Potassium channel blocker TEA prevents CA1 hippocampal injury following transient forebrain ischemia in adult rats

Acupuncture ameliorates neurological function in rats with cerebral ischemia-reperfusion by regulating the opening of large-conductance Ca2+ -activated potassium channels

Acupuncture has a good effect on improving neurological function after cerebral ischemia-reperfusion, but there are few studies on the neuroprotective effect of acupuncture from the perspective of ion channel cellular electrophysiology. Studies have shown that the over activation of large-conductance Ca²⁺ -activated potassium channel （BKCa） after cerebral ischemia-reperfusion can reduce the excitability of neurons and induce apoptosis. This study intends to establish middle cerebral artery occlusion/reperfusion (MCAO/R) model, with acupuncture at GV26 as the intervention measure, using patch-clamp technique to record the electrophysiological changes of BKCa channel. The results showed that the neurological function score of MCAO/R rats was significantly decreased, and the conductance, open dwell time and open probability of BKCa channel in hippocampal CA1 neurons of MCAO/R rats were significantly increased. Acupuncture at GV26 could significantly improve the neurological function scores of MCAO/R rats, and reduce the conductance, open dwell time, and open probability of BKCa channel. The effect of acupuncture at GV26 was significantly better than acupuncture at non-acupuncture point. The neuroprotective effect of acupuncture at GV26 after cerebral ischemia-reperfusion may be related to regulating the electrophysiological characteristics of BKCa channel opening.

Disruption of KV2.1 somato-dendritic clusters prevents the apoptogenic increase of potassium currents

As the predominant mediator of the delayed rectifier current, K_V2.1 is an important regulator of neuronal excitability. K_V2.1, however, also plays a well-established role in apoptotic cell death. Apoptogenic stimuli induce syntaxin-dependent trafficking of K_V2.1, resulting in an augmented delayed rectifier current that acts as a conduit for K⁺ efflux required for pro-apoptotic protease/nuclease activation. Recent evidence suggests that K_V2.1 somato-dendritic clusters regulate the formation of endoplasmic reticulum-plasma membrane junctions that function as scaffolding sites for plasma membrane trafficking of ion channels, including K_V2.1. However, it is unknown whether K_V2.1 somato-dendritic clusters are required for apoptogenic trafficking of K_V2.1. By overexpression of a protein derived from the C-terminus of the cognate channel K_V2.2 (K_V2.2CT), we induced calcineurin-independent disruption of K_V2.1 somato-dendritic clusters in rat cortical neurons, without altering the electrophysiological properties of the channel. We observed that K_V2.2CT-expressing neurons are less susceptible to oxidative stress-induced cell death. Critically, expression of K_V2.2CT effectively blocked the increased current density of the delayed rectifier current associated with oxidative injury, supporting a vital role of K_V2.1-somato-dendritic clusters in apoptogenic increases in K_V2.1-mediated currents.

Enhancement of an outwardly rectifying chloride channel in hippocampal pyramidal neurons after cerebral ischemia

Cerebral ischemia induces delayed, selective neuronal death in the CA1 region of the hippocampus. The underlying molecular mechanisms remain unclear, but it is known that apoptosis is involved in this process. Chloride efflux has been implicated in the progression of apoptosis in various cell types. Using both the inside-out and whole-cell configurations of the patch-clamp technique, the present study characterized an outwardly rectifying chloride channel (ORCC) in acutely dissociated pyramid neurons in the hippocampus of adult rats. The channel had a nonlinear current-voltage relationship with a conductance of 42.26±1.2pS in the positive voltage range and 18.23±0.96pS in the negative voltage range, indicating an outward rectification pattern. The channel is Cl(-) selective, and the open probability is voltage-dependent. It can be blocked by the classical Cl(-) channel blockers DIDS, SITS, NPPB and glibenclamide. We examined the different changes in ORCC activity in CA1 and CA3 pyramidal neurons at 6, 24 and 48h after transient forebrain ischemia. In the vulnerable CA1 neurons, ORCC activity was persistently enhanced after ischemic insult, whereas in the invulnerable CA3 neurons, no significant changes occurred. Further analysis of channel kinetics suggested that multiple openings are a major contributor to the increase in channel activity after ischemia. Pharmacological blockade of the ORCC partly attenuated cell death in the hippocampal neurons. We propose that the enhanced activity of ORCC might contribute to selective neuronal damage in the CA1 region after cerebral ischemia, and that ORCC may be a therapeutic target against ischemia-induced cell death.

