Rapamycin suppresses mossy fiber sprouting but not seizure frequency in a mouse model of temporal lobe epilepsy

Mechanistic target of rapamycin complex 1 and 2 in human temporal lobe epilepsy

OBJECTIVE

Temporal lobe epilepsy (TLE) is a chronic epilepsy syndrome defined by seizures and progressive neurological disabilities, including cognitive impairments, anxiety, and depression. Here, human TLE specimens were investigated focusing on the mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) and complex 2 (mTORC2) activities in the brain, given that both pathways may represent unique targets for treatment.

METHODS

Surgically resected hippocampal and temporal lobe samples from therapy-resistant TLE patients were analyzed by western blotting to quantify the expression of established mTORC1 and mTORC2 activity markers and upstream or downstream signaling pathways involving the two complexes. Histological and immunohistochemical techniques were used to assess hippocampal and neocortical structural abnormalities and cell-specific expression of individual biomarkers. Samples from patients with focal cortical dysplasia (FCD) type II served as positive controls.

RESULTS

We found significantly increased expression of phospho-mTOR (Ser2448), phospho-S6 (Ser235/236), phospho-S6 (Ser240/244), and phospho-Akt (Ser473) in TLE samples compared to controls, consistent with activation of both mTORC1 and mTORC2. Our work identified the phosphoinositide 3-kinase and Ras/extracellular signal-regulated kinase signaling pathways as potential mTORC1 and mTORC2 upstream activators. In addition, we found that overactive mTORC2 signaling was accompanied by induction of two protein kinase B-dependent prosurvival pathways, as evidenced by increased inhibitory phosphorylation of forkhead box class O3a (Ser253) and glycogen synthase kinase 3 beta (Ser9).

INTERPRETATION

Our data demonstrate that mTOR signaling is significantly dysregulated in human TLE, offering new targets for pharmacological interventions. Specifically, clinically available drugs that suppress mTORC1 without compromising mTOR2 signaling, such as rapamycin and its analogs, may represent a new group of antiepileptogenic agents in TLE patients. Ann Neurol 2018;83:311-327.

Epilepsy-associated alterations in hippocampal excitability

The hippocampus exhibits a wide range of epilepsy-related abnormalities and is situated in the mesial temporal lobe, where limbic seizures begin. These abnormalities could affect membrane excitability and lead to overstimulation of neurons. Multiple overlapping processes refer to neural homeostatic responses develop in neurons that work together to restore neuronal firing rates to control levels. Nevertheless, homeostatic mechanisms are unable to restore normal neuronal excitability, and the epileptic hippocampus becomes hyperexcitable or hypoexcitable. Studies show that there is hyperexcitability even before starting recurrent spontaneous seizures, suggesting although hippocampal hyperexcitability may contribute to epileptogenesis, it alone is insufficient to produce epileptic seizures. This supports the concept that the hippocampus is not the only substrate for limbic seizure onset, and a broader hyperexcitable limbic structure may contribute to temporal lobe epilepsy (TLE) seizures. Nevertheless, seizures also occur in conditions where the hippocampus shows a hypoexcitable phenotype. Since TLE seizures most often originate in the hippocampus, it could therefore be assumed that both hippocampal hypoexcitability and hyperexcitability are undesirable states that make the epileptic hippocampal network less stable and may, under certain conditions, trigger seizures.

Converging early responses to brain injury pave the road to epileptogenesis

Epilepsy, characterized by recurrent seizures and abnormal electrical activity in the brain, is one of the most prevalent brain disorders. Over two million people in the United States have been diagnosed with epilepsy and 3% of the general population will be diagnosed with it at some point in their lives. While most developmental epilepsies occur due to genetic predisposition, a class of "acquired" epilepsies results from a variety of brain insults. A leading etiological factor for epilepsy that is currently on the rise is traumatic brain injury (TBI), which accounts for up to 20% of all symptomatic epilepsies. Remarkably, the presence of an identified early insult that constitutes a risk for development of epilepsy provides a therapeutic window in which the pathological processes associated with brain injury can be manipulated to limit the subsequent development of recurrent seizure activity and epilepsy. Recent studies have revealed diverse pathologies, including enhanced excitability, activated immune signaling, cell death, and enhanced neurogenesis within a week after injury, suggesting a period of heightened adaptive and maladaptive plasticity. An integrated understanding of these processes and their cellular and molecular underpinnings could lead to novel targets to arrest epileptogenesis after trauma. This review attempts to highlight and relate the diverse early changes after trauma and their role in development of epilepsy and suggests potential strategies to limit neurological complications in the injured brain.

Cerebral Malaria Causes Enduring Behavioral and Molecular Changes in Mice Brain Without Causing Gross Histopathological Damage

Malaria, parasitic disease considered a major health public problem, is caused by Plasmodium protozoan genus and transmitted by the bite of infected female Anopheles mosquito genus. Cerebral malaria (CM) is the most severe presentation of malaria, caused by P. falciparum and responsible for high mortality and enduring development of cognitive deficits which may persist even after cure and cessation of therapy. In the present study we evaluated selected behavioral, neurochemical and neuropathologic parameters after rescue from experimental cerebral malaria caused by P. berghei ANKA in C57BL/6 mice. Behavioral tests showed impaired nest building activity as well as increased marble burying, indicating that natural behavior of mice remains altered even after cure of infection. Regarding the neurochemical data, we found decreased α2/α3 Na⁺,K⁺-ATPase activity and increased immunoreactivity of phosphorylated Na⁺,K⁺-ATPase at Ser⁹⁴³ in cerebral cortex after CM. In addition, [³H]-Flunitrazepam binding assays revealed a decrease of benzodiazepine/GABA_A receptor binding sites in infected animals. Moreover, in hippocampus, dot blot analysis revealed increased levels of protein carbonyls, suggesting occurrence of oxidative damage to proteins. Interestingly, no changes in the neuropathological markers Fluoro-Jade C, Timm staining or IBA-1 were detected. Altogether, present data indicate that behavioral and neurochemical alterations persist even after parasitemia clearance and CM recovery, which agrees with available clinical findings. Some of the molecular mechanisms reported in the present study may underlie the behavioral changes and increased seizure susceptibility that persist after recovery from CM and may help in the future development of therapeutic strategies for CM sequelae.

