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In epilepsy models, organic calcium antagonists regularly induce a transient activity increase before suppression of epileptiform discharges. This action was speculated to be mediated by a modulation of potassium currents. Since A-type currents potently regulate neuronal excitability, their modulation by calcium channel blockers was investigated in acutely isolated human neocortical temporal lobe neurons and CA1 neurons of guinea pigs using the whole-cell voltage-clamp technique. In human neurons, 40 microM nifedipine caused an amplitude reduction by 28% at a command potential of -6 mV and produced a biexponential, markedly accelerated current inactivation with time constants of 8.4 +/- 1.1 ms (n = 6) and 62.9 +/- 6.4 ms (n = 5). The time constant under control conditions was 50.1 +/- 8.5 ms (n = 6). Verapamil (40 microM) did not affect the current amplitude, but accelerated the monoexponential current inactivation from 40.2 +/- 7.1 ms to 13.3 +/- 0.8 ms (n = 9). Accordingly, verapamil accelerated the inactivation from 42.3 +/- 5.9 ms to 15.0 +/- 1.3 ms (n = 11) in guinea pig CA1 neurons, without affecting the current amplitude. In this preparation, it was shown that the two enantiomers of verapamil do not differ in their actions. The results show that the A-type current in human neocortical and in guinea pig hippocampal neurons is reduced by organic calcium channel blockers.

Spreading depressions (SD) occur in association with ischaemia, epilepsy and migraine. Intracellular calcium oscillations have been suggested to be involved in the generation and propagation of SD. The present study was performed to study the mechanism of conditioning guinea pig hippocampal slices by the T-type calcium channel blockers NiCl2 and amiloride. SD-like fluctuations of DC potential were recorded by inserting microelectrodes into the CA1 and CA3 regions. The SD occurrence was significantly greater with 10 micromol/l NiCl2 as well as with 25 and 50 micromol/l amiloride than with other concentrations of these substances. The concentration response curve was inversely U-shaped with the maximum repetition rates of SDs being achieved at 10 micromol/l NiCl2 as well as at 25 and 50 micromol/l amiloride. SD occurrence could be completely blocked by the NMDA antagonist APV (10 micromol/l) in all cases. These data demonstrate that modulation of the Ca2+ dynamics conditioned guinea pig hippocampal slices and increased their susceptibility to generate SD.

Organic calcium channel antagonists block epileptiform activity in adult tissue, suggesting an essential role of L-type channels in epileptogenesis in the mature CNS. By contrast, this remains doubtful for neonatal tissue, as the density of calcium channels changes markedly with ontogenesis. The paper addresses this question by exploring the antiepileptic efficacy of the L-type calcium channel blockers verapamil and nifedipine in low-Mg(2+)-epilepsy in rat hippocampal slices of different postnatal (PN) ages. Field (CA3, CA1) and membrane potentials (CA3) were recorded. Washout of Mg(2+) induced epileptiform potentials, which were blocked age-dependently: Verapamil suppressed activity in all preparations of PN1-5 and PN13-30+, but only in 70% of PN6-12. Nifedipine depressed activity in >75% of slices of PN13-30+, but only in 33% of PN1-12. The findings indicate a role of L-type calcium channels in epileptogenesis from PN13 onwards, with phenylalkylamine-sensitive calcium channels also being involved during PN1-5., (Copyright 2000 Academic Press.)

The antiepileptic effect of the dihydropyridine calcium channel blocker nifedipine was tested in neocortical slice preparations (n=27) from patients ranging in age from four to 46 years (mean=25) who underwent surgery for the treatment of intractable epilepsy. Epileptiform events consisted of spontaneously occurring rhythmic sharp waves as well as of untriggered epileptiform field potentials induced by omission of Mg(2+) from the superfusate, or epileptiform field potentials elicited by application of bicuculline and triggered by single electrical stimuli. (1) Spontaneous rhythmic sharp waves (n=6): with nifedipine (40micromol/l), the repetition rate was decreased down to 30% of initial value, whereas the area under the field potential remained nearly unchanged. (2) Untriggered low Mg(2+) epileptiform field potentials (n=6): with nifedipine (40micromol/l) the area under the field potentials was reduced while the action on the repetition rate was ambiguous. (3) Triggered bicuculline epileptiform field potentials (n=15): with nifedipine (40micromol/l; n=4), no antiepileptic effect was found. There was, however, a marked increase in the area under the epileptiform field potentials. The area under the field potentials was reduced only at a dosage of 60micromol/l (n=11). This effect was stronger when nifedipine was applied with a K(+) concentration raised from 4 to 8mmol/l. The results show that the calcium channel blocker nifedipine is able to reduce differential epileptiform discharges in human neocortical tissue. These observations are in line with previous findings, suggesting that calcium flux into neurons is involved in epileptogenesis. The present results therefore support the idea that some organic calcium antagonists may be useful in human epilepsy therapy, although the etiology of epileptic seizures seems to be a critical factor for the efficacy of the drug.

Alongside GABA, glycine is the major inhibitory transmitter in the central nervous system. Application of the glycine receptor blocker strychnine is known to evoke epileptiform phenomena. The present paper addresses the question whether postsynaptic calcium currents through L-type channels contribute to strychnine-induced epileptiform field potentials (EFP). To test for this, the antiepileptic effect of the specific L-type calcium channel blocker, verapamil, in hippocampal and neocortical slices was investigated. In parallel with this, the antiepileptic efficacy of the unspecific calcium channel modulator, flunarizine, was tested with respect to pharmacotherapy of epilepsies. In both preparations, the L-type calcium channel blocker, verapamil, was able to suppress EFP. In neocortical slices, EFP were blocked in all experiments, whereas in hippocampal slices, in 3 out of 11 experiments, no complete suppression occurred. Flunarizine acted in a similar way. It is concluded that L-type calcium channels are involved in strychnine-induced epilepsy, but to a greater extent in the neocortex than in the hippocampus.

The specific L-type calcium channel blocker verapamil exerts an antiepileptic effect on neurons. This effect is assumed to depend on the blockade of transmembraneous calcium flux during epileptic discharges. In order to test this hypothesis, fura-dextran loaded snail neurons were rendered epileptic by pentylenetetrazole (40 mmol/l). The effect of verapamil (20 or 40 mumol/l) on free intracellular calcium ([Ca2+]i) transients was investigated by means of fluorescence ratio-imaging and simultaneous intracellular membrane potential recording. During epileptic depolarization [Ca2+]i increased especially in the outermost submembraneous areas of the neuron. [Ca2+]i reached peak values 6-22 s after the onset of epileptic depolarizations. Application of verapamil progressively shortened the epileptic depolarizations. This shortening of epileptic depolarizations developed along with a diminution of the submembraneous calcium signals down to noise level. The effect was found to be reversible. It is concluded that the antiepileptic effect of verapamil depends largely on its ability to block transmembraneous calcium flux.

With epileptic activity, neurons show paroxysmal depolarization shifts (PDSs) corresponding to epileptic field potentials (EFPs) generated by the neuronal population. Experimental results indicate that calcium and calcium-dependent currents participate in the generation of these events. Consequently neuronal PDS/EFP were depressed by organic calcium channel blockers. This justifies the hope that calcium channel blockers might be useful in the treatment of human epilepsies.

The results demonstrate that organic calcium antagonists are able to reduce epileptic activity at the level of single neurons and of neuronal populations. This holds true also for human cortical tissue. Among other observations already published this justifies the hope that calcium antagonistic agents might be useful in the treatment of human epilepsies (28).