Extracellular ions during veratridine-induced neurotoxicity in hippocampal slices: neuroprotective effects of flunarizine and tetrodotoxin

Neuronal Swelling: A Non-osmotic Consequence of Spreading Depolarization

An acute reduction in plasma osmolality causes rapid uptake of water by astrocytes but not by neurons, whereas both cell types swell as a consequence of lost blood flow (ischemia). Either hypoosmolality or ischemia can displace the brain downwards, potentially causing death. However, these disorders are fundamentally different at the cellular level. Astrocytes osmotically swell or shrink because they express functional water channels (aquaporins), whereas neurons lack functional aquaporins and thus maintain their volume. Yet both neurons and astrocytes immediately swell when blood flow to the brain is compromised (cytotoxic edema) as following stroke onset, sudden cardiac arrest, or traumatic brain injury. In each situation, neuronal swelling is the direct result of spreading depolarization (SD) generated when the ATP-dependent sodium/potassium ATPase (the Na+/K+ pump) is compromised. The simple, and incorrect, textbook explanation for neuronal swelling is that increased Na+ influx passively draws Cl- into the cell, with water following by osmosis via some unknown conduit. We first review the strong evidence that mammalian neurons resist volume change during acute osmotic stress. We then contrast this with their dramatic swelling during ischemia. Counter-intuitively, recent research argues that ischemic swelling of neurons is non-osmotic, involving ion/water cotransporters as well as at least one known amino acid water pump. While incompletely understood, these mechanisms argue against the dogma that neuronal swelling involves water uptake driven by an osmotic gradient with aquaporins as the conduit. Promoting clinical recovery from neuronal cytotoxic edema evoked by spreading depolarizations requires a far better understanding of molecular water pumps and ion/water cotransporters that act to rebalance water shifts during brain ischemia.

Non-canonical glutamate signaling in a genetic model of migraine with aura

Migraine with aura is a common but poorly understood sensory circuit disorder. Monogenic models allow an opportunity to investigate its mechanisms, including spreading depolarization (SD), the phenomenon underlying migraine aura. Using fluorescent glutamate imaging, we show that awake mice carrying a familial hemiplegic migraine type 2 (FHM2) mutation have slower clearance during sensory processing, as well as previously undescribed spontaneous "plumes" of glutamate. Glutamatergic plumes overlapped anatomically with a reduced density of GLT-1a-positive astrocyte processes and were mimicked in wild-type animals by inhibiting glutamate clearance. Plume pharmacology and plume-like neural Ca2+ events were consistent with action-potential-independent spontaneous glutamate release, suggesting plumes are a consequence of inefficient clearance following synaptic release. Importantly, a rise in basal glutamate and plume frequency predicted the onset of SD in both FHM2 and wild-type mice, providing a novel mechanism in migraine with aura and, by extension, the other neurological disorders where SD occurs.

Systematic review of the pharmacological agents that have been tested against spreading depolarizations

Spreading depolarization (SD) occurs alongside brain injuries and it can lead to neuronal damage. Therefore, pharmacological modulation of SD can constitute a therapeutic approach to reduce its detrimental effects and to improve the clinical outcome of patients. The major objective of this article was to produce a systematic review of all the drugs that have been tested against SD. Of the substances that have been examined, most have been shown to modulate certain SD characteristics. Only a few have succeeded in significantly inhibiting SD. We present a variety of strategies that have been proposed to overcome the notorious harmfulness and pharmacoresistance of SD. Information on clinically used anesthetic, sedative, hypnotic agents, anti-migraine drugs, anticonvulsants and various other substances have been compiled and reviewed with respect to the efficacy against SD, in order to answer the question of whether a drug at safe doses could be of therapeutic use against SD in humans.

ITH14001, a CGP37157-Nimodipine Hybrid Designed to Regulate Calcium Homeostasis and Oxidative Stress, Exerts Neuroprotection in Cerebral Ischemia

