Mitochondrial Na+/Ca2+-exchanger blocker CGP37157 protects against chromaffin cell death elicited by veratridine.

Mitochondrial calcium (Ca(2+)) dyshomeostasis constitutes a critical step in the metabolic crossroads leading to cell death. Therefore, we have studied here whether 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157; CGP), a blocker of the mitochondrial Na(+)/Ca(2+)-exchanger (mNCX), protects against veratridine-elicited chromaffin cell death, a model suitable to study cell death associated with Ca(2+) overload. Veratridine produced a concentration-dependent cell death, measured as lactate dehydrogenase released into the medium after a 24-h incubation period. CGP rescued cells from veratridine-elicited death in a concentration-dependent manner; its EC(50) was approximately 10 microM, and 20 to 30 microM caused near 100% cytoprotection. If preincubated for 30 min and washed out for 3 min before adding veratridine, CGP still afforded significant cytoprotection. At 30 microM, CGP blocked the veratridine-elicited free radical production, mitochondrial depolarization, and cytochrome c release. At this concentration, CGP also inhibited the Na(+) and Ca(2+) currents by 50 to 60% and the veratridine-elicited oscillations of cytosolic Ca(2+). This drastic cytoprotective effect of CGP could be explained in part through its regulatory actions on the mNCX.

Lithium ion entry through the sodium channel of cultured mouse neuroblastoma cells: a biochemical study.

Lithium ion entry at low concentrations (1 to 5 mM) into an electrically active adrenergic clone of mouse neuroblastoma cells was stimulated by veratridine; and this stimulation was blocked by tetrodotoxin, These data provide biochemical evidence that lithium ions enter by way of the sodium channel which may be a major pathway for entry of this ion into electrically active cells.

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.

Neuronal electrical high frequency stimulation enhances GABA outflow from human neocortical slices.

Electrical high frequency stimulation of the globus pallidus internus or the subthalamic nucleus has beneficial motor effects in advanced Parkinson's disease. The mechanisms underlying these clinical results remain, however, unclear. From previous studies it is proposed that the gamma-aminobutyric acid (GABA) system is involved in the effectiveness of electrical high frequency stimulation. In these experiments, human neocortical slices were stimulated electrically (130 Hz) in vitro, and GABA outflow was measured after o-phthaldialdehyde sulphite derivatization using HPLC with electrochemical detection. Our results could demonstrate that high frequency stimulation (HFS) significantly increased basal GABA outflow in the presence of submaximal concentrations of the voltage-gated sodium channel opener veratridine. This effect could be abolished by the GABA antagonists bicuculline or picrotoxin. These results suggest that HFS has an activating effect on GABAergic neuronal terminals in human neocortical slices, depending on sodium and chloride influx. Since GABA plays a role in CNS disorders of basal ganglia, anxiety and epilepsy, its neocortical modulation by HFS may be (patho)physiologically relevant.

Depolarization preconditioning produces cytoprotection against veratridine-induced chromaffin cell death.

The hypothesis that K(+) channels and cell depolarization are involved in neuronal death and neuroprotection was tested in bovine chromaffin cells subjected to two treatment periods: the first period (preconditioning period) lasted 6 to 48 h and consisted of treatment with high K(+) solutions or with tetraethylammonium (TEA), a K(+) channel blocker; the second period consisted of incubation with veratridine for 24 h, to cause cell damage. Preconditioning with high K(+) (20-80 mM) or TEA (10-30 mM) for 24 h caused 20-60% cytoprotection against veratridine-induced cell death in bovine chromaffin cells. The absence of Ca(2+) ions during the first 9 h of an 18-h preconditioning period abolished the cytoprotection. Preconditioning with K(+) or TEA increased by 2.5-fold the expression of brain-derived neurotrophic factor and by nearly 2-fold the expression of the antiapoptotic protein Bcl-2. However, preconditioning did not modify the veratridine-evoked Ca(2+) signal. High K(+) shifted the Em by about 10 mV and TEA evoked a transient burst of action potentials superimposed on a sustained depolarization. We conclude that preconditioning may protect chromaffin cells from death by blocking K(+) channels that depolarize the cell and cause a cytosolic Ca(2+) signal, leading to enhanced expression of BDNF and Bcl-2.

The effect of long-chain N-acylethanolamines on some membrane-associated functions of neuroblastoma C1300 N18 cells.

