Influence of the organic calcium antagonist verapamil on N-methyl-D-aspartate (NMDA) induced cortical field potentials (neocortical slice, guinea pig)

Modulation of calcium and potassium currents by lamotrigine

Actions of the new antiepileptic drug lamotrigine (LTG) were characterized using extracellular and whole cell patch clamp recordings from rat CA1 and CA3 pyramidal cells in vitro. The results suggest that LTG, beside its previously described effect on the fast sodium inward current, also modulates - presumably voltage-gated - calcium currents and the transient potassium outward current ID. These may be effective mechanisms to inhibit pathological excitation in epilepsy and may be of potential benefit in treating underlying cellular disturbances in bipolar disorder.

Effects of the atypical antidepressant trimipramine on field potentials in the low Mg2+-model in guinea pig hippocampal slices

Trimipramine has been classified as an atypical tricyclic antidepressant, because only weak inhibitory effects on serotonin and/or noradrenaline reuptake have been found. Since some antidepressive drugs (e.g. imipramine) and other agents used in the treatment of affective disorders (e.g. carbamazepine) modulate neuronal calcium channels, trimipramine was tested on field potential changes (fp) in the low Mg2+-model of epilepsy which has been shown to be affected by calcium antagonists. Trimipramine reduced the frequency of occurrence of fp in a dose dependent manner (5-100 microM). The threshold concentration of trimipramine which did not decrease the firing rate was approximately 1 microM. Simultaneous application of subthreshold concentrations (2 microM) of the organic calcium antagonist verapamil with trimipramine decreased the firing rate to 37.0+/-22.7% (means+/-SEM, n=7) with respect to baseline values. In contrast, no additive effects of N-methyl-D-aspartate antagonists and trimipramine were observed. In conclusion, the data suggests that the antidepressive effects observed with trimipramine treatment may be due to its inhibitory action on neuronal calcium channels.

Strychnine-induced epileptiform activity in hippocampal and neocortical slice preparations: suppression by the organic calcium antagonists verapamil and flunarizine

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.

A calcium antagonistic effect of the new antiepileptic drug lamotrigine

The new antiepileptic drug lamotrigine (LTG; 3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazine) has been shown to be effective in the treatment of focal epilepsies with or without secondary generalization. Furthermore, some case reports indicate an efficacy in the treatment of bipolar affective disorders. It has been suggested that the main mechanism of action of LTG is the inhibition of glutamate release through blockade of voltage sensitive sodium channels and stabilisation of the neuronal membrane. Since some antidepressant drugs and the antiepileptic substance carbamazepine have calcium antagonistic properties, which may be of significance in the pathophysiology of epilepsies and affective disorders, the interaction of lamotrigine with carbamazepine and the organic calcium channel blocker verapamil was analyzed in the low Mg(2+)-induced model epilepsy which has been shown to be suppressed specifically by organic calcium antagonists. Lamotrigine reduced the frequency of occurrence of low-magnesium induced field potentials in CA1 and CA3 areas of the hippocampus slice preparation (guinea pigs) in a dose-dependent manner. The subthreshold concentrations which yielded no effect were 1 mumol/l for lamotrigine, 10 mumol/l for carbamazepine and 2 mumol/l for verapamil. Combinations of these subthreshold concentrations elicited a reduction in the repetition rate of field potentials. The results indicate that lamotrigine behaves additive with verapamil and carbamazepine what can be due to a common action on the same subtype of calcium channels. It can be assumed that lamotrigine may have besides its action on high-frequency sodium dependent action potentials also effects on calcium channels.

