Alterations of hippocampal GAbaergic system contribute to development of spontaneous recurrent seizures in the rat lithium-pilocarpine model of temporal lobe epilepsy

Fragile X cortex is characterized by decreased parvalbumin-expressing interneurons

Fragile X syndrome is a genetic neurodevelopmental disorder caused by a mutation of the fragile X messenger ribonucleoprotein 1 (FMR1) gene in the X chromosome. Many fragile X syndrome cases present with autism spectrum disorder and fragile X syndrome cases account for up to 5% of all autism spectrum disorder cases. The cellular composition of the fragile X syndrome cortex is not well known. We evaluated alterations in the number of Calbindin, Calretinin, and Parvalbumin expressing interneurons across 5 different cortical areas, medial prefrontal cortex (BA46), primary somatosensory cortex (BA3), primary motor cortex (BA4), superior temporal cortex (BA22), and anterior cingulate cortex (BA24) of fragile X syndrome and neurotypical brains. Compared with neurotypical cases, fragile X syndrome brains displayed a significant reduction in the number of PV+ interneurons in all areas and of CR+ interneurons in BA22 and BA3. The number of CB+ interneurons did not differ. These findings are the first to demonstrate that fragile X syndrome brains are characterized by cortical wide PV+ interneuron deficits across multiple cortical areas. These add to the idea that deficits in PV+ interneurons could disrupt the cortical balance and promote clinical deficits in fragile X syndrome patients and help to develop novel therapies for neurodevelopmental disorders like fragile X syndrome and autism spectrum disorder.

Current advances in rodent drug-resistant temporal lobe epilepsy models: Hints from laboratory studies

Anti-seizure drugs (ASDs) are the first choice for the treatment of epilepsy, but there is still one-third of patients with epilepsy (PWEs) who are resistant to two or more appropriately chosen ASDs, named drug-resistant epilepsy (DRE). Temporal lobe epilepsy (TLE), a common type of epilepsy usually associated with hippocampal sclerosis (HS), shares the highest proportion of drug resistance (approximately 70%). In view of the key role of the temporal lobe in memory, emotion, and other physiological functions, patients with drug-resistant temporal lobe epilepsy (DR-TLE) are often accompanied by serious complications, and surgical procedures also yield extra considerations. The exact mechanisms for the genesis of DR-TLE remain unillustrated, which makes it hard to manage patients with DR-TLE in clinical practice. Animal models of DR-TLE play an irreplaceable role in both understanding the mechanism and searching for new therapeutic strategies or drugs. In this review article, we systematically summarized different types of current DR-TLE models, and then recent advances in mechanism investigations obtained in these models were presented, especially with the development of advanced experimental techniques and tools. We are deeply encouraged that novel strategies show great therapeutic potential in those DR-TLE models. Based on the big steps reached from the bench, a new light has been shed on the precise management of DR-TLE.

Advances in understanding the pathogenesis of post-traumatic epilepsy: a literature review

Severe head trauma can lead to seizures. Persistent epileptic seizures and their progression are associated with the severity of trauma. Although case reports have revealed that early use of anti-seizure drugs after trauma can prevent epilepsy, clinical case-control studies have failed to confirm this phenomenon. To date, many brain trauma models have been used to study the correlation between post-traumatic seizures and related changes in neural circuit function. According to these studies, neuronal and glial responses are activated immediately after brain trauma, usually leading to significant cell loss in injured brain regions. Over time, long-term changes in neural circuit tissues, especially in the neocortex and hippocampus, lead to an imbalance between excitatory and inhibitory neurotransmission and an increased risk of spontaneous seizures. These changes include alterations in inhibitory interneurons and the formation of new, over-recurrent excitatory synaptic connections. In this study, we review the progress of research related to post-traumatic epilepsy to better understand the mechanisms underlying the initiation and development of post-traumatic seizures and to provide theoretical references for the clinical treatment of post-traumatic seizures.

Increasing adult neurogenesis protects mice from epilepsy

Neurogenesis occurs in the adult brain in the hippocampal dentate gyrus, an area that contains neurons which are vulnerable to insults and injury, such as severe seizures. Previous studies showed that increasing adult neurogenesis reduced neuronal damage after these seizures. Because the damage typically is followed by chronic lifelong seizures (epilepsy), we asked if increasing adult neurogenesis would prevent epilepsy. Adult neurogenesis was selectively increased by deleting the pro-apoptotic gene Bax from Nestin-expressing progenitors. Tamoxifen was administered at 6 weeks of age to conditionally delete Bax in Nestin-CreERT2Baxfl/fl mice. Six weeks after tamoxifen administration, severe seizures (status epilepticus; SE) were induced by injection of the convulsant pilocarpine. Mice with increased adult neurogenesis exhibited fewer chronic seizures. Postictal depression was reduced also. These results were primarily female mice, possibly because they were the more affected by Bax deletion than males, consistent with sex differences in Bax in development. The female mice with enhanced adult neurogenesis also showed less neuronal loss of hilar mossy cells and hilar somatostatin-expressing neurons than wild type females or males, which is notable because these two cell types are implicated in epileptogenesis. The results suggest that increasing adult neurogenesis in the normal adult brain can reduce experimental epilepsy, and the effect shows a striking sex difference. The results are surprising in light of past studies showing that suppressing adult-born neurons can also reduce chronic seizures.

Cell-autonomous and non-cell-autonomous roles of NKCC1 in regulating neural stem cell quiescence in the hippocampal dentate gyrus

Quiescence is a hallmark of adult neural stem cells (NSCs) in the mammalian brain, and establishment and maintenance of quiescence is essential for life-long continuous neurogenesis. How NSCs in the dentate gyrus (DG) of the hippocampus acquire their quiescence during early postnatal stages and continuously maintain quiescence in adulthood is poorly understood. Here, we show that Hopx-CreER^T2-mediated conditional deletion of Nkcc1, which encodes a chloride importer, in mouse DG NSCs impairs both their quiescence acquisition at early postnatal stages and quiescence maintenance in adulthood. Furthermore, PV-CreER^T2-mediated deletion of Nkcc1 in PV interneurons in the adult mouse brain leads to activation of quiescent DG NSCs, resulting in an expanded NSC pool. Consistently, pharmacological inhibition of NKCC1 promotes NSC proliferation in both early postnatal and adult mouse DG. Together, our study reveals both cell-autonomous and non-cell-autonomous roles of NKCC1 in regulating the acquisition and maintenance of NSC quiescence in the mammalian hippocampus.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Alterations in the Properties of the Rat Hippocampus Glutamatergic System in the Lithium-Pilocarpine Model of Temporal Lobe Epilepsy

Status epilepticus (SE) triggers many not yet fully understood pathological changes in the nervous system that can lead to the development of epilepsy. In this work, we studied the effects of SE on the properties of excitatory glutamatergic transmission in the hippocampus in the lithium-pilocarpine model of temporal lobe epilepsy in rats. The studies were performed 1 day (acute phase), 3 and 7 days (latent phase), and 30 to 80 days (chronic phase) after SE. According to RT-qPCR data, expression of the genes coding for the AMPA receptor subunits GluA1 and GluA2 was downregulated in the latent phase, which may lead to the increased proportion of calcium-permeable AMPA receptors that play an essential role in the pathogenesis of many CNS diseases. The efficiency of excitatory synaptic neurotransmission in acute brain slices was decreased in all phases of the model, as determined by recording field responses in the CA1 region of the hippocampus in response to the stimulation of Schaffer collaterals by electric current of different strengths. However, the frequency of spontaneous excitatory postsynaptic potentials increased in the chronic phase, indicating an increased background activity of the glutamatergic system in epilepsy. This was also evidenced by a decrease in the threshold current causing hindlimb extension in the maximal electroshock seizure threshold test in rats with temporal lobe epilepsy compared to the control animals. The results suggest a series of functional changes in the properties of glutamatergic system associated with the epilepsy development and can be used to develop the antiepileptogenic therapy.