As reported earlier (Gulaya, N.M., Vaskovsky, V.E., Vystosky, M.V., Volkov, G.L., Govseeva, N.N. and Artemenko, I.P. (1988) Ukr. Biochim. J. 60, 58-63), N-acylphosphatidylethanolamines (NAPE) and products of their catabolism, N-acylethanolamines (NAE), are present in the lipids of neuroblastoma C1300 N18 undifferentiated cells. The present paper describes the distribution of NAE added to culture medium of differentiated cells and its effect on the fast sodium channels and some other membrane characteristics. It is shown that NAE inhibits the destroying action of veratridine on membranes.

PAN-811 (3-aminopyridine-2-carboxaldehyde thiosemicarbazone), a novel neuroprotectant, elicits its function in primary neuronal cultures by up-regulating Bcl-2 expression.

Neurotoxicity in primary neurons was induced using hypoxia/hypoglycemia (H/H), veratridine (10microM), staurosporine (1microM) or glutamate (100microM), which resulted in 72%, 67%, 75% and 66% neuronal injury, respectively. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (PAN-811; 10microM; Panacea Pharmaceuticals, Gaithersburg, MD) pretreatment for 24 h provided maximal neuroprotection of 89%, 42%, 47% and 89% against these toxicities, respectively. Glutamate or H/H treatment of cells increased cytosolic cytochrome c levels, which was blocked by pretreatment of cells with PAN-811. Pretreatment of neurons with PAN-811 produced a time-dependent increase in the protein level of Bcl-2, which was evident even after glutamate or H/H treatments. An up-regulation in the expression of the p53 and Bax genes was also observed following exposure to these neurotoxic insults; however, this increase was not suppressed by PAN-811 pretreatment. Functional inhibition of Bcl-2 by HA14-1 reduced the neuroprotective efficacy of PAN-811. PAN-811 treatment also abolished glutamate or H/H-mediated internucleosomal DNA fragmentation.

Ability of some plant extracts, traditionally used to treat ciguatera fish poisoning, to prevent the in vitro neurotoxicity produced by sodium channel activators.

The effects of 31 plant extracts, which most are traditionally used to treat ciguatera fish poisoning in the Pacific area, were studied on the cytotoxicity of mouse neuroblastoma cells produced by ouabain, veratridine and/or brevetoxin-3 or Pacific ciguatoxin-1. The cell viability was determined using a quantitative colorimetric method. A marked cytotoxicity of seven of the 31 plant extracts studied, was observed. Despite this, these plant extracts were suspected to contain active compound(s) against the cytotoxicity produced by brevetoxin (2 extracts), brevetoxin, ouabain and/or veratridine (3 extracts), or only against that of ouabain and/or veratridine (2 extracts). Among the 24 plant extracts that exhibited by themselves no cytotoxicity, 22 were active against the effect of brevetoxin or against that of both veratridine and brevetoxin. Similar results were obtained when the seven most active plant extracts were reassayed using ciguatoxin instead of brevetoxin. In conclusion, the present work reports the first activity assessment of some plant extracts, achieved in vitro on a quite large scale. The fact that 27 plant extracts were found to exert, in vitro, a protective effect against the action of ciguatoxin and/or brevetoxin, paves the way for finding new active compounds to treat ciguatera fish poisoning, provided these compounds also reverse the effects of sodium channel activators.

Neomycin inhibits K+- and veratridine-stimulated noradrenaline release in rat brain slices and rat brain synaptosomes.

The possible involvement of phosphoinositides' turnover in the process of neurotransmitter release in the central nervous system (CNS) was studied using rat brain slices and synaptosomes. A depolarizing concentration of potassium chloride (25 mM) induces an 8.6 +/- 0.4% increase of [3H]noradrenaline [( 3H]NA) fractional release in cerebral cortical slices above spontaneous release, and 15 mM KCl induces a 3-fold increase of [3H]NA release in rat brain synaptosomes. Neomycin, an aminoglycoside which binds phosphoinositides, inhibits the potassium-induced release in cortical slices with an IC50 = 0.5 +/- 0.07 mM and with IC50 = 0.2 +/- 0.03 mM in synaptosomes. Veratridine, a veratrum alkaloid which increases membrane permeability to sodium ions and causes depolarization of neuronal cells, induces a net 13.4 +/- 0.3% increase of [3H]NA fractional release above spontaneous release in cortical slices. In analogy to K+ stimulation, neomycin inhibits the veratridine-stimulated release in cortical slices with an IC50 = 0.65 +/- 0.1 mM. It appears that the recycling of phosphoinositides, which is necessary for Ca2+ mobilization, participates in the Ca2+-dependent induced neurotransmitter release in the central nervous system.

Dihydropyridines change the uptake of calcium induced by depolarization into primary cultures of cerebellar granule cells.

