neuronal viability from 54 +/- 2% to 84 +/- 2% and 92 +/- 3%, respectively (n = 25-40). These effects were not inhibited by the putative mitoK(ATP) channel blocker 5-hydroxydecanoic acid. Diazoxide application increased production of reactive oxygen species (ROS) and coapplication of M40401, a superoxide dismutase mimetic, prevented delayed preconditioning. The 24 hr preconditioned neurons showed significantly reduced ROS production upon glutamate stimulation compared to that in untreated cells. These results suggest that diazoxide induces delayed preconditioning in cultured cortical neurons via increased ROS production and attenuation of oxidative stress upon glutamate stimulation.

Pharmacological manipulations of ATP-dependent potassium channels and adenosine A1 receptors do not impact hippocampal ischemic preconditioning in vivo: evidence in a highly quantitative gerbil model

Ischemic preconditioning models have been characterized in brain, heart, and other tissues, and previous pharmacologic studies have suggested an involvement of adenosine and ATP dependent potassium (KATP) channels in such tolerance phenomena. This question was reexamined in a reproducible gerbil model in which the duration of ischemic depolarization defined the severity of preconditioning and test insults. Agents studied were glibenclamide, a blocker of KATP channels; 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), an adenosine A1 receptor antagonist; and N6-cyclopentyladenosine (CPA), an A1 agonist. Intraventricular glibenclamide injections aggravated neuron damage after brief priming insults, in parallel with a dose-dependent prolongation of ischemic depolarization. However, the depolarization thresholds for ischemic neuronal injury were identical in vehicle- and glibenclamide-treated animals, and glibenclamide did not affect preconditioning when equivalent insult severity was maintained during priming insults. Neither DPCPX nor CPA had any effect on the onset or duration of depolarization after intraperitoneal injection in this model, and neither drug affected neuron damage. In the case of CPA, it was necessary to maintain temperature for 4 to 6 hours of recirculation to avoid significant confounding hypothermia. These results fail to support a direct involvement of A1 receptors or KATP channels during early stages in the development of ischemic tolerance in vivo, and emphasize the need for robust, well-controlled, and quantitative models in such studies.

Ion channels and membrane rafts in apoptosis

Ion channels have been demonstrated to be a central element in the induction and the execution of apoptosis. In particular, mitochondrial ion channels, including not only the permeability transition pore but also a mitochondrial, ATP-sensitive (mKATP) channel as well as a mitochondrial calcium-activated potassium channel are involved critically in apoptotic changes in mitochondria. Ion channels in the cell membrane that are altered by induction of apoptosis include potassium, chloride and calcium channels. The Kv1.3 potassium channel belongs to the best-characterized ion channels involved in apoptosis and a genetic model of cells deficient for Kv1.3 has indicated a critical role for Kv1.3, at least in some forms of apoptosis. The mechanisms regulating ion channels during apoptosis are, however, still poorly defined. Recent studies have suggested a function for distinct membrane domains, termed rafts, in the cell membrane for the regulation of ion channels during apoptosis. Small sphingolipid- and cholesterol-enriched membrane domains are modified by many apoptotic stimuli to form large ceramide-enriched membrane platforms. These platforms serve to cluster receptor molecules, to re-organize intracellular signalling molecules including ion channels, to bring ion channels into close contact with their regulators and/or to separate proteins from a specific ion channel. Finally, the lipid composition of the cell membrane might be involved directly in ion channel regulation.

Diazoxide induces delayed pre-conditioning in cultured rat cortical neurons

We investigated the effect of diazoxide on neuronal survival in primary cultures of rat cortical neurons against oxygen-glucose deprivation (OGD). Diazoxide pre-treatment induced delayed pre-conditioning and almost entirely attenuated the OGD-induced neuronal death. Diazoxide inhibited succinate dehydrogenase and induced mitochondrial depolarization, free radical production and protein kinase C activation. The putative mitochondrial ATP-sensitive potassium channel blocker 5-hydroxydecanoate abolished the protective effect of diazoxide while the non-selective KATP channel blocker glibenclamide did not. The non-selective KATP channel openers nicorandil and cromakalim did not improve viability. Superoxide dismutase mimetic, M40401, or protein kinase C inhibitor, chelerythrine, prevented the neuroprotective effect of diazoxide. Diazoxide did not increase reduced glutathione and manganese-superoxide dismutase levels but we found significantly higher reduced glutathione levels in diazoxide-pre-conditioned neurons after OGD. In pre-conditioned neurons free radical production was reduced upon glutamate stimulation. The succinate dehydrogenase inhibitor 3-nitropropionic acid also induced pre-conditioning and free radical production in neurons. Here, we provide the first evidence that diazoxide induces delayed pre-conditioning in neurons via acute generation of superoxide anion and activation of protein kinases and subsequent attenuation of oxidant stress following OGD. The succinate dehydrogenase-inhibiting effect of diazoxide is more likely to be involved in this neuroprotection than the opening of mitochondrial ATP-sensitive potassium channels.

