In epilepsy research, emphasis is put on exploring non-neuronal targets such as astrocytic proteins, since many patients remain pharmacoresistant to current treatments, which almost all target ...neuronal mechanisms. This paper reviews available data on astrocytic connexin43 (Cx43) signaling in seizures and epilepsy. Cx43 is a widely expressed transmembrane protein and the constituent of gap junctions (GJs) and hemichannels (HCs), allowing intercellular and extracellular communication, respectively. A plethora of research papers show altered Cx43 mRNA levels, protein expression, phosphorylation state, distribution and/or functional coupling in human epileptic tissue and experimental models. Human Cx43 mutations are linked to seizures as well, as 30% of patients with oculodentodigital dysplasia (ODDD), a rare genetic condition caused by mutations in the GJA1 gene coding for Cx43 protein, exhibit neurological symptoms including seizures. Cx30/Cx43 double knock-out mice show increased susceptibility to evoked epileptiform events in brain slices due to impaired GJ-mediated redistribution of K
and glutamate and display a higher frequency of spontaneous generalized chronic seizures in an epilepsy model. Contradictory, Cx30/Cx43 GJs can traffic nutrients to high-energy demanding neurons and initiate astrocytic Ca
waves and hyper synchronization, thereby supporting proconvulsant effects. The general connexin channel blocker carbenoxolone and blockers from the fenamate family diminish epileptiform activity in vitro and improve seizure outcome in vivo. In addition, interventions with more selective peptide inhibitors of HCs display anticonvulsant actions. To conclude, further studies aiming to disentangle distinct roles of HCs and GJs are necessary and tools specifically targeting Cx43 HCs may facilitate the search for novel epilepsy treatments.
The communication between the immune and central nervous system (CNS) is affected in many neurological disorders. Peripheral injections of the endotoxin lipopolysaccharide (LPS) are widely used to ...study this communication: an LPS challenge leads to a biphasic syndrome that starts with acute sickness and is followed by persistent brain inflammation and chronic behavioral alterations such as depressive‐like symptoms. In vitro, the response to LPS treatment has been shown to involve enhanced expression of system
xc−. This cystine‐glutamate antiporter, with xCT as specific subunit, represents the main glial provider of extracellular glutamate in mouse hippocampus. Here we injected male xCT knockout and wildtype mice with a single intraperitoneal dose of 5 mg/kg LPS. LPS‐injection increased hippocampal xCT expression but did not alter the mainly astroglial localization of the xCT protein. Peripheral and central inflammation (as defined by cytokine levels and morphological activation of microglia) as well as LPS‐induced sickness and depressive‐like behavior were significantly attenuated in xCT‐deficient mice compared with wildtype mice. Our study is the first to demonstrate the involvement of system
xc− in peripheral and central inflammation in vivo and the potential therapeutic relevance of its inhibition in brain disorders characterized by peripheral and central inflammation, such as depression.
Main Points
Peripheral injections of LPS increase hippocampal xCT expression while decreasing GLT‐1 protein.
Neurological consequences of peripheral LPS, such as sickness, depressive‐like behavior and neuroinflammation, require a functional system
xc−.
Astrocytes are active players in higher brain function as they can release gliotransmitters, which are essential for synaptic plasticity. Various mechanisms have been proposed for gliotransmission, ...including vesicular mechanisms as well as non-vesicular ones, for example by passive diffusion via connexin hemichannels (HCs). We here investigated whether interfering with connexin43 (Cx43) HCs influenced hippocampal spatial memory. We made use of the peptide Gap19 that blocks HCs but not gap junction channels and is specific for Cx43. To this end, we microinfused transactivator of transcription linked Gap19 (TAT-Gap19) into the brain ventricle of male NMRI mice and assessed spatial memory in a Y maze. We found that the
blockade of Cx43 HCs did not affect the locomotor activity or spatial working memory in a spontaneous alternation Y maze task. Cx43 blockade did however significantly impair the spatial short-term memory in a delayed spontaneous alternation Y maze task. These results indicate that Cx43 HCs play a role in spatial short-term memory.
