Sodium and potassium channel modulators: their role in neuroprotection

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.

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.

Traumatic brain injury in mice lacking the K channel, TREK-1

The purpose of this study was to determine whether the potassium channel, TREK-1, was neuroprotective after traumatic brain injury (TBI). Since there are no selective blockers, we used TREK-1 knockout (KO) mice for our study. Wild-type (WT) and TREK-1 KO mice were anesthetized and subjected to controlled-cortical impact injury (deformation of the brain by 1.5 mm by a 3-mm diameter rod traveling at a 3 m/s). Laser Doppler perfusion (LDP) decreased by ∼80% in the injured cortex and remained at that level in both WT and TREK-1 KO mice (n=10 and 11, respectively). Laser Doppler perfusion decreased by 50% to 60% in cortical areas directly adjacent to the site of injury. There were no statistical differences in LDP between genotype. The contusion volume, determined 15 days after the TBI using hematoxylin and eosin-stained coronal brain sections, was 4.1±0.8 (n=10) and 5.1±0.5 (n=11) mm(3) for WT and TREK-1 KO, respectively (not significant, P=0.34). Cell counts of viable neurons in the CA1 and CA3 regions of the hippocampus were similar between WT and TREK-1 KO mice (P=0.51 and 0.84 for CA1 and CA3, respectively). We conclude that TREK-1 expression does not provide brain protection after TBI.

Neuroprotection (including hypothermia)

There have been over 2000 publications in the last year addressing the topic of neuroprotection. Novel and emerging therapeutic targets that have been explored include cerebral inflammation, hypothermia, neural transplantation and repair and gene therapy. Unfortunately, with few exceptions, the successes of experimental neuroprotection have not been translated into clinical practice. The possible reasons for the discrepancy between experimental success and clinical benefit are explored.

Neuropathology in Drosophila membrane excitability mutants

Mutations affecting ion channels and neuronal membrane excitability have been identified in Drosophila as well as in other organisms and characterized for their acute effects on behavior and neuronal function. However, the long-term effect of these perturbations on the maintenance of neuronal viability has not been studied in detail. Here we perform an initial survey of mutations affecting Na+ channels and K+ channels in Drosophila to investigate their effects on life span and neuronal viability as a function of age. We find that mutations that decrease membrane excitability as well as those that increase excitability can trigger neurodegeneration to varying degrees. Results of double-mutant interactions with dominant Na+/K+ ATPase mutations, which themselves cause severe neurodegeneration, suggest that excitotoxicity owing to hyperexcitability is insufficient to explain the resultant phenotype. Although the exact mechanisms remain unclear, our results suggest that there is an important link between maintenance of proper neuronal signaling and maintenance of long-term neuronal viability. Disruption of these signaling mechanisms in any of a variety of ways increases the incidence of neurodegeneration.

Effect of new O-superfamily conotoxin SO3 on sodium and potassium currents of cultured hippocampal neurons

The effects of a new O-superfamily conotoxin SO3 on sodium and potassium currents were examined in cultured rat hippocampal neurons using the whole-cell patch clamp technique. SO3 caused a concentration-dependent, rapidly developing and reversible inhibition of sodium currents (I(Na)). The IC(50) value for the blockage of I(Na) was calculated to be 0.49 and the Hill coefficient was 1.7. Using electrophysiological and pharmacological protocols, transient A-type potassium currents (I(A)) and delayed rectifiers potassium currents (I(K)) were isolated. SO3 caused a concentration-dependent, and reversible inhibition of I(K). The IC(50) value for the blockage of I(K) was calculated to be 1.6 and the Hill coefficient was 0.6, with no significant effect on I(A). These results indicate that SO3 can selectively inhibit neuronal sodium and potassium currents.

Neuroprotective effects of anticonvulsants in rat hippocampal slice cultures exposed to oxygen/glucose deprivation

Some anticonvulsants show neuroprotective effects, and may be of use in reducing neuronal death resulting from stroke or traumatic brain injury. Here I report that a broad range of anticonvulsants protect cells in hippocampal slice cultures from death induced by oxygen/glucose deprivation (OGD). Hippocampal slice cultures were submitted to 1 h OGD and the resulting cell death was quantified 24 h later using a novel automated fluorescent scanning method. The classical anticonvulsants phenobarbital, phenytoin, ethosuximide, chlordiazepoxide and midazolam all significantly and dose-dependently reduced cell death induced by OGD. The newer anticonvulsants carbamazepine, felbamate, lamotrigine, tiagabine, and oxcarbazepine also had significant neuroprotective effects, but gabapentin, valproic acid (10 mM), levetiracetam and retigabine were not neuroprotective at a concentration up to 300 microM. In conclusion, several classical and newer anticonvulsants have neuroprotective properties in an in vitro model that simulates cerebral ischemia.