During brain ischemia, oxygen and glucose deprivation induces calcium overload, extensive oxidative stress, neuroinflammation, and, finally, massive neuronal loss. In the search of a neuroprotective compound to mitigate this neuronal loss, we have designed and synthesized a new multitarget hybrid (ITH14001) directed at the reduction of calcium overload by acting on two regulators of calcium homeostasis; the mitochondrial Na⁺/Ca²⁺ exchanger (mNCX) and L-type voltage dependent calcium channels (VDCCs). This compound is a hybrid of CGP37157 (mNCX inhibitor) and nimodipine (L-type VDCCs blocker), and its pharmacological evaluation revealed a moderate ability to selectively inhibit both targets. These activities conferred concentration-dependent neuroprotection in two models of Ca²⁺ overload, such as toxicity induced by high K⁺ in the SH-SY5Y cell line (60% protection at 30 μM) and veratridine in hippocampal slices (26% protection at 10 μM). It also showed neuroprotective effect against oxidative stress, an activity related to its nitrogen radical scavenger effect and moderate induction of the Nrf2-ARE pathway. Its Nrf2 induction capability was confirmed by the increase of the expression of the antioxidant and anti-inflammatory enzyme heme-oxygenase I (3-fold increase). In addition, the multitarget profile of ITH14001 led to anti-inflammatory properties, shown by the reduction of nitrites production induced by lipopolysaccharide in glial cultures. Finally, it showed protective effect in two acute models of cerebral ischemia in hippocampal slices, excitotoxicity induced by glutamate (31% protection at 10 μM) and oxygen and glucose deprivation (76% protection at 10 μM), reducing oxidative stress and iNOS deleterious induction. In conclusion, our hybrid derivative showed improved neuroprotective properties when compared to its parent compounds CGP37157 and nimodipine.

In vitro detection of cardiotoxins or neurotoxins affecting ion channels or pumps using beating cardiomyocytes as alternative for animal testing

The present study investigated if and to what extent murine stem cell-derived beating cardiomyocytes within embryoid bodies can be used as a broad screening in vitro assay for neurotoxicity testing, replacing for example in vivo tests for marine neurotoxins. Effect of nine model compounds, acting on either the Na(+), K(+), or Ca(2+) channels or the Na(+)/K(+) ATP-ase pump, on the beating was assessed. Diphenhydramine, veratridine, isradipine, verapamil and ouabain induced specific beating arrests that were reversible and none of the concentrations tested induced cytotoxicity. Three K(+) channel blockers, amiodarone, clofilium and sematilide, and the Na(+)/K(+) ATPase pump inhibitor digoxin had no specific effect on the beating. In addition, two marine neurotoxins i.e. saxitoxin and tetrodotoxin elicited specific beating arrests in cardiomyocytes. Comparison of the results obtained with cardiomyocytes to those obtained with the neuroblastoma neuro-2a assay revealed that the cardiomyocytes were generally somewhat more sensitive for the model compounds affecting Na(+) and Ca(2+) channels, but less sensitive for the compounds affecting K(+) channels. The stem cell-derived cardiomyocytes were not as sensitive as the neuroblastoma neuro-2a assay for saxitoxin and tetrodotoxin. It is concluded that the murine stem cell-derived beating cardiomyocytes provide a sensitive model for detection of specific neurotoxins and that the neuroblastoma neuro-2a assay may be a more promising cell-based assay for the screening of marine biotoxins.

Mechanism of the persistent sodium current activator veratridine-evoked Ca elevation: implication for epilepsy

Although the role of Na(+) in several aspects of Ca(2+) regulation has already been shown, the exact mechanism of intracellular Ca(2+) concentration ([Ca(2+)](i)) increase resulting from an enhancement in the persistent, non-inactivating Na(+) current (I(Na,P)), a decisive factor in certain forms of epilepsy, has yet to be resolved. Persistent Na(+) current, evoked by veratridine, induced bursts of action potentials and sustained membrane depolarization with monophasic intracellular Na(+) concentration ([Na(+)](i)) and biphasic [Ca(2+)](i) increase in CA1 pyramidal cells in acute hippocampal slices. The Ca(2+) response was tetrodotoxin- and extracellular Ca(2+)-dependent and ionotropic glutamate receptor-independent. The first phase of [Ca(2+)](i) rise was the net result of Ca(2+) influx through voltage-gated Ca(2+) channels and mitochondrial Ca(2+) sequestration. The robust second phase in addition involved reverse operation of the Na(+)-Ca(2+) exchanger and mitochondrial Ca(2+) release. We excluded contribution of the endoplasmic reticulum. These results demonstrate a complex interaction between persistent, non-inactivating Na(+) current and [Ca(2+)](i) regulation in CA1 pyramidal cells. The described cellular mechanisms are most likely part of the pathomechanism of certain forms of epilepsy that are associated with I(Na,P). Describing the magnitude, temporal pattern and sources of Ca(2+) increase induced by I(Na,P) may provide novel targets for antiepileptic drug therapy.

