Contribution of L-type calcium channels to epileptiform activity in hippocampal and neocortical slices of guinea-pigs

L-type voltage-gated calcium channel regulation of in vitro human cortical neuronal networks

The combination of in vitro multi-electrode arrays (MEAs) and the neuronal differentiation of stem cells offers the capability to study human neuronal networks from patient or engineered human cell lines. Here, we use MEA-based assays to probe synaptic function and network interactions of hiPSC-derived neurons. Neuronal network behaviour first emerges at approximately 30 days of culture and is driven by glutamate neurotransmission. Over a further 30 days, inhibitory GABAergic signalling shapes network behaviour into a synchronous regular pattern of burst firing activity and low activity periods. Gene mutations in L-type voltage gated calcium channel subunit genes are strongly implicated as genetic risk factors for the development of schizophrenia and bipolar disorder. We find that, although basal neuronal firing rate is unaffected, there is a dose-dependent effect of L-type voltage gated calcium channel inhibitors on synchronous firing patterns of our hiPSC-derived neural networks. This demonstrates that MEA assays have sufficient sensitivity to detect changes in patterns of neuronal interaction that may arise from hypo-function of psychiatric risk genes. Our study highlights the utility of in vitro MEA based platforms for the study of hiPSC neural network activity and their potential use in novel compound screening.

The Glycolytic Metabolite, Fructose-1,6-bisphosphate, Blocks Epileptiform Bursts by Attenuating Voltage-Activated Calcium Currents in Hippocampal Slices

Manipulation of metabolic pathways (e.g., ketogenic diet (KD), glycolytic inhibition) alters neural excitability and represents a novel strategy for treatment of drug-refractory seizures. We have previously shown that inhibition of glycolysis suppresses epileptiform activity in hippocampal slices. In the present study, we aimed to examine the role of a “branching” metabolic pathway stemming off glycolysis (i.e., the pentose-phosphate pathway, PPP) in regulating seizure activity, by using a potent PPP stimulator and glycolytic intermediate, fructose-1,6-bisphosphate (F1,6BP). Employing electrophysiological approaches, we investigated the action of F1,6BP on epileptiform population bursts, intrinsic neuronal firing, glutamatergic and GABAergic synaptic transmission and voltage-activated calcium currents (I_Ca) in the CA3 area of hippocampal slices. Bath application of F1,6BP (2.5–5 mM) blocked epileptiform population bursts induced in Mg²⁺-free medium containing 4-aminopyridine, in ~2/3 of the slices. The blockade occurred relatively rapidly (~4 min), suggesting an extracellular mechanism. However, F1,6BP did not block spontaneous intrinsic firing of the CA3 neurons (when synaptic transmission was eliminated with DNQX, AP-5 and SR95531), nor did it significantly reduce AMPA or NMDA receptor-mediated excitatory postsynaptic currents (EPSC_AMPA and EPSC_NMDA). In contrast, F1,6BP caused moderate reduction (~50%) in GABA_A receptor-mediated current, suggesting it affects excitatory and inhibitory synapses differently. Finally and unexpectedly, F1,6BP consistently attenuated I_Ca by ~40% without altering channel activation or inactivation kinetics, which may explain its anticonvulsant action, at least in this in vitro seizure model. Consistent with these results, epileptiform population bursts in CA3 were readily blocked by the nonspecific Ca²⁺ channel blocker, CdCl₂ (20 μM), suggesting that these bursts are calcium dependent. Altogether, these data demonstrate that the glycolytic metabolite, F1,6BP, blocks epileptiform activity via a previously unrecognized extracellular effect on I_Ca, which provides new insight into the metabolic control of neural excitability.

Cav1.2 and Cav1.3 L-type calcium channels operate in a similar voltage range but show different coupling to Ca(2+)-dependent conductances in hippocampal neurons

