Induction of seizures in mice by intracerebroventricular administration of the calcium channel agonist BAY k 8644

A Genetically Encoded Actuator Selectively Boosts L-type Calcium Channels in Diverse Physiological Settings

L-type Ca 2+ channels (Ca V 1.2/1.3) convey influx of calcium ions (Ca 2+ ) that orchestrate a bevy of biological responses including muscle contraction and gene transcription. Deficits in Ca V 1 function play a vital role in cardiac and neurodevelopmental disorders. Yet conventional pharmacological approaches to upregulate Ca V 1 are limited, as excessive Ca 2+ influx leads to cytotoxicity. Here, we develop a genetically encoded enhancer of Ca V 1.2/1.3 channels (GeeC) to manipulate Ca 2+ entry in distinct physiological settings. Specifically, we functionalized a nanobody that targets the Ca V macromolecular complex by attaching a minimal effector domain from a Ca V enhancer-leucine rich repeat containing protein 10 (Lrrc10). In cardiomyocytes, GeeC evoked a 3-fold increase in L-type current amplitude. In neurons, GeeC augmented excitation-transcription (E-T) coupling. In all, GeeC represents a powerful strategy to boost Ca V 1.2/1.3 function in distinct physiological settings and, in so doing, lays the groundwork to illuminate new insights on neuronal and cardiac physiology and disease.

Ginsenoside Re blocks Bay k-8644-induced neurotoxicity via attenuating mitochondrial dysfunction and PKCδ activation in the hippocampus of mice: Involvement of antioxidant potential

Although the anticonvulsant effects of ginsenosides are recognized, little is known about their effects on the convulsive behaviors induced by the activation of L-type Ca²⁺ channels. Here, we investigated whether ginsenoside Re (GRe) modulates excitotoxicity induced by the L-type Ca²⁺ channel activator Bay k-8644. GRe significantly attenuated Bay k-8644-induced convulsive behaviors and hippocampal oxidative stress in mice. GRe-mediated antioxidant potential was more pronounced in the mitochondrial fraction than cytosolic fraction. As L-type Ca²⁺ channels are thought to be targets of protein kinase C (PKC), we investigated the role of PKC under excitotoxic conditions. GRe attenuated Bay k-8644-induced mitochondrial dysfunction, PKCδ activation, and neuronal loss. The PKCδ inhibition and neuroprotection mediated by GRe were comparable to those by the ROS inhibitor N-acetylcysteine, the mitochondrial protectant cyclosporin A, the microglial inhibitor minocycline, or the PKCδ inhibitor rottlerin. Consistently, the GRe-mediated PKCδ inhibition and neuroprotection were counteracted by the mitochondrial toxin 3-nitropropionic acid or the PKC activator bryostatin-1. GRe treatment did not have additional effects on PKCδ gene knockout-mediated neuroprotection, suggesting that PKCδ is a molecular target of GRe. Collectively, our results suggest that GRe-mediated anticonvulsive/neuroprotective effects require the attenuation of mitochondrial dysfunction and altered redox status and inactivation of PKCδ.

Molecular targets for antiepileptic drug development

This review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the alpha subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, alpha2-delta voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABA(A) receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABA(B) and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated current Ih; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABA(A) receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

Pharmacological modulators of voltage-gated calcium channels and their therapeutical application

Calcium channels (CCs) play an important role in the transduction of action potential to the cytosol. An influx of Ca(2+) is essential for muscle contraction, neurotransmitter, and hormonal release. Level of cytosolic Ca(2+) controls activities of many enzymes and regulatory proteins. Voltage-gated calcium channels (VGCCs) serve as sensors for membrane depolarization. Blood pressure reduction is due to relaxation of actomyosine filaments in vascular smooth muscles. Calcium channel blockers (CCBs) are traditionally used for treatment of cardiovascular diseases. Neurotransmitter release from presynaptic neurons is triggered by Ca(2+) influx. Blockers of neuronal CCs may be applied for pain treatment. Overload of neurons by Ca(2+) is toxic. CCBs may be applied for prevention of some neurodegenerative disorders.

Background potassium concentrations and epileptiform discharges. II. Involvement of calcium channels

Potassium- and calcium conductances regulate neuronal excitability and epileptiform activity. In this study, the effects of different extracellular potassium concentrations ([K(+)](o)) were investigated on the modulatory effect of the L-type transmembranous calcium currents on epileptiform discharges. The in vitro brain slice technique was used to examine the effects of calcium channel blockers, verapamil and nifedipine, on the repetition rate, amplitude, and duration of epileptiform field potentials (EFP) in the presence of low, physiological, and high background [K(+)](o) in guinea pig hippocampal slices. Epileptiform activity was induced by omission of Mg(2+) from artificial cerebrospinal fluid contained 2, 4, and 8 mM [K(+)](o). Both verapamil and nifedipine suppressed EFP after a transient increase in repetition rate. The extent of EFP frequency rate acceleration significantly increased with reduction of [K(+)](o). The increase in EFP frequency rate induced by application of verapamil and nifedipine was accompanied by a reduction in the EFP amplitude and a reversible increase in the burst discharge duration. The extent of burst discharge prolongation was also significantly higher with decreasing [K(+)](o). Further application of verapamil and nifedipine suppressed the epileptiform burst activity in the presence of different [K(+)](o). The latency of EFP depression was significantly diminished both with increased and decreased background potassium concentrations. The data indicate the importance of the effect of the L-type transmembranous calcium currents on the regulatory effect of background [K(+)](o) on epileptiform burst discharge frequency and duration.

