Effects of the sodium channel blocker tetrodotoxin (TTX) on cellular ion homeostasis in rat brain subjected to complete ischemia

Neuroprotection--rationale for pharmacological modulation of Na(+)-channels

The primary factor detrimental to neurons in neurological disorders associated with deficient oxygen supply or mitochondrial dysfunction is insufficient ATP production relative to their requirement. As a large part of the energy consumed by brain cells is used for maintenance of the Na+ gradient across the cellular membrane, reduction of energy demand by down-modulation of voltage-gated Na(+)-channels is a rational strategy for neuroprotection. In addition, preservation of the inward Na+ gradient may be beneficial because it is an essential driving force for vital ion exchanges and transport mechanisms such as Ca2+ homeostasis and neurotransmitter uptake.

Inhibition of major cationic inward currents prevents spreading depression-like hypoxic depolarization in rat hippocampal tissue slices

Hypoxia-induced spreading depression-like depolarization (hypoxic SD, or anoxic depolarization) is accompanied by the near-loss of membrane potential, severe reduction of membrane resistance, and influx of Na+, Ca2+, Cl- and water into neurons. The biophysical nature of these membrane changes is incompletely understood. In the present study we applied a pharmacological mixture (10 microM DNQX, 10 microM CPP, 1 microM TTX, and 2 mM Ni2+) to rat hippocampal tissue slices to inhibit major Na+ and Ca2+ inward currents. This inhibitory cocktail slightly depolarized CA1 pyramidal neurons and completely blocked all evoked potentials. In its presence severe hypoxia of up to 20 min duration failed to induce hypoxic SD and the accompanying intrinsic optical signal. Instead, only moderate, very slow negative shifts of the extracellular DC potential were observed. Following 10 min hypoxia and 1 hour wash-out of the inhibitors antidromic and orthodromic responses were still blocked but hypoxic SD with markedly delayed onset could be induced in most slices. In current-clamped CA1 pyramidal cells hypoxia induced a rapid, near-complete depolarization and decreased the input resistance by 89%. In the presence of the cocktail, however, hypoxia caused a gradual, partial depolarization, to about -40 mV; the membrane resistance decreased by only 37%. We conclude that simultaneous blockade of the known major Na+ and Ca2+ channels consistently prevents hypoxic SD. The hypothesis that SD initiation is the consequence of general loss of selective permeability or general membrane breakdown becomes unlikely. Instead, influx of Na+ and Ca2+ might play a crucial role in the generation of the rapid SD-like depolarization.

Neuroprotective strategies: voltage-gated Na+-channel down-modulation versus presynaptic glutamate release inhibition

Insufficient ATP production relative to cellular requirements is the key factor detrimental to neurons in neurological disorders associated with deficient oxygen/glucose supply or mitochondrial dysfunction. As a large part of the energy consumed by brain cells is used to maintain the Na+ gradient across the cellular membrane, reduction of energy demand by down-modulation of voltage-gated Na+-channels is a rational strategy for neuroprotection against these conditions. Preservation of the inward Na+ gradient is likely to be also beneficial as it is an essential driving force for vital ion exchanges and transport mechanisms (e.g. Ca2+-homeostasis and cell volume regulation). From these elements, I propose that use-dependent Na+-channel blockers increase the resilience of nerve cells to the primary insult and/or subsequent deleterious events, and that reduced efflux of glutamate and other compounds is only a consequence of cellular stress attenuation. The widespread hypothesis that down-modulation of Na+-channels is neuroprotective primarily through reduction of presynaptic glutamate release conflicts with strong experimental evidence.

Sodium channel blockade unmasks two temporally distinct mechanisms of striatal dopamine release during hypoxia/hypoglycaemia in vitro