Organic and inorganic calcium antagonists inhibit veratridine-induced epileptiform activity in CA3 neurons of the guinea pig

Veratridine is believed to cause epileptiform discharges via its effects on sodium channels. We addressed the question whether calcium currents, known to contribute to the generation of paroxysmal depolarization shifts (PDS) in most models of epilepsies, also contribute to veratridine-induced epileptiform activity. Therefore, we recorded from CA3 neurons (n=50) of veratridine-treated hippocampal slices and analyzed the effects of two calcium antagonists. Veratridine (0.5-1.0 microM) elicited spontaneous epileptiform bursts, paroxysmal depolarization shifts (PDS) lasting 100-300 ms, and depolarizations (LD) lasting up to several minutes. Most often PDS directly preceded LD which resulted in typical composite depolarizations termed veratridine-induced complexes (VC). VC persisted even in the presence of CNQX and APV (25 micromol/l, both), or in nominally calcium-free saline, revealing the non-synaptic nature of these potentials. Cobalt (1-2mM) abolished VC within minutes, but allowed LD type-like potentials to be elicited by depolarizing current pulses. Verapamil (50 microM) also diminished or abolished amplitudes of VC. All inhibitory effects of cobalt and verapamil were at least partly reversible. Due to the effects of both calcium antagonists we conclude that veratridine-induced epileptiform activity depends not only on sodium, but also on calcium currents.

The nootropic drug vinpocetine inhibits veratridine-induced [Ca2+]i increase in rat hippocampal CA1 pyramidal cells

The alkaloid derivative vinpocetine (14-ethoxycarbonyl-(3alpha,16alpha-ethyl)-14,15-eburnamine; Cavinton) has a well known beneficial effect on brain function in hypoxic and ischemic conditions. While it increases CNS blood flow and improves cellular metabolism, relatively little is known about vinpocetine's underlying molecular mechanisms on the single cell level. Since apoptotic and necrotic cell damage is always preceded by an increase in [Ca2+]i, this study investigated the effect of vinpocetine on [Ca2+]i increases in acute brain slices. Sodium influx is an early event in the biochemical cascade that takes place during ischemia. The alkaloid veratridine can activate this Na+ influx, causing depolarization and increasing [Ca2+]i in the cells. Therefore, it can be used to simulate an ischemic attack in brain cells. Using a cooled CCD camera-based ratio imaging system and cell loading with fura 2/AM, the effect of vinpocetine on [Ca2+]i changes in single pyramidal neurons in the vulnerable CA1 region of rat hippocampal slices was investigated. Preperfusion and continuous administration of vinpocetine (10 microM) significantly inhibited the elevation in [Ca2+]i induced by veratridine (10 microM). When the drug was administered after veratridine, it could accelerate the recovery of cellular calcium levels. Piracetam, another nootropic used in clinical practice, could attenuate the elevation of [Ca2+]i only at a high, 1 mM, concentration. We have concluded that vinpocetine, at a pharmacologically relevant concentration, can decrease pathologically high [Ca2+]i levels in individual rat hippocampal CA1 pyramidal neurons; this effect might contribute to the neuroprotective property of the drug.

Role of voltage-gated Na+ channels in hypoxia-induced neuronal injuries

1. Mammalian neurons in the central nervous system are vulnerable to oxygen deprivation. In clinical conditions, such as stroke or apnoea, permanent loss of neuronal functions can occur within minutes of severe hypoxia. 2. Recent studies have focused on the role of Na+ in acute neuronal responses to hypoxia. These studies have shown that the influx of extracellular Na+ is an important factor in hypoxia-induced injury and that blockade of voltage-gated Na+ channels reduces hypoxic responses and injury of neurons. Yet, the mechanism underlying the effect of blockade of Na+ channels on hypoxic injury is unclear. 3. The aim of the present review is to discuss the above topics given the current understanding of the role of Na+ channels in hypoxia and its implications on therapeutic strategy for preventing hypoxia-induced neurological damage. 4. It has been known that the maintenance of ionic homeostasis and membrane properties in neurons are improved by reducing the activity of voltaged-gated Na+ channels during acute hypoxia. 5. Recent studies suggest that persistent Na+ current and Na+-dependent exchangers may play a role in Na+ influx and neuronal injury during hypoxia. 6. The neuroprotective action of blockers of the Na+ channel may also be via the improved maintainance of intracellular energy levels because the action is dependent on cellular energy levels and extracellular glucose during hypoxia. 7. Hence, the blockade of voltage-gated Na+ channels reduces the excitability of neurons, Na+ influx and the accumulation of intracellular Na+. These improve the ionic homeostasis and cellular energy levels and, thus, prevent hypoxia-induced neuronal injury and neuronal damage mediated by Ca2+ overload.

