Cell domain-dependent changes in the glutamatergic and GABAergic drives during epileptogenesis in the rat CA1 region

Alterations in GABAA receptor-mediated inhibition triggered by status epilepticus and their role in epileptogenesis and increased anxiety

The triggers of status epilepticus (SE) in non-epileptic patients can vary widely, from idiopathic causes to exposure to chemoconvulsants. Regardless of its etiology, prolonged SE can cause significant brain damage, commonly resulting in the development of epilepsy, which is often accompanied by increased anxiety. GABAA receptor (GABAAR)-mediated inhibition has a central role among the mechanisms underlying brain damage and the ensuing epilepsy and anxiety. During SE, calcium influx primarily via ionotropic glutamate receptors activates signaling cascades which trigger a rapid internalization of synaptic GABAARs; this weakens inhibition, exacerbating seizures and excitotoxicity. GABAergic interneurons are more susceptible to excitotoxic death than principal neurons. During the latent period of epileptogenesis, the aberrant reorganization in synaptic interactions that follow interneuronal loss in injured brain regions, leads to the formation of hyperexcitable, seizurogenic neuronal circuits, along with disturbances in brain oscillatory rhythms. Reduction in the spontaneous, rhythmic "bursts" of IPSCs in basolateral amygdala neurons is likely to play a central role in anxiogenesis. Protecting interneurons during SE is key to preventing both epilepsy and anxiety. Antiglutamatergic treatments, including antagonism of calcium-permeable AMPA receptors, can be expected to control seizures and reduce excitotoxicity not only by directly suppressing hyperexcitation, but also by counteracting the internalization of synaptic GABAARs. Benzodiazepines, as delayed treatment of SE, have low efficacy due to the reduction and dispersion of their targets (the synaptic GABAARs), but also because themselves contribute to further reduction of available GABAARs at the synapse; furthermore, benzodiazepines may be completely ineffective in the immature brain.

Ligand-gated mechanisms leading to ictogenesis in focal epileptic disorders

We review here the neuronal mechanisms that cause seizures in focal epileptic disorders and, specifically, those involving limbic structures that are known to be implicated in human mesial temporal lobe epilepsy. In both epileptic patients and animal models, the initiation of focal seizures - which are most often characterized by a low-voltage fast onset EEG pattern - is presumably dependent on the synchronous firing of GABA-releasing interneurons that, by activating post-synaptic GABA_A receptors, cause large increases in extracellular [K⁺] through the activation of the co-transporter KCC2. A similar mechanism may contribute to seizure maintenance; accordingly, inhibiting KCC2 activity transforms seizure activity into a continuous pattern of short-lasting epileptiform discharges. It has also been found that interactions between different areas of the limbic system modulate seizure occurrence by controlling extracellular [K⁺] homeostasis. In line with this view, low-frequency electrical or optogenetic activation of limbic networks restrain seizure generation, an effect that may also involve the activation of GABA_B receptors and activity-dependent changes in epileptiform synchronization. Overall, these findings highlight the paradoxical role of GABA_A signaling in both focal seizure generation and maintenance, emphasize the efficacy of low-frequency activation in abating seizures, and provide experimental evidence explaining the poor efficacy of antiepileptic drugs designed to augment GABAergic function in controlling seizures in focal epileptic disorders.

Evolution of interictal activity in models of mesial temporal lobe epilepsy

Interictal activity and seizures are the hallmarks of focal epileptic disorders (which include mesial temporal lobe epilepsy, MTLE) in humans and in animal models. Interictal activity, which is recorded with cortical and intracerebral EEG recordings, comprises spikes, sharp waves and high-frequency oscillations, and has been used in clinical practice to identify the epileptic zone. However, its relation with seizures remains debated. Moreover, it is unclear whether specific EEG changes in interictal activity occur during the time preceding the appearance of spontaneous seizures. This period, which is termed "latent", has been studied in rodent models of MTLE in which spontaneous seizures start to occur following an initial insult (most often a status epilepticus induced by convulsive drugs such as kainic acid or pilocarpine) and may mirror epileptogenesis, i.e., the process leading the brain to develop an enduring predisposition to seizure generation. Here, we will address this topic by reviewing experimental studies performed in MTLE models. Specifically, we will review data highlighting the dynamic changes in interictal spiking activity and high-frequency oscillations occurring during the latent period, and how optogenetic stimulation of specific cell populations can modulate them in the pilocarpine model. These findings indicate that interictal activity: (i) is heterogeneous in its EEG patterns and thus, presumably, in its underlying neuronal mechanisms; and (ii) can pinpoint to the epileptogenic processes occurring in focal epileptic disorders in animal models and, perhaps, in epileptic patients.

Before the first seizure: The developmental imprint of infant epilepsy on neurodevelopment

In light of the heterogeneity of epilepsy, both from a clinical and from an etiological perspective, it is difficult to establish a link between epilepsy and development that can be generalized to all infantile epilepsies. In general however, early-onset epilepsy has a poor developmental prognosis that is significantly linked to several parameters: age at first seizure, drug resistance, treatment, and etiology. This paper discusses the relationship between visible epilepsy parameters (those that allow the diagnosis of epilepsy) and neurodevelopment in infants, with special focus on Dravet syndrome and KCNQ2-related epilepsy, two common developmental and epileptic encephalopathies; and focal epilepsy caused by focal cortical dysplasia, which often begins during infancy. There are a number of reasons why it is difficult to dissect the relationship between seizures and their causes, and we suggest a conceptual model in which epilepsy is a neurodevelopmental disorder whose severity is determined by how the disease imprints itself on the developmental process rather than by the symptoms or etiology. The precocity of this developmental imprint may explain why treating seizures once they occur can have a very slight beneficial effect on development.

Clptm1, a new target in suppressing epileptic seizure by regulating GABAA R-mediated inhibitory synaptic transmission in a PTZ-induced epilepsy model

Disruption of gamma-amino butyric acid type A receptors (GABAA Rs) synaptic clustering and a decrease in the number of GABAA Rs in the plasma membrane are thought to contribute to alteration of the balance between excitatory and inhibitory neurotransmission, which promotes seizure induction and propagation. The multipass transmembrane protein cleft lip and palate transmembrane protein 1 (Clptm1) controls the forward trafficking of GABAA R, thus decaying miniature inhibitory postsynaptic current (mIPSC) of inhibitory synapses. In this study, using a pentylenetetrazol (PTZ)-induced epilepsy rat model, we found that Clptm1 regulates epileptic seizures by modulating GABAA R-mediated inhibitory synaptic transmission. First, we showed that Clptm1 expression was elevated in the PTZ-induced epileptic rats. Subsequently, we found that downregulation of Clptm1 expression protected against PTZ-induced seizures, which was attributed to an increase in the number of GABAA Rγ2s in the plasma membrane and the amplitude of mIPSC. Taken together, our findings identify a new anti-seizure target that provides a theoretical basis for the development of novel strategies for the prevention and treatment of epilepsy.

