R56865 and flunarizine as Na(+)-channel blockers in isolated Purkinje neurons of rat cerebellum

Voltage-gated sodium channel as a target for metastatic risk reduction with re-purposed drugs

Objective: To determine the exact role of sodium channel proteins in migration, invasion and metastasis and understand the possible anti-invasion and anti-metastatic activity of repurposed drugs with voltage gated sodium channel blocking properties.

Material and methods: A review of the published medical literature was performed searching for pharmaceuticals used in daily practice, with inhibitory activity on voltage gated sodium channels. For every drug found, the literature was reviewed in order to define if it may act against cancer cells as an anti-invasion and anti-metastatic agent and if it was tested with this purpose in the experimental and clinical settings.

Results: The following pharmaceuticals that fulfill the above mentioned effects, were found: phenytoin, carbamazepine, valproate, lamotrigine, ranolazine, resveratrol, ropivacaine, lidocaine, mexiletine, flunarizine, and riluzole. Each of them are independently described and analyzed.

Conclusions: The above mentioned pharmaceuticals have shown anti-metastatic and anti-invasion activity and many of them deserve to be tested in well-planned clinical trials as adjunct therapies for solid tumors and as anti-metastatic agents. Antiepileptic drugs like phenytoin, carbamazepine and valproate and the vasodilator flunarizine emerged as particularly useful for anti-metastatic purposes.

Termination of a tachyarrhythmia by flunarizine is not a specific marker for a triggered mechanism

BACKGROUND

Prior studies have indicated that tachyarrhythmia termination by flunarizine demonstrates a triggered mechanism. This concept was not confirmed in atrial tachyarrhythmias.

OBJECTIVE

The purpose of this study was to test the hypothesis that flunarizine will not terminate reentrant atrial flutter (AFL).

METHODS

We administered flunarizine (2 mg/kg intravenously over 2 minutes) in 11 episodes of reproducibly inducible, sustained AFL in eight canines with sterile pericarditis. If flunarizine terminated AFL, we studied AFL reinducibility. We also studied pacing thresholds, refractoriness, and intra-atrial conduction time during closed-chest studies and pacing at selected cycle lengths (CLs) from selected sites before and after flunarizine administration. Atrial mapping (510 electrodes) assessed the epicardial activation sequence during AFL and its termination in six episodes. Four AFL episodes were studied in the closed-chest state.

RESULTS

Flunarizine increased AFL CL in all episodes (mean 21 ms; range 7-49 ms), which is explained by slowing conduction in the AFL reentrant circuit, principally in the area of slow conduction. AFL was terminated in 10/11 episodes after drug initiation (mean 3.7 minutes; range 0.5-6.5 minutes) by block in the area of slow conduction. AFL was then not immediately reinducible until >20 minutes after drug administration. Flunarizine had no meaningful effect on atrial pacing thresholds for capture or refractoriness and only affected conduction time in the area of slow conduction in the reentrant circuit.

CONCLUSIONS

Flunarizine (1) causes progressive slowing and block in the area of slow conduction of the AFL reentrant circuit in the canine sterile pericarditis model and (2) is effective in terminating reentrant AFL and so is not a specific marker for a triggered mechanism.

Flunarizine is a Highly Potent Inhibitor of Cardiac hERG Potassium Current

Flunarizine has been widely used for the management of a variety of disorders such as peripheral vascular diseases, migraine, and epilepsy. The majority of its beneficial effects have been attributed to its ability to inhibit voltage-gated Ca2+ channels in the low micromolar range, albeit non-selectively, as flunarizine has been shown to inhibit a variety of ion channels. We examined the effects of flunarizine on potassium currents through cardiac channels encoded by the human ether-a-go-go related gene (hERG) stably expressed in CHO cells. In this study, we have characterized the effect of flunarizine on biophysical properties of hERG potassium currents with standard whole-cell voltage-clamp techniques. Notably, flunarizine is a highly potent inhibitor of hERG current with an IC50 value of 5.7 nM. The effect of flunarizine on hERG potassium current is concentration and time dependent, and displays voltage dependence over the voltage range between -40 and 0 mV. At concentrations near or above the IC50, flunarizine causes a negative shift in the voltage dependence of hERG current activation and accelerates tail current deactivation. Flunarizine preferentially blocks the activated state of the channel and displays weak frequency dependence of inhibition. Flunarizine also inhibits KCNQ1/KCNE1 channel current with an IC50 of 0.76 microM.