The protective action of tetrodotoxin and (+/-)-kavain on anaerobic glycolysis, ATP content and intracellular Na+ and Ca2+ of anoxic brain vesicles

Because recent reports point to Na+ channel blockers as protective agents directed against anoxia-induced neuronal damage including protection of anaerobic glycolysis, the influences of tetrodotoxin (TTX) and (+/-)-kavain on anoxic rat brain vesicles were investigated with respect to lactate synthesis, vesicular ATP content and cytosolic free Na+ and Ca2+ ([Na+]i, [Ca2+]i), both of the latter determined fluorometrically employing SBFI and FURA-2, respectively. After anoxia, basal lactate production was increased from 2.9 to 9.8 nmol lactate/min/mg protein. Although lactate synthesis seemed to be stable for at least 45 min of anoxia, as deduced from the linearity of lactate production, the ATP content declined continuously with a half life (tau 1/2) of 14.5 min, indicating that anaerobic glycolysis was insufficient to cover the energy demand of anoxic vesicles. Correspondingly, [Na+]i and [Ca2+]i increased persistently after anoxia by 22.1 mmol/l Na+ and 274.9 nmol/l Ca2+, determined 6.3 min after onset. An additional stimulation of vesicles with veratridine accelerated the drop of ATP (tau 1/2 = 5.1 min) and provoked a massive Na+ overload, which levelled off to 119 mmol/l Na+ within a few minutes. Concomitantly, [Ca2+]i increased linearly with a rate of 355 nmol Ca2+/l/min. Despite the massive perturbation of ion homeostasis, lactate production was unaffected during the first 8 min of veratridine stimulation. However, complete inhibition of lactate synthesis took place 30 min after veratridine was added. The Na+ channel blockers TTX and (+/-)-kavain, if applied before anoxia, preserved vesicular ATP content, diminished anoxia-induced increases in [Na+]i and [Ca2+]i and prevented both the veratridine-induced increases of [Na+]i and [Ca2+]i and the inhibition of lactate production. The data indicate a considerable Na+ influx via voltage-dependent Na+ channels during anoxia, which speeds up the decline in ATP and provokes an increase in [Ca2+]i. A massive Na+ and Ca2+ overload induced by veratridine failed to influence lactate synthesis directly, but initiated its inhibition.

Sodium overload through voltage-dependent Na(+) channels induces necrosis and apoptosis of rat superior cervical ganglion cells in vitro

Using the failure to exclude trypan blue as a criterion for cell death, we found that veratridine, the voltage-dependent Na(+) channel activator, exerted its toxicity to cultured sympathetic neurons in a dose-dependent manner (half-maximal toxicity occurred at 2 microM). The co-presence of tetrodotoxin completely reversed the toxicity only at concentrations of veratridine < 20 microM. Veratridine neurotoxicity was due to the influx of Na(+); a medium low in Na(+) (36 mM) completely abolished its neurotoxicity, whereas a Ca(2+)-free medium did not attenuate its neurotoxicity. Furthermore, the buffering action of 1, 2-Bis-(2-aminophenoxy)ethane-N,N,N',N',-tetraacetate (BAPTA) on veratridine-induced increase in intracellular Ca(2+) levels neither blocked veratridine neurotoxicity in normal medium, nor attenuated the low Na(+) effect. Elevated K(+) effectively blocked veratridine neurotoxicity in a Ca(2+)-dependent manner. Cytoplasmic pH measurements using a fluorescent pH indicator demonstrated that cellular acidification (from pH 7.0 to pH 6.5) occurred upon treatment with veratridine. Both veratridine-induced acidification and cell death were ameliorated by 5-(N-ethyl-N-isopropyl)amiloride, the specific inhibitor of the Na(+)/H(+) exchanger (IC(50) = 0.5 microM). Finally, necrosis occurred predominantly in veratridine neurotoxicity, but both staining with bis-benzimide and TUNEL analysis showed nuclear features of apoptosis in sympathetic neurons undergoing cell death.

Plasma concentration and CNS effects of Ca antagonists darodipine and nimodipine after single-dose oral administration to healthy volunteers

The dynamics at the brain level (quantitative EEG), plasma kinetics and effects on blood pressure and heart rate of the Ca antagonists, darodipine (slow-release, 50- 200 mg) and nimodipine (30 mg), were compared in a double-blind cross-over study on healthy volunteers during a 9-hour period following single drug/placebo administration. Increased EEG total power was observed after 100 and 200 mg daropidine; a concomitant decrease of 14.5-32.0 Hz relative power was observed at 100 mg. The 50-mg dose proved ineffective. These effects were correlated with the darodipine plasma concentration only at the 100-mg dose, with indications of an active concentration interval at approximately 5-10 ng/ml; a reduction in diastolic blood pressure and increased heart rate proved to be linearly correlated with the drug plasma concentration throughout the entire concentration range. Comparable EEG effects were observed after nimodipine, but they did not correlate with the plasma concentration. Implications of the predictability of the brain effect from the drug plasma concentration and differential thresholds for the brain action and effects on (peripheral) circulation are suggested.

