Metabotropic glutamate receptors(mGluRs)in epileptogenesis:an update on abnormal mGluRs signaling and its therapeutic implications

Epilepsy is a neurological disorder characterized by high morbidity,high recurrence,and drug resistance.Enhanced signaling through the excitatory neurotransmitter glutamate is intricately associated with epilepsy.Metabotropic glutamate receptors(mGluRs)are G protein-coupled receptors activated by glutamate and are key regulators of neuronal and synaptic plasticity.Dysregulated mGluR signaling has been associated with various neurological disorders,and numerous studies have shown a close relationship between mGluRs expression/activity and the development of epilepsy.In this review,we first introduce the three groups of mGluRs and their associated signaling pathways.Then,we detail how these receptors influence epilepsy by describing the signaling cascades triggered by their activation and their neuroprotective or detrimental roles in epileptogenesis.In addition,strategies for pharmacological manipulation of these receptors during the treatment of epilepsy in experimental studies is also summarized.We hope that this review will provide a foundation for future studies on the development of mGluR-targeted antiepileptic drugs.

Astrocyte and ions metabolism during epileptogenesis:A review for modeling studies

As a large group of cells in a central nervous system, astrocytes have a great influence on ion and energy metabolism in a nervous system. Disorders of neuronal ion and energy metabolism caused by impaired astrocytes play a key role in the pathogenesis of epilepsy. This paper reviews the existing computational models of epileptogenesis resulting from impaired astrocytes and presents several open perspectives with regard to ion and energy metabolism-induced epileptogenesis in a neuron-astrocyte-capillary coupled model.

Changes in synaptic and extrasynaptic N-methyl-D-aspartate receptor-mediated currents at early-stage epileptogenesis in adult mice

Previous reports have shown that N-methyl-D-aspartate （NMDA） receptors are extensively involved in epilepsy genesis and recurrence. Recent studies have shown that synaptic and extrasynaptic NMDA receptors play different, or even opposing, roles in various signaling pathways, including synaptic plasticity and neuronal death. The present study analyzed changes in synaptic and extrasynaptic NMDA receptor-mediated currents during epilepsy onset. Mouse models of lithium chloride pilocarpLne-induced epilepsy were established, and hippocampal slices were prepared at 24 hours after the onset of status epilepticus. Synaptic and extrasynaptic NMDA receptor-mediated excitatory post-synaptic currents （NMDA-EPSCs） were recorded in CA1 pyramidal neurons by whole-cell patch clamp technique. Results demonstrated no significant difference in rise and delay time of synaptic NMDA-EPSCs compared with normal neurons. Peak amplitude, area-to-peak ratio, and rising time of extrasynaptic NMDA-EPSCs remained unchanged, but decay of extrasynaptic NMDA-EPSCs was faster than that of normal neurons, These results suggest that extrasynaptic NMDA receptors play a role in epileptogenesis.

Epileptogenesis in Sturge-Weber syndrome

Sturge-Weber syndrome(SWS)is a sporadic congenital neurocutaneous disorder characterized by facial port-wine stain,glaucoma and leptomeningeal angioma.It is hypothesized that somatic mutation in GNAQ(p.R183Q),which is associated with the disruption of vascular development,may be a possible mechanism of SWS.The neurological course of this disease may be progressive,and its major morbidity includes epilepsy,stroke-like episodes and intellectual retardation.The earlier the time point of the mutation,the severer the disease presents itself later in life.However,the relationship between SWS and epileptogenesis is still unknown.

The Neurovascular Unit Dysfunction in the Molecular Mechanisms of Epileptogenesis and Targeted Therapy

Epilepsy is a multifaceted neurological syndrome characterized by recurrent,spontaneous,and synchronous seizures.The pathogenesis of epilepsy,known as epileptogenesis,involves intricate changes in neurons,neuroglia,and endothelium,leading to structural and functional disorders within neurovascular units and culminating in the development of spontaneous epilepsy.Although current research on epilepsy treatments primarily centers around anti-seizure drugs,it is imperative to seek effective interventions capable of disrupting epileptogenesis.To this end,a comprehensive exploration of the changes and the molecular mechanisms underlying epileptogenesis holds the promise of identifying vital biomarkers for accurate diagnosis and potential therapeutic targets.Emphasizing early diagnosis and timely intervention is paramount,as it stands to significantly improve patient prognosis and alleviate the socioeconomic burden.In this review,we highlight the changes and molecular mechanisms of the neurovascular unit in epileptogenesis and provide a theoretical basis for identifying biomarkers and drug targets.