K+-dependent cerebellar granule neuron apoptosis. Role of task leak K+ channels

Rat mature cerebellar granule, unlike hippocampal neurons, die by apoptosis when cultured in a medium containing a physiological concentration of K+ but survive under high external K+ concentrations. Cell death in physiological K+ parallels the developmental expression of the TASK-1 and TASK-3 subunits that encode the pH-sensitive standing outward K+ current IKso. Genetic transfer of the TASK subunits in hippocampal neurons, lacking IKso, induces cell death, while their genetic inactivation protects cerebellar granule neurons. Neuronal death of cultured rat granule neurons is also prevented by conditions that specifically reduce K+ efflux through the TASK-3 channels such as extracellular acidosis and ruthenium red. TASK leak K+ channels thus play an important role in K+-dependent apoptosis of cerebellar granule neurons in culture.

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

CNS sensing and regulation of peripheral glucose levels

It is clear that the brain has evolved a mechanism for sensing levels of ambient glucose. Teleologically, this is likely to be a function of its requirement for glucose as a primary metabolic substrate. There is no question that the brain can sense and mount a counterregulatory response to restore very low levels of plasma and brain glucose. But it is less clear that the changes in glucose associated with normal diurnal rhythms and feeding cycles are sufficient to influence either ingestive behavior or the physiologic responses involved in regulating plasma glucose levels. Glucosensing neurons are clearly a distinct class of metabolic sensors with the capacity to respond to a variety of intero- and exteroceptive stimuli. This makes it likely that these glucosensing neurons do participate in physiologically relevant homeostatic mechanisms involving energy balance and the regulation of peripheral glucose levels. It is our challenge to identify the mechanisms by which these neurons sense and respond to these metabolic cues.

A potent protective role of lysophospholipids against global cerebral ischemia and glutamate excitotoxicity in neuronal cultures

Lysophospholipids (LPLs) are important intermediates in the synthesis and degradation of membrane phospholipids. Here we show that certain LPLs, particularly lysophosphatidylcholine and lysophosphatidylinositol, prevent neuronal death both in an in vivo model of transient global ischemia and in an in vitro model of excitotoxicity using primary cultures of cerebellar granule cells exposed to high extracellular concentrations of glutamate (20-40 micromol/L). The intravenous injection of lysophosphatidylcholine or lysophosphatidylinositol at a concentration of 200 nmol/kg induced a survival of CA1 pyramidal neurons as high as approximately 95%, even when the treatment was started 30 minutes after 15-minute global ischemia. In contrast, lysophosphatidic acid induced no protection. This work also provides evidence that a pretreatment with lysophosphatidylcholine or lysophosphatidylinositol (200 nmol/kg) injected as long as 3 days before a severe 6-minute ischemia provided a potent tolerance against neurodegeneration. Neuroprotection was also observed in in vitro experiments with LPLs. Taken together, in vivo and in vitro data suggest a potential therapeutic use of LPLs as antiischemic compounds. The potential role of 2P-domain K+ channels as targets of LPLs in this potent neuroprotective effect is discussed.