Migraine prophylactic drugs work via ion channels

In recent decades, the concept of a vascular origin of migraine has been replaced by theories based on a neuronal pathophysiology. These theories all involve rapid changes in the functioning of the brain, particularly the brain stem, and the trigeminal nerves. While such paroxysmal changes in function could be the result of altered synaptic transmission, or other physiological changes, they could also be due to changes in the function of voltage-regulated sodium and calcium ion channels. Support for this view of migraine as a channelopathy comes from an examination of the likely mechanism of action of migraine prophylactic drugs. It is the present hypothesis that most of the widely used drugs for migraine prevention work by inhibiting the function of one or both of these ion channels. A review of the laboratory research done on most of the commonly used migraine prophylactic drugs, divided into five classes, reveals that they all may work on sodium channels, calcium channels, or both. If this is the common mechanism of action of migraine prophylactics, it should lead toward the development of more effective prophylactic drugs.

Quality of pronase dissociation of mature inferior colliculus neurons

One major advantage of acutely dissociated inferior colliculus (IC) neurons in electrophysiological investigations is their complete isolation from the surrounding cellular network. In this way, patch-clamp recordings can be performed under controlled conditions to study membrane properties of IC neurons in more detail. The aim of the present study was to adapt a dissociation method for immature IC neurons to the highly sensitive, fragile and vulnerable mature IC neurons of mammals (mice). The modification of a pronase-based dissociation protocol with respect to concentration, incubation time and handling (trituration) of the cells yielded intact, live IC neurons with a clean cell surface so that they were well suited for further electrophysiological investigations in our study. The largely modified dissociation protocol is described in detail and critically discussed.

Anticonvulsant profile of flunarizine and relation to Na(+) channel blocking effects

The present study will summarize our findings concerning the anticonvulsant properties of the Ca2+ channel blocker flunarizine in a variety of experimental models of epilepsy. Flunarizine exhibits anticonvulsant effects against tonic seizures induced by electroshock or various chemoconvulsants in mice, however, did not protect against pentylenetetrazol-induced clonic seizures. In the MES test, the efficacy of clinically established antiepileptics was increased by co-medication. In the rotarod test, a minimal "neurotoxic" dose (TD50) of 18.0 mg/kg intraperitoneally was determined. In models of complex partial seizures like the hippocampal stimulation and the amygdala kindling in rats, flunarizine showed only a moderate activity. Thus, it can be suggested that the anticonvulsant potency of flunarizine in various screening tests is lower than that of standard antiepileptics such as carbamazepine and phenytoin. Concerning the possible mode of action, whole-cell patch-clamp experiments with cultured neonatal rat cardiomyocytes showed that flunarizine depressed the fast inward Na+ current in a concentration- and frequency-dependent manner well comparable with the action of phenytoin. It is concluded that the use-dependent inhibition of voltage-dependent Na+ channels may essentially contribute to the anticonvulsant activity of flunarizine in models for generalized tonic-clonic seizures. The clinical efficacy as add-on therapy is critically discussed in view of the present data.