Brain Injury-Induced Synaptic Reorganization in Hilar Inhibitory Neurons Is Differentially Suppressed by Rapamycin

Following traumatic brain injury (TBI), treatment with rapamycin suppresses mammalian (mechanistic) target of rapamycin (mTOR) activity and specific components of hippocampal synaptic reorganization associated with altered cortical excitability and seizure susceptibility. Reemergence of seizures after cessation of rapamycin treatment suggests, however, an incomplete suppression of epileptogenesis. Hilar inhibitory interneurons regulate dentate granule cell (DGC) activity, and de novo synaptic input from both DGCs and CA3 pyramidal cells after TBI increases their excitability but effects of rapamycin treatment on the injury-induced plasticity of interneurons is only partially described. Using transgenic mice in which enhanced green fluorescent protein (eGFP) is expressed in the somatostatinergic subset of hilar inhibitory interneurons, we tested the effect of daily systemic rapamycin treatment (3 mg/kg) on the excitability of hilar inhibitory interneurons after controlled cortical impact (CCI)-induced focal brain injury. Rapamycin treatment reduced, but did not normalize, the injury-induced increase in excitability of surviving eGFP+ hilar interneurons. The injury-induced increase in response to selective glutamate photostimulation of DGCs was reduced to normal levels after mTOR inhibition, but the postinjury increase in synaptic excitation arising from CA3 pyramidal cell activity was unaffected by rapamycin treatment. The incomplete suppression of synaptic reorganization in inhibitory circuits after brain injury could contribute to hippocampal hyperexcitability and the eventual reemergence of the epileptogenic process upon cessation of mTOR inhibition. Further, the cell-selective effect of mTOR inhibition on synaptic reorganization after CCI suggests possible mechanisms by which rapamycin treatment modifies epileptogenesis in some models but not others.

Seizure frequency correlates with loss of dentate gyrus GABAergic neurons in a mouse model of temporal lobe epilepsy

Epilepsy occurs in one of 26 people. Temporal lobe epilepsy is common and can be difficult to treat effectively. It can develop after brain injuries that damage the hippocampus. Multiple pathophysiological mechanisms involving the hippocampal dentate gyrus have been proposed. This study evaluated a mouse model of temporal lobe epilepsy to test which pathological changes in the dentate gyrus correlate with seizure frequency and help prioritize potential mechanisms for further study. FVB mice (n = 127) that had experienced status epilepticus after systemic treatment with pilocarpine 31-61 days earlier were video-monitored for spontaneous, convulsive seizures 9 hr/day every day for 24-36 days. Over 4,060 seizures were observed. Seizure frequency ranged from an average of one every 3.6 days to one every 2.1 hr. Hippocampal sections were processed for Nissl stain, Prox1-immunocytochemistry, GluR2-immunocytochemistry, Timm stain, glial fibrillary acidic protein-immunocytochemistry, glutamic acid decarboxylase in situ hybridization, and parvalbumin-immunocytochemistry. Stereological methods were used to measure hilar ectopic granule cells, mossy cells, mossy fiber sprouting, astrogliosis, and GABAergic interneurons. Seizure frequency was not significantly correlated with the generation of hilar ectopic granule cells, the number of mossy cells, the extent of mossy fiber sprouting, the extent of astrogliosis, or the number of GABAergic interneurons in the molecular layer or hilus. Seizure frequency significantly correlated with the loss of GABAergic interneurons in or adjacent to the granule cell layer, but not with the loss of parvalbumin-positive interneurons. These findings prioritize the loss of granule cell layer interneurons for further testing as a potential cause of temporal lobe epilepsy.

Rapamycin inhibits spermatogenesis by changing the autophagy status through suppressing mechanistic target of rapamycin-p70S6 kinase in male rats

Rapamycin (sirolimus) is an antiproliferative drug that has been widely used in the clinic as an immunosuppressant and a potential anticancer agent. Certain reports have indicated that rapamycin may induce male infertility through impairing sperm quality. The present study investigated the mechanism of male infertility caused by rapamycin and examined whether withdrawal of rapamycin could recover the number of sperm in rats. Male Sprague-Dawley rats (n=100) were divided randomly into 5 groups: 3 rapamycin-treated groups (2, 4 and 6 mg/kg) and 2 control groups [Blank and dimethyl sulfoxide (DMSO)]. Organ coefficients of the testes, number of sperm and hematoxylin-eosin staining analyses demonstrated that rapamycin treatment markedly damaged the structure of the seminiferous tubule and reduced the number of sperm. Immunohistochemistry of mechanistic target of rapamycin (mTOR) and Ki67 in testes tissue, and western blotting of phosphorylated-p70S6K and p70S6K, supported the hypothesis that rapamycin causes sperm reduction through inhibiting proliferation of spermatogonia. Unfortunately, 24 weeks after cessation of rapamycin treatment, only the number of sperm in 2 mg/kg group was restored back to the normal level. In addition, to the best of our knowledge, the present study was the first to demonstrate that low doses rapamycin leads to activation of autophagy in rat testes. This may be a self-protective mechanism of the cell in response to external stress. Thus, spermatogenesis can be recovered in the testes from rats in the low dose group. High doses of rapamycin resulted in excessive consumption of autophagy proteins, and the damage could not be compensated. In addition, it was revealed that cell apoptosis increased after treatment with rapamycin. In conclusion, the present study demonstrated that rapamycin inhibits spermatogenesis through suppressing phosphorylation of p70S6K and changing the autophagy status, ultimately reducing the number of sperm. These findings provide important guidance for the clinical application of rapamycin.

Novel therapeutic approaches for disease-modification of epileptogenesis for curing epilepsy

This article describes the recent advances in epileptogenesis and novel therapeutic approaches for the prevention of epilepsy, with a special emphasis on the pharmacological basis of disease-modification of epileptogenesis for curing epilepsy. Here we assess animal studies and human clinical trials of epilepsy spanning 1982-2016. Epilepsy arises from a number of neuronal factors that trigger epileptogenesis, which is the process by which a brain shifts from a normal physiologic state to an epileptic condition. The events precipitating these changes can be of diverse origin, including traumatic brain injury, cerebrovascular damage, infections, chemical neurotoxicity, and emergency seizure conditions such as status epilepticus. Expectedly, the molecular and system mechanisms responsible for epileptogenesis are not well defined or understood. To date, there is no approved therapy for the prevention of epilepsy. Epigenetic dysregulation, neuroinflammation, and neurodegeneration appear to trigger epileptogenesis. Targeted drugs are being identified that can truly prevent the development of epilepsy in at-risk people. The promising agents include rapamycin, COX-2 inhibitors, TRK inhibitors, epigenetic modulators, JAK-STAT inhibitors, and neurosteroids. Recent evidence suggests that neurosteroids may play a role in modulating epileptogenesis. A number of promising drugs are under investigation for the prevention or modification of epileptogenesis to halt the development of epilepsy. Some drugs in development appear rational for preventing epilepsy because they target the initial trigger or related signaling pathways as the brain becomes progressively more prone to seizures. Additional research into the target validity and clinical investigation is essential to make new frontiers in curing epilepsy.

