Valproate suppresses status epilepticus induced by 4-aminopyridine in CA1 hippocampus region

Increasing glutathione levels by a novel posttranslational mechanism inhibits neuronal hyperexcitability

Glutathione (GSH) depletion, and impaired redox homeostasis have been observed in experimental animal models and patients with epilepsy. Pleiotropic strategies that elevate GSH levels via transcriptional regulation have been shown to significantly decrease oxidative stress and seizure frequency, increase seizure threshold, and rescue certain cognitive deficits. Whether elevation of GSH per se alters neuronal hyperexcitability remains unanswered. We previously showed that thiols such as dimercaprol (DMP) elevate GSH via post-translational activation of glutamate cysteine ligase (GCL), the rate limiting GSH biosynthetic enzyme. Here, we asked if elevation of cellular GSH by DMP altered neuronal hyperexcitability in-vitro and in-vivo. Treatment of primary neuronal-glial cerebrocortical cultures with DMP elevated GSH and inhibited a voltage-gated potassium channel blocker (4-aminopyridine, 4AP) induced neuronal hyperexcitability. DMP increased GSH in wildtype (WT) zebrafish larvae and significantly attenuated convulsant pentylenetetrazol (PTZ)-induced acute 'seizure-like' swim behavior. DMP treatment increased GSH and inhibited convulsive, spontaneous 'seizure-like' swim behavior in the Dravet Syndrome (DS) zebrafish larvae (scn1Lab). Furthermore, DMP treatment significantly decreased spontaneous electrographic seizures and associated seizure parameters in scn1Lab zebrafish larvae. We investigated the role of the redox-sensitive mammalian target of rapamycin (mTOR) pathway due to the presence of several cysteine-rich proteins and their involvement in regulating neuronal excitability. Treatment of primary neuronal-glial cerebrocortical cultures with 4AP or l-buthionine-(S,R)-sulfoximine (BSO), an irreversible inhibitor of GSH biosynthesis, significantly increased mTOR complex I (mTORC1) activity which was rescued by pre-treatment with DMP. Furthermore, BSO-mediated GSH depletion oxidatively modified the tuberous sclerosis protein complex (TSC) consisting of hamartin (TSC1), tuberin (TSC2), and TBC1 domain family member 7 (TBC1D7) which are critical negative regulators of mTORC1. In summary, our results suggest that DMP-mediated GSH elevation by a novel post-translational mechanism can inhibit neuronal hyperexcitability both in-vitro and in-vivo and a plausible link is the redox sensitive mTORC1 pathway.

Divergent paths to seizure-like events

Much debate exists about how the brain transitions into an epileptic seizure. One source of confusion is that there are likely to be critical differences between experimental seizure models. To address this, we have compared the evolving activity patterns in two widely used in vitro models of epileptic discharges. Brain slices from young adult mice were prepared in the same way and bathed either in 0 Mg2+ or 100 µmol/L 4AP artificial cerebrospinal fluid. We have found that while local field potential recordings of epileptiform discharges in the two models appear broadly similar, patch-clamp analysis reveals an important difference in the relative degree of glutamatergic involvement. 4AP affects parvalbumin-expressing interneurons more than other cortical populations, destabilizing their resting state and inducing spontaneous bursting behavior. Consequently, the most prominent pattern of transient discharge ("interictal event") in this model is almost purely GABAergic, although the transition to seizure-like events (SLEs) involves pyramidal recruitment. In contrast, interictal discharges in 0 Mg2+ are only maintained by a very large glutamatergic component that also involves transient discharges of the interneurons. Seizure-like events in 0 Mg2+ have significantly higher power in the high gamma frequency band (60-120Hz) than these events do in 4AP, and are greatly delayed in onset by diazepam, unlike 4AP events. We, therefore, conclude that the 0 Mg2+ and 4AP models display fundamentally different levels of glutamatergic drive, demonstrating how ostensibly similar pathological discharges can arise from different sources. We contend that similar interpretative issues will also be relevant to clinical practice.

Beneficial Effects of Selective Orexin-A Receptor Antagonist in 4-aminopyridine-induced Seizures in Male Rats

Background

Orexins are excitatory neuropeptides which stimulate the central regulatory pathways. Orexins increase the penicillin-induced epileptic activity in rats. Orexin-A increases in different types of seizures and its elevated level is the characteristic feature in the epileptic children during polysomnography. Recently, the orexin receptor blockage has been reported to increase seizure threshold in mice; however, effect of the selective orexin-A receptor antagonist (SB-334867) on 4-aminopyridine (4-AP)-induced seizures has not been investigated.