Differential synaptic mechanism underlying the neuronal modulation of prefrontal cortex, amygdala, and hippocampus in response to chronic postsurgical pain with or without cognitive deficits in rats

Chronic Postsurgical Pain (CPSP) is well recognized to impair cognition, particularly memory. Mounting evidence suggests anatomic and mechanistic overlap between pain and cognition on several levels. Interestingly, the drugs currently used for treating chronic pain, including opioids, gabapentin, and NMDAR (N-methyl-D-aspartate receptor) antagonists, are also known to impair cognition. So whether pain-related cognitive deficits have different synaptic mechanisms as those underlying pain remains to be elucidated. In this context, the synaptic transmission in the unsusceptible group (cognitively normal pain rats) was isolated from that in the susceptible group (cognitively compromised pain rats). It was revealed that nearly two-thirds of the CPSP rats suffered cognitive impairment. The whole-cell voltage-clamp recordings revealed that the neuronal excitability and synaptic transmission in the prefrontal cortex and amygdala neurons were enhanced in the unsusceptible group, while these parameters remained the same in the susceptible group. Moreover, the neuronal excitability and synaptic transmission in hippocampus neurons demonstrated the opposite trend. Correspondingly, the levels of synaptic transmission-related proteins demonstrated a tendency similar to that of the excitatory and inhibitory synaptic transmission. Furthermore, morphologically, the synapse ultrastructure varied in the postsynaptic density (PSD) between the CPSP rats with and without cognitive deficits. Together, these observations indicated that basal excitatory and inhibitory synaptic transmission changes were strikingly different between the CPSP rats with and without cognitive deficits.

Caloric restriction: Anti-inflammatory and antioxidant mechanisms against epileptic seizures

Caloric restriction (CR) possesses different cellular mechanisms. Though there are still gaps in the literature regarding its plausible beneficial effects, the suggestion that this alternative therapy can improve the inflammatory and antioxidant response to control epileptic seizures is explored throughout this study. Epilepsy is the second most prevalent neurodegenerative disease in the world. However, the appropriate mechanisms for it to be fully controlled are still unknown. Neuroinflammation and oxidative stress promote epileptic seizures' appearance and might even aggravate them. There is growing evidence that caloric restriction has extensive anti-inflammatory and antioxidant properties. For instance, nuclear factor erythroid 2-related factor 2 (Nrf2) and all-trans retinoic acid (ATRA) have been proposed to induce antioxidant processes and ulteriorly improve the disease progression. Caloric restriction can be an option for those patients with refractory epilepsy since it allows for anti-inflammatory and antioxidant properties to evolve within the brain areas involved.

Excitatory synaptic transmission in hippocampal area CA1 is enhanced then reduced as chronic epilepsy progresses

This study examines changes in synaptic transmission with progression of the chronic epileptic state. Male Sprague-Dawley rats (P40-45) were injected with either saline or pilocarpine. In rats injected with pilocarpine, status epilepticus ensued. Hippocampal slices were cut 20-60 days or 80-110 days post-treatment. Evoked and miniature EPSCs (mEPSCs) were recorded from CA1 pyramidal neurons using whole-cell voltage-clamp. Fiber volleys were also recorded from stratum radiatum. Evoked EPSCs from the pilocarpine-treated cohort showed enhanced amplitudes 20-60 days post-treatment compared to the saline-treated cohort, whereas mEPSCs recorded from the same age group showed no change in event frequency and a slight but significant decrease in mEPSC amplitude distribution. In contrast, comparing evoked EPSCs and mEPSCs recorded 80-110 days after treatment indicated reduced amplitudes from pilocarpine-treated animals compared to controls. mEPSC inter-event interval decreased. This could be explained by a partial depletion of the ready releasable pool of neurotransmitter vesicles in Schaffer collateral presynaptic terminals of the pilocarpine-treated rats. In both saline- and pilocarpine-treated cohorts, concomitant decreases in mEPSC amplitudes as time after treatment progressed suggest that age-related changes in CA1 circuitry may be partially responsible for changes in synaptic transmission that may influence the chronic epileptic state.

Regulation of chemoconvulsant-induced seizures by store-operated Orai1 channels

KEY POINTS

Temporal lobe epilepsy is a complex neurological disease caused by imbalance of excitation and inhibition in the brain. Growing literature implicates altered Ca²⁺ signalling in many aspects of epilepsy but the diversity of Ca²⁺ channels that regulate this syndrome are not well-understood. Here, we report that mice lacking the store-operated Ca²⁺ channel, Orai1, in the brain show markedly stronger seizures in response to the chemoconvulsants, kainic acid and pilocarpine. Electrophysiological analysis reveals that selective deletion of Orai1 channels in inhibitory neurons disables chemoconvulsant-induced excitation of GABAergic neurons in the CA1 hippocampus. Likewise, deletion of Orai1 in GABAergic neurons abrogates the chemoconvulsant-induced burst of spontaneous inhibitory postsynaptic currents (sIPSCs) on CA1 pyramidal neurons in the hippocampus. This loss of chemoconvulsant inhibition likely aggravates status epilepticus in Orai1 KO mice. These results identify Orai1 channels as regulators of hippocampal interneuron excitability and seizures.

ABSTRACT

Store-operated Orai1 channels are a major mechanism for Ca²⁺ entry in many cells and mediate numerous functions including gene expression, cytokine production and gliotransmitter release. Orai1 is expressed in many regions of the mammalian brain; however, its role in regulating neuronal excitability, synaptic function and brain disorders has only now begun to be investigated. To investigate a potential role of Orai1 channels in status epilepticus induced by chemoconvulsants, we examined acute seizures evoked by intraperitoneal injections of kainic acid (KA) and pilocarpine in mice with a conditional deletion of Orai1 (or its activator STIM1) in the brain. Brain-specific Orai1 and STIM1 knockout (KO) mice exhibited significantly stronger seizures (P = 0.00003 and P < 0.00001), and higher chemoconvulsant-induced mortality (P = 0.02) compared with wildtype (WT) littermates. Electrophysiological recordings in hippocampal brain slices revealed that KA stimulated the activity of inhibitory interneurons in the CA1 hippocampus (P = 0.04) which failed to occur in Orai1 KO mice. Further, KA and pilocarpine increased the frequency of spontaneous IPSCs in CA1 pyramidal neurons >twofold (KA: P = 0.04; pilocarpine: P = 0.0002) which was abolished in Orai1 KO mice. Mice with selective deletion of Orai1 in GABAergic neurons alone also showed stronger seizures to KA (P = 0.001) and pilocarpine (P < 0.00001) and loss of chemoconvulsant-induced increases in sIPSC responses compared with WT controls. We conclude that Orai1 channels regulate chemoconvulsant-induced excitation in GABAergic neurons and that destabilization of the excitatory/inhibitory balance in Orai1 KO mice aggravates chemoconvulsant-mediated seizures. These results identify Orai1 channels as novel molecular regulators of hippocampal neuronal excitability and seizures.

The two faces of synaptic failure in AppNL-G-F knock-in mice

BACKGROUND

Intensive basic and preclinical research into Alzheimer's disease (AD) has yielded important new findings, but they could not yet been translated into effective therapies. One of the reasons is the lack of animal models that sufficiently reproduce the complexity of human AD and the response of human brain circuits to novel treatment approaches. As a step in overcoming these limitations, new App knock-in models have been developed that avoid transgenic APP overexpression and its associated side effects. These mice are proposed to serve as valuable models to examine Aß-related pathology in "preclinical AD."

METHODS

Since AD as the most common form of dementia progresses into synaptic failure as a major cause of cognitive deficits, the detailed characterization of synaptic dysfunction in these new models is essential. Here, we addressed this by extracellular and whole-cell patch-clamp recordings in AppNL-G-F mice compared to AppNL animals which served as controls.