New Insights on Temporal Lobe Epilepsy Based on Plasticity-Related Network Changes and High-Order Statistics

Epilepsy is a disorder of the brain characterized by the predisposition to generate recurrent unprovoked seizures, which involves reshaping of neuronal circuitries based on intense neuronal activity. In this review, we first detailed the regulation of plasticity-associated genes, such as ARC, GAP-43, PSD-95, synapsin, and synaptophysin. Indeed, reshaping of neuronal connectivity after the primary, acute epileptogenesis event increases the excitability of the temporal lobe. Herein, we also discussed the heterogeneity of neuronal populations regarding the number of synaptic connections, which in the theoretical field is commonly referred as degree. Employing integrate-and-fire neuronal model, we determined that in addition to increased synaptic strength, degree correlations might play essential and unsuspected roles in the control of network activity. Indeed, assortativity, which can be described as a condition where high-degree correlations are observed, increases the excitability of neural networks. In this review, we summarized recent topics in the field, and data were discussed according to newly developed or unusual tools, as provided by mathematical graph analysis and high-order statistics. With this, we were able to present new foundations for the pathological activity observed in temporal lobe epilepsy.

Reorganization of the septohippocampal cholinergic fiber system in experimental epilepsy

The septohippocampal cholinergic neurotransmission has long been implicated in seizures, but little is known about the structural features of this projection system in epileptic brain. We evaluated the effects of experimental epilepsy on the areal density of cholinergic terminals (fiber varicosities) in the dentate gyrus. For this purpose, we used two distinct post-status epilepticus rat models, in which epilepsy was induced with injections of either kainic acid or pilocarpine. To visualize the cholinergic fibers, we used brain sections immunostained for the vesicular acetylcholine transporter. It was found that the density of cholinergic fiber varicosities was higher in epileptic rats versus control rats in the inner and outer zones of the dentate molecular layer, but it was reduced in the dentate hilus. We further evaluated the effects of kainate treatment on the total number, density, and soma volume of septal cholinergic cells, which were visualized in brain sections stained for either vesicular acetylcholine transporter or choline acetyltransferase (ChAT). Both the number of septal cells with cholinergic phenotype and their density were increased in epileptic rats when compared to control rats. The septal cells stained for vesicular acetylcholine transporter, but not for ChAT, have enlarged perikarya in epileptic rats. These results revealed previously unknown details of structural reorganization of the septohippocampal cholinergic system in experimental epilepsy, involving fiber sprouting into the dentate molecular layer and a parallel fiber retraction from the dentate hilus. We hypothesize that epilepsy-related neuroplasticity of septohippocampal cholinergic neurons is capable of increasing neuronal excitability of the dentate gyrus.

Short-Term Depression of Sprouted Mossy Fiber Synapses from Adult-Born Granule Cells

Epileptic seizures potently modulate hippocampal adult neurogenesis, and adult-born dentate granule cells contribute to the pathologic retrograde sprouting of mossy fiber axons, both hallmarks of temporal lobe epilepsy. The characteristics of these sprouted synapses, however, have been largely unexplored, and the specific contribution of adult-born granule cells to functional mossy fiber sprouting is unknown, primarily due to technical barriers in isolating sprouted mossy fiber synapses for analysis. Here, we used DcxCreER^T2 transgenic mice to permanently pulse-label age-defined cohorts of granule cells born either before or after pilocarpine-induced status epilepticus (SE). Using optogenetics, we demonstrate that adult-born granule cells born before SE form functional recurrent monosynaptic excitatory connections with other granule cells. Surprisingly, however, although healthy mossy fiber synapses in CA3 are well characterized "detonator" synapses that potently drive postsynaptic cell firing through their profound frequency-dependent facilitation, sprouted mossy fiber synapses from adult-born cells exhibited profound frequency-dependent depression, despite possessing some of the morphological hallmarks of mossy fiber terminals. Mature granule cells also contributed to functional mossy fiber sprouting, but exhibited less synaptic depression. Interestingly, granule cells born shortly after SE did not form functional excitatory synapses, despite robust sprouting. Our results suggest that, although sprouted mossy fibers form recurrent excitatory circuits with some of the morphological characteristics of typical mossy fiber terminals, the functional characteristics of sprouted synapses would limit the contribution of adult-born granule cells to hippocampal hyperexcitability in the epileptic hippocampus.SIGNIFICANCE STATEMENT In the hippocampal dentate gyrus, seizures drive retrograde sprouting of granule cell mossy fiber axons. We directly activated sprouted mossy fiber synapses from adult-born granule cells to study their synaptic properties. We reveal that sprouted synapses from adult-born granule cells have a diminished ability to sustain recurrent excitation in the epileptic hippocampus, which raises questions about the role of sprouting and adult neurogenesis in sustaining seizure-like activity.

A calpain inhibitor ameliorates seizure burden in an experimental model of temporal lobe epilepsy

In this study, we used the pilocarpine model of epilepsy to evaluate the involvement of calpain dysregulation on epileptogenesis. Detection of spectrin breakdown products (SBDPs, a hallmark of calpain activation) after induction of pilocarpine-induced status epilepticus (SE) and before appearance of spontaneous seizure suggested the existence of sustained calpain activation during epileptogenesis. Acute treatment with a cell permeable inhibitor of calpain, MDL-28170, resulted in a partial but significant reduction on seizure burden. The reduction on seizure burden was associated with a limited reduction on the generation of SBDPs but was correlated with a reduction in astrocytosis, microglia activation and cell sprouting. Together, these observations provide evidence for the role of calpain in epileptogenesis. In addition, provide proof-of-principle for the use of calpain inhibitors as a novel strategy to prevent epileptic seizures and its associated pathologies.

Bumetanide reduce the seizure susceptibility induced by pentylenetetrazol via inhibition of aberrant hippocampal neurogenesis in neonatal rats after hypoxia-ischemia

Hypoxia-ischemia brain damage (HIBD) is one of prevalent causes of neonatal mortality and morbidity. Our data demonstrated that hypoxia-ischemia (HI) induced Na⁺-K⁺-Cl^--co-transporter 1 (NKCC1) increasing in hippocampus. Previous studies demonstrated that NKCC1 regulates various stages of neurogenesis. In this study, we studied the role of increased NKCC1 in regulating of HI-induced neurogenesis. HIBD model was established in 7days old Sprague-Dawley rat pup, and the expression of NKCC1 was detected by western blot and qPCR. Brain electrical activity in freely rats was monitored by electroencephalography (EEG) recordings. HI-induced neurogenesis was detected by immunofluorescence staining. Neurobehavioral test was to investigate the neuro-protective role of bumetanide, an inhibitor of NKCC1, on neonatal rats after HI. The results showed that bumetanide treatment significantly reduced brain electrical activity and the seizure stage of epilepsy induced by pentylenetetrazol (PTZ) in vivo after HI. In addition, bumetanide restored aberrant hippocampal neurogenesis and associated cognitive function. Our data demonstrated that bumetanide reduces the susceptibility of epilepsy induced by PTZ in rats suffering from HI injury during neonatal period via restoring the ectopic newborn neurons in dentate gyrus (DG) and cognitive function.