Materials and Methods

We used the intraperitoneal injection of 4-AP to induce seizure in male rats. Under urethane anesthesia, SB-334867 (50 and 100 nmol) was injected stereotaxically into the ventral hippocampal commissure. Using video recording, the effects of SB-334867 on electroencephalogram and tonic-clonic convulsions were compared to those that received diazepam or dimethyl sulfoxide (DMSO).

Results

SB-334867 significantly decreased the duration of spike trains compared to DMSO-treated rats (P < 0.001) and reduced the duration of convulsive seizures (P < 0.05). Seizure onset was increased significantly by SB-334867, 50 nmol, compared to DMSO (P < 0.05) and diazepam (P < 0.01) treated rats.

Conclusion

Antagonism of orexin-A receptor by a low-dose SB-334867 showed protective effects in 4-AP-induced seizure-like activities in anesthetized rats.

Metyrapone prevents acute glucose hypermetabolism and short-term brain damage induced by intrahippocampal administration of 4-aminopyridine in rats

Intracerebral administration of the potassium channel blocker 4-aminopyridine (4-AP) triggers neuronal depolarization and intense acute seizure activity followed by neuronal damage. We have recently shown that, in the lithium-pilocarpine rat model of status epilepticus (SE), a single administration of metyrapone, an inhibitor of the 11β-hydroxylase enzyme, had protective properties of preventive nature against signs of brain damage and neuroinflammation. Herein, our aim was to investigate to which extent, pretreatment with metyrapone (150 mg/kg, i.p.) was also able to prevent eventual changes in the acute brain metabolism and short-term neuronal damage induced by intrahippocampal injection of 4-AP (7 μg/5 μl). To this end, regional brain metabolism was assessed by 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG) positron emission tomography (PET) during the ictal period. Three days later, markers of neuronal death and hippocampal integrity and apoptosis (Nissl staining, NeuN and active caspase-3 immunohistochemistry), neurodegeneration (Fluoro-Jade C labeling), astrogliosis (glial fibrillary acidic protein (GFAP) immunohistochemistry) and microglia-mediated neuroinflammation (in vitro [¹⁸F]GE180 autoradiography) were evaluated. 4-AP administration acutely triggered marked brain hypermetabolism within and around the site of injection as well as short-term signs of brain damage and inflammation. Most important, metyrapone pretreatment was able to reduce ictal hypermetabolism as well as all the markers of brain damage except microglia-mediated neuroinflammation. Overall, our study corroborates the neuroprotective effects of metyrapone against multiple signs of brain damage caused by seizures triggered by 4-AP. Ultimately, our data add up to the consistent protective effect of metyrapone pretreatment reported in other models of neurological disorders of different etiology.

Effect of the Anti-depressant Sertraline, the Novel Anti-seizure Drug Vinpocetine and Several Conventional Antiepileptic Drugs on the Epileptiform EEG Activity Induced by 4-Aminopyridine

Seizures are accompanied by an exacerbated activation of cerebral ion channels. 4-aminopyridine (4-AP) is a pro-convulsive agent which mechanism of action involves activation of Na(+) and Ca(2+) channels, and several antiepileptic drugs control seizures by reducing these channels permeability. The antidepressant, sertraline, and the anti-seizure drug vinpocetine are effective inhibitors of cerebral presynaptic Na(+) channels. Here the effectiveness of these compounds to prevent the epileptiform EEG activity induced by 4-AP was compared with the effectiveness of seven conventional antiepileptic drugs. For this purpose, EEG recordings before and at three intervals within the next 30 min following 4-AP (2.5 mg/kg, i.p.) were taken in anesthetized animals; and the EEG-highest peak amplitude values (HPAV) calculated. In control animals, the marked increase in the EEG-HPAV observed near 20 min following 4-AP reached its maximum at 30 min. Results show that this epileptiform EEG activity induced by 4-AP is prevented by sertraline and vinpocetine at a dose of 2.5 mg/kg, and by carbamazepine, phenytoin, lamotrigine and oxcarbazepine at a higher dose (25 mg/kg). In contrast, topiramate (25 mg/kg), valproate (100 mg/kg) and levetiracetam (100 mg/kg) failed to prevent the epileptiform EEG activity induced by 4-AP. It is concluded that 4-AP is a useful tool to elicit the mechanism of action of anti-seizure drugs at clinical meaningful doses. The particular efficacy of sertraline and vinpocetine to prevent seizures induced by 4-AP is explained by their high effectiveness to reduce brain presynaptic Na(+) and Ca(2+) channels permeability.

