Effects of antiepileptic drugs on pentylenetetrazole-induced epileptiform activity in the in vitro hippocampus

Factors influencing the acute pentylenetetrazole-induced seizure paradigm and a literature review

OBJECTIVE

To confirm the critical factors affecting seizure susceptibility in acute pentylenetetrazole (PTZ) mouse epilepsy models and evaluate the prior literature for these factors.

METHODS

Serial cohorts of wild-type mice administered intraperitoneal (IP)-PTZ were aggregated and analyzed by multivariate logistic regression for the effect of sex, age, background strain, dose, and physiologic stress (i.e., EEG implantation and/or single-housing) on seizure response. We assessed the reporting of these factors in a comprehensive literature review over the last 10 years (2010-2020).

RESULTS

We conducted aggregated analysis of pooled data of 307 mice (220 C57BL/6J mice and 87 mixed background mice; 202 males, 105 females) with median age of 10 weeks (range: 6-49 weeks) with acute PTZ injection (dose range 40-65 mg/kg). Significance in multivariate analysis was found between seizures and increased PTZ dose (odds ratio (OR) 1.149, 95% confidence interval (CI) 1.102-1.205), older age (OR 1.1, 95% CI 1.041-1.170), physiologic stress (OR 17.36, 95% CI 7.349-44.48), and mixed background strain (OR 0.4725, 95% CI 0.2315-0.9345). Literature review identified 97 papers using acute PTZ-seizure models. Age, housing, sex, and background were omitted by 61% (59/97), 51% (49/97), 18% (17/97), and 8% (8/97) papers, respectively. Only 17% of publications specified all four factors (16/97).

INTERPRETATION

Our analysis and literature review demonstrate a critical gap in standardization of acute PTZ-induced seizure paradigm in mice. We recommend that future studies specify and control for age, background strain, sex, and housing conditions of experimental animals.

How do we use in vitro models to understand epileptiform and ictal activity? A report of the TASK1-WG4 group of the ILAE/AES Joint Translational Task Force

In vitro brain tissue preparations allow the convenient and affordable study of brain networks and have allowed us to garner molecular, cellular, and electrophysiologic insights into brain function with a detail not achievable in vivo. Preparations from both rodent and human postsurgical tissue have been utilized to generate in vitro electrical activity similar to electrographic activity seen in patients with epilepsy. A great deal of knowledge about how brain networks generate various forms of epileptiform activity has been gained, but due to the multiple in vitro models and manipulations used, there is a need for a standardization across studies. Here, we describe epileptiform patterns generated using in vitro brain preparations, focusing on issues and best practices pertaining to recording, reporting, and interpretation of the electrophysiologic patterns observed. We also discuss criteria for defining in vitro seizure‐like patterns (i.e., ictal) and interictal discharges. Unifying terminologies and definitions are proposed. We suggest a set of best practices for reporting in vitro studies to favor both efficient across‐lab comparisons and translation to in vivo models and human studies.

Therapeutic potential of an anti-high mobility group box-1 monoclonal antibody in epilepsy

Brain inflammation is a major factor in epilepsy, and the high mobility group box-1 (HMGB1) protein is known to contribute significantly to the generation of seizures. Here, we investigated the therapeutic potential of an anti-HMGB1 monoclonal antibody (mAb) in epilepsy. anti-HMGB1 mAb attenuated both acute seizure models (maximal electroshock seizure, pentylenetetrazole-induced and kindling-induced), and chronic epilepsy model (kainic acid-induced) in a dose-dependent manner. Meanwhile, the anti-HMGB1 mAb also attenuated seizure activities of human brain slices obtained from surgical resection from drug-resistant epilepsy patients. The mAb showed an anti-seizure effect with a long-term manner and appeared to be minimal side effects at even very high dose (no disrupted physical EEG rhythm and no impaired basic physical functions, such as body growth rate and thermoregulation). This anti-seizure effect of mAb results from its inhibition of translocated HMGB1 from nuclei following seizures, and the anti-seizure effect was absent in toll-like receptor 4 knockout (TLR4^-/-) mice. Interestingly, the anti-HMGB1 mAb also showed a disease-modifying anti-epileptogenetic effect on epileptogenesis after status epileptics, which is indicated by reducing seizure frequency and improving the impaired cognitive function. These results indicate that the anti-HMGB1 mAb should be viewed as a very promising approach for the development of novel therapies to treat refractory epilepsy.