In view of the clinical need for new antiseizure drugs (ASDs) with novel modes of action, we used a zebrafish seizure model to screen the anticonvulsant activity of medicinal plants used by ...traditional healers in the Congo for the treatment of epilepsy, and identified a crude plant extract that inhibited pentylenetetrazol (PTZ)-induced seizures in zebrafish larvae. Zebrafish bioassay-guided fractionation of this anticonvulsant Fabaceae species, Indigofera arrecta, identified indirubin, a compound with known inhibitory activity of glycogen synthase kinase (GSK)-3, as the bioactive component. Indirubin, as well as the more potent and selective GSK-3 inhibitor 6-bromoindirubin-3′-oxime (BIO-acetoxime) were tested in zebrafish and rodent seizure assays. Both compounds revealed anticonvulsant activity in PTZ-treated zebrafish larvae, with electroencephalographic recordings revealing reduction of epileptiform discharges. Both indirubin and BIO-acetoxime also showed anticonvulsant activity in the pilocarpine rat model for limbic seizures and in the 6-Hz refractory seizure mouse model. Most interestingly, BIO-acetoxime also exhibited anticonvulsant actions in 6-Hz fully kindled mice. Our findings thus provide the first evidence for anticonvulsant activity of GSK-3 inhibition, thereby implicating GSK-3 as a potential therapeutic entry point for epilepsy. Our results also support the use of zebrafish bioassay-guided fractionation of antiepileptic medicinal plant extracts as an effective strategy for the discovery of new ASDs with novel mechanisms of action.
Parkinson's disease (PD) is a neurodegenerative disorder, characterized by a loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). Caloric restriction (CR) has been shown to ...exert ghrelin-dependent neuroprotective effects in the 1-methyl-4-phenyl-1,2,3,6-tetrathydropyridine (MPTP)-based animal model for PD. We here investigated whether CR is neuroprotective in the lactacystin (LAC) mouse model for PD, in which proteasome disruption leads to the destruction of the DA neurons of the SNc, and whether this effect is mediated via the ghrelin receptor. Adult male ghrelin receptor wildtype (WT) and knockout (KO) mice were maintained on an ad libitum (AL) diet or on a 30% CR regimen. After 3 weeks, LAC was injected unilaterally into the SNc, and the degree of DA neuron degeneration was evaluated 1 week later. In AL mice, LAC injection significanty reduced the number of DA neurons and striatal DA concentrations. CR protected against DA neuron degeneration following LAC injection. However, no differences were observed between ghrelin receptor WT and KO mice. These results indicate that CR can protect the nigral DA neurons from toxicity related to proteasome disruption; however, the ghrelin receptor is not involved in this effect.
Summary
Objective
Besides seizures, patients with epilepsy are affected by a variety of cognitive and psychiatric comorbidities that further impair their quality of life. The present study provides ...an in‐depth characterization of the behavioral alterations induced by 6 Hz corneal kindling. Furthermore, we correlate these behavioral changes to alterations in c‐Fos protein expression throughout the brain following kindling.
Methods
Adolescent male Naval Medical Research Institute (NMRI) mice were kindled via repetitive subconvulsive 6 Hz corneal stimulations until they reached the fully kindled state (defined as 10 consecutive generalized seizures). Afterwards we performed an elaborate battery of behavioral tests and we evaluated c‐Fos expression throughout the brain using immunohistochemistry.
Results
Fully kindled mice display an abnormal behavioral phenotype, characterized by basal and amphetamine‐induced hyperlocomotion, anhedonia, social withdrawal, and deficits in short‐ and long‐term memory. Moreover, 6 Hz corneal kindling enhances c‐Fos immunoreactivity in the visual, parahippocampal, and motor cortices and the limbic system, whereas c‐Fos+ cells are decreased in the orbital cortex of fully kindled mice.
Significance
The behavioral outcomes of 6 Hz corneal kindling cluster into 3 main categories: positive symptoms, negative symptoms, and cognitive impairment. These symptoms are accompanied by c‐Fos activation in relevant brain regions once the fully kindled state is established. Based on the face validity of this model, we speculate that 6 Hz corneal kindling can be used to model not only pharmacoresistant limbic seizures, but also several neurobehavioral comorbidities that affect patients with epilepsy.