The inhibitory effect of propofol on Kv2.1 potassium channel in rat parietal cortical neurons

Excessive K(+) efflux via activated voltage-gated K(+) channels can deplete intracellular K(+) and lead to long-lasting membrane depolarization which will promote neuronal apoptosis during ischemia/hypoxia injury. The Kv2.1 potassium channel was the major component of delayed rectifier potassium current (Ik) in pyramidal neurons in cortex and hippocampus. The neuronal protective effect of propofol has been proved. Delayed rectifier potassium current (Ik) has been shown to have close relationship with neuronal damage. The study was designed to test the inhibitory effect of propofol on Kv2.1 potassium channel in rat parietal cortical neurons. Whole-cell patch clamp recordings and Western blot analysis were used to investigate the electrophysiological function and protein expression of Kv2.1 in rat parietal cortical neurons after propofol treatment. We found that propofol concentration-dependently inhibited Ik in pyramidal neurons. Propofol also caused a downward shift of the I-V curve of Ik at 30μM concentration. Propofol significantly inhibited the expression of Kv2.1 protein level at 30μM, 50μM, 100μM concentration. In conclusion, our data showed that propofol could inhibit Ik, probably via depressing the expression of Kv2.1 protein in rat cerebral parietal cortical neurons.

Functional study of TREK-1 potassium channels during rat heart development and cardiac ischemia using RNAi techniques

To explore the physiological and pathological significance of the 2-pore domain potassium channel TWIK-related K(+) (TREK)-1 in rat heart, its expression and role during heart development and cardiac ischemia were investigated. In the former study, the ventricles of Sprague Dawley rats were collected from embryo day 19 to postnatal 18 months and examined for mRNA and protein expression of TREK-1. It was found that both increased during development, reached a maximum at postnatal day 28, and remained higher at postnatal day 3 through to postnatal 18 months. In the latter study, protein expression of TREK-1 was examined after initiation of acute heart ischemia by ligation of the left anterior descending coronary artery. TREK-1 expression was found to be increased in the endocardium but unchanged in the epicardium. In primary cultured rat neonatal ventricular myocytes subjected to hypoxia (oxygen-glucose deprivation), TREK-1 expression was increased. In cultured neonatal cardiomyocytes, silencing of the TREK-1 gene by lentivirus delivery of the short-hairpin RNAs, L-sh-492 and L-sh-605, was found to promote their viability and number. In addition, both short-hairpin RNA provided protection against hypoxia-induced injury to cardiomyocytes in vitro. These results suggest that TREK-1 plays an important role in neonatal rat heart development and downregulation of TREK-1 may provide protection against ischemic injury. It seems that TREK-1 is a potential drug target for treatment of acute heart ischemia.

Glibenclamide in cerebral ischemia and stroke

The sulfonylurea receptor 1 (Sur1)-transient receptor potential 4 (Trpm4) channel is an important molecular element in focal cerebral ischemia. The channel is upregulated in all cells of the neurovascular unit following ischemia, and is linked to microvascular dysfunction that manifests as edema formation and secondary hemorrhage, which cause brain swelling. Activation of the channel is a major molecular mechanism of cytotoxic edema and "accidental necrotic cell death." Blockade of Sur1 using glibenclamide has been studied in different types of rat models of stroke: (i) in conventional non-lethal models (thromboembolic, 1-2 h temporary, or permanent middle cerebral artery occlusion), glibenclamide reduces brain swelling and infarct volume and improves neurological function; (ii) in lethal models of malignant cerebral edema, glibenclamide reduces edema, brain swelling, and mortality; (iii) in models with rtPA, glibenclamide reduces swelling, hemorrhagic transformation, and death. Retrospective studies of diabetic patients who present with stroke have shown that those whose diabetes is managed with a sulfonylurea drug and who are maintained on the sulfonylurea drug during hospitalization for stroke have better outcomes at discharge and are less likely to suffer hemorrhagic transformation. Here, we provide a comprehensive review of the basic science, preclinical experiments, and retrospective clinical studies on glibenclamide in focal cerebral ischemia and stroke. We also compare the preclinical work in stroke models to the updated recommendations of the Stroke Therapy Academic Industry Roundtable (STAIR). The findings reviewed here provide a strong foundation for a translational research program to study glibenclamide in patients with ischemic stroke.

Decreased calcium-activated potassium channels by hypoxia causes abnormal firing in the spontaneous firing medial vestibular nuclei neurons

Vertebrobasilar insufficiency (VBI) presents complex varied clinical symptoms, including vertigo and hearing loss. Little is known, however, about how Ca(2+)-activated K(+) channel attributes to the medial vestibular nucleus (MVN) neural activity in VBI. To address this issue, we performed whole-cell patch clamp and quantitative polymerase chain reaction (qPCR) to examine the effects of hypoxia on neural activity and the changes of the large conductance Ca(2+) activated K(+) channels (BKCa channels) in the MVN neurons in brain slices of male C57BL/6 mice. Brief hypoxic stimuli of the brain slices containing MVN were administrated by switching the normoxic artificial cerebrospinal fluid (ACSF) equilibrated with 21% O2/5% CO2 to hypoxic ACSF equilibrated with 5% O2/5% CO2 (balance N2). 3-min hypoxia caused a depolarization in the resting membrane potential (RM) in 8/11 non-spontaneous firing MVN neurons. 60/72 spontaneous firing MVN neurons showed a dramatic increase in firing frequency and a depolarization in the RM following brief hypoxia. The amplitude of the afterhyperpolarization (AHPA) was significantly decreased in both type A and type B spontaneous firing MVN neurons. Hypoxia-induced firing response was alleviated by pretreatment with NS1619, a selective BKCa activator. Furthermore, brief hypoxia caused a decrease in the amplitude of iberiotoxin-sensitive outward currents and mRNA level of BKCa in MVN neurons. These results suggest that BKCa channels protect against abnormal MVN neuronal activity induced by hypoxia, and might be a key target for treatment of vertigo and hearing loss in VBI.

