Effect of anticonvulsant drugs on inhibitory and excitatory pathways

Epileptiform activity of veratridine model in rat brain slices: effects of antiepileptic drugs

The effects of the antiepileptic drugs valproic acid (VPA), phenytoin (PHT), and ethosuximide (ESM) on evoked and spontaneous seizure-like (epileptiform) activity were studied in the veratridine epileptiform model in rat brain slices, using conventional electrophysiological intracellular recording techniques. The veratridine model is generated by treatment of brain slices with a low concentration (0.3 microM) of the alkaloid veratridine. The drug modifies sodium channel function so that a brief current injection in hippocampal CA1 pyramidal neurons evokes bursts of epileptiform activity. Therapeutic concentrations of VAP (50-200 microM) inhibited both evoked and spontaneous bursting in a voltage-dependent manner without affecting membrane resting potential or input resistance. Similarly, therapeutic concentrations of PHT (4-15 microM) inhibited both evoked and spontaneous bursting in a voltage-dependent fashion with no apparent change in the membrane resting potential. However, PHT increased the membrane input resistance and elevated the firing threshold of neurons. The antiepileptic drug ESM failed to inhibit evoked or spontaneous bursting even at high concentrations. The results suggest that the veratridine model of epileptiform activity is sensitive only to antiepileptic drugs that primarily affect the sodium channels.

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.

Gabapentin: discussion

Gabapentin (GBP, Neurontin) is a novel antiepileptic drug (AED) that was shown to be effective against refractory partial seizures in five placebo-controlled trials. However, a number of patients with complex partial seizures experienced an increase in seizure frequency, suggesting that patients suffering from complex partial seizures are not a homogeneous group. In fact, we found that currently available AEDs are likely to be ineffective when staring is a prominent component of complex partial seizures. The poor response of this group of patients may reflect the fact that staring spells are inhibitory seizures and that the AEDs prescribed for partial seizures appear to facilitate inhibitory mechanisms. GBP resembles phenytoin (PHT) and carbamazepine (CBZ) in depressing segmental and reticular excitatory mechanisms and facilitating segmental inhibitory mechanisms, just as it resembles PHT and CBZ in efficacy against some partial seizures and against secondarily generalized seizures. Perhaps the patients in whom GBP increased seizure frequency had complex partial seizures with staring and were therefore unlikely to benefit from drugs such as GBP, CBZ, and PHT, which enhance inhibitory mechanisms in the brain. These findings suggest that future AED trials would greatly benefit from a categorization of complex partial seizures into nosologically distinct groups.

Valproic acid inhibits the depolarizing rectification in neurons of rat amygdala

The actions of valproic acid (VPA) on neuronal membrane properties and synaptic transmission were studied using intracellular recording techniques in rat basolateral neurons of the amygdala slices. In therapeutically attainable concentrations (10-100 microM), VPA decreased synaptically-induced epileptiform bursting in the presence of bicuculline. Additionally, the frequency of repetitive discharge induced by direct superthreshold depolarizing current pulses was decreased by VPA. However, evoked excitatory and inhibitory postsynaptic potentials were not affected at this level of drug concentration. The current-voltage relationship of untreated neurons revealed rectification of membrane potential when neuronal membrane was depolarized with cathodal current pulses. This depolarizing rectification was blocked by VPA. High medium calcium or addition of the sodium channel blocker tetrodotoxin (TTX) also blocked the depolarizing rectification, whereas the calcium channel antagonist diltiazem had no effect on the rectification. Elevation of medium calcium concentration also blocked the bicuculline-induced bursting. These results indicate that the inhibition by VPA of subthreshold slow sodium current and membrane depolarizing rectification results in suppression of neuronal membrane excitability which is probably a major mechanism for its anticonvulsant action.