Decreased alpha-endosulfine, an endogenous regulator of ATP-sensitive potassium channels, in brains from adult Down syndrome patients

Alpha-endosulfine has the ability to block ATP-sensitive potassium (K(ATP)) channels and stimulate insulin release in beta cells like sulfonylurea. Alpha-endosulfine is expressed in a wide range of tissue, including brain and endocrine tissues. Although K(ATP) channels are also present in brain and its regulators have been reported to be involved in the release of neurotransmitters such as acetylcholine that plays an important role in cognitive function, the neurobiological role of alpha-endosulfine has not been studied yet. We examined the expression levels of alpha-endosulfine protein in frontal cortex and cerebellum from patients with Down syndrome (DS) showing Alzheimer's disease (AD) pathology using Western blotting. In frontal cortex, alpha-endosulfine was detected in all of 10 controls, but only 1 (from female) out of 8 DS with weak density. In cerebellum, alpha-endosulfine was also detected in all of 9 controls, but only 1 (from male) out of 6 DS with weak density. The considerably decreased alpha-endosulfine could result in the continuous opening of K(ATP) channels and the subsequent decrease of neurotransmitters release associated with cognition. This study is of significance providing evidence for a biological role of alpha-endosulfine in brain and alpha-endosulfine protein could be a pharmacological target for therapeutic intervention.

Polyunsaturated fatty acids induce ischemic and epileptic tolerance

The findings reported in this work show that pretreatment with polyunsaturated fatty acids, particularly linolenic acid, present in vegetable oils, can provide a potent tolerance against neurodegeneration in two models of neuronal death-generating treatments such as kainic acid injection and global ischemia. Rats were injected i.v. with 500 nmol/kg of linolenic acid as long as 3 days prior to 6 min global ischemia or received an injection of linolenic acid as long as 3 days prior to a dose of 7.5 mg/kg kainic acid. Neuronal degeneration, assessed by analysis of neuronal density on Cresyl Violet-stained hippocampal sections, was significantly reduced in linolenic acid-treated rats (94-85% of cell survival in the ischemic model and 99-79% of cell survival in the epileptic model in respective CA1 and CA3 subfields). The neuroprotection observed following the injection of linolenic acid 3 days prior to induction of a severe ischemic or epileptic challenge was associated with the induction of the neuroprotective HSP70 heat shock protein within the time window of protection. The injection of 500 nmol/kg of linolenic acid induced a maximal HSP70 expression of 387% at 72 h. In contrast, the overexpression of one well-known protein inducer of neuronal cell death, Bax, which is induced by both ischemic and kainic acid-induced epileptic insults, was prevented by linolenic acid in the 3-day window of protection. These results strengthen the idea of an interesting potential therapeutical value of polyunsaturated fatty acids in neuronal protection.

ASIC-like, proton-activated currents in rat hippocampal neurons

The expression of mRNA for acid sensing ion channels (ASIC) subunits ASIC1a, ASIC2a and ASIC2b has been reported in hippocampal neurons, but the presence of functional hippocampal ASIC channels was never assessed. We report here the first characterization of ASIC-like currents in rat hippocampal neurons in primary culture. An extracellular pH drop induces a transient Na(+) current followed by a sustained non-selective cation current. This current is highly sensitive to pH with an activation threshold around pH 6.9 and a pH(0.5) of 6.2. About half of the total peak current is inhibited by the spider toxin PcTX1, which is specific for homomeric ASIC1a channels. The remaining PcTX1-resistant ASIC-like current is increased by 300 microM Zn(2+) and, whereas not fully activated at pH 5, it shows a pH(0.5) of 6.0 between pH 7.4 and 5. We have previously shown that Zn(2+) is a co-activator of ASIC2a-containing channels. Thus, the hippocampal transient ASIC-like current appears to be generated by a mixture of homomeric ASIC1a channels and ASIC2a-containing channels, probably heteromeric ASIC1a+2a channels. The sustained non-selective current suggests the involvement of ASIC2b-containing heteromeric channels. Activation of the hippocampal ASIC-like current by a pH drop to 6.9 or 6.6 induces a transient depolarization which itself triggers an initial action potential (AP) followed by a sustained depolarization and trains of APs. Zn(2+) increases the acid sensitivity of ASIC channels, and consequently neuronal excitability. It is probably an important co-activator of ASIC channels in the central nervous system.