Differential effects of dihydropyridine calcium channel blockers in kainic acid-induced experimental seizures in rats

The anticonvulsant effects of the dihydropyridine calcium channel blockers nifedipine, nicardipine and nimodipine were studied on experimental seizures induced by intra-hippocampal injection of kainic acid (KA) in chloralhydrate anesthetized Wistar rats. The rats were anesthetized and placed in a stereotaxic apparatus. After midline incision four screw electrodes were placed over the left and right frontal and parietal cortex and KA was injected into left dorsal hippocampus via 5-microliter Hamilton microsyringe. The changes in electroencephalograph (EEG) activity and EEG power spectra were recorded in basal conditions and 5, 10, 15, 30 and 60 min following KA injection. KA-induced excitatory changes in the surface EEG activity were associated with the marked increase in EEG power spectra in the frequency range from 14.5-22 Hz. Pretreatment with nifedipine, nicardipine and nimodipine revealed that they exerted certain differences in their anticonvulsant properties. Nimodipine significantly delayed the onset of seizures and prevented the KA-induced changes in EEG and in EEG power spectra in all recorded channels and in a dose dependent manner. Nifedipine exerted significant anticonvulsant effect only in channel four, while nicardipine was ineffective. Our results suggest that anticonvulsant action of some dihydropyridine calcium channel blockers, especially nimodipine may be in part independent of its antagonism on L-type voltage-gated calcium (Ca(2+)) channels.

Self-biting induced by activation of L-type calcium channels in mice: dopaminergic influences

The L-type calcium channel activator +/-Bay K 8644 induces repetitive self-biting and self-injurious behavior in young mice. Since dopaminergic systems have been implicated in prior studies of these behaviors in both humans and animals, the present experiments were designed to test whether drugs influencing the dopaminergic systems could modify the behavioral responses to +/-Bay K 8644. The ability of +/-Bay K 8644 to provoke self-biting and self-injurious behavior was increased by amphetamine and GBR 12909, drugs that augment synaptic dopaminergic concentrations by blocking the reuptake and/or stimulating the release of dopamine. Conversely, self-biting and self-injurious behavior were decreased by tetrabenazine or reserpine, two drugs that deplete vesicular stores of dopamine. These results suggest that dopaminergic systems may play a role in the ability of +/-Bay K 8644 to provoke self-biting and self-injurious behavior.

Effects of dextromethorphan on the seizures induced by kainate and the calcium channel agonist BAY k-8644: comparison with the effects of dextrorphan

BAY k-8644 (an L-type Ca(2+) channel agonist of the dihydropyridine class) is recognized as a potent convulsant agent. In this study, we used BAY k-8644 to explore the effects of dextromethorphan (DM) and its major metabolite, dextrorphan (DX), on the (pro)convulsant activity regulated by calcium channels. BAY k-8644 (2 mg/kg, s.c) potentiated seizures induced in rats by kainic acid (KA) (10 mg/kg, i.p.). DM appears more efficacious than DX in attenuation of KA-induced seizures. The anticonvulsant effect of a low dose (12.5 mg/kg, s.c.) of DM was reversed by BAY k-8644 (2 mg/kg) challenge. In contrast, BAY k-8644 (1 or 2 mg/kg) did not significantly affect an anticonvulsant effect from a higher dose (25 mg/kg) of either DM or DX. Intracerebroventricular injection of BAY k-8644 (37.5 microg) significantly induced seizures in mice. DM (12.5 or 25 mg/kg) pretreatment more significantly attenuated seizures evoked by BAY k-8644 than did DX (12.5 or 25 mg/kg). Furthermore, seizure activity induced by KA or BAY k-8644 was consistent with respective activator protein-1 DNA binding activity of the hippocampus. Therefore, our results suggest that the anticonvulsant effects of the morphinans involve, at least in part, the L-type calcium channel. They also suggest that DM is a more potent anticonvulsant than DX in the KA and BAY k-8644 seizure models.

Calcium channel agonists and dystonia in the mouse

Systemic administration of the L-type calcium channel agonists +/-Bay K 8644 or FPL 64176 causes a characteristic pattern of motor dysfunction in normal C57BL/6J mice that resembles generalized dystonia. There is no associated change in the electroencephalogram, confirming that the motor disorder does not reflect epileptic seizures. However, the electromyogram reveals an increase in baseline motor unit activity with prolonged phasic discharges consistent with dystonia. The duration and severity of dystonia is dependent on the dose administered and the age of the animal at testing. The effects are transient, with the return of normal motor behavior 1-4 hours after treatment. Similar effects can be provoked by intracerebral administration of small amounts of the drugs, indicating a centrally mediated response. Dystonia can be attenuated by co-administration of dihydropyridine L-type calcium channel antagonists (nifedipine, nimodipine, and nitrendipine) but not by non-dihydropyridine antagonists (diltiazem, verapamil, and flunarizine). These results implicate abnormal function of L-type calcium channels in the expression of dystonia in this model.

Valproic acid intensifies epileptiform activity in the hippocampal pyramidal neurons

The effects of large concentrations of valproic acid (VPA) on veratridine-induced epileptiform activity (veratridine model) were investigated in rat hippocampal CA1 pyramidal neurons. Studies were performed on the veratridine model in rat brain slices using conventional electrophysiological intracellular techniques. Large concentrations of VPA (5 mM or more) enhanced rather than inhibited epileptiform activity induced by veratridine. During the proepileptic phase of VPA, a membrane depolarization accompanied by a decrease in membrane input resistance were evident. The voltage-dependent proepileptic effect of VPA was blocked by tetrodotoxin (TTX; 100 nM) but not by the calcium channel blockers, diltiazem (5 microM) or omega-conotoxin GVIA (5 microM). VPA did not induce a proepileptic effect when it was superfused at high concentration (0.5-10 mM) on sodium channel-independent models such as the bicuculline or magnesium-free artificial cerebrospinal fluid. Large concentrations of VPA had no significant effect on untreated neurons. The VPA-enhanced veratridine bursting is probably related to the reported proepileptic activities observed in patients taking high doses of this drug. These data also suggest the involvement of sodium channels in the proepileptic effect of VPA.

