Action of anticonvulsants on hippocampal slices in Mg-free medium

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

A potential role for astrocytes in mediating the antiepileptic actions of furosemide in vitro

Epileptic seizures are characterized by abnormal electrical discharge. In previous studies we established a powerful antiepileptic action for a commonly used diuretic (furosemide). However, it remains unclear precisely how furosemide terminates abnormal electrical discharges. To address this issue, we performed in vitro experiments to examine conditions where furosemide exerts antiepileptic activity and patch-clamp studies to analyze the effect of furosemide on neuronal membrane properties, synaptic function and inward potassium current. Furosemide was not found to alter synaptic field responses, excitatory postsynaptic currents or intrinsic membrane properties of principal hippocampal neurons. Our in vitro studies indicate that furosemide does not abolish spontaneous epileptiform bursting during co-application of Ba2+ or Cs+ ions (to block inwardly rectifying potassium channels). Our patch-clamp data indicate that furosemide enhances the function of astrocytic, but not neuronal, inward potassium channels and that this modulation may be required for its antiepileptic activity. Although a variety of antiepileptic drugs are already available, none of these compounds selectively target astrocytes while preserving synaptic/neuronal function. Thus, furosemide-mediated modulation of inward potassium current (on astrocytes) represents a new target for control of abnormal electrical discharge in the CNS.

Effects of carbamazepine and novel 10,11-dihydro-5H-dibenz[b,f]azepine-5-carboxamide derivatives on synaptic transmission in rat hippocampal slices

The effects of carbamazepine on synaptic transmission in rat hippocampal slices were compared with those of two novel analogues (BIA2-093 and BIA2-024) with equivalent anticonvulsant efficacy but with fewer side effects. Carbamazepine (10-1000 microM) inhibited in a concentration-dependent manner the field excitatory postsynaptic potential (fPSP) response, with an EC50 of 263 microM, and also attenuated the presynaptic volley with a similar EC50 value. Carbamazepine was more potent to inhibit the NMDA receptor component of the fPSP (fPSPNMDA), with an EC50 of 160 microM. BIA2-093 and BIA2-024 were nearly equipotent with carbamazepine to inhibit synaptic transmission, and displayed similar potency to inhibit the fPSP (EC50 of 145 microM and 205 microM) and fPSPNMDA responses (EC50 of 198 microM and 206 microM). As with carbamazepine, BIA2-093 and BIA2-024 also attenuated the presynaptic volley with EC50 values ranging from 142 to 322 microM. These results indicate that carbamazepine and its analogues mostly inhibit synaptic transmission through inhibition of conduction, although carbamazepine, but not BIA2-093 and BIA2-024, may also depress NMDA receptor-mediated responses.

Lowering of the potassium concentration induces epileptiform activity in guinea-pig hippocampal slices

Extra- and intracellular recording techniques were used to study the epileptiform activity generated by guinea-pig hippocampal slices perfused with low potassium containing artificial cerebrospinal fluid. Extracellular field potentials were recorded in CA1 and CA3 regions along with intracellular recordings in CA3 subfield. Reduction of the extracellular potassium concentration [K(+)](o) from 4 to 2 mM caused a transient neuronal hyperpolarisation which was followed by a repolarisation and subsequent depolarisation period. Paroxysmal depolarisation shifts occurred during the transient hyperpolarisation period while epileptic field potentials (EFP) appeared in the late repolarisation or early depolarisation phase. EFP elicited by reduction of [K(+)](o) were neither affected by blockade of N-methyl-D-aspartate (NMDA) glutamate-subreceptor or gamma aminobutyric acid receptor, nor by application of the organic calcium channel blocker nifedipine or the anticonvulsant drugs carbamazepine and valproic acid. Upon application of non-NMDA glutamate-subreceptor blocker the EFP were abolished in all trials, while application of the organic calcium channel blocker verapamil only suppressed the EFP in some cases. The results point to a novel mechanism of epileptogenesis and may provide an in vitro model for the development of new drugs against difficult-to-treat epilepsy.

Action of carbamazepine on epileptiform activity of the verartidine model in CA1 neurons

The veratridine epileptiform model was utilized to assess the antiepileptic effect of Carbamazepine (CBZ) in rat hippocampal CA1 pyramidal neurons using conventional intracellular recording techniques. In the veratridine model, where brain slices are treated with veratridine (0.3 microM), a single intracellular stimulus evokes epileptiform bursting. Additionally, spontaneous epileptiform activity commonly appears on prolonged exposure to veratridine in this model. In this model, therapeutic (7-15 microM) and high (50 microM) concentrations of CBZ inhibited the evoked and spontaneous epileptiform bursting in a concentration- and voltage-dependent manner. At all concentrations tested, CBZ produced inhibition of epileptiform activity without affecting the membrane resting potential or input resistance. However, at 50 microM, the drug increased the firing threshold of neurons. These results confirm the suitability of this model for testing sodium channel-dependent antiepileptic agents.