Temporal lobe epileptiform activity following systemic administration of 4-aminopyridine in rats

PURPOSE

  The K(+) channel blocker 4-aminopyridine (4AP) induces epileptiform synchronization in brain slices maintained in vitro without interfering with γ-aminobutyric acid (GABA)A receptor-mediated inhibition and, actually, even enhancing it. The hypothesis that similar electrographic epileptiform patterns occur in vivo following systemic 4AP injection was tested here.

METHODS

  Sprague-Dawley rats (n = 13) were implanted with bipolar electrodes aimed at the hippocampal CA3 region, entorhinal cortex, subiculum, dentate gyrus, and amygdala. They were then injected with a single dose of 4AP (4-5 mg/kg, i.p.), and video-monitoring/electroencephalography (EEG) recordings were performed.

KEY FINDINGS

  4AP induced convulsive or nonconvulsive seizures in 12 of 13 rats, along with generalized fascicular twitching, wet-dog shakes, and myoclonic jerks. On EEG, we observed in 7 (58.3%) of 12 animals long-lasting interictal spikes from the subiculum before the occurrence of the first seizure. Once seizures had started, interictal spikes occurred in all areas with no fixed site of origin. Most seizures (41/60, 68.3%) were characterized by a low-voltage fast-activity onset pattern and were convulsive (48/60, 80%). 4AP also induced highly rhythmic theta (6-11 Hz) oscillations in CA3 and entorhinal cortex before seizure occurrence.

SIGNIFICANCE

  Our study shows that systemic 4AP administration in vivo can enhance theta oscillations and induce slow interictal spikes and low-voltage fast-onset seizures similar to those reported in brain slices. We propose that these effects may reflect, at least in part, enhanced GABAA receptor-mediated inhibition as reported in in vitro studies.

High frequency stimulation can suppress globally seizures induced by 4-AP in the rat hippocampus: an acute in vivo study

BACKGROUND

High frequency stimulation (HFS) on the hippocampus can locally suppress epileptiform activity in-vitro and decrease seizure frequency in vivo. In-vitro HFS on the ventral commissural tract, a novel target, was shown to block the axonal conduction and suppress activity in the CA1 and CA3 neuron.

OBJECTIVE

To study the spatial extent of seizure suppression by HFS applied on the tract and focus site in an in vivo experiment.

METHODS

Five adult Sprague-Dawley rats were used for the study. Six electrodes were placed on the septal, middle, and temporal hippocampus bilaterally to simultaneously record seizure activity in the entire hippocampus. Seizure activity was induced by injecting 4-aminopyridine (4-AP) into the right middle part of the hippocampus. Following induction, HFS (100 Hz) was applied to the tract and the focus site at 100, 300 and 500 μA.

RESULTS

The induced seizure activity was dominated by two patterns, high frequency spiking and pseudo-periodic spikes. Either tract or focus site stimulation could generate suppression of only the pseudo-periodic spikes. The suppression rates were dependent on stimulation amplitude (P < 0.005, chi square test). However, HFS also caused conversion of the seizure pattern. The conversion rates increased with higher stimulation amplitudes and were higher with focus site stimulation (P < 0.01, Fisher's exact test).

CONCLUSIONS

The results of this study have two practical implications [1], both tract and focus site stimulation can produce global suppression of hippocampus and [2] the choice of stimulation parameters is critical in order to produce suppression, but not conversion, of seizure pattern.