In the central nervous system, L-type voltage-gated calcium channels (LTCCs) come in two isoforms, namely Cav1.2 and Cav1.3 channels. It has been shown previously that these channels differ in biophysical properties, in subcellular localization, and in the coupling to the gene transcription machinery. In previous work on rat hippocampal neurons we have identified an excitatory cation conductance and an inhibitory potassium conductance as important LTCC coupling partners. Notably, a stimulus-dependent interplay of LTCC-mediated Ca(2+) influx and activation of these Ca(2+)-dependent conductances was found to give rise to characteristic voltage responses. However, the contribution of Cav1.2 and Cav1.3 to these voltage responses remained unknown. Hence, the relative contribution of the LTCC isoforms therein was the focus of the current study on hippocampal neurons derived from genetically modified mice, which either lack a LTCC isoform (Cav1.3 knockout mice) or express a dihydropyridine-insensitive LTCC isoform (Cav1.2DHP(-)-knockin mice). We identified common and alternate ion channel couplings of Cav1.2 and Cav1.3 channels. Whereas hyperpolarizing Ca(2+)-dependent conductances were coupled to both Cav1.2 and Cav1.3 channels, an afterdepolarizing potential was only induced by the activity of Cav1.2 channels. Unexpectedly, the activity of Cav1.2 channels was found at relatively hyperpolarized membrane voltages. Our data add important information about the differences between Cav1.2 and Cav1.3 channels that furthers our understanding of the physiological and pathophysiological neuronal roles of these calcium channels. Moreover, our findings suggest that Cav1.3 knockout mice together with Cav1.2DHP(-)-knockin mice provide valuable models for future investigation of hippocampal LTCC-dependent afterdepolarizations.

Can we improve the outcome of epileptic encephalopathies?

The Canadian pediatric epilepsy network defines an epileptic encephalopathy (EE) as: “a brain dysfunction manifested by a progressive decline in cognitive and behavioral functions that is associated with frequent seizures and/or interictal epileptiform discharges in a child with medically refractory epilepsy.” Outcomes of these EEs are usually poor owing to the presence of continuous epileptic discharges and/or very frequent seizures, leading to excitotoxic cell death. In addition, early diagnosis is often delayed because of the subtle initial manifestations, and an inadequate choice of treatment not only worsens the seizures acutely but possibly alters long-term prognosis. Steroids, a ketogenic diet and vagal nerve stimulation are possible alternatives to improve the outcome of certain EEs, but I believe developing new therapeutic agents that target the mechanisms implicated in animal models should be favored. Possible targets include γ-aminobutyric acid B receptors, calcium channels or intracellular calcium stores.

Paradoxical nifedipine facilitation of 45Ca uptake into rat hippocampal synaptosomes

Nifedipine has a high incidence of neurologic adverse reactions as compared with other dihydropyridine Cav1 (L-type) channel blockers used for treating cardiovascular diseases. The mechanism mediating neuronal excitation by nifedipine is still in debate. Nifedipine caused a dual role on veratridine-induced 45Ca uptake by rat hippocampal synaptosomes. In the nanomolar range (0.001-0.3 microM), nifedipine decreased 45Ca uptake in a cadmium-sensitive manner. In contrast with nitrendipine (0.001-10 microM), nifedipine consistently facilitated 45Ca accumulation when used in low micromolar concentrations (0.3-10 microM). The cadmium-insensitive nifedipine facilitation became less evident upon increasing veratridine concentration from 5 to 20 microM and was not detected when the synaptosomes where depolarised with 30 mM KCl. Na+ substitution by N-methyl-D-glucamine (132 mM) or blockade of Na+ currents with tetrodotoxin (1 microM) both prevented nifedipine excitation. The Na+/Ca2+-exchanger inhibitor, KB-R7943 (3-50 microM), did not reproduce nifedipine actions. Data suggest that tetrodotoxin-sensitive Na+ channels may operate paradoxical nifedipine facilitation of 45Ca uptake by rat hippocampal synaptosomes.

Increased seizure susceptibility of the hippocampus compared with the neocortex of the immature mouse brain in vitro

PURPOSE

The temporal lobe seems particularly susceptible to seizure activity. Mesial temporal lobe structures, including the hippocampus, have the lowest seizure thresholds in the brain. Conversely, thresholds in the frontal neocortex are significantly higher. The development of intact, isolated preparations of hippocampus and neocortex in vitro allows for study into mechanisms governing seizure threshold.

METHODS

Epileptiform discharges in isolated mouse neocortical blocks were compared with the contralateral intact hippocampus, isolated from the same brain, by using the low-Mg2+, 4 aminopyridine (4-AP), and low-Ca2+ in vitro seizure models. The pharmacology of low Mg(2+)-induced ictal-like events (ILEs) generated in the hippocampus and neocortex was then compared by using glutamatergic antagonists DL-2-amino-5-phosphonovaleric acid (APV) and 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), and the Ca2+ channel antagonist, nifedipine.