Modulation of GABAB receptors in the insula bidirectionally affects associative memory of epilectic rats in both spatial and non-spatial operant tasks

Background

Stimulation of gamma-aminobutyric acid (GABA) activity through GABA receptor agonists is the basic mechanism of many anticonvulsant drugs. Nevertheless, many of these GABergic drugs have adverse cognitive effects. We previously found that GABAB receptors (GABA_BRs) in the insula regulate operant associative memory in healthy rats. The present study aimed at investigating the effects of GABA_BR modulation in the insula on operant associative memory in epileptic rats, along with the underlying mechanisms.

Methods

The lithium-pilocarpine model of temporal lobe epilepsy (TLE) was established in male Sprague-Dawley rats. A 22-gauge stainless-steel guide cannula was surgically implanted into the granular insula cortex of the epileptic rats. Baclofen (125 ng/μl, 1 μl), CGP35348 (12.5 μg/μl, 1 μl), or saline (1 μl) was slowly infused through the guide cannula. The Intellicage automated behavioral testing system was used to evaluate operant associative memory of the epileptic rats, including non-spatial operant tasks (basic nosepoke learning and skilled nosepoke learning) and spatial operant tasks (chamber position learning). The expression of the GABA_BR subunits GB1 and GB2 in the insula was examined by immunofluorescence and Western blotting.

Results

The Intellicage tests demonstrated that baclofen significantly impaired basic nosepoke learning, skilled nosepoke learning and chamber position learning of the epileptic rats, while CGP35348 boosted these functions. Immunofluorescence staining revealed that GB1 and GB2 were expressed in the insula of the epileptic rats, and Western blotting analysis showed that baclofen enhanced while CGP35348 inhibited the expression of these subunits.

Conclusion

GABA_BRs in the insula bidirectionally regulate both spatial and non-spatial operant associative memory of epileptic rats. Effects of GABA_BRs on cognition should be taken into account when evaluating new possible treatments for people with epilepsy.

Deep brain stimulation of the anterior nuclei of the thalamus can alleviate seizure severity and induce hippocampal GABAergic neuronal changes in a pilocarpine-induced epileptic mouse brain

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) has been widely used as an effective treatment for refractory temporal lobe epilepsy. Despite its promising clinical outcome, the exact mechanism of how ANT-DBS alleviates seizure severity has not been fully understood, especially at the cellular level. To assess effects of DBS, the present study examined electroencephalography (EEG) signals and locomotor behavior changes and conducted immunohistochemical analyses to examine changes in neuronal activity, number of neurons, and neurogenesis of inhibitory neurons in different hippocampal subregions. ANT-DBS alleviated seizure activity, abnormal locomotor behaviors, reduced theta-band, increased gamma-band EEG power in the interictal state, and increased the number of neurons in the dentate gyrus (DG). The number of parvalbumin- and somatostatin-expressing inhibitory neurons was recovered to the level in DG and CA1 of naïve mice. Notably, BrdU-positive inhibitory neurons were increased. In conclusion, ANT-DBS not only could reduce the number of seizures, but also could induce neuronal changes in the hippocampus, which is a key region involved in chronic epileptogenesis. Importantly, our results suggest that ANT-DBS may lead to hippocampal subregion-specific cellular recovery of GABAergic inhibitory neurons.

Dysregulation of the hippocampal neuronal network by LGI1 auto-antibodies

LGI1 is a neuronal secreted protein highly expressed in the hippocampus. Epileptic seizures and LGI1 hypo-functions have been found in both ADLTE, a genetic epileptogenic syndrome and LGI1 limbic encephalitis (LE), an autoimmune disease. Studies, based mainly on transgenic mouse models, investigated the function of LGI1 in the CNS and strangely showed that LGI1 loss of function, led to a decreased AMPA-receptors (AMPA-R) expression. Our project intends at better understanding how an altered function of LGI1 leads to epileptic seizures. To reach our goal, we infused mice with LGI1 IgG purified from the serum of patients diagnozed with LGI1 LE. Super resolution imaging revealed that LGI1 IgG reduced AMPA-R expression at the surface of inhibitory and excitatory neurons only in the dentate gyrus of the hippocampus. Complementary electrophysiological approaches indicated that despite reduced AMPA-R expression, LGI1 IgG increased the global hyperexcitability in the hippocampal neuronal network. Decreased AMPA-R expression at inhibitory neurons and the lack of LGI1 IgG effect in presence of GABA antagonist on excitability, led us to conclude that LGI1 function might be essential for the proper functioning of the overall network and orchestrate the imbalance between inhibition and excitation. Our work suggests that LGI1 IgG reduced the inhibitory network activity more significantly than the excitatory network shedding lights on the essential role of the inhibitory network to trigger epileptic seizures in patients with LGI1 LE.

Bilateral optogenetic activation of inhibitory cells favors ictogenesis

Mesial temporal lobe epilepsy (MTLE) is the most common type of focal refractory epilepsy and is characterized by recurring seizures that are often refractory to medication. Since parvalbumin-positive (PV) interneurons were recently shown to play significant roles in ictogenesis, we established here how bilateral optogenetic stimulation of these interneurons in the hippocampus CA3 regions modulates seizures, interictal spikes and high-frequency oscillations (HFOs; ripples: 80-200 Hz, fast ripples: 250-500 Hz) in the pilocarpine model of MTLE. Bilateral optogenetic stimulation of CA3 PV-positive interneurons at 8 Hz (lasting 30 s, every 2 min) was implemented in PV-ChR2 mice for 8 consecutive days starting on day 7 (n = 8) or on day 13 (n = 6) after pilocarpine-induced status epilepticus (SE). Seizure occurrence was higher in both day 7 and day 13 groups of PV-ChR2 mice during periods of optogenetic stimulation ("ON"), compared to when stimulation was not performed ("OFF") (day 7 group = p < 0.01, day 13 group = p < 0.01). In the PV-ChR2 day 13 group, rates of seizures (p < 0.05), of interictal spikes associated with fast ripples (p < 0.01), and of isolated fast ripples (p < 0.01) during optogenetic stimulations were significantly higher than in the PV-ChR2 day 7 group. Our findings reveal that bilateral activation of PV-interneurons in the hippocampus (leading to a presumptive increase in GABA signaling) favors ictogenesis. These effects may also mirror the neuropathological changes that occur over time after SE in this animal model.

