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

Hypoxia with inflammation and reperfusion alters membrane resistance by dynamically regulating voltage-gated potassium channels in hippocampal CA1 neurons

Hypoxia typically accompanies acute inflammatory responses in patients and animal models. However, a limited number of studies have examined the effect of hypoxia in combination with inflammation (Hypo-Inf) on neural function. We previously reported that neuronal excitability in hippocampal CA1 neurons decreased during hypoxia and greatly rebounded upon reoxygenation. We attributed this altered excitability mainly to the dynamic regulation of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels and input resistance. However, the molecular mechanisms underlying input resistance changes by Hypo-Inf and reperfusion remained unclear. In the present study, we found that a change in the density of the delayed rectifier potassium current (IDR) can explain the input resistance variability. Furthermore, voltage-dependent inactivation of A-type potassium (IA) channels shifted in the depolarizing direction during Hypo-Inf and reverted to normal upon reperfusion without a significant alteration in the maximum current density. Our results indicate that changes in the input resistance, and consequently excitability, caused by Hypo-Inf and reperfusion are at least partially regulated by the availability and voltage dependence of KV channels. Moreover, these results suggest that selective KV channel modulators can be used as potential neuroprotective drugs to minimize hypoxia- and reperfusion-induced neuronal damage.

Comparison of Neuronal Death, Blood-Brain Barrier Leakage and Inflammatory Cytokine Expression in the Hippocampal CA1 Region Following Mild and Severe Transient Forebrain Ischemia in Gerbils

Transient ischemia in the brain causes blood-brain barrier (BBB) breakdown and dysfunction, which is related to ischemia-induced neuronal damage. Leakage of plasma proteins following transient ischemia is one of the indicators that is used to determine the extent of BBB dysfunction. In this study, neuronal damage/death, leakage of albumin and IgG, microgliosis, and inflammatory cytokine expression were examined in the hippocampal CA1 region, which is vulnerable to transient ischemia, following 5-min (mild) and 15-min (severe) ischemia in gerbils induced by transient common carotid arteries occlusion (tCCAo). tCCAo-induced neuronal damage/death occurred earlier and was more severe after 15-min tCCAo vs. after 5-min tCCAo. Significant albumin and IgG leakage (albumin and IgG immunoreactivity) took 1 or 2 days to begin, and immunoreactivity was markedly increased 5 days after 5-min tCCAo. While, albumin and IgG leakage began to increase 6 h after 15-min tCCAo and remained significantly higher over time than that seen in 5-min tCCAo. IgG immunoreactivity was observed in degenerating neurons and activated microglia after tCCAo, and microglia were activated to a greater extent after 15-min tCCAo than 5-min tCCAo. In addition, following 15-min tCCAo, pro-inflammatory cytokines [tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β)] immunoreactivity was significantly higher than that seen following 5-min tCCAo, whereas immunoreactivity of anti-inflammatory cytokines (IL-4 and IL-13) was lower in 15-min than 5-min tCCAo. These results indicate that duration of tCCAo differentially affects the timing and degree of neuronal damage or loss, albumin and IgG leakage and inflammatory cytokine expression in brain tissue. In addition, more severe BBB leakage is closely related to acceleration of neuronal damage through increased microglial activation and pro-inflammatory cytokine expression in the ischemic hippocampal CA1 region.