RESULTS

Neocortical blocks generated both recurrent, spontaneous ILEs and interictal-like events under low-Mg2+ artificial CSF (aCSF) perfusion, distinct from those generated in the hippocampus. ILEs from the hippocampus displayed lower thresholds and longer durations as compared with isolated neocortical blocks. Similar results were obtained during 4-AP perfusion. Perfusion with low-Ca2+ ACSF did not produce stereotypical ILEs in the neocortical block, producing instead recurrent, slow depolarizations. Both ILEs and recurrent, slow depolarizations were produced in the hippocampus. Application of APV and nifedipine exacerbated low Mg(2+)-induced ILEs in the hippocampus but not the neocortex, indicating a distinct pharmacology for partial seizures of different brain regions.

CONCLUSIONS

The developing mouse hippocampus demonstrates increased ictogenesis compared with the developing neocortex in vitro, consistent with clinical observations and in vivo experimental models.

Model of frequent, recurrent, and spontaneous seizures in the intact mouse hippocampus

This study presents a model of chronic, recurrent, spontaneous seizures in the intact isolated hippocampal preparation from mice aged P8-P25. Field activity from the CA1 pyramidal cell layer was recorded and recurrent, spontaneous seizure-like events (SLEs) were observed in the presence of low Mg2+ (0.25 mM) artificial cerebrospinal fluid (ACSF). Hippocampi also showed interictal epileptiform discharges (IEDs) of 0.9-4.2 Hz occurring between seizures. No age-specific differences were found in SLE occurrence (2 SLEs per 10 min, on average), duration, and corresponding frequencies. After long exposure to low Mg2+ ACSF (>3 h), SLEs were completely reversible within minutes with the application of normal (2 mM Mg2+) ACSF. The AMPA antagonist, CNQX, blocked all epileptiform activity, whereas the NMDA antagonist, APV, did not. The gamma-aminobutyric acid (GABA)A antagonist, bicuculline, attenuated and fragmented SLEs, implicating interneurons in SLE generation. The L-type Ca2+ blocker, nifedipine, enhanced epileptiform activity. Analysis of dual site recordings along the septotemporal hippocampus demonstrated that epileptiform activity began first in the temporal pole of the hippocampus, as illustrated by disconnection experiments. Once an SLE had been established, however, the septal hippocampus was sometimes seen to lead the epileptiform activity. The whole hippocampus with intact local circuitry, treated with low Mg2+, provides a realistic model of recurrent spontaneous seizures, which may be used, in normal and genetically modified mice, to study the dynamics of seizures and seizure evolution, as well as the mechanisms of action of anti-epileptic drugs and other therapeutic modalities.

Reduction of human neocortical and guinea pig CA1-neuron A-type currents by organic calcium channel blockers

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.

High-frequency synaptic input contributes to seizure initiation in the low-[Mg2+] model of epilepsy

High-frequency field potential activity between 50 and 400 Hz occurs throughout seizure-like events recorded from the CA3 region of juvenile rat hippocampal slices under low-[Mg(2+)] condition. Another (400-800 Hz) component occurred mainly during preictal paroxysmal spiking and the onsets of seizure-like events (97%) and less frequently during tonic and clonic phases (38% and 70%, respectively). Short epochs of oscillations in this range were associated with fast negative field potential deflections at the start of field potential transients. Voltage-clamp recordings from putative CA3 pyramidal cells showed the occurrence of synaptic inputs in the same frequency range at the onset of seizure-like events and the beginning of preictal or clonic paroxysmal spikes, while the frequency of action potentials never reached that range. The amplitude of fast negative field potential deflection, the rise time of membrane potential or voltage-clamp current changes and the mean phase coherence were consistent with an increase of synchronization towards the onset of a seizure-like event. Their parallel changes indicate the involvement of both synaptic and nonsynaptic mechanisms in the synchronization of neuronal activity and the development of seizure-like events in the low-[Mg(2+)] model of epilepsy.