Effect of valproate, lamotrigine and levetiracetam on excitability and firing properties of CA1 neurons in rat brain slices

The purpose of this study was to analyze the rapid effects of the antiepileptic drugs valproate, lamotrigine, and levetiracetam on excitability and firing properties of hippocampal neurons. The drug effects on resting potential, action potential, and repetitive firing properties were studied in whole-cell current-clamp recordings of CA1 neurons in rat brain slices. Lamotrigine changed action potential rising slope by -24 ± 38 V/s (mean ± SD), peak amplitude by -6.8 ± 5.0 mV, and maximum firing frequency by -60 ± 13%. Lamotrigine thereto increased the voltage threshold by 4.3 ± 4.2 mV and augmented the action potential attenuation during repetitive firing. All effects were significant (P < 0.01 to P < 0.0002) compared to control cells. Valproate and levetiracetam showed no significant effects on these parameters. None of the tested drugs had a significant effect on the resting potential. The lamotrigine effects are consistent with sodium channel blocking which may explain or contribute to the antiepileptic mode of action. Valproate and levetiracetam did not show these effects and the mechanism of their antiepileptic action need to be different. These findings (valproate) differ in some respects from findings reported in cultured or dissociated neurons. In a slice where the neurons have largely preserved connections, drug effects are likely to be more similar to the therapeutic action in the brain.

Intravenous valproate for status epilepticus ... An effective, yet still merely empirical alternative!

Valproate Is an Effective, Well-Tolerated Drug for Treatment of Status Epilepticus/Serial Attacks in Adults. Olsen KB, Taubøll E, Gjerstad L. Acta Neurol Scand Suppl 2007;187:51–54. OBJECTIVE: Status epilepticus (SE) and serial attacks (SA) represent neurological emergencies, and mortality rate for SE/SA is high, ranging from 3% to 25%, depending on cause and co-morbidity. As SE/SA become more refractory to treatment over time, rapid, appropriate treatment is extremely important. Here, we report a prospective registration of the effect of intravenous (IV) valproate (VPA) on SE/SA in a group of Norwegian patients. PATIENTS AND METHODS: Forty-one adult patients (18 males, 23 females) were included in the study. All had previously been unsuccessfully treated with diazepam. For 19, the main SE/SA seizure type was generalized tonic-clonic, while 16 had complex-partial seizures. Six had seizures that were difficult to classify. The treatment protocol recommended 25 mg/kg of VPA loading dose over 30 min, followed by continuous infusion of 100 mg/h for at least 24 h, then per oral administration. If seizures persisted after the loading dose, general anaesthesia (barbiturates/propofol/midazolam) was administered. RESULTS: No serious side effects were reported. In 76% of the cases (31 of 41), SE/SA stopped and anaesthesia was not required. Of the patients treated within 3 h, only 5% needed anaesthesia, whereas of those treated after 3–24 h, 38% needed anaesthesia. Of those who waited for more than 24 h before treatment, 60% required anaesthesia. Furthermore, 60% of the patients who needed anaesthesia were given loading doses below 2100 mg. CONCLUSIONS: VPA seems to be a safe, effective treatment of SE/SA, but efficacy is dependent on time lapse between symptoms and VPA treatment, and administration of a sufficiently high loading dose.

Mechanistic and pharmacologic aspects of status epilepticus and its treatment with new antiepileptic drugs

We review recent advances in our understanding and treatment of status epilepticus (SE). Repeated seizures cause an internalization of gamma-aminobutyric acid (GABA)(A) receptors, together with a movement of N-methyl-d-aspartate (NMDA) receptors to the synapse. As a result, the response of experimental SE to treatment with GABAergic drugs (but not with NMDA antagonists) fades with increasing seizure duration. Prehospital treatment, which acts before these changes are established, is finding increased acceptance, and solid evidence of its efficacy is available, particularly in children. Rational polypharmacy aims at multiple receptors or ion channels to increase inhibition and simultaneously reduce excitation. Combining GABA(A) agonists with NMDA antagonists and with agents acting at other sites is successful in treating experimental SE, and in reducing SE-induced brain damage and epileptogenesis. The relevance of these experimental data to clinical SE is actively debated. Valproate and levetiracetam have recently become available for intravenous use, and the use of ketamine and of other agents (topiramate, felbamate, etc.) have seen renewed interest. A rapidly increasing but largely anecdotal body of literature reports success in seizure control at the price of relatively few complications with the clinical use of those agents in refractory SE.