Rapid tolerance to focal cerebral ischemia in rats is attenuated by adenosine A1 receptor antagonist

Two types of ischemic tolerance in the brain, rapid and delayed, have been reported in terms of the interval between the conditioning and test insults. Although many reports showed that delayed-phase neuroprotection evoked by preconditioning is evident after 1 week or longer, there have been a few investigations about rapidly induced tolerance, and the reported neuroprotective effects become ambiguous 7 days after the insults. The authors examined whether this rapid ischemic tolerance exists after 7 days of reperfusion in a rat focal ischemic model, and investigated modulating effects of the adenosine A 1 receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine). Preconditioning with 30 minutes of middle cerebral artery occlusion reduced infarct volume 7 days after 180 minutes of subsequent focal ischemia given after 1-hour reperfusion. The rapid preconditioning also improved neurologic outcome. These beneficial effects were attenuated by pretreatment of 0.1 mg/kg DPCPX, which did not influence the infarct volume after conditioning (30 minutes) or test (180 minutes) ischemia when given alone. The results show that preconditioning with a brief focal ischemia induces rapid tolerance to a subsequent severe ischemic insult, the effect of which is still present after 7 days of reperfusion, and that the rapid ischemic tolerance is possibly mediated through an adenosine A 1 receptor-related mechanism.

The adenosine triphosphate-sensitive potassium channel opener nicorandil protects the ischemic rabbit spinal cord

OBJECTIVE

We investigated the protective effects of an adenosine triphosphate-sensitive potassium channel opener nicorandil in the rabbit model of spinal cord ischemia.

METHODS

Rabbits were randomized into 4 groups (each n = 6): the nicorandil group (100 microg/kg intravenous nicorandil 10 minutes before ischemia); the glibenclamide plus nicorandil group (3 mg/kg intravenous glibenclamide, an antagonist of adenosine triphosphate-sensitive potassium channels, 10 minutes before nicorandil administration); the vehicle group (vehicle alone); and the sham operation group (without spinal cord ischemia). Spinal cord ischemia was induced by balloon occlusion of the infrarenal abdominal aorta for 15 minutes at 39 degrees C. Neurologic function was graded into Johnson's score at 8 hours, 1 day, and 2 days. Histopathologic examination was performed at 2 days, and the number of intact motor neuron cells was compared.

RESULTS

Johnson scores of the glibenclamide plus nicorandil and vehicle groups were significantly lower than those of the sham operation and nicorandil groups at each time point, and no statistically significant difference was observed between the glibenclamide plus nicorandil and vehicle groups. Histopathologic examination revealed that motor neurons were almost normal in the nicorandil group, whereas about 55% of motor neurons were lost in the vehicle and glibenclamide plus nicorandil groups.

CONCLUSIONS

Nicorandil has a protective effect on the ischemic rabbit spinal cord, and the beneficial effect seems mediated through the activation of adenosine triphosphate-sensitive potassium channels.

Ion homeostasis and apoptosis

Alterations in the transmembrane gradients of several physiological ions may influence programmed cell death. In particular, recent data suggest that increases in intracellular calcium may either promote or inhibit apoptosis, depending on the level, timing and location, whereas loss of intracellular potassium promotes apoptosis.

Delivery of genes encoding cardiac K(ATP) channel subunits in conjunction with pinacidil prevents membrane depolarization in cells exposed to chemical hypoxia-reoxygenation

Metabolic injury is a complex process affecting various: tissues with membrane depolarisation recognised as a common trigger event leading to cell death. To examine whether, under metabolic challenge, membrane potential homeostasis can be maintained by an activator of channel proteins, we here delivered Kir6.2 and SUR2A genes, which encode cardiac K(ATP) channel subunits, into a somatic cell line lacking native K(ATP) channels (COS-7 cells). Chemical hypoxia-reoxygenation was simulated in COS-7 cells by addition and removal of the mitochondrial poison 2,4 dinitrophenol (DNP). The membrane potential of COS-7 cells at rest was -31 +/- 3 mV. This value did not change following 3 min-long exposure to DNP (-32 +/- 4 mV). In contrast, washout of DNP induced significant membrane depolarisation (-17 +/- 2 mV). Delivery of Kir6.2/SUR2A genes did not change cellular response to hypoxia-reoxygenation. Similarly, pinacidil, potassium channel opener, did not have effect on hypoxia-reoxygenation-induced membrane depolarisation in cells lacking recombinant K(ATP) channel subunits. However, gene delivery combined with pinacidil prevented membrane depolarisation induced by hypoxia-reoxygenation. This effect of pinacidil, in cells expressing Kir6.2/SUR2A, was observed regardless of whether pinacidil was added only during hypoxia or reoxygenation. The present study demonstrates that combined use of K(ATP) channel subunits gene delivery and pharmacological targeting of recombinant proteins can be used to efficiently control membrane potential under hypoxia-reoxygenation.