Investigation of AM-36: a novel neuroprotective agent

1. The neurochemical sequelae following cerebral ischaemia are complex, involving excess release of excitatory amino acids, particularly glutamate, disruption of ionic homeostasis due to Na+ and Ca2+ influx and generation of toxic free radicals, ultimately leading to cell death by both necrosis and apoptosis. 2. Drugs that block components of this biochemical cascade, such as glutamate receptor antagonists, sodium channel blockers and free radical scavengers, have been investigated as putative neuroprotective agents. The knowledge that multiple mechanisms contribute to neuronal injury in ischaemia have led to the general recognition that a single drug treatment is unlikely to be beneficial in the treatment of cerebral ischaemia. 3. AM-36 [1-(2-(4-chlorophenyl)-2-hydroxy)ethyl-4-(3,5-bis(1,1-dimethyl)-4-hydroxyphenyl)methylpiperazine] is one of a series of hybrid molecules designed to incorporate multiple neuroprotective mechanisms within the one structure. Primary screening tests demonstrated that AM-36 inhibited binding to the polyamine site of glutamate receptors, blocked neuronal sodium channels and had potent anti-oxidant activity. In neuronal cell cultures, AM-36 inhibited toxicity induced by N-methyl-D-aspartate (NMDA) and the sodium channel opener veratridine and, in addition, inhibited veratridine-induced apoptosis. 4. In a middle cerebral artery occlusion model of stroke in conscious rats, systemic administration of AM-36 markedly reduced both cortical and striatal infarct volume and significantly improved functional outcome in motor performance, neurological deficit and sensorimotor neglect tests. AM-36 was neuroprotective even when administration was delayed until 3 h systemically, or 5 h intravenously, after induction of stroke. 5. These studies indicate that AM-36 is a unique neuroprotective agent with multiple modes of action, making it an attractive candidate for the treatment of acute stroke in humans.

The interaction of AR-A008055 and its enantiomers with the GABA(A) receptor complex and their sedative, muscle relaxant and anticonvulsant activity

AR-A008055 [(+/-)-1-(4-methyl-5-thiazolyl)-1-phenylmethylamine] is structurally related to clomethiazole and has been used to probe the mechanism of the neuroprotective effect of clomethiazole. Clomethiazole, (+/-)-AR-A008055 and (S)-(-)-AR-A008055 all displaced [35S]-t-butyl-bicyclophosphorothionate ([35S]TBPS) from rat cerebral cortex tissue (IC50 values: GABA, 8.1+/-0.04 microM; clomethiazole, 130+/-30 microM; (+/-)-AR-A008055, 494+/-7 microM; (S)-(-)-AR-A008055, 221+/-14 microM. (R)-(+)-AR-A008055 was without significant effect (IC50>1000 microM). None of the compounds interacted with NMDA or AMPA receptors or with sodium or calcium (N, P/Q) channels. Brain penetration of both enantiomers following their i.p. administration was excellent, with brain and plasma concentrations being similar. Clomethiazole dose-dependently inhibited spontaneous locomotor activity in rats and was approximately 10 times more sedative than either enantiomer of AR-A008055. Clomethiazole was more potent than (S)-(-)-AR-A008055 in the "pull-up" test (muscle relaxation) and in producing loss of righting reflex, while (R)-(+)-AR-A008055 had little effect. The time animals remained on a Rota-rod was of the order: clomethiazole<(S)-(-)-AR-A008055<(R)-(+)-AR-A008055. (S)-(-)-AR-A008055 (210 micromol/kg) raised seizure threshold to pentylenetetrazole (i.v.) by 119+/-21%. The (R)-(+)- enantiomer was not anticonvulsant. Overall, (S)-(-)-AR-A008055 exhibited a similar pharmacology to clomethiazole. However, its sedative and muscle relaxant effects were substantially less than clomethiazole, emphasising that these properties are not directly related to neuroprotective efficacy. The current data suggest that the proposed GABA uptake inhibitory property of (R)-(+)-AR-A008055 fails to produce significant sedative, myorelaxant or anticonvulsant activity.