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.

Sulfonylurea receptor 1 in central nervous system injury: a focused review

The sulfonylurea receptor 1 (Sur1)-regulated NC(Ca-ATP) channel is a nonselective cation channel that is regulated by intracellular calcium and adenosine triphosphate. The channel is not constitutively expressed, but is transcriptionally upregulated de novo in all cells of the neurovascular unit, in many forms of central nervous system (CNS) injury, including cerebral ischemia, traumatic brain injury (TBI), spinal cord injury (SCI), and subarachnoid hemorrhage (SAH). The channel is linked to microvascular dysfunction that manifests as edema formation and delayed secondary hemorrhage. Also implicated in oncotic cell swelling and oncotic (necrotic) cell death, the channel is a major molecular mechanism of 'accidental necrotic cell death' in the CNS. In animal models of SCI, pharmacological inhibition of Sur1 by glibenclamide, as well as gene suppression of Abcc8, prevents delayed capillary fragmentation and tissue necrosis. In models of stroke and TBI, glibenclamide ameliorates edema, secondary hemorrhage, and tissue damage. In a model of SAH, glibenclamide attenuates the inflammatory response due to extravasated blood. Clinical trials of an intravenous formulation of glibenclamide in TBI and stroke underscore the importance of recent advances in understanding the role of the Sur1-regulated NC(Ca-ATP) channel in acute ischemic, traumatic, and inflammatory injury to the CNS.

Effects of carvedilol on delayed rectifier and transient inactivating potassium currents in rat hippocampal CA1 neurons

1. The aims of the present study were to investigate the mechanism(s) underlying the protective effect of carvedilol against neural damage. 2. The transient inactivating potassium current (I(A) ) and the delayed rectifier potassium current (I(K) ) in rat hippocampal CA1 pyramidal neurons were recorded using whole-cell patch-clamp techniques. 3. Carvedilol (0.1-3 μmol/L) significantly inhibited I(K) with an IC(50) of 1.3 μmol/L and the inhibition was voltage independent. Over the same concentration range, carvedilol had no effect on the amplitude of I(A). At 1 μmol/L, carvedilol did not significantly change the steady state activation curves of I(A) and I(K), but did negatively shift their steady state inactivation curves. Recovery from inactivation was slowed for both I(A) and I(K). The inhibitory effect of carvedilol on I(K) was not affected by the adrenoceptor agonists phenylephrine and prazosin or the adrenoceptor antagonist isoproterenol, but propranolol was able to shift the dose-response curve of carvedilol for I(K) to the right. 4. Because I(K) is the main pathway for loss of intracellular potassium from depolarized neurons, selective obstruction of I(K) by carvedilol could be useful for neuroprotection.

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.

delta-Opioid receptors protect from anoxic disruption of Na+ homeostasis via Na+ channel regulation

Hypoxic/ischemic disruption of ionic homeostasis is a critical trigger of neuronal injury/death in the brain. There is, however, no promising strategy against such pathophysiologic change to protect the brain from hypoxic/ischemic injury. Here, we present a novel finding that activation of delta-opioid receptors (DOR) reduced anoxic Na+ influx in the mouse cortex, which was completely blocked by DOR antagonism with naltrindole. Furthermore, we co-expressed DOR and Na+ channels in Xenopus oocytes and showed that DOR expression and activation indeed play an inhibitory role in Na+ channel regulation by decreasing the amplitude of sodium currents and increasing activation threshold of Na+ channels. Our results suggest that DOR protects from anoxic disruption of Na+ homeostasis via Na+ channel regulation. These data may potentially have significant impacts on understanding the intrinsic mechanism of neuronal responses to stress and provide clues for better solutions of hypoxic/ischemic encephalopathy, and for the exploration of acupuncture mechanism since acupuncture activates opioid system.

Arylbenzazepines are potent modulators for the delayed rectifier K+ channel: a potential mechanism for their neuroprotective effects