Intrahippocampal infusions of k-atp channel modulators influence spontaneous alternation performance: relationships to acetylcholine release in the hippocampus

One mechanism by which administration of glucose enhances cognitive functions may be by modulating central ATP-sensitive potassium (K-ATP) channels. K-ATP channels appear to couple glucose metabolism and neuronal excitability, with channel blockade increasing the likelihood of neurosecretion. The present experiment examined the effects of glucose and the direct K-ATP channel modulators glibenclamide and lemakalim on spontaneous alternation performance and hippocampal ACh release. Rats received either artificial CSF vehicle or vehicle plus drug for two consecutive 12 min periods via microdialysis probes (3 mm; flow rate of 2.1 microliter/min) implanted in the left hippocampus. During the second 12 min period, rats were tested for spontaneous alternation performance. Dialysate was simultaneously collected for later analysis of ACh content. Both glucose (6.6 mm) and glibenclamide (100 micrometer) significantly increased alternation scores compared with those of controls. Conversely, lemakalim (200 micrometer) significantly reduced alternation scores relative to those of controls. Simultaneous administration of lemakalim with either glucose or glibenclamide resulted in alternation scores not significantly different from control values. All drug treatments enhanced hippocampal ACh output relative to control values. The results demonstrate that K-ATP channel modulators influence behavior when administered directly into the hippocampus, with channel blockers enhancing and openers impairing spontaneous alternation performance, thus supporting the hypothesis that glucose enhances memory via action at central K-ATP channels. That lemakalim, as well as glibenclamide and glucose, increased hippocampal ACh output suggests a dissociation between the effects of K-ATP channel modulators on behavior and hippocampal ACh release.

Mice transgenically overexpressing sulfonylurea receptor 1 in forebrain resist seizure induction and excitotoxic neuron death

The ability of the sulfonylurea receptor (SUR) 1 to suppress seizures and excitotoxic neuron damage was assessed in mice transgenically overexpressing this receptor. Fertilized eggs from FVB mice were injected with a construct containing SUR cDNA and a calcium-calmodulin kinase IIalpha promoter. The resulting mice showed normal gross anatomy, brain morphology and histology, and locomotor and cognitive behavior. However, they overexpressed the SUR1 transgene, yielding a 9- to 12-fold increase in the density of [(3)H]glibenclamide binding to the cortex, hippocampus, and striatum. These mice resisted kainic acid-induced seizures, showing a 36% decrease in average maximum seizure intensity and a 75% survival rate at a dose that killed 53% of the wild-type mice. Kainic acid-treated transgenic mice showed no significant loss of hippocampal pyramidal neurons or expression of heat shock protein 70, whereas wild-type mice lost 68-79% of pyramidal neurons in the CA1-3 subfields and expressed high levels of heat shock protein 70 after kainate administration. These results indicate that the transgenic overexpression of SUR1 alone in forebrain structures significantly protects mice from seizures and neuronal damage without interfering with locomotor or cognitive function.

Seizures and neurodegeneration induced by 4-aminopyridine in rat hippocampus in vivo: role of glutamate- and GABA-mediated neurotransmission and of ion channels