Role of sodium ion influx in depolarization-induced neuronal cell death by high KCI or veratridine

Intracellular Na+ concentration plays an important role in the regulation of cellular energy metabolism; i.e., increased intracellular Na+ concentration stimulates glucose utilization both in cultured neurons and astrocytes. Both high KCI and veratridine, which have been known to cause neuronal damage, elicit increased glucose utilization, presumably via increased intracellular Na+ concentration. In the present study, we examined the role of intracellular Na+ influx in the mechanisms of neuronal cell damage induced by high KCl or veratridine assayed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric method. Rat primary cultures of striatal neurons were incubated with high KCl (final concentrations: 25, 50 mM) or veratridine (0.1-100 microM) with or without various inhibitors. High KCl depolarizes cell membrane, thus, leading to Na+ influx through an activation of voltage-sensitive Na+ channels, while veratridine elicits Na+ influx by directly opening these channels. After 24-h incubation with elevated [K+]o or veratridine, glucose contents in the medium decreased significantly (approximately by 7 mM), but remained higher than 18 mM. High [K+]o reduced percent cell viability significantly (approximately 50% at 25 mM, approximately 40% at 50 mM [K+]o, P<0.01), but tetrodotoxin (100 nM) had no protective effect, indicating that Na+ influx was not essential to high K+ -induced cell death. DL-2-Amino-5-phosponovaleric acid (APV) (1 mM) completely blocked cell death induced by elevated [K+]o, while 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) did not. In contrast, veratridine (>10 microM) caused cell damage in a dose-dependent and tetrodotoxin-sensitive manner, but none of APV, CNQX, or bepridil (Na+ -Ca2+ exchanger blocker) had any protective effect. Nifedipine (50 approximately 100 microM), however, reduced percent cell damage induced by veratridine.

Effects of dotarizine and flunarizine on chromaffin cell viability and cytosolic Ca2+

Dotarizine (a novel piperazine derivative with antimigraine properties) and flunarizine (a Ca2+ channel antagonist) were compared concerning: first, their ability to cause chromaffin cell damage in vitro; second, the possible correlation of their octanol/water partition coefficients and those of another 28 compounds (i.e., Ca2+ channel antagonists, blockers of histamine H1 receptors, antimycotics, beta-adrenoceptor antagonists, neuroleptics), with their ability to cause cell damage; third, their capacity to protect the cells against the damaging effects of veratridine; and fourth, their capabilities to enhance the basal cytosolic Ca2+ concentration in fura-2-loaded single chromaffin cells, or to modify the pattern of [Ca2+]i oscillations elicited by veratridine. After 24-h exposure to 1-30 microM dotarizine, the viability of bovine adrenal chromaffin cells (measured under phase contrast or as lactate dehydrogenase, released into the medium) was similar to that of control, untreated cells; at 100 microM, 80% lactate dehydrogenase release was produced. At 1-3 microM flunarizine caused no cell damage; however 10 microM caused 20% lactate dehydrogenase release and 30 and 100 microM over 90% lactate dehydrogenase release. The time course of cell damage was considerably faster for flunarizine, in comparison to dotarizine. Out of 30 molecules tested (at 10 microM), having different octanol/water partition coefficients (log P), dotarizine was among the molecules causing no cell damage; flunarizine caused 20% cell loss, lidoflazine and verapamil over 50% cell loss, and penfluridol, draflazine, astemizole or nifedipine over 80% cell loss. No correlation was found between log P and cytotoxicity. Both dotarizine (10-30 microM) and flunarizine (3-10 microM) provided protection against veratridine-induced cell death; however, at 30 microM dotarizine afforded a pronounced protection while flunarizine enhanced the cytotoxic effects of veratridine. Dotarizine (30 microM) (but not flunarizine) caused a prompt transient elevation of the basal [Ca2+]i. Both compounds abolished the K+-induced increases of [Ca2+]i as well as the oscillations of [Ca2+]i induced by veratridine. The blocking effects of dotarizine were readily reversed after washout, while those of flunarizine were long-lasting. These differences might be relevant to the clinical use of dotarizine as an antimigraine drug.

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.