Late adolescence mortality in mice with brain-specific deletion of the volume-regulated anion channel subunit LRRC8A

The leucine-rich repeat-containing family 8 member A (LRRC8A) is an essential subunit of the volume-regulated anion channel (VRAC). VRAC is critical for cell volume control, but its broader physiological functions remain under investigation. Recent studies in the field indicate that Lrrc8a disruption in the brain astrocytes reduces neuronal excitability, impairs synaptic plasticity and memory, and protects against cerebral ischemia. In the present work, we generated brain-wide conditional LRRC8A knockout mice (LRRC8A bKO) using Nestin^Cre -driven Lrrc8a^flox/flox excision in neurons, astrocytes, and oligodendroglia. LRRC8A bKO animals were born close to the expected Mendelian ratio and developed without overt histological abnormalities, but, surprisingly, all died between 5 and 9 weeks of age with a seizure phenotype, which was confirmed by video and EEG recordings. Brain slice electrophysiology detected changes in the excitability of pyramidal cells and modified GABAergic inputs in the hippocampal CA1 region of LRRC8A bKO. LRRC8A-null hippocampi showed increased immunoreactivity of the astrocytic marker GFAP, indicating reactive astrogliosis. We also found decreased whole-brain protein levels of the GABA transporter GAT-1, the glutamate transporter GLT-1, and the astrocytic enzyme glutamine synthetase. Complementary HPLC assays identified reduction in the tissue levels of the glutamate and GABA precursor glutamine. Together, these findings suggest that VRAC provides vital control of brain excitability in mouse adolescence. VRAC deletion leads to a lethal phenotype involving progressive astrogliosis and dysregulation of astrocytic uptake and supply of amino acid neurotransmitters and their precursors.

Sleep, neuronal hyperexcitability, inflammation and neurodegeneration: Does early chronic short sleep trigger and is it the key to overcoming Alzheimer's disease?

Evidence links neuroinflammation to Alzheimer's disease (AD); however, its exact contribution to the onset and progression of the disease is poorly understood. Symptoms of AD can be seen as the tip of an iceberg, consisting of a neuropathological build-up in the brain of extracellular amyloid-β (Aβ) plaques and intraneuronal hyperphosphorylated aggregates of Tau (pTau), which are thought to stem from an imbalance between its production and clearance resulting in loss of synaptic health and dysfunctional cortical connectivity. The glymphatic drainage system, which is particularly active during sleep, plays a key role in the clearance of proteinopathies. Poor sleep can cause hyperexcitability and promote Aβ and tau pathology leading to systemic inflammation. The early neuronal hyperexcitability of γ-aminobutyric acid (GABA)-ergic inhibitory interneurons and impaired inhibitory control of cortical pyramidal neurons lie at the crossroads of excitatory/inhibitory imbalance and inflammation. We outline, with a prospective framework, a possible vicious spiral linking early chronic short sleep, neuronal hyperexcitability, inflammation and neurodegeneration. Understanding the early predictors of AD, through an integrative approach, may hold promise for reducing attrition in the late stages of neuroprotective drug development.

Semaphorins in Adult Nervous System Plasticity and Disease

Semaphorins, originally discovered as guidance cues for developing axons, are involved in many processes that shape the nervous system during development, from neuronal proliferation and migration to neuritogenesis and synapse formation. Interestingly, the expression of many Semaphorins persists after development. For instance, Semaphorin 3A is a component of perineuronal nets, the extracellular matrix structures enwrapping certain types of neurons in the adult CNS, which contribute to the closure of the critical period for plasticity. Semaphorin 3G and 4C play a crucial role in the control of adult hippocampal connectivity and memory processes, and Semaphorin 5A and 7A regulate adult neurogenesis. This evidence points to a role of Semaphorins in the regulation of adult neuronal plasticity. In this review, we address the distribution of Semaphorins in the adult nervous system and we discuss their function in physiological and pathological processes.

Anxious Profile Influences Behavioral and Immunohistological Findings in the Pilocarpine Model of Epilepsy

Anxiety and epilepsy have a complex bidirectional relationship, where a depressive/anxious condition is a factor that can trigger seizures which in turn can aggravate the depressive/anxious condition. In addition, brain structures such as the hippocampus and amygdala might have a critical relevance in both epilepsy and anxiety. The aim of the present work was to investigate the influence of different anxious profiles to epileptogenesis. Initially, animals were screened through the elevated plus-maze anxiety test, and then seizure development was evaluated using the pilocarpine model of epilepsy. There were no differences in the susceptibility to status epilepticus, mortality rate or frequency of spontaneous recurrent seizures between animals characterized as anxious as compared to the non-anxious animals. Next, we evaluated immunohistological patterns related to seizures and anxiety in various related brain areas. Despite a decrease in the density of neuropeptide Y and parvalbumin expression in epileptic animals, those presenting greater neuropeptide Y immunoreactivity in various brain regions, also showed higher spontaneous recurrent seizures frequency. Differences on the anxious profile showed to interfere with some of these findings in some regions. In addition, animals that were injected with pilocarpine, but did not develop status epilepticus, had behavioral and neuroanatomical alterations as compared to control animals, indicating its importance as an additional tool for investigating the heterogeneity of the epileptogenic response after an initial insult. This study allowed to better understand the association between anxiety and temporal lobe epilepsy and might allow for therapeutic targets to be developed to minimize the negative impacts associated with it.

Comparative profile of refractory status epilepticus models following exposure of cholinergic agents pilocarpine, DFP, and soman

Status epilepticus (SE) is a medical emergency with continuous seizure activity that causes profound neuronal damage, morbidity, or death. SE incidents can arise spontaneously but mostly are elicited by seizurogenic triggers. Chemoconvulsants such as the muscarinic agonist pilocarpine and, organophosphates (OP) such as the pesticide diisopropylfluorophosphate (DFP) and, the nerve agent soman, can induce SE. Pilocarpine, DFP, and soman share a common feature of cholinergic crisis that transitions into a state of refractory SE, but their comparative profiles remain unclear. Here, we evaluated the comparative convulsant profile of pilocarpine, DFP, and soman to produce refractory SE and brain damage in rats. Behavioral and electrographic seizures were monitored for 24 h after exposure, and the extent of brain injury was determined by histological markers of neuronal injury and degeneration. Seizures were elicited rather slowly after pilocarpine as compared to DFP or soman, which caused rapid onset of spiking that swiftly developed into persistent SE. Time-course of SE activity after DFP was comparable to that after soman, a potent nerve agent. Diazepam controlled pilocarpine-induced SE, but it was ineffective in reducing OP-induced SE. All three agents produced modestly different degrees of neuronal injury and neurodegeneration in the brain. These results reveal distinct convulsant and neuronal injury patterns following exposure to cholinergic agonists, OP pesticides, and nerve agents. A battery of SE models, especially SE induced by cholinergic agents and other etiologies including epilepsy and brain tumors, is essential to identify novel anticonvulsant therapies for the management of refractory SE.

Gastrodin alleviates seizure severity and neuronal excitotoxicities in the rat lithium-pilocarpine model of temporal lobe epilepsy via enhancing GABAergic transmission

ETHNOPHARMACOLOGICAL RELEVANCE

Temporal lobe epilepsy remains one of the most drug-resistant focal epilepsy, leading to enormous healthcare burden. Among traditional herb medicine, some ingredients have the potential to treat seizure and alleviate the neuronal excitoxicity. The dried rhizome of Gastrodia elata Blume has been used to treat convulsive disorder, dizziness, dementia and migraine in eastern Asia.

AIM OF THE STUDY

To determine whether gastrodin, an active ingredient of Gastrodia elata Blume, can reduce lithium-pilocarpine induced seizure severity and neuronal excitotoxicity and explore the underlying mechanism.