RESULTS

We found a beginning synaptic impairment (LTP deficit) at 3-4 months in the prefrontal cortex of AppNL-G-F mice that is further aggravated and extended to the hippocampus at 6-8 months. Measurements of miniature EPSCs and IPSCs point to a marked increase in excitatory and inhibitory presynaptic activity, the latter accompanied by a moderate increase in postsynaptic inhibitory function.

CONCLUSIONS

Our data reveal a marked impairment of primarily postsynaptic processes at the level of synaptic plasticity but the dominance of a presumably compensatory presynaptic upregulation at the level of elementary miniature synaptic function.

VIP Modulation of Hippocampal Synaptic Plasticity: A Role for VIP Receptors as Therapeutic Targets in Cognitive Decline and Mesial Temporal Lobe Epilepsy

Vasoactive intestinal peptide (VIP) is an important modulatory peptide throughout the CNS acting as a neurotransmitter, neurotrophic or neuroprotective factor. In the hippocampus, a brain area implicated in learning and memory processes, VIP has a crucial role in the control of GABAergic transmission and pyramidal cell activity in response to specific network activity by either VIP-containing basket cells or interneuron-selective (IS) interneurons and this appears to have a differential impact in hippocampal-dependent cognition. At the cellular level, VIP regulates synaptic transmission by either promoting disinhibition, through activation of VPAC₁ receptors, or enhancing pyramidal cell excitability, through activation of VPAC₂ receptors. These actions also control several important synaptic plasticity phenomena such as long-term potentiation (LTP) and long-term depression (LTD). This paper reviews the current knowledge on the activation and multiple functions of VIP expressing cells in the hippocampus and their role in controlling synaptic transmission, synaptic plasticity and learning and memory processes, discussing also the role of VPAC₁ and VPAC₂ VIP receptors in the regulation of these different processes. Furthermore, we address the current knowledge regarding changes in VIP mediated neurotransmission in epileptogenesis and mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS), and discuss the therapeutic opportunities of using selective VIP receptor ligands to prevent epileptogenesis and cognitive decline in MTLE-HS.

The expression of keratan sulfate reveals a unique subset of microglia in the mouse hippocampus after pilocarpine-induced status epileptics

Induction of keratan sulfate in microglia has been found in several animal models of neurological disorders. However, the significance of keratan sulfate-expressing microglia is not fully understood. To address this issue, we analyzed the characteristics of microglia labeled by the 5D4 epitope, a marker of high-sulfated keratan sulfate, in the mouse hippocampus during the latent period after pilocarpine-induced status epilepticus (SE). Only 5D4-negative (5D4^- ) microglia were found in the CA1 region of vehicle-treated controls and pilocarpine-treated mice at 1 day after SE onset. A few 5D4⁺ microglia appeared in the strata oriens and radiatum at 5 days post-SE, and they were distributed into the stratum pyramidale at 14 days post-SE. The expressions of genes related to both anti- and pro-inflammatory cytokines were higher in 5D4⁺ cells than in 5D4^- cells at 5 but not 14 days post-SE. The expressions of genes related to phagocytosis were higher in 5D4⁺ cells than in 5D4^- cells throughout the latent period. The phagocytic activity of microglia, as measured by engulfment of the zymosan bioparticles, was higher in 5D4⁺ cells than in 5D4^- cells. The contact ratios between excitatory synaptic boutons and microglia were also higher in 5D4⁺ cells than in 5D4^- cells at 5 and 14 days post-SE. The excitatory/inhibitory ratios of synaptic boutons within the microglial domain were lower in 5D4⁺ cells than in 5D4^- cells at 14 days post-SE. Our findings indicate that 5D4⁺ microglia may play some role in epileptogenesis via pruning of excitatory synapses during the latent period after SE.

Seizure-Induced Potentiation of AMPA Receptor-Mediated Synaptic Transmission in the Entorhinal Cortex

Excessive excitation is considered one of the key mechanisms underlying epileptic seizures. We investigated changes in the evoked postsynaptic responses of medial entorhinal cortex (ERC) pyramidal neurons by seizure-like events (SLEs), using the modified 4-aminopyridine (4-AP) model of epileptiform activity. Rat brain slices were perfused with pro-epileptic solution contained 4-AP and elevated potassium and reduced magnesium concentration. We demonstrated that 15-min robust epileptiform activity in slices leads to an increase in the amplitude of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated component of the evoked response, as well as an increase in the polysynaptic γ-aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) receptor-mediated components. The increase in AMPA-mediated postsynaptic conductance depends on NMDA receptor activation. It persists for at least 15 min after the cessation of SLEs and is partly attributed to the inclusion of calcium-permeable AMPA receptors in the postsynaptic membrane. The mathematical modeling of the evoked responses using the conductance-based refractory density (CBRD) approach indicated that such augmentation of the AMPA receptor function and depolarization by GABA receptors results in prolonged firing that explains the increase in polysynaptic components, which contribute to overall network excitability. Taken together, our data suggest that AMPA receptor enhancement could be a critical determinant of sustained status epilepticus (SE).

Negative effects of interictal spikes on theta rhythm in human temporal lobe epilepsy

Interictal spike is a biomarker of epilepsy that can occur frequently between seizures. Its potential effects on brain oscillations, especially on theta rhythm (4-8 Hz) that is related to a variety of cognitive processes, remain controversial. Using local field potentials recorded from patients with temporal lobe epilepsy (TLE), we investigated here the impact of spikes on theta rhythm immediately after spikes and during the prolonged periods (lasting 4-36 s) between adjacent spikes. Local field potentials (LFPs) were recorded in different epileptogenic areas including the anterior hippocampus (aH) and the entorhinal cortex (EC) as well as in the extended propagation pathway. We found that interictal spikes had a significant inhibitory effect on theta rhythm. Power of theta rhythm was reduced immediately after spikes, and the inhibitory effect on theta rhythm might sustain during the prolonged between-spike periods. The inhibitory effect was more severe when the epileptogenic areas involved both the aH and EC compared to that involved only a single structure. These observations suggest that interictal spikes have a significant negative impact on theta rhythm and may thus play a role in theta-related cognition changes in patients with TLE.

Nano-CuO impairs spatial cognition associated with inhibiting hippocampal long-term potentiation via affecting glutamatergic neurotransmission in rats

Manufactured metal nanoparticles and their applications are continuously expanding because of their unique characteristics while their increasing use may predispose to potential health problems. Several studies have reported the adverse effects of copper oxide nanoparticles (nano-CuO) relative to ecotoxicity and cell toxicity, whereas little is known about the neurotoxicity of nano-CuO. The present study aimed to examine its effects on spatial cognition, hippocampal function, and the possible mechanisms. Male Wistar rats were used to establish an animal model, and nano-CuO was administered at a dose of 0.5 mg/kg/day for 2 weeks. The Morris water maze (MWM) test was employed to evaluate learning and memory. The long-term potentiation (LTP) from Schaffer collaterals to the hippocampal CA1 region, and the effects of nano-CuO on synases were recorded in the hippocampal CA1 neurons of rats. MWM test showed that learning and memory abilities were impaired significantly by nano-CuO ( p < 0.05). The LTP test demonstrated that the field excitatory postsynaptic potential (fEPSP) slopes were significantly lower in nano-CuO-treated groups compared with the control group ( p < 0.01). Furthermore, the data of whole-cell patch-clamp experiments showed that nano-CuO markedly depressed the frequencies of both spontaneous excitatory postsynaptic currents (sEPSCs) and miniature EPSCs (mEPSCs), indicating an effect of nano-CuO on inhibiting the release frequency of glutamate presynapticly ( p < 0.01). Meanwhile, the amplitudes of both sEPSC and mEPSC were significantly reduced in nano-CuO-treated animals, which suggested that the effect of nano-CuO modulates postsynaptic receptor kinetics ( p < 0.01). Paired pulse facilitation (PPF) ( p < 0.05) and the expression of NR2A, but not NR2B, of N-methyl-d-aspartate (NMDA) subunits ( p < 0.05), were decreased significantly. In conclusion, nano-CuO impaired glutamate transmission presynapticly and postsynapticly, which may contribute importantly to diminished LTP and other induced cognitive deficits.