Massive striatal dopamine release during cerebral ischaemia has been implicated in the resulting neuronal damage. Sodium influx is an early event in the biochemical cascade during ischaemia and blockade of sodium channels may increase resistance to ischaemia by reducing energy demand involved in compensation for sodium and potassium fluxes. In this study, we have determined the effects of opening and blockade of voltage-gated sodium channels on hypoxia/hypoglycaemia-induced dopamine release. Slices of rat caudate nucleus were maintained in a slice chamber superfused by an oxygenated artificial cerebrospinal fluid containing 4 mM glucose. Ischaemia (hypoxia/hypoglycaemia) was mimicked by a switch to a deoxygenated artificial cerebrospinal fluid containing 2 mM glucose and dopamine release was measured using fast cyclic voltammetry. In drug-free (control) slices, there was a 2-3 min delay after the onset of hypoxia/hypoglycaemia followed by a rapid dopamine release event which was associated with anoxic depolarization. In slices treated with the Na+ channel opener, veratridine (1 microM), the time to onset of dopamine release was shortened (101 +/- 20 s, compared with 171 +/- 8 s in controls, P < 0.05). Conversely, phenytoin (100 microM), lignocaine (200 microM) and the highly selective sodium channel blocker, tetrodotoxin (1 microM) markedly delayed and slowed dopamine release vs paired controls. In the majority of cases, dopamine release was biphasic after sodium channel blockade: a slow phase preceded a more rapid dopamine release event. The latter was associated with anoxic depolarization. Neither the fast nor the slow release events were affected by pretreatment with the selective dopamine uptake blocker GBR 12935 (0.2 microM), suggesting that uptake carrier reversal did not contribute to these events. In conclusion, sodium channel antagonism delays and slows hypoxia/hypoglycaemia-induced dopamine release in vitro. Furthermore, sodium channel blockade delays anoxic depolarization and its associated neurotransmitter release, revealing an earlier dopamine release event that does not result from reversal of the uptake carrier.

Zonisamide as a neuroprotective agent in an adult gerbil model of global forebrain ischemia: a histological, in vivo microdialysis and behavioral study

Brief periods of global cerebral ischemia are known to produce characteristic patterns of neuronal injury both in human studies and in experimental animal models. Ischemic damage to vulnerable areas such as the CA1 sector of the hippocampus is thought to result from excitotoxic amino acid neurotransmission. The objective of this study was to determine the ability of a novel sodium channel blocking compound, zonisamide, to reduce neuronal damage by preventing the ischemia-associated accumulation of extracellular glutamate. Using a gerbil model, animals were subjected to 5 min ischemic insults. Both pre- and post-ischemic drug administration (zonisamide 150 mg/kg) were studied. Histological brain sections were prepared using a silver stain at 7 and 28 days post ischemia. The animals sacrificed at 28 days also underwent behavioral testing using a modified Morris water maze. In vivo microdialysis was performed on a separate group of animals in order to determine the patterns of ischemia-induced glutamate accumulation in the CA1 sector of the hippocampus. Pyramidal cell damage scores in the CA1 region of the hippocampus were significantly reduced in animals pre-treated with zonisamide compared to saline-treated controls, both at 7 days (drug pre-treated: 0.812 +/- 0.28, n = 8; controls: 1.625 +/- 0.24, n = 8; *P < 0.05) and 28 (drug pre-treated: 0.833 +/- 0.22, n = 12; controls: 1.955 +/- 0.26, n = 11; **P < 0.01) days post ischemia. However, animals receiving zonisamide post-treatment did not display significant differences from controls. Behavioral studies also showed significant preservation of function in drug-treated animals. Microdialysis studies confirmed a reduction in glutamate release in drug-treated animals compared to saline-treated controls. Our data suggest that zonisamide is effective in reducing neuronal damage by a mechanism involving decreased ischemia-induced extracellular glutamate accumulation and interruption of excitotoxic pathways.

Effects of increased extracellular glutamate levels on the local field potential in the brain of anaesthetized rats

1. It is generally considered that glutamate-mediated transmission can be altered from a physiological to neurotoxic action when extracellular glutamate levels become excessive subsequent to impaired uptake and/or excessive release. However, high extracellular glutamate does not consistently correlate with neuronal dysfunction and death in vivo. The purpose of this study was to examine in situ the local depolarizations, as indicated by negative shifts of the extracellular field (d.c.) potential, produced by local inhibition of high-affinity glutamate uptake, with or without co-application of exogenous glutamate, in three brain regions of anaesthetized rats. 2. Microdialysis probes incorporating an electrode were used to apply exogenous glutamate and/or its uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC), and to monitor the resulting changes in extracellular glutamate and d.c. potential at the sites of application within the cortex, striatum and hippocampus. 3. Perfusion of 1 to 10 mM L-trans-PDC markedly and concentration-dependently increased extracellular glutamate levels (by up to 1700% of basal level in the parietal cortex). Despite their large magnitude, glutamate changes were associated with minor negative shifts of the d.c. potential (< 2 mV), which were not suppressed by the N-methyl-D-aspartate (NMDA)-channel blocker, dizocilpine (MK-801, 2 mg kg-1, i.v.), or the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/ kainate-receptor antagonist, 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX, 30 mg kg-1, i.p.). L-trans-PDC had virtually identical concentration-dependent effects on dialysate glutamate in the hippocampus and striatum, but those induced in the cortex were around 40% larger (P < 0.002). In contrast, the associated depolarizations were around twice as large in the striatum and cortex as in the hippocampus (P < 0.002). Finally, co-application of L-trans-PDC did not enhance the d.c. potential changes evoked by perfusion of 5 or 20 mM glutamate. 4. As the neurotoxic potency of glutamate agonists is considered to be linked to excessive opening of glutamate-operated ion channels, these results challenge the notion that high extracellular glutamate levels may be the key to excitotoxicity in neurological disorders. In particular, they do not support the hypothesis that high extracellular glutamate causes the sudden negative shifts of the d.c. potential associated with ischaemia (i.e. anoxic depolarization), traumatic brain injury and spreading depression. Impaired uptake and excessive release of glutamate may well lead to excitotoxicity, but only at the synaptic level, not by spreading through the interstitial fluid.

