Decreased fast ripples in the hippocampus of rats with spontaneous recurrent seizures treated with carbenoxolone and quinine

High-Frequency Oscillations and Epileptogenic Network

Epilepsy is a network disease caused by aberrant neocortical large-scale connectivity spanning regions on the scale of several centimeters. High-frequency oscillations, characterized by the 80-600 Hz signals in electroencephalography, have been proven to be a promising biomarker of epilepsy that can be used in assessing the severity and susceptibility of epilepsy as well as the location of the epileptogenic zone. However, the presence of a high-frequency oscillation network remains a topic of debate as high-frequency oscillations have been previously thought to be incapable of propagation, and the relationship between high-frequency oscillations and the epileptogenic network has rarely been discussed. Some recent studies reported that high-frequency oscillations may behave like networks that are closely relevant to the epileptogenic network. Pathological highfrequency oscillations are network-driven phenomena and elucidate epileptogenic network development; high-frequency oscillations show different characteristics coincident with the epileptogenic network dynamics, and cross-frequency coupling between high-frequency oscillations and other signals may mediate the generation and propagation of abnormal discharges across the network.

The role of astrocytes in epileptic disorders

Epilepsy affects about 1% of the population and approximately 30% of epileptic patients are resistant to current antiepileptic drugs. As a hallmark in epileptic tissue, many of the epileptic patients show changes in glia morphology and function. There are characteristic changes in different types of glia in different epilepsy models. Some of these changes such as astrogliosis are enough to provoke epileptic seizures. Astrogliosis is well known in mesial temporal lobe epilepsy (MTLE), the most common form of refractory epilepsy. A better understanding of astrocytes alterations could lead to novel and efficient pharmacological approaches for epilepsy. In this review, we present the alterations of astrocyte morphology and function and present some instances of targeting astrocytes in seizure and epilepsy.

The Anti-Epileptic Effects of Carbenoxolone In Vitro and In Vivo

Gap junctions (GJs) are intercellular junctions that allow the direct transfer of ions and small molecules between neighboring cells, and GJs between astrocytes play an important role in the development of various pathologies of the brain, including regulation of the pathological neuronal synchronization underlying epileptic seizures. Recently, we found that a pathological change is observed in astrocytes during the ictal and interictal phases of 4-aminopyridin (4-AP)-elicited epileptic activity in vitro, which was correlated with neuronal synchronization and extracellular epileptic electrical activity. This finding raises the question: Does this signal depend on GJs between astrocytes? In this study we investigated the effect of the GJ blocker, carbenoxolone (CBX), on epileptic activity in vitro and in vivo. Based on the results obtained, we came to the conclusion that the astrocytic syncytium formed by GJ-associated astrocytes, which is responsible for the regulation of potassium, affects the formation of epileptic activity in astrocytes in vitro and epileptic seizure onset. This effect is probably an important, but not the only, mechanism by which CBX suppresses epileptic activity. It is likely that the mechanisms of selective inhibition of GJs between astrocytes will show important translational benefits in anti-epileptic therapies.

Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying Epileptogenesis

Astrocytes are key homeostatic regulators in the central nervous system and play important roles in physiology. After brain damage caused by e.g., status epilepticus, traumatic brain injury, or stroke, astrocytes may adopt a reactive phenotype. This process of reactive astrogliosis is important to restore brain homeostasis. However, persistent reactive astrogliosis can be detrimental for the brain and contributes to the development of epilepsy. In this review, we will focus on physiological functions of astrocytes in the normal brain as well as pathophysiological functions in the epileptogenic brain, with a focus on acquired epilepsy. We will discuss the role of astrocyte-related processes in epileptogenesis, including reactive astrogliosis, disturbances in energy supply and metabolism, gliotransmission, and extracellular ion concentrations, as well as blood-brain barrier dysfunction and dysregulation of blood flow. Since dysfunction of astrocytes can contribute to epilepsy, we will also discuss their role as potential targets for new therapeutic strategies.