Calcium channel activation and self-biting in mice

The L type calcium channel agonist (+/-)Bay K 8644 has been reported to cause characteristic motor abnormalities in adult mice. The current study shows that administration of this drug can also cause the unusual phenomenon of self-injurious biting, particularly when given to young mice. Self-biting is provoked by injecting small quantities of (+/-)Bay K 8644 directly into the lateral ventricle of the brain, suggesting a central effect of the drug. Similar behaviors can be provoked by administration of another L type calcium channel agonist, FPL 64176. The self-biting provoked by (+/-)Bay K 8644 can be inhibited by pretreating the mice with dihydropyridine L type calcium channel antagonists such as nifedipine, nimodipine, or nitrendipine. However, self-biting is not inhibited by nondihydropyridine antagonists including diltiazem, flunarizine, or verapamil. The known actions of (+/-)Bay K 8644 as an L type calcium channel agonist, the reproduction of similar behavior with another L type calcium channel agonist, and the protection afforded by certain L type calcium channel antagonists implicate calcium channels in the mediation of the self-biting behavior. This phenomenon provides a model for studying the neurobiology of this unusual behavior.

Effect of amlodipine upon the protective activity of antiepileptic drugs against maximal electroshock-induced seizures in mice

Amlodipine, a calcium channel antagonist of the dihydropyridine class, up to 10 mg kg(-1)(i.p.) did not significantly affect the threshold for electroconvulsions. However, this calcium channel antagonist (10 mg kg(-1)) enhanced the anticonvulsive activity of carbamazepine, valproate and phenobarbital against maximal electroshock-induced seizures in mice. Furthermore, amlodipine (5 mg kg(-1)) intensified the protection offered by carbamazepine. This effect was associated with the increased free plasma level of carbamazepine in the presence of amlodipine. Amlodipine did not influence the free or total plasma level of phenobarbital and valproate, so a pharmacokinetic interaction is not probable for valproate and phenobarbital. The anticonvulsive action and free plasma level of diphenylhydantoin was not modified by amlodipine. The combined treatment of the calcium channel antagonist and antiepileptics caused motor impairment (evaluated in the chimney test). Long-term memory (assessed in the passive avoidance test) in case of combinations of amlodipine with carbamazepine or diphenylhydantoin was not affected. The combination of amlodipine with valproate or phenobarbital significantly influenced the retention in this test. A possible usefulness of amlodipine as add-on therapy in epileptic patients may be limited by its considerable adverse effect revealed by behavioural tests. The pharmacokinetic interaction between carbamazepine and amlodipine might have some clinical importance for patients treated with these drugs.

Felbamate demonstrates low propensity for interaction with methylxanthines and Ca2+ channel modulators against experimental seizures in mice

The aim of this study was to determine the interaction potential of the new antiepileptic drug felbamate (2-phenyl-1,3-propanediol dicarbamate) with three Ca2+ channel blockers (nicardipine, nifedipine, and flunarizine), one Ca2+ channel activator (Bay K 8644; 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridi ne carboxylic acid), and two methylxanthines (caffeine and aminophylline (theophylline2 . ethylenediamine)) which are all known to markedly change protective effects of conventional antiepileptic drugs. To do so, the maximal electroshock seizure test in mice (an experimental model predicting drug efficacy in the treatment of human generalized tonic-clonic seizures) was employed to (1) quantify changes in the protective efficacy and potency of felbamate produced by adjunct drugs and (2) assess the ability of aminophylline and caffeine to affect protective efficacy afforded by a submaximal protective dose of felbamate against maximal electroshock-induced seizures. Doses of adjunct drugs were selected based on their effects on the threshold for electroconvulsions and on appropriate literature. Nicardipine (10-30 mg/kg), nifedipine (5-20 mg/kg), flunarizine (2.5-10 mg/kg), Bay K 8644 (2.5-5 mg/kg), and aminophylline (50-75 mg/kg) did not change the protective efficacy and potency of felbamate against maximal electroshock-induced tonic convulsions. Aminophylline in the dose of 100 mg/kg, however, diminished the protective potency of felbamate as evidenced by a statistically significant increase in the protective ED50 value of felbamate (a dose, in mg/kg, predicted to protect 50% of mice against convulsive stimulus) from 79.6 to 118 mg/kg; P < 0.05). Aminophylline and caffeine only at high doses (100 and 161.7 mg/kg, respectively) significantly diminished the protective efficacy of felbamate (110 mg/kg) from 96% to 27% and 40% (P < 0.05), respectively. In conclusion, felbamate shows low interaction potential with Ca2+ channel modulators and methylxanthines. Such low interaction potential clearly differentiates felbamate from conventional antiepileptic drugs where protective effects are readily altered by the compounds tested in the present study.

The effect of isradipine on maximal electroshock seizures in mice

1. The aim of this study was to investigate the possible anticonvulsant effect of a dihydropyridine calcium antagonist, isradipine, which easily crosses the blood-brain barrier displaying high affinity and specificity for the brain L-type voltage-sensitive calcium channel, on maximal electroshock seizures in mice. 2. Isradipine at i.p. doses of 2.5 mg/kg and 5.0 mg/kg was found to cause a statistically significant increase in the convulsion threshold of maximal electroshock seizures in a dose-dependent manner (P = 0.047 and P = 0.022, respectively). 3. It was concluded that the mode of action of the anticonvulsant effect of isradipine is related to blockade of the intraneuronal calcium currents, which play an important role in epileptic activity.