Low-frequency Stimulation at the Subiculum Prevents Extensive Secondary Epileptogenesis in Temporal Lobe Epilepsy

Secondary epileptogenesis is characterized by increased epileptic susceptibility and a tendency to generate epileptiform activities outside the primary focus.It is one of the major resultants of pharmacoresistance and failure of surgical outcomes in epilepsy,but still lacks effective treatments.Here,we aimed to test the effects of low-frequency stimulation(LFS)at the subiculum for secondary epileptogenesis in a mouse model.Here,secondary epileptogenesis was simulated at regions both contralateral and ipsilateral to the primary focus by applying successive kindling stimuli.Mice kindled at the right CA3 showed higher seizure susceptibilities at both the contralateral CA3 and the ipsilateral entorhinal cortex and had accelerated kindling processes compared with naive mice.LFS at the ipsilateral subiculum during the primary kindling progress at the right CA3 effectively prevented secondary epileptogenesis at both the contralateral CA3 and the ipsilateral entorhinal cortex,characterized by decreased seizure susceptibilities and a retarded kindling process at those secondary foci.Only application along with the primary epileptogenesis was effective.Notably,the effects of LFS on secondary epileptogenesis were associated with its inhibitory effect at the secondary focus through interfering with the enhancement of synaptic connections between the primary and secondary foci.These results imply that LFS at the subiculum is an effective preventive strategy for extensive secondary epileptogenesis in temporal lobe epilepsy and present the subiculum as a target with potential translational importance.

Generation of Febrile Seizures and Subsequent Epileptogenesis

Febrile seizures（FSs） occur commonly in children aged from 6 months to 5 years. Complex（repetitive or prolonged） FSs, but not simple FSs, can lead to permanent brain modification. Human infants and immature rodents that have experienced complex FSs have a high risk of subsequent temporal lobe epilepsy. However, the causes of FSs and the mechanisms underlying the subsequent epileptogenesis remain unknown. Here, we mainly focus on two major questions concerning FSs： how fever triggers seizures, and how epileptogenesis occurs after FSs. The risk factors responsible for the occurrence of FSs and the epileptogenesis after prolonged FSs are thoroughly summarized and discussed. An understanding of these factors can provide potential therapeutic targets for the prevention of FSs and also yield biomarkers for identifying patients at risk of epileptogenesis following FSs.

The role of inflammation in epileptogenesis

Epilepsy is a chronic neurological disorder that has an extensive impact on a patient’s life.Accumulating evidence has suggested that inflammation participates in the progression of spontaneous and recurrent seizures.Proconvulsant incidences can stimulate immune cells,augment the release of pro-inflammatory cytokines,elicit neuronal excitation as well as blood-brain barrier(BBB)dysfunction,and finally trigger the generation or recurrence of seizures.Understanding the pathogenic roles of inflammatory mediators,including inflammatory cytokines,cells,and BBB,in epileptogenesis will be beneficial for the treatment of epilepsy.In this systematic review,we performed a literature search on the PubMed database using the following keywords:“epilepsy”or“seizures”or“epileptogenesis”,and“immunity”or“inflammation”or“neuroinflammation”or“damage-associated molecular patterns”or“cytokines”or“chemokines”or“adhesion molecules”or“microglia”or“astrocyte”or“blood-brain barrier”.We summarized the classic inflammatory mediators and their pathogenic effects in the pathogenesis of epilepsy,based on the most recent findings from both human and animal model studies.