Infusion of the K(+) channel blocker 4-aminopyridine in the hippocampus induces the release of glutamate, as well as seizures and neurodegeneration. Since an imbalance between excitation and inhibition, as well as alterations of ion channels, may be involved in these effects of 4-aminopyridine, we have studied whether they are modified by drugs that block glutamatergic transmission or ion channels, or drugs that potentiate GABA-mediated transmission. The drugs were administered to anesthetized rats subjected to intrahippocampal infusion of 4-aminopyridine through microdialysis probes, with simultaneous collection of dialysis perfusates and recording of the electroencephalogram, and subsequent histological analysis. Ionotropic glutamate receptor antagonists clearly diminished the intensity of seizures and prevented the neuronal damage, but did not alter substantially the enhancement of extracellular glutamate induced by 4-aminopyridine. None of the drugs facilitating GABA-mediated transmission, including uptake blockers, GABA-transaminase inhibitors and agonists of the A-type receptor, was able to reduce the glutamate release, seizures or neuronal damage produced by 4-aminopyridine. In contrast, nipecotate, which notably increased extracellular levels of the amino acid, potentiated the intensity of seizures and the neurodegeneration. GABA(A) receptor antagonists partially reduced the extracellular accumulation of glutamate induced by 4-aminopyridine, but did not exert any protective action. Tetrodotoxin largely prevented the increase of extracellular glutamate, the electroencephalographic epileptic discharges and the neuronal death in the CA1 and CA3 hippocampal regions. Valproate and carbamazepine, also Na(+) channel blockers that possess general anticonvulsant action, failed to modify the three effects of 4-aminopyridine studied. The N-type Ca(2+) channel blocker omega-conotoxin, the K(+) channel opener diazoxide, and the non-specific ion channel blocker riluzole diminished the enhancement of extracellular glutamate and slightly protected against the neurodegeneration. However, the two former compounds did not antagonize the 4-aminopyridine-induced epileptiform discharges, and riluzole instead markedly increased the intensity and duration of the disharges. Moreover, at the highest dose tested (8mg/kg, i.p.), riluzole caused a 75% mortality of the rats. We conclude that 4-aminopyridine stimulates the release of glutamate from nerve endings and that the resultant augmented extracellular glutamate is directly related to the neurodegeneration and is involved in the generation of epileptiform discharges through the concomitant overactivation of glutamate receptors. Under these conditions, a facilitated GABA-mediated transmission may paradoxically boost neuronal hyperexcitation. Riluzole, a drug used to treat amyotrophic lateral sclerosis, seems to be toxic when combined with neuronal hyperexcitation.

K(ATP) channel openers, adenosine agonists and epileptic preconditioning are stress signals inducing hippocampal neuroprotection

Many models of induced ischemic and epileptic tolerance have now been described in the brain. Although detailed mechanisms underlying such protections still remain largely unknown, induction of heat shock proteins is amongst the endogenous responses believed to play an important role in cellular defense mechanisms. This study reveals that the development of epileptic tolerance also coincides with the induction of the 70,000 mol. wt heat shock protein expression within the time window of protection. Adenosine agonists or ATP-sensitive potassium channel openers have also been shown to exert strong neuroprotective effects when injected shortly prior to a severe ischemic or epileptic insult. The present work shows that adenosine receptor activation and ATP-sensitive potassium channel opening induce 70,000 mol. wt heat shock protein expression in the rat hippocampus and are able to mimic neuroprotection driven by preconditioning. R-phenylisopropyladenosine, a purine agonist, or (-)cromakalim, an ATP-sensitive potassium channel opener, was administered three days prior to a lethal ischemic or epileptic episode to mimic preconditioning. Neurodegeneration was assessed using Cresyl Violet staining and cellular DNA fragmentation visualized by the terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling method. 70, 000 mol. wt heat shock protein expression was analysed by western blotting and immunohistochemistry. The results show a long-lasting neuroprotection induced by activation of adenosine receptors or ATP-sensitive K(+) channels as early as three days prior to induction of a severe ischemic or epileptic challenge. This protective effect is associated with enhanced 70,000 mol. wt heat shock protein expression also occurring three days following administration of R-phenylisopropyladenosine or (-)cromakalim. These findings support the idea that preconditioning doses of R-phenylisopropyladenosine and (-)cromakalim act as mild cellular stresses inducing neuroprotection in a manner similar to a mild kainate treatment prior to a lethal ischemic or severe epileptic insult three days later. They also suggest that a delayed 70,000 mol. wt heat shock protein expression induced by excitatory neuronal stresses such as short ischemia, mild kainic acid treatment or activation of adenosine receptors and ATP-sensitive potassium channels is predictive of neuronal survival against a subsequent lethal injury.