MATERIALS AND METHODS

We divided the Sprague-Dawley rats into an experimental group (gastrodin group) and a control group (Dimethyl sulfoxide, vehicle group) and performed the behavioral analysis and electroencephalography to determine the effect of gastrodin on the seizure severity induced by lithium-pilocarpine injection. Nissl-stained histopathology elucidated the degree of rat hippocampal neuronal damage as markers of acute and subacute neuronal excitotoxicity. Besides, the Western blotting of dissected hippocampus was carried out to demonstrate the protein expression involving GABAergic transmission and metabolic pathway.

RESULTS

Gastrodin reduced the acute seizure severity in lithium-pilocarpine-induced seizure model. In electroencephalography recording, gastrodin exerted inhibitory action on epileptiform discharge. Compared with control group, gastrodin exhibited neuroprotective effect against seizure related hippocampal neuronal damage at acute and subacute stages. The Western blotting showed that gastrodin reversed the degradation of GABAA receptor after pilocarpine-induced seizures.

CONCLUSIONS

In the experimental seizure model, gastrodin showed anti-seizure and neuroprotective abilities. Enhancing the expression of GABAA receptor plays an important role in its antiepileptic mechanism. The results offer a new insight of developing new antiepileptic drugs from traditional Chinese medicine.

Short-Term Amygdala Low-Frequency Stimulation Does not Influence Hippocampal Interneuron Changes Observed in the Pilocarpine Model of Epilepsy

Temporal lobe epilepsy (TLE) is characterized by changes in interneuron numbers in the hippocampus. Deep brain stimulation (DBS) is an emerging tool to treat TLE seizures, although its mechanisms are not fully deciphered. We aimed to depict the effect of amygdala DBS on the density of the most common interneuron types in the CA1 hippocampal subfield in the lithium-pilocarpine model of epilepsy. Status epilepticus was induced in male Wistar rats. Eight weeks later, a stimulation electrode was implanted to the left basolateral amygdala of both pilocarpine-treated (Pilo, n = 14) and age-matched control rats (n = 12). Ten Pilo and 4 control animals received for 10 days 4 daily packages of 50 s 4 Hz regular stimulation trains. At the end of the stimulation period, interneurons were identified by immunolabeling for parvalbumin (PV), neuropeptide Y (NPY), and neuronal nitric oxide synthase (nNOS). Cell density was determined in the CA1 subfield of the hippocampus using confocal microscopy. We found that PV+ cell density was preserved in pilocarpine-treated rats, while the NPY+/nNOS+ cell density decreased significantly. The amygdala DBS did not significantly change the cell density in healthy or in epileptic animals. We conclude that DBS with low frequency applied for 10 days does not influence interneuron cell density changes in the hippocampus of epileptic rats.

mTOR-driven neural circuit changes initiate an epileptogenic cascade

Mutations in genes regulating mTOR pathway signaling are now recognized as a significant cause of epilepsy. Interestingly, these mTORopathies are often caused by somatic mutations, affecting variable numbers of neurons. To better understand how this variability affects disease phenotype, we developed a mouse model in which the mTOR pathway inhibitor Pten can be deleted from 0 to 40 % of hippocampal granule cells. In vivo, low numbers of knockout cells caused focal seizures, while higher numbers led to generalized seizures. Generalized seizures coincided with the loss of local circuit interneurons. In hippocampal slices, low knockout cell loads produced abrupt reductions in population spike threshold, while spontaneous excitatory postsynaptic currents and circuit level recurrent activity increased gradually with rising knockout cell load. Findings demonstrate that knockout cells load is a critical variable regulating disease phenotype, progressing from subclinical circuit abnormalities to electrobehavioral seizures with secondary involvement of downstream neuronal populations.

Etiology-related Degree of Sprouting of Parvalbumin-immunoreactive Axons in the Human Dentate Gyrus in Temporal Lobe Epilepsy

In the present study, we examined parvalbumin-immunoreactive cells and axons in the dentate gyrus of surgically resected tissues of therapy-resistant temporal lobe epilepsy (TLE) patients with different etiologies. Based on MRI results, five groups of patients were formed: (1) hippocampal sclerosis (HS), (2) malformation of cortical development, (3) malformation of cortical development + HS, (4) tumor-induced TLE, (5) patients with negative MRI result. Four control samples were also included in the study. Parvalbumin-immunoreactive cells were observed mostly in subgranular location in the dentate hilus in controls, in tumor-induced TLE, in malformation of cortical development and in MR-negative cases. In patients with HS, significant decrease in the number of hilar parvalbumin-immunoreactive cells and large numbers of ectopic parvalbumin-containing neurons were detected in the dentate gyrus' molecular layer. The ratio of ectopic/normally-located cells was significantly higher in HS than in other TLE groups. In patients with HS, robust sprouting of parvalbumin-immunoreactive axons were frequently visible in the molecular layer. The extent of sprouting was significantly higher in TLE patients with HS than in other groups. Strong sprouting of parvalbumin-immunoreactive axons were frequently observed in patients who had childhood febrile seizure. Significant correlation was found between the level of sprouting of axons and the ratio of ectopic/normally-located parvalbumin-containing cells. Electron microscopy demonstrated that sprouted parvalbumin-immunoreactive axons terminate on proximal and distal dendritic shafts as well as on dendritic spines of granule cells. Our results indicate alteration of target profile of parvalbumin-immunoreactive neurons in HS that contributes to the known synaptic remodeling in TLE.

Ictal onset sites and γ-aminobutyric acidergic neuron loss in epileptic pilocarpine-treated rats

OBJECTIVE

The present study tested whether ictal onset sites are regions of more severe interneuron loss in epileptic pilocarpine-treated rats, a model of human temporal lobe epilepsy.

METHODS

Local field potential recordings were evaluated to identify ictal onset sites. Electrode sites were visualized in Nissl-stained sections. Adjacent sections were processed with proximity ligation in situ hybridization for glutamic acid decarboxylase 2 (Gad2). Gad2 neuron profile numbers at ictal onset sites were compared to contralateral regions. Other sections were processed with immunocytochemistry for reelin or nitric oxide synthase (NOS), which labeled major subtypes of granule cell layer-associated interneurons. Stereology was used to estimate numbers of reelin and NOS granule cell layer-associated interneurons per hippocampus.

RESULTS

Ictal onset sites varied between and within rats but were mostly in the ventral hippocampus and were frequently bilateral. There was no conclusive evidence of more severe Gad2 neuron profile loss at sites of earliest seizure activity compared to contralateral regions. Numbers of granule cell layer-associated NOS neurons were reduced in the ventral hippocampus.

SIGNIFICANCE

In epileptic pilocarpine-treated rats, ictal onset sites were mostly in the ventral hippocampus, where there was loss of granule cell layer-associated NOS interneurons. These findings suggest the hypothesis that loss of granule cell layer-associated NOS interneurons in the ventral hippocampus is a mechanism of temporal lobe epilepsy.

