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.

Involvement of calcium in the cardiac depressant actions of a garlic dialysate.

In order to elucidate a possible role for calcium on the negative cardiotropic effects of a garlic (Allium sativum L., Liliaceae) dialysate in rat atria we studied: (a) the effects of our extract 15 min after preincubation with high and low concentrations of extracellular calcium ([Ca2+]o) on left and right activity of rat atria. The negative inotropism of garlic dialysate increased with calcium 0.75 mM; in contrast, high level of calcium (4.5 mM) induced a significant reduction of this depressant effect. None of these treatments modified the negative chronotropism of garlic; (b) nifedipine (10(-9) to 10(-7) M, verapamil (10(-9) to 10(-7) M) and diltiazem (10(-9) to 10(-7) M) induced a concentration-dependent synergism of the log concentration-effect of garlic dialysate on left atria. Verapamil and diltiazem (10(-7)M), but not nifedipine increased the inhibitory chronotropism of garlic in right atria; (c) negative inotropic and chronotropic effects demonstrated by nifedipine (1 x 10(-10) to 1.1 x 10(-6) M) were antagonized as expected by preincubation with Bay K-8644. Depressant actions of garlic were not modified with this pretreatment. These results suggest that the negative inotropic effect of our garlic dialysate is related to [Ca2+]o availability. It is possible that a restriction of intracellular calcium contributes to this effect. However, the negative chronotropic effect of garlic is scarcely affected by these modifications.

Role of voltage dependent calcium channels and cholinergic receptors in the convulsant action of BAY k-8644.

The study was designed to investigate the effects of administration of cholinomimetic agents and nifedipine on seizures induced by BAY k-8644 in rats. Injection of pilocarpine (3 mg/kg) increased seizures induced by BAY k-8644 (2 mg/kg). Administration of nifedipine (10 mg/kg) did not affect the convulsions and mortality elicited by coadministration of BAY k-8644 and pilocarpine. However, the facilitating effect of pilocarpine on BAY k-8644-induced seizures was fully prevented by pretreatment of rats with a cholinergic antagonist atropine (5 mg/kg). No similar effects were observed after injection of another cholinomimetic agent physostigmine (0.3 mg/kg). This finding implies, that facilitation of convulsant action of BAY k-8644 by pilocarpine may be related to activity of cholinergic system, but not strictly to activity of voltage dependent calcium channels.

NMDA receptor involvement in endothelin neurotoxicity in rat striatal slices.

The high K(+)-evoked dopamine release from rat striatal slices remained impaired by 50% up to 2 h after pulse exposure of the tissues to endothelin-3, under conditions of hypoglycemia/hypoxia. This striatal dysfunction was significantly improved by D-2-amino-5-phosphonopentanoic acid, a NMDA receptor antagonist, at a much lower concentration than that providing protection against NMDA-evoked dysfunction. In light of these findings, the important role of glutamatergic mechanisms, especially NMDA receptors, in mediating endothelin neurotoxicity warrants further attention.

Anticonvulsant activity of calmodulin antagonist W-7 in convulsions induced by lindane and BayK-8644: effects in c-fos expression.

The anticonvulsant activity of calmodulin antagonist W-7, was investigated on convulsions induced in mice by the insecticide lindane and by the calcium channel agonist BayK-8644. We also studied the inhibitory effect of W-7 on on c-fos mRNA expression induced by both convulsants. We observed a good correlation between doses and the acute convulsive effects of lindane and BayK-8644. The incidence rate and time to onset were clearly dose-dependent. W-7 antagonized the convulsive effects of lindane and BayK-8644 in all the parameters studied. A significant decrease in the incidence rate and time to onset were observed when they are compared with the values obtained with the ED100 of lindane- and BayK-8644 induced seizures. Both were able to activate the mRNA expression of the proto-oncogene. The pattern of this expression displayed by in situ hybridization was very similar. A dramatic increase was found in dentate gyrus and high levels of mRNA expression also occurring in hippocampal fields and cortical regions. In accordance with the behavioural results, W-7 antagonized also the c-fos expression induced by lindane and BayK-8644. Our results suggest that lindane as BayK-8644 may activate voltage-dependent calcium channels leading to calmodulin activation.

Opiate tolerance-induced modulation of free intracellular calcium in synaptosomes.

Synaptosomes were prepared from morphine-tolerant and non-tolerant mice. Significantly higher levels of basal free intracellular calcium were observed in the synaptosomes from the opiate-tolerant mice compared to synaptosomes from non-tolerant mice (468 nM versus 328 nM, respectively). In addition, morphine (1 microM) failed to attenuate KCl-induced rises in intracellular calcium in the synaptosomes from the tolerant mice. Conversely, morphine produced a concentration-related, naloxone-reversible attenuation of 50 mM KCl-induced rises in intracellular calcium in the synaptosomes from the non-tolerant mice. Omega conotoxin, which blocks both "L" and "N" type calcium channels, attenuated KCl-stimulated rises in intracellular calcium only in synaptosomes from non-tolerant mice. BAY-K 8644, an "L-type" calcium channel agonist, produced nifedipine-reversible increases in intracellular calcium in the synaptosomes from the tolerant animals only. These data suggest that chronic exposure to morphine results in an alteration in either the number of the activation state of calcium channels in the membrane. Changes in intracellular free calcium may be the final common pathway through which neurons adapt to the chronic exposure to morphine.