Effects of glutamate application on the rhythm of low magnesium-induced epileptiform activity in hippocampal slices of guinea-pigs

The extracellular concentration of glutamate has previously been reported to increase to more than 10-fold the basal level during seizure activity. In the present study, we tested whether localized increases in extracellular glutamate concentration influence the rhythm of epileptiform discharges in the low-magnesium epilepsy model. In hippocampal slices of guinea-pigs, epileptiform activity was induced by omission of magnesium from the bath fluid. Glutamate and its subreceptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were ejected into different strata of the CA3 and CA1 regions using microiontophoretic and micropressure application. Glutamate, NMDA and AMPA applied to the CA3 region, but not to the CA1 region, induced a short-lasting increase in epileptiform discharge frequency, often followed by a transient reduction. The effect was most pronounced with application into the stratum lacunosum-moleculare of the CA3 region and could only be evoked in slices exceeding 400 microns in thickness. The effects on the rhythm of epileptiform discharges induced by NMDA and AMPA were blocked by their specific receptor antagonists. They were not influenced by application of GABAA and GABAB receptor antagonists. Changes in somatic membrane potential of CA3 pyramidal neurons did not correlate with changes in the rhythm of epileptiform discharges elicited in this region. The transient suppression of epileptiform discharges that followed the increase in discharge frequency was abolished by an adenosine A1 receptor antagonist. We propose that localized increases in extracellular glutamate concentration modify the rhythm of epileptiform discharges due to changes in neuronal network activity.

Changes of extracellular calcium concentration induced by application of excitatory amino acids in the human neocortex in vitro

The influence of the glutamate subreceptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on cortical field potentials and on changes in extracellular free calcium concentration ([Ca2+]o) was tested on human neocortical slices (eleven from nine different patients). The tissue used was a small portion of that which is normally removed for the treatment of a brain tumor. [Ca2+]o and field potentials were measured by Ca(2+)-selective microelectrodes. Local pressure-microejection of NMDA (100 mumol/l)- and AMPA (1 mmol/l)-induced negative field potentials with maximal amplitudes of 0.9 +/- 0.1 mV (11 slices, mean +/- S.E.M.) and 1.0 +/- 0.1 mV (nine slices), respectively. The negative field potentials induced by NMDA were accompanied by monophasic decreases of [Ca2+]o (0.8 +/- 0.1 mmol/l, nine slices). AMPA elicited no (three slices) or only minor decreases of [Ca2+]o (0.2 +/- 0.1 mmol/l, five slices). The responses to the glutamate subreceptor agonists NMDA and AMPA were reversibly depressed by adding their specific antagonists DL-2-amino-5-phosphonovalerate (APV, 100 mumol/l, six slices) and 6-cyano-7-nitroquinoxalin-2,3-dion (CNQX, 5 mumol/l, four slices), respectively. The results correspond to findings in animal experiments and are consistent with the interpretation that in the human neocortex the Ca2+ permeability of channels gated by NMDA is higher than those gated by AMPA.

Picrotoxin-induced epileptic activity in hippocampal and neocortical slices (guinea pig): suppression by organic calcium channel blockers

Epileptic activity induced by the GABAA receptor antagonist bicuculline is known to be blocked by organic calcium antagonists. To further analyse the mechanism underlying convulsant activity induced by substances reducing GABA-mediated synaptic transmission, the effect of organic calcium channel blockers on epileptic activity induced by the GABAA channel blocker picrotoxin in hippocampal and neocortical slices of guinea pigs were investigated. Verapamil and flunarizine suppressed paroxysmal depolarization shifts (PDS) of single neurons and accompanying epileptic field potentials (EFP). As a measure of drug action the repetition rate of epileptic events were used. The depression down to 10% the initial value (90% depression) is indicated. In the hippocampus verapamil suppressed PDS/EFP within 70 +/- 16 min (40 mumol/l) and within 39 +/- 5 min (60 mumol/l). This suppression was reversible with washout of verapamil. Flunarizine irreversibly blocked EFP/PDS within 108 +/- 14 min (18 mumol/l). In the neocortex verapamil reversibly suppressed EFP within 146 +/- 6 min (40 mumol/l) and 127 +/- 26 min (60 mumol/l). Flunarizine irreversibly blocked EFP within 181 +/- 30 min (3 mumol/l) and 109 +/- 13 min (18 mumol/l). The results suggest that voltage dependent calcium channels are essentially involved in picrotoxin-induced epileptic activity.