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.

Persistent seizure control in epileptic mice transplanted with gamma-aminobutyric acid progenitors

OBJECTIVE

A significant proportion of the more than 50 million people worldwide currently suffering with epilepsy are resistant to antiepileptic drugs (AEDs). As an alternative to AEDs, novel therapies based on cell transplantation offer an opportunity for long-lasting modification of epileptic circuits. To develop such a treatment requires careful preclinical studies in a chronic epilepsy model featuring unprovoked seizures, hippocampal histopathology, and behavioral comorbidities.

METHODS

Transplantation of progenitor cells from embryonic medial or caudal ganglionic eminence (MGE, CGE) were made in a well-characterized mouse model of status epilepticus-induced epilepsy (systemic pilocarpine). Behavioral testing (handling and open field), continuous video-electroencephalographic (vEEG) monitoring, and slice electrophysiology outcomes were obtained up to 270 days after transplantation (DAT). Post-hoc immunohistochemistry was used to confirm cell identity.

RESULTS

MGE progenitors transplanted into the hippocampus of epileptic mice rescued handling and open field deficits starting at 60 DAT. In these same mice, an 84% to 88% reduction in seizure activity was observed between 180 and 210 DAT. Inhibitory postsynaptic current frequency, measured on pyramidal neurons in acute hippocampal slices at 270 DAT, was reduced in epileptic mice but restored to naïve levels in epileptic mice receiving MGE transplants. No reduction in seizure activity was observed in epileptic mice receiving intrahippocampal CGE progenitors.

INTERPRETATION

Our findings demonstrate that transplanted MGE progenitors enhance functional GABA-mediated inhibition, reduce spontaneous seizure frequency, and rescue behavioral deficits in a chronic epileptic animal model more than 6 months after treatment. Ann Neurol 2017;82:530-542.

Transition Dynamics of a Dentate Gyrus-CA3 Neuronal Network during Temporal Lobe Epilepsy

In temporal lobe epilepsy (TLE), the variation of chemical receptor expression underlies the basis of neural network activity shifts, resulting in neuronal hyperexcitability and epileptiform discharges. However, dynamical mechanisms involved in the transitions of TLE are not fully understood, because of the neuronal diversity and the indeterminacy of network connection. Hence, based on Hodgkin–Huxley (HH) type neurons and Pinsky–Rinzel (PR) type neurons coupling with glutamatergic and GABAergic synaptic connections respectively, we propose a computational framework which contains dentate gyrus (DG) region and CA3 region. By regulating the concentration range of N-methyl-D-aspartate-type glutamate receptor (NMDAR), we demonstrate the pyramidal neuron can generate transitions from interictal to seizure discharges. This suggests that enhanced endogenous activity of NMDAR contributes to excitability in pyramidal neuron. Moreover, we conclude that excitatory discharges in CA3 region vary considerably on account of the excitatory currents produced by the excitatory pyramidal neuron. Interestingly, by changing the backprojection connection, we find that glutamatergic type backprojection can promote the dominant frequency of firings and further motivate excitatory counterpropagation from CA3 region to DG region. However, GABAergic type backprojection can reduce firing rate and block morbid counterpropagation, which may be factored into the terminations of TLE. In addition, neuronal diversity dominated network shows weak correlation with different backprojections. Our modeling and simulation studies provide new insights into the mechanisms of seizures generation and connectionism in local hippocampus, along with the synaptic mechanisms of this disease.

Transient reduction in theta power caused by interictal spikes in human temporal lobe epilepsy

The inhibitory impacts of spikes on LFP theta rhythms(4-8Hz) are investigated around sporadic spikes(SSs) based on intracerebral EEG of 4 REM sleep patients with temporal lobe epilepsy(TLE) under the pre-surgical monitoring. Sequential interictal spikes in both genesis area and extended propagation pathway are collected, that, SSs genesis only in anterior hippocampus(aH)(possible propagation pathway in Entorhinal cortex(EC)), only in EC(possible propagation pathway in aH), and in both aH and EC synchronously. Instantaneous theta power was estimated by using Gabor wavelet transform, and theta power level was estimated by averaged over time and frequency before SSs(350ms pre-spike) and after SSs(350ms post-spike). The inhibitory effect around spikes was evaluated by the ratio of theta power level difference between pre-spike and post-spike to pre-spike theta power level. The findings were that theta power level was reduced across SSs, and the effects were more sever in the case of SSs in both aH and EC synchronously than either SSs only in EC or SSs only in aH. It is concluded that interictal spikes impair LFP theta rhythms transiently and directly. The work suggests that the reduction of theta power after the interictal spike might be an evaluation indicator of damage of epilepsy to human cognitive rhythms.

Electrocorticographic Dynamics as a Novel Biomarker in Five Models of Epileptogenesis

Postinjury epilepsy (PIE) is a devastating sequela of various brain insults. While recent studies offer novel insights into the mechanisms underlying epileptogenesis and discover potential preventive treatments, the lack of PIE biomarkers hinders the clinical implementation of such treatments. Here we explored the biomarker potential of different electrographic features in five models of PIE. Electrocorticographic or intrahippocampal recordings of epileptogenesis (from the insult to the first spontaneous seizure) from two laboratories were analyzed in three mouse and two rat PIE models. Time, frequency, and fractal and nonlinear properties of the signals were examined, in addition to the daily rate of epileptiform spikes, the relative power of five frequency bands (theta, alpha, beta, low gamma, and high gamma) and the dynamics of these features over time. During the latent pre-seizure period, epileptiform spikes were more frequent in epileptic compared with nonepileptic rodents; however, this feature showed limited predictive power due to high inter- and intra-animal variability. While nondynamic rhythmic representation failed to predict epilepsy, the dynamics of the theta band were found to predict PIE with a sensitivity and specificity of >90%. Moreover, theta dynamics were found to be inversely correlated with the latency period (and thus predict the onset of seizures) and with the power change of the high-gamma rhythm. In addition, changes in theta band power during epileptogenesis were associated with altered locomotor activity and distorted circadian rhythm. These results suggest that changes in theta band during the epileptogenic period may serve as a diagnostic biomarker for epileptogenesis, able to predict the future onset of spontaneous seizures.SIGNIFICANCE STATEMENT Postinjury epilepsy is an unpreventable and devastating disorder that develops following brain injuries, such as traumatic brain injury and stroke, and is often associated with neuropsychiatric comorbidities. As PIE affects as many as 20% of brain-injured patients, reliable biomarkers are imperative before any preclinical therapeutics can find clinical translation. We demonstrate the capacity to predict the epileptic outcome in five different models of PIE, highlighting theta rhythm dynamics as a promising biomarker for epilepsy. Our findings prompt the exploration of theta dynamics (using repeated electroencephalographic recordings) as an epilepsy biomarker in brain injury patients.