Ethosuximide alters intrathalamic and thalamocortical synchronizing mechanisms: a possible explanation of its antiabsence effect

Effects of systemic administration of a single dose (50 mg/kg) of ethosuximide (ESM) on extracellularly recorded thalamic (nucleus centralis lateralis, CL; nucleus reticularis, RE) and cortical neurons and on cortical EEG activity of acute cats, have been studied. In intact animals ESM led to: (a) desynchronization of cortical EEG activity; (b) reduction of cortical recruiting responses to 6 Hz stimulation of nucleus centralis medialis (CeM); (c) increased firing rate of CL units; and (d) reduction of incremental responses (IRs) of CL neurons to CeM stimulation. In midbrain reticular formation (MRF)-lesioned animals, ESM induced: (a) reduction of cortical spindle waves; (b) increment of their intraburst frequency; (c) reduction of the IR of CL neurons to 3 and 6 Hz CeM stimulation; (d) shortening of the inhibitory period following each response; and (e) no increment of spontaneous firing rate of CL units. Moreover, ESM led to important changes in the spontaneous activity of RE neurons: spike barrages, typical of these neurons in MRF-lesioned animals, became less frequent and of longer duration, being also constituted by longer interspike intervals. However, responses of RE neurons to low frequency CeM stimulation, when present, did not show any incremental phenomenon and appeared unchanged after ESM. Responses of cortical neurons to paired stimuli, applied with different interstimulus intervals, to nucleus ventralis posterolateralis or in animals with isolated cortex, to subcortical white matter, disclosed a reduction of the cortical inhibitory period following the response to the conditioning stimulus. These data suggest that ESM exerts a moderate diffuse anti-inhibitory action at both cortical and thalamic levels and an activating effect on MRF, which could also be accomplished through disinhibition. The reduction of the inhibitory phases in thalamic nuclei would alter spontaneous intrathalamic synchronizing mechanisms, leading to a decreased effectiveness of thalamocortical volleys, which are believed to be fundamental for the appearance of cortical spike and wave discharges. This hypothesis would therefore explain the specific efficacy of ESM against absence seizures.

Failure of the antiepileptic drug valproic acid to modify synaptic and non-synaptic responses of CA1 hippocampal pyramidal cells maintained 'in vitro'

The mechanisms of action of the antiepileptic drug valproic acid (VPA) were analyzed in 24 CA1 pyramidal neurons of the 'in vitro' hippocampal slice by using standard intracellular recording techniques. VPA (0.5-2 mM) failed to induce any significant change in the amplitude of the orthodromic EPSPs and the amplitude and duration of the IPSPs evoked by orthodromic or antidromic stimuli. The repetitive firing induced by depolarizing current pulses and the subsequent long lasting afterhyperpolarization were also not affected by VPA. We conclude that VPA, at doses within the therapeutic range, does not potentiate GABA-mediated inhibition in this preparation and probably acts on mechanisms which are not operating or fully expressed in normal (i.e., non-epileptic) situations.

Hypothesis: the classification of epileptic seizures according to systems of the CNS

Several comprehensive classifications of the epilepsies and epileptic syndromes have been proposed and the purpose of the last one (1985) was to supplement the International Classification of Epileptic Seizures as accepted by the General Assembly of the International League Against Epilepsy in 1981. These efforts represent a major contribution to a difficult problem, which must be continued as further scientific advances permit. Categorization according to clinical seizure type is a logical approach, inasmuch as response to antiepileptic medication depends more on seizure type than on etiological or other factors. A limitation of this approach, however, has arisen as a result of the Commission's failure to relate clinical seizure types to the underlying neurophysiology that mediates them. This has resulted in the categorization of absence as a generalized form of seizure along with generalized tonic-clonic seizures (GTCS). On the other hand, temporal lobe seizures (including automatisms) have been categorized as a complex form of partial seizures, which emphasizes their focal origin. It is the belief of the authors that a further clarification might be achieved, if a differentiation of seizure types is made with respect to the underlying CNS circuitry which mediates the sustained discharge that determines the clinical manifestations of each type. Focal origins, while significant, assume a secondary importance, since they do not explain the sustained discharge that mediates clinical seizure types, except in the case of elementary partial seizures (EPS) that remain discretely localized. In contradistinction to GTCS that essentially involve the entire CNS, absence and complex partial seizures (CPS) should be categorized as limited forms of seizures, mediated by bilateral, regional systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of zonisamide (CI-912) on a synaptic system model