The uptake of calcium (Ca++) into cerebellar granule cells in primary culture was increased by depolarizing the cells with either 60 mM KC1 or veratridine. Nitrendipine, at concentrations of 100 nM or greater, antagonized approximately 40 percent of the depolarization induced Ca++ uptake. The half maximal concentration of nitrendipine was 7nM. Furthermore, another dihydropyridine derivative, BAY K 8644 enhanced the uptake of Ca++ and in the presence of nitrendipine, this facilitation of Ca++ uptake was reduced. Thus, these data indicate the existence of voltage dependent Ca++ channels which are sensitive to dihydropyridines in primary cultures of cerebellar granule cells.

Flunarizine and R 56865 suppress veratridine-induced increase in oxygen consumption and uptake of 45Ca2+ in rat cortical synaptosomes.

The effect of the anti-ischemic compounds flunarizine and R 56865 on the veratridine-induced uptake of Ca2+ and Na+ was observed in cortical synaptosomes in the rat. The veratridine-induced uptake of Na+ and Ca2+ was determined by means of a measurement of synaptosomal oxygen consumption and a method for the uptake of 45Ca2+, respectively. Veratridine (10(-5) M) was found to induce a 3-fold increase in synaptosomal oxygen consumption (uptake of Na+) and uptake of 45Ca2+, both of which were inhibited by tetrodotoxin (10(-5) M). Nitrendipine (10(-5) M) and omega-conotoxin (5 x 10(-7) M) were ineffective on the veratridine-induced response. Nimodipine (10(-5) M) suppressed the veratridine-induced uptake of 45Ca2+ but also diminished the unstimulated uptake of 45Ca2+. The veratridine-induced uptake of Na+ was not influenced by nimodipine. Flunarizine (3 x 10(-6)-10(-5) M), as well as R 56865 (10(-6)-10(-5) M), attenuated the veratridine-induced uptake of both Na+ and 45Ca2+. In conclusion, the veratridine-induced uptake of Na+ and 45Ca2+ was shown to be closely correlated to the activity of Na+ channels but not to voltage-operated Ca2+ channels. Secondly, flunarizine and R 56865 seemed to evoke their effects by interfering with the permeability of Na+ channels. Since veratridine-induced uptake of Na+ and Ca2+ shares some similarities with ischaemia-induced uptake of Na+ and Ca2+, it is proposed, that flunarizine and R 56865 exert their anti-ischaemic effects by reducing ischaemia-induced Na+ and Ca2+ load, probably by inhibiting a TTX-sensitive Na+ channel.

The effect of cocaine on membrane potential, on membrane depolarization by veratridine or elevated [K]o and on sodium/potassium permeability ratios in synaptosomes from the limbic cortex of the rat.

Effects of cocaine on the synaptosomal membrane potential (Em), on membrane depolarization induced by veratridine or elevated [K]o and on sodium/potassium permeability ratios (pNa/pK), have been measured in buffer containing either low or high [Na]. Fluorescence of the dye rhodamine 6G was used to measure the membrane potential. Cocaine began to reduce the Em (depolarized) at concentrations between 10(-6) and 10(-5) M in low [Na] buffer and between 10(-5) and 10(-4) M in high [Na] buffer. Maximum depolarization (with 10(-3) M cocaine) was 21 mV in low [Na] buffer and 11 mV in high [Na] buffer. Cocaine also reduced the depolarization caused by veratridine or elevated [K]o; the effective concentration of cocaine in reducing the response to veratridine was one-tenth that necessary to reduce the response to elevated [K]o. The antagonism by cocaine of the response to veratridine was similar to that found by other investigators; however, this action would tend to oppose depolarization and thus cannot explain the depolarizing effect of cocaine alone. The antagonism by cocaine of the depolarization caused by elevated [K] was consistent with a reduction in pK; such a change in pK could explain the observed reduction in Em caused by cocaine alone. The effect of cocaine (10(-3) M) on the Em was also measured as a function of [K]o at low and high [Na]o. Cocaine caused membrane depolarization at all [K]o's (3.9-19.2 mV), an effect that was somewhat greater in the low [Na] medium. These measurements of Em were fitted to the Goldman equation and the ratio of pNa/pK estimated. The presence of cocaine increased the estimate of pNa/pK by 45.7%, presumably by reducing pK.

Synthesis and pharmacological evaluation of phenylacetamides as sodium-channel blockers.