(+/-) SKF83959, like many other arylbenzazepines, elicits powerful neuroprotection in vitro and in vivo. The neuroprotective action of the compound was found to partially depend on its D(1)-like dopamine receptor agonistic activity. The precise mechanism for the (+/-) SKF83959-mediated neuroprotection remains elusive. We report here that (+/-) SKF83959 is a potent blocker for delayed rectifier K(+) channel. (+/-) SKF83959 inhibited the delayed rectifier K(+) current (I(K)) dose-dependently in rat hippocampal neurons. The IC(50) value for inhibition of I(K) was 41.9+/-2.3 microM (Hill coefficient = 1.81+/-0.13, n = 6), whereas that for inhibition of I(A) was 307.9+/-38.5 microM (Hill coefficient = 1.37+/-0.08, n = 6). Thus, (+/-) SKF83959 is 7.3-fold more potent in suppressing I(K) than I(A). Moreover, the inhibition of I(K) by (+/-) SKF83959 was voltage-dependent and not related to dopamine receptors. The rapidly onset of inhibition and recovery suggests that the inhibition resulted from a direct interaction of (+/-) SKF83959 with the K(+) channel. The intracellular application of (+/-) SKF83959 had no effects of on I(K), indicating that the compound most likely acts at the outer mouth of the pore of K(+) channel. We also tested the enantiomers of (+/-) SKF83959, R-(+) SKF83959 (MCL-201), and S-(-) SKF83959 (MCL-202), as well as SKF38393; all these compounds inhibited I(K). However, (+/-) SKF83959, at either 0.1 or 1 mM, exhibited the strongest inhibition on the currents among all tested drug. The present findings not only revealed a new potent blocker of I(K) , but also provided a novel mechanism for the neuroprotective action of arylbenzazepines such as (+/-) SKF83959.

Gene profiles and electrophysiology of doublecortin-expressing cells in the subventricular zone after ischemic stroke

Stroke increases neuroblasts in the subventricular zone (SVZ) of the lateral ventricle and these neuroblasts migrate toward the ischemic boundary to replace damaged neurons. Using brain slices from the nonischemic adult rat and transgenic mice that expressed enhanced green fluorescent protein (EGFP) concomitantly with doublecortin (DCX), a marker for migrating neuroblasts, we recorded electrophysiological characteristics while simultaneously analyzing the gene expression in single SVZ cells. We found that SVZ cells expressing the DCX gene from the nonischemic rat had a mean resting membrane potential (RMP) of -30 mV. DCX-EGFP-positive cells in the nonischemic SVZ of the transgenic mouse had a mean RMP of -25+/-7 mV and did not exhibit Na(+) currents, characteristic of immature neurons. However, DCX-EGFP-positive cells in the ischemic SVZ exhibited a hyperpolarized mean RMP of -54+/-18 mV and displayed Na(+) currents, indicative of more mature neurons. Single-cell multiplex RT-PCR analysis revealed that DCX-EGFP-positive cells in the nonischemic SVZ of the transgenic mouse expressed high neural progenitor marker genes, Sox2 and nestin, but not mature neuronal marker genes. In contrast, DCX-EGFP-positive cells in the ischemic SVZ expressed tyrosine hydroxylase, a mature neuronal marker gene. Together, these data indicate that stroke changes gene profiles and the electrophysiology of migrating neuroblasts.

Electrophysiological characterization of a novel Kv channel blocker N,N'-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl) ]bis(4-methyl)-benzenesulfonamide found in virtual screening

AIM

N,No-[oxybis(2,1-ethanediyloxy-2,1-ethanediyl)]bis(4-methyl)- benzenesulfonamide (OMBSA) is a hit compound with potent voltage-gated K+ (Kv) channel-blocking activities that was found while searching the MDL Available Chemicals Directory with a virtual screening approach. In the present study, the blocking actions of OMBSA on Kv channels and relevant mechanisms were characterized.

METHODS

Whole-cell voltage-clamp recording was made in acutely dissociated hippocampal CA1 pyramidal neurons of newborn rats.

RESULTS

Superfusion of OMBSA reversibly inhibited both the delayed rectifier (I(K)) and fast transient K+ currents (I(A)) with IC50 values of 2.1+/-1.1 micromol/L and 27.8+/-1.5 micromol/L, respectively. The inhibition was voltage independent. OMBSA markedly accelerated the decay time course of IK, without a significant effect on that of I(A). OMBSA did not change the activation, steady-state inactivation of IK, and its recovery from inactivation, but the compound caused a significant hyperpolarizing shift of the voltage dependence of the steady-state inactivation of I(A) and slowed down its recovery from inactivation. Intracellular dialysis of OMBSA had no effect on both I(K) and I(A).

CONCLUSION

The results demonstrate that OMBSA blocks both I(K) and I(A) through binding to the outer mouth of the channel pore, as predicted by the molecular docking model used in the virtual screening. In addition, the compound differentially moderates the inactivation kinetics of the K+ channels through allosteric mechanisms.

Activation of DOR attenuates anoxic K+ derangement via inhibition of Na+ entry in mouse cortex

We have recently found that in the mouse cortex, activation of delta-opioid receptor (DOR) attenuates the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation. This novel observation suggests that DOR may protect neurons from hypoxic/ischemic insults via the regulation of K(+) homeostasis because the disruption of K(+) homeostasis plays a critical role in neuronal injury under hypoxic/ischemic stress. The present study was performed to explore the ionic mechanism underlying the DOR-induced neuroprotection. Because anoxia causes Na(+) influx and thus stimulates K(+) leakage, we investigated whether DOR protects the cortex from anoxic K(+) derangement by targeting the Na(+)-based K(+) leakage. By using K(+)-sensitive microelectrodes in mouse cortical slices, we showed that 1) lowering Na(+) concentration and substituting with impermeable N-methyl-D-glucamine caused a concentration-dependent attenuation of anoxic K(+) derangement; 2) lowering Na(+) concentration by substituting with permeable Li(+) tended to potentiate the anoxic K(+) derangement; and 3) the DOR-induced protection against the anoxic K(+) responses was largely abolished by low-Na(+) perfusion irrespective of the substituted cation. We conclude that external Na(+) concentration greatly influences anoxic K(+) derangement and that DOR activation likely attenuates anoxic K(+) derangement induced by the Na(+)-activated mechanisms in the cortex.