Dynamic structural neuroplasticity during and after epileptogenesis in a pilocarpine rat model of epilepsy

Background:The role of neuroplasticity in epilepsy has been widely studied in experimental models and human brain samples.However,the results are contradictory and it remains unclear if neuroplasticity is more related to the cause or the consequence of epileptic seizures.Clarifying this issue can provide insights into epilepsy therapies that target the disease mechanism and etiology rather than symptoms.Therefore,this study was aimed to investigate the dynamic changes of structural plasticity in a pilocarpine rat model of epilepsy.Methods:A single acute dose of pilocarpine(380 mg/kg,i.p.)was injected into adult male Wistar rats to induce status epilepticus(SE).Animal behavior was monitored for 2 h.Immunohistochemical staining was performed to evaluate neurogenesis in the CA3 and dentate gyrus(DG)regions of hippocampus using biomarkers Ki67 and doublecortin(DCX).The Golgi-Cox method was performed to analyze dendritic length and complexity.All experiments were performed in control rats(baseline),at 24 h after SE,on day 20 after SE(latent phase),after the first and 10th spontaneous recurrent seizures(SRS;chronic phase),and in non-epileptic rats(which did not manifest SRS 36 days after pilocarpine injection).Results:SE significantly increased the number of Ki67 and DCX-positive cells,suggesting neurogenesis during the latent phase.The dendritic complexity monitoring showed that plasticity was altered differently during epilepsy and epileptogenesis,suggesting that the two processes are completely separate at molecular and physiological levels.The numbers of spines and mushroom-type spines were increased in the latent phase.However,the dendritogenesis and spine numbers did not increase in rats that were unable to manifest spontaneous seizures after SE.Conclusion:All parameters of structural plasticity that increase during epileptogenesis,are reduced by spontaneous seizure occurrence,which suggests that the development of epilepsy involves maladaptive plastic changes.Therefore,the maladaptive plasticity biomarkers can be used to predict epilepsy before development of SRS in the cases of serious brain injury.

Biomarkers for epileptogenesis in patients with autoimmune epilepsy

Autoimmune epilepsy(AE)is a general term to describe recurrent seizures that have an immune-mediated origin.It is increasingly being recognized as a cause of epilepsy due to accumulating evidence supporting an immune-mediated pathogenesis in patients who have shown resistance to traditional antiepileptic drugs(AEDs).The diagnosis of AE is one of the exclusions.Currently,there are no strict diagnostic guidelines for AE,and it is similarly under-recognized.The importance of early diagnosis of AE cannot be overstated,as prompt immunotherapy is important for seizure reduction.Further investigations into potential biomarkers are needed for early detection of AE and include targeted immunotherapies in combination with AEDs.The goal of this review was to provide an overview of the following biomarkers that have been associated with AE:AMPAR,LGl1,CASPR2,DPPX,GABAAR,GABABR,GFAP,GlyR,mGluR5,NMDAR,VGCC(P/Q types),amphiphysin,ANNA-1,CRMP-5,GAD65,and Ma1/Ma2 antibodies.

Pro-and anti-epileptic roles of microglia

Microglia are brain-resident immune cells that contribute to the maintenance of brain homeostasis.In the epileptic brain, microglia show various activation phenotypes depending on the stage of epileptogenesis.Therefore, it remains unclear whether microglial activation acts in a pro-epileptic or anti-epileptic manner.In mesial temporal lobe epilepsy, one of the most common form of epilepsies, microglia exhibit at least two distinct morphologies, amoeboid shape and ramified shape.Amoeboid microglia are often found in sclerotic area, whereas ramified microglia are mainly found in non-sclerotic area;however, it remains unclear whether these structurally distinct microglia share separate roles in the epileptic brain.Here, we review the roles of the two distinct microglial phenotypes, focusing on their pro-and anti-epileptic roles in terms of inflammatory response, regulation of neurogenesis and microglia-neuron interaction.

Cyclooxygenase-2 inhibitor inhibits hippocampal synaptic reorganization in pilocarpine-induced status epilepticus rats