SYNTHESIS OF 1,3,4-OXADIAZOLES AS SELECTIVE T-TYPE CALCIUM CHANNEL INHIBITORS

Neuropathic pain, epilepsy, insomnia, and tremor disorder may arrive from an increase of intracellular Ca2+ concentration through a dysfunction of T-type Ca2+ channels. Thus, T-type calcium channels could be a target in drug discovery for the treatments of neuropathic pain and epilepsy. From rational drug design approach, a group of 2,5-disubstituted 1,3,4-oxadiazole molecules was synthesized and their selective T-type channel inhibitions were evaluated. The synthetic strategy consists of a short sequence of three reactions: (i) condensation of thiosemicarbazide with acid chlorides; (ii) ring closing by 1,3-dibromo-5,5- dimethylhydantoin; and (iii) coupling with various acid chlorides. 5-Chloro-N-(5- phenyl-1,3,4-oxadiazol-2-yl)thiophene-2-carboxamide (11) was found to selectively inhibit T-type Ca2+ channel over Na+ and K+ channels in mouse dorsal root ganglion neurons and/or human embryonic kidney (HEK)-293 cells and to suppress seizure-induced death in mouse model. Consequently, compound 11 is a useful probe for investigation of physiologic and pathophysiologic roles of the T-channel, and provides a basis to develop a novel therapeutic to treat chronic neuropathic and inflammatory pains.

Oxygen-glucose deprivation enhancement of cell death/apoptosis in PC12 cells and hippocampal neurons correlates with changes in neuronal excitatory amino acid neurotransmitter signaling and potassium currents

Neuronal death is a pathophysiological process that is often caused by hypoxia/ischemia. However, the causes of hypoxia/ischemia-induced neuronal death are debated, and additional experimental data are needed to resolve this debate. In the present study, we applied oxygen-glucose deprivation (OGD) to PC12 cells and hippocampal neurons to establish a hypoxia/ischemia model. We evaluated the effects of OGD on cell death/apoptosis and on the levels of two excitatory amino acid neurotransmitters, aspartic acid and glutamic acid, in both hippocampal neurons and the medium used to culture the hippocampal neurons. We also evaluated GluR2 expression in hippocampal neurons as well as the effects of OGD on whole-cell potassium currents in PC12 cells and hippocampal neurons. Our experimental results showed that OGD significantly decreased cell viability and markedly enhanced apoptosis in PC12 cells and hippocampal neurons. OGD treatment for 3 h increased the levels of Asp and Glu in the medium used to culture hippocampal neurons, but decreased both the levels of Asp and Glu and GluR2 expression in hippocampal neurons. Furthermore, OGD altered the electrophysiological properties of voltage-dependent potassium channels in PC12 cells and hippocampal neurons in different ways; OGD decreased the voltage-dependent potassium current in PC12 cells, but increased this current in hippocampal neurons. On the basis of these results, we concluded that OGD enhanced neuronal cell death/apoptosis in addition to altering neuronal excitatory amino acid neurotransmitter signaling and whole-cell voltage-dependent potassium currents.

Genistein inhibits hypoxia, ischemic-induced death, and apoptosis in PC12 cells

A hypoxia/ischemia neuronal model was established in PC12 cells using oxygen-glucose deprivation (OGD). OGD-induced neuronal death, apoptosis, glutamate receptor subunit GluR2 expression, and potassium channel currents were evaluated in the present study to determine the effects of genistein in mediating the neuronal death and apoptosis induced by hypoxia and ischemia, as well as its underlying mechanism. OGD exposure reduced the cell viability, increased apoptosis, decreased the GluR2 expression, and decreased the voltage-activated potassium currents. Genistein partially reversed the effects induced by OGD. Therefore, genistein may prevent hypoxia/ischemic-induced neuronal apoptosis that is mediated by alterations in GluR2 expression and voltage-activated potassium currents.

Local plasticity of dendritic excitability can be autonomous of synaptic plasticity and regulated by activity-based phosphorylation of Kv4.2

While plasticity is typically associated with persistent modifications of synaptic strengths, recent studies indicated that modulations of dendritic excitability may form the other part of the engram and dynamically affect computational processing and output of neuronal circuits. However it remains unknown whether modulation of dendritic excitability is controlled by synaptic changes or whether it can be distinct from them. Here we report the first observation of the induction of a persistent plastic decrease in dendritic excitability decoupled from synaptic stimulation, which is localized and purely activity-based. In rats this local plasticity decrease is conferred by CamKII mediated phosphorylation of A-type potassium channels upon interaction of a back propagating action potential (bAP) with dendritic depolarization.