Models of drug-induced epileptiform synchronization in vitro

Models of epileptiform activity in vitro have many advantages for recording and experimental manipulation. Neural tissues can be maintained in vitro for hours, and in neuronal or organotypic slice cultures for several weeks. A variety of drugs and other agents increase activity in these in vitro conditions, in many cases resulting in epileptiform activity, thus providing a direct model of symptomatic seizures. We review these preparations and the experimental manipulations used to induce epileptiform activity. The most common of drugs used are GABAA receptor antagonists and potassium channel blockers (notably 4-aminopyridine). Muscarinic agents also can induce epileptiform synchronization in vitro, and include potassium channel inhibition amongst their cellular actions. Manipulations of extracellular ions are reviewed in another paper in this special issue, as are ex vivo slices prepared from chronically epileptic animals and from people with epilepsy. More complex slices including extensive networks and/or several connected brain structures can provide insights into the dynamics of long range connections during epileptic activity. Visualization of slices also provides opportunities for identification of living neurons and for optical recording/stimulation and manipulation. Overall, the analysis of the epileptiform activity induced in brain tissue in vitro has played a major role in advancing our understanding of the cellular and network mechanisms of epileptiform synchronization, and it is expected to continue to do so in future.

Heterogeneous effects of antiepileptic drugs in an in vitro epilepsy model--a functional multineuron calcium imaging study

Epilepsy is a chronic brain disease characterised by recurrent seizures. Many studies of this disease have focused on local neuronal activity, such as local field potentials in the brain. In addition, several recent studies have elucidated the collective behavior of individual neurons in a neuronal network that emits epileptic activity. However, little is known about the effects of antiepileptic drugs on neuronal networks during seizure-like events (SLEs) at single-cell resolution. Using functional multineuron Ca(2+) imaging (fMCI), we monitored the activities of multiple neurons in the rat hippocampal CA1 region on treatment with the proconvulsant bicuculline under Mg(2+) -free conditions. Bicuculline induced recurrent synchronous Ca(2+) influx, and the events were correlated with SLEs. Other proconvulsants, such as 4-aminopyridine, pentetrazol, and pilocarpine, also induced synchronous Ca(2+) influx. We found that the antiepileptic drugs phenytoin, flupirtine, and ethosuximide, which have different mechanisms of action, exerted heterogeneous effects on bicuculline-induced synchronous Ca(2+) influx. Phenytoin and flupirtine significantly decreased the peak, the amount of Ca(2+) influx and the duration of synchronous events in parallel with the duration of SLEs, whereas they did not abolish the synchronous events themselves. Ethosuximide increased the duration of synchronous Ca(2+) influx and SLEs. Furthermore, the magnitude of the inhibitory effect of phenytoin on the peak synchronous Ca(2+) influx level differed according to the peak amplitude of the synchronous event in each individual cell. Evaluation of the collective behavior of individual neurons by fMCI seems to be a powerful tool for elucidating the profiles of antiepileptic drugs.

Neurohypophyseal hormones protect against pentylenetetrazole-induced seizures in zebrafish: role of oxytocin-like and V1a-like receptor

Oxytocin (OT) and arginine-vasopressin (AVP) are involved in the physiological response to different stressors like the occurrence of seizures which is regarded as a severe stress factor. Zebrafish (Danio rerio) is recently featured as a model of epilepsy but the role of neurohypophyseal hormones on this teleost is still unknown. We attempted to determine whether non-mammalian homologues like isotocin (IT) and vasotocin (AVT) affected pentylenetetrazole (PTZ)-induced seizures in adult zebrafish in comparison with OT/AVP. The mechanism was studied using the most selective OT and AVP receptor antagonists. Zebrafish were injected i.m. with increasing doses (0.1-40 ng/kg) of the neuropeptides 10 min before PTZ exposure. DesGly-NH2-d(CH2)5-[D-Tyr2,Thr4]OVT (desglyDTyrOVT) for OT receptor and SR49059 for V1a subtype receptor, were injected together with each agonist 20 min before PTZ exposure. All the peptides significantly decreased the number of seizures, increased the mean latency time to the first seizure and decreased lethality. This protective effect led to a dose-response curve following a U-shaped form. IT was approximately 40 times more active than OT while AVT was 20 times more potent than AVP in reducing the number of seizures. DesglyDTyrOVT was more effective in antagonizing OT/IT, while SR49059 mainly blocked AVP/AVT-induced protection against PTZ-induced seizures. The present findings provide direct evidence of an important involvement of IT/OT and AVP/AVT as anticonvulsant agents against PTZ-induced seizures with a receptor-mediated mechanism in zebrafish. These data reinforce zebrafish as an emerging experimental model to study and identify new antiepileptic drugs.