Topiramate hyperpolarizes and modulates the slow poststimulus AHP of rat olfactory cortical neurones in vitro

1. The effects of the novel antiepileptic drug topiramate (TPM) were investigated in rat olfactory cortex neurones in vitro using a current/voltage clamp technique. 2. In 80% of recorded cells, bath application of TPM (20 microm) reversibly hyperpolarized and inhibited neuronal repetitive firing by inducing a slow outward membrane current, accompanied by a conductance increase. The response was reproducible after washout, and was most likely carried largely by K(+) ions, although other ionic conductances may also have contributed. 3. In 90% of cells, TPM (20 microm) also enhanced and prolonged the slow (Ca(2+)-dependent) poststimulus afterhyperpolarization (sAHP) and underlying slow outward tail current (sI(AHP)). This effect was due to a selective enhancement/prolongation of an underlying L-type Ca(2+) current that was blocked by nifedipine (20 microm); the TPM response was unlikely to involve an interaction at PKA-dependent phosphorylation sites. 4. The carbonic anhydrase (CA) inhibitor acetazolamide (ACTZ, 20 microm) and the poorly membrane permeant inhibitor benzolamide (BZ, 50 microm) both mimicked the membrane effects of TPM, in generating a slow hyperpolarization (slow outward current under voltage clamp) and sAHP enhancement. ACTZ and BZ occluded the effects of TPM in generating the outward current response, but were additive in producing the sAHP modulatory effect, suggesting different underlying response mechanisms. 5. In bicarbonate/CO(2)-free, HEPES-buffered medium, all the membrane effects of TPM and ACTZ were reproducible, therefore not dependent on CA inhibition. 6. We propose that both novel effects of TPM and ACTZ exerted on cortical neurones may contribute towards their clinical effectiveness as anticonvulsants.

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.

Cellular biology of epileptogenesis

The ionic currents that underlie the mechanisms of epileptogenesis have been systematically characterised in different experimental preparations. The recent elucidation of the molecular structures of most membrane channels and receptors has enabled structure-function analyses in both physiological and pathophysiological conditions. The neurophysiological and biomolecular features of epileptogenic mechanisms that putatively account for human epilepsies are summarised in this review. Particular emphasis is given to epilepsies that are associated with genetically determined alterations of ligand-gated and voltage-gated ion channels. Changes in ionic currents that flow through sodium, potassium, and calcium channels can lead to different types of epilepsies. Inherited or acquired changes that alter the function of receptors for acetylcholine, glutamate, and gamma-aminobutryic acid are also involved. better understanding of the role of these epileptogenic mechanisms will promote new advances in the development of selective and targeted antiepileptic drugs.

The role of Ca2+/calmodulin-dependent protein kinase II in mechanisms underlying neuronal hyperexcitability induced by repeated, brief exposure to hypoxia or high K+ in rat hippocampal slices

Analysis of extracellular recordings of evoked excitatory postsynaptic potentials and population spikes from rat hippocampal slices has previously revealed that repeated, brief exposures to high extracellular K(+) or to episodes of hypoxia induce a sustained (more than 3 h) hyperexcitability of CA1 pyramidal neurons accompanied with epileptiform activity which was dependent on activation of L-type Ca(2+) channels and N-methyl-D-aspartate receptors. Using in vitro phosphorylation assay we have found the significant increase of Ca(2+)-independent activity of Ca(2+)/calmodulin-dependent protein kinase II in CA1 region of hippocampal slices 60 min after the high extracellular K(+) and 60-80 min after the hypoxic episodes. These data suggest possible involvement of Ca(2+)/calmodulin-dependent protein kinase II in Ca(2+)-dependent mechanisms of the maintenance phase of the observed epileptiform activity.

Inter-ictal- and ictal-like epileptic discharges in the dendritic tree of neocortical pyramidal neurons