Interaction of GABAA and GABAB antagonists after status epilepticus in immature rats

Among neurotransmitter systems affected by status epilepticus (SE) in adult rats are both GABAergic systems. To analyze possible changes of GABA_A and GABA_B systems in developing rats lithium-pilocarpine SE was induced at postnatal day 12 (P12). Seizures were elicited by a GABA_A antagonist pentylenetetrazol (PTZ) 3, 6, 9, and 13 days after SE (i.e., in P15, P18, P21, and P25 rats), and their possible potentiation by a GABA_B receptor antagonist CGP46381 was studied. Pilocarpine was replaced by saline in control animals (lithium-paraldehyde [LiPAR]). Pentylenetetrazol in a dose of 50 mg/kg s.c. elicited generalized seizures in nearly all 15-day-old naive rats and in 40% of 18-day-old ones but not in older animals. After SE, PTZ no longer elicited seizures in these two younger groups, i.e., sensitivity of GABA_A system was diminished. The GABA_B antagonist exhibited proconvulsant effect in P15 and P18 SE as well as LiPAR rats returning the incidence of PTZ-induced seizures to values of control animals. A decrease in the incidence of minimal clonic seizures was seen in P21 LiPAR animals; these seizures in the oldest group were not affected. Change of the effect from proconvulsant to anticonvulsant (or at least to no action) took place before postnatal day 21. Both SE and LiPAR animals exhibited similar changes but their intensity differed, effects in LiPAR controls were usually more expressed than in SE rats.

Altered Dynamics of Canonical Feedback Inhibition Predicts Increased Burst Transmission in Chronic Epilepsy

Inhibitory interneurons, organized into canonical feedforward and feedback motifs, play a key role in controlling normal and pathological neuronal activity. We demonstrate prominent quantitative changes in the dynamics of feedback inhibition in a rat model of chronic epilepsy (male Wistar rats). Systematic interneuron recordings revealed a large decrease in intrinsic excitability of basket cells and oriens-lacunosum moleculare interneurons in epileptic animals. Additionally, the temporal dynamics of interneuron recruitment by recurrent feedback excitation were strongly altered, resulting in a profound loss of initial feedback inhibition during synchronous CA1 pyramidal activity. Biophysically constrained models of the complete feedback circuit motifs of normal and epileptic animals revealed that, as a consequence of altered feedback inhibition, burst activity arising in CA3 is more strongly converted to a CA1 output. This suggests that altered dynamics of feedback inhibition promote the transmission of epileptiform bursts to hippocampal projection areas.SIGNIFICANCE STATEMENT We quantitatively characterized changes of the CA1 feedback inhibitory circuit in a model of chronic temporal lobe epilepsy. This study shows, for the first time, that dynamic recruitment of inhibition in feedback circuits is altered and establishes the cellular mechanisms for this change. Computational modeling revealed that the observed changes are likely to systematically alter CA1 input-output properties leading to (1) increased seizure propagation through CA1 and (2) altered computation of synchronous CA3 input.

Proportional loss of parvalbumin-immunoreactive synaptic boutons and granule cells from the hippocampus of sea lions with temporal lobe epilepsy

One in 26 people develop epilepsy and in these temporal lobe epilepsy (TLE) is common. Many patients display a pattern of neuron loss called hippocampal sclerosis. Seizures usually start in the hippocampus but underlying mechanisms remain unclear. One possibility is insufficient inhibition of dentate granule cells. Normally parvalbumin-immunoreactive (PV) interneurons strongly inhibit granule cells. Humans with TLE display loss of PV interneurons in the dentate gyrus but questions persist. To address this, we evaluated PV interneuron and bouton numbers in California sea lions (Zalophus californianus) that naturally develop TLE after exposure to domoic acid, a neurotoxin that enters the marine food chain during harmful algal blooms. Sclerotic hippocampi were identified by the loss of Nissl-stained hilar neurons. Stereological methods were used to estimate the number of granule cells and PV interneurons per dentate gyrus. Sclerotic hippocampi contained fewer granule cells, fewer PV interneurons, and fewer PV synaptic boutons, and the ratio of granule cells to PV interneurons was higher than in controls. To test whether fewer boutons was attributable to loss versus reduced immunoreactivity, expression of synaptotagmin-2 (syt2) was evaluated. Syt2 is also expressed in boutons of PV interneurons. Sclerotic hippocampi displayed proportional losses of syt2-immunoreactive boutons, PV boutons, and granule cells. There was no significant difference in the average numbers of PV- or syt2-positive boutons per granule cell between control and sclerotic hippocampi. These findings do not address functionality of surviving synapses but suggest reduced granule cell inhibition in TLE is not attributable to anatomical loss of PV boutons.

Changing effect of GABA B receptor antagonist CGP46381 after status epilepticus in immature rats

Possible proconvulsant action of GABA_B receptor antagonist CGP46381 was studied 3 and 13 days after status epilepticus elicited in 12-day-old rats. GABAA-dependent activity was tested by pentylenetetrazol administration and found different in 15-day-old rats after status epiolepticus but not in the older group. The interaction of the two GABAergic systems should be studied in detail.

Subfield-specific tractography of the hippocampus in epilepsy patients at 7 Tesla

PURPOSE

MRI-negative epilepsy patients could benefit from advanced imaging techniques such as high-resolution diffusion magnetic resonance imaging (dMRI). Our aim was to perform hippocampal subfield-specific tractography and quantify connectivity of the subfields in MRI-negative patients. Abnormal connectivity of the hippocampal subfields may help inform seizure focus hypothesis and provide information to guide surgical intervention.

METHODS

Hippocampal structural imaging and dMRI was acquired in 25 drug resistant MRI-negative patients and 25 healthy volunteers. The hippocampi of each subject was segmented on high-resolution structural images and dMRI-based probabilistic tractography was performed in each subfield. The degrees of connectivity and fiber densities of the hippocampal subfields were quantified and compared between epilepsy patients and healthy volunteers.

RESULTS

We were able to perform subfield-specific hippocampal tractography in each subject that participated in this study. These methods identified some hippocampal subfields that are abnormally connected in MRI-negative patients. In particular patients suspected of left temporal seizure focus exhibited increased connectivity of certain ipsilateral subfields, especially the subiculum, presubiculum, and parasubiculum, and reduced connectivity of some contralateral subfields, such as CA1 and subiculum.

CONCLUSIONS

Our data suggest that the hippocampal subfields are connected in distinct ways in different types of epilepsy. These results may provide important information that could help inform seizure focus hypothesis and in the surgical treatment of MRI-negative patients. These observations suggest that high-resolution dMRI-based tractography of the hippocampal subfields can detect subtle abnormalities in otherwise normal-appearing MRI-negative patients.

Docosanoids and elovanoids from omega-3 fatty acids are pro-homeostatic modulators of inflammatory responses, cell damage and neuroprotection

The functional significance of the selective enrichment of the omega-3 essential fatty acid docosahexaenoic acid (DHA; 22C and 6 double bonds) in cellular membrane phospholipids of the nervous system is being clarified by defining its specific roles on membrane protein function and by the uncovering of the bioactive mediators, docosanoids and elovanoids (ELVs). Here, we describe the preferential uptake and DHA metabolism in photoreceptors and brain as well as the significance of the Adiponectin receptor 1 in DHA retention and photoreceptor cell (PRC) survival. We now know that this integral membrane protein is engaged in DHA retention as a necessary event for the function of PRCs and retinal pigment epithelial (RPE) cells. We present an overview of how a) NPD1 selectively mediates preconditioning rescue of RPE and PR cells; b) NPD1 restores aberrant neuronal networks in experimental epileptogenesis; c) the decreased ability to biosynthesize NPD1 in memory hippocampal areas of early stages of Alzheimer's disease takes place; d) NPD1 protection of dopaminergic circuits in an in vitro model using neurotoxins; and e) bioactivity elicited by DHA and NPD1 activate a neuroprotective gene-expression program that includes the expression of Bcl-2 family members affected by Aβ42, DHA, or NPD1. In addition, we highlight ELOVL4 (ELOngation of Very Long chain fatty acids-4), specifically the neurological and ophthalmological consequences of its mutations, and their role in providing precursors for the biosynthesis of ELVs. Then we outline evidence of ELVs ability to protect RPE cells, which sustain PRC integrity. In the last section, we present a summary of the protective bioactivity of docosanoids and ELVs in experimental ischemic stroke. The identification of early mechanisms of neural cell survival mediated by DHA-synthesized ELVs and docosanoids contributes to the understanding of cell function, pro-homeostatic cellular modulation, inflammatory responses, and innate immunity, opening avenues for prevention and therapeutic applications in neurotrauma, stroke and neurodegenerative diseases.