Genetically Induced Retrograde Amnesia of Associative Memories After Neuroplastin Ablation

BACKGROUND

Neuroplastin cell recognition molecules have been implicated in synaptic plasticity. Polymorphisms in the regulatory region of the human neuroplastin gene (NPTN) are correlated with cortical thickness and intellectual abilities in adolescents and in individuals with schizophrenia.

METHODS

We characterized behavioral and functional changes in inducible conditional neuroplastin-deficient mice.

RESULTS

We demonstrate that neuroplastins are required for associative learning in conditioning paradigms, e.g., two-way active avoidance and fear conditioning. Retrograde amnesia of learned associative memories is elicited by inducible neuron-specific ablation of Nptn gene expression in adult mice, which shows that neuroplastins are indispensable for the availability of previously acquired associative memories. Using single-photon emission computed tomography imaging in awake mice, we identified brain structures activated during memory recall. Constitutive neuroplastin deficiency or Nptn gene ablation in adult mice causes substantial electrophysiologic deficits such as reduced long-term potentiation. In addition, neuroplastin-deficient mice reveal profound physiologic and behavioral deficits, some of which are related to depression and schizophrenia, which illustrate neuroplastin's essential functions.

CONCLUSIONS

Neuroplastins are essential for learning and memory. Retrograde amnesia after an associative learning task can be induced by ablation of the neuroplastin gene. The inducible neuroplastin-deficient mouse model provides a new and unique means to analyze the molecular and cellular mechanisms underlying retrograde amnesia and memory.

Regulation of the cell surface expression of chloride transporters during epileptogenesis

The process is commonly known as epileptogenesis refers to the cascade of molecular and cellular changes that transform the brain to make it hyperexcitable and capable of generate recurrent spontaneous seizures. Unfortunately, our understanding of the molecular changes that affect the brain during epileptogenesis remains incomplete. Recent evidence suggests that dysfunction of cation-chloride transporters (CCCs) might be one of the factors that contribute to the deficits in inhibitory neurotransmission observed during epileptogenesis. This study analyzed the cell surface expression of CCCs during epileptogenesis and during chronic epilepsy to evaluate if a loss of CCCs from the plasma membrane might contribute to hyperexcitability. Alterations in the plasma membrane expression of CCCs were mostly detected during the early phase of the epileptogenic period, suggesting that dysfunction of CCCs might contribute to the alterations in the chloride gradient previously detected. Together, the findings presented here suggest that aberrant regulation of the plasma membrane levels of CCCs might contribute to the impartment of GABAergic neurotransmission and that CCCs dysfunction might be relevant for the initial appearance of spontaneous seizures.

Analysis and Enhancements of a Prolific Macroscopic Model of Epilepsy

Macroscopic models of epilepsy can deliver surprisingly realistic EEG simulations. In the present study, a prolific series of models is evaluated with regard to theoretical and computational concerns, and enhancements are developed. Specifically, we analyze three aspects of the models: (1) Using dynamical systems analysis, we demonstrate and explain the presence of direct current potentials in the simulated EEG that were previously undocumented. (2) We explain how the system was not ideally formulated for numerical integration of stochastic differential equations. A reformulated system is developed to support proper methodology. (3) We explain an unreported contradiction in the published model specification regarding the use of a mathematical reduction method. We then use the method to reduce the number of equations and further improve the computational efficiency. The intent of our critique is to enhance the evolution of macroscopic modeling of epilepsy and assist others who wish to explore this exciting class of models further.

Functional Integration of Adult-Born Hippocampal Neurons after Traumatic Brain Injury(1,2,3)

Traumatic brain injury (TBI) increases hippocampal neurogenesis, which may contribute to cognitive recovery after injury. However, it is unknown whether TBI-induced adult-born neurons mature normally and functionally integrate into the hippocampal network. We assessed the generation, morphology, and synaptic integration of new hippocampal neurons after a controlled cortical impact (CCI) injury model of TBI. To label TBI-induced newborn neurons, we used 2-month-old POMC-EGFP mice, which transiently and specifically express EGFP in immature hippocampal neurons, and doublecortin-CreER^T2 transgenic mice crossed with Rosa26-CAG-tdTomato reporter mice, to permanently pulse-label a cohort of adult-born hippocampal neurons. TBI increased the generation, outward migration, and dendritic complexity of neurons born during post-traumatic neurogenesis. Cells born after TBI had profound alterations in their dendritic structure, with increased dendritic branching proximal to the soma and widely splayed dendritic branches. These changes were apparent during early dendritic outgrowth and persisted as these cells matured. Whole-cell recordings from neurons generated during post-traumatic neurogenesis demonstrate that they are excitable and functionally integrate into the hippocampal circuit. However, despite their dramatic morphologic abnormalities, we found no differences in the rate of their electrophysiological maturation, or their overall degree of synaptic integration when compared to age-matched adult-born cells from sham mice. Our results suggest that cells born after TBI participate in information processing, and receive an apparently normal balance of excitatory and inhibitory inputs. However, TBI-induced changes in their anatomic localization and dendritic projection patterns could result in maladaptive network properties.

Impaired dendritic inhibition leads to epileptic activity in a computer model of CA3

Temporal lobe epilepsy (TLE) is a common type of epilepsy with hippocampus as the usual site of origin. The CA3 subfield of hippocampus is reported to have a low epileptic threshold and hence initiates the disorder in patients with TLE. This study computationally investigates how impaired dendritic inhibition of pyramidal cells in the vulnerable CA3 subfield leads to generation of epileptic activity. A model of CA3 subfield consisting of 800 pyramidal cells, 200 basket cells (BC) and 200 Oriens-Lacunosum Moleculare (O-LM) interneurons was used. The dendritic inhibition provided by O-LM interneurons is reported to be selectively impaired in some TLEs. A step-wise approach is taken to investigate how alterations in network connectivity lead to generation of epileptic patterns. Initially, dendritic inhibition alone was reduced, followed by an increase in the external inputs received at the distal dendrites of pyramidal cells, and finally additional changes were made at the synapses between all neurons in the network. In the first case, when the dendritic inhibition of pyramidal cells alone was reduced, the local field potential activity changed from a theta-modulated gamma pattern to a prominently gamma frequency pattern. In the second case, in addition to this reduction of dendritic inhibition, with a simultaneous large increase in the external excitatory inputs received by pyramidal cells, the basket cells entered a state of depolarization block, causing the network to generate a typical ictal activity pattern. In the third case, when the dendritic inhibition onto the pyramidal cells was reduced and changes were simultaneously made in synaptic connectivity between all neurons in the network, the basket cells were again observed to enter depolarization block. In the third case, impairment of dendritic inhibition required to generate an ictal activity pattern was lesser than the two previous cases. Moreover, the ictal like activity began earlier in the third case. Hence, our study suggests that greater synaptic plasticity occurring in the whole network due to increase in reception of external excitatory inputs (due to impaired dendritic inhibition) makes the network more susceptible to generation of epileptic activity.