The synthesis and structure-activity relationships of a series of phenylacetamides related to N-[3-(2,6-dimethyl-1-piperidinyl)propyl]-alpha-phenylbenzeneacetamide (1) (PD85639) acting at the voltage-dependent Na+ channel are described. All structural variations for this study were made in the phenylacetic acid portion of these molecules, and the compounds were synthesized by coupling the appropriately substituted phenylacetic acid derivative with 3-[1-(2,6-dimethyl)piperidinyl]-propanamine using standard methods of amide formation. Compounds were tested as inhibitors of [3H]batrachtoxinin binding in rat neocortical membranes and also as inhibitors of veratridine-induced Na+ influx in Chinese hamster ovary cells expressing type IIA Na+ channels. Diphenylacetic acid derivatives with halogenated aromatic rings (12-15) were very potent in both assays, while alkoxy and alkyl substitution did not affect activity (16 and 17). Selected compounds were tested as potential neuroprotective agents in two cell culture assays involving inhibition of veratridine-induced and hypoxia-induced lactate dehydrogenase release. Compound 15 was equipotent with flunarizine, a reference compound in both neuroprotection assays.

Activation of voltage-dependent sodium channels in cultured cerebellar poffule cells induces neurotoxicity that is not mediated by glutamate release.

Exposure of rat cerebellar granule cell cultures to neurotoxins that specifically enhance the open state probability of voltage-dependent Na+ channels, resulted in neuronal death as estimated by a cell viability assay based on fluorescent staining and 51Cr-uptake. Toxicity was detected within 1 h after addition of 100 microM veratridine and was complete within 10-18 h; it was dose-dependent and was found to be completely abolished by tetrodotoxin, an Na+ channel blocker. When veratridine was replaced by an alpha-scorpion toxin, similar observations were done. In contrast, when cultured neurons prepared ffom the cerebral hemisphere of fetal rat brain were exposed to either veratridine or alpha-scorpion toxin for 18 h or even for a longer time of incubation, no neuronal death was observed. DNA fragmentation analysis showed that the toxicity was not mediated by apoptosis. Neuronal death was neither prevented by glutamate receptor antagonists, nor by depletion of endogenous glutamate, nor by voltage sensitive calcium channel antagonists such as omega-Conotoxin-GVIA (N-type channels), omega-Agatoxin-IVA (P-type channels), nimodipine and nitrendipine (L-type channels). Our study indicates that prolonged opening of Na+ channels induced neuronal death of cerebellar granule cells which is not mediated by glutamate and reveals novel neurotoxic mechanism in addition to the well-established excitatory amino acid receptor pathway.

Tetrapentylammonium block of chloramine-T and veratridine modified rat brain type IIA sodium channels.

Tetrapentylammonium (TPeA) block of rat brain type IIA sodium channel alpha subunit was studied using whole cell patch clamp. Results indicate that TPeA blocks the inactivating brain sodium channel in a potential and use-dependent manner similar to that of the cardiac sodium channel. Removal of inactivation using chloramine-T (CT) unmasks a time-dependent block by TPeA consistent with slow blocking kinetics. On the other hand, no time dependence is observed when inactivation is abolished by modification with veratridine. TPeA does not bind in a potential-dependent fashion to veratridine-modified channels and does not significantly affect gating of veratridine-modified channels suggesting that high affinity binding of TPeA to the brain sodium channel is lost after veratridine modification.

The effect of veratridine on the release of catecholamines from the perfused adrenal gland.

1. Experiments on perfused adrenal glands of guinea-pigs were carried out to study the catecholamine output induced by veratridine in the presence of hexamethonium and atropine. 2. Veratridine (10 micrometer to 200 micrometer) caused a dose-dependent increase in catecholamine output. 3. The addition of veratridine to the perfusion medium for a period of 3 min caused an increase in catecholamine output which reached a maximum 5 min to 10 min after withdrawal of the drug. The catecholamine output then gradually declined and reached near resting values within 30 minutes. It was never sustained for a longer period, even when veratridine was infused for 1 hour. 4. Veratridine failed to increase the catecholamine output in the absence of extracellular Ca2+. However, the addition of Ca2+ after an infusion of veratridine (100 micrometer) in the absence of Ca2+ caused an increase in the catecholamine output which was proportional to the concentration of Ca2+ (0.55 mM to 8.8 mM) used. 5. Veratridine did not increase the catecholamine output in the absence of extracellular Na+ ions, NaCl being replaced by equimolar choline chloride or LiCl. Veratridine also failed to evoke catecholamine output in a Na+-free solution in which Na+ was replaced by sucrose; this was the case even in the presence of a high concentration of Ca2+ (8.8 mM). 6. Tetrodotoxin (0.1 micrometer) and excess Mg2+ (20 mM) reversibly inhibited the catecholamine output induced by veratridine. 7. Ouabain (10 micrometer) significantly potentiated the veratridine-induced catecholamine output. 8. It is suggested that Na+-dependent Ca2+ influx as well as voltage-dependent Ca2+ influx mechanisms may be involved in the catecholamine output induced by veratridine.