Voltage-Dependent Calcium Channels, Calcium Binding Proteins, and Their Interaction in the Pathological Process of Epilepsy

As an important second messenger, the calcium ion (Ca²⁺) plays a vital role in normal brain function and in the pathophysiological process of different neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), and epilepsy. Ca²⁺ takes part in the regulation of neuronal excitability, and the imbalance of intracellular Ca²⁺ is a trigger factor for the occurrence of epilepsy. Several anti-epileptic drugs target voltage-dependent calcium channels (VDCCs). Intracellular Ca²⁺ levels are mainly controlled by VDCCs located in the plasma membrane, the calcium-binding proteins (CBPs) inside the cytoplasm, calcium channels located on the intracellular calcium store (particular the endoplasmic reticulum/sarcoplasmic reticulum), and the Ca²⁺-pumps located in the plasma membrane and intracellular calcium store. So far, while many studies have established the relationship between calcium control factors and epilepsy, the mechanism of various Ca²⁺ regulatory factors in epileptogenesis is still unknown. In this paper, we reviewed the function, distribution, and alteration of VDCCs and CBPs in the central nervous system in the pathological process of epilepsy. The interaction of VDCCs with CBPs in the pathological process of epilepsy was also summarized. We hope this review can provide some clues for better understanding the mechanism of epileptogenesis, and for the development of new anti-epileptic drugs targeting on VDCCs and CBPs.

Midazolam-Resistant Seizures and Brain Injury after Acute Intoxication of Diisopropylfluorophosphate, an Organophosphate Pesticide and Surrogate for Nerve Agents

Organophosphates (OP) such as the pesticide diisopropylfluorophosphate (DFP) and the nerve agent sarin are lethal chemicals that induce seizures, status epilepticus (SE), and brain damage. Midazolam, a benzodiazepine modulator of synaptic GABA-A receptors, is currently considered as a new anticonvulsant for nerve agents. Here, we characterized the time course of protective efficacy of midazolam (0.2-5 mg/kg, i.m.) in rats exposed to DFP, a chemical threat agent and surrogate for nerve agents. Behavioral and electroencephalogram (EEG) seizures were monitored for 24 hours after DFP exposure. The extent of brain injury was determined 3 days after DFP exposure by unbiased stereologic analyses of valid markers of neurodegeneration and neuroinflammation. Seizures were elicited within ∼8 minutes after DFP exposure that progressively developed into persistent SE lasting for hours. DFP exposure resulted in massive neuronal injury or necrosis, neurodegeneration of principal cells and interneurons, and neuroinflammation as evident by extensive activation of microglia and astrocytes in the hippocampus, amygdala, and other brain regions. Midazolam controlled seizures, neurodegeneration, and neuroinflammation when given early (10 minutes) after DFP exposure, but it was less effective when given at 40 minutes or later. Delayed therapy (≥40 minutes), a simulation of the practical therapeutic window for first responders or hospital admission, was associated with reduced seizure protection and neuroprotection. These results strongly reaffirm that the DFP-induced seizures and brain damage are progressively resistant to delayed treatment with midazolam, confirming the benzodiazepine refractory SE after OP intoxication. Thus, novel anticonvulsants superior to midazolam or adjunct therapies that enhance its efficacy are needed for effective treatment of refractory SE.

Trajectory of Parvalbumin Cell Impairment and Loss of Cortical Inhibition in Traumatic Brain Injury

Many neuropsychiatric symptoms that follow traumatic brain injury (TBI), including mood disorders, sleep disturbance, chronic pain, and posttraumatic epilepsy (PTE) are attributable to compromised cortical inhibition. However, the temporal trajectory of cortical inhibition loss and its underlying mechanisms are not known. Using paired-pulse transcranial magnetic stimulation (ppTMS) and immunohistochemistry, we tracked functional and cellular changes of cortical inhibitory network elements after fluid-percussion injury (FPI) in rats. ppTMS revealed a progressive loss of cortical inhibition as early as 2 weeks after FPI. This profile paralleled the increasing levels of cortical oxidative stress, which was accompanied by a gradual loss of parvalbumin (PV) immunoreactivity in perilesional cortex. Preceding the PV loss, we identified a degradation of the perineuronal net (PNN)-a specialized extracellular structure enwrapping cortical PV-positive (PV+) inhibitory interneurons which binds the PV+ cell maintenance factor, Otx2. The trajectory of these impairments underlies the reduced inhibitory tone, which can contribute to posttraumatic neurological conditions, such as PTE. Taken together, our results highlight the use of ppTMS as a biomarker to track the course of cortical inhibitory dysfunction post-TBI. Moreover, the neuroprotective role of PNNs on PV+ cell function suggests antioxidant treatment or Otx2 enhancement as a promising prophylaxis for post-TBI symptoms.

Disease Modifying Effects of the Spider Toxin Parawixin2 in the Experimental Epilepsy Model

(1) Background: Temporal lobe epilepsy (TLE) is the most common type of epilepsy in adults. It is also the one with the highest percentage of drug-resistance to the current available anti-epileptic drugs (AED). Additionaly, most antiepileptic drugs are only able to control seizures in epileptogenesis, but do not decrease the hippocampal neurodegenerative process. TLE patients have a reduced population of interneuronal cells, which express Parvalbumin (PV) proteins. This reduction is directly linked to seizure frequency and severity in the chronic period of epilepsy. There is therefore a need to seek new therapies with a disease-modifying profile, and with efficient antiepileptic and neuroprotective properties. Parawixin2, a compound isolated from the venom of the spider Parawixia bistriata, has been shown to inhibit GABA transporters (GAT) and to have acute anticonvulsant effects in rats. (2) Methods: In this work, we studied the effects of Parawixin2 and Tiagabine (an FDA- approved GAT inhibitor), and compared these effects in a TLE model. Rats were subjected to lithium-pilocarpine TLE model and the main features were evaluated over a chronic period including: (a) spontaneous recurrent seizures (SRS), (b) neuronal loss, and (c) PV cell density in different regions of the hippocampus (CA1, CA3, DG and Hilus). (3) Results: Parawixin2 treatment reduced SRS frequency whereas Tiagabine did not. We also found a significant reduction in neuronal loss in CA3 and in the hilus regions of the hippocampus, in animals treated with Parawixin2. Noteworthy, Parawixin2 significantly reversed PV cell loss observed particularly in DG layers. (4) Conclusions: Parawixin2 exerts a promising neuroprotective and anti-epileptic effect and has potential as a novel agent in drug design.