Differential involvement of L-type calcium channels in epileptogenesis of rat hippocampal slices during ontogenesis

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.

The anticonvulsant retigabine potently suppresses epileptiform discharges in the low Ca ++ and low Mg++ model in the hippocampal slice preparation

Retigabine (N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester, D-23129) is a broad acting anticonvulsant currently undergoing phase II clinical trials. An opening effect on leakage conductance K+ channels, potentiation of GABA induced currents and a weak blocking effect on Na+ and Ca++ channels were previously reported. The goal of this study was to investigate whether retigabine is capable of blocking epileptiform discharges in the low Ca++ and low Mg++ model in the hippocampal slice preparations and whether the anti-burst activity can be related to the K+ channel opening effect. In the low Ca++ model, synaptic transmission is blocked and discharges evolve from ephaptically-coupled neurons. Compounds which directly interfere with the threshold for action potential induction via alteration of ion channel function (i.e. Na+ channel blocker) may alter the discharges, while compounds interfering with synaptic transmission are not active. Retigabine suppressed the discharges in a concentration-dependent manner. A significant reduction in frequency without effect on amplitude was observed after application of 1 microM, and a full block of all discharges after application of 25 microM. The opener of the ATP sensitive K+ channels cromakalim was also active. Application of 300 microM cromakalim yielded to a lower frequency with no effects on the amplitude of discharges. Treatment with phenytoin and carbamazepine resulted in a marked reduction in amplitude accompanied by a rise in frequency; only at higher concentrations was a full block observed. The effect of retigabine therefore differs from sodium channel blockers and can be related to the K+ channel opening effect. In the low Mg++ model, excitatory neurotransmission is augmented by reducing the Mg++ block of NMDA channels. This results in development of interictal-like epileptiform activity in area CA1 in isolated hippocampal slices. Treatment with retigabine 10 microM resulted in a significant reduction of the discharges, and discharges were fully blocked after application of 25 microM. Qualitatively similar effects were observed with cromakalim and valproate, albeit at higher concentrations. The data indicate that retigabine exerts potent broad spectrum activity making it an interesting candidate for treatment of drug resistant patients.

Electrophysiologic analysis of the actions of valproate on pyramidal neurons in the rat hippocampal slice

PURPOSE

Studies in invertebrates and cultured mammalian neurons suggested that valproate (VPA) mediates its main antiepileptic effect by slowing the recovery from inactivation of voltage-dependent sodium channels. This predicts an effect on the refractory period of the action potential and, consequently, on the bursting behavior of neurons.

METHODS

We investigated this prediction using intracellular and extracellular recording techniques in hippocampal slices prepared from adult rats. The refractory period (RFP) and the ratio of the slopes (SR) of a pair of action potentials were used as indices of the recovery from inactivation of sodium channels. They were measured by injecting a series of paired depolarizing current pulses into CA1 pyramidal neurons.

RESULTS

No significant changes were observed in the RFP or SR measured during a 1-h recording period when VPA was bath-applied (1 mM), or when it was present in the recording electrode (10-50 mM). Lowering the temperature from 34.5 degrees C to 26.4 degrees C resulted in an increase of the RFP by 100% and a decrease of the SR by 40%. However, VPA did not affect any of the measured action potential parameters at this lower temperature. VPA was also without effect on the presynaptic fiber volley of axons recorded extracellularly in the stratum radiatum. The antidromic population spike was unaffected by VPA (2 mM), whereas phenytoin (50 microM) clearly affected this spike in the same slices. The absence of effect of VPA on each of the measured parameters could not be attributed to poor penetration through the slice because bath-applied VPA reduced the frequency of extracellularly recorded spontaneous interictal bursts, induced by bicuculline and elevated K+, within 10 min.

CONCLUSIONS

These findings suggest that at least in the hippocampal slice the drug's principal antiepileptic effect cannot be explained by its action on voltage-dependent sodium channels.

Inhibition of neuronal activity in rat hippocampal slices by Aconitum alkaloids

The structurally related Aconitum alkaloids aconitine, lappaconitine, and 6-benzoylheteratisine inhibited the orthodromic and antidromic population spike in hippocampal CA1 area in a frequency-dependent manner. Aconitine (1 microM) completely suppressed epileptiform activity induced by omission of Mg2+ as well as normal neuronal activity, whereas lappaconitine (10 microM) and 6-benzoylheteratisine (10 microM) diminished epileptiform activity by sparing normal neuronal activity.