The anticonvulsant effect of sparteine on pentylenetetrazole-induced seizures in rats: a behavioral, electroencephalographic, morphological and molecular study

Abnormal synchronous activity in neurons generates epileptic seizures. Antiepileptic drugs (AEDs) are effective in 70% of patients, but this percentage is drastically lower in developing countries. Sparteine is a quinolizidine alkaloid synthesized from most Lupine species and has a probable anticonvulsive effect. For this reason, the objective of the present work was to study the anticonvulsant effect of sparteine using a dose-effect curve and to determine its effectiveness against seizures using behavioral, electroencephalographic, morphological and molecular data. Wistar rats were grouped into control [saline solution (0.9%), pentylenetetrazole (90 mg/kg), and sparteine (13, 20 and 30 mg/kg), intraperitoneal (i.p.)] and experimental (sparteine + pentylenetetrazole) groups. The rats were implanted with surface electrodes to register electrical activity, and convulsive behavior was evaluated according to Velisek's scale. The rats were perfused to obtain brain slices for cresyl violet staining and cellular density quantification as well as for immunohistochemistry for NeuN and GFAP. Other animals were used to determine the hippocampal mRNA expression of the M2 and M4 acetylcholine receptors by qPCR. Sparteine exhibited a better anticonvulsant effect at a dose of 30 mg/kg (i.p.) than at the other doses used. This anticonvulsant effect was characterized by a decrease in the severity of convulsive behavior, 100% survival, an inhibitory effect on epileptiform activity 75 min after pentylenetetrazole administration, and the conservation of the cellular layers of CA1, CA3 and the dentate gyrus (DG); however, astrogliosis was observed after 30 mg/kg sparteine treatment. In addition, sparteine treatment increased the mRNA expression of the M4 receptor three hours after administration. According to our findings, the effective dose of sparteine as an anticonvulsant agent by i.p. injection is 30 mg/kg. The astrogliosis that was observed after sparteine administration may be a compensatory mechanism to diminish excitability and maintain neuronal homeostasis, possibly through redistributing potassium and glutamate. The increase in the mRNA expression of the M4 receptor may suggest the participation of the M4 receptor in the anticonvulsive effect of sparteine, as the activation of this receptor may inhibit acetylcholine release and facilitate the subsequent release of GABA. However, the precise mechanisms by which sparteine produces these effects are not known, and therefore, further experiments are necessary.

Evolving Mechanistic Concepts of Epileptiform Synchronization and their Relevance in Curing Focal Epileptic Disorders

The synchronized activity of neuronal networks under physiological conditions is mirrored by specific oscillatory patterns of the EEG that are associated with different behavioral states and cognitive functions. Excessive synchronization can, however, lead to focal epileptiform activity characterized by interictal and ictal discharges in epileptic patients and animal models. This review focusses on studies that have addressed epileptiform synchronization in temporal lobe regions by employing in vitro and in vivo recording techniques. First, we consider the role of ionotropic and metabotropic excitatory glutamatergic transmission in seizure generation as well as the paradoxical role of GABAA signaling in initiating and perhaps maintaining focal seizure activity. Second, we address non-synaptic mechanisms (which include voltage-gated ionic currents and gap junctions) in the generation of epileptiform synchronization. For each mechanism, we discuss the actions of antiepileptic drugs that are presumably modulating excitatory or inhibitory signaling and voltage-gated currents to prevent seizures in epileptic patients. These findings provide insights into the mechanisms of seizure initiation and maintenance, thus leading to the development of specific pharmacological treatments for focal epileptic disorders.

Connexins-Based Hemichannels/Channels and Their Relationship with Inflammation, Seizures and Epilepsy

Connexins (Cxs) are a family of 21 protein isoforms, eleven of which are expressed in the central nervous system, and they are found in neurons and glia. Cxs form hemichannels (connexons) and channels (gap junctions/electric synapses) that permit functional and metabolic coupling between neurons and astrocytes. Altered Cx expression and function is involved in inflammation and neurological diseases. Cxs-based hemichannels and channels have a relevance to seizures and epilepsy in two ways: First, this pathological condition increases the opening probability of hemichannels in glial cells to enable gliotransmitter release, sustaining the inflammatory process and exacerbating seizure generation and epileptogenesis, and second, the opening of channels favors excitability and synchronization through coupled neurons. These biological events highlight the global pathological mechanism of epilepsy, and the therapeutic potential of Cxs-based hemichannels and channels. Therefore, this review describes the role of Cxs in neuroinflammation and epilepsy and examines how the blocking of channels and hemichannels may be therapeutic targets of anti-convulsive and anti-epileptic treatments.

The protective effect of carbenoxolone on gap junction damage in the hippocampal CA1 area of a temporal lobe epilepsy rat model

Background

Astrocytes are one of the most important types of neural cells in the central nervous system (CNS). Dysfunctional gap junction (GJ) communication could play an underlying role in epileptogenesis. Carbenoxolone (CBX) is a conventional chemical GJ blocker, and its target is connexin 43 (Cx43). Previous studies have shown that CBX can inhibit status epilepticus (SE) and spontaneous epileptic seizures (SESs). However, there is little information about the direct interaction between CBX and Cxs in temporal lobe epilepsy (TLE).