Ca2+ channel blockade and the antielectroshock activity of NMDA receptor antagonists, CGP 40116 and CGP 43487, in mice

Nicardipine, nifedipine and flunarizine showed anticonvulsive activity (reflected by significant elevations of the seizure threshold for tonic hindlimb extension) in doses of 20, 20 and 15 mg/kg, respectively. In combination studies, CGP 40116 [D-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid] or its methyl ester derivative (CGP 43487) was administered in a constant dose of 0.25 and 3.5 mg/kg, respectively. At these doses both competitive NMDA receptor antagonists were able to elevate significantly the convulsive threshold. Nicardipine, nifedipine, and flunarizine were administered at maximal doses (or lower) not affecting the convulsive threshold (15, 15 and 10 mg/kg, respectively). The protective activity of CGP 40116 and CGP 43487 was dose dependently potentiated by all three Ca2+ channel inhibitors. The combined treatment caused motor impairments (evaluated in the chimney test) and long-term memory deficits (measured in the passive avoidance task) similar to these produced by CGP 40116 or CGP 43487 alone. Our results indicate that nicardipine, nifedipine and flunarizine significantly potentiate the protective activity, but not the adverse effects, of CGP 40116 and CGP 43487 in mice.

Influence of nicardipine, nimodipine and flunarizine on the anticonvulsant efficacy of antiepileptics against pentylenetetrazol in mice

Among three calcium channel inhibitors, only nicardipine (10-40 mg/kg) significantly inhibited clonic seizures induced by pentylenetetrazol administered at its CD97 (convulsive dose 97%) of 81 mg/kg, subcutaneously. Nimodipine and flunarizine (both up to 80 mg/kg) did not suppress pentylenetetrazol-induced clonic seizures per se. Co-administration of nicardipine (5 mg/kg) resulted in a significant enhancement of the protective potency of either ethosuximide (50 mg/kg) or valproate (100 mg/kg) against clonic seizures in this test. Similar effects were noted in case of combined treatment of nimodipine (20-40 mg/kg) with these antiepileptics. On the contrary, flunarizine (up to 20 mg/kg) did not modify the anticonvulsive action of these antiepileptic drugs. Moreover, none of the studied calcium channel inhibitors influenced the protective activity of clonazepam (0.01 mg/kg). The antiepileptic drugs, administered alone in above doses, were ineffective against pentylenetetrazol-induced clonic convulsions. In case of ethosuximide and valproate, the motor performance in the chimney test was worsened by co-administration of nimodipine (40 mg/kg). We found no pharmacokinetic interactions (at least in relation to the plasma levels of ethosuximide and valproate) that could explain the observed results. Thus, we conclude that a combination of some calcium channel inhibitors and antiepileptic drugs may provide more efficient protection against experimental seizures which may bear a potential clinical significance.

Influence of BAY k-8644, a calcium channel agonist, on the anticonvulsant activity of conventional anti-epileptics against electroconvulsions in mice

BAY k-8644, an agonist at the dihydropyridine binding site of the L-type voltage dependent calcium channel, at the dose of 5 mg/kg (s.c.) did not significantly affect the threshold for electroconvulsions, but impaired the protective efficacy of flunarizine (15 and 20 mg/kg, i.p.) in the electroconvulsive test. Interestingly, the calcium channel agonist (at 1 and 5 mg/kg) distinctly diminished the protection offered by conventional anti-epileptic drugs (carbamazepine, diphenylhydantoin and phenobarbital) against maximal electroshock-induced seizures in mice. A pharmacokinetic interaction does not seem to be involved in the effect of BAY k-8644, since total plasma levels of these anti-epileptics (measured by immunofluorescence) were not affected by the calcium channel agonist. The only anti-epileptic drug resistant to BAY k-8644 (up to 5 mg/kg) was valproate, whose ED50 (in mg/kg) was not changed in the presence of the calcium channel agonist. Further, BAY k-8644 (5 mg/kg) did not influence the flunarizine (a calcium channel blocker)-induced potentiation of the protective action of valproate against maximal electroshock-induced convulsions. The calcium channel agonist (5 mg/kg) reversed the flunarizine-induced augmentation of the anticonvulsive activity of carbamazepine. It may be concluded that carbamazepine, diphenylhydantoin and phenobarbital partially exert their anticonvulsive effects via blockade of calcium influx whilst valproate does not seem to. In this context, the flunarizine-induced potentiation of the anticonvulsive activity of valproate is probably independent of calcium channel blockade.

Influence of a Ca2+ channel agonist, Bay k-8644, on the anticonvulsant activity of NMDA and non-NMDA receptor antagonists

Bay k-8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoro- methylphenyl)-pyridine-5-carboxylate) (a Ca2+ channel agonist of the dihydropyridine class) at 5 mg/kg (s.c.) impaired the anticonvulsant activities of two competitive NMDA receptor antagonists, CGP 37849 (D,L-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid) and D-CPP-ene (3-(2-carboxypiperazine-4-yl)-1-propenyl-1-phosphonic acid) (given i.p.), against electroconvulsions. In contrast, the Ca2+ channel agonist did not affect the protection afforded by the AMPA receptor antagonists, NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline) and GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylendioxy-5-2,3-benzodiazepine ), or by a non-competitive NMDA receptor antagonist, MK-801 (dizocilpine), all being injected i.p. It may be concluded that the anticonvulsive activity of competitive NMDA receptor antagonists can be impaired by Ca2+ ion influx.

Effects of diltiazem in convulsive states differ from those previously reported for dihydropyridine calcium channel antagonists

Unlike the dihydropyridine calcium channel antagonists studied previously, the benzothiazepine calcium channel antagonist, diltiazem, increased the incidence of convulsions caused by bicuculline, N-methyl-DL-aspartate or 4-aminopyridine. However, the latencies to convulsions were also increased. Diltiazem increased the ratings of convulsive behaviour on handling after intraperitoneal administration of bicuculline, or pentylenetetrazol and after the calcium channel activator, Bay K 8644, administered ICV. When the binding of the dihydropyridine, [3H]-nitrendipine in the CNS was measured in vivo, this was increased by diltiazem. This compound therefore showed a different pattern of interaction with convulsant drugs then that previously demonstrated for other calcium channel antagonists, appearing to possess both pro- and anticonvulsant actions, and a different pattern of interaction with the dihydropyridine receptor complex.