Sodium channel modulators prevent oxygen and glucose deprivation injury and glutamate release in rat neocortical cultures

Neocortical cultures were deprived of oxygen and glucose to model ischemic neuronal injury. We used a graded series of periods of oxygen and glucose deprivation, providing graded insults. Cell death was measured by release of lactate dehydrogenase (LDH). One hundred and twenty to 240 min of deprivation caused graded increases in glutamate overflow, LDH release and 45Ca influx. Curves of LDH release with respect to deprivation time were shifted to longer intervals by treatment with tetrodotoxin (TTX; 3, 30 or 300 nM), phenytoin (10, 30 or 100 microM), lidocaine (10, 30 or 100 microM) or the N-methyl-D-aspartate antagonist CPP [3(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid, 3, 10, 30 or 100 microM]. Combined treatment with TTX and CPP caused pronounced rightward shifts of LDH deprivation curves. Our results indicate that Na+ channel blockade is neuroprotective in neocortex cultures. Our results also suggest that neuroprotection with Na+ channel blockers may be due to inhibition of glutamate release.

Anoxia-induced depolarization in CA1 hippocampal neurons: role of Na+-dependent mechanisms

We have previously shown that (1) removal of extracellular sodium (Na+) reduces the anoxia-induced depolarization in neurons in brain-slice preparations and (2) amiloride, which blocks Na+-dependent exchangers, prevents anoxic injury in cultured neocortical neurons. Since anoxia-induced depolarization has been linked to neuronal injury, we have examined in this study the role of Na+-dependent exchangers and voltage-gated Na+ channels in the maintenance of membrane properties of CA1 neurons at rest and during acute hypoxia. We recorded intracellularly from CA1 neurons in hippocampal slices, monitored Vm and measured input resistance (Rm) with periodic injections of negative current. We found that tetrodotoxin (TTX, 1 microM) hyperpolarized CA1 neurons at rest and significantly attenuated both the rate of depolarization (delta Vm/dt) and the rate of decline of Rm (delta Rm/dt) by about 60% during the early phase of hypoxia. The effect of TTX was dose-dependent. Amiloride (1 mM) decreased Vm and increased Rm in the resting condition but changed little the effect of hypoxia on neuronal function. Benzamil and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride (EIPA), two specific inhibitors of Na+ dependent exchangers, were similar to amiloride in their effect. We conclude that neuronal membrane properties are better maintained during anoxia by reducing the activity of TTX-sensitive channels and not by the action of Na+-dependent exchangers.

Na(+)-Ca2+ exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate

Na(+)-Ca2+ exchanger-associated membrane currents were studied in cultured murine neocortical neurons, using whole-cell recording combined with intracellular perfusion. A net inward current specifically associated with forward (Na+(o)-Ca2+(i)) exchange was evoked at -40 mV by switching external 140 mM Li+ to 140 mM Na+. The voltage dependence of this current was consistent with that predicted for 3Na+:1Ca2+ exchange. As expected, the current depended on internal Ca2+, and could be blocked by intracellular application of the exchanger inhibitory peptide, XIP. Raising internal Na+ from 3 to 20 mM or switching the external solution from 140 mM Li+ to 30 mM Na+ activated outward currents, consistent with reverse (Na+(i)-Ca2+(o)) exchange. An external Ca2(+)-sensitive current was also identified as associated with reverse Na(+)-Ca2+ exchange based on its internal Na+ dependence and sensitivity to XIP. Combined application of external Na+ and Ca2+ in the absence of internal Na+ triggered a 3.3-fold larger inward current than the current activated in the presence of 3 mM internal Na+, raising the intriguing possibility that Na(+)-Ca2+ exchangers might concurrently operate in both the forward and the reverse direction, perhaps in different subcellular locations. With this idea in mind, we examined the effect of excitotoxic glutamate receptor activation on exchanger operation. After 3-5 min of exposure to 100-200 microM glutamate, the forward exchanger current was significantly increased even when external Na+ was reduced to 100 mM, and the external Ca2(+)-activated reverse exchanger current was eliminated.