delta-, but not mu-, opioid receptor stabilizes K(+) homeostasis by reducing Ca(2+) influx in the cortex during acute hypoxia

Past work has shown that delta-opioid receptor (DOR) activation by [D-Ala(2),D-Leu(5)]-enkephalin (DADLE) attenuated the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation (OGD) in the cortex, while naltrindole, a DOR antagonist blocked this effect, suggesting that DOR activity stabilizes K(+) homeostasis in the cortex during hypoxic/ischemic stress. However, several important issues remain unclear regarding this new observation, especially the difference between DOR and other opioid receptors in the stabilization of K(+) homeostasis and the underlying mechanism. In this study, we asked whether DOR is different from micro-opioid receptors (MOR) in stabilizing K(+) homeostasis and which membrane channel(s) is critically involved in the DOR effect. The main findings are that (1) similar to DADLE (10 microM), H-Dmt-Tic-NH-CH (CH(2)--COOH)-Bid (1-10 microM), a more specific and potent DOR agonist significantly attenuated anoxic K(+) derangement in cortical slice; (2) [D-Ala(2), N-Me-Phe(4), glycinol(5)]-enkephalin (DAGO; 10 microM), a MOR agonist, did not produce any appreciable change in anoxic disruption of K(+) homeostasis; (3) absence of Ca(2+) greatly attenuated anoxic K(+) derangement; (4) inhibition of Ca(2+)-activated K(+) (BK) channels with paxilline (10 microM) reduced anoxic K(+) derangement; (5) DADLE (10 microM) could not further reduce anoxic K(+) derangement in the Ca(2+)-free perfused slices or in the presence of paxilline; and (6) glybenclamide (20 microM), a K(ATP) channel blocker, decreased anoxia-induced K(+) derangement, but DADLE (10 microM) could further attenuate anoxic K(+) derangement in the glybenclamide-perfused slices. These data suggest that DOR, but not MOR, activation is protective against anoxic K(+) derangement in the cortex, at least partially via an inhibition of hypoxia-induced increase in Ca(2+) entry-BK channel activity.

Anion channel blockers attenuate delayed neuronal cell death induced by transient forebrain ischemia

Chloride efflux is known to be involved in the progression of apoptosis in various cell types. We have recently shown that the volume-sensitive outwardly rectifying (VSOR) anion channel serves as the pathway for apoptotic chloride efflux in some cells. In the present study, we tested the neuroprotective effects of drugs that can block the VSOR anion channel, on delayed neuronal death (DND) induced by transient forebrain ischemia. The functional expression of the VSOR anion channel was first examined in hippocampal neurons in both primary culture and hippocampal slice preparations, by the whole-cell patch-clamp technique. We then tested the channel's sensitivity to an anion channel blocker, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and a tyrosine kinase blocker, genistein. By histological examinations and cytochrome c release assessments, the protective effects of these drugs on the DND of hippocampal CA1 neurons in mice subjected to transient ischemia were examined. Drugs were administered via the jugular vein prior to ischemic treatment and into the peritoneal cavity after reperfusion. Hippocampal neurons were found to express the volume-sensitive Cl(-) channel, which exhibits outward rectification and is sensitive to DIDS and genistein. Administration of DIDS or genistein reduced cytochrome c release and the number of damaged neurons in the CA1 region after transient forebrain ischemia. This fact suggests that the DND induction mechanism involves the activity of the VSOR anion channel and that this channel may provide a therapeutic target for the treatment of stroke.

Calcineurin-independent inhibition of the delayed rectifier K+ current by the immunosuppressant FK506 in rat hippocampal neurons

The immunosuppressant drug FK506 was found to be a potent neuroprotective agent in animal models of brain ischemia. However, the mechanisms underlying the action remain to be elucidated. The delayed rectifier K(+) channel has been implicated in ischemic injury and neuronal death in the brain. The aim of the present study is to investigate whether the neuroprotective action of FK506 results from blocking the K(+) channel. In acutely dissociated CA1 pyramidal neurons of rat hippocampus, superfusion of FK506 (0.01-100 microM) selectively inhibited the delayed rectifier K(+) current (I(K)) with an IC(50) value of 13.2+/-4.9 microM. The inhibition of I(K) by FK506 (10 microM) had a rapid onset, and then gradually reached a steady-state level. The inhibition was voltage-dependent, became more potent when the currents were elicited by strong depolarization. Moreover, FK506 (10 microM) caused marked negative shifts of the steady-state activation and inactivation curves of I(K), and accelerated its recovery from inactivation. Intracellular dialysis of FK506 (30 microM) was ineffective. The inhibition of I(K) by FK506 (10 microM) persisted under the low-Ca(2+) conditions that blocked the basal activity of protein phosphatase 2B (calcineurin). Rapamycin did not antagonize FK506 but mimicked it. Cyclosporin A inhibited I(K) only at 30 and 100 microM. Taken together, the results suggest that FK506 exert a direct inhibition on the delayed rectifier K(+) channel without involvement of calcineurin.