The effect of the experimental antiepileptic drug zonisamide (1,2-benzisoxazole-3-methanesulfonamide, ZNS) on the trigeminal complex of cats was compared with the effect of established antiepileptic drugs. Intravenous administration of 10-40 mg/kg ZNS significantly depresses descending excitatory mechanisms, as well as segmental and descending inhibitory mechanisms, but has only a minor effect on segmental excitatory mechanisms. This spectrum of activity is similar to that of valproate, and suggests that ZNS should also be a broad-spectrum antiepileptic drug. In agreement with our experimental observations, it has been found that ZNS is effective against complex partial, generalized tonic clonic, and myoclonic seizures. The antiepileptic profile of ZNS in conventional screening tests resembles that of carbamazepine (CBZ) and phenytoin. However, CBZ exacerbates rather than prevents myoclonic seizures. Our experimental model thus provides a more accurate prediction of ZNS's clinical spectrum of activity. The relationship of these findings to the mechanism of action of antiepileptic drugs is discussed.

Acute effect of anticonvulsants on amygdaloid kindled seizures induced with low-frequency stimulations

Acute effects of several antiepileptic drugs on low-frequency amygdaloid-kindled seizures were assessed. The number of stimulating pulses required for the provocation of epileptic afterdischarge (pulse-number threshold, PNT) was used as an indicator for the seizure-generating threshold. The duration of epileptic afterdischarge (AD duration) was used as an indicator for the severity of the induced seizures. Phenytoin (PHT) and carbamazepine (CBZ) reduced AD duration more than did elevating PNT. Conversely, phenobarbital (PB) and diazepam (DZP) elevated PNT more than did reducing AD duration. Weak effects on the two indicators, valproic acid (VPA) and ethosuximide (ESM), were observed. Low-frequency kindling may be a useful experimental model of epilepsy in drug-assessments.

Intracellular recordings in pericruciate neurons during spike and wave discharges of feline generalized penicillin epilepsy

Concurrent EEG and intracellular recordings from pericruciate neurons of cats obtained before and after i.m. injection of penicillin inducing the syndrome of feline generalized penicillin epilepsy (FGPE) characterized by spike and wave (SW) discharge in the EEG, display large excitatory postsynaptic potentials (EPSPs) at the time of the EEG 'spike' which alternate with hyperpolarizing potentials occurring in coincidence with the EEG 'wave' component of the SW complex. The large EPSPs trigger discharges of single or multiple high-frequency action potentials which do not show a progressive decrement in amplitude nor an appreciable increase in duration. These bursts thus differ in some respects from typical paroxysmal depolarization shifts. The hyperpolarizing potentials show an early phase which is reversed by intracellular Cl- injection or diffusion and thus behaves like a classical inhibitory postsynaptic potential (IPSP). The late phase is unaffected by Cl-. Hyperpolarizing potentials of pericruciate neurons induced by antidromic activation of the cerebral peduncle (CP) or by direct cortical stimulation are not altered after i.m. injections of penicillin at doses sufficient to induce generalized SW discharge. The early phase of hyperpolarization both before and after i.m. penicillin is reversed by intracellular Cl- injection or diffusion, the late phase remains unchanged. The early phase thus represents a classical IPSP, which does not appear to be affected by the low brain penicillin concentrations sufficient to induce generalized SW discharge. It is concluded that this form of epileptic discharge cannot be attributed to blockage of phasic (presumably somatic) postsynaptic inhibition by penicillin. These results indicate that to regard all forms of epileptic discharge as the consequence of a blockage of gamma-aminobutyric acid-mediated phasic postsynaptic inhibition acting on the soma represents an unduly restrictive view of epileptogenesis.