Electrophysiological effects of aconitine in rat hippocampal slices

The electrophysiological effects of aconitine were investigated in the rat hippocampal slice and compared with those of veratridine. Both alkaloids are known to bind at site 2 of sodium channels and to block its inactivation. Extracellular recordings revealed that aconitine and veratridine exert inhibitory effects on neuronal excitability. Aconitine slowly and reversibly decreased the population spike recorded in the CA1 pyramidal cell layer. The reduction of the spike amplitude was similar whether orthodromically or antidromically activated. The aconitine-induced inhibition did not differ from that of veratridine. However, following washout of aconitine, the amplitude of the antidromic spike was increased compared to the control amplitude. The veratridine-induced inhibition was only partially reversible. This inhibition was also observed during suppression of synaptic transmission by a low Ca2+/high Mg2+-medium, indicating an inhibition of axonal conductance. The results show that in the absence of synaptic transmission the antidromic (alvear) spike is more sensitive to the inhibitory action of aconitine than the presynaptic fiber spike elicited by stimulation of the Schaffer collaterals. Furthermore, it is shown that aconitine acts in an activity-dependent manner, in that the latency of onset of the inhibition is prolonged when the stimulation frequency is decreased. Field excitatory postsynaptic potentials were also suppressed by aconitine, whereas excitatory postsynaptic currents recorded by the patch clamp technique were not influenced by aconitine when cells were held at -60 mV.

The effects of a purified toxic extract of Prymnesium patelliferum on transport of ions through the plasma membrane of synaptosomes

Extract of the ichthyotoxic marine alga Prymnesium patelliferum has been shown to have several different effects on the transport of neurotransmitters across nerve membranes. It inhibits the sodium dependent uptake of L-glutamate and GABA and enhances the calcium-dependent release of acetylcholine. We have therefore investigated the effects of a purified toxic extract of P. patelliferum on some membrane properties using rat brain synaptosomes. We found that under conditions where the algal extract inhibited the uptake of L-glutamate, it increased the intracellular concentrations of Na+ and Ca2+, stimulated efflux of K+ determined as 86Rb efflux, and depolarized the synaptosomal membrane. There was no effect on Na+/K(+)-ATPase or ouabain-insensitive ATPase activities. Further, there was no leakage of the cytosolic marker LDH, indicating that the various effects of the algal extract were not due to nonspecific leakage or lysis of the synaptosomes. The rise in the cytosolic concentration of free Ca2+ induced by the algal extract was dependent on extracellular Ca2+, and was inhibited by flunarizine (1-100 microM) but not by the Ca2+ channel blockers omega-conotoxin GVIA (1 microM), diltiazem (100 microM), nifedipine (100 microM) or verapamil (100-500 microM). The increase in Na+ influx induced by the algal extract was insensitive to tetrodotoxin (3 microM) and procaine (100 microM), whereas both the Na+ influx and the membrane depolarization were inhibited by flunarizine (1-100 microM). The increase in K+ efflux was insensitive to flunarizine (5-100 microM). From these results it appears that the toxic extract of P. patelliferum increases the permeability of synaptosomes to Ca2+, Na+ and K+ and that these effects may be responsible for the plasma membrane depolarization and the disturbance of the neurotransmitter transport processes.

Neuroprotective properties of lifarizine compared with those of other agents in a mouse model of focal cerebral ischaemia

1. Changes in the peripheral type benzodiazepine binding site density following middle cerebral artery occlusion in the mouse, have been used as a marker of neuronal damage. These sites can be identified using the selective ligand [3H]-PK 11195 located on non neuronal cells, macrophages and astroglia, within the CNS. Glial cell proliferation and macrophage invasion is an unvoidable sequelae to cerebral ischaemic injury, secondary to neuronal loss. Following occlusion of the left middle cerebral artery (left MCA) a reproducible lesion was found in the parietal cortex within 7 days which gave rise to a significant increase in [3H]-PK 11195 binding. 2. Treatment of animals with the sodium channel blocker, lifarizine, significantly reduced the ischaemia-induced increase in [3H]-PK 11195 binding when given either 30 min pre-ischaemia and three times daily for 7 days at 0.5 mg kg-1, i.p. (P < 0.01) or delayed until 15 min post-ischaemia and three times daily for 7 days at 0.5 mg kg-1, i.p. (P < 0.001). Lifarizine was an effective neuroprotective agent in this model of focal ischaemia in the mouse. 3. Lifarizine also showed a dose-related protection against the ischaemia-induced increase in [3H]-PK 11195 binding with significant protection at doses of 0.1 mg kg-1, i.p. (P < 0.05), 0.25 mg kg-1, i.p. (P < 0.01) or 0.5 mg kg-1, i.p. (P < 0.01) 15 min post-ischaemia and b.i.d. for 7 days. No significant change is seen in the Kd for [3H]-PK 11195. The first dose could be delayed for up to 4 h after cerebralartery cauterization and protection was maintained.4. Phenytoin (28 mg kg-1, i.v. 15 min and 24 h post-ischaemia) was also neuroprotective in this model(P<0.01). This agent is thought to interact with voltage-dependent sodium channels to effect its anticonvulsantactions and this mechanism may also underlie its neuroprotective actions in focal cerebralischaemia.5. Agents with other mechanisms of action were also shown to have significant neuroprotection in this model. The non-competitive NMDA antagonist, MK 801, showed significant neuroprotection in the model when given at 0.5 mg kg-1, i.p. 30 min pre-ischaemia with t.i.d. dosing for 7 days (P< 0.001). The dihydropyridine calcium antagonist, nimodipine was not protective when given using the same dosing protocol as MK 801, 0.5 mg kg-1 30 min pre-occlusion and three times daily for 7 days but showed significant protection when given at 0.05 mg kg-1 15 min post-ischaemia and three times daily for 7days. The lipid peroxidation inhibitor, tirilazad (single dose 1 mg kg-1, i.v.) showed significant neuroprotection when given 5 min post-ischaemia but not when the first dose was delayed for 4 h.