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.

Cortical delta-opioid receptors potentiate K+ homeostasis during anoxia and oxygen-glucose deprivation

Central neurons are extremely vulnerable to hypoxic/ischemic insult, which is a major cause of neurologic morbidity and mortality as a consequence of neuronal dysfunction and death. Our recent work has shown that delta-opioid receptor (DOR) is neuroprotective against hypoxic and excitotoxic stress, although the underlying mechanisms remain unclear. Because hypoxia/ischemia disrupts ionic homeostasis with an increase in extracellular K(+), which plays a role in neuronal death, we asked whether DOR activation preserves K(+) homeostasis during hypoxic/ischemic stress. To test this hypothesis, extracellular recordings with K(+)-sensitive microelectrodes were performed in mouse cortical slices under anoxia or oxygen-glucose deprivation (OGD). The main findings in this study are that (1) DOR activation with [D-Ala(2), D-Leu(5)]-enkephalinamide attenuated the anoxia- and OGD-induced increase in extracellular K(+) and decrease in DC potential in cortical slices; (2) DOR inhibition with naltrindole, a DOR antagonist, completely abolished the DOR-mediated prevention of increase in extracellular K(+) and decrease in DC potential; (3) inhibition of protein kinase A (PKA) with N-(2-[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide dihydrochloride had no effect on the DOR protection; and (4) inhibition of protein kinase C (PKC) with chelerythrine chloride reduced the DOR protection, whereas the PKC activator (phorbol 12-myristate 13-acetate) mimicked the effect of DOR activation on K(+) homeostasis. These data suggest that activation of DOR protects the cortex against anoxia- or ODG-induced derangement of potassium homeostasis, and this protection occurs via a PKC-dependent and PKA-independent pathway. We conclude that an important aspect of DOR-mediated neuroprotection is its early action against derangement of K(+) homeostasis during anoxia or ischemia.

The Blockade of K+‐ATP Channels has Neuroprotective Effects in an In Vitro Model of Brain Ischemia

There is a common belief that the opening of K(+)-ATP channels during an ischemic episode has protective effects on neuronal functions by inducing a reduction in energy consumption. However, recent studies have also proposed that activation of these channels might have deleterious effects on cell's survival possibly after a stroke or during long-lasting neurodegenerative processes. Considering these contrasting results, we have used a hippocampal in vitro slice preparation in order to investigate the possible effects of K(+)-ATP channel blockers on the electrophysiological and morphological changes induced by a transient episode of ischemia (oxygen and glucose deprivation) on CA1 pyramidal neurons. Therefore, we found that tolbutamide and glibenclamide, both nonselective K(+)-ATP channel blockers, produce neuroprotective effects against in vitro ischemia. Interestingly, the mitochondrial K(+)-ATP channel blocker 5-hydroxydecanoate and various K(+) channel blockers did not exert neuroprotection. Our results are consistent with the concept that a decreased activity of the plasmalemmal K(+)-ATP conductances may have a protective effect during episodes of transient cerebral ischemia.

Calcium- and metabolic state-dependent modulation of the voltage-dependent Kv2.1 channel regulates neuronal excitability in response to ischemia

Ischemic stroke is often accompanied by neuronal hyperexcitability (i.e., seizures), which aggravates brain damage. Therefore, suppressing stroke-induced hyperexcitability and associated excitoxicity is a major focus of treatment for ischemic insults. Both ATP-dependent and Ca2+-activated K+ channels have been implicated in protective mechanisms to suppress ischemia-induced hyperexcitability. Here we provide evidence that the localization and function of Kv2.1, the major somatodendritic delayed rectifier voltage-dependent K+ channel in central neurons, is regulated by hypoxia/ischemia-induced changes in metabolic state and intracellular Ca2+ levels. Hypoxia/ischemia in rat brain induced a dramatic dephosphorylation of Kv2.1 and the translocation of surface Kv2.1 from clusters to a uniform localization. In cultured rat hippocampal neurons, chemical ischemia (CI) elicited a similar dephosphorylation and translocation of Kv2.1. These events were reversible and were mediated by Ca2+ release from intracellular stores and calcineurin-mediated Kv2.1 dephosphorylation. CI also induced a hyperpolarizing shift in the voltage-dependent activation of neuronal delayed rectifier currents (IK), leading to enhanced IK and suppressed neuronal excitability. The IK blocker tetraethylammonium reversed the ischemia-induced suppression of excitability and aggravated ischemic neuronal damage. Our results show that Kv2.1 can act as a novel Ca2+- and metabolic state-sensitive K+ channel and suggest that dynamic modulation of IK/Kv2.1 in response to hypoxia/ischemia suppresses neuronal excitability and could confer neuroprotection in response to brief ischemic insults.