Advances in targeting voltage-gated sodium channels with small molecules

Blockade of voltage-gated sodium channels (VGSCs) has been used successfully in the clinic to enable control of pathological firing patterns that occur in conditions as diverse as chronic pain, epilepsy, and arrhythmias. Herein we review the state of the art in marketed sodium channel inhibitors, including a brief compendium of their binding sites and of the cellular and molecular biology of sodium channels. Despite the preferential action of this drug class toward over-excited cells, which significantly limits potential undesired side effects on other cells, the need to develop a second generation of sodium channel inhibitors to overcome their critical clinical shortcomings is apparent. Current approaches in drug discovery to deliver novel and truly innovative sodium channel inhibitors is next presented by surveying the most recent medicinal chemistry breakthroughs in the field of small molecules and developments in automated patch-clamp platforms. Various strategies aimed at identifying small molecules that target either particular isoforms of sodium channels involved in specific diseases or anomalous sodium channel currents, irrespective of the isoform by which they have been generated, are critically discussed and revised.

Improvement of pentylenetetrazol-induced learning deficits by valproic acid in the adult zebrafish

Pentylenetetrazol (PTZ) has been shown to induce seizure-like behavior, learning deficits in passive avoidance response test, and an increase in hsp70 (heat shock protein 70) mRNA expression in the adult zebrafish; PTZ has been increasingly appreciated as an excellent model system for the study of seizures. In this study, we demonstrate that valproic acid (VPA), an antiepileptic drug, suppresses seizure-like behavior and improves learning ability in adult zebrafish treated with PTZ. Pretreatment with VPA significantly reduces rapid involuntary movement and abrupt changes in moving direction in the PTZ-treated zebrafish. PTZ-induced learning impairments were also improved in the zebrafish pretreated with 200 or 500 microM VPA. However, the scopolamine-induced impairments of learning ability were not improved by VPA pretreatment. It is worth noting that while the zebrafish treated with 500 microM VPA for 1-3 weeks learned the passive avoidance response, those treated with 1 or 2mM VPA for 3h didn't. Furthermore, the increased level of hsp70 expression induced by PTZ, a stress marker protein, was significantly reduced in the VPA-pretreated zebrafish brains. Collectively, our data show the antiepileptic effects of VPA in the adult zebrafish, which coincides with reduced hsp70 mRNA expression, rescued learning impairment under PTZ-treated conditions.

Subtype-selective targeting of voltage-gated sodium channels

Voltage-gated sodium channels are key to the initiation and propagation of action potentials in electrically excitable cells. Molecular characterization has shown there to be nine functional members of the family, with a high degree of sequence homology between the channels. This homology translates into similar biophysical and pharmacological properties. Confidence in some of the channels as drug targets has been boosted by the discovery of human mutations in the genes encoding a number of them, which give rise to clinical conditions commensurate with the changes predicted from the altered channel biophysics. As a result, they have received much attention for their therapeutic potential. Sodium channels represent well-precedented drug targets as antidysrhythmics, anticonvulsants and local anaesthetics provide good clinical efficacy, driven through pharmacology at these channels. However, electrophysiological characterization of clinically useful compounds in recombinant expression systems shows them to be weak, with poor selectivity between channel types. This has led to the search for subtype-selective modulators, which offer the promise of treatments with improved clinical efficacy and better toleration. Despite developments in high-throughput electrophysiology platforms, this has proven very challenging. Structural biology is beginning to offer us a greater understanding of the three-dimensional structure of voltage-gated ion channels, bringing with it the opportunity to do real structure-based drug design in the future. This discipline is still in its infancy, but developments with the expression and purification of prokaryotic sodium channels offer the promise of structure-based drug design in the not too distant future.