Objective： To examine modulations caused by cyclooxygenase-2 （COX-2） inhibitors on altered microenvironments and overbalanced neurotransmitters in pilocarpine-induced epileptic status rats and to investigate possible mechanisms. Methods： Celecoxib （a COX-2 inhibitor） was administered 45 min prior to pilocarpine administration. The effects of COX-2 inhibitors on mlPSCs （miniature GABAergic inhibitory postsynaptic currents） of CA3 pyramidal cells in the hippocampus were recorded. Expressions of COX-2, c-Fos, newly generated neurons, and activated microgliosis were analyzed by immunohistochemistry, and expressions of c^-subunit of y-amino butyric acid （GABAA） receptors and mitogen-activated protein kinase/extracellular signal-regulated protein kinase （MAPK/ERK） activity were detected by Western blotting. Results： Pretreatment with celecoxib showed protection against pilocarpine-induced seizures. Celecoxib prevented microglia activation in the hilus and inhibited the abnormal neurogenesis and astrogliosis in the hippocampus by inhibiting MAPK/ERK activity and c-Fos transcription. Celecoxib also up-regulated the expression of GABAA receptors. NS-398 （N-2-cyclohexyloxy-4-nitrophenyl-methanesulfonamide）, another COX-2 inhibitor, enhanced the frequency and decay time of mIPSCs. Conclusion： The COX-2 inhibitor celecoxib decreased neuronal excitability and prevented epileptogenesis in pilocarpine-induced status epilepticus rats. Celecoxib regulates synaptic reorganization by inhibiting astrogliosis and ectopic neurogenesis by attenuating MAPK/ERK signal activity, mediated by a GABAergic mechanism.

Glycolysis in energy metabolism during seizures

Studies have shown that glycolysis increases during seizures, and that the glycolytic metabolite lactic acid can be used as an energy source. However, how lactic acid provides energy for seizures and how it can participate in the termination of seizures remains unclear. We reviewed possible mechanisms of glycolysis involved in seizure onset. Results showed that lactic acid was involved in seizure onset and provided energy at early stages. As seizures progress, lactic acid reduces the pH of tissue and induces metabolic acidosis, which terminates the seizure. The specific mechanism of lactic acid-induced acidosis involves several aspects, which include lactic acid-induced inhibition of the glycolytic enzyme 6-diphosphate kinase-1, inhibition of the N-methyl-D-aspartate receptor, activation of the acid-sensitive 1A ion channel, strengthening of the receptive mechanism of the inhibitory neurotransmitter Y-aminobutyric acid, and changes in the intra- and extracellular environment.

The Contributions of Thrombospondin-1 to Epilepsy Formation

Epilepsy is a neural network disorder caused by uncontrolled neuronal hyperexcitability induced by an imbalance between excitatory and inhibitory networks.Abnormal synaptogenesis plays a vital role in the formation of overexcited networks.Recent evidence has confirmed that thrombospondin-1(TSP-1),mainly secreted by astrocytes,is a critical cytokine that regulates synaptogenesis during epileptogenesis.Furthermore,numerous studies have reported that TSP-1 is also involved in other processes,such as angiogenesis,neuroinflammation,and regulation of Ca^(2+)homeostasis,which are closely associated with the occurrence and development of epilepsy.In this review,we summarize the potential contributions of TSP-1 to epilepsy development.

One hour of pilocarpine-induced status epilepticus is sufficient to develop chronic epilepsy in mice, and is associated with mossy fiber sprouting but not neuronal death

Determining the minimal duration of status epilepticus （SE） that leads to the development of subsequent spontaneous seizures （i.e., epilepsy） is important, because it provides a critical timewindow for seizure intervention and epilepsy prevention. In the present study, male ICR （imprinting Control Region） mice were injected with pilocarpine to induce acute sei zures. SE was terminated by diazepam at 10 min, 30 min, 1 h, 2 h and 4 h after seizure onset. Spon taneous seizures occurred in the 1, 2 and 4 h SE groups, and the seizure frequency increased with the prolongation of SE. Similarly, the Morris water maze revealed that the escape latency was significantly increased and the number of target quadrant cross ings was markedly decreased in the 1, 2 and 4 h SE groups. Robust mossy fiber sprouting was observed in these groups, but not in the 10 or 30 min group. In contrast, FluoroJade B staining revealed significant cell death only in the 4 h SE group. The incidence and frequency of spontaneous seizures were corre lated with Timm score （P = 0.004） and escape latency （P = 0.004）. These data suggest that SE longer than one hour results in spontaneous motor seizures and memory deficits, and spontaneous seizures are likely associated with robust mossy fiber sprouting but not neuronal death.