Trans-resveratrol, a red wine ingredient, inhibits voltage-activated potassium currents in rat hippocampal neurons

The red wine ingredient trans-resveratrol was found to exert potent neuroprotective effects in different in vivo and in vitro models. Thus far, the mechanisms underlying the neuroprotection were attributed mainly to its antioxidant properties. The aim of this study was to investigate the actions of trans-resveratrol on voltage-gated K(+) channels, which have been implicated in neuronal apoptosis. Superfusion of trans-resveratrol reversibly inhibited both the delayed rectifier (I(K)) and fast transient K(+) current (I(A)) in rat dissociated hippocampal neurons with IC(50) values of 13.6 +/- 1.0 microM and 45.7 +/- 7.5 microM, respectively. The inhibition on I(K) had a slow onset, was neither voltage dependent nor use dependent. Trans-resveratrol (30 microM) shifted the steady-state inactivation curve of I(K) to the hyperpolarizing direction by 20 mV and slowed down its recovery from inactivation. The inhibition on I(A) was similar to that on I(K), but voltage dependent. Superfusion of trans-resveratrol (30 microM) shifted the steady-state activation curve of I(A) to the depolarizing direction by 17 mV. Intracellular application of trans-resveratrol (30 microM) was ineffective. Based on the comparable effective concentrations, the inhibition of voltage-activated K(+) currents by trans-resveratrol may contribute to its neuroprotective effects.

Neuronal transmission stimulates the phosphorylation of Kv1.4 channel at Ser229 through protein kinase A1

Phosphorylation of voltage-gated K+ channels (Kv) is involved in regulation of neuronal excitability, synaptic plasticity and neuronal survival. Among Kv channels expressed in the CNS, Kv1.4 is located in the soma, dendrite and axon terminus of neurones in most regions of the brain. Here, we show that Ser229 found within the highly conserved T1 domain of Kv1.4 in cultured rat cortical neurones is phosphorylated by protein kinase A (PKA), as demonstrated by in vitro protein kinase assay and Western blotting with a polyclonal antibody specific against phosphorylated Ser229. Glutamate, high concentrations of K+ or K+ channel blockers known to increase neurotransmission all stimulated the phosphorylation of Kv1.4 at Ser229 via N-methyl-D-aspartate (NMDA), but not alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptor, whereas tetradotoxin (TTX), known to block neuronal transmission, and depletion of extracellular Ca2+ inhibited phosphorylation induced by tetraethylammonium (TEA), a non-selective K+ channel blocker. Mutation of Ser229 to Ala229 enhanced the current density. Taken together, elevation of the neuronal transmission stimulates the phosphorylation of Kv1.4 at Ser229 via the Ca2+ influx through NMDA receptor. Thus, it is possible that neuronal transmission regulates neuronal excitability partially through the phosphorylation of Kv1.4S229.

Alterations of potassium currents in ischemia-vulnerable and ischemia-resistant neurons in the hippocampus after ischemia

CA1 pyramidal neurons in the hippocampus die 2-3 days following transient forebrain ischemia, whereas CA3 pyramidal neurons and granule cells in the dentate gyrus remain viable. Excitotoxicity is the major cause of ischemic cell death, and potassium currents play important roles in regulating the neuronal excitability. The present study compared the changes of potassium currents in acutely dissociated hippocampal neurons at different intervals after ischemia. In CA1 neurons, the amplitude of rapid inactivating potassium currents (I(A)) was significantly increased at 14 h and returned to control levels at 38 h after ischemia; the rising slope and decay time constant of I(A) were accordingly increased after ischemia. The activation curve of I(A) in CA1 neurons shifted to the depolarizing direction at 38 h after ischemia. In granule cells, the amplitude and rising slope of I(A) were significantly increased at 38 h after ischemia; the inactivation curves of I(A) shifted toward the depolarizing direction accordingly at 38 h after ischemia. The I(A) remained unchanged in CA3 neurons after ischemia. The amplitudes of delayed rectifier potassium currents (I(Kd)) in CA1 neurons were progressively increased after ischemia. No significant difference in I(Kd) was detected in CA3 and granule cells at any time points after reperfusion. These results indicated that the voltage dependent potassium currents in hippocampal neurons were differentially altered after cerebral ischemia. The up-regulation of I(A) in dentate granule cells might have protective effects. The increase of I(Kd) in CA1 neurons might be associated with the neuronal damage after ischemia.