Blockade of U50488H on potassium currents of acutely isolated mouse hippocampal CA3 pyramidal neurons

The actions of the opioid agonist U50488H on IA and IK were examined in acutely isolated mouse hippocampal CA3 pyramidal neurons using the whole-cell patch clamp technique. U50488H caused a concentration dependent, rapidly developing and reversible inhibition of voltage-activated IA and IK. The inhibitory actions were still observed in the presence of 30 microM naloxone or 5 microM nor-binaltorphimine dihydrochloride. The IC50 values for the blockade of IA and IK were calculated as 20.1.9 and 3.7 microM, respectively. In the presence of 3.3 microM U50488H, repetitive stimulation induced use-dependent inhibition of IA and IK. A 10 microM concentration of U50488H positively shifted the half-activation membrane potential of IA by +11 mV, but negatively shifted IK by -14 mV. These results demonstrate that U50488H can directly inhibit neuronal IA and IK without involvement of the activation of kappa-opioid receptors.

Evaluation of the neuroprotective effects of sodium channel blockers after spinal cord injury: improved behavioral and neuroanatomical recovery with riluzole

OBJECT

Persistent activation of voltage-sensitive Na+ channels is associated with cellular toxicity and may contribute to the degeneration of neural tissue following traumatic brain and spinal cord injury (SCI). Pharmacological blockade of these channels can attenuate secondary pathophysiology and reduce functional deficits acutely.

METHODS

To determine the therapeutic effects of Na+ channel blockers on long-term tissue sparing and functional neurological recovery after traumatic SCI, the authors injected Wistar rats intraperitoneally with riluzole (5 mg/kg), phenytoin (30 mg/kg), CNS5546A, a novel Na+ channel blocker (15 mg/kg), or vehicle (2-HP3CD; 5 mg/kg) 15 minutes after induction of compressive SCI at C7-T1. Functional neurological recovery of coordinated hindlimb function and strength, assessed 1 week postinjury and weekly thereafter for 6 weeks, was significantly enhanced in animals treated with riluzole compared with the other treatment groups. Seven weeks postinjury the preservation of residual tissue and integrity of descending axons were determined with digital morphometrical and fluorescent histochemical analysis. All three Na+ channel blockers significantly enhanced residual tissue area at the injury epicenter compared with control. Riluzole significantly reduced tissue loss in rostrocaudal regions surrounding the epicenter, with overall sparing of gray matter and selective sparing of white matter. Also, counts of red nuclei neurons retrogradely labeled with fluorogold introduced caudal to the injury site were significantly increased in the riluzole group.

CONCLUSIONS

Systemic Na+ channel blockers, in particular riluzole, can confer significant neuroprotection after in vivo SCI and result in behavioral recovery and sparing of both gray and white matter.

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.

Moclobemide reduces intracellular pH and neuronal activity of CA3 neurones in guinea-pig hippocampal slices-implication for its neuroprotective properties

Mechanisms underlying the neuroprotective properties of the weak MAO-A inhibitor moclobemide are not understood. Increasing evidence suggests that a moderate increase in intracellular free protons may contribute to neuroprotective properties due to a proton-mediated decrease in neuronal activity. Therefore, we studied effects of 10-700 microM moclobemide (i) on the intracellular pH (pH(i)) of BCECF-AM loaded CA3 neurones as well as (ii) on spontaneous action potentials and epileptiform activity (induced by bicuculline-methiodide, caffeine, or 4-aminopyridine) of CA3 neurones in the stratum pyramidale. Moclobemide-concentrations of > or = 300 microM reversibly reduced the steady-state pH(i) by up to 0. 25 pH-units within 5-20 min. Simultaneously, the frequency of spontaneous action potentials and epileptiform discharges became depressed. Moclobemide also abolished 4-aminopyridine-induced GABA-mediated hyperpolarisations suggesting that the inhibitory and acidifying effects of moclobemide do not result from an amplification of the GABA system. The stronger MAO-A inhibitors clorgyline or pargyline (both 10 microM) mimicked the moclobemide-effects. Investigating effects on pH(i)-regulation we found that 700 microM moclobemide impaired the recovery from intracellular acidification elicited by an ammonium prepulse which demonstrates an impairment of transmembrane acid extrusion. We suggest that the latter effect is responsible for the moderate decrease in the steady-state pH(i) which in turn reduced neuronal activity. This mechanism may substantially contribute to the neuroprotective properties of moclobemide.