Intracellular calcium chelator BAPTA protects cells against toxic calcium overload but also alters physiological calcium responses

The effect of the membrane-permeant calcium chelator 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl) ester (BAPTA/AM) on ionomycin-induced cellular calcium overload was studied in single differentiated NH15-CA2 neuroblastoma x glioma hybrid cells. To monitor [Ca2+]i we used the fluorescent indicator Fura-2. Preincubation of the cells with 3 microM BAPTA/AM reduced the number of cells showing deregulation of [Ca2+]i during ionomycin-induced calcium influx. The calcium transients elicited by application of KCl were also severely affected by the chelator. These transients, although varying from cell to cell in shape, amplitude and duration, are well reproducible in individual cells. After incubation of cells for 1 h with 0.3-30 microM BAPTA/AM the time course of these cellular transients was markedly slowed. At 1 microM BAPTA/AM, the time constant of decline of [Ca2+]i was increased by a factor of 4.1 +/- 2.4 (n = 14) and the amplitude was reduced to about 50%. With 30 microM BAPTA/AM, the K(+)-induced calcium transients were almost completely inhibited. We conclude that intracellularly loaded calcium chelators may be used for the prevention of [Ca2+]i-induced cell damage, however, at the expense of a disturbed calcium signalling.

Anoxic injury in the rat spinal cord: pharmacological evidence for multiple steps in Ca(2+)-dependent injury of the dorsal columns

To examine anoxic injury in spinal cord white matter, we studied axonal conduction in the dorsal columns during and following a standard 60 min anoxic insult at 36 degrees C. Perfusion of the spinal cord in 0-Ca2+ Ringer solution resulted in significantly improved recovery of the compound action potential. Similarly, removal of Na+ from the perfusate resulted in significantly improved recovery of conduction in dorsal column axons. Exposure of the anoxic spinal cord to the Na+ channel blocker tetrodotoxin (TTX), the Na-Ca exchange blockers benzamil and bepridil, Na(+)-H+ exchange blockers amiloride and harmaline, and perfusion in Ringer solution with pH adjusted to 6.4, all resulted in improved recovery. The tertiary anesthetics procaine and lidocaine, as well as phenytoin and carbamazepine, also resulted in improved recovery of compound action potential amplitude after 60 min of anoxia. These results demonstrate that a significant component of irreversible loss of conduction, following anoxic injury of the dorsal columns, is Ca(2+)-dependent. Moreover, these results demonstrate that TTX-inhibitable Na+ channels participate in the pathophysiology of anoxic injury in spinal cord white matter, and indicate that reverse Na-Ca exchange provides a route for at least part of the damaging influx of Ca2+ into an intracellular compartment in anoxic spinal cord white matter. Our results also suggest that extracellular acidosis may have a protective effect on anoxic spinal cord white matter, and support the hypothesis that anoxic injury of spinal cord white matter may involve the Na(+)-H+ exchanger.

Altered Na(+)-channel function as an in vitro model of the ischemic penumbra: action of lubeluzole and other neuroprotective drugs

Veratridine blocks Na(+)-channel inactivation and causes a persistant Na(+)-influx. Exposure of hippocampal slices to 10 microM veratridine led to a failure of synaptic transmission, repetitive spreading depression (SD)-like depolarizations of increasing duration, loss of Ca(+)-homeostasis, a large reduction of membrane potential, spongious edema and metabolic failure. Normalization of the amplitude of the negative DC shift evoked by high K+ ACSF 80 min after veratridine exposure was taken as the primary endpoint for neuroprotection. Compounds whose mechanisms of action includes Na(+)-channel modulation were neuroprotective (IC50-values in microM): tetrodotoxin 0.017, verapamil 1.18, riluzole 1.95, lamotrigine > or = 10, and diphenylhydantoin 16.1. Both NMDA (MK-801 and PH) and non-NMDA (NBQX) excitatory amino acid antagonists were inactive, as were NOS-synthesis inhibitor (nitro-L-arginine and L-NAME) Ca(2+)-channel blockers (cadmium, nimodipine) and a K(+)-channel blocker (TEA). Lubeluzole significantly delayed in time before the slices became epileptic, postponed the first SD-like depolarization, allowed the slices to better recover their membrane potential after a larger number of SD-like DC depolarizations, preserved Ca2+ and energy homeostasis, and prevented the neurotoxic effects of veratridine (IC50-value 0.54 microM). A concentration of lubeluzole, which was 40 x higher than its IC50-value for neuroprotection against veratridine, had no effect on repetitive Na(+)-dependent action potentials induced by depolarizing current in normal ACSF. The ability of lubeluzole to prevent the pathological consequences of excessive Na(+)-influx, without altering normal Na(+)- channel function may be of benefit in stroke.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Phenytoin pretreatment prevents hypoxic-ischemic brain damage in neonatal rats