Vinpocetine is a highly potent neuroprotectant against veratridine-induced cell death in primary cultures of rat cerebral cortex

The effects of vinpocetine and phenytoin against veratridine-induced cell death were investigated in primary cultures of rat cerebral cortex. Toxicity was evaluated by phase contrast microscopy and quantified by measuring lactic dehydrogenase leakage from damaged cells. Vinpocetine was highly potent in inhibiting the cell death evoked by veratridine. The concentrations of the drug evoking 50% protection (IC50 values) against 100 microM (maximal response) and 50 microM (half-maximal response) veratridine were 490 nM and 63 nM, respectively. The protective efficacy of vinpocetine exceeded about 100-fold that of phenytoin (IC50 = 44.2 microM against 100 microM veratridine), a prototype sodium-channel blocker. These data suggest that the blockade of voltage-gated sodium channels is a possible mechanism of action for the well-known neuroprotective and anticonvulsant properties of vinpocetine.

Effects of Ca2+ channel antagonists on chromaffin cell death and cytosolic Ca2+ oscillations induced by veratridine

Exposure of bovine chromaffin cells to 30 microM veratridine for 24 h led to 70-80% cell death as reflected by phase contrast microscopy, trypan blue exclusion, lactate dehydrogenase (LDH) release and cell catecholamine contents. Na+ deprivation, Ca2+ deletion or tetrodotoxin (5 microM) prevented the veratridine-induced cell damage. Nimodipine and verapamil, but not omega-conotoxin GVIA afforded 20-30% protection. Flunarizine protected the cells by 80% and R56865 by 60%. Stimulation of fura-2-loaded single bovine chromaffin cells with 30 microM of 1,1-dimethyl-4-phenylpiperazinium (DMPP) or 59 mM K+ caused fast increases in cytosolic Ca2+ concentrations, ([Ca2+]i). The [Ca2+]i rose from 0.1 to peaks of 1.9 microM, which quickly declined to near basal levels with a t1/2 of around 30 s. In spite of sustained stimulation with these two depolarizing agents, the [Ca2+]i remained low and did not undergo oscillations. In contrast, veratridine (30 microM) caused large and frequent oscillatory changes in the [Ca2+]i which were long-lasting and did not disappear even 30 min after washing out the toxin. The [Ca2+]i oscillations were reversibly suppressed by Na+ or Ca2+ removal and by 5 microM tetrodotoxin. Selective L-type Ca2+ channel blockers (10 microM nimodipine or verapamil) or N-type Ca2+ channel blockers (1 microM omega-conotoxin GVIA) did not affect the [Ca2+]i oscillations. In contrast, flunarizine or R56865 (10 microM each) suppressed the oscillations of [Ca2+]i. The results demonstrate that bovine chromaffin cells have the necessary machinery to develop prolonged and repetitive [Ca2+]i oscillations in the presence of veratridine; however, 'physiological' depolarizing stimuli did not cause oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Anoxic depolarization (AD) and failure of the cellular ion homeostasis are suggested to play a key role in ischemia-induced neuronal death. Recent studies show that the blockade of Na+ influx significantly improved the neuronal outcome. In the present study, we investigated the effects of 10 microM tetrodotoxin (TTX) on ischemia-induced disturbances of ion homeostasis in the isolated perfused rat brain. TTX inhibited the spontaneous EEG activity, delayed the ischemia-induced tissue acidification, and significantly postponed the occurrence of AD by 65%. The [Ca2+]e elevation prior to AD was attenuated from 17.8% to 6% while the increase of the [Na+]e in this period was enhanced (from 2.9% to 7.3%). These findings implied that the ischemia-induced early cellular sodium load and the corresponding shrinkage of the extracellular space was counteracted by TTX. Our results suggest that the Na+ influx via voltage-dependent channels preceding complete breakdown of ion homeostasis is one major factor leading to cell depolarization. The massive Na+ influx coinciding with AD, however, may be mainly via non-selective cation channels or/and receptor-operated channels. Persistent Na+ influx deteriorates neuronal tissue integrity by favouring Ca2+ influx and edema formation. Blockade of ischemia-induced excessive Na+ influx is, therefore, a promising pharmacological approach for stroke treatment.