Role of L-type calcium channels on yohimbine-precipitated clonidine withdrawal in vivo and in vitro

This study was designed to elucidate the possible participation of L-type calcium channels in the expression of clonidine-withdrawal precipitated by yohimbine in clonidine-dependent animals. Mice implanted for 5 days with osmotic minipumps containing the alpha 2-adrenoceptor agonist clonidine showed symptoms of a withdrawal syndrome (jerks, headshakes, defecations and weight loss) when yohimbine, an alpha 2-adrenoceptor antagonist, was injected. Similarly, isolated rat ilea incubated with clonidine in vitro showed a withdrawal contracture when yohimbine was added to the organ bath. The effects of L-type calcium channel blockers (verapamil and diltiazem) and the stimulant Bay K 8644 on these two different types of withdrawal responses were evaluated. A dose-dependent decrease in yohimbine-precipitated clonidine withdrawal in vivo was observed when verapamil (10-40 mg/kg, s.c. and 120 micrograms/mouse, i.c.v.) or diltiazem (5-20 mg/kg, s.c. and 160 micrograms/mouse, i.c.v.) were administered to mice dependent on clonidine. No effect was found after Bay K 8644 (0.5-5 mg/kg, s.c. and 1-5 micrograms/mouse) was injected under these conditions. In vitro, both verapamil (0.1-5 microM) and D-cis-diltiazem (1-50 microM) concentration-dependently reduced the height of the yohimbine-precipitated withdrawal contracture in rat ileum incubated with clonidine. Furthermore, the effect of diltiazem was stereospecific, as D-cis-diltiazem 10 microM markedly inhibited clonidine withdrawal, whereas the same concentration of L-cis-diltiazem had no effect. In contrast, the calcium channel stimulant Bay K 8644 (0.1-1 microM) increased the height of the ileum withdrawal contracture.(ABSTRACT TRUNCATED AT 250 WORDS)

[Role of voltage-dependent ion channels in epileptogenesis]

The aim of this review is to gather information in favour of the involvement of voltage-dependent ion channels in epileptogenesis. Although, up to now, no study has shown that epilepsy is accompanied by a modification in the activity to these channels, the recently acquired knowledge of their physiology allows to presume would favor their involvement in epileptogenesis. The results from electrophysiological studies are as follows: a persistent sodium current increases neuronal excitability whereas potassium currents have an inhibitory role. In particular, calcium-dependent potassium current are involved in the post-hyperpolarization phases which follows PDS. Calcium currents are also involved in the genesis of the "bursting pacemaker" activity displayed by the neurons presumed to be inducers of the epileptic activity. Biochemical data has shown that as a consequence of epileptic activity, sodium and calcium channels are down regulated. This down-regulation could be a way to reduces neuronal hyperexcitability. Pharmacological data demonstrate the drugs which activate calcium channels or which inhibit potassium channels have a convusilvant effect. On the contrary, agents which block calcium or sodium channels or which properties. Among the latter ones, some antiepileptic drugs can be found. In summary situations which lead to increase in calcium and sodium currents and/or to an inhibition in potassium currents are potentially epileptogenic.

Anticonvulsant properties of calcium channel blockers in mice: N-methyl-D-,L-aspartate- and Bay K 8644-induced convulsions are potently blocked by the dihydropyridines

Ten calcium channel blockers were evaluated in mice after intraperitoneal (i.p.) administration for prevention of seizures induced by various convulsants. The dihydropyridines (class II calcium antagonists, i.e., nisoldipine, nitrendipine, nicardipine, nifedipine, and nimodipine) selectively prevented seizures elicited by administration of pentylenetetrazol (PTZ), N-methyl-D,L-aspartate (NMDLA) and the dihydropyridine calcium channel agonist BAY K 8644. With regard to prevention of NMDLA-induced seizures and the subsequent mortality, these compounds were similar in potency to the noncompetitive NMDA receptor antagonist MK801. Unlike MK801 (IC50 = 0.014 microM), the dihydropyridines did not inhibit in vitro binding of MK801 to synaptic membrane fractions prepared from rat cerebrohippocampal tissue. The dihydropyridines did not influence seizures elicited by maximal electroshock (MES). Flunarizine (diphenyl-alkylamine, class IV) was selectively active in the MES test, considerably less potent against NMDLA-induced convulsions/mortality, exhibited weak noncompetitive NMDA antagonism in vitro (IC50 = 28 microM), and was inactive in the PTZ and BAY K 8644 testing paradigms. Diltiazem, a class III benzothiazepine, possessed relatively weak broad spectra of activity against MES, PTZ, NMDLA, and BAY K 8644 test situations. It was inactive in vitro as a noncompetitive NMDA antagonist. The class I compound verapamil (phenylalkylamine) displayed only moderate inhibition of NMDLA-evoked seizures/mortality. Prenylamine (class V) was moderately active against convulsions produced by MES and NMDLA while retaining a degree (IC50 = 16 microM) of noncompetitive NMDA antagonism. Lidoflazine (class VI) was inactive in all tests. The Ca2+ channel blockers and MK801 were inconsistent in their ability to prevent bicuculline (BIC)-elicited convulsions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of antiepileptic drugs, calcium channel blockers and other compounds on seizures induced by activation of voltage-dependent L calcium channel in DBA/2 mice

1. The convulsant activity of the calcium voltage L-channel agonist Bay k 8644 was studied in genetically epilepsy prone DBA/2 mice. 2. Seizures were induced by intracerebroventricular injection of Bay k 8644. 3. These seizures were reversed by some calcium channel blockers such as dihydropyridines, some excitatory amino acid antagonists such as 2-amino-7-phosphonoeptanoate and CPPene, 2-chloro-adenosine, some anticonvulsant drugs such as magnesium valproate, diazepam and clonazepam and two kappa opioid agonists (U-50488H and U-54494A). 4. The remaining antiepileptic drugs (carbamazepine, phenytoin, phenobarbital and trimethadione) were ineffective in this respect. Other anticonvulsant compounds such as dizocilpine (MK 801), ketamine and drugs enhancing GABAergic transmission did not significantly affect the clonic phase of the seizures induced by Bay k 8644. 5. These results show that Bay k 8644 seizures are relatively resistant to some anticonvulsant compounds. The role of some neurotransmitters on seizures induced by Bay k 8644 is discussed.