Curcumin inhibits amygdaloid kindled seizures in rats

Background Curcumin can reduce the severity of seizures induced by kainate acid （KA）, but the role of curcumin in amygdaloid kindled models is still unknown. This study aimed to explore the effect of curcumin on the development of kindling in amygdaloid kindled rats. Methods With an amygdaloid kindled Sprague-Dawley （SD） rat model and an electrophysiological method, different doses of curcumin （10 mg·kg^-1·d^-1 and 30 mg·kg^-1·d^-1 as low dose groups, 100 mg·kg^-1·d^-1 and 300 mg·kg^-1·d^-1 as high dose groups） were administrated intraperitoneally during the whole kindling days, by comparison with the course of kindling, afterdischarge （AD） thresholds and the number of ADs to reach the stages of class I to V seizures in the rats between control and experimental groups. One-way or two-way ANOVA and Fisher＇s least significant difference post hoc test were used for statistical analyses. Results Curcumin （both 100 mg·kg^-1·d^-1 and 300 mg·kg^-1·d^-1 ） significantly inhibited the behavioral seizure development in the （19.80±2.25） and （21.70±2.21） stimulations respectively required to reach the kindled state. Rats treated with 100mg·kg^-1·d^-1 curcumin 30 minutes before kindling stimulation showed an obvious increase in the stimulation current intensity required to evoke AD from （703.3±85.9） μA to （960.0±116.5） μA during the progression to class V seizures. Rats treated with 300 mg·kg^-1·d^-1 curcumin showed a significant increase in the stimulation current intensity required to evoke AD from （735.0±65.2） μA to （867.0±93.4） μA during the progression to class V seizures. Rats treated with 300mg·kg^-1·d^-1 curcumin required much more evoked ADs to reach the stage of class both IV （as （199.83±12.47） seconds） and V seizures （as （210.66±10.68） seconds）. Rats treated with 100 mg·kg^-1·d^-1 curcumin required much more evoked ADs to reach the stage of class V seizures （as （219.56±18.24） seconds）. Conclusion Our study suggests that curcumin has a potential antiepileptogenic effect on kindling-induced epileptogenesis.

Biomolecular mechanisms of epileptic seizures and epilepsy: a review

Epilepsy is a recurring neurological disease caused by the abnormal electrical activity in the brain. This disease has caused about 50 new cases in 100,000 populations every year with the clinical manifestations of awareness loss, bruising, and mobility abnormalities. Due to the lack understanding of the pathophysiology behind the illness, a wide variety of medications are available to treat epilepsy. Epileptogenesis is the process by which a normally functioning brain undergoes alterations leading to the development of epilepsy, involving various factors. This is related to the inflammation which is driven by cytokines like IL-1 and tumor necrosis factor-α (TNF-α) leads to neuronal hyperexcitability. Pro-inflammatory cytokines from activated microglia and astrocytes in epileptic tissue initiate an inflammatory cascade, heightening neuronal excitability and triggering epileptiform activity. The blood-brain barrier (BBB) maintains central nervous system integrity through its tight endothelial connections, but inflammation impact BBB structure and function which leads to immune cell infiltration. The mammalian target of rapamycin (mTOR) pathway’s excessive activation influences epileptogenesis, impacting neuronal excitability, and synapse formation, with genetic mutations contributing to epilepsy syndromes and the modulation of autophagy playing a role in seizure onset. The apoptotic pathway contribute to cell death through glutamate receptor-mediated excitotoxicity, involving pro-apoptotic proteins like p53 and mitochondrial dysfunction, leading to the activation of caspases and the disruption of calcium homeostasis. Ionic imbalances within neural networks contribute to the complexity of epileptic seizures, involving alterations in voltage-gated sodium and potassium channels, and the formation of diverse ion channel subtypes. Epileptogenesis triggers molecular changes in hippocampus, including altered neurogenesis and enhanced expression of neurotrophic factors and proteins. Oxidative stress leads to cellular damage, disrupted antioxidant systems, and mitochondrial dysfunction, making it a key player in epileptogenesis and potential neuroprotective interventions. Thalamocortical circuitry disruption is central to absence epilepsy, the normal circuit becomes faulty and results in characteristic brain wave patterns.