Methods

The behavior of epileptic rats was observed. Moreover, micromorphological changes in the hippocampal cornu ammonis 1 (CA1) area of epileptic rats following CBX injection were determined through transmission electron microscopy (TEM). To illustrate the possible mechanism of these changes, the Western blot method was used.

Results

After the injection of CBX, the seizure frequency, seizure duration, latency period to the first instance of SES, SESs behavioral score according to a scoring system developed by Velíšková and microstructures in the CA1 area were shown to be improved 60 days after SE by TEM. Furthermore, the dynamic expression patterns of Cx43 and Cx43 phosphorylated at Ser368 continuously declined after the injection of CBX until 60 days after SE.

Conclusions

CBX may contribute to the improvement of GJ dysfunction during epileptogenesis in the hippocampal CA1 area in a TLE rat model.

Targeting gap junction in epilepsy: Perspectives and challenges

Gap junctions (GJs) are multiple cellular intercellular connections that allow ions to pass directly into the cytoplasm of neighboring cells. Electrical coupling mediated by GJs plays a role in the generation of highly synchronous electrical activity. Accumulative investigations show that GJs in the brain are involved in the generation, synchronization and maintenance of seizure events. At the same time, GJ blockers exert potent curative potential on epilepsy in vivo or in vitro. This review aims to shed light on the role of GJs in epileptogenesis. Targeting GJs is likely to be served as a novel therapeutic approach on epileptic patients.

Gap Junction Blockers: An Overview of their Effects on Induced Seizures in Animal Models

BACKGROUND

Gap junctions are clusters of intercellular channels allowing the bidirectional pass of ions directly into the cytoplasm of adjacent cells. Electrical coupling mediated by gap junctions plays a role in the generation of highly synchronized electrical activity. The hypersynchronous neuronal activity is a distinctive characteristic of convulsive events. Therefore, it has been postulated that enhanced gap junctional communication is an underlying mechanism involved in the generation and maintenance of seizures. There are some chemical compounds characterized as gap junction blockers because of their ability to disrupt the gap junctional intercellular communication.

OBJECTIVE

Hence, the aim of this review is to analyze the available data concerning the effects of gap junction blockers specifically in seizure models.

RESULTS

Carbenoxolone, quinine, mefloquine, quinidine, anandamide, oleamide, heptanol, octanol, meclofenamic acid, niflumic acid, flufenamic acid, glycyrrhetinic acid and retinoic acid have all been evaluated on animal seizure models. In vitro, these compounds share anticonvulsant effects typically characterized by the reduction of both amplitude and frequency of the epileptiform activity induced in brain slices. In vivo, gap junction blockers modify the behavioral parameters related to seizures induced by 4-aminopyridine, pentylenetetrazole, pilocarpine, penicillin and maximal electroshock.

CONCLUSION

Although more studies are still required, these molecules could be a promising avenue in the search for new pharmaceutical alternatives for the treatment of epilepsy.

Protective Effect of Quinine on Chemical Kindling and Passive Avoidance Test in Rats

Background

In humans, convulsive diseases such as temporal lobe epilepsy are usually accompanied by learning and memory impairments. In recent years, the role of gap junction channels as an important target of antiepileptic drugs has been studied and discussed. Quinine, as a gap junction blocker of connexin 36, can abolish ictal epileptiform activity in brain slices.

Objectives

The role of quinine in memory retrieval in pentylenetetrazole (PTZ)-kindled rats was examined using a step-through passive avoidance task.

Methods

Forty rats were used in this experimental study in groups of 10 animals. Quinine (15, 30, and 60 mg/kg, i.p.) and PTZ (35 mg/kg, i.p.) were injected into the rats before the start of the learning test. Then, retention tests were conducted after the treatments ended.

Results

Quinine could attenuate seizure severity at doses of 15, 30 and 60 mg/kg compared with the control at the beginning of the kindling experiment by lowering the mean seizure stages (P < 0.01, P < 0.001, P < 0.001). Quinine at doses of 15 and 30 mg/kg could significantly increase memory retrieval compared with the control in the retention test 24 and 48 hours after training (P < 0.05). Quinine at a dose of 60 mg/kg increased latency to enter the dark chamber 24 and 48 hours after training (P < 0.001). The results of the retention test one and two weeks after training of quinine were not significant (P > 0.05).

Conclusions

Quinine may decrease the severity of seizure and improve the memory retrieval of animals by inhibiting the gap junction channel. However, further studies are needed to evaluate the molecular mechanism underlying the effects of quinine.