Anticonvulsant effect of BmK IT2, a sodium channel-specific neurotoxin, in rat models of epilepsy

BACKGROUND AND PURPOSE

The sodium channel is a primary target for treating central nervous system disorders such as epilepsy. In this study the anticonvulsant effect of BmK IT2, a sodium channel-specific neurotoxin, was evaluated in different animal models of epilepsy.

EXPERIMENTAL APPROACH

Experiments were performed on freely moving rats made epileptic by administration of either pentylenetetrazole (PTZ) or pilocarpine. BmK IT2 (0.05-0.5 microg in 2 microl) was microinjected into the CA1 area and its effects on PTZ-induced widespread, seizure-like behaviour and cortex epileptiform EEG, as well as on pilocarpine-induced seizure-like behaviour and c-Fos expression were studied.

KEY RESULTS

Intrahippocampal application of BmK IT2 dose-dependently inhibited PTZ-induced seizure-like behaviour, and reduced the numbers and duration of the high amplitude and frequency discharges (HAFDs) of the epileptiform EEG component induced by PTZ. Similarly, in the pilocarpine-induced status epilepticus (SE) model, BmK IT2 significantly prolonged the latency to onset of the SE, reduced the severity of SE and suppressed hippocampal c-Fos expression during SE.

CONCLUSIONS AND IMPLICATIONS

BmK IT2 showed anticonvulsant activity as it inhibited the widespread seizures induced by PTZ and pilocarpine-induced SE in rats. This activity might be due to the modulation of sodium channels in the hippocampus. Hence, BmK IT2 could be used as a novel tool to explore the molecular and pathological mechanisms of epilepsy with regard to the involvement of sodium channels.

Pentylenetetrazole induced changes in zebrafish behavior, neural activity and c-fos expression

Rodent seizure models have significantly contributed to our basic understanding of epilepsy. However, medically intractable forms of epilepsy persist and the fundamental mechanisms underlying this disease remain unclear. Here we show that seizures can be elicited in a simple vertebrate system e.g. zebrafish larvae (Danio rerio). Exposure to a common convulsant agent (pentylenetetrazole, PTZ) induced a stereotyped and concentration-dependent sequence of behavioral changes culminating in clonus-like convulsions. Extracellular recordings from fish optic tectum revealed ictal and interictal-like electrographic discharges after application of PTZ, which could be blocked by tetrodotoxin or glutamate receptor antagonists. Epileptiform discharges were suppressed by commonly used antiepileptic drugs, valproate and diazepam, in a concentration-dependent manner. Up-regulation of c-fos expression was also observed in CNS structures of zebrafish exposed to PTZ. Taken together, these results demonstrate that chemically-induced seizures in zebrafish exhibit behavioral, electrographic, and molecular changes that would be expected from a rodent seizure model. Therefore, zebrafish larvae represent a powerful new system to study the underlying basis of seizure generation, epilepsy and epileptogenesis.

Basic pharmacology of valproate: a review after 35 years of clinical use for the treatment of epilepsy