Effects of valproate in a model nervous system (buccal ganglia of Helix pomatia): II. Epileptogenic actions

High concentrations of valproate (VPA; greater than 20 mM) depolarized identified neuronal individuals in the buccal ganglia of Helix pomatia and transiently induced paroxysmal depolarization shifts (PDS). Threshold concentration of VPA for the induction of PDS was decreased (a) by increased seizure susceptibility, (b) by increased concentrations of derivatives of VPA, and (c) by increased H+ concentrations. Intrasomatic injection of VPA did not induce PDS. The epileptogenic action of VPA is believed to be exerted from the extracellular side of the cell membrane.

Effects of some calcium antagonists upon the activity of common antiepileptic compounds on sound-induced seizures in DBA/2 mice

1. Flunarizine (2.65 mumol/kg, i.p.) and nimodipine (5.25 mumol/kg, i.p.) potentiated the anticonvulsant properties of phenytoin, phenobarbital and valproate against audiogenic seizures in DBA/2 mice. 2. Diltiazem (5.25 mumol/kg, i.p.) was able to potentiate the antiseizure activity of phenytoin but was not effective against the anticonvulsant action of phenobarbital and valproate. 3. Verapamil (5.25 mumol/kg, i.p.) was unable to potentiate the anticonvulsant properties of all antiepileptic drugs studied. 4. Bay K 8644 (1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluorophenyl)-pyridine- 5-carboxylic acid), a calcium agonist at a dose of 2.65 mumol/kg, i.p., induced a reduction of anticonvulsant potency of phenytoin, phenobarbital and valproate. 5. None of the calcium antagonists used significantly increased the plasma levels of antiepileptic compounds or significantly affected the body temperature changes induced by anticonvulsant drugs. 6. It may be concluded that some calcium antagonists enhance the anticonvulsant properties of some antiepileptic drugs against audiogenic seizures. A pharmacokinetic interaction does not seem responsible for these effects.

Psychopharmacological properties of calcium channel inhibitors

The previous decade has witnessed a major expansion of knowledge of the role played by voltage-sensitive calcium channels in the function of the central nervous system. Significant progress in the field has been made possible with the broadening use of organic calcium channel inhibitors (CCIs, Ca2+ antagonists), until recently considered almost exclusively as peripherally active antianginal and antiarrhythmic drugs. CCIs, however, do penetrate the blood-brain barrier from the periphery. Autoradiographic studies have established a highly heterogeneous distribution of CCI recognition sites within the brain. The existing evidence suggests that CCIs have marked psychotropic properties. The profile of their central activity is unique and spans a wide range of effects. Nevertheless, question regarding potentially confounding potent peripheral effects of these drugs remain. This paper reviews the psychopharmacology of CCIs, concentrating on preclinical data, but including supportive clinical and biochemical evidence as well. It focuses on these drugs' antidepressant, antidopaminergic (neuroleptic-like), anxiolytic and anticonvulsant effects. CCIs may also modify the reinforcing properties of some addictive drugs.

Effect of nifedipine and anticonvulsants on kainic acid-induced seizures in mice

The calcium-channel inhibitor nifedipine and several anticonvulsant drugs were evaluated for effects on seizures induced by intracerebroventricular injection of 0.14 microgram of kainic acid. These seizures were markedly exacerbated by valproic acid and moderately inhibited by diazepam. Nifedipine decreased the duration of each individual seizure episode, but did not block the development of seizures. It is concluded that nifedipine prevents the maintenance or propagation of kainate-induced seizures.

The effects of dihydropyridine calcium channel modulators on pentylenetetrazole convulsions

The effects of low doses of dihydropyridine (DHP) calcium channel antagonists nimodipine, nifedipine, (-)-R-202-791, and amlodipine, the DHP calcium channel agonist BAY K 8644 were investigated on clonic convulsions to pentylenetetrazole (PTZ) in mice. Nimodipine (2-20 mg/kg) produced a dose-dependent increase in the onset time for convulsions, but did not decrease the number of mice convulsing. Nifedipine, amlodipine (10 mg/kg) and BAY K 8644 (2 mg/kg) also produced an increase in the onset time for convulsions. (-)-R-202-791 (10 mg/kg) was without effect on clonic convulsions to PTZ. BAY K 8644 increased the number of mice dying from tonic-extension convulsions to PTZ. Nimodipine did not affect convulsions elicited by strychnine. Thus, low doses of DHP calcium antagonists possess anticonvulsant properties which are structurally dependent, while DHP calcium channel activators may act to promote convulsions. These observations suggest and support previous evidence that DHP receptors are important modulatory sites for the convulsive state.

Seizures and wet-dog shakes induced by 4-aminopyridine, and their potentiation by nifedipine

The behavioral and electrographic effects of 4-aminopyridine (4-AP) administered i.p. or microinjected into the hippocampal CA1 region (i.h.) were studied in rats. The modification of such effects by the systemic administration of the Ca2+ antagonist dihydropyridine, nifedipine, was also studied. 4-AP i.p. (5 mg/kg) induced generalized tonic convulsions in 74% of the animals and death in 13%. Convulsions were characterized by electrical discharges of relatively short duration in all structures studied (frontal cortex, amygdala, dorsal hippocampus and dorsal raphe). Limbic seizures and frequent wet-dog shakes were observed when 4-AP was administered i.h. (2-4 nmol) and this behavior was correlated with hippocampal discharges, which rapidly propagated to the other structures. Pretreatment with nifedipine (7.5-50 mg/kg s.c.) markedly potentiated the effects of 4-AP. The percentage of rats that died during generalized convulsion after i.p. 4-AP increased to 56-87% and the frequency of wet-dog shakes increased after i.h. microinjection of 4-AP. Moreover, nifedipine-treated rats showed long-lasting (greater than 60 min) continuous discharges in all structures studied (status epilepticus). These results are discussed in the light of the possible participation of Ca2+ channels in the convulsant effect of 4-AP and its potentiation by nifedipine.