The role of the basal ganglia in the control of seizure

Epilepsy is a network disorder and each type of seizure involves distinct cortical and subcortical network, differently implicated in the control and propagation of the ictal activity. The role of the basal ganglia has been revealed in several cases of focal and generalized seizures. Here, we review the data that show the implication of the basal ganglia in absence, temporal lobe, and neocortical seizures in animal models (rodent, cat, and non-human primate) and in human. Based on these results and the advancement of deep brain stimulation for Parkinson's disease, basal ganglia neuromodulation has been tested with some success that can be equally seen as promising or disappointing. The effect of deep brain stimulation can be considered promising with a 76% in seizure reduction in temporal lobe epilepsy patients, but also disappointing, since only few patients have become seizure free and the antiepileptic effects have been highly variable among patients. This variability could probably be explained by the heterogeneity among the patients included in these clinical studies. To illustrate the importance of specific network identification, electrophysiological activity of the putamen and caudate nucleus has been recorded during penicillin-induced pre-frontal and motor seizures in one monkey. While an increase of the firing rate was found in putamen and caudate nucleus during pre-frontal seizures, only the activity of the putamen cells was increased during motor seizures. These preliminary results demonstrate the implication of the basal ganglia in two types of neocortical seizures and the necessity of studying the network to identify the important nodes implicated in the propagation and control of each type of seizure.

Early morphological and functional changes in the GABAergic system of hippocampus in the rat lithium-pilocarpine model of epilepsy

We studied early alterations in the GABAergic system of the rat hippocampus in the lithium-pilocarpine model of epilepsy. Twenty-four hours after the pilocarpine treatment, a decrease in the number of calretinin-positive interneurons was observed in the CA1 field of the hippocampus, whereas the number of parvalbumin-positive interneurons remained unchanged. The decreased levels of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD67) and the membrane GABA transporter GAT1 were revealed using Western blot analysis. These data indicate an altered excitation/inhibition balance in the hippocampus with excitation dominance.

Glial GABA Transporters as Modulators of Inhibitory Signalling in Epilepsy and Stroke

Imbalances in GABA-mediated tonic inhibition are involved in several pathophysiological conditions. A classical way of controlling tonic inhibition is through pharmacological intervention with extrasynaptic GABA_A receptors that sense ambient GABA and mediate a persistent GABAergic conductance. An increase in tonic inhibition may, however, also be obtained indirectly by inhibiting glial GABA transporters (GATs). These are sodium-coupled membrane transport proteins that normally act to terminate GABA neurotransmitter action by taking up GABA into surrounding astrocytes. The aim of the review is to provide an overview of glial GATs in regulating tonic inhibition, especially in epilepsy and stroke. This entails a comprehensive summary of changes known to occur in GAT expression levels and signalling following epileptic and ischemic insults. Further, we discuss the accumulating pharmacological evidence for targeting GATs in these diseases.

Accelerated cognitive decline in a rodent model for temporal lobe epilepsy

OBJECTIVE

Cognitive impairment is frequently observed in patients with temporal lobe epilepsy. It is hypothesized that cumulative seizure exposure causes accelerated cognitive decline in patients with epilepsy. We investigated the influence of seizure frequency on cognitive decline in a rodent model for temporal lobe epilepsy.

METHODS

Neurobehavioral assessment was performed before and after surgery, after the induction of self-sustaining limbic status epilepticus (SSLSE), and in the chronic phase in which rats experienced recurrent seizures. Furthermore, we assessed potential confounders of memory performance.

RESULTS

Rats showed a deficit in spatial working memory after the induction of the SSLSE, which endured in the chronic phase. A progressive decline in recognition memory developed in SSLSE rats. Confounding factors were absent. Seizure frequency and also the severity of the status epilepticus were not correlated with the severity of cognitive deficits.

SIGNIFICANCE

The effect of the seizure frequency on cognitive comorbidity in epilepsy has long been debated, possibly because of confounders such as antiepileptic medication and the heterogeneity of epileptic etiologies. In an animal model of temporal lobe epilepsy, we showed that a decrease in spatial working memory does not relate to the seizure frequency. This suggests for other mechanisms are responsible for memory decline and potentially a common pathophysiology of cognitive deterioration and the occurrence and development of epileptic seizures. Identifying this common denominator will allow development of more targeted interventions treating cognitive decline in patients with epilepsy. The treatment of interictal symptoms will increase the quality of life of many patients with epilepsy.

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.

Differential susceptibility of cortical and subcortical inhibitory neurons and astrocytes in the long term following diffuse traumatic brain injury

Long-term diffuse traumatic brain injury (dTBI) causes neuronal hyperexcitation in supragranular layers in sensory cortex, likely through reduced inhibition. Other forms of TBI affect inhibitory interneurons in subcortical areas but it is unknown if this occurs in cortex, or in any brain area in dTBI. We investigated dTBI effects on inhibitory neurons and astrocytes in somatosensory and motor cortex, and hippocampus, 8 weeks post-TBI. Brains were labeled with antibodies against calbindin (CB), parvalbumin (PV), calretinin (CR) and neuropeptide Y (NPY), and somatostatin (SOM) and glial fibrillary acidic protein (GFAP), a marker for astrogliosis during neurodegeneration. Despite persistent behavioral deficits in rotarod performance up to the time of brain extraction (TBI = 73.13 ± 5.23% mean ± SEM, Sham = 92.29 ± 5.56%, P < 0.01), motor cortex showed only a significant increase, in NPY neurons in supragranular layers (mean cells/mm² ± SEM, Sham = 16 ± 0.971, TBI = 25 ± 1.51, P = 0.001). In somatosensory cortex, only CR⁺ neurons showed changes, being decreased in supragranular (TBI = 19 ± 1.18, Sham = 25 ± 1.10, P < 0.01) and increased in infragranular (TBI = 28 ± 1.35, Sham = 24 ± 1.07, P < 0.05) layers. Heterogeneous changes were seen in hippocampal staining: CB⁺ decreased in dentate gyrus (TBI = 2 ± 0.382, Sham = 4 ± 0.383, P < 0.01), PV⁺ increased in CA1 (TBI = 39 ± 1.26, Sham = 33 ± 1.69, P < 0.05) and CA2/3 (TBI = 26 ± 2.10, Sham = 20 ± 1.49, P < 0.05), and CR⁺ decreased in CA1 (TBI = 10 ± 1.02, Sham = 14 ± 1.14, P < 0.05). Astrogliosis significantly increased in corpus callosum (TBI = 6.7 ± 0.69, Sham = 2.5 ± 0.38; P = 0.007). While dTBI effects on inhibitory neurons appear region- and type-specific, a common feature in all cases of decrease was that changes occurred in dendrite targeting interneurons involved in neuronal integration. J. Comp. Neurol. 524:3530-3560, 2016. © 2016 Wiley Periodicals, Inc.