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.

Up-regulation of A-type potassium currents protects neurons against cerebral ischemia

Excitotoxicity is the major cause of many neurologic disorders including stroke. Potassium currents modulate neuronal excitability and therefore influence the pathological process. A-type potassium current (I(A)) is one of the major voltage-dependent potassium currents, yet its roles in excitotoxic cell death are not well understood. We report that, following ischemic insults, the I(A) increases significantly in large aspiny (LA) neurons but not medium spiny (MS) neurons in the striatum, which correlates with the higher resistance of LA neurons to ischemia. Activation of protein kinase Cα increases I(A) in LA neurons after ischemia. Cultured neurons from transgenic mice lacking both Kv1.4 and Kv4.2 subunits exhibit an increased vulnerability to ischemic insults. Increase of I(A) by recombinant expression of Kv1.4 or Kv4.2 is sufficient in improving the survival of MS neurons against ischemic insults both in vitro and in vivo. These results, taken together, provide compelling evidence for a protective role of I(A) against ischemia.

Downregulation of Kv4.2 channels mediated by NR2B-containing NMDA receptors in cultured hippocampal neurons

Somatodendritic Kv4.2 channels mediate transient A-type potassium currents (I(A)), and play critical roles in controlling neuronal excitability and modulating synaptic plasticity. Our studies have shown an NMDA receptor-dependent downregulation of Kv4.2 and I(A). NMDA receptors are heteromeric complexes of NR1 combined with NR2A-NR2D, mainly NR2A and NR2B. Here, we investigate NR2B receptor-mediated modulation of Kv4.2 and I(A) in cultured hippocampal neurons. Application of glutamate caused a reduction in total Kv4.2 protein levels and Kv4.2 clusters, and produced a hyperpolarized shift in the inactivation curve of I(A). The effects of glutamate on Kv4.2 and I(A) were inhibited by pretreatment of NR2B-selective antagonists. NR2B-containing NMDA receptors are believed to be located predominantly extrasynaptically. Like application of glutamate, selective activation of extrasynaptic NMDA receptors caused a reduction in total Kv4.2 protein levels and Kv4.2 clusters, which was also blocked by NR2B-selective antagonists. In contrast, specific stimulation of synaptic NMDA receptors had no effect on Kv4.2. In addition, the influx of Ca(2+) was essential for extrasynaptic modulation of Kv4.2. Calpain inhibitors prevented the reduction of total Kv4.2 protein levels following activation of extrasynaptic NMDA receptors. These results demonstrate that the glutamate-induced downregulation of Kv4.2 and I(A) is mediated by NR2B-containing NMDA receptors and is linked to proteolysis by calpain, which might contribute to the development of neuronal hyperexcitability and neurodegenerative diseases.

Regulation of Kv4.2 channels by glutamate in cultured hippocampal neurons

Somatodendritic voltage-dependent K(+) currents (Kv4.2) channels mediate transient A-type K(+) currents and play critical roles in controlling neuronal excitability. Accumulating evidence has indicated that Kv4.2 channels are key regulatory components of the signaling pathways that lead to synaptic plasticity. In contrast to the extensive studies of glutamate-induced AMPA [(+/-) alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrate] receptors redistribution, less is known about the regulation of Kv4.2 by glutamate. In this study, we report that brief treatment with glutamate rapidly reduced total Kv4.2 levels in cultured hippocampal neurons. The glutamate effect was mimicked by NMDA, but not by AMPA. The effect of glutamate on Kv4.2 was dramatically attenuated by pre-treatment of NMDA receptors antagonist MK-801 [(5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate] or removal of extracellular Ca(2+). Immunocytochemical analysis showed a loss of Kv4.2 clusters on the neuronal soma and dendrites following glutamate treatment, which was also dependent on the activation of NMDA receptors and the influx of Ca(2+). Furthermore, whole-cell patch-clamp recordings revealed that glutamate caused a hyperpolarized shift in the inactivation curve of A-type K(+) currents, while the activation curve remained unchanged. These results demonstrate a glutamate-induced alteration of Kv4.2 channels in cultured hippocampal neurons, which might be involved in activity-dependent changes of neuronal excitability and synaptic plasticity.