Cortical negative DC deflections following middle cerebral artery occlusion and KCl-induced spreading depression: effect on blood flow, tissue oxygenation, and electroencephalogram

In the periphery of ischemic brain lesions, transient spreading depression-like direct current (DC) deflections occur that may be of pathophysiological importance for determining the volume of the ischemic infarct. The effect of these deflections on cerebral blood flow, tissue oxygen tension, and electrophysiology was studied in rats submitted to intraluminal thread occlusion of the middle cerebral artery (MCA) and compared with the changes following potassium chloride (KCl)-induced spreading depression of intact animals. Immediately after MCA occlusion, cortical laser-Doppler flow (LDF) in the periphery of the MCA territory sharply decreased to 35 +/- 14% of control (mean +/- SD; p < 0.05), tissue PO2 declined from 28 +/- 4 to 21 +/- 3 mm Hg (p < 0.05), and EEG power fell to approximately 80% of control. During 7-h occlusion, 3-11 DC deflections with a mean duration of 5.2 +/- 4.8 min occurred at irregular intervals, and EEG power gradually declined to 66 +/- 16% of control (p < 0.05). During the passage of DC deflections, LDF did not change, but PO2 further declined to 19 +/- 4 mm Hg (p < 0.05). KCl-induced depolarizations of intact rats were significantly shorter (1.4 +/- 0.5 min; p < 0.05) and were accompanied by a 43% increase in LDF (p < 0.05) and a slight but significant increase in tissue PO2 from 22 +/- 4 to 25 +/- 4 mm Hg (p < 0.05). The comparison of periinfarct and KCl-induced depolarizations demonstrates that oxygen requirements are not coupled to an appropriate flow response in the periinfarct zone with severely reduced blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Flunarizine inhibits both calcium-dependent and -independent release of glutamate from synaptosomes and cultured neurones

Flunarizine, an established Ca2+ channel antagonist, blocks both exocytotic glutamate release from mammalian cultured cerebellar granule cells and isolated presynaptic nerve endings (synaptosomes) prepared from two distinct areas of the mammalian brain. This blockade of release displays the same flunarizine concentration dependency in synaptosomes in the presence or absence of Ca2+, with total inhibition at a concentration of 10 microM. In cultured neurones, a selective effect on the L-channel-coupled component of the KCl-evoked rise in intracellular Ca2+, [Ca2+]c, can be demonstrated between flunarizine concentrations of 100 nM and 10 microM, while at concentrations above 10 microM, the remaining residual and transient components are affected. In synaptosomes, flunarizine blocks the KCl-evoked elevation in [Ca2+]c in a concentration-dependent manner. Additionally, 10 microM flunarizine directly antagonises ouabain-induced tetrodotoxin (TTX)-sensitive Na+ influx, glutamate, aspartate and GABA release from synaptosomes, whilst inhibiting veratridine-induced Ca(2+)-independent TTX-sensitive Na+ influx and glutamate release at 15 microM and 10 microM in cells and synaptosomes, respectively. In both cultured neurones and synaptosomes, the ability of flunarizine to block both neurotransmitter and cytoplasmic glutamate release is due to a direct antagonism of both voltage dependent Ca2+ channels and tetrodotoxin-sensitive Na+ channels.

Veratridine-induced intoxication: an in vitro model for the characterization of anti-ischemic compounds?

Due to the complexity of ischemia-induced cellular dysfunction many different pharmacological approaches have been tested to improve cellular ischemia resistance. However, despite the importance of [Na+]i for ischemia-induced dysfunction, only very few studies have investigated the contribution of the Na+ channel to ischemia-induced failure of intracellular ion homeostasis. Since an activation of Na+ channels by veratridine also results in a failure of intracellular ion homeostasis, the veratridine- and ischemia-induced alterations of cellular function were compared. Moreover, despite the difference in the electrophysiological changes induced by veratridine and ischemia, the possible involvement of a slowly inactivating, less selective Na+ channel in both veratridine- and ischemia-induced cellular dysfunction is discussed. As a conclusion it is suggested that veratridine intoxication could be a helpful in vitro method for the characterization of putative anti-ischemic compounds.