Since its first marketing as an antiepileptic drug (AED) 35 years ago in France, valproate has become established worldwide as one of the most widely used AEDs in the treatment of both generalised and partial seizures in adults and children. The broad spectrum of antiepileptic efficacy of valproate is reflected in preclinical in vivo and in vitro models, including a variety of animal models of seizures or epilepsy. There is no single mechanism of action of valproate that can completely account for the numerous effects of the drug on neuronal tissue and its broad clinical activity in epilepsy and other brain diseases. In view of the diverse molecular and cellular events that underlie different seizure types, the combination of several neurochemical and neurophysiological mechanisms in a single drug molecule might explain the broad antiepileptic efficacy of valproate. Furthermore, by acting on diverse regional targets thought to be involved in the generation and propagation of seizures, valproate may antagonise epileptic activity at several steps of its organisation. There is now ample experimental evidence that valproate increases turnover of gamma-aminobutyric acid (GABA) and thereby potentiates GABAergic functions in some specific brain regions thought to be involved in the control of seizure generation and propagation. Furthermore, the effect of valproate on neuronal excitation mediated by the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors might be important for its anticonvulsant effects. Acting to alter the balance of inhibition and excitation through multiple mechanisms is clearly an advantage for valproate and probably contributes to its broad spectrum of clinical effects. Although the GABAergic potentiation and glutamate/NMDA inhibition could be a likely explanation for the anticonvulsant action on focal and generalised convulsive seizures, they do not explain the effect of valproate on nonconvulsive seizures, such as absences. In this respect, the reduction of gamma-hydroxybutyrate (GHB) release reported for valproate could be of interest, because GHB has been suggested to play a critical role in the modulation of absence seizures. Although it is often proposed that blockade of voltage-dependent sodium currents is an important mechanism of antiepileptic action of valproate, the exact role played by this mechanism of action at therapeutically relevant concentrations in the mammalian brain is not clearly elucidated. By the experimental observations summarised in this review, most clinical effects of valproate can be explained, although much remains to be learned at a number of different levels about the mechanisms of action of valproate. In view of the advances in molecular neurobiology and neuroscience, future studies will undoubtedly further our understanding of the mechanisms of action of valproate.

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.

Valproic acid intensifies epileptiform activity in the hippocampal pyramidal neurons

The effects of large concentrations of valproic acid (VPA) on veratridine-induced epileptiform activity (veratridine model) were investigated in rat hippocampal CA1 pyramidal neurons. Studies were performed on the veratridine model in rat brain slices using conventional electrophysiological intracellular techniques. Large concentrations of VPA (5 mM or more) enhanced rather than inhibited epileptiform activity induced by veratridine. During the proepileptic phase of VPA, a membrane depolarization accompanied by a decrease in membrane input resistance were evident. The voltage-dependent proepileptic effect of VPA was blocked by tetrodotoxin (TTX; 100 nM) but not by the calcium channel blockers, diltiazem (5 microM) or omega-conotoxin GVIA (5 microM). VPA did not induce a proepileptic effect when it was superfused at high concentration (0.5-10 mM) on sodium channel-independent models such as the bicuculline or magnesium-free artificial cerebrospinal fluid. Large concentrations of VPA had no significant effect on untreated neurons. The VPA-enhanced veratridine bursting is probably related to the reported proepileptic activities observed in patients taking high doses of this drug. These data also suggest the involvement of sodium channels in the proepileptic effect of VPA.

Effects of new valproate derivatives on epileptiform discharges induced by pentylenetetrazole or low Mg2+ in rat entorhinal cortex-hippocampus slices

The effects of four valproic acid derivatives were studied on pentylenetetrazole-induced epileptiform discharges in combined entorhinal cortex hippocampus slices. The two new sugar-esters of valproic acid, dimethylenexylitol valproate (VDMX, 0.5 mM) and glucose valproate (VG, 2 mM) abolished the epileptiform activity. These two new derivatives were compared to two clinically used anticonvulsant drugs, valpromide (2 mM) which suppressed the activity and valproic acid (2 mM), which was ineffective. The new drugs VDMX and VG were also tested on different patterns of epileptiform activity induced by lowering of [Mg2+]0. A 1 mM concentration of VDMX and 2 mM VG, reversibly suppressed the recurrent short discharges in area CA1 and the seizure-like events in the entorhinal cortex. A concentration of 2 mM VDMX was required to abolish the late recurrent discharges in entorhinal cortex. VG at 2 mM reduced the frequency of these discharges by 58.5+/-9.5%.

Characterization of the in vitro antiepileptic activity of new and old anticonvulsant drugs

1. The in vitro antiepileptiform effects of some old and new anticonvulsants in the experimental model of the "epileptiform" hippocampal slice have been reviewed. 2. On the basis of their influence on in vitro epileptogenesis and basal neuronal excitability, anticonvulsants can be classified into three main categories: (1) anticonvulsants (prototypical drug phenytoin) affecting basal neuronal excitability but not epileptogenesis; (2) anticonvulsants (prototypical drugs barbiturates) affecting basal neuronal excitability and epileptogenesis; (3) anticonvulsants (prototypical drug felbamate) affecting epileptogenesis but not basal neuronal excitability. 3. It is concluded that the model of the "epileptiform" hippocampal slices can be considered a previsional test for the study and the screening of new anticonvulsant drugs.