Persistent Hyperactivity of Hippocampal Dentate Interneurons After a Silent Period in the Rat Pilocarpine Model of Epilepsy

Profile of GABAergic interneuron activity after pilocarpine-induced status epilepticus (SE) was examined in the rat hippocampal dentate gyrus by analyzing immediate early gene expression and recording spontaneous firing at near resting membrane potential (REM). SE for exact 2 h or more than 2 h was induced in the male Sprague-Dawley rats by an intraperitoneal injection of pilocarpine. Expression of immediate early genes (IEGs) was examined at 1 h, 1 week, 2 weeks or more than 10 weeks after SE. For animals to be examined at 1 h after SE, SE lasted for exact 2 h was terminated by an intraperitoneal injection of diazepam. Spontaneous firing at near the REM was recorded in interneurons located along the border between the granule cell layer and the hilus more than 10 weeks after SE. Results showed that both c-fos and activity-regulated cytoskeleton associated protein (Arc) in hilar GABAergic interneurons were up-regulated after SE in a biphasic manner; they were increased at 1 h and more than 2 weeks, but not at 1 week after SE. Ten weeks after SE, nearly 60% of hilar GABAergic cells expressed c-fos. With the exception of calretinin (CR)-positive cells, percentages of hilar neuronal nitric oxide synthase (nNOS)-, neuropeptide Y (NPY)-, parvalbumin (PV)-, and somatostatin (SOM)-positive cells with c-fos expression are significantly higher than those of controls more than 10 weeks after SE. Without the REM to be more depolarizing and changed threshold potential level in SE-induced rats, cell-attached recording revealed that nearly 90% of hilar interneurons fired spontaneously at near the REM while only 22% of the same cell population did so in the controls. In conclusion, pilocarpine-induced SE eventually leads to a state in which surviving dentate GABAergic interneurons become hyperactive with a subtype-dependent manner; this implies that a fragile balance between excitation and inhibition exists in the dentate gyrus and in addition, the activity-dependent up-regulation of IEGs may underlie plastic changes seen in some types of GABAergic cells in the pilocarpine model of epilepsy.

RNA sequencing reveals region-specific molecular mechanisms associated with epileptogenesis in a model of classical hippocampal sclerosis

We report here the first complete transcriptome analysis of the dorsal (dDG) and ventral dentate gyrus (vDG) of a rat epilepsy model presenting a hippocampal lesion with a strict resemblance to classical hippocampal sclerosis (HS). We collected the dDG and vDG by laser microdissection 15 days after electrical stimulation and performed high-throughput RNA-sequencing. There were many differentially regulated genes, some of which were specific to either of the two sub-regions in stimulated animals. Gene ontology analysis indicated an enrichment of inflammation-related processes in both sub-regions and of axonal guidance and calcium signaling processes exclusively in the vDG. There was also a differential regulation of genes encoding molecules involved in synaptic function, neural electrical activity and neuropeptides in stimulated rats. The data presented here suggests, in the time point analyzed, a remarkable interaction among several molecular components which takes place in the damaged hippocampi. Furthermore, even though similar mechanisms may function in different regions of the DG, the molecular components involved seem to be region specific.

Tonic GABAA Receptors as Potential Target for the Treatment of Temporal Lobe Epilepsy

Tonic GABA_A receptors are a subpopulation of receptors that generate long-lasting inhibition and thereby control network excitability. In recent years, these receptors have been implicated in various neurological and psychiatric disorders, including Parkinson’s disease, schizophrenia, and epilepsy. Their distinct subunit composition and function, compared to phasic GABA_A receptors, opens the possibility to specifically modulate network properties. In this review, the role of tonic GABA_A receptors in epilepsy and as potential antiepileptic target will be discussed.

The role of TRPC6 in seizure susceptibility and seizure-related neuronal damage in the rat dentate gyrus

Transient receptor potential canonical channel-6 (TRPC6) forms Ca(2+)-permeable non-selective cation channels in neurons. Although TRPC6 plays an important role in neurite outgrowth and neuronal survival during development, TRPC6 expression profiles available to identify distinctive hippocampal neuronal damage and hippocampal excitability in epilepsy are less defined. As compared to normal animals, TRPC6 expression was down-regulated in chronic epileptic rats showing spontaneous recurrent seizures. TRPC6 knockdown increased seizure susceptibility, excitability ratio and paired-pulse inhibition in the dentate gyrus (DG) of normal animals. Furthermore, TRPC6 knockdown promoted programmed neuronal necrosis in dentate granule cells, but prevented it in CA1 and CA3 neurons following status epilepticus. The present data suggest for the first time that TRPC6 may inhibit seizure susceptibility and neuronal vulnerability in the rat DG.

Hippocampal neuro-networks and dendritic spine perturbations in epileptogenesis are attenuated by neuroprotectin d1

Purpose

Limbic epileptogenesis triggers molecular and cellular events that foster the establishment of aberrant neuronal networks that, in turn, contribute to temporal lobe epilepsy (TLE). Here we have examined hippocampal neuronal network activities in the pilocarpine post-status epilepticus model of limbic epileptogenesis and asked whether or not the docosahexaenoic acid (DHA)-derived lipid mediator, neuroprotectin D1 (NPD1), modulates epileptogenesis.

Methods

Status epilepticus (SE) was induced by intraperitoneal administration of pilocarpine in adult male C57BL/6 mice. To evaluate simultaneous hippocampal neuronal networks, local field potentials were recorded from multi-microelectrode arrays (silicon probe) chronically implanted in the dorsal hippocampus. NPD1 (570 μg/kg) or vehicle was administered intraperitoneally daily for five consecutive days 24 hours after termination of SE. Seizures and epileptiform activity were analyzed in freely-moving control and treated mice during epileptogenesis and epileptic periods. Then hippocampal dendritic spines were evaluated using Golgi-staining.

Results

We found brief spontaneous microepileptiform activity with high amplitudes in the CA1 pyramidal and stratum radiatum in epileptogenesis. These aberrant activities were attenuated following systemic NPD1 administration, with concomitant hippocampal dendritic spine protection. Moreover, NPD1 treatment led to a reduction in spontaneous recurrent seizures.

Conclusions

Our results indicate that NPD1 displays neuroprotective bioactivity on the hippocampal neuronal network ensemble that mediates aberrant circuit activity during epileptogenesis. Insight into the molecular signaling mediated by neuroprotective bioactivity of NPD1 on neuronal network dysfunction may contribute to the development of anti-epileptogenic therapeutic strategies.

Methanol extract of Ficus platyphylla ameliorates seizure severity, cognitive deficit and neuronal cell loss in pentylenetetrazole-kindled mice

Decoctions of Ficus plathyphylla are used in Nigeria's folk medicine to manage epilepsy for many years and their efficacies are widely acclaimed among the rural communities of Northern Nigeria. In this study, we examined the ameliorative effects of the standardized methanol extract of Ficus platyphylla (FP) stem bark on seizure severity, cognitive deficit and neuronal cell loss in pentylenetetrazole-kindled mice. The (35)S-GTPγS, glutamate and γ-aminobutyric acid receptors binding properties of the extract were also evaluated. Male CD-1 mice were kindled with an initial subeffective dose of pentylenetetrazole (PTZ, 37.5mg/kg, i.p.) for a total of 13 convulsant injections and the treatment groups concurrently received FP (100 and 200mg/kg). Control animals received the same number of saline injections. Twenty-four h after kindling completion the animals' learning performance was tested in a two-way shuttle-box. The animals were challenged with another subeffective dose of PTZ (32.5mg/kg, i.p.) on day 7 after kindling completion. Animals were sacrificed a day after the challenged experiment and the brains were processed for histological investigation. FP ameliorates seizure severity, cognitive deficits and neuronal cell loss in PTZ kindled mice. Components of the extract showed affinity for GABAergic and glutamatergic receptors. Glutamate release was diminished and the (35)S-GTPγS binding assay revealed no intrinsic activity at glutamatergic receptors. Our results revealed that FP contains psychoactive secondary metabolites with anticonvulsant properties, thus supporting the isolation and development of the biologically active components of this medicinal plant as antiepileptic agents.