Axonal sprouting in commissurally projecting parvalbumin-expressing interneurons

Previous research has shown that in vivo on-demand optogenetic stimulation of inhibitory interneurons expressing parvalbumin (PV) is sufficient to suppress seizures in a mouse model of temporal lobe epilepsy (TLE). Surprisingly, this intervention was capable of suppressing seizures when PV-expressing interneurons were stimulated ipsilateral or contralateral to the presumed seizure focus, raising the possibility of commissural inhibition in TLE. There are mixed reports regarding commissural PV interneuron projections in the healthy hippocampus, and it was previously unknown whether these connections would be maintained or modified following the network reorganization associated with TLE. Using retrograde labeling and viral vector technology in both sexes and the intrahippocampal kainate mouse model of TLE, we therefore examined these issues. Our results reveal that healthy controls possess a population of commissurally projecting hippocampal PV interneurons. Two weeks post kainate injection, we observed a slight, but not statistically significant decrease in retrogradely labeled PV interneurons in the hippocampus contralateral to kainate and tracer injection. By 6 months post kainate, however, there was a significant increase in retrogradely labeled PV interneurons, suggesting commissural inhibitory axonal sprouting. Using viral green fluorescent protein expression selectively in PV neurons, we demonstrated sprouting of commissural PV projections in the dentate gyrus of the kainate-injected hippocampus 6 months post kainate. These findings indicate that PV interneurons supply direct inhibition to the contralateral hippocampus and undergo sprouting in a mouse model of TLE. © 2017 Wiley Periodicals, Inc.

Inflammatory aspects of epileptogenesis: contribution of molecular inflammatory mechanisms

OBJECTIVE

Epilepsy is a chronic neurological disease characterised with seizures. The aetiology of the most generalised epilepsies cannot be explicitly determined and the seizures are pronounced to be genetically determined by disturbances of receptors in central nervous system. Besides, neurotransmitter distributions or other metabolic problems are supposed to involve in epileptogenesis. Lack of adequate data about pharmacological agents that have antiepileptogenic effects point to need of research on this field. Thus, in this review, inflammatory aspects of epileptogenesis has been focussed via considering several concepts like role of immune system, blood-brain barrier and antibody involvement in epileptogenesis.

METHODS

We conducted an evidence-based review of the literatures in order to evaluate the possible participation of inflammatory processes to epileptogenesis and also, promising agents which are effective to these processes. We searched PubMed database up to November 2015 with no date restrictions.

RESULTS

In the present review, 163 appropriate articles were included. Obtained data suggests that inflammatory processes participate to epileptogenesis in several ways like affecting fibroblast growth factor-2 and tropomyosin receptor kinase B signalling pathways, detrimental proinflammatory pathways [such as the interleukin-1 beta (IL-1β)-interleukin-1 receptor type 1 (IL-1R1) system], mammalian target of rapamycin pathway, microglial activities, release of glial inflammatory proteins (such as macrophage inflammatory protein, interleukin 6, C-C motif ligand 2 and IL-1β), adhesion molecules that are suggested to function in signalling pathways between neurons and microglia and also linkage between these molecules and proinflammatory cytokines.

CONCLUSION

The literature research indicated that inflammation is a part of epileptogenesis. For this reason, further studies are necessary for assessing agents that will be effective in clinical use for therapeutic treatment of epileptogenesis.

Molecular neurobiology of mTOR

Mammalian/mechanistic target of rapamycin (mTOR) is a serine-threonine kinase that controls several important aspects of mammalian cell function. mTOR activity is modulated by various intra- and extracellular factors; in turn, mTOR changes rates of translation, transcription, protein degradation, cell signaling, metabolism, and cytoskeleton dynamics. mTOR has been repeatedly shown to participate in neuronal development and the proper functioning of mature neurons. Changes in mTOR activity are often observed in nervous system diseases, including genetic diseases (e.g., tuberous sclerosis complex, Pten-related syndromes, neurofibromatosis, and Fragile X syndrome), epilepsy, brain tumors, and neurodegenerative disorders (Alzheimer's disease, Parkinson's disease, and Huntington's disease). Neuroscientists only recently began deciphering the molecular processes that are downstream of mTOR that participate in proper function of the nervous system. As a result, we are gaining knowledge about the ways in which aberrant changes in mTOR activity lead to various nervous system diseases. In this review, we provide a comprehensive view of mTOR in the nervous system, with a special focus on the neuronal functions of mTOR (e.g., control of translation, transcription, and autophagy) that likely underlie the contribution of mTOR to nervous system diseases.

Neural Progenitor Cells Rptor Ablation Impairs Development but Benefits to Seizure-Induced Behavioral Abnormalities

AIMS

Previous study suggests that mTOR signaling pathway may play an important role in epileptogenesis. The present work was designed to explore the contribution of raptor protein to the development of epilepsy and comorbidities.

METHODS

Mice with conditional knockout of raptor protein were generated by cross-bred Rptor^flox/flox mice with nestin-CRE mice. The expression of raptor protein was analyzed by Western blotting in brain tissue samples. Neuronal death and mossy fiber sprouting were detected by FJB staining and Timm staining, respectively. Spontaneous seizures were recorded by EEG-video system. Morris water maze, open field test, and excitability test were used to study the behaviors of Rptor CKO mice.

RESULTS

As the consequence of deleting Rptor, downstream proteins of raptor in mTORC1 signaling were partly blocked. Rptor CKO mice exhibited decrease in body and brain weight under 7 weeks old and accordingly, cortical layer thickness. After kainic acid (KA)-induced status epilepticus, overactivation of mTORC1 signaling was markedly reversed in Rptor CKO mice. Although low frequency of spontaneous seizure and seldom neuronal cell death were observed in both Rptor CKO and control littermates, KA seizure-induced mossy fiber spouting were attenuated in Rptor CKO mice. Additionally, cognitive-deficit and anxiety-like behavior after KA-induced seizures were partly reversed in Rptor CKO mice.

CONCLUSION

Loss of the Rptor gene in mice neural progenitor cells affects normal development in young age and may contribute to alleviate KA seizure-induced behavioral abnormalities, suggesting that raptor protein plays an important role in seizure comorbidities.

Neuroprotective levels of IGF-1 exacerbate epileptogenesis after brain injury

Exogenous Insulin-Like Growth Factor-1 (IGF-1) is neuroprotective in animal models of brain injury, and has been considered as a potential therapeutic. Akt-mTOR and MAPK are downstream targets of IGF-1 signaling that are activated after brain injury. However, both brain injury and mTOR are linked to epilepsy, raising the possibility that IGF-1 may be epileptogenic. Here, we considered the role of IGF-1 in development of epilepsy after brain injury, using the organotypic hippocampal culture model of post-traumatic epileptogenesis. We found that IGF-1 was neuroprotective within a few days of injury but that long-term IGF-1 treatment was pro-epileptic. Pro-epileptic effects of IGF-1 were mediated by Akt-mTOR signaling. We also found that IGF-1 - mediated increase in epileptic activity led to neurotoxicity. The dualistic nature of effects of IGF-1 treatment demonstrates that anabolic enhancement through IGF-1 activation of mTOR cascade can be beneficial or harmful depending on the stage of the disease. Our findings suggest that epilepsy risk may need to be considered in the design of neuroprotective treatments for brain injury.