Seizure-related regulation of GABAA receptors in spontaneously epileptic rats

In this study, we analyzed the impact that spontaneous seizures might have on the plasma membrane expression, composition and function of GABAA receptors (GABAARs). For this, the tissue of chronically epileptic rats was collected within 3h of seizure occurrence (≤3h group) or at least 24h after seizure occurrence (≥24h group). A retrospective analysis of seizure frequency revealed that selecting animals on the bases of seizure proximity also grouped animals in terms of overall seizure burden with a higher seizure burden observed in the ≤3h group. A biochemical analysis showed that although animals with more frequent/recent seizures (≤3h group) had similar levels of GABAAR at the plasma membrane they showed deficits in inhibitory neurotransmission. By contrast, the tissue obtained from animals experiencing infrequent seizures (≥24h group) had increased plasma membrane levels of GABAAR and showed no deficit in inhibitory function. Together, our findings offer an initial insight into the molecular changes that might help to explain how alterations in GABAAR function can be associated with differential seizure burden. Our findings also suggest that increased plasma membrane levels of GABAAR might act as a compensatory mechanism to more effectively maintain inhibitory function, repress hyperexcitability and reduce seizure burden. This study is an initial step towards a fuller characterization of the molecular events that trigger alterations in GABAergic neurotransmission during chronic epilepsy.

The vulnerability of calretinin-containing hippocampal interneurons to temporal lobe epilepsy

This review focuses on the vulnerability of a special interneuron type—the calretinin (CR)-containing interneurons—in temporal lobe epilepsy (TLE). CR is a calcium-binding protein expressed mainly by GABAergic interneurons in the hippocampus. Despite their morphological heterogeneity, CR-containing interneurons form a distinct subpopulation of inhibitory cells, innervating other interneurons in rodents and to some extent principal cells in the human. Their dendrites are strongly connected by zona adherentiae and presumably by gap junctions both in rats and humans. CR-containing interneurons are suggested to play a key role in the hippocampal inhibitory network, since they can effectively synchronize dendritic inhibitory interneurons. The sensitivity of CR-expressing interneurons to epilepsy was discussed in several reports, both in animal models and in humans. In the sclerotic hippocampus the density of CR-immunopositive cells is decreased significantly. In the non-sclerotic hippocampus, the CR-containing interneurons are preserved, but their dendritic tree is varicose, segmented, and zona-adherentia-type contacts can be less frequently observed among dendrites. Therefore, the dendritic inhibition of pyramidal cells may be less effective in TLE. This can be partially explained by the impairment of the CR-containing interneuron ensemble in the epileptic hippocampus, which may result in an asynchronous and thus less effective dendritic inhibition of the principal cells. This phenomenon, together with the sprouting of excitatory pathway axons and enhanced innervation of principal cells, may be involved in seizure generation. Preventing the loss of CR-positive cells and preserving the integrity of CR-positive dendrite gap junctions may have antiepileptic effects, maintaining proper inhibitory function and helping to protect principal cells in epilepsy.

Regular-spiking cells in the presubiculum are hyperexcitable in a rat model of temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is the most common form of adult epilepsy, characterized by recurrent seizures originating in the temporal lobes. Here, we examine TLE-related changes in the presubiculum (PrS), a less-studied parahippocampal structure that both receives inputs from and projects to regions affected by TLE. We assessed the state of PrS neurons in TLE electrophysiologically to determine which of the previously identified cell types were rendered hyperexcitable in epileptic rats and whether their intrinsic and/or synaptic properties were altered. Cell types were characterized based on action potential discharge profiles followed by unsupervised hierarchical clustering. PrS neurons in epileptic animals could be divided into three major groups comprising of regular-spiking (RS), irregular-spiking (IR), and fast-adapting (FA) cells. RS cells, the predominant cell type encountered in PrS, were the only cells that were hyperexcitable in TLE. These neurons were previously identified as sending long-range axonal projections to neighboring structures including medial entorhinal area (MEA), and alterations in intrinsic properties increased their propensity for sustained firing of action potentials. Frequency and amplitude of both spontaneous excitatory and inhibitory synaptic events were reduced. Further analysis of nonaction potential-dependent miniature currents (in tetrodotoxin) indicated that reduction in excitatory drive to these neurons was mediated by decreased activity of excitatory neurons that synapse with RS cells concomitant with reduced activity of inhibitory neurons. Alterations in physiological properties of PrS neurons and their ensuing hyperexcitability could entrain parahippocampal structures downstream of PrS, including the MEA, contributing to temporal lobe epileptogenesis.

On the nature of seizure dynamics

Seizures can occur spontaneously and in a recurrent manner, which defines epilepsy; or they can be induced in a normal brain under a variety of conditions in most neuronal networks and species from flies to humans. Such universality raises the possibility that invariant properties exist that characterize seizures under different physiological and pathological conditions. Here, we analysed seizure dynamics mathematically and established a taxonomy of seizures based on first principles. For the predominant seizure class we developed a generic model called Epileptor. As an experimental model system, we used ictal-like discharges induced in vitro in mouse hippocampi. We show that only five state variables linked by integral-differential equations are sufficient to describe the onset, time course and offset of ictal-like discharges as well as their recurrence. Two state variables are responsible for generating rapid discharges (fast time scale), two for spike and wave events (intermediate time scale) and one for the control of time course, including the alternation between 'normal' and ictal periods (slow time scale). We propose that normal and ictal activities coexist: a separatrix acts as a barrier (or seizure threshold) between these states. Seizure onset is reached upon the collision of normal brain trajectories with the separatrix. We show theoretically and experimentally how a system can be pushed toward seizure under a wide variety of conditions. Within our experimental model, the onset and offset of ictal-like discharges are well-defined mathematical events: a saddle-node and homoclinic bifurcation, respectively. These bifurcations necessitate a baseline shift at onset and a logarithmic scaling of interspike intervals at offset. These predictions were not only confirmed in our in vitro experiments, but also for focal seizures recorded in different syndromes, brain regions and species (humans and zebrafish). Finally, we identified several possible biophysical parameters contributing to the five state variables in our model system. We show that these parameters apply to specific experimental conditions and propose that there exists a wide array of possible biophysical mechanisms for seizure genesis, while preserving central invariant properties. Epileptor and the seizure taxonomy will guide future modeling and translational research by identifying universal rules governing the initiation and termination of seizures and predicting the conditions necessary for those transitions.

Reorganization of supramammillary-hippocampal pathways in the rat pilocarpine model of temporal lobe epilepsy: evidence for axon terminal sprouting

In mesial temporal lobe epilepsy (MTLE), spontaneous seizures likely originate from a multi-structural epileptogenic zone, including several regions of the limbic system connected to the hippocampal formation. In this study, we investigate the structural connectivity between the supramammillary nucleus (SuM) and the dentate gyrus (DG) in the model of MTLE induced by pilocarpine in the rat. This hypothalamic nucleus, which provides major extracortical projections to the hippocampal formation, plays a key role in the regulation of several hippocampus-dependent activities, including theta rhythms, memory function and emotional behavior, such as stress and anxiety, functions that are known to be altered in MTLE. Our findings demonstrate a marked reorganization of DG afferents originating from the SuM in pilocarpine-treated rats. This reorganization, which starts during the latent period, is massive when animals become epileptic and continue to evolve during epilepsy. It is characterized by an aberrant distribution and an increased number of axon terminals from neurons of both lateral and medial regions of the SuM, invading the entire inner molecular layer of the DG. This reorganization, which reflects an axon terminal sprouting from SuM neurons, could contribute to trigger spontaneous seizures within an altered hippocampal intrinsic circuitry.