Interactions of benztropine, atropine and ketamine with veratridine-activated sodium channels: effects on membrane depolarization, K+-efflux and neurotransmitter amino acid release.

1. The effect of benztropine, atropine and ketamine on veratridine-induced efflux of K+, membrane depolarization and release of amino acid neurotransmitters was investigated in the preparation of rat brain synaptosomes. 2. All three drugs inhibited in a concentration-dependent manner the processes measured: the most effective compound was benztropine which exhibited an approximate Kd of 2 microM. The inhibition was not competitive in nature. 3. The veratridine titration curves in the presence of drugs were sigmoid with Hill coefficients of about 1.4. 4. At higher concentrations, benztropine, atropine and ketamine blocked uptake of amino acid neurotransmitters into synaptosomes. 5. It is postulated that benztropine, atropine and ketamine interfere with the veratridine-activated influx of sodium into synaptosomes through voltage-dependent channels by acting at the same site as local anaesthetics. Interactions at this site alter allosterically binding and action of veratridine. In addition, at higher concentrations the drugs interact with the carrier proteins for amino acid neurotransmitters and block their transport.

Effects of besipirdine at the voltage-dependent sodium channel.

1. Besipirdine (HP 749) is a compound undergoing clinical trials for efficacy in treating Alzheimer's disease. Among other pharmacological effects, besipirdine inhibits voltage-dependent sodium and potassium channels. This paper presents a pharmacological study of the interaction of besipirdine with voltage-dependent sodium channels. 2. Besipirdine inhibited [3H]-batrachotoxin binding (IC50 = 5.5 +/- 0.2 microM) in a rat brain vesicular preparation and concentration-dependently inhibited veratridine (25 microM)-stimulated increases in intracellular free sodium ([Na+]i) and calcium ([Ca2+]i) in primary cultured cortical neurones of rat. 3. Besipirdine (30-100 microM) concentration-dependently inhibited (up to 100%) veratridine-stimulated release of [3H]-noradrenaline (NA) from rat cortical slices. 4. When examined in greater detail, besipirdine was found to inhibit [3H]-batrachotoxin binding in vesicular membranes competitively. However, when examined in rat brain synaptosomes, we found that the antagonism by besipirdine was not competitive; that is, the maximal stimulation of [Ca2+]i induced by veratridine decreased with increasing concentrations of besipirdine. 5. These results show that besipirdine is an inhibitor of voltage-sensitive sodium channels and appears to bind to a site close to the batrachotoxin/veratridine binding site.

Antagonism by riluzole of entry of calcium evoked by NMDA and veratridine in rat cultured granule cells: evidence for a dual mechanism of action.

1. Intracellular calcium levels were measured in cultured cerebellar granule cells of the rat by use of the fluorescent dye, indo-1/AM. 2. Intracellular calcium levels were increased by depolarizing stimuli such as N-methyl-D-aspartate (NMDA) (100 microM), glutamic acid (20 microM), and veratridine (10 microM). This increase was essentially due to entry of external calcium. 3. Riluzole (10 microM) blocked responses to all the depolarizing agents. 4. Riluzole could still block the increase in intracellular calcium evoked by NMDA or glutamic acid when sodium channels were blocked by tetrodotoxin, suggesting that this effect is not mediated by a direct action of riluzole on the voltage-dependent sodium channel. 5. Pretreatment of the cells with pertussis toxin (0.1 micrograms ml-1) did not modify the increases in intracellular calcium evoked by NMDA, glutamic acid or veratridine. 6. In pertussis toxin-treated cells, riluzole could no longer block responses to excitatory amino acids, but still blocked responses to veratridine. 7. It is concluded that riluzole has a dual action on cerebellar granule cells, both blocking voltage-dependent sodium channels and interfering with NMDA receptor-mediated responses via a pertussis toxin-sensitive mechanism. Furthermore, these two processes have been shown to be independent.

Protection against veratridine toxicity in rat cortical cultures: relationship to sodium channel blockade.

Neuroprotection against 100 microM veratridine-induced cell death and inhibition of voltage-dependent sodium currents by phenytoin, carbamazepine, lidocaine and vinpocetine were studied in rat primary cerebrocortical cultures. Neuroprotective efficacies and sodium channel blocking potencies of these drugs failed to show a correlation, suggesting that (i) mechanisms other than sodium channel blockade may be involved in the neuroprotection, and/or (ii) inhibitory efficacy against veratridine- and voltage-activated channels may differ remarkably.