Electrophysiological actions of phenytoin on N-methyl-D-aspartate receptor-mediated responses in rat hippocampus in vitro

1. The effects of the anticonvulsant, phenytoin, have been examined on N-methyl-D-aspartate (NMDA) receptor-mediated population spikes in the CA1 region of the rat hippocampus in vitro. 2. The 'conventional' (AMPA receptor-mediated) CA1 population spike, evoked by electrical stimulation of the Schaffer collateral/commissural pathway, was abolished by 5 min treatment with 5 x 10(-6) M 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), after which superfusion with a nominally Mg(2+)-free Krebs solution (containing 5 x 10(-6) M CNQX) led to the appearance of an epileptiform population spike which was fully developed by 30-40 min. 3. The epileptiform population spike was abolished by the non-competitive NMDA antagonist, dizocilpine (1 x 10(-6) M, 20-30 min) and inhibited by the competitive NMDA receptor antagonist, D-CPP (IC50 for reducing the amplitude of the first spike in the train = 8.3 x 10(-7) M), demonstrating that the response was mediated by activation of NMDA receptors and validating its use as an assay for antagonists acting at the NMDA receptor/channel complex. 4. Phenytoin (0.1, 0.3 and 1 x 10(-4) M applied cumulatively for 30 min at each concentration) failed to inhibit the NMDA receptor-mediated epileptiform population response (n = 7 slices). 5. Phenytoin (3 x 10-6 M to 1 x 10-4M) attenuated the effects of the sodium channel activator,veratridine (2 x 10-6 M), on the CAl population spike amplitude (recorded in normal Krebs solution),indicating that the previously observed lack of effect of phenytoin on the NMDA receptor-mediated response was not due to impaired access of phenytoin to the biophase.6. These data support the conclusion that antagonism of NMDA receptor-mediated events is not a pharmacological property of phenytoin and that such an action is therefore unlikely to contribute to the anticonvulsant activity of this drug.

Flunarizine shows increased antiepileptic efficacy with elevated K+ levels in low magnesium induced epileptic activity (neocortical slices, guinea pig)

Organic calcium channel blockers have been demonstrated to abolish epileptic activity in various experimental models. Furthermore, it was shown that the antiepileptic efficacy of the organic calcium channel blocker verapamil was significantly augmented when the KCl concentration background was elevated to levels normally occurring during epileptic seizures. The aim of the present investigation was to test whether flunarizine, which in contrast to verapamil is able to penetrate the blood brain barrier, suppresses epileptic activity in neocortical slice preparations, and whether this effect would be enhanced by raising the KCl background concentration. Epileptic activity was induced in neocortical slices of guinea pigs by omission of Mg2+ from the superfusate. As a measure of epileptic activity, field potentials were recorded from layers III and V. They appeared within approx 30 min after omission of Mg2+ from the bath solutions. The frequency of occurrence in normal and elevated KCl concentration was 47 +/- 10/5 min and 46 +/- 9/5 min, respectively. Flunarizine, in concentrations of 3.2 and 18 mumol/l, abolished epileptiform activity dose dependently. A 90% depression occurred within 194 +/- 27 and 376 +/- 27 min for flunarizine concentrations of 18 and 3.2 mumol/l, respectively. Elevating the KCl back-ground concentration to 8 mmol/l significantly enhanced the antiepileptic efficacy of flunarizine. Under these conditions, a 90% depression occurred within 67 +/- 14 and 165 +/- 37 min for flunarizine. Under these conditions, a 90% depression occurred within 67 +/- 14 and 165 +/- 37 min for flunarizine concentrations of 18 and 3.2 mumol/l, respectively. The experiments demonstrate that flunarizine suppresses epileptic activity in neocortical preparations, with enhanced action in elevated K+ levels.

Effects of carbamazepine and baclofen on 4-aminopyridine-induced epileptic activity in rat hippocampal slices

1. Rat transverse hippocampal slices exposed to 100 microM 4-aminopyridine (4-AP) generate spontaneous epileptic discharges ranging in duration from short 50 ms 'interictal' bursts to long 0.5-2 s 'polyspike' activity. 2. Here we compared the effects of the commonly used anticonvulsant, carbamazepine (40 microM) and the antispastic drug, baclofen (2 microM) on the various types of burst. 3. Carbamazepine completely abolished long bursts whilst leaving shorter bursts intact. This is consistent with its known anticonvulsant properties. 4. Baclofen greatly reduced the frequency of short bursts but did not block the long bursts. Rather, they became significantly more prolonged, indicating that baclofen does not have an anticonvulsant action, and may be proconvulsant. 5. These results conflict with conclusions based on studies using models that exhibited only interictal bursts, and emphasize the need to use experimental epilepsies which generate several types of epileptic discharge to evaluate the effects of putative anticonvulsant drugs. 6. The present findings suggest that GABAB receptors play a role in the transition of benign interictal bursts to longer polyspike activity which could develop into seizures in the whole animal.