Alteration of intracellular pH and activity of CA3-pyramidal cells in guinea pig hippocampal slices by inhibition of transmembrane acid extrusion

Transmembrane acid extruders, such as electroneutral operating Na(+)/H(+)-exchangers (NHE) and Na(+)-dependent Cl(-)/HCO(3)(-)-exchangers (NCHE) are essential for the maintenance and regulation of cell volume and intracellular pH (pH(i)). Both of them are hypothesised to be closely linked to the control of excitability. To get further information about the relation of neuronal pH(i) and activity of cortical neurones we investigated the effect of NHE- and/or NCHE-inhibition on (i) spontaneous action potentials and epileptiform burst-activity (induced by bicuculline-methiodide, caffeine or 4-aminopyridine) and (ii) on pH(i) of CA3-neurones. NHE-inhibition by amiloride (0.25-0.5 mM) or its more potent derivative dimethylamiloride (50 microM) and NCHE-inhibition by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 0.25-0.5 mM) induced a biphasic alteration of neuronal activity: an initial, up to 30 min lasting, increase in frequency of action potentials and bursts preceded a growing and partially reversible suppression of neuronal activity. In BCECF-loaded neurones the pH(i), however, continuously decreased during either amiloride- or DIDS-treatment and reached its steady-state (DeltapH(i) up to 0.3 pH-units) when the neuronal activity was markedly suppressed. Combined treatment with amiloride (0.5 mM) and DIDS (0.5 mM) or treatment with harmaline alone (0.25-0.5 mM), which also continuously acidified neurones via inhibition of an amiloride-insensitive NHE-subtype, induced a monophasic and partially reversible suppression of neuronal activity. As an initial excitatory period failed to occur during combined NHE/NCHE-inhibition we speculate that its occurrence during amiloride- or DIDS-treatment resulted rather from disturbances in volume- than in pH(i)-regulation. The powerful inhibitory and anticonvulsive properties of NHE- and NCHE-inhibitors, however, very likely based upon intracellular acidification - as derived from our previous findings that a moderate increase in intracellular free protons is sufficient to reduce membrane excitability of CA3-neurones.

Neuronal death and perinatal lethality in voltage-gated sodium channel alpha(II)-deficient mice

Neural activity is crucial for cell survival and fine patterning of neuronal connectivity during neurodevelopment. To investigate the role in vivo of sodium channels (NaCh) in these processes, we generated knockout mice deficient in brain NaChalpha(II). NaChalpha(II)(-/-) mice were morphologically and organogenically indistinguishable from their NaChalpha(+/-) littermates. Notwithstanding, NaChalpha(II)(-/-) mice died perinatally with severe hypoxia and massive neuronal apoptosis, notably in the brainstem. Sodium channel currents recorded from cultured neurons of NaChalpha(II)(-/-) mice were sharply attenuated. Death appears to arise from severe hypoxia consequent to the brainstem deficiency of NaChalpha(II). NaChalpha(II) expression is, therefore, redundant for embryonic development but essential for postnatal survival.

An investigation of the possible interaction of clomethiazole with glutamate and ion channel sites as an explanation of its neuroprotective activity

The activity of the neuroprotective agent clomethiazole at glutamate and ion channel sites has been investigated. Dizocilpine (3.25 mg/kg intraperitoneally) provided almost total protection against the damage produced by infusion of N-methyl-DL-aspartate (NMDLA; 75 micrograms) into the right hippocampus. In contrast, clomethiazole (96 mg/kg intraperitoneally) was without effect. Using ligand binding techniques, no evidence was found for clomethiazole interacting with NMDA, AMPA or sigma binding sites. Clomethiazole did inhibit the stimulatory effect of the metabotropic glutamate receptor agonist 1S3R-aminocyclopentone-1,3-dicarboxylic acid (ACPD) on phosphoinositol hydrolysis, but only at a concentration of 10(-3) M, which is unlikely to have functional relevance. Clomethiazole was also without effect on ligand binding to Ca2+ channels (N- or L- type), Na+ channels or ATP-sensitive K+ channels. Potentiation of GABA function therefore remains the most plausible explanation for the neuroprotective activity of clomethiazole.