Rapamycin Reduced Ischemic Brain Damage in Diabetic Animals Is Associated with Suppressions of mTOR and ERK1/2 Signaling

The objectives of the present study are to investigate the activation of mTOR and ERK1/2 signaling after cerebral ischemia in diabetic rats and to examine the neuroprotective effects of rapamycin. Ten minutes transient global cerebral ischemia was induced in straptozotocin-induced diabetic hyperglycemic rats and non-diabetic, euglycemic rats. Brain samples were harvested after 16 h of reperfusion. Rapamycin or vehicle was injected 1 month prior to the induction of ischemia. The results showed that diabetes increased ischemic neuronal cell death and associated with elevations of p-P70S6K and Ras/ERK1/2 and suppression of p-AMPKα. Rapamycin ameliorated diabetes-enhanced ischemic brain damage and suppressed phosphorylation of P70S6K and ERK1/2. It is concluded that diabetes activates mTOR and ERK1/2 signaling pathways in rats subjected to transient cerebral ischemia and inhibition of mTOR by rapamycin reduces ischemic brain damage and suppresses the mTOR and ERK1/2 signaling in diabetic settings.

Optogenetic study of networks in epilepsy

Currently, approximately 30% of patients with epilepsy do not have adequate seizure control. A greater understanding of the underlying mechanisms by which seizures start or propagate could lead to new therapeutic strategies. The recent development of optogenetics, because of its unprecedented precision for controlling activity within distinct neuronal populations, has revolutionized neuroscience, including epilepsy research. This Review discusses recent breakthroughs made with optogenetics in epilepsy research. These breakthroughs include new insights into the key roles that different cell types play in mediating seizures as well as in the development of epilepsy. Subsequently, we discuss how targeting different brain regions and cell populations has opened up the possibility of highly specific therapies that can stop seizures on demand. Finally, we illustrate how combining newly available neuroscience tools with whole-brain imaging techniques will allow researchers to understand better the spread of seizures on a network level. © 2016 The Authors. Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Organization and control of epileptic circuits in temporal lobe epilepsy

When studying the pathological mechanisms of epilepsy, there are a seemingly endless number of approaches from the ultrastructural level-receptor expression by EM-to the behavioral level-comorbid depression in behaving animals. Epilepsy is characterized as a disorder of recurrent seizures, which are defined as "a transient occurrence of signs and/or symptoms due to abnormal excessive or synchronous neuronal activity in the brain" (Fisher et al., 2005). Such abnormal activity typically does not occur in a single isolated neuron; rather, it results from pathological activity in large groups-or circuits-of neurons. Here we choose to focus on two aspects of aberrant circuits in temporal lobe epilepsy: their organization and potential mechanisms to control these pathological circuits. We also look at two scales: microcircuits, ie, the relationship between individual neurons or small groups of similar neurons, and macrocircuits, ie, the organization of large-scale brain regions. We begin by summarizing the large body of literature that describes the stereotypical anatomical changes in the temporal lobe-ie, the anatomical basis of alterations in microcircuitry. We then offer a brief introduction to graph theory and describe how this type of mathematical analysis, in combination with computational neuroscience techniques and using parameters obtained from experimental data, can be used to postulate how microcircuit alterations may lead to seizures. We then zoom out and look at the changes which are seen over large whole-brain networks in patients and animal models, and finally we look to the future.

1400W, a highly selective inducible nitric oxide synthase inhibitor is a potential disease modifier in the rat kainate model of temporal lobe epilepsy

Status epilepticus (SE) initiates epileptogenesis to transform normal brain to epileptic state which is characterized by spontaneous recurrent seizures (SRS). Prior to SRS, progressive changes occur in the brain soon after SE, for example, loss of blood-brain barrier (BBB) integrity, neuronal hyper-excitability (epileptiform spiking), neuroinflammation [reactive gliosis, high levels of reactive oxygen/nitrogen species (ROS/RNS)], neurodegeneration and synaptic re-organization. Our hypothesis was that modification of early epileptogenic events will alter the course of disease development and its progression. We tested the hypothesis in the rat kainate model of chronic epilepsy using a novel disease modifying drug, 1400W, a highly selective inhibitor of inducible nitric oxide synthase (iNOS/NOS-II). In an in vitro mouse brain slice model, using a multi-electrode array system, co-application of 1400W with kainate significantly suppressed kainate-induced epileptiform spiking. In the rats, in vivo, 4h after the induction of SE with kainate, 1400W (20mg/kg, i.p.) was administered twice daily for three days to target early events of epileptogenesis. The rats were subjected to continuous (24/7) video-EEG monitoring, remotely, for six months from epidurally implanted cortical electrodes. The 1400W treatment significantly reduced the epileptiform spike rate during the first 12-74h post-SE, which resulted in >90% reduction in SRS in long-term during the six month period when compared to the vehicle-treated control group (257±113 versus 19±10 episodes). Immunohistochemistry (IHC) of brain sections at seven days and six months revealed a significant reduction in; reactive astrogliosis and microgliosis (M1 type), extravascular serum albumin (a marker for BBB leakage) and neurodegeneration in the hippocampus, amygdala and entorhinal cortex in the 1400W-treated rats when compared to the vehicle control. In the seven day group, hippocampal Western blots revealed downregulation of inwardly-rectifying potassium (Kir 4.1) channels and glutamate transporter-1 (GLT-1) levels in the vehicle group, and 1400W treatment partially reversed Kir 4.1 levels, however, GLT-1 levels were unaffected. In the six month group, a significant reduction in mossy fiber staining intensity in the inner molecular layer of the dentate gyrus was observed in the 1400W-treated group. Overall these findings demonstrate that 1400W, by reducing the epileptiform spike rate during the first three days of post-insult, potentially modifies epileptogenesis and the severity of chronic epilepsy in the rat kainate model of TLE.

Non-invasive treatment options for focal cortical dysplasia

Focal cortical dysplasia (FCD) presents a strong clinical challenge especially for the treatment of the associated epilepsy. Epilepsy in FCD is often treatment-resistant and constitutes 50% of treatment-resistant cases. Antiepileptic drugs (AEDs) have been widely used in the treatment of FCD. However, evidence to suggest their specific effect on the treatment of FCD remains to be established. In view of this resistance, several alternative treatments have been suggested. Although treatment currently involves surgical management, non-invasive treatments have been identified. The aim of the present review, was to assess non-invasive management strategies including, i) mammalian target of rapamycin (mTOR) inhibitors, ii) ketogenic diet (KD), and iii) vagus nerve stimulation (VNS). In addition, we discussed the literature available regarding the use of AEDs in FCD. Experiments conducted with mammals detailing rapamycin gene mutations in FCD have produced vital information for exploring treatment options using mTOR inhibitors. Of note is the importance of KD in children with FCD. This diet has been shown to modify disease progression by attenuating chromatin modification, a master regulator for gene expression and functional adaptation of the cell. FCD has also been studied widely with neurostimulation techniques. The outcomes of these techniques have been found to be variable. For widespread dysplasias, VNS has been shown to produce responder rates of >50%. Nevertheless, non-invasive cranial nerve stimulation techniques such as transcutaneous VNS and non-invasive VNS are gaining better patient compatibility, albeit their efficacy remains to be established.