Attenuation of phospholipid signaling provides a novel mechanism for the action of valproic acid

Valproic acid (VPA) is used to treat epilepsy and bipolar disorder and to prevent migraine. It is also undergoing trials for cancer therapy. However, the biochemical and molecular biological actions of VPA are poorly understood. Using the social amoeba Dictyostelium discoideum, we show that an acute effect of VPA is the inhibition of chemotactic cell movement, a process partially dependent upon phospholipid signaling. Analysis of this process shows that VPA attenuates the signal-induced translocation of PH(Crac)-green fluorescent protein from cytosol to membrane, suggesting the inhibition of phosphatidylinositol-(3,4,5)-trisphosphate (PIP(3)) production. Direct labeling of lipids in vivo also shows a reduction in PIP and PIP(2) phosphorylation following VPA treatment. We further show that VPA acutely reduces endocytosis and exocytosis-processes previously shown to be dependent upon PIP(3) production. These results suggest that in Dictyostelium, VPA rapidly attenuates phospholipid signaling to reduce endocytic trafficking. To examine this effect in a mammalian model, we also tested depolarization-dependent neurotransmitter release in rat nerve terminals, and we show that this process is also suppressed upon application of VPA and an inhibitor of phosphatidylinositol 3-kinase. Although a more comprehensive analysis of the effect of VPA on lipid signaling will be necessary in mammalian systems, these results suggest that VPA may function to reduce phospholipid signaling processes and thus may provide a novel therapeutic effect for this drug.

Animal models for the development of new neuropharmacological therapeutics in the status epilepticus

Status epilepticus (SE) is a major medical emergency associated with significant morbidity and mortality. SE is best defined as a continuous, generalized, convulsive seizure lasting > 5 min, or two or more seizures during which the patient does not return to baseline consciousness. The relative efficacy and safety of different drugs in the treatment of human SE should be determined in a prospective, randomized, blinded study. However, complementary animal models of SE are required to answer important questions concerning the treatment of SE because of the obvious difficulties of setting up such studies in clinical emergency conditions. This review offers an overview of the implementation and characteristics of some of the most prevalent animal models of SE currently in use. A description is also provide about how animal models of SE may facilitate the use of neurobiological techniques to successfully address critical questions in the drug treatment of SE. In particular, the experience with recently introduced drugs such as intravenous valproate will be addressed. Finally, the importance of some animal models and pharmacological approaches is explained and we discuss their impact in the development of therapeutic strategies to improve pharmacological treatment for SE is discussed.

Valproate reduced synaptic activity increase induced by 4-aminopyridine at the hippocampal CA3-CA1 synapse

PURPOSE

We investigated the effects of valproate (VPA) on excitatory synaptic transmission changes induced by 4-aminopyridine (4-AP) to determine whether the antiepileptic effects shown by VPA can be ascribed to a modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in the CA3-CA1 synapse.

METHODS

Rat hippocampal slices were prepared and maintained in vitro with standard methods. Whole-cell current and voltage-clamp recordings were obtained from CA1 pyramidal neurons by using the "blind" patch-clamp technique in an immersion recording chamber. Increase in the spontaneous excitatory synaptic activity was induced by addition of 4-AP to the medium.

RESULTS

Perfusion with VPA significantly counteracted the increase of frequency and amplitude of the sEPSCs induced by application of 4-AP and suppressed the epileptiform activity.

CONCLUSIONS

We conclude that VPA decreases the 4-AP-induced enhancement of excitatory synaptic activity at the CA3-CA1 synapse, and that this reduction of excitation input to CA1 contributes to the anticonvulsant effects of VPA.

Valproate reduced excitatory postsynaptic currents in hippocampal CA1 pyramidal neurons

Valproate (VPA) is one of the most widely used antiepileptic drugs, and it is also increasingly used for the treatment of neuropsychological disorders and neuropathic pain, as well as migraine prophylaxis. However, the underlying cellular mechanisms of VPA on the synaptic physiology remain unclear. We investigated the effects of VPA on synaptic transmission using the in vitro rat hippocampal slice technique and whole-cell patch clamp recordings from CA1 pyramidal neurons. Perfusion with VPA, at therapeutically attainable concentrations, decreased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by Schaffer collateral stimulation, without modifying inhibitory postsynaptic currents (IPSCs). Furthermore, VPA induced a significant reduction of the non-NMDA EPSC (non-NMDA(EPSC)) component, without modifying the NMDA EPSC (NMDA(EPSC)) component. Paired pulse facilitation and EPSC variance were not significantly affected by VPA, indicating that VPA did not decrease transmitter release probability, which suggests a postsynaptic mechanism of action. We therefore conclude that VPA decreases excitatory synaptic activity through the modulation of postsynaptic non-NMDA receptors, without modifying synaptic inhibition, and that this reduction of excitation is, at least in part, responsible for the effects of VPA.