Calcium antagonistic effects of carbamazepine as a mechanism of action in neuropsychiatric disorders: studies in calcium dependent model epilepsies

Carbamazepine (CBZ) is used in neurology for the treatment of epilepsies and trigeminal neuralgia and in psychiatry for the prophylactic treatment of affective and schizoaffective psychoses. Since a common mechanism of epilepsies and affective psychoses might be increased intracellular calcium ion levels, CBZ action was analyzed in penicillin, caffeine and low Mg2+ induced model epilepsies which have been shown to be suppressed specifically by organic calcium antagonists. In CA3 and CA1 areas of hippocampal slice preparations of guinea pigs CBZ reduced paroxysmal depolarizations and extracellular field potentials (EFP) in a typical time and concentration dependent manner as it is known from calcium antagonists. Furthermore, subthreshold concentrations of the organic calcium antagonist verapamil intensified the action of CBZ. NMDA induced increases of the discharge rate of EFP were, however, unaffected by CBZ.

Effects of antiepileptic drugs on 4-aminopyridine-induced epileptiform activity in young and adult rat hippocampus

Extracellular field potential recordings were used to study the effects of the antiepileptic drugs (AEDs) carbamazepine (CBZ), phenytoin (PHT), phenobarbital (PhB) and valproic acid (VPA) on the epileptiform activity evoked by 4-aminopyridine (4-AP, 50 microM) in the CA3 subfield of rat hippocampal slices obtained from young (8-23-day-old) and adult (> 60-day-old) male rats. Ictal (duration: 3-20 s; rate of occurrence: 3-12 x 10(-3) s-1) and interictal (duration: 0.2-0.8 s; rate of occurrence: 0.2-0.8 s-1) discharges were recorded in young slices, while only interictal activity (duration: 70-90 ms; rate of occurrence: 0.5-0.9 s-1) discharges were observed in adult slices. In addition, in both young and adult slices 4-AP disclosed a synchronous long-lasting potential (duration and rate of occurrence: 0.6-3 s, 7-70 x 10(-3) s-1 and 260-660 ms, 8-60 x 10(-3) s-1, respectively) that was caused by the activation of the gamma-aminobutyric acid type A (GABAA) receptor. In young slices, ictal discharges were blocked by CBZ (0.05 mM), PHT (0.1 mM), PhB (0.5 mM) and VPA (0.5 mM). With the exception of PhB, higher concentrations were necessary in these experiments for blocking the interictal activity (i.e., CBZ: 0.1 mM; PHT: > 0.2 mM; VPA: 2 mM). At these concentrations, none of the AEDs blocked the interictal activity in the adult hippocampus, but only reduced the rate of occurrence. PhB enhanced the rate of occurrence of the synchronous GABA-mediated long-lasting potentials both in young (increase: 190%) and in adult (increase: 145%) slices, while VPA increased their occurrence by 54% only in young slices. CBZ decreased the rate of occurrence of this long-lasting potential only in adult hippocampus. Our data indicate that the effects of the AEDs on 4-AP-induced epileptiform discharges are both pattern- and age-dependent. The rank order of potencies of the four AEDs was: (a) in young: CBZ > PHT > PhB > VPA; (b) in adult: CBZ > PhB > PHT > VPA.

Low magnesium induced epileptiform discharges in neocortical slices (guinea pig): Increased antiepileptic efficacy of organic calcium antagonist verapamil with elevation of extracellular K+ concentration

1. The antiepileptic effect of the organic calcium antagonist verapamil on low Mg2+ induced epileptiform discharges in the neocortex was tested. 2. The experiments were carried out on slices of the frontal neocortex (guinea pigs). Verapamil was tested at normal (4 mmol/l) and elevated (8 mmol/l) KCl levels. 3. Verapamil (40, 60 mumol/l) suppressed epileptiform activity in any case. 4. With elevated K+ concentration the suppressive effect of verapamil was significantly accelerated. 5. Epileptic activity reappeared when verapamil was omitted from the Mg(2+)-free superfusate.

Anticonvulsants do not suppress long-term potentiation (LTP) in the rat hippocampus

Long-term potentiation (LTP) of population spikes in the CA1 area of rat hippocampus was induced by tetanic stimulation of stratum radiatum in slices kept submerged in a perfusion chamber. Addition of the two antiepileptic drugs phenytoin or the diazepine midazolam to the medium did not significantly alter this phenomenon within 22 min after the tetanus. The early enhancement (post-tetanic potentiation, PTP) was reduced only by phenytoin. Therefore an interaction of these drugs with N-methyl-D-aspartate (NMDA) receptors and LTP induction is unlikely.