Analysis of single K(ATP) channels in mammalian dentate gyrus granule cells

ATP-sensitive potassium (K(ATP)) channels are heteromultimer complexes of subunits from members of the inwardly rectifying K(+) channel and the ATP-binding cassette protein superfamilies. K(ATP) channels couple metabolic state to membrane excitability, are distributed widely, and participate in a variety of physiological functions. Understood best in pancreatic beta cells, where their activation inhibits insulin release, K(ATP) channels have been implicated also in postischemia cardio- and neuroprotection. The dentate gyrus (DG) is a brain region with a high density of K(ATP) channels and is relatively resistant to ischemia/reperfusion-induced cell death. Therefore we were interested in describing the characteristics of single K(ATP) channels in DG granule cells. We recorded single K(ATP) channels in 59/105 cell-attached patches from DG granule cells in acutely prepared hippocampal slices. Single-channel openings had an E(K) close to 0 mV (symmetrical K(+)) and were organized in bursts with a duration of 19.3 +/- 1.6 (SE) ms and a frequency of 3.5 +/- 0.8 Hz, a unitary slope conductance of 27 pS, and a low, voltage-independent, probability of opening (P(open), 0.04 +/- 0.01). Open and closed dwell-time histograms were fitted best with one (tau(open) = 1.3 +/- 0.2 ms) and the sum of two (tau(closed,fast) = 2.6 +/- 0.9 ms, tau(closed,slow) = 302.7 +/- 67. 7 ms) exponentials, respectively, consistent with a kinetic model having at least a single open and two closed states. The P(open) was reduced ostensibly to zero by the sulfonylureas, glybenclamide (500 nM, 2/6; 10 microM,11/14 patches) and tolbutamide (20 microM, 4/6; 100 microM, 4/4 patches). The blocking dynamics for glybenclamide included transition to a subconductance state (43.3 +/- 2.6% of control I(open channel)). Unlike glybenclamide, the blockade produced by tolbutamide was reversible. In 5/5 patches, application of diazoxide (100 microM) increased significantly P(open) (0.12 +/- 0.02), which was attributable to a twofold increase in the frequency of bursts (8.3 +/- 2.0 Hz). Diazoxide was without effect on tau(open) and tau(closed,fast) but decreased significantly tau(closed,slow) (24.4 +/- 2.6 ms). We observed similar effects in 6/7 patches after exposure to hypoxia/hypoglycemia, which increased significantly P(open) (0.09 +/- 0.03) and the frequency of bursts (7.1 +/- 1.7 Hz) and decreased significantly tau(closed,slow) (29.5 +/- 1.8 ms). We have presented convergent evidence consistent with single K(ATP) channel activity in DG granule cells. The subunit composition of K(ATP) channels native to DG granule cells is not known; however, the characteristics of the channel activity we recorded are representative of Kir6.1/SUR1, SUR2B-based channels.

Glucose-regulated dopamine release from substantia nigra neurons

Glucose modulates substantia nigra (SN) dopamine (DA) neuronal activity and GABA axon terminal transmitter release by actions on an ATP-sensitive potassium channel (K(ATP)). Here, the effect of altering SN glucose levels on striatal DA release was assessed by placing microdialysis probes into both the SN and striatum of male Sprague-Dawley rats. Reverse dialysis of 20 mM glucose through the SN probes transiently decreased striatal DA efflux by 32% with a return to baseline after 45 min despite constant glucose levels. During 50 mM glucose infusion, striatal DA efflux increased transiently by 50% and returned to baseline after 60 min. Infusion of 100 mM glucose produced a transient 25% decrease in striatal DA efflux followed by a sustained 50% increase above baseline. Efflux increased by a further 30% when the GABA(A) antagonist bicuculline (50 microM) was added to the 100 mM glucose infusate. At basal glucose levels, nigral bicuculline alone raised striatal DA efflux by 31% suggesting a tonic GABA inhibitory input to the DA neurons. The sulfonylurea glipizide (50 microM) produced a transient 25% increase in striatal DA release that became sustained when bicuculline was added. Thus, striatal DA release is affected by changing SN glucose levels. This response may well reflect the known effect of glucose on K(ATP) channel activity on both SN DA neurons and GABA axon terminals in the substantia nigra. These interactions could provide a mechanism whereby glucose modulates motor activity involved in food intake.

The adenosine-induced mechanism for the acquisition of ischemic tolerance in primary rat neuronal cultures

Neurons can be preconditioned by various procedures to resist ischemic insult. The preconditioning mechanism induced by adenosine ("the adenosine mechanism") was characterized in primary rat neuronal cultures, employing a model of chemical ischemia. The protective mechanism, initiated by activation of adenosine receptors, consists of a signal transduction pathway, involving activation of protein kinase C (PKC) and opening of ATP-sensitive potassium (K(ATP)) channels. Direct activation (and inhibition) of PKC, as well as opening of K(ATP) channels, also confers protection. The opening of the K(ATP) channels mediates the signal activated by the adenosine receptors, and probably also that activated by PKC. The acquired ischemic resistance lasts up to 5 days, depending on the activating substance. The adenosine-activated cascade of events leading to ischemic tolerance in neurons is similar to that operating in cardiomyocytes.