Differential effect of ethosuximide and of electrical stimulation on inhibitory and excitatory mechanisms

Much longer trains of conditioning stimuli are required to elicit inhibition descending from the reticular formation than to elicit segmental inhibition in the trigeminal nucleus. In contrast, a single conditioning stimulus is the most effective in eliciting descending facilitation, while the test stimulus alone is most effective in eliciting segmental excitation. Ethosuximide (ESM) selectively depresses descending inhibition and to a lesser extent segmental inhibition. Thus, ESM only depresses pathways requiring repetitive stimulation, such as inhibitory pathways in the reticular formation. This action would account for ESM's specificity for absence seizures, which are probably due to paroxysmal activity in inhibitory pathways.

Mechanisms of anticonvulsant drug action. II. Drugs primarily used for absence epilepsy

The usefulness of the anticonvulsant drugs is determined by the mechanisms by which the agent acts and its pharmacokinetics. The general mechanisms of action of these agents include (1) effects on neurotransmitter action, (2) effects on repetitive neuronal firing mechanisms, (3) effects on neuronal networks, and (4) effects on neuronal ionic transport. Ethosuximide, valproic acid and clonazepam are used primarily in absence epilepsy. Valproic acid is also effective against generalized tonic-clonic epilepsy. Diazepam is used primarily in status epilepticus. Valproic acid enhances gamma aminobutyric acid (GABA)-mediated inhibition, reduces repetitive firing, and reduces both inhibition and excitation in neuronal networks. Clonazepam and diazepam enhance the inhibitory action of GABA, decrease inhibition in neuronal networks and affect calcium ion transport with lesser effects on repetitive firing. Ethosuximide reduces inhibition in neuronal networks, may interact with dopamine, and possibly affects sodium and potassium ion transport. Further work is needed to assess the degree of involvement of these effects in the anticonvulsant action versus the adverse effects of these agents.

Neuronal sensitivity to GABA and glutamate in generalized epilepsy with spike and wave discharges

In awake but painlessly immobilized cats the extracellular activity of the same cortical neurons was recorded before and for 2 to 5 h after the injection of penicillin G (350,000 IU/kg, i.m.) during the development of generalized epilepsy with bilaterally synchronous spike and wave discharges. Possible changes in their sensitivity to microiontophoretically applied glutamate and GABA during this period were searched for using computer-generated periejection histograms at intervals of about 30 min. In contrast to reported studies in other models of epilepsy, glutamate excited and GABA depressed virtually all neurons tested during fully developed spike and wave epilepsy. Spike height was not apparently affected either by the amino acids or by the development of epilepsy. Comparison of relative thresholds for the above effects on rhythmical neuronal activity associated with spike and wave discharge versus effects on random neuronal activity during the interburst periods, supported the idea that spikes and waves result from strong excitatory and inhibitory synaptic drives of the neurons. In all neurons until the appearance of spike and wave discharges, changes in the effect of amino acids, if observed, were small and statistically nonsignificant. This suggests that the hyperexcitability of cortical neurons which reportedly leads to the appearance of spike and wave discharges depends on mechanisms other than an increase in sensitivity to glutamate or a desensitization to GABA. Sometimes the sensitivity to GABA decreased later in this experimental model when the very frequent appearance of spike and wave discharges eventually led to EEG tonic-clonic seizures.