Spatiotemporal profile of Map2 and microglial changes in the hippocampal CA1 region following pilocarpine-induced status epilepticus

Status epilepticus (SE) triggers pathological changes to hippocampal dendrites that may promote epileptogenesis. The microtubule associated protein 2 (Map2) helps stabilize microtubules of the dendritic cytoskeleton. Recently, we reported a substantial decline in Map2 that coincided with robust microglia accumulation in the CA1 hippocampal region after an episode of SE. A spatial correlation between Map2 loss and reactive microglia was also reported in human cortex from refractory epilepsy. New evidence supports that microglia modulate dendritic structures. Thus, to identify a potential association between SE-induced Map2 and microglial changes, a spatiotemporal profile of these events is necessary. We used immunohistochemistry to determine the distribution of Map2 and the microglia marker IBA1 in the hippocampus after pilocarpine-induced SE from 4 hrs to 35 days. We found a decline in Map2 immunoreactivity in the CA1 area that reached minimal levels at 14 days post-SE and partially increased thereafter. In contrast, maximal microglia accumulation occurred in the CA1 area at 14 days post-SE. Our data indicate that SE-induced Map2 and microglial changes parallel each other’s spatiotemporal profiles. These findings may lay the foundation for future mechanistic studies to help identify potential roles for microglia in the dendritic pathology associated with SE and epilepsy.

Evaluation of the pentylenetetrazole seizure threshold test in epileptic mice as surrogate model for drug testing against pharmacoresistant seizures

Resistance to antiepileptic drugs (AEDs) is a major problem in epilepsy therapy, so that development of more effective AEDs is an unmet clinical need. Several rat and mouse models of epilepsy with spontaneous difficult-to-treat seizures exist, but because testing of antiseizure drug efficacy is extremely laborious in such models, they are only rarely used in the development of novel AEDs. Recently, the use of acute seizure tests in epileptic rats or mice has been proposed as a novel strategy for evaluating novel AEDs for increased antiseizure efficacy. In the present study, we compared the effects of five AEDs (valproate, phenobarbital, diazepam, lamotrigine, levetiracetam) on the pentylenetetrazole (PTZ) seizure threshold in mice that were made epileptic by pilocarpine. Experiments were started 6 weeks after a pilocarpine-induced status epilepticus. At this time, control seizure threshold was significantly lower in epileptic than in nonepileptic animals. Unexpectedly, only one AED (valproate) was less effective to increase seizure threshold in epileptic vs. nonepileptic mice, and this difference was restricted to doses of 200 and 300 mg/kg, whereas the difference disappeared at 400mg/kg. All other AEDs exerted similar seizure threshold increases in epileptic and nonepileptic mice. Thus, induction of acute seizures with PTZ in mice pretreated with pilocarpine does not provide an effective and valuable surrogate method to screen drugs for antiseizure efficacy in a model of difficult-to-treat chronic epilepsy as previously suggested from experiments with this approach in rats.

Differential effects of rapamycin treatment on tonic and phasic GABAergic inhibition in dentate granule cells after focal brain injury in mice

The cascade of events leading to post-traumatic epilepsy (PTE) after traumatic brain injury (TBI) remains unclear. Altered inhibition in the hippocampal formation and dentate gyrus is a hallmark of several neurological disorders, including TBI and PTE. Inhibitory synaptic signaling in the hippocampus is predominately driven by γ-aminobutyric acid (GABA) neurotransmission, and is prominently mediated by postsynaptic type A GABA receptors (GABAAR's). Subsets of these receptors involved in tonic inhibition of neuronal membranes serve a fundamental role in maintenance of inhibitory state, and GABAAR-mediated tonic inhibition is altered functionally in animal models of both TBI and epilepsy. In this study, we assessed the effect of mTOR inhibition on hippocampal hilar inhibitory interneuron loss and synaptic and tonic GABAergic inhibition of dentate gyrus granule cells (DGCs) after controlled cortical impact (CCI) to determine if mTOR activation after TBI modulates GABAAR function. Hilar inhibitory interneuron density was significantly reduced 72h after CCI injury in the dorsal two-thirds of the hemisphere ipsilateral to injury compared with the contralateral hemisphere and sham controls. Rapamycin treatment did not alter this reduction in cell density. Synaptic and tonic current measurements made in DGCs at both 1-2 and 8-13weeks post-injury indicated reduced synaptic inhibition and THIP-induced tonic current density in DGCs ipsilateral to CCI injury at both time points post-injury, with no change in resting tonic GABAAR-mediated currents. Rapamycin treatment did not alter the reduced synaptic inhibition observed in ipsilateral DGCs 1-2weeks post-CCI injury, but further reduced synaptic inhibition of ipsilateral DGCs at 8-13weeks post-injury. The reduction in THIP-induced tonic current after injury, however, was prevented by rapamycin treatment at both time points. Rapamycin treatment thus differentially modifies CCI-induced changes in synaptic and tonic GABAAR-mediated currents in DGCs.

Exploring novel AEDs from drugs used for treatment of non-epileptic disorders

Epilepsy is a chronic neurological disease. Although many anti-epileptic drugs (AEDs) have been developed for clinical use, they have no effect on 20-30% of patients and do not generally prevent epileptogenesis. Because of the long development cycle for new AEDs and the high cost, increasing efforts are being made to find anti-epileptic effects among drugs that are already listed for the treatment of other diseases and repurpose them as potential anti-epileptic treatments. Here, we review the progress that has been made in this field as a result of animal and clinical trials of drugs such as rapamycin, everolimus, losartan, celecoxib, bumetanide and other non-epileptic drugs. These drugs can prevent the epileptogenesis, reduce the epileptic pathological changes, and even be used to treat intractable epilepsy. Their mechanisms of action are completely different from those of existing AEDs, prompting researchers to change their perspectives in the search for new AEDs.