This study was performed to investigate whether the anticonvulsant phenytoin has neuroprotective effect in a model of hypoxia-ischemia with neonatal rats. The left carotid artery of each rat was ligated, followed by 3 h of hypoxic exposure (8% O2) in a temperature-regulated environment (36 degrees C). Two weeks later, brain damage was assessed by measuring loss of brain hemisphere weight. Phenytoin had no effect on body temperature or plasma glucose, but attenuated brain damage in a dose-dependent manner (3, 10, and 30 mg/kg i.p.) when administered before the hypoxic episode. Phenytoin administered during or after hypoxia did not alter hypoxic brain damage significantly. A parallel experiment using histological examination of frozen brain sections demonstrated less brain infarction after phenytoin treatment (30 mg/kg i.p.). In an additional experiment measuring breakdown of an endogenous brain calpain substrate, spectrin, phenytoin treatment reduced this measure of early cellular damage. Our results indicate that pretreatment with phenytoin is neuroprotective at a plasma phenytoin concentration of approximately 12 micrograms/ml. These results are consistent with the hypothesis that blockade of voltage-dependent sodium channels reduces brain damage following ischemia.

Voltage-dependent Na+/K+ ion channel blockade fails to ameliorate behavioral deficits after traumatic brain injury in the rat

Traumatic brain injury (TBI) induces massive, transient ion flux, after impact. This may be via agonist gated channels, such as the muscarinic, cholinergic or NMDA receptor, or via voltage-dependent channels. Pharmacological blockade of the former, is neuroprotective in most TBI models, but the role of voltage-dependent Na+/K+ channels has not been tested. We have therefore tested the hypothesis that intraventricular tetrodotoxin (TTX) (20 microliters, 5 mM) induced blockade of post-TBI ion flux will prevent cytotoxic cell swelling, Na+ and K+ flux, and behavioral deficit. Microdialysis demonstrated blockade of [K+]d flux in the TTX group compared to controls. Behavioral evaluation of motor (days 1-5) and memory function (days 11-15) after TBI revealed no beneficial effect in the TTX group compared to controls. Thus, although evidence of reduced ionic flux was demonstrated in the TTX group, memory and behavior were unaffected, suggesting that agonist-operated channel-mediated ion flux is more important after TBI.

Na+ channels as targets for neuroprotective drugs

Drugs that block voltage-dependent Na+ channels are well known as local anaesthetics, antiarrhythmics and anticonvulsants. Recent studies show that these compounds also provide a powerful mechanism of cytoprotection in animal models of cerebral ischaemia, hypoxia or head trauma. In this article Charles Taylor and Brian Meldrum review evidence indicating that Na+ channel modulators are neuroprotective and describe recent ideas for the molecular sites of action of voltage-dependent Na+ channel blockers. Clinical trials with several compounds are now in progress for stroke and traumatic head injury, and the therapeutic potential for this group of compounds is discussed.

Ion channel involvement in anoxic depolarization induced by cardiac arrest in rat brain

Anoxic depolarization (AD) and failure of ion homeostasis play an important role in ischemia-induced neuronal injury. In the present study, different drugs with known ion-channel-modulating properties were examined for their ability to interfere with cardiac-arrest-elicited AD and with the changes in the extracellular ion activity in rat brain. Our results indicate that only drugs primarily blocking membrane Na+ permeability (NBQX, R56865, and flunarizine) delayed the occurrence of AD, while compounds affecting cellular Ca2+ load (MK-801 and nimodipine) did not influence the latency time. The ischemia-induced [Na+]e reduction was attenuated by R56865. Blockade of the ATP-sensitive K+ channels with glibenclamide reduced the [K+]e increase upon ischemia, indicating an involvement of the KATP channels in ischemia-induced K+ efflux. The KATP channel opener cromakalim did not affect the AD or the [K+]e concentration. The ischemia-induced rapid decline of extracellular calcium was attenuated by receptor-operated Ca2+ channel blockers MK-801 and NBQX, but not by the voltage-operated Ca2+ channel blocker nimodipine, R56865, and flunarizine.