Accumulating evidence shows a key function for astrocytic connexin43 (Cx43) signaling in epilepsy. However, the lack of experimental distinction between Cx43 gap junction channels (GJCs) and ...hemichannels (HCs) has impeded the identification of the exact contribution of either channel configurations to epilepsy. We therefore investigated whether TAT‐Gap19, a Cx mimetic peptide that inhibits Cx43 HCs but not the corresponding Cx43 GJCs, influences experimentally induced seizures in rodents. Dye uptake experiments in acute hippocampal slices of mice demonstrated that astroglial Cx43 HCs open in response to the chemoconvulsant pilocarpine and this was inhibited by TAT‐Gap19. In vivo, pilocarpine‐induced seizures as well as the accompanying increase in D‐serine microdialysate levels were suppressed by Cx43 HC inhibition. Moreover, the anticonvulsant action of TAT‐Gap19 was reversed by exogenous D‐serine administration, suggesting that Cx43 HC inhibition protects against seizures by lowering extracellular D‐serine levels. The anticonvulsive properties of Cx43 HC inhibition were further confirmed in electrical seizure mouse models, i.e. an acute 6 Hertz (Hz) model of refractory seizures and a chronic 6 Hz corneal kindling model. Collectively, these results indicate that Cx43 HCs play a role in seizures and underscore their potential as a novel and druggable target in epilepsy treatment.
Main Points
Astrocytic Cx43 HCs open in response to pilocarpine.
Cx43 HC inhibition diminishes pilocarpine‐induced seizures and extracellular D‐serine levels in vivo.
Cx43 HC inhibition suppresses electrically‐induced refractory seizures.
Summary Objective Besides seizures, patients with epilepsy are affected by a variety of cognitive and psychiatric comorbidities that further impair their quality of life. The present study provides ...an in-depth characterization of the behavioral alterations induced by 6 Hz corneal kindling. Furthermore, we correlate these behavioral changes to alterations in c-Fos protein expression throughout the brain following kindling. Methods Adolescent male Naval Medical Research Institute (NMRI) mice were kindled via repetitive subconvulsive 6 Hz corneal stimulations until they reached the fully kindled state (defined as 10 consecutive generalized seizures). Afterwards we performed an elaborate battery of behavioral tests and we evaluated c-Fos expression throughout the brain using immunohistochemistry. Results Fully kindled mice display an abnormal behavioral phenotype, characterized by basal and amphetamine-induced hyperlocomotion, anhedonia, social withdrawal, and deficits in short- and long-term memory. Moreover, 6 Hz corneal kindling enhances c-Fos immunoreactivity in the visual, parahippocampal, and motor cortices and the limbic system, whereas c-Fos+ cells are decreased in the orbital cortex of fully kindled mice. Significance The behavioral outcomes of 6 Hz corneal kindling cluster into 3 main categories: positive symptoms, negative symptoms, and cognitive impairment. These symptoms are accompanied by c-Fos activation in relevant brain regions once the fully kindled state is established. Based on the face validity of this model, we speculate that 6 Hz corneal kindling can be used to model not only pharmacoresistant limbic seizures, but also several neurobehavioral comorbidities that affect patients with epilepsy.
•Validation of 6Hz model for blood–brain barrier impermeable compounds.•Seizure evocation in NMRI mice at 49mA with ECT Unit 57800 Ugo Basile stimulator.•Seizure duration as novel, accurate parameter ...to express results in the 6Hz model.•Levetiracetam decreases seizure duration after i.p. and i.c.v. administration.•I.p. and i.c.v. administered sodium phenytoin does not alter seizure duration.
The six hertz (6Hz) refractory seizure model is considered an indispensable chain of the Anticonvulsant Screening Project. We here describe an adapted protocol using the intracerebroventricular (i.c.v.) delivery route, which will allow researchers to perform targetvalidation or proof-of-principle studies using promising compounds with unknown or limited blood–brain barrier permeability (e.g. neuropeptides and peptidomimetics) in this model. Seizures were induced by single application of a current intensity of 49mA to i.c.v.-implanted NMRI mice using an ECT Unit 57800 Ugo Basile stimulator. By applying these key parameters, c-Fos immunohistochemistry revealed the recruitment of the dentate gyrus, ratifying this model as a valuable tool for testing i.c.v. administered compounds against therapy-resistant seizures. This finding was further strengthened, since i.c.v. administration of levetiracetam suppressed 6Hz-evoked seizure severity but sodium phenytoin did not. We also propose to use “seizure duration” as an alternative, accurate parameter to express the results within this model.
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