Dendritic mechanisms have been implied to play a key role in the formation of epileptic discharges. However, presently only a handful of direct dendritic recordings have been reported during epileptic discharges. In this study, I performed simultaneous voltage recordings from the soma and apical dendrite of the same neuron combined with calcium-imaging measurements to investigate inter-ictal- and ictal-like epileptic discharges in dendrites of layer 5 pyramidal neurons. Neocortical brain slices treated with bicuculline (BCC) produced both isolated "inter-ictal" paroxysymal depolarization shift (PDS) responses and electrographic seizures. Concomitant voltage recordings from the soma and apical dendrite revealed that PDS responses developed in both the apical dendrites and soma. However, the two responses differed from one another. In apical dendrites, the PDS was significantly higher in amplitude and shorter in duration compared with the somatic PDS. The PDS response in dendrites had a peak amplitude of 68.9 +/- 2.2 (SD) mV, peak voltage value of 9.3 +/- 2.7 mV, and half-width of 203.8 +/- 38.4 ms. In contrast, the somatic PDS had a peak amplitude of 48.7 +/- 2.7 mV, peak voltage value of -11.9 +/- 3.1 mV, and half-width of 247.8 +/- 57.3 ms (P < 0.01, n = 18). In addition the apical dendritic PDS always preceded the somatic counterpart in all 18 neurons examined. Concomitant calcium-imaging measurements showed the PDS evoked large calcium influx into the entire dendritic tree including the apical tuft, basal, and oblique dendrites. The PDS evoked [Ca(2+)](i) were not uniform along the dendritic tree, being highest in the oblique dendrites (71.3 +/- 14.5 microM) and lowest at the distal tuft branches (9.3 +/- 0.7 microM). The PDS responses persisted after blockade of voltage-gated sodium channels by intracellular QX-314 but became narrower (by 69.6 +/- 9.7%) following intracellular administration of the voltage-gated calcium channel blocker D600. Electrographic seizures recorded in the soma and apical dendrites were composed of recurrent bursts. The initial bursts represented PDS responses. During the seizure the amplitude of bursts gradually attenuated and reached an average value of 26 +/- 13% of the initial ictal PDS burst. Double recordings during electrographic seizures revealed the initial one to four ictal bursts appeared first at the apical dendrite while later ictal bursts were always observed first at the soma. In conclusion, the results of this study show "inter-ictal" PDS responses originated in the apical dendritic tree, were partially mediated by voltage-gated calcium channels and spread throughout the dendritic tree including the fine tuft, basal, and oblique dendrites. During electrographic seizures the origin of epileptic bursts shifted from the apical dendritic tree to the soma-basal region.

Ca(2+) entry through L-type Ca(2+) channels helps terminate epileptiform activity by activation of a Ca(2+) dependent afterhyperpolarisation in hippocampal CA3

In CA3 neurons of disinhibited hippocampal slice cultures the slow afterhyperpolarisation, following spontaneous epileptiform burst events, was confirmed to be Ca(2+) dependent and mediated by K(+) ions. Apamin, a selective blocker of the SK channels responsible for part of the slow afterhyperpolarisation reduced, but did not abolish, the amplitude of the post-burst afterhyperpolarisation. The result was an increased excitability of individual CA3 cells and the whole CA3 network, as measured by burst duration and burst frequency. Increases in excitability could also be achieved by strongly buffering intracellular Ca(2+) or by minimising Ca(2+) influx into the cell, specifically through L-type (but not N-type) voltage operated Ca(2+) channels. Notably the L-type Ca(2+) channel antagonist, nifedipine, was more effective than apamin at reducing the post-burst afterhyperpolarisation. Nifedipine also caused a greater increase in network excitability as determined from measurements of burst duration and frequency from whole cell and extracellular recordings. N-methyl D-aspartate receptor activation contributed to the depolarisations associated with the epileptiform activity but Ca(2+) entry via this route did not contribute to the activation of the post-burst afterhyperpolarisation. We suggest that Ca(2+) entry through L-type channels during an epileptiform event is selectively coupled to both apamin-sensitive and -insensitive Ca(2+) activated K(+) channels. Our findings have implications for how the route of Ca(2+) entry and subsequent Ca(2+) dynamics can influence network excitability during epileptiform discharges.

Effects of nifedipine on rhythmic synchronous activity of human neocortical slices

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.

Vigabatrin reduces epileptiform activity in brain slices from pharmacoresistant epilepsy patients

Human neocortical temporal lobe tissue resected for treatment of pharmacoresistant epilepsy was investigated. In slices prepared from this tissue, epileptiform field potentials (EFP) were induced by omission of magnesium from the artificial cerebrospinal fluid (ACSF). The effects of the gamma-aminobutyric acid transaminase inhibitor vigabatrin on EFP were tested. Vigabatrin exerted a dose-dependent reduction of the repetition rate of EFP: after 3 h of administration of vigabatrin in concentrations of 100 and 200 micromol/l, the repetition rate of EFP was reduced to 35% and 18% of the initial values, respectively. This effect was not reversible. In control experiments with neocortical slices from rats, vigabatrin reduced EFP in a comparable range. The results demonstrate a strong antiepileptic effect of vigabatrin on EFP in tissues from pharmacoresistant epilepsy patients.

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.