Impact of rapamycin on status epilepticus induced hippocampal pathology and weight gain

Growing evidence implicates the dentate gyrus in temporal lobe epilepsy (TLE). Dentate granule cells limit the amount of excitatory signaling through the hippocampus and exhibit striking neuroplastic changes that may impair this function during epileptogenesis. Furthermore, aberrant integration of newly-generated granule cells underlies the majority of dentate restructuring. Recently, attention has focused on the mammalian target of rapamycin (mTOR) signaling pathway as a potential mediator of epileptogenic change. Systemic administration of the mTOR inhibitor rapamycin has promising therapeutic potential, as it has been shown to reduce seizure frequency and seizure severity in rodent models. Here, we tested whether mTOR signaling facilitates abnormal development of granule cells during epileptogenesis. We also examined dentate inflammation and mossy cell death in the dentate hilus. To determine if mTOR activation is necessary for abnormal granule cell development, transgenic mice that harbored fluorescently-labeled adult-born granule cells were treated with rapamycin following pilocarpine-induced status epilepticus. Systemic rapamycin effectively blocked phosphorylation of S6 protein (a readout of mTOR activity) and reduced granule cell mossy fiber axon sprouting. However, the accumulation of ectopic granule cells and granule cells with aberrant basal dendrites was not significantly reduced. Mossy cell death and reactive astrocytosis were also unaffected. These data suggest that anti-epileptogenic effects of mTOR inhibition may be mediated by mechanisms other than inhibition of these common dentate pathologies. Consistent with this conclusion, rapamycin prevented pathological weight gain in epileptic mice, suggesting that rapamycin might act on central circuits or even peripheral tissues controlling weight gain in epilepsy.

Hyperactive mTOR signals in the proopiomelanocortin-expressing hippocampal neurons cause age-dependent epilepsy and premature death in mice

Epilepsy is a frequent comorbidity in patients with focal cortical dysplasia (FCD). Recent studies utilizing massive sequencing data identified subsets of genes that are associated with epilepsy and FCD. AKT and mTOR-related signals have been recently implicated in the pathogenic processes of epilepsy and FCD. To clarify the functional roles of the AKT-mTOR pathway in the hippocampal neurons, we generated conditional knockout mice harboring the deletion of Pten (Pten-cKO) in Proopiomelanocortin-expressing neurons. The Pten-cKO mice developed normally until 8 weeks of age, then presented generalized seizures at 8–10 weeks of age. Video-monitored electroencephalograms detected paroxysmal discharges emerging from the cerebral cortex and hippocampus. These mice showed progressive hypertrophy of the dentate gyrus (DG) with increased expressions of excitatory synaptic markers (Psd95, Shank3 and Homer). In contrast, the expression of inhibitory neurons (Gad67) was decreased at 6–8 weeks of age. Immunofluorescence studies revealed the abnormal sprouting of mossy fibers in the DG of the Pten-cKO mice prior to the onset of seizures. The treatment of these mice with an mTOR inhibitor rapamycin successfully prevented the development of seizures and reversed these molecular phenotypes. These data indicate that the mTOR pathway regulates hippocampal excitability in the postnatal brain.

Deletion of mTOR in Reactive Astrocytes Suppresses Chronic Seizures in a Mouse Model of Temporal Lobe Epilepsy

Germline and somatic mutations in key genes of the mammalian target of rapamycin (mTOR) pathway have been identified in seizure-associated disorders. mTOR mutations lead to aberrant activation of mTOR signaling, and, although affected neurons are critical for epileptogenesis, the role of mTOR activation in glial cells remains poorly understood. We previously reported a consistent activation of the mTOR pathway in astrocytes in the epileptic foci of temporal lobe epilepsy. In this study, it was demonstrated that mTOR deletion from reactive astrocytes prevents increases in seizure frequency over the disease course. By using a tamoxifen-inducible mTOR conditional knockout system and kainic acid, a model was developed that allowed astrocyte-specific mTOR gene deletion in mice with chronic epilepsy. Animals in which mTOR was deleted from 44 % of the astrocyte population exhibited a lower seizure frequency compared with controls. Down-regulation of mTOR significantly ameliorated astrogliosis in the sclerotic hippocampus but did not rescue mossy fiber sprouting. In cultured astrocytes, the mTOR pathway modulated the stability of the astroglial glutamate transporter 1 (Glt1) and influenced the ability of astrocytes to remove extracellular glutamate. Taken together, these data indicate that astrocytes with activated mTOR signaling may provide conditions that are favorable for spontaneous recurrent seizures.

Axonal plasticity of age-defined dentate granule cells in a rat model of mesial temporal lobe epilepsy

Dentate granule cell (DGC) mossy fiber sprouting (MFS) in mesial temporal lobe epilepsy (mTLE) is thought to underlie the creation of aberrant circuitry which promotes the generation or spread of spontaneous seizure activity. Understanding the extent to which populations of DGCs participate in this circuitry could help determine how it develops and potentially identify therapeutic targets for regulating aberrant network activity. In this study, we investigated how DGC birthdate influences participation in MFS and other aspects of axonal plasticity using the rat pilocarpine-induced status epilepticus (SE) model of mTLE. We injected a retrovirus (RV) carrying a synaptophysin-yellow fluorescent protein (syp-YFP) fusion construct to birthdate DGCs and brightly label their axon terminals, and compared DGCs born during the neonatal period with those generated in adulthood. We found that both neonatal and adult-born DGC populations participate, to a similar extent, in SE-induced MFS within the dentate gyrus inner molecular layer (IML). SE did not alter hilar MF bouton density compared to sham-treated controls, but adult-born DGC bouton density was greater in the IML than in the hilus after SE. Interestingly, we also observed MF axonal reorganization in area CA2 in epileptic rats, and these changes arose from DGCs generated both neonatally and in adulthood. These data indicate that both neonatal and adult-generated DGCs contribute to axonal reorganization in the rat pilocarpine mTLE model, and indicate a more complex relationship between DGC age and participation in seizure-related plasticity than was previously thought.

Effects of Rapamycin Treatment on Neurogenesis and Synaptic Reorganization in the Dentate Gyrus after Controlled Cortical Impact Injury in Mice

Post-traumatic epilepsy (PTE) is one consequence of traumatic brain injury (TBI). A prominent cell signaling pathway activated in animal models of both TBI and epilepsy is the mammalian target of rapamycin (mTOR). Inhibition of mTOR with rapamycin has shown promise as a potential modulator of epileptogenesis in several animal models of epilepsy, but cellular mechanisms linking mTOR expression and epileptogenesis are unclear. In this study, the role of mTOR in modifying functional hippocampal circuit reorganization after focal TBI induced by controlled cortical impact (CCI) was investigated. Rapamycin (3 or 10 mg/kg), an inhibitor of mTOR signaling, was administered by intraperitoneal injection beginning on the day of injury and continued daily until tissue collection. Relative to controls, rapamycin treatment reduced dentate granule cell area in the hemisphere ipsilateral to the injury two weeks post-injury. Brain injury resulted in a significant increase in doublecortin immunolabeling in the dentate gyrus ipsilateral to the injury, indicating increased neurogenesis shortly after TBI. Rapamycin treatment prevented the increase in doublecortin labeling, with no overall effect on Fluoro-Jade B staining in the ipsilateral hemisphere, suggesting that rapamycin treatment reduced posttraumatic neurogenesis but did not prevent cell loss after injury. At later times post-injury (8–13 weeks), evidence of mossy fiber sprouting and increased recurrent excitation of dentate granule cells was detected, which were attenuated by rapamycin treatment. Rapamycin treatment also diminished seizure prevalence relative to vehicle-treated controls after TBI. Collectively, these results support a role for adult neurogenesis in PTE development and suggest that suppression of epileptogenesis by mTOR inhibition includes effects on post-injury neurogenesis.