Dynamics of interictal spikes and high-frequency oscillations during epileptogenesis in temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is characterized in humans and in animal models by a seizure-free latent phase that follows an initial brain insult; this period is presumably associated to plastic changes in temporal lobe excitability and connectivity. Here, we analyzed the occurrence of interictal spikes and high frequency oscillations (HFOs; ripples: 80-200Hz and fast ripples: 250-500Hz) from 48h before to 96h after the first seizure in the rat pilocarpine model of MTLE. Interictal spikes recorded with depth EEG electrodes from the hippocampus CA3 area and entorhinal cortex (EC) were classified as type 1 (characterized by a spike followed by a wave) or type 2 (characterized by a spike with no wave). We found that: (i) there was a switch in the distribution of both types of interictal spikes before and after the occurrence of the first seizure; during the latent phase both types of interictal spikes predominated in the EC whereas during the chronic phase both types of spikes predominated in CA3; (ii) type 2 spike duration decreased in both regions from the latent to the chronic phase; (iii) type 2 spikes associated to fast ripples occurred at higher rates in EC compared to CA3 during the latent phase while they occurred at similar rates in both regions in the chronic phase; and (iv) rates of fast ripples outside of spikes were higher in EC compared to CA3 during the latent phase. Our findings demonstrate that the transition from the latent to the chronic phase is paralleled by dynamic changes in interictal spike and HFO expression in EC and CA3. We propose that these changes may represent biomarkers of epileptogenicity in MTLE.

Transient impact of spike on theta rhythm in temporal lobe epilepsy

Epileptic spike is an indicator of hyper-excitability and hyper-synchrony of neural networks. While cognitive deficit in epilepsy is a common observation, how spikes transiently influence brain oscillations, especially those essential for cognitive functions, remains obscure. Here we aimed to quantify the transient impacts of sporadic spikes on theta oscillations and investigate how such impacts may evolve during epileptogenesis. Longitudinal depth EEG data were recorded in the CA1 area of pilocarpine temporal lobe epilepsy (TLE) rat models. Phase stability, a measure of synchrony, and theta power were estimated around spikes as well as in the protracted spike-free periods (FP) at least 1h after spike bursts. We found that the change in theta power did not correlate with the change in phase stability. More importantly, the impact of spikes on theta rhythm was highly time-dependent. While theta power decreased abruptly after spikes both in the latent and chronic stages, changes of theta phase stability demonstrated opposite trends in the latent and chronic stages, potentially due to the substantial reorganization of neural circuits along epileptogenesis. During FP, theta phase stability was significantly higher than the baseline level before injections, indicating that hyper-synchrony remained even hours after the spike bursts. We concluded that spikes have transient negative effects on theta rhythm, however, impacts are different during latent and chronic stages, implying that its influence on cognitive processes may also change over time during epileptogenesis.

Global hyper-synchronous spontaneous activity in the developing optic tectum

Studies of patterned spontaneous activity can elucidate how the organization of neural circuits emerges. Using in vivo two-photon Ca²⁺ imaging, we studied spatio-temporal patterns of spontaneous activity in the optic tectum of Xenopus tadpoles. We found rhythmic patterns of global synchronous spontaneous activity between neurons, which depends on visual experience and developmental stage. By contrast, synchronous spontaneous activity between non-neuronal cells is mediated more locally. To understand the source of the neuronal spontaneous activity, input to the tectum was systematically removed. Whereas removing input from the visual or mechanosensory system alone had little effect on patterned spontaneous activity, removing input from both systems drastically altered it. These results suggest that either input is sufficient to maintain the intrinsically generated spontaneous activity and that patterned spontaneous activity results from input from multisensory systems. Thus, the amphibian midbrain differs from the mammalian visual system, whose spontaneous activity is controlled by retinal waves.

The possible role of GABAA receptors and gephyrin in epileptogenesis

The term epileptogenesis refers to a dynamic alteration in neuronal excitability that promotes the appearance of spontaneous seizures. Temporal lobe epilepsy, the most common type of acquired epilepsy, often develops after an insult to the brain such as trauma, febrile seizures, encephalitis, or status epilepticus. During the pre-epileptic state (also referred as latent or silent period) there is a plethora of molecular, biochemical, and structural changes that lead to the generation of recurrent spontaneous seizures (or epilepsy). The specific contribution of these alterations to epilepsy development is unclear, but a loss of inhibition has been associated with the increased excitability detected in the latent period. A rapid increase in neuronal hyperexcitability could be due, at least in part, to a decline in the number of physiologically active GABAA receptors (GABAAR). Altered expression of scaffolding proteins involved in the trafficking and anchoring of GABAAR could directly impact the stability of GABAergic synapses and promote a deficiency in inhibitory neurotransmission. Uncovering the molecular mechanisms operating during epileptogenesis and its possible impact on the regulation of GABAAR and scaffolding proteins may offer new targets to prevent the development of epilepsy.

Phase shift in the 24-hour rhythm of hippocampal EEG spiking activity in a rat model of temporal lobe epilepsy

For over a century epileptic seizures have been known to cluster at specific times of the day. Recent studies have suggested that the circadian regulatory system may become permanently altered in epilepsy, but little is known about how this affects neural activity and the daily pattern of seizures. To investigate, we tracked long-term changes in the rate of spontaneous hippocampal EEG spikes (SPKs) in a rat model of temporal lobe epilepsy. In healthy animals, SPKs oscillated with near 24-h period; however, after injury by status epilepticus, a persistent phase shift of ∼12 h emerged in animals that later went on to develop chronic spontaneous seizures. Additional measurements showed that global 24-h rhythms, including core body temperature and theta state transitions, did not phase shift. Instead, we hypothesized that locally impaired circadian input to the hippocampus might be responsible for the SPK phase shift. This was investigated with a biophysical computer model in which we showed that subtle changes in the relative strengths of circadian input could produce a phase shift in hippocampal neural activity. MRI provided evidence that the medial septum, a putative circadian relay center for the hippocampus, exhibits signs of damage and therefore could contribute to local circadian impairment. Our results suggest that balanced circadian input is critical to maintaining natural circadian phase in the hippocampus and that damage to circadian relay centers, such as the medial septum, may disrupt this balance. We conclude by discussing how abnormal circadian regulation may contribute to the daily rhythms of epileptic seizures and related cognitive dysfunction.

Epileptic synapsin triple knockout mice exhibit progressive long-term aberrant plasticity in the entorhinal cortex

Studying epileptogenesis in a genetic model can facilitate the identification of factors that promote the conversion of a normal brain into one chronically prone to seizures. Synapsin triple-knockout (TKO) mice exhibit adult-onset epilepsy, thus allowing the characterization of events as preceding or following seizure onset. Although it has been proposed that a congenital reduction in inhibitory transmission is the underlying cause for epilepsy in these mice, young TKO mice are asymptomatic. We report that the genetic lesion exerts long-term progressive effects that extend well into adulthood. Although inhibitory transmission is initially reduced, it is subsequently strengthened relative to its magnitude in control mice, so that the excitation to inhibition balance in adult TKOs is inverted in favor of inhibition. In parallel, we observed long-term alterations in synaptic depression kinetics of excitatory transmission and in the extent of tonic inhibition, illustrating adaptations in synaptic properties. Moreover, age-dependent acceleration of the action potential did not occur in TKO cortical pyramidal neurons, suggesting wide-ranging secondary changes in brain excitability. In conclusion, although congenital impairments in inhibitory transmission may initiate epileptogenesis in the synapsin TKO mice, we suggest that secondary adaptations are crucial for the establishment of this epileptic network.