Propofol inhibits epileptiform activity in rat hippocampal slices

Propofol (2,6 di-isopropylphenol) is an intravenous general anesthetic used widely in neuroanesthesia, as a sedative in intensive care units, and has successfully aborted refractory status epilepticus. We investigated the effects of propofol on epileptiform activity in rat hippocampal slices. Interictal epileptiform activity was produced by bath applying one of the following: picrotoxin (PTX; 10 and 50 microM), bicucculine methiodide (BMI; 10 and 50 microM), 4-aminopyridine (4-AP; 50 microM), 8.5 mM [K+]o or 0 [Mg2+]o artificial cerebrospinal fluid. Propofol was then added in increasing concentrations and the effect on the rate of extracellular field epileptiform discharges was measured. Ictal-like discharges (> 2 Hz for > 2 s) were produced by 7.5 mM [K+]o and pilocarpine (10 microM). Propofol (30 micrograms/ml, 168 microM) completely abolished discharges induced by 8.5 mM [K+]o and at 60 micrograms/ml (337 mM) completely suppressed discharges induced by 4-AP and 0 [Mg2+]o. Propofol was less effective in reducing discharges produced by GABAA/Cl- receptor complex antagonists. Propofol at a concentration of 300 micrograms/ml (1.7 mM) was needed to reduce BMI-induced (50 microM) discharges by 77% and only reduced PTX-induced (50 microM) discharges by 20%. Ictal-like discharges produced by pilocarpine were disrupted by low concentrations of propofol (3-10 micrograms/ml, 16.9-56.2 microM) and the duration of the ictal-like discharge period was significantly reduced. We found that propofol has significant in vitro antiepileptic effects. Additionally, propofol was less effective against GABAA antagonists suggesting that the GABAA receptor complex is the site of its action.

Effects of various valproic acid derivatives on low-calcium spontaneous epileptiform activity in hippocampal slices

Lowering of extracellular calcium induces the development of spontaneous epileptiform activities in rat hippocampal slices. The antiepileptogenic effect of four new sugar-ester derivatives of valproic acid--dimethylenexylitol valproate, monoacetoneglucose valproate, diacetoneglucose valproate and glucose valproate--were investigated on such activity through 20-min bath applications and their effect compared to that of valproate, valpromide and phenytoin. Sodium valproate, 5 mM, did not completely suppress the spontaneous epileptiform activity. Valpromide, 2.5 mM, and phenytoin, 0.25 mM, produced complete cessation of seizure activity. Dimethylenexylitol valproate, 0.1 mM, completely suppressed spontaneous epileptiform activities. The other derivatives were less potent: concentrations of 0.25 mM of monoacetoneglucose valproate and 1 mM of diacetoneglucose valproate and glucose valproate were required for complete cessation of activity. The sugar carriers alone were devoid of effect. The data show that these molecules have a direct action on the nervous tissue and their antiepileptogenic efficacy in the low-calcium model is far larger than that of valproic acid itself. Such derivatives, especially dimethylenexylitol valproate, appear to be promising for development of new antiepileptic molecules.

Chemical models of epilepsy with some reference to their applicability in the development of anticonvulsants

This paper reviews chemical models of epilepsy and their relevance in the identification and characterization of anticonvulsants. For each convulsant we discuss possible modes of administration, clinical type(s) of seizures induced, proposed mechanism(s) of epileptogenesis and, where available, responsiveness of the induced seizures to anticonvulsants. The following compounds are reviewed: pentylenetetrazol, bicuculline, penicillin, picrotoxin, beta-carbolines, 3-mercaptopropionic acid, hydrazides, allylglycine; the glycine antagonist strychnine; gamma-hydroxybutyrate; excitatory amino acids (glutamate, aspartate, N-methyl-D-aspartate, quisqualate, kainate, quinolinic acid); monosubstituted guanidino compounds, metals (alumina, cobalt, zinc, iron); neuropeptides (opioid peptides, corticotropin releasing factor, somatostatin, vasopressin); cholinergic agents (acetylcholine, acetylcholinesterase inhibitors, pilocarpine); tetanus toxin; flurothyl; folates; homocysteine and colchicine. Although there are a multitude of chemical models of epilepsy, only a limited number are applied in the routine screening of potential anticonvulsants. Some chemical models have a predictive value with regard to the clinical profile of efficacy of the tested anticonvulsants. Some chemical models may contribute to a better understanding of possible mechanisms of epileptogenesis.