Effects of calcium channel inhibitors upon the efficacy of common antiepileptic drugs

Diltiazem and nifedipine (both 1.25 mg/kg) markedly potentiated the protective action of carbamazepine and diphenylhydantoin against maximal electroshock-induced seizures in mice. These calcium channel inhibitors retained their activity at lower doses. Diltiazem and nifedipine (2.5 mg/kg) also moderately potentiated the efficacy of phenobarbital and valproate. Verapamil (up to 10 mg/kg) was not effective against the action carbamazepine, diphenylhydantoin, phenobarbital, and valproate. None of the calcium channel inhibitors used (up to 40 mg/kg) influenced aminophylline-induced convulsions and mortality. Moreover, the anti-aminophylline activity of valproate and phenobarbital was not potentiated by the calcium channel inhibitors in doses up to 10 mg/kg. Further, combination of carbamazepine, ethosuximide, and trimethadione with the calcium channel inhibitors (up to 10 mg/kg) did not offer any protection against aminophylline-induced convulsions. It can be concluded that calcium channel inhibitors enhance the protective efficacy of some antiepileptics against electroconvulsions. A pharmacokinetic interaction does not seem to be responsible for this effect.

Anticonvulsant effects of dihydropyridine Ca2+ antagonists in electrocortical shock seizures

The dihydropyridine calcium antagonists nimodipine (NMD), PN200-110, and nicardipine were compared with phenytoin (PHT) as potential anticonvulsants in electrocortical shock (ECS)-induced seizures in the white New Zealand rabbit. Before treatment, seizure duration ranged from 43.8 +/- 5.1 to 49.6 +/- 5.2 s with an ECS stimulus of 10-V, 100-Hz, 0.1-ms pulses for 5 s. Each drug was administered into the right internal intracarotid artery 2 min before the ECS. A cumulative nimodipine dose of 440 micrograms/kg decreased seizure discharge to 6.6 +/- 5.0 s (p less than 0.001), whereas a total dose of 1.0 mg/kg PN200-110 was required to achieve a similar effect. Nicardipine was ineffective. A cumulative dose of 7 mg/kg phenytoin was required to suppress seizure discharge. These results indicate that Ca2+ channels modulated by dihydropyridines play a facilitating role in ECS-induced seizures. We propose that the anticonvulsant effects of nimodipine and PN200-110 are due to inhibition of neuronal calcium L-channels. Dihydropyridine Ca2+ antagonists that penetrate the blood-brain barrier (BBB) and bind to neuronal tissue may emerge as a novel class of anticonvulsants.

Effects of diltiazem on hyperthermia induced seizures in the rat pup

1. We studied the effects of a calcium antagonist diltiazem, as well as diazepam and phenytoin on hyperthermia induced seizures in unrestrained 15 day-old rats. 2. Saline injected animals exposed to an ambient temperature of 40 degrees C showed a gradual increase in body temperature reaching a maximum of 42 +/- 0.1 degree C at 50 min. 3. At this time all rats pups had generalized seizures. 4. Similar results were obtained when the animals were pretreated with phenytoin (100% showed seizures). 5. Animals receiving diltiazem had a temperature of 41.5 +/- 0.1 degree C at 90 min of exposure to 40 degrees C environment. 6. However, diltiazem completely prevented seizures. 7. The rats treated with diazepam showed lower temperature than in saline, diltiazem and phenytoin groups and no seizures were observed in this experimental group.

Neuropsychological profiles of calcium antagonists

The present study aims at reviewing the preclinical evidence suggesting that calcium antagonists exert bio-behavioural effects that may have some relevance to CNS pharmacology, and thus to psychiatry. We briefly address the question of whether calcium antagonists share the following profiles; anxiolytic, antidepressant, neuroleptic, anticonvulsant, analgesic and memory-enhancing. This survey suggests that calcium antagonists and, more especially, dihydropyridine derivatives share all these profiles together. There are, however, important limitations in the interpretation of these preclinical data. Whether the various calcium antagonists may have varying profiles, and thus varying potential psychiatric applications, cannot be explored in depth as there are few comparative data on these drugs on a large variety of animal models. In addition, the doses of calcium antagonists reported to produce behavioural responses are generally higher than the doses sufficient to produce other pharmacodynamic actions. Thus, the possibility that these former responses could be secondary to these latter actions cannot be excluded.

Calcium, neuronal hyperexcitability and ischemic injury

Due to tight regulatory controls, a 10,000-fold concentration gradient exists between intracellular and extracellular free Ca2+ concentrations. With appropriate stimulus Ca2+ will rapidly flow into neurons through various types of membrane channels including voltage-dependent and receptor-operated channels. Intracellular Ca2+ concentrations are then quickly restored primarily through Ca2+-ATPase, Na+/Ca2+ exchange, and endoplasmic reticulum sequestration. It is well-known that Ca2+ is essential for neurotransmitter release. More recent investigations indicate that Ca2+ influx is essential for neuronal excitability independent from synaptic function. In fact, abnormal Ca2+ metabolism may play a dominant role in both the initiation and propagation of seizure discharge. Accordingly, Ca2+ channel blockers may represent a new therapeutic modality to treat epilepsy. Analyzed in this article are the major mechanisms by which neurons control Ca2+ fluxes and the evidence supporting the role of Ca2+ in seizure phenomena. Thereafter, an integrative theory for the role of calcium in neuronal hyperexcitability and ischemic cell death is constructed.