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.

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.

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.

Caspase-3 immunoreactivity in different cortical areas of young and aging macaque (Macaca mulatta) monkeys

It has been shown that cytochrome-c-dependent caspase-3 activation is significantly elevated in the aging macaque brain. To assess the underlying age-related changes in the cellular distribution of caspase-3, we have examined the motor cortex, cerebellum and hippocampus of young (4-year-old, n = 4) and old (20-year-old, n = 4)rhesus monkeys by immunohistochemistry. Western blot analyses of brain homogenate showed that the antibody reacted only with inactive 32-kDa procaspase and its active 20- and 17-kDa subunits, formed after granzyme B exposure. In the motor cortex, pyramidal cells of layers III and V were moderately labeled; the underlying white matter contained weakly stained astrocytes. In the hippocampus, hilar neurons and pyramidal cells in CA3 showed the strongest immunoreaction, pyramidal cells in CA1 and granule cells of the dentate gyrus were also strongly labeled. In contrast, CA2 pyramidal cells were only weakly stained, and neurons of the molecular layer were unlabeled. Weak caspase-3 immunoreaction of CA2 neurons parallels known decreased susceptibility to apoptosis. In the cerebellar cortex, clusters of strongly labeled Purkinje cells were observed next to groups of weakly and unstained cells; granule cells were generally unstained. The brains of aging monkeys displayed a similar pattern of caspase-3 immunoreactivity. In neocortical layer V, however, scattered very strongly labeled pyramidal cells were regularly detected, which were not observed in younger animals. This clustering of caspase-3 indicates increased vulnerability of a subset of pyramidal cells in the aging brain.

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.

Folic acid pretreatment prevents the reduction of Na+,K+‐ATPase and butyrylcholinesterase activities in rats subjected to acute hyperhomocysteinemia

The main objective of the present study was to evaluate the effect of folic acid pretreatment on parietal cortex Na(+),K(+)-ATPase and serum butyrylcholinesterase activities in rats subjected to acute hyperhomocysteinemia. Animals were pretreated daily with an intraperitoneal injection of folic acid (5 mg/kg) or saline from the 22th to the 28th day of age. Twelve hours after the last injection of folic acid or saline, the rats received a single subcutaneous injection of homocysteine (0.6 micromol/g of weight body) or saline and were killed 1h later. Serum was collected and the brain was quickly removed and parietal cortex dissected. Results showed that acute homocysteine administration significantly decreased the activities of Na(+),K(+)-ATPase and butyrylcholinesterase on parietal cortex and serum, respectively. Furthermore, folic acid pretreatment totally prevented these inhibitory effects. We also evaluated the effect of acute homocysteine administration on some parameters of oxidative stress, namely thiobarbituric acid-reactive substances and total thiol content in parietal cortex of rats. No alteration of these parameters were observed in parietal cortex of homocysteinemic animals, indicating that these oxidative stress parameters were probably not responsible for the reduction of Na(+),K(+)-ATPase and butyrylcholinesterase activities. The presented results confirm previous findings that acute hyperhomocysteinemia produces an inhibition of Na(+),K(+)-ATPase and butyrylcholinesterase activities and that pretreatment with folic acid prevents such effects. Assuming that homocysteine might also reduce the activities of these enzymes in human beings, our results support a new potential therapeutic strategy based on folic acid supplementation to prevent the neurological damage found in hyperhomocysteinemia.

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.