An examination of the anticonvulsant properties of voltage-sensitive calcium channel inhibitors in amygdala kindled seizures

Voltage-sensitive calcium (VSC) channels may contribute to epileptogenesis. A systematic examination of the anticonvulsant efficacy of different classes of VSC channel inhibitors, however, is lacking in chronic seizure models. The present study evaluated representatives from three different classes of VSC channel inhibitors for their protection against amygdala kindled seizures. Adult male rats (n = 12) were kindled to stage 5 seizures (GS), and a threshold intensity required to evoke a GS was determined. The Ca(++)-channel inhibitors (verapamil 0, 10, 20, 40 mg/kg; nimodipine 0, 5, 25, 50 mg/kg; nitrendipine 0, 25, 50, 100 mg/kg and flunarizine 0, 20, 40, 80 mg/kg) were administered 60-90 min prior to amygdala stimulation at the established threshold. None of the drugs altered threshold for inducing a seizure. Verapamil, a phenylalkylamine, and the dihydropyridines nimodipine and nitrendipine were without effect on kindled seizures. The diphenylalkylamine, flunarizine, was found to be the most efficacious, reducing AD duration and duration of clonic seizure activity by more than 60% in most animals. Flunarizine also decreased the severity of behavioral seizures, with 40% of the animals displaying Stage 1-2 seizures only. It is concluded that some VSC Ca(++)-channel inhibitors do possess anticonvulsant potential. Thus influx of extracellular calcium through VSC channels may contribute to the expression of kindled seizures.

Pitfalls in the use of brain slices

In vitro brain slices are the preparation of choice for the detailed examination of local circuit properties in mammalian brain. However it is the investigator's responsibility to verify that the circuits under investigation are indeed confined within the boundaries of the functional region of the slice used. The medium in which the slice is maintained is under the full control of the investigator. This places the burden on the investigator to ensure that: (1) the properties of the medium are fully under control; (2) the effects of the medium on the slice are known; (3) the conditions under which the slice is being maintained bear some reasonable relation to those it enjoys (or endures) in vivo. Generalizations to in vivo conditions must be made with caution. If at all possible, similar studies (perhaps less extensive, due to the greater technical difficulties) should be done in vivo to provide a basis for comparison. Investigators using drugs should be aware of, and respect, the basic pharmacological principles cited in the text. In particular, the substantial freedom the investigator has in defining the extracellular medium should not be abused.

Differential effects of phenobarbital and pentobarbital on stimulus train-induced bursting in the hippocampal slice

Stimulus train-induced bursting (STIB) in the hippocampal slice is an in vitro model of epileptiform activity that is analogous to kindling and requires no manipulation of the neurochemical or ionic environment. The bursts recorded from slices after STIB closely resemble the discharges observed in vivo after kindling and may provide a convenient and accessible model for studies of the electrophysiological underpinnings of epileptiform bursting in local neural circuits. One critical element in the validation of this model is the responsiveness of STIB to traditional sedatives and anticonvulsants. In this study we assessed the effects of the barbiturates phenobarbital and pentobarbital on both spontaneous and stimulus-triggered bursts after STIB. Concentration-response studies indicated that both drugs had inhibitory effects on stimulus-triggered bursting, although at significantly different concentrations. The rate of spontaneous bursting, however, was not significantly affected by phenobarbital at any of the concentrations tested, whereas pentobarbital reduced the frequency of spontaneous bursting in a concentration-dependent manner. These results indicate that STIB is responsive to the barbiturates tested, although the drugs have differential potencies depending on the type of bursting measured. These effects may be the result of differential potencies of these drugs on gamma-aminobutyric acid-mediated inhibition within local hippocampal circuits, as well as different effects on the biophysical characteristics of pacemaker cells within these circuits.