Changes in interictal spike features precede the onset of temporal lobe epilepsy

OBJECTIVE

One cornerstone event during epileptogenesis is the occurrence of the first spontaneous seizure (SZ1). It is therefore important to identify biomarkers of the network alterations leading to SZ1. In experimental models of temporal lobe epilepsy (TLE), interictal-like activity (ILA) precedes SZ1 by several days. The goal of this study was to determine whether ILA dynamics bore electrophysiological features signaling the impeding transition to SZ1.

METHODS

Experimental TLE was triggered by pilocarpine- or kainic acid-induced status epilepticus (SE). Continuous electroencephalographic recordings were performed 7 days before and up to 40 days after SE. The amplitude and duration of the spike and wave components of interictal spikes were analyzed.

RESULTS

Two types of interictal spikes were distinguished: type 1, with a spike followed by a long-lasting wave, and type 2, with a spike without wave. The number, amplitude, and duration of type 1 spikes started to decrease, whereas the number of type 2 spikes increased, several days before SZ1, reaching their minimum/maximum values just before SZ1.

INTERPRETATION

The change in ILA pattern could constitute a predictive biomarker of SZ1. The mechanisms underlying these dynamic modifications and their functional impact are discussed in the context of the construction of an epileptogenic network.

Potential role of drebrin a, an f-actin binding protein, in reactive synaptic plasticity after pilocarpine-induced seizures: functional implications in epilepsy

Several neurological disorders characterized by cognitive deficits, including Alzheimer's disease, down syndrome, and epilepsy exhibit abnormal spine density and/or morphology. Actin-based cytoskeleton network dynamics is critical for the regulation of spine morphology and synaptic function. In this paper, I consider the functions of drebrin A in cell shaping, spine plasticity, and synaptic function. Developmentally regulated brain protein (drebrin A) is one of the most abundant neuron-specific binding proteins of F-actin and its expression is increased in parallel with synapse formation. Drebrin A is particularly concentrated in dendritic spines receiving excitatory inputs. Our recent findings point to a critical role of DA in dendritic spine structural integrity and stabilization, likely via regulation of actin cytoskeleton dynamics, and glutamatergic synaptic function that underlies the development of spontaneous recurrent seizures in pilocarpine-treated animals. Further research into this area may provide useful insights into the pathology of status epilepticus and epileptogenic mechanisms and ultimately may provide the basis for future treatment options.

Transition to seizure: from "macro"- to "micro"-mysteries

One of the most terrifying aspects of epilepsy is the sudden and apparently unpredictable transition of the brain into the pathological state of an epileptic seizure. The pathophysiology of the transition to seizure still remains mysterious. Herein we review some of the key concepts and relevant literatures dealing with this enigmatic transitioning of brain states. At the "MACRO" level, electroencephalographic (EEG) recordings at time display preictal phenomena followed by pathological high-frequency oscillations at the seizure onset. Numerous seizure prediction algorithms predicated on identifying changes prior to seizure onset have met with little success, underscoring our lack of understanding of the dynamics of transition to seizure, amongst other inherent limitation. We then discuss the concept of synchronized hyperexcited oscillatory networks underlying seizure generation. We consider these networks as weakly coupled oscillators, a concept which forms the basis of some relevant mathematical modeling of seizure transitions. Next, the underlying "MICRO" processes involved in seizure generation are discussed. The depolarization of the GABA(A) chloride reversal potential is a major concept, facilitating epileptogenesis, particularly in immature brain. Also the balance of inhibitory and excitatory local neuronal networks plays an important role in the process of transitioning to seizure. Gap junctional communication, including that which occurs between glia, as well as ephaptic interactions are increasingly recognized as critical for seizure generation. In brief, this review examines the evidence regarding the characterization of the transition to seizure at both the "MACRO" and "MICRO" levels, trying to characterize this mysterious yet critical problem of the brain state transitioning into a seizure.

Neuron-restrictive silencer factor-mediated hyperpolarization-activated cyclic nucleotide gated channelopathy in experimental temporal lobe epilepsy

OBJECTIVE

Enduring, abnormal expression and function of the ion channel hyperpolarization-activated cyclic adenosine monophosphate gated channel type 1 (HCN1) occurs in temporal lobe epilepsy (TLE). We examined the underlying mechanisms, and investigated whether interfering with these mechanisms could modify disease course.

METHODS

Experimental TLE was provoked by kainic acid-induced status epilepticus (SE). HCN1 channel repression was examined at mRNA, protein, and functional levels. Chromatin immunoprecipitation was employed to identify the transcriptional mechanism of repressed HCN1 expression, and the basis for their endurance. Physical interaction of the repressor, NRSF, was abolished using decoy oligodeoxynucleotides (ODNs). Video/electroencephalographic recordings were performed to assess the onset and initial pattern of spontaneous seizures.

RESULTS

Levels of NRSF and its physical binding to the Hcn1 gene were augmented after SE, resulting in repression of HCN1 expression and HCN1-mediated currents (I(h) ), and reduced I(h) -dependent resonance in hippocampal CA1 pyramidal cell dendrites. Chromatin changes typical of enduring, epigenetic gene repression were apparent at the Hcn1 gene within a week after SE. Administration of decoy ODNs comprising the NRSF DNA-binding sequence (neuron restrictive silencer element [NRSE]), in vitro and in vivo, reduced NRSF binding to Hcn1, prevented its repression, and restored I(h) function. In vivo, decoy NRSE ODN treatment restored theta rhythm and altered the initial pattern of spontaneous seizures.

INTERPRETATION

Acquired HCN1 channelopathy derives from NRSF-mediated transcriptional repression that endures via chromatin modification and may provide insight into the mechanisms of a number of channelopathies that coexist with, and may contribute to, the conversion of a normal brain into an epileptic one.

Melamine impairs spatial cognition and hippocampal synaptic plasticity by presynaptic inhibition of glutamatergic transmission in infant rats

The scandal of melamine-contamination has not been quite blown out, since the toxicity of melamine continues to raise concerns for public health. It has been well known that fetus and infant periods play the most important roles in brain development, whereas little has been done on the harmful effects of melamine on the center nervous system (CNS) of children. In the present study, we investigated the effects of melamine on behavioral and electrophysiology alternations in rats, and the effects of melamine on synaptic transmission were examined using whole-cell patch-clamp technique in the hippocampal CA1 neurons of infant rats. Morris water maze (MWM) test showed that learning and memory abilities were impaired significantly by melamine. The long-time potentiation (LTP) test exhibited that the field excitatory postsynaptic potential (fEPSP) slopes were significantly lower in melamine group compared to that in control group. Furthermore, the data of whole-cell patch-clamp experiments showed that melamine decreased the frequencies of both spontaneous EPSCs (sEPSCs) and minitura EPSCs (mEPSCs) to the same extent (about 76% and 78% respectively). However, there were no significant changes in sEPSC or mEPSC amplitude or kinetics after melamine addition, indicating that the effect of melamine on glutamatergic transmission was probably presynaptic. In conclusion, melamine reduced the release of glutamate in presynaptic transmission of hippocampus, which partly resulted in diminished LTP and further damaged the function of learning and memory.