Increase of delayed rectifier potassium currents in large aspiny neurons in the neostriatum following transient forebrain ischemia

Large aspiny (LA) neurons in the neostriatum are resistant to cerebral ischemia whereas spiny neurons are highly vulnerable to the same insult. Excitotoxicity has been implicated as the major cause of neuronal damage after ischemia. Voltage-dependent potassium currents play important roles in controlling neuronal excitability and therefore influence the ischemic outcome. To reveal the ionic mechanisms underlying the ischemia-resistance, the delayed rectifier potassium currents (Ik) in LA neurons were studied before and at different intervals after transient forebrain ischemia using brain slices and acute dissociation preparations. The current density of Ik increased significantly 24 h after ischemia and returned to control levels 72 h following reperfusion. Among currents contributing to Ik, the margatoxin-sensitive currents increased 24 h after ischemia while the KCNQ/M current remained unchanged after ischemia. Activation of protein kinase A (PKA) down-regulated Ik in both control and ischemic LA neurons, whereas inhibition of PKA only up-regulated Ik and margatoxin-sensitive currents 72 h after ischemia, indicating an active PKA regulation on Ik at this time. Protein tyrosine kinases had a tonic inhibition on Ik to a similar extent before and after ischemia. Compared with that of control neurons, the spike width was significantly shortened 24 h after ischemia due to facilitated repolarization, which could be reversed by blocking margatoxin-sensitive currents. The increase of Ik in LA neurons might be one of the protective mechanisms against ischemic insult.

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.

Neuroprotective effects of Buyang Huanwu Decoction on neuronal injury in hippocampus after transient forebrain ischemia in rats

Buyang Huanwu Decoction (BYHWD), a traditional Chinese medicine, has been developed as a drug to be used for treatment of stroke for hundreds of years. However, the underlying mechanisms remain unknown. In the present study, the effects of BYHWD on delayed neuronal death of hippocampus after transient forebrain ischemia were examined in rats. Transient forebrain ischemia in a duration of 15 min was induced with the four-vessel occlusion method. BYHWD (per 6.65 g/kg) was given orally to rats twice each day for 7 days before ischemia. In BYHWD-pretreated rats, the neuronal injury in the hippocampal CA1 region was significantly less than that of controls. Oral administration of BYHWD also markedly attenuated the number of TUNEL-positive neurons and suppressed the expression of caspase-3p20, a product of catalytically active caspase-3, in the CA1 region. Our results suggest that an inhibition of caspase-3 and apoptosis by BYHWD may partially account for its neuroprotection against ischemic injury in the hippocampal CA1 region.

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.

Potassium channel blockers attenuate hypoxia- and ischemia-induced neuronal death in vitro and in vivo

BACKGROUND AND PURPOSE

In light of recent evidence suggesting that an upregulation of K+ efflux mediated by outward delayed rectifier (I(K)) channels promotes central neuronal apoptosis, we sought to test the possibility that blockers of I(K) channels might be neuroprotective against hypoxia/ischemia-induced neuronal death.

METHODS

Membrane currents were recorded with the use of patch clamp recordings in cultured murine cortical neurons. Protective effects of K+ channel blockers were examined in rats subjected to transient middle cerebral artery occlusion followed by 14-day reperfusion.

RESULTS

The K+ channel blocker tetraethylammonium (TEA) (5 mmol/L) selectively blocked I(K) without affecting N-methyl-D-aspartate receptor-mediated current or voltage-gated Ca2+ currents. Both TEA and a lipophilic K+ channel blocker, clofilium, attenuated neuronal apoptosis induced by hypoxia in vitro and infarct volume induced by ischemia in vivo.

CONCLUSIONS

These data are consistent with the idea that K+ channel-mediated K+ efflux may contribute to ischemia-triggered apoptosis and suggest that preventing excessive K+ efflux through K+ channels may constitute a therapeutic approach for the treatment of stroke.

Transient forebrain ischemia induces persistent hyperactivity of large conductance Ca2+-activated potassium channels via oxidation modulation in rat hippocampal CA1 pyramidal neurons

The present study examined temporal changes in activity of large conductance, Ca2+-activated potassium (BKCa) channels in postischemic CA1 pyramidal neurons at 2, 6, 24 and 48 h after reperfusion. These changes in activity and possible cellular mechanisms were examined using the inside--out configuration of patch clamp. The unitary conductance of postischemic BKCa channels increased transiently to 119% of the control at 2 h after reperfusion, and recovered to the control level thereafter. A persistent increase in [Ca2+]i sensitivity of BKCa channels was observed in postischemic CA1 neurons with the maximal sensitivity to [Ca2+]i at 6 h after reperfusion while channel voltage- dependence showed no obvious changes. Kinetic analyses showed that the postischemic enhancement of BKCa channel activity was due to longer open times and shorter closed times as there was no significant changes in opening frequency after ischemia. Glutathione disulphide markedly increased BKCa channel activity in normal CA1 neurons, while reducing glutathione caused a decrease in BKCa channel activity by reducing the sensitivity of this channel to [Ca2+]i in postischemic CA1 neurons. Similar modulatory effects on postischemic BKCa channels were also observed with another redox couple, DTNB and DTT, suggesting an oxidation modulation of BKCa channel function after ischemia. The present results indicate that a persistent enhancement in activity of BKCa channels, probably via oxidation of channels, in postischemic CA1 pyramidal neurons may account for the decrease in neuronal excitability and increase in fAHP after ischemia. The ischemia-induced augmentation in BKCa channel activity may be also associated with the postischemic neuronal injury.