Comparison of progabide with other antiepileptic and GABAergic drugs

The effect of the experimental antiepileptic gamma-aminobutyric acid (GABA) agonist drug progabide, [alpha-(chloro-4-phenyl)fluor-5-hydroxy-2-benzilideneamino]-4-buty ramide, on the trigeminal complex of cats was compared with the effect of established antiepileptic drugs and with the effect of various GABA agonists and antagonists. Intravenous administration of 10-40 mg/kg progabide depressed excitatory transmission and descending periventricular inhibition, similar to carbamazepine and phenytoin. However, progabide depressed, rather than facilitated, segmental inhibition. The serum levels of progabide were comparable with those in patients receiving long-term treatment with progabide. The GABA antagonist bicuculline had the opposite effect of progabide on our experimental model, but the other GABA agonists THIP (4,5,6,7-tetrahydroisoxazolo-5,4-C-pyridine-3-ol) and muscimol did not have the same effects as progabide. THIP had no effect on excitatory transmission, periventricular inhibition, or segmental inhibition, whereas muscimol facilitated periventricular inhibition and sometimes segmental inhibition and had no effect on excitatory transmission. Our experiments thus indicate that progabide, but not THIP or muscimol, should have antiepileptic properties, in agreement with the clinical experiences that have been reported. The reason for the differential effect of these three GABA agonists remains to be elucidated.

Depression of hippocampal low calcium field bursts by the antiepileptic drug valproic acid

The antiepileptic drug valproic acid (VPA) reduces the occurrence of the rhythmic and synchronous bursts produced by hippocampal neurons maintained 'in vitro' and bathed in Ringer-containing low-Ca2+ (0.2 mM), high-Mg2+ (4.0 mM). In this medium, synaptic transmission is blocked, thus demonstrating an action of VPA unrelated to potentiation of GABAergic phenomena. This conclusion is reenforced by the persistence of VPA effects in the presence of bicuculline. Also, the VPA doses effective in reducing the low-calcium synchronous burst in the hippocampal slice are similar to the free plasma levels of VPA observed to exert anticonvulsant effects in kindled rats.

Effects of acute and chronic phenytoin on the electrolyte content and the activities of Na+, K+-, Ca2+, Mg2+-, and HCO3- -ATPases and carbonic anhydrase of neonatal and adult rat cerebral cortex

Various parameters of anion and cation transport were measured in the cerebral cortex of neonatal (3-day-old) and adult rats following acute and chronic treatment with phenytoin (PHT). Acutely, PHT significantly inhibited the enzyme Na+, K+-ATPase in both neonatal and adult rats. This effect was accompanied by a significant increase in cerebral cortical Na+ content and a decrease in K+ content only in neonatal animals. Chronic treatment (two and four times a day for 7 days) of adult rats with PHT significantly reduced Na+ content without affecting whole homogenate Na+, K+-ATPase activity. The activity of this enzyme was markedly increased in the myelin- (glial product) and slightly decreased in the synaptosomal- (neuronal) fractions following chronic (four times a day for 7 days) PHT treatment. These results suggest that PHT differentially affects the two forms (neuronal and glial) of the enzyme Na+, K+-ATPase. The possible relevance of this hypothesis in relationship to the anticonvulsant and excitatory properties of PHT is discussed. Chronic (two and four times a day for 7 days) PHT treatment increased both DNA content and activity of the glial marker enzyme carbonic anhydrase. Activity of the mitochondrial enzyme HCO3- -ATPase was also increased following chronic PHT treatment. These two enzymes are intimately involved in the regulation of HCO3- -Cl- transport across glial cell and mitochondrial membranes, and these results suggest that PHT is able to affect beneficially glial regulatory processes. The ability to enhance glial regulation of anions and cations in extracellular fluid provides new and important insights into the mechanism of the anticonvulsant action of PHT.