Influence of lidocaine on ouabain-induced inotropic response in rat atria

In this paper we demonstrated that lidocaine broadens the therapeutic range of ouabain action having a protective effect on ouabain-induced toxicity on rat atria. The lidocaine effect on therapeutic ouabain action was associated with the increase in the sensitivity of Na(+)-K(+)-ATPase related to a decreased in the equilibrium dissociation constant (K(d)) of high affinity binding sites. Lidocaine suppressed the ouabain-induced tonotropic effect and arrhythmias, decreasing the number of low affinity binding sites (B(max)) without changes in K(d). Blockade of Na(+)-Ca(2+) exchange with KB-R7943 or dual Na(+)-Ca(2+) channel with flunarizine, mimicked lidocaine effect increasing ouabain therapeutic action, extending its concentration range tolerated, delaying the onset of contracture. Lidocaine itself triggered negative inotropic response at high concentration. This effect was increased in the presence of flunarizine and verapamil but not by the inhibition of calcium/calmodulin with W-7. The mechanism underlying the lidocaine-induced negative inotropic response, appears to be different that underlying the positive inotropic effect on ouabain action. This study provides evidence that lidocaine can interact with the same or similar binding sites for ouabain in rat atrial tissue, providing a protective effect on ouabain-induced changes in contractility. The contribution of Na(+)-Ca(2+) exchange and/or Ca(2+) overload on lidocaine effect is discussed.

Immunohistochemical study on the distribution of the type I and type II voltage-gated sodium channels in the gerbil cerebellum

In this study, we investigated the distribution of the type I and type II Na+ channels in the gerbil cerebellum by immunohistochemistry. Strong uniform staining for type I was observed in the granular layer, whereas there was little evidence of concentrated labeling in the cell bodies and processes of Purkinje cells. The most intense staining for type II was observed in the cell bodies and dendrites of Purkinje cells, with a strong signal in the molecular layer. This localization study has shown clearly that the type I and type II Na+ channel subunits have differential distribution in the gerbil cerebellum, for the first time. The present study may provide useful data for the future investigations to understand the roles of voltage-gated sodium channels in neurological pathways.

High affinity interaction of mibefradil with voltage-gated calcium and sodium channels

Mibefradil is a novel Ca(2+) antagonist which blocks both high-voltage activated and low voltage-activated Ca(2+) channels. Although L-type Ca(2+) channel block was demonstrated in functional experiments its molecular interaction with the channel has not yet been studied. We therefore investigated the binding of [(3)H]-mibefradil and a series of mibefradil analogues to L-type Ca(2+) channels in different tissues. [(3)H]-Mibefradil labelled a single class of high affinity sites on skeletal muscle L-type Ca(2+) channels (K(D) of 2.5+/-0.4 nM, B(max)=56.4+/-2.3 pmol mg(-1) of protein). Mibefradil (and a series of analogues) partially inhibited (+)-[(3)H]-isradipine binding to skeletal muscle membranes but stimulated binding to brain L-type Ca(2+) channels and alpha1C-subunits expressed in tsA201 cells indicating a tissue-specific, non-competitive interaction between the dihydropyridine and mibefradil binding domain. [(3)H]-Mibefradil also labelled a heterogenous population of high affinity sites in rabbit brain which was inhibited by a series of nonspecific Ca(2+) and Na(+)-channel blockers. Mibefradil and its analogue RO40-6040 had high affinity for neuronal voltage-gated Na(+)-channels as confirmed in binding (apparent K(i) values of 17 and 1.0 nM, respectively) and functional experiments (40% use-dependent inhibition of Na(+)-channel current by 1 microM mibefradil in GH3 cells). Our data demonstrate that mibefradil binds to voltage-gated L-type Ca(2+) channels with very high affinity and is also a potent blocker of voltage-gated neuronal Na(+)-channels. More lipophilic mibefradil analogues may possess neuroprotective properties like other nonselective Ca(2+)-/Na(+)-channel blockers.

Electrophysiological actions of N-[1-[4-(4-fluorophenoxy)butyl]-4-piperidinyl]-N-methyl-2-benzothiazola mine (R56865) on CA1 neurons of the rat hippocampal slice during hypoxia

The electrophysiological effects of N-[1-[4-(4-fluorophenoxy)butyl]-4-piperidinyl]-N-methyl-2-benzothiazo lamine (R56865), a drug which protects heart cells from ischemia-induced arrhythmias, was studied on intracellularly-recorded CA1 neurons of the rat hippocampal slice under normal or hypoxic conditions. On normoxic cells R56865 (1 microM) reduced firing accommodation without changing passive membrane properties, spike characteristics or synaptic transmission. On hypoxic cells R56865 selectively reduced the amplitude of hypoxia-induced membrane depolarization and partly counteracted the depression of synaptic transmission evoked by Schaffers collateral stimulation. Despite its influence on repetitive firing properties, R56865 might be useful to limit the extent of cellular depolarizing responses to hypoxia.

Late sodium current inhibition in human isolated cardiomyocytes by R 56865

R 56865, a cytoprotective agent, has been shown to prevent myocardial ischemia and reperfusion injury by blockade of the late sodium current (I(Nal)). The effect of R 56865 on I(Nal) in isolated human atrial myocytes was investigated by using the whole-cell patch-clamp technique. I(Nal) recorded at the end of a 350-ms test pulse evoked from -100 to +20 mV was significantly increased by the addition of veratrine (100 microg/ml: quantity of charge corresponding to total I(Nal): 6.1 +/- 1.2 at baseline vs. 86.9 +/- 15; p < 0.001). Tetrodotoxin (TTX; 1 microM) fully prevented veratrine-induced increases in I(Nal). R 56865 (0.1-10 microM, n = 14) significantly and reversibly decreased veratrine-induced I(Nal) (42.01 +/- 8.6%, n = 6; p < 0.001 at 10 microM). Moreover, R 56865 reduced I(Nal) without significantly affecting kinetic parameters of inactivation [tau1 = 1.04 +/- 0.1 ms and tau2 = 119.3 +/- 2.3 ms (baseline) vs. tau1 = 1.57 +/- 0.5 ms and tau2 = 134.4 +/- 14 ms in the presence of 10 microM R 56865; NS]. The data indicate that R 56865 is a potent blocker of the late inducible component of sodium current in human cardiomyocytes.

Kava extract ingredients, (+)-methysticin and (+/-)-kavain inhibit voltage-operated Na(+)-channels in rat CA1 hippocampal neurons

The action of synthetic kava pyrones, (+)-methysticin and (+/-)-kavain, on voltage-operated Na(+)-channels was studied in whole-cell patch-clamped CA1 hippocampal neurons. In doses of 1-400 microM, both compounds exerted a rapid and reversible inhibition of the peak amplitude of Na(+)-currents. Shifting holding membrane potential (Vhold) to more positive values enhanced their blocking effect. The drugs studied did not demonstrate use-dependent properties at 10 Hz stimulation but shifted H infinity curve toward more negative potentials, accelerated time-course of inactivation and slowed down the recovery from inactivation. Voltage-dependence of Na(+)-channel inhibition can be explained by interaction of (+)-methysticin and (+/-)-kavain with resting closed and inactivated states of Na(+)-channel.

Altered subthreshold sodium currents and disrupted firing patterns in Purkinje neurons of Scn8a mutant mice

Sodium currents and action potentials were characterized in Purkinje neurons from ataxic mice lacking expression of the sodium channel Scn8a. Peak transient sodium current was approximately 60% of that in normal mice, but subthreshold sodium current was affected much more. Steady-state current elicited by voltage ramps was reduced to approximately 30%, and resurgent sodium current, an unusual transient current elicited on repolarization following strong depolarizations, was reduced to 8%-18%. In jolting mice, with a missense mutation in Scn8a, steady-state and resurgent current were also reduced, with altered voltage dependence and kinetics. Both spontaneous firing and evoked bursts of spikes were diminished in cells from null and jolting mice. Evidently Scn8a channels carry most subthreshold sodium current and are crucial for repetitive firing.

Role of Ca2+ in metabolic inhibition-induced norepinephrine release in rat brain synaptosomes

Ischemia and simulated ischemic conditions induce enhanced release of norepinephrine (NE) in the brain and the heart. Although studies with neuronal preparations demonstrated a rise in [Ca2+]i under energy-depleted conditions, such release of NE in the heart appears to be predominantly Ca2+ independent. Since Ca2+ overload occurs in ischemia or energy depletion and since a rise in [Ca2+]i triggers exocytosis without membrane depolarization, we tested the possibility, using brain synaptosomes, that increased NE release could be, at least in part, a consequence of raised [Ca2+]i. Brain synaptosomes were incubated with Krebs-Henseleit medium, and ischemia was mimicked by treatment with metabolic inhibitors. NE content in incubation medium (supernatant) and synaptosomes was analyzed chromatographically. Treatment with metabolic inhibitors reduced ATP content by 75% and increased [Ca2+]i by more than fourfold within minutes. Metabolic inhibition elicited NE release, which started within 10 minutes and reached a maximum after 30 minutes, with a corresponding 55% reduction in synaptosomal NE content after 40 minutes. NE release, together with a marked increase in [Ca2+]i, was also induced in energy-depleted synaptosomes by Ca2+ repletion after incubation with the Ca(2+)-free medium. Effects on NE release of various interventions to prevent Ca2+ overload were tested. Omission of Ca2+ from the incubation medium or loading synaptosomes with the Ca2+ chelator BAPTA-AM (20 and 100 mumol/L) prevented NE release, indicating a Ca(2+)-dependent mechanism. Inhibition of Ca2+ channels with omega-conotoxin, cadmium, or nifedipine had no effect on NE release during energy depletion. In contrast, nickel and 3,4-dichlorobenzamil, Na(+)-Ca2+ exchange inhibitors, dose-dependently inhibited NE release. In conclusion, this study provides evidence that under energy-depleted conditions, Ca2+ overload in synaptosomes of noradrenergic neurons from the brain is an important mechanism for the enhanced release of NE and that a reversal of Na(+)-Ca2+ exchange may be the key pathway leading to intraneuronal Ca2+ overload.

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.

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.

Neuroprotective profile of lifarizine (RS-87476) in rat cerebrocortical neurones in culture

1. The ability of the neuroprotective agent, lifarizine (RS-87476), to mitigate veratridine-, cyanide- and glutamate-induced toxicity in rat embryonic cerebrocortical neurones in primary culture has been compared with that of tetrodotoxin (TTX), nitrendipine, (+)-MK-801 and (-)-MK-801. Lactate dehydrogenase (LDH) released into the culture medium was used as the indicator of cell viability. 2. Incubation of cultures for 16 h in a medium containing veratridine (10(-4) M), sodium glutamate (10(-3) M) or sodium cyanide (10(-3) M) resulted in consistent elevations of LDH activity in the culture medium. The ability of compounds to attenuate these elevations was expressed as the concentration required to inhibit the increases in LDH release by 50% (IC50). 3. Neurotoxicity induced by veratridine was inhibited by lifarizine (IC50 = 4 x 10(-7) M), TTX (IC50 = 3 x 10(-8) M) and nitrendipine (IC50 = 3 x 10(-5) M). In contrast, (+)-MK-801 (up to 3 x 10(-5) M) was ineffective against this insult. 4. Glutamate-induced neurotoxicity was inhibited by (+)-MK-801 (IC50 = 1.4 x 10(-8) M) and to a lesser extent by (-)-MK-801 (IC50 = 1 x 10(-7) M), but was unaffected by lifarizine, TTX or nitrendipine (up to 10(-6) M). 5. (+)-MK-801 was effective against sodium cyanide-induced neurotoxicity (IC50 = 1.9 x 10(-8) M), whereas lifarizine and TTX (up to 10(-6) M) and nitrendipine (up to 3 x 10(-6) M) were without protective activity against this insult. 6. The results demonstrate that lifarizine potently protects rat cortical neurones in vitro against a neurotoxic insult that requires activation of sodium channels for its expression, and that the compound is ineffective against insults mediated by N-methyl-D-aspartate receptor activation. The weak efficacy of nitrendipine against veratridine-induced cell death argues against the involvement of L-type calcium channels in this insult. These data are consistent with the notion that the neuroprotective activity oflifarizine observed in vivo may be mediated by inhibition of neuronal sodium currents.