Status epilepticus, blood-brain barrier disruption, inflammation, and epileptogenesis

Over the last 15 years, attention has been focused on dysfunction of the cerebral vasculature and inflammation as important players in epileptogenic processes, with a specific emphasis on failure of the blood-brain barrier (BBB; Fig. 1) (Seiffert et al., 2004; Marchi et al., 2007; Oby and Janigro, 2006; van Vliet et al., 2014; Vezzani et al., 2011) [3-7]. Here, we discuss how the BBB is disrupted as a consequence of SE and how this BBB breakdown may be involved in epileptogenesis. This article is part of a Special Issue entitled "Status Epilepticus".

Structural MRI biomarkers of shared pathogenesis in autism spectrum disorder and epilepsy

Etiological factors that contribute to a high comorbidity between autism spectrum disorder (ASD) and epilepsy are the subject of much debate. Does epilepsy cause ASD or are there common underlying brain abnormalities that increase the risk of developing both disorders? This review summarizes evidence from quantitative MRI studies to suggest that abnormalities of brain structure are not necessarily the consequence of ASD and epilepsy but are antecedent to disease expression. Abnormal gray and white matter volumes are present prior to onset of ASD and evident at the time of onset in pediatric epilepsy. Aberrant brain growth trajectories are also common in both disorders, as evidenced by blunted gray matter maturation and white matter maturation. Although the etiological factors that explain these abnormalities are unclear, high heritability estimates for gray matter volume and white matter microstructure demonstrate that genetic factors assert a strong influence on brain structure. In addition, histopathological studies of ASD and epilepsy brain tissue reveal elevated rates of malformations of cortical development (MCDs), such as focal cortical dysplasia and heterotopias, which supports disruption of neuronal migration as a contributing factor. Although MCDs are not always visible on MRI with conventional radiological analysis, quantitative MRI detection methods show high sensitivity to subtle malformations in epilepsy and can be potentially applied to MCD detection in ASD. Such an approach is critical for establishing quantitative neuroanatomic endophenotypes that can be used in genetic research. In the context of emerging drug treatments for seizures and autism symptoms, such as rapamycin and rapalogs, in vivo neuroimaging markers of subtle structural brain abnormalities could improve sample stratification in human clinical trials and potentially extend the range of patients that might benefit from treatment. This article is part of a Special Issue entitled "Autism and Epilepsy".

In vivo evaluation of the dentate gate theory in epilepsy

The dentate gyrus is a region subject to intense study in epilepsy because of its posited role as a 'gate', acting to inhibit overexcitation in the hippocampal circuitry through its unique synaptic, cellular and network properties that result in relatively low excitability. Numerous changes predicted to produce dentate hyperexcitability are seen in epileptic patients and animal models. However, recent findings question whether changes are causative or reactive, as well as the pathophysiological relevance of the dentate in epilepsy. Critically, direct in vivo modulation of dentate 'gate' function during spontaneous seizure activity has not been explored. Therefore, using a mouse model of temporal lobe epilepsy with hippocampal sclerosis, a closed-loop system and selective optogenetic manipulation of granule cells during seizures, we directly tested the dentate 'gate' hypothesis in vivo. Consistent with the dentate gate theory, optogenetic gate restoration through granule cell hyperpolarization efficiently stopped spontaneous seizures. By contrast, optogenetic activation of granule cells exacerbated spontaneous seizures. Furthermore, activating granule cells in non-epileptic animals evoked acute seizures of increasing severity. These data indicate that the dentate gyrus is a critical node in the temporal lobe seizure network, and provide the first in vivo support for the dentate 'gate' hypothesis.

mTOR signaling in epilepsy: insights from malformations of cortical development

Over the past decade enhanced activation of the mammalian target of rapamycin (mTOR)-signaling cascade has been identified in focal malformations of cortical development (MCD) subtypes, which have been collectively referred to as "mTORopathies." Mutations in mTOR regulatory genes (e.g., TSC1, TSC2, AKT3, DEPDC5) have been associated with several focal MCD highly associated with epilepsy such as tuberous sclerosis complex (TSC), hemimegalencephaly (HME; brain malformation associated with dramatic enlargement of one brain hemisphere), and cortical dysplasia. mTOR plays important roles in the regulation of cell division, growth, and survival, and, thus, aberrant activation of the cascade during cortical development can cause dramatic alterations in cell size, cortical lamination, and axon and dendrite outgrowth often observed in focal MCD. Although it is widely believed that structural alterations induced by hyperactivated mTOR signaling are critical for epileptogenesis, newer evidence suggests that mTOR activation on its own may enhance neuronal excitability. Clinical trials with mTOR inhibitors have shown efficacy in the treatment of seizures associated with focal MCD.

Short-term treatment with the GABAA receptor antagonist pentylenetetrazole produces a sustained pro-cognitive benefit in a mouse model of Down's syndrome

BACKGROUND AND PURPOSE

Down's syndrome is a common genetic cause of intellectual disability, for which there are no drug therapies. Mechanistic studies in a model of Down's syndrome [Ts65Dn (TS) mice] demonstrated that impaired cognitive function was due to excessive neuronal inhibitory tone. These deficits were normalized by low doses of GABAA receptor antagonists in adult animals. In this study, we explore the therapeutic potential of pentylenetetrazole, a GABAA receptor antagonist with a history of safe use in humans.

EXPERIMENTAL APPROACH

Long-term memory was assessed by the novel object recognition test in different cohorts of TS mice after a delay following a short-term chronic treatment with pentylenetetrazole. Seizure susceptibility, an index of treatment safety, was studied by means of EEG, behaviour and hippocampus morphology. EEG spectral analysis was used as a bio-marker of the treatment.

KEY RESULTS

PTZ has a wide therapeutic window (0.03-3 mg·kg(-1)) that is >10-1000-fold below its seizure threshold and chronic pentylenetetrazole treatment did not lower the seizure threshold. Short-term, low, chronic dose regimens of pentylenetetrazole elicited long-lasting (>1 week) normalization of cognitive function in young and aged mice. Pentylenetetrazole effectiveness was dependent on the time of treatment; cognitive performance improved after treatment during the light (inactive) phase, but not during the dark (active) phase. Chronic pentylenetetrazole treatment normalized EEG power spectra in TS mice.

CONCLUSIONS AND IMPLICATIONS

Low doses of pentylenetetrazole were safe, produced long-lasting cognitive improvements and have the potential of fulfilling an unmet therapeutic need in Down's syndrome.