Differential effect of antiepileptic and non-antiepileptic drugs on the reticular formation

The effect of the antiepileptic drugs carbamazepine and phenytoin, and of the non-antiepileptic drug baclofen, was compared on various inhibitory and excitatory mechanisms in the feline trigeminal nucleus. Baclofen resembled carbamazepine and phenytoin in depressing segmental excitatory and facilitating segmental inhibitory mechanisms. However, baclofen facilitated the periventricular and periaqueductal inhibition of the trigeminal nucleus, while carbamazepine and phenytoin depressed these descending inhibitory mechanisms. Baclofen also resembles carbamazepine and phenytoin in its effectiveness in trigeminal neuralgia, but baclofen is not a clinically effective antiepileptic agent. Our experiments indicate that the ability to depress the reticular formation of the diencephalon and midbrain is an important characteristic of antiepileptic drugs. This suggests that the reticular core is involved in the spread and generalization of seizures.

The perspective of GABA replenishment therapy in the epilepsies: a critical evaluation of hopes and concerns

Impaired GABA-mediated inhibition is probably one of the cellular abnormalities leading to Focal Epilepsy. The role of GABA in generalized seizures, particularly of Petit Mal type, is unknown. Various approaches are available to potentiate GABA function. Merits and flaws of each one of them are critically evaluated. In some forms of epilepsy, GABA agonists may replenish depleted pools, and in some others may nonspecifically raise the general excitability threshold of the brain, yet in other forms they may exert a glutamate/aspartate antagonistic effect. The available experimental evidence suggests that in bilaterally synchronous spike and wave epilepsies, GABA agonists are either ineffective or pejorative.

Effects of phenytoin, carbamazepine, and clonazepam on cortex- and amygdala-evoked potentials

Multiple doses of phenytoin, carbamazepine, and clonazepam were tested against single- and double-pulse evoked potentials. Evoked responses were triggered from either the amygdala or the cortex and were recorded in both monosynaptic sites and the mesencephalic reticular formation (MRF). In accordance with previous studies of spinal potentials, it was expected that all three drugs would display a depressant action. Contrary to expectation, only clonazepam had any significant effect, and this drug was effective only against potentials triggered in the amygdala and recorded in the MRF. These data appear inconsistent with the traditional belief that anticonvulsants block seizure spread by antagonizing transmission in neural pathways.

Effect of cinromide on inhibitory and excitatory mechanisms

The effect of the experimental anticonvulsant cinromide (3-bromo-N-ethylcinnamamide) on various inhibitory and excitatory mechanisms was investigated in the trigeminal nucleus of cats. Intravenous administration of 20-80 mg/kg cinromide depressed excitatory transmission and facilitated segmental inhibition to the same extent as phenytoin, but less than carbamazepine. These doses of cinromide also depressed periventricular inhibition, similar to valproate and ethosuximide. In addition, cinromide had a marked depressant effect on the EEG, suggesting a pronounced sedative effect. The serum levels of cinromide and of its active metabolites (3-bromocinnamamide and 3-bromocinnamic acid) were comparable to those in patients receiving long-term treatment with cinromide. Our results agree with those in other experimental models, which also suggest that cinromide is a broad-spectrum anticonvulsant, and with cinromide's effect in the clinical trials reported so far.

Do phenytoin and carbamazepine depress excitation and/or facilitate inhibition?

The effect of phenytoin and carbamazepine on the segmental and the periventricular inhibition was investigated in interneurons of the spinal trigeminal nucleus oralis of cats. The segmental inhibition was elicited by delivering a conditioning stimulus to the maxillary nerve 100 ms prior to the test stimulus to the maxillary nerve, and the periventricular inhibition was elicited by delivering a conditioning stimulus to the periventricular gray matter. Therapeutic serum levels of phenytoin and carbamazepine facilitated the segmental inhibition, but depressed the periventricular inhibition, while also depressing the response of trigeminal nucleus neurons to an unconditioned maxillary nerve stimulus. These results indicate that the depression of neural activity by phenytoin and carbamazepine is selective, leaving some inhibitory pathways relatively untouched and thus greatly enhancing their effectiveness. It is suggested that the anticonvulsant properties of phenytoin and carbamazepine are due to their ability to produce a relative facilitation of inhibitory feedback mechanisms.