Central and peripheral effects of the dihydropyridine calcium channel activator BAY K 8644 in the rat

Following intraperitoneal (i.p.) administration BAY K 8644 (0.5-4 mg/kg) induced an increase in blood pressure associated with bradycardia, increased tail-flick latency in response to radiant heat, decreased locomotion, induced muscle contraction, postural changes and also reduced reflex activity. Only the postural changes and reduced locomotion were seen after intracerebroventricular administration (5-20 micrograms/kg), suggesting that the other effects are mediated peripherally. All the above effects were antagonised by the calcium channel blocker nifedipine. BAY K 8644 (4 mg/kg i.p.) also significantly increased homovanillic acid and 3,4-dihydroxyphenylacetic acid concentrations in the cortex and striatum, an effect which could also be reversed by nifedipine. Apart from inducing hypotension and tachycardia, nifedipine alone had no effect on any of the above parameters. The analgesic-like activity of BAY K 8644 observed in the tail-flick test appears to be related to its vasoconstrictor effects as the peripherally acting vasodilator phenylephrine had similar analgesic activity. These results show that both central and peripheral dihydropyridine-sensitive calcium channels mediate the effects of BAY K 8644. Although a physiological role for the dihydropyridine-sensitive voltage-operated calcium channel in the CNS remains to be demonstrated, activation of these channels can clearly have functional effects.

Enantiomer selectivity and the development of tolerance to the behavioral effects of the calcium channel activator BAY K 8644

The putative behavioral effects of the enantiomers of BAY K 8644 and the behavioral responses to (+/-)-BAY K 8644 following chronic injection were assessed on motor function in mice. The interaction of the enantiomers of BAY K 8644 with mouse brain dihydropyridine binding sites was also evaluated. The calcium channel activating enantiomer (-)-S-BAY K 8644 impaired rotarod and motor activity with an ED50 value of 0.5 mg/kg. The calcium channel blocker enantiomer (+)-R-BAY K 8644 neither affected rotarod nor motor activity. (+)-R-BAY K 8644, and the structurally related dihydropyridine calcium channel blockers nifedipine and (-)-202-791 inhibited the impairment of rotarod activity by (-)-S-BAY K 8644 in a dose-dependent manner. (+/-)-BAY K 8644 produced convulsions in mice with a CD50 of 5 mg/kg. Chronic injection of (+/-)-BAY K 8644 (8 mg/kg IP once each day for four days) resulted in a significant tolerance to, and increase in recovery from, the motor deficits produced by (+/-)-BAY K 8644. Furthermore, chronic treatment with (+/-)-BAY K 8644 increased the onset time, but did not reduce the number of mice having convulsions to (+/-)-BAY K 8644. Chronic injection of nifedipine did not affect the motor deficit and convulsive activity of (+/-)-BAY K 8644. The behavioral effects of (+/-)-BAY K 8644 were observed at significant brain levels of drug. [3H]Nitrendipine binding to mouse brain dihydropyridine binding sites was unchanged in mice chronically injected with either (+/-)-BAY K 8644 or nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of various calcium channel blockers on three different models of limbic seizures in rats

Voltage-dependent calcium channel-blockers were studied for their ability to modulate limbic seizures induced in rats by injection of quinolinic acid and kainic acid into the hippocampus or by hippocampal kindling. Flunarizine, at 40 mg/kg (but not 20 mg/kg), reduced the total number of seizures and total time spent in seizures induced by quinolinic acid by 75%; at 60 mg/kg, both parameters were reduced more than 90%, while at 80 mg/kg seizures induced by kainic acid were not affected. Forty and 60 mg/kg of flunarizine protected hippocampal-kindled rats from fully developed convulsions (Stage 5). Nifedipine, at 20 and 40 mg/kg, was ineffective on seizures induced by both quinolinate and kainate. However, at 20 mg/kg, 57% of the kindled animals were protected from Stage 5 and total protection was achieved at 40 mg/kg. Verapamil, at 40 mg/kg, reduced by respectively, 88% and 78%, the total number of seizures and the total time spent in seizures induced by quinolinic acid, but had no effect on seizures induced by kainate and Stage 5 seizures. The results suggest that, while seizures induced by kainic acid were refractory to all voltage-dependent calcium channel blockers, binding sites affected by flunarizine and verapamil in the brain may selectively facilitate ictal activity induced by quinolinic acid. Binding sites for dihydropyridine might contribute to the increased hippocampal excitability in kindled animals. The role of calcium entry through voltage-dependent calcium channels in the occurrence of seizures in these models of limbic epilepsy is discussed.

The effects of chemically and electrically-induced convulsions on [3H]nitrendipine binding in mouse brain

The effects of chemically and electrically-evoked seizures on [3H]nitrendipine binding to voltage-dependent calcium channels in mouse brain were determined 30 and 60 min following the initiation of convulsions. While maximal electroconvulsive shock, pentylenetetrazol and strychnine exhibited either no or marginal effects, Ro 5-4864 produced a decrease (14%) in the Bmax of [3H]nitrendipine at 30 min but not 60 min. The convulsant dihydropyridine calcium channel activator, BAY K 8644, produced a significant increase in the Kd (31%) of [3H]nitrendipine at 30 min, and a significant increase in both the Bmax (21%) and Kd (28%) of [3H]nitrendipine 60 min following the initiation of convulsions. While maximal electroconvulsive shock, pentylenetetrazol and strychnine exhibited either no or marginal effects, Ro 5-4864 produced a decrease (14%) in the Bmax of [3H]nitrendipine at 30 min but not 60 min following the initiation of convulsions. These findings indicate that modulation of voltage-dependent calcium channels by certain convulsants may be important in the